QUEENSLAND UNIVERSITY OF TECHNOLOGY

Vibrational spectroscopic investigation of polymer melt processing

A thesis presented in fulfilment of the requirements for the degree of

Doctor of Philosophy

Lalehvash Moghaddam

Under the Supervision of:

A/Professor Peter M. Fredericks Professor Graeme A. George

A/Professor Peter Halley

School of Physical and Chemical Sciences

September 2008

ii

iii

ABSTRACT

A polymer is rarely used as a pure material and the baseline physical, chemical and

rheological properties such as molecular weight, strength, stiffness and viscosity are

often modified by the addition of fillers or by blending with another polymer.

However, as many polymers are immiscible, compatibilisation and graft processing

polymer blends are very important techniques to increase miscibility of the blends as

well as to improve chemical, physical and mechanical properties.

Reactive extrusion, or melt-state processing, is one of the most appropriate

techniques for improving polymer properties. Compatibilisation and graft polymer

processing are often carried out under reactive extrusion conditions. This technique

is an efficient approach because it is easy, inexpensive and has a short processing

time. Although reactive extrusion has numerous advantages one of the limitations is

degradation of the polymer under the high temperatures and mechanical stresses

encountered.

In the polymer industry, because of increasing customer demand for improved

product quality, optimising the polymerisation process by decreasing product costs

and controlling the reaction during polymerisation has become more important. It

can be said that any method used for monitoring the polymerisation process has to be

fast, accurate and reliable. Both in-line and on-line methods may be involved in in-

process monitoring. The primary information from in-process monitoring is used for

identifying and understanding molecular structure and changes, optimising and

improving process modelling and understanding whether the process is under

control. This also involves considering whether the products have the required

properties.

This thesis describes research in a number of aspects of melt processing of polymers,

including examination of extruded products, an in situ spectroscopic study of the

reaction of MAH and PP, a study of the melt processing of TPU, and a study of the

use of nitroxide radicals as probes for degradation reactions.

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As mentioned previously, a suitable method for improving polymer properties is

polymer blending. Starch is a hydrophilic biodegradable polymer which may be

blended with other polymers to produce biodegradable products. In spite of its

benefits, it is immiscible with most synthetic polymers, such as polyesters. The main

technique for improving the miscibility of starch with the other polymer is a grafting

reaction.

The reactive extrusion technique was applied to the production of starch and

polyester blends, the product of which was a biodegradable aliphatic polyester. In

this process dicumyl peroxide (DCP) and maleic anhydride (MAH) were used as an

initiator and cross-linker, respectively. Extruded samples were investigated by

infrared microscopic mapping using the attenuated total reflectance (ATR)

technique. Measurement of various band parameters from the spectra allowed IR

maps to be constructed with semi-quantitative information about the distribution of

blend components. IR maps were generated by measuring the band area ratio of O-H

and C=O stretching bands which are related to starch and polyester, respectively.

This was the first time this method has been used for understanding the homogeneity

of a polymer blend system. This method successfully indicated that the

polyester/starch blend was not a homogenised blend. It was concluded that to

improve the homogeneity the reaction conditions should be modified.

Another important compatibilisation reaction is the reaction between a polyolefin

and MAH. This was investigated by combining a near infrared (NIR) spectrometer

with a small laboratory scale extruder, a Haake Minilab. The NIR spectra were

collected in situ during melt processing by the use of a fibre optic cable. In addition

to this the viscosity of the polymer melt was measured continuously during

processing through two pressure transducers within the Minilab extruder. The vinyl

C-H stretch overtone of the MAH was clearly seen in the NIR spectra near 6100 cm-1

and diminished over time as the MAH reacted with PP. The spectra obtained were

analysed by two techniques: principal component analysis (PCA); and peak area

ratios. The peak area ratios were calculated using the =C-H first overtone of MAH

with respect to the band observed between 6600 and 7400 cm-1. This band

corresponds to a combination band of CH2 and CH3 in PP and was unchanged during

the reaction. These data facilitated interpretation of the reaction kinetics and

v

experiments at different temperatures allowed determination of the activation energy

of the reaction. These results have thrown new light on the PP-MAH reaction

mechanism. It was also shown that although the presence of DCP causes production

of a high concentration of macro-radicals it does not have any effect on the rate and

kinetics of the reaction.

As mentioned previously, one of the limitations of reactive extrusion is degradation

of the polymer under high temperatures and shear rates. Hindered amine stabilisers

(HAS) are often used as inhibitors to control the thermal-oxidative degradation of

polymers. They are used in various polymeric materials but were primarily

developed for polyolefins, particularly PP. The stabilisation mechanism of HAS

involves interaction firstly with the alkyl peroxyl radicals produced during oxidative

degradation so that the hindered amine converts to the corresponding nitroxide. The

nitroxide is then able to capture a carbon-centred radical and so retard the subsequent

degradation chain reaction.

1,1,3,3- tetramethyldibenzo[e,g]isoindoline-2-yloxyl (TMDBIO) was used as a probe

for investigation of PP during reactive extrusion conditions. The TMDBIO is a

profluorescent compound that has been used previously to identify polymer

degradation. In the radical form, there is no fluorescence since the unpaired spin on

the nitroxide quenches the fluorescence of the phenanthrene moiety. When the

radical is removed (by radical trapping or reduction) fluorescence is observed. As a

result, the location and intensity of fluorescence can be used as a probe for

identification of degradation and to determine the concentration of carbon-centred

radicals produced during thermal or mechanical degradation such as occurs during

reaction processing. This novel method shows that, the degradation of PP started at

the early stage of processing. Also this method can be used as a useful technique to

modify the processing conditions to decrease degradation of the polymer during

processing.

The second system investigated using in situ monitoring via the NIR fibre optic was

the melt processing of a TPU nano-composite. This was the first time that the in situ

monitoring of TPU nano-composite had been examined. In this investigation the

effect of temperature during processing on the TPU molecular structure and

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rheological behaviour was again investigated. In addition, dispersion of clay nano-

particles through the TPU matrix and rheological changes due to this was

investigated. This investigation was successful in that it was found that several

factors affected the viscosity of the nano-composite. However, to fully understand

the degradation mechanism and viscosity changes further studies must be performed.

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LIST OF PUBLICATIONS

Papers

• L. Moghaddam, L. Rintoul, P.J. Halley, P.M. Fredericks; Infrared microspectroscopic mapping of the homogeneity of extruded blends: Application to starch/polyester blends; Polymer Testing, 25 (2006) 16-21

Oral Presentations:

• “Investigation of Polypropylene Degradation in Reactive Extrusion by using a novel Nitroxide as a probe” at 29th Australasian Polymer Symposium (APS2007), Hobart, 11 – 15 February 2007

• “In-situ monitoring of reactive extrusion- laboratory scale” at 7th

Australasian Vibrational Spectroscopy (ACOVS2007), Wollongong, Australia, 26 – 28 September 2007

Poster Presentations:

• “Homogeneity of Extruded Starch-Polyester Blends” at the 6th Australian Conference on Vibrational Spectroscopy , University of Sydney, 28 – 30 September 2005

• “Investigation of Homogeneity of Extruded Starch-Polyester blends by

FTIR/ATR Microspectroscopy” at 28th Australasian Polymer Symposium (APS2006) and the Australasian Society for Biomaterials 16th Annual Conference, Rotorua New Zealand, 5 – 9 February 2006

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DECLARATION OF ORIGINAL AUTHORSHIP

The work submitted in this thesis has not been previously submitted for a degree or

diploma at this or any other educational institution. To the best of my knowledge and

belief, the information contained in this thesis contains no material previously

published or written by any other person except where due reference is made.

Signed ………………………..

Dated ………………………..

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ACKNOWLEDGEMENTS This document is the result of the contributions from many people – those that have

contributed direct information that is described within and those that have shaped me

to be able to produce this document. My work has spanned across several

universities, with the majority of work being performed at the Queensland University

of Technology (QUT) and the University of Queensland (UQ) and thus I have

collaborated with many people that have provided positive support for the research

that I was undertaking. I appreciate their assistance and thank them all.

A/Prof. Peter M. Fredericks, who not only took on the large role of being my

principal supervisor, but also for providing constant help and guidance through the

project and in particular for poring over my thesis for many hours. Moreover, I am

grateful for him encouraging and challenging me to get out of my comfort zone and

explore the international nature of research.

Prof. Graeme A. George, my co-supervisor, for his guidance and dedication in

ensuring I was doing things right. In addition, I appreciate him spending his own

time to assist with the research, thesis and paper writing.

A/Prof. Peter Halley, my co-supervisor, for initiating the new direction of the

research project and guidance with polymer rheology and reactive extrusion, and

without whom it would have been impossible to perform the large proportion of this

project. He also provided me with a laboratory, facilities and office at UQ for the

majority of my candidature.

I appreciate Dr Llew Rintoul for training me in spectroscopic techniques and

spectrum interpretation skills and his great help and encouragements throughout the

project.

I acknowledge A/Prof. Darren. J. Martin for sharing his experience and knowledge

and providing kind support for my understanding of the thermoplastic polyurethane

and nano-composites behaviour.

I appreciate A/Prof. Steven Bottle and James Blinco for sharing their experience and

knowledge, providing the nitroxide compounds and helping me to understand how

nitroxides behave.

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I appreciate Dr Sarah Ede for her special help editing my thesis.

I also wish to acknowledge the members of the QUT and UQ polymer groups who

afforded me a friendly and very supportive environment. In addition, thanks to my

other colleagues who provided enormous entertainment and motivation.

Thank you to QUT for providing financial support and a fee waiver scholarship.

And finally, I thank my family and my friends who all have been very supportive

over the course of the research.

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TABLE OF CONTENTS LIST OF PUBLICATIONS ...................................................................................... vii

DECLARATION OF ORIGINAL AUTHORSHIP .............................................. viii

ACKNOWLEDGEMENTS ........................................................................................ ix

TABLE OF CONTENTS ............................................................................................ xi

TABLE OF FIGURES ............................................................................................... xv

TABLE OF TABLES ................................................................................................. xx

LIST OF ABBREVIATIONS .................................................................................. xxi

Chapter 1 Introduction ................................................................................................ 1

1.1 Introduction .................................................................................................... 1

1.2 Extrusion technology ..................................................................................... 1

1.3 Reactive extrusion .......................................................................................... 2

1.4 Modification or functionalisation using reactive extrusion ........................... 5

1.4.1 Examples of modification in reactive extrusion ..................................... 6

1.5 Compatibilisation process ............................................................................ 10

1.5.1 Example of compatibilisation extrusion............................................... 10

1.6 Process conditions in reactive extrusion ...................................................... 15

1.7 Features of reactive extrusion ...................................................................... 16

1.8 Rheological modelling of extruders ............................................................. 21

1.8.1 Flow ..................................................................................................... 21

1.8.2 Extruder size ........................................................................................ 22

1.8.3 Extruder volume ................................................................................... 22

1.8.4 Shear rate .............................................................................................. 23

1.9 Degradation of polymers .............................................................................. 24

1.9.1 Degradation during reactive extrusion ................................................. 25

1.10 Monitoring reactive extrusion ...................................................................... 29

1.10.1 Off-line monitoring .............................................................................. 30

1.10.2 On-line monitoring ............................................................................... 31

1.11 Vibrational spectroscopy .............................................................................. 33

1.11.1 Infrared spectroscopy ........................................................................... 33

1.11.2 Raman spectroscopy ............................................................................ 37

1.12 Project outline .............................................................................................. 37

Chapter 2 Materials and experimental techniques ................................................. 39

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2.1 Introduction .................................................................................................. 39

2.2 Materials ....................................................................................................... 39

2.2.1 Polypropylene ...................................................................................... 39

2.2.2 Maleic anhydride .................................................................................. 39

2.2.3 Dicumyl peroxide, xylene and acetone ................................................ 39

2.2.4 Nitroxide .............................................................................................. 39

2.2.5 Diethyl ether ......................................................................................... 40

2.3 Experiments .................................................................................................. 40

2.3.1 Laboratory scale melt processing: Minilab extruder ............................ 40

2.3.2 Near-IR spectroscopy ........................................................................... 44

2.3.3 ATR/FTIR spectroscopy ...................................................................... 44

2.3.4 Principal Components Analysis ........................................................... 44

2.3.5 Fluorescence spectroscopy ................................................................... 44

2.3.6 Raman spectroscopy ............................................................................ 45

2.3.7 Thermogravimetric Analysis ................................................................ 45

Chapter 3 Investigation of homogeneity of extruded starch/polyester blends by

using infrared microspectroscopic mapping ........................................................... 46

3.1 Introduction .................................................................................................. 46

3.2 Experimental ................................................................................................ 48

3.2.1 Materials ............................................................................................... 48

3.2.2 Sample preparation............................................................................... 49

3.2.3 Micro-ATR/FTIR ................................................................................. 49

3.2.4 Construction of images ........................................................................ 50

3.3 Results and discussion .................................................................................. 51

3.3.1 ATR spectra ......................................................................................... 51

3.3.2 IR images ............................................................................................. 53

3.4 Conclusions .................................................................................................. 55

Chapter 4 Grafting of maleic anhydride onto polypropylene by reactive

extrusion: a laboratory scale study ........................................................................... 56

4.1 Introduction .................................................................................................. 56

4.2 Experimental ................................................................................................ 58

4.2.1 Laboratory scale melt state processing and NIR spectroscopy ............ 58

4.2.2 ATR/FTIR ............................................................................................ 59

4.2.3 Determination of grafting efficiency .................................................... 60

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4.2.4 Data analysis ........................................................................................ 60

4.3 Results and discussion .................................................................................. 60

4.3.1 In situ NIR Spectroscopy ..................................................................... 60

4.3.2 NIR monitoring and viscosity investigation of PP processing............. 63

4.3.3 Investigation of final product by ATR/FTIR ....................................... 67

4.3.4 Investigation into the effect of initiator on final product ..................... 69

4.3.5 Effect of temperature on PP-MAH graft processing ............................ 70

4.3.6 Effect of DCP on PP-MAH graft processing ....................................... 74

4.3.7 Investigation into the effect of MAH and DCP concentration on graft

processing ..................................................................................... 78

4.3.8 Viscosity ............................................................................................... 80

4.3.9 TGA investigation of products ............................................................. 84

4.3.10 Mechanism of the grafting reaction ..................................................... 86

4.3.11 Kinetics of the MAH-grafted PP process ............................................. 90

4.3.12 Activation energy of MAH graft modification of PP........................... 98

4.3.13 Conclusions .......................................................................................... 99

Chapter 5 Investigation of PP degradation using a novel nitroxide probe during

melt processing in reactive extrusion: A laboratory scale study ......................... 101

5.1 Introduction ................................................................................................ 101

5.1.1 Degradation of PP .............................................................................. 103

5.2 Experimental .............................................................................................. 104

5.2.1 Sample preparation............................................................................. 104

5.2.2 Minilab extruder conditions ............................................................... 105

5.3 Results and discussion ................................................................................ 106

5.3.1 Polypropylene .................................................................................... 106

5.3.2 Fluorescence spectrometry ................................................................. 107

5.3.3 Raman and ATR/FTIR spectrometry ................................................. 110

5.4 Fluorescence spectrometry ......................................................................... 111

5.4.1 PP mixed with TMDBIO (0.05% w/w).............................................. 111

5.4.2 PP mixed with TMDBIO (0.1% w/w)................................................ 116

5.5 Viscosity ..................................................................................................... 117

5.5.1 PP-TMDBIO (0.05% w/w) ................................................................ 117

5.5.2 Comparison between viscosity changes for PP and PP with 0.05%

(w/w) TMDBIO ......................................................................... 119

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5.5.3 Comparison between viscosity changes for PP and PP with 0.05% and

0.1% (w/w) TMDBIO ................................................................ 122

5.6 Conclusions ................................................................................................ 123

Chapter 6 Segmented polyurethane nano-composites: laboratory scale melt

processing .................................................................................................................. 124

6.1 Introduction ................................................................................................ 124

6.2 Experimental .............................................................................................. 125

6.2.1 Materials ............................................................................................. 125

6.2.2 Sample preparation............................................................................. 126

6.2.3 Minilab extruder conditions ............................................................... 127

6.2.4 Data analysis ...................................................................................... 127

6.3 Results and discussion ................................................................................ 127

6.3.1 NIR spectroscopy of starting materials .............................................. 128

6.3.2 NIR melt-processing investigations ................................................... 130

6.3.3 ATR/FTIR investigation .................................................................... 131

6.3.4 PCA of NIR spectra from TPU clay nano-composites ...................... 137

6.3.5 Viscosity measurements ..................................................................... 143

6.4 Conclusions ................................................................................................ 146

Chapter 7 Conclusions and future work ................................................................ 148

7.1 Conclusions ................................................................................................ 148

7.2 Future work ................................................................................................ 150

References ................................................................................................................. 152

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TABLE OF FIGURES Figure 1.1 Chemical structures of HDPE, LLDPE and EPR 13 ................................... 8

Figure 1.2 Schematic representation of an extruder showing the different zones

within the barrel (picture was modified from reference 98)98 ................... 17

Figure 1.3 The parameters of an extruder screw (picture was modifieded from

reference 124) 124 ...................................................................................... 18

Figure 1.4 The sections of a screw (picture was modifieded from reference 124) 124

.................................................................................................................... 19

Figure 1.5 Diagram of different dies showing hole configuration and size within the

break plate (picture was modifieded from reference 54)54 ........................ 20

Figure 1.6 Total internal reflection elements. Schematic A shows single TIR and

schematic B shows multi-TIR (picture was modifieded from reference

188).188 ....................................................................................................... 35

Figure 2.1 Haake Minilab Laboratory Scale Extruder: External view ....................... 41

Figure 2.2 Inside the barrel showing twin screws ...................................................... 41

Figure 2.3 Photographs of (a) The NIR spectrometer linked to the Haake Minilab

extruder by a fibre optic cable. (b) close-up of fibre optic probe inserted

into Minilab. ............................................................................................... 42

Figure 2.4 A diagrammatic representation of the modified Minilab connected to the

NIR spectrometer with a fibre optic. .......................................................... 43

Figure 2.5 A diagrammatic representation of the modified upper barrel and sapphire

window, showing how the light beam passes through to the polymer ...... 43

Figure 3.1 The four received polyester/starch blend samples .................................... 49

Figure 3.2 The ATR/FTIR objective used for mapping ............................................. 50

Figure 3.3 Example ATR/FTIR spectra of the extruded material. (A) A high starch

point of the sample; (B) a high polyester point of the sample. (The spectra

have been offset for clarity.) ...................................................................... 52

Figure 3.4 Examples of infrared images from various sections of the extruded

material. The colours show different ranges of the infrared band area ratio

for the O–H/C=O stretching bands, as shown in the scale. Blue indicates

high starch, while red indicates high polyester. White areas indicate a

defect (hole) in the sample. ........................................................................ 54

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Figure 4.1 An NIR spectrum of pure PP at 200 °C: assignments shown in Table 4.2

.................................................................................................................... 61

Figure 4.2 NIR spectra of (a) MAH and (b) DCP in powder form. ........................... 62

Figure 4.3 NIR spectrum of the PP-MAH grafting process after 2 minutes showing

the presence of MAH vibrational bands .................................................... 63

Figure 4.4 On-line NIR spectra of the PP reactive extrusion at 200 °C without MAH

and DCP. Spectra were collected at (A) 2 minutes and (B) 90 minutes of

processing. .................................................................................................. 64

Figure 4.5 PC1 (a) factor loadings and (b) scores plot of PP on-line monitoring at

200 °C at the range of 4700 – 8700 cm-1, normalized at 6500 – 7400 cm-1.

.................................................................................................................... 64

Figure 4.6 Viscosity changes for PP and PP in the presence of 0.5% DCP at 200 °C

.................................................................................................................... 65

Figure 4.7 ATR/FTIR spectra of (a) pure PP, (b) PP-g-MAH without any

purification, (c) PP-g-MAH after 24 hr heating in vacuum oven at 110 °C,

and (d) PP-g-MAH purified by xylene and heated in vacuum oven for 24

hr at 110 °C. (Spectra are offset for clarity.) .............................................. 68

Figure 4.8 Hydrolysis reaction scheme of MAH grafted onto the PP backbone ....... 68

Figure 4.9 ATR/FTIR spectra of PP-MAH processed at 200 °C in the absence of

DCP after purification of final product ...................................................... 69

Figure 4.10 ATR/FTIR spectra of PP-MAH processed at 200 °C in the presence of

DCP after purification of final product ...................................................... 70

Figure 4.11 In situ monitoring NIR spectra of PP-MAH grafting process after (a) 2,

(b)30, (c) 60 and (d) 90 minutes at 200 °C. (Spectra are offset for clarity.)

.................................................................................................................... 71

Figure 4.12 Second derivative PC1 (a) scores and (b) factor loadings plots for PP and

MAH grafting without DCP at 200 °C....................................................... 71

Figure 4.13 Second derivative PC1 scores plots for PP and MAH grafting without

DCP at 200, 210 and 220 °C ...................................................................... 72

Figure 4.14 Spectral subtract of PP-MAH after 2 and 90 minutes processing at 200

°C ............................................................................................................... 73

Figure 4.15 Peak area ratio measurements for PP-MAH at 200, 210 and 220 °C ..... 74

Figure 4.16 Spectra of PP-MAH and PP-MAH-DCP at 200 °C. (Spectra are offset

for clarity.) ................................................................................................. 75

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Figure 4.17 PC1 factor loadings plots of (a) PP-MAH and (b) PP-MAH-DCP ........ 76

Figure 4.18 PC1 scores plots for PP-MAH-DCP graft processing at 200, 210 and 220

°C ............................................................................................................... 77

Figure 4.19 Peak area ratio measurements for PP-MAH-DCP graft processing at 200,

210 and 220 °C ........................................................................................... 77

Figure 4.20 PC1 scores plots for PP-MAH (3.5 wt% )-DCP (0.5 wt%), PP-MAH (7

wt%)-DCP (0.25 wt %) and PP-MAH (7 wt%)-DCP (0.5 wt%) graft

processing at 200 ºC ................................................................................... 79

Figure 4.21 Peak area ratio measurements for PP-MAH (3.5 wt% )-DCP (0.5 wt%),

PP-MAH (7 wt%)-DCP (0.25 wt %) and PP-MAH (7 wt%)-DCP (0.5

wt%) graft processing at 200 ºC ................................................................. 80

Figure 4.22 Viscosity measurements for PP-MAH at 200, 210 and 220 °C and

viscosity changes for PP at 200 °C ............................................................ 81

Figure 4.23 MAH linkage between two macro-radicals. ........................................... 82

Figure 4.24 Viscosity measurements for PP-MAH-DCP at 200, 210 and 220 °C .... 83

Figure 4.25 Viscosity changes for PP-MAH (3.5% of PP)-DCP, PP-MAH-DCP

(0.25% of PP) and PP-MAH-DCP graft processing at 200 °C .................. 84

Figure 4.26 TGA thermogram of PP grafted with MAH in the absence of DCP at 200

°C after (a) 6 and (b) 90 minutes processing. Also shown is (c) pure PP (d)

and MAH. ................................................................................................... 85

Figure 4.27 Proposed mechanisms for MAH grafted onto PP backbone13,29,71,72,80,89

.................................................................................................................... 87

Figure 4.28 Peroxide MAH radical species ............................................................... 87

Figure 4.29 Produced PP macroradical, (a) tertiary, (b) secondary and (c) primary

carbon radical13 .......................................................................................... 88

Figure 4.30 Arrhenius plots for the apparent rate constant (k) where T is absolute

temperature determined according to peak area ratio equations for PP-

MAH in presence and absence DCP. ......................................................... 98

Figure 4.31 Arrhenius plots for the apparent rate constant (k) where T is absolute

temperature determined according to PC1 score equations for PP-MAH in

presence and absence DCP. ....................................................................... 99

Figure 5.1 Carbon-centred free-radical scavenging by TMDBIO191 ....................... 103

Figure 5.2 PP ATR/FT-IR spectra at (a) 4 minutes and (b) 76 minutes after

processing at 260 ºC. (Spectra have been offset for clarity.) ................... 106

xviii

Figure 5.3 PP Raman spectra at (a) 4 minutes and (b) 76 minutes of processing at

260 ºC. (Spectra have been offset for clarity.) ......................................... 107

Figure 5.4 Fluorescence emission of PP at 200 ºC after 20 minutes ....................... 108

Figure 5.5 PP viscosity changes during the processing at 190, 200, 210 and 260 ºC

.................................................................................................................. 108

Figure 5.6 Fluorescence emission of TMDBIO-PP at 190 ºC after 8 minutes

processing in the Minilab ......................................................................... 111

Figure 5.7 The peak area of each sample slice and the average of the slices at 190 °C

.................................................................................................................. 112

Figure 5.8 Calculated fluorescence peak area graphs related to PP-TMDBIO (0.05%

w/w) at 190, 200, 210, 230, 260 °C ......................................................... 113

Figure 5.9 Possible reversible reaction which occurred between nitroxide radical and

carbon centred-radicals at temperatures higher than 200 °C ................... 115

Figure 5.10 Average fluorescence peak areas of PP-TMDBIO at a concentration of

0.05% w/w and 0.1% w/w, processed at 200 ºC ...................................... 117

Figure 5.11 PP-TMDBIO (0.05% w/w) viscosity changes during processing at 190,

200, 210 and 260 ºC ................................................................................. 118

Figure 5.12 Comparison between viscosity changes of PP and PP-TMDBIO

(0.05%w/w) at (a) 190, (b) 200, (c) 210 and (d) 260 ºC .......................... 121

Figure 5.13 Viscosity changes of PP and PP-TMDBIO with a concentration of

0.05% w/w and 0.1% w/w at 200 ºC ........................................................ 122

Figure 6.1 The chemical structure of TPU ............................................................... 126

Figure 6.2 The chemical structure of MEE surfactant ............................................. 126

Figure 6.3 The NIR spectrum of pure TPU (TPU-control) before processing. ....... 128

Figure 6.4 NIR spectra of (a) TPU-control (pure TPU), (b) TPU-MEE 30 and (c)

MEE 30. (Spectra have been offset for clarity.) ...................................... 129

Figure 6.5 NIR spectra of TPU containing MEE clay with a particle size of (a) 30,

(b) 75, (c) 200, and (d) 650 nm. (Spectra have been offset for clarity.) .. 130

Figure 6.6 Difference NIR spectrum from melt processing of TPU-MEE (75 nm) at

190 ºC obtained by subtracting the 18 minutes spectrum from the 2

minutes spectrum ..................................................................................... 131

Figure 6.7 (I) ATR/FTIR spectra of (a) TPU-control and (b) TPU-MEE (75 nm)

before processing and (II) difference spectrum (a – b) ............................ 133

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Figure 6.8 (I) ATR/FTIR spectra of TPU-control (a) before processing and (b) after

processing at 190 °C and (II) difference spectrum (a – b) ....................... 135

Figure 6.9 (I) ATR/FTIR spectra of TPU-MEE (200 nm) (a) before processing and

(b) after processing at 190 °C and (II) difference spectrum (a – b) ......... 136

Figure 6.10 Factor loadings plot for TPU-control processed at 200 ºC ................... 137

Figure 6.11 PC1 scores plot versus time for NIR spectra obtained from TPU-control

at 190 and 200 °C ..................................................................................... 138

Figure 6.12 Factor loadings plot of TPU-MEE (200 nm) which was melt processed

at 200 ºC ................................................................................................... 139

Figure 6.13 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE

(30 nm) at 190 and 200 °C ....................................................................... 140

Figure 6.14 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE

(75 nm) at 190 and 200 °C ....................................................................... 141

Figure 6.15 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE

(200 nm) at 190 and 200 °C ..................................................................... 142

Figure 6.16 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE

(650 nm) at 190 and 200 °C ..................................................................... 142

Figure 6.17 Viscosity changes for TPU-MEE with different particle sizes at 190 °C

.................................................................................................................. 143

Figure 6.18 TPU nano-composite with (a) MEE (650nm) with the inter-gallery space

size of 3.5 nm (b) and MEE (30nm) with inter-gallery space of 10 nm. . 144

Figure 6.19 Viscosity changes for TPU-MEE with different particle sizes at 200 °C

.................................................................................................................. 145

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TABLE OF TABLES Table 3.1 The composition ratio of materials in blends (parts by weight %) ............ 48

Table 3.2 Infrared band area ratios for O–H/C=O stretching bands from randomly

selected sections of the four samples of extruded material ........................ 53

Table 4.1 Reactive graft processing settings used for PP, MAH, and DCP .............. 59

Table 4.2 Peak assignments of PP NIR spectrum90 ................................................... 61

Table 4.3 PC1 scores and peak area ratio plot equations related to PP-g-MAH in the

absence of DCP at three different temperatures. ........................................ 73

Table 4.4 PC1 scores and peak area ratio plot equations for PP-g-MAH in the

presence of DCP at three different temperatures. ...................................... 76

Table 4.5 PC1 scores and peak area ratio plot equations related to differing

concentrations of MAH and DCP at 200 ºC .............................................. 79

Table 4.6 Calculated .................................................................................................. 99

Table 6.1 Assignments of TPU vibrational bands227,228,232,233 ................................. 128

xxi

LIST OF ABBREVIATIONS ATR Attenuated total reflection

DCP Dicumyl peroxide

DMAc Dimethylacetamide

EPR Ethylene propylene rubber

FTIR Fourier transform infrared spectrometer

HAS Hindered amine stabiliser

HDPE High density PE

IR Infrared

IRE Internal reflection element

IRS Internal reflection spectroscopy

LDPE Low density PE

LLDPE Linear LDPE

MAH Maleic anhydride

MFI Melt flow index

MIR Mid-IR

NIR Near infrared

NMR Nuclear magnetic resonance

PBSA Poly (butylene succinate adipate)

PC1 First principal component

PCA Principal Components Analysis

PCL Polycaprolactone

PE Polyethylene

PET Polyethylene terephthalate

PP Polypropylene

PVC Polyvinylchloride

Si Silicon

TGA Thermogravimetric Analysis

TMDBIO 1,1,3,3-tetramethyl-2,3-dibenzo[e,g]-

isoindoline-2-yloxyl

TMIO 1,1,3,3-tetramethylisoindolin-2-yloxyl

MIR Multiple internal reflection

ZnSe Zinc selenide

Chapter 1

1

Chapter 1 Introduction

1.1 Introduction

Polymer science, which is an integration of pure and applied chemistry, physics and

chemical engineering, has grown quickly as an independent branch of science since

the 1950s.1,2 Because of their useful properties such as low cost, elasticity, toughness

and light-weight, polymers are extremely valuable as industrial materials.1 Despite

the growing utility of polymers in industry and daily life, they are rarely used as a

pure material but are typically modified to improve some physical or rheological

property.1,3 One of the most important techniques for modification of polymers is

reactive extrusion.

This literature review will give a basic introduction to the technique of reactive

extrusion. Some factors which influence the rheological modelling of extruders in

reactive extrusion will be explained. In addition, one of the most important

limitations of reactive extrusion, degradation, will be briefly discussed. Also,

monitoring or ways of process controlling during polymerisation will be considered.

Lastly, vibrational spectroscopy, which is one of the most powerful techniques for

monitoring polymerisation processes, particularly for reactive extrusion, will be

explained.

1.2 Extrusion technology

Extrusion technology has been utilised by industry since the 18th century. However,

it was not until the 1930s that the manufacturing industry attempted to significantly

improve extrusion processes. Nowadays, applications of extrusion processes include

the production of foodstuffs, plastics and many other manufactured goods.4,5

Extrusion is the process of producing and shaping specific products from raw

materials by forcing material through a die or an orifice under controlled

conditions.4,6

Chapter 1

2

In commercial terms, extrusion processes are divided into three broad categories.

Ram and cylinder extrusion was the earliest extrusion process and is still widely used

today. Ram and cylinder extrusion operates as a batch production system under high

pressure. Pump and spinneret extrusion is the second type of extrusion process. This

process operates under low pressures and is used for starting material or solvents

with low viscosity. If the low viscosity melt or solution is filtered and then pumped

through a special die, the extrusion process is termed spinneret extrusion. Generally,

pump extrusion is used for special processes such as regenerated cellulose, cellulose

acetate, nylon, etc. The third type of extrusion process is rotating screw extrusion.

Fundamentally, this involves a screw which is situated and rotated in a hot barrel.4

The raw materials enter via a hopper and exit through a die. This process will be

described in more detail in further sections.

Extrusion techniques can be divided into two sub-categories. Wet extrusion, which is

commonly employed by industry, is the dissolution of raw material ingredients in a

solvent. Heat and pressure may be applied according to the special requirements of

the reaction. Wet extrusion is often carried out as a batch process and may be quite

time-consuming.6,7 Dry extrusion, on the other hand, does not require a solvent,

instead relying on heat to form a melt. Screws are used to create homogeneous

melts.4,6,8

1.3 Reactive extrusion

Reactive extrusion, also known as melt-state processing, has been an important

method in materials processing for several decades.9,10 Moreover, reactive extrusion

has been investigated within several technological and scientific areas of research.7

One of these areas is polymer technology, which makes extensive use of this

process.11 Since the 1970s, due to increasing demands for manufacture of specific

polymers and improvement of their physical, chemical and mechanical properties,

numerous studies of the modification and melt-state processing of polymers were

performed.12 Moad,13 van Duin and co-workers12 and Seker5 explain that the

technique involves the synthesis of polymers by using a heated extruder as a

chemical reactor and is based on a melt-phase reaction. This technique involves

mixing monomers or other reactants such as a compatibiliser and additives with a

Chapter 1

3

molten polymer in an extruder at high temperature. This is generally conducted in

the presence of an initiator.9,11,14-16 The reactive extrusion process is a continuous

process and involves numerous steps such as: feeding, transferring, heating or

melting, shaping, mixing and reaction.5,12,17-19 In addition, improvement of product

properties, such as chemical, mechanical or rheological properties, polymerisation of

monomers, synthesis of new polymers and modification of the polymer can be

carried out simultaneously.17,20,21

The reactive extrusion technique is one of the most important achievements in the

polymer industry.12,22-27 From a technological point of view, in comparison with

solution-state processing, reactive extrusion is a successful and low cost technique

and has multiple advantages. One of these advantages is the absence or minimal use

of solvents so that the technique differs considerably from the more common

approach of solvent-state processing. As a result, there is no need to expend energy

for heating, cooling, and removing solvent, which leads to economic and

environmental benefits. Moreover, reactive extrusion has a very short reaction time,

so it is a very fast technique, leading to a high production rate. This type of technique

can not only be done under batch-process conditions, but also, as mentioned

previously, can be performed as a continuous process.7,9,28-32 Another advantage of

this process, compared with a non-reactive extrusion process, is that it breaks up

reactant particles and thoroughly mixes them. Hence, this can increase the reactant

surface area, so the reaction can propagate faster and more efficiently.33 Likewise,

reactive extrusion is a flexible process and as a result, a wide range of process

conditions can be utilised. For example, temperatures between 70 ºC and 500 ºC and

pressures between 0 atm and 500 atm can be used. Another benefit of reactive

extrusion is the flexibility to change and control the degree of mixing by increasing

or decreasing the screw speed.8,12,19

Although there are numerous advantages for reactive extrusion, there are a few

limitations. This technique is very complicated because it involves several

convoluted processes such as melting, dispersion, heating and mass transportation.34

According to Moad,13 to perform polymerisation under reactive extrusion conditions

the polymer needs to be in the melt-state, and hence the reactor must usually operate

at a high temperature. On the other hand, reactive extrusion is a non-isothermal

Chapter 1

4

system and as a result, temperature control is extremely difficult.8,17,19 Also, for the

reaction to proceed smoothly the reactants need to have a small particle size and

should be intimately mixed.13 Another limitation for reactive extrusion is that the

potential for side-reactions, such as cross-linking, chain scission and degradation

during the polymerisation process may be increased.34-44 Additionally, as mentioned

before, polymerisation and modification of high viscosity polymers can be easily

performed using reactive extrusion. However, for polymers with a very low

Reynolds number,* mixing and polymerisation is very difficult.8,12 Residence time

must also be pointed out as another limitation of reactive extrusion. Because the

residence times are very short, only relatively fast reactions can be carried out using

this process.19

There are various types of reactions which can be performed under reactive extrusion

conditions. The most important of these are listed below, with examples:8,17,28,45-47

a) Degradation control

- Control of polypropylene (PP) degradation by shear-heating or

peroxide addition.

b) Cross-linking or coupling polymerisation

- Cross-linking polymerisation of polystyrene with trimethylol propane

and triacrylate.

c) Grafting, modification or functionalisation reactions

- Graft copolymer of polystyrene and maleic anhydride (MAH)

- Graft copolymer of polyolefins and MAH

- Graft copolymer of polyolefins and vinylsilanes

- Graft copolymers of polyolefins and acrylic or methacrylate

monomers

d) Bulk polymerisation

- Polyetherimide

- Polyesters

- Poly(ethylene terephthalate) (PET)

- Polyamide 6.6

* Reynolds number is described as the ratio of inertial forces to viscous forces.

Chapter 1

5

e) Copolymerisation by inter-chain formation

- Copolymerisation of polystyrene and polyolefins

- Copolymerisation of PP grafted with MAH and nylon-6

f) Control of molecular weight

- PP

- Polyethylene (PE) Among these reactions and polymerisations, grafting or modification reactions are of

considerable importance and are the topic for much recent research..48-60 Numerous

approaches, such as monitoring for accurate residence time, residence time

distribution or rheological properties,61-67 extruder and screw designs, vibrational

spectroscopic investigations and degradation studies have been used by a number of

investigators. These will be mentioned in detail in the following sections.

1.4 Modification or functionalisation using reactive extrusion

Some polymers, such as polyolefins, have very poor properties when blended with

other, more polar, polymers. In addition to this, a wide range of polymers are

immiscible and as a result they have very poor mechanical properties after blending.

They also have very poor adhesion to glass, metal, inorganic materials and other

polar surfaces. This immiscibility is due to the lack of adhesion between the two

polymer phases. In other words, the dispersed polymer phase cannot properly mix

with the matrix phase.9,21,35,53,59,68-76

From a thermodynamic perspective, two or more polymers are immiscible when the

Gibbs free energy change from mixing is positive,51 as explained by the Gibbs free

energy equation for a reversible system:

Δ Δ ΔG H T Sm m m= − 1.1

where:

ΔGm is Gibbs free energy of mixing,

ΔHm is the enthalpy of mixing and

ΔSm is the entropy of mixing.

Chapter 1

6

If ΔGm for blending two polymers at a given temperature is positive, then a two-

phase polymer will be formed. Conversely, when two or more polymers are

miscible, ΔGm will be negative.

The second factor which is necessary to consider is a change of the Gibbs free

energy of mixing corresponding to the volume fraction of the polymer:

∂∂

2

22 0Δ

ΦGm

T P

⎡

⎣⎢

⎤

⎦⎥ ⟩

,

1.2

where:

Φ2 is the volume fraction of polymer,

T is temperature and

P is pressure.

Note that both of these conditions are necessary for two polymers to be miscible. It

means if both ΔGm and Equation 1.2 are positive, the blend will be separated into two

phases.61 When this occurs, modification or functionalisation of the polymer is a

common technique for improving miscibility.9,29,31,45,59 Functionalising a polymer

usually involves linking or grafting small, polar groups onto the polymer

chain.7,28,45,60 When used for the purpose of improving blending characteristics this

process is called compatibilisation.

According to Moad13 and Machado et al30 one of the best methods for introducing

functional groups onto a polymer chain is by free-radical grafting. The main

reactants when using this method are: an initiator, typically an organic peroxide; a

monomer or macro-monomer; and a polymer, such as a polyolefin.9,13,18,56,60,67,72,77,78

1.4.1 Examples of modification in reactive extrusion

A common polymer which is often modified by reactive extrusion is PP. PP is a

polyolefin, which because of its flexibility, low cost and desirable physical and

chemical properties such as stiffness, low specific gravity, non-toxicity and

resistance to corrosive chemicals, is widely used.13,14,28,57,70,73,77,79-81 It has a range of

manufacturing applications such as tubes, injection-moulded products, films, and

fibres.5,72,82,83 Because of its desirable properties PP is usually preferred over PE,

polyurethane and polyvinylchloride (PVC).9,14,30,53,71,84 Although, PP has numerous

Chapter 1

7

applications in industry, it often requires modification as it has poor miscibility with

other polymers due to low hydrophilicity and a lack of reaction sites. It also has a

high susceptibility to degradation, especially photo-oxidation.9,35,53,71,72,81,85

PP can be functionalised by a free radical reactive extrusion process.9,14,29,30,58,70,77,86-

88 The melt-state or reactive extrusion technique is preferred because of low cost,

high controllability and high production rate.9,28,29,31,86 Although a wide number of

monomers have been used for PP functionalisation via reactive extrusion, many

polymer investigations have focused on MAH.9,13,35,53,57,58,85,87-95 This is because it is

extremely difficult to homo-polymerise MAH at high temperatures.13,53,89,91,92

Although the functionalisation of PP by MAH is a well known process, the chemical

mechanism involved in this process is not completely understood.9,14,30,35,71,85 It has

been reported by da Costa et al96 that temperature has a strong influence on the

rheological properties of PP and was demonstrated that by increasing the

temperature, viscosity was reduced and polymer elasticity decreased. Tselios and co-

workers79 studied the compatibilisation of PP and PE blends by reactive extrusion.

They reported that the mechanical properties of the blend, such as impact strength

and tensile strength, were improved in comparison with PP or PE on their own.

The main reaction mechanism for grafting MAH onto a PP backbone consists of the

following steps:13

1. Thermal decomposition of initiator to form radical species

ROOR 2 RO 1.3

2. Making a carbon radical by abstraction of a PP tertiary hydrogen

RO +C

+ ROH 1.4

Δ

Chapter 1

8

3. Grafting of MAH onto PP

CO

OO

+

C

O OO

1.5

Polyethylene (PE) is another polyolefin polymer which is commonly functionalised

by reactive extrusion. It is widely used in industrial applications, especially in the

food packaging industry.56,97 According to Moad13 and Fried98 PE can be classified

into four groups. The first is high density PE (HDPE). It has a linear structure and is

produced by the coordination homo-polymerisation of ethylene.13,56 The most

important industrial applications of HDPE are blow-moulded containers, crates, gas

tanks and blown films.98 The second type of PE is low density PE (LDPE) or

branched-ethylene polymer. LDPE is also produced by free-radical

polymerisation.13,99 Its molecular weight is between 6000 and 40000 g/mol.98 LDPE

is usually used in blown-film, coating, blow-moulding and foaming operations.99

Compared with HDPE, LDPE has a lower crystalline-melting temperature.98 Linear

LDPE (LLDPE) is the third type of PE which is produced by coordination

copolymerisation of ethylene with an olefin such as butene, hexene or octene. In

contrast with LDPE, LLDPE has a faster cycling time during the molding of

containers. It has very extensive branching with different lengths of branches along

the backbone.13,56,98,100 The last type of PE is ethylene propylene rubber (EPR) which

is produced by coordination polymerisation of ethylene and PP. This type of PE is

commercially available as segmented and block copolymers of ethylene and

propylene .13,100 Figure 1.1 shows the chemical structure of HDPE, LLDPE and EPR.

HDPE LLDPE EPR Figure 1.1 Chemical structures of HDPE, LLDPE and EPR 13

PE, like PP has no functional groups in its main chain suitable for enabling

miscibility with polar polymers.56,101 Modification of PE using reactive extrusion

Chapter 1

9

occurs via free radical processing.102 During the modification of PE by reactive

extrusion, a dominant side-reaction which can occur is cross-linking. The probability

of this is increased when there is a high peroxide concentration.56,103 Cross-linking

can change physical and rheological properties of the modified polymer.52,102,104-107

The principal mechanism of grafting MAH onto the PE chain is:13

1. Initiator decomposition

ROOR 2 RO 1.6

2. Abstraction of hydrogen from PE chain

RO + C + ROH 1.7

3. Grafting of MAH onto PE

OOO

C

O OO

C + 1.8

Increasing the degree of grafting and reducing side-reactions is important in reactive

extrusion modification. Many researchers have reported that they have achieved

these results by increasing initiator and monomer concentrations and by using co-

agents, such as styrene, during reactive extrusion.13,28,29,52,53,56,58,60,108

Pesetskii et al18 have investigated the effect of initiator solubility and free-radical

grafting of itaconic acid onto LDPE. They found that the reactive extrusion was

affected not only by solubility of the initiator, but also by its thermal stability.

Villarreal et al109 studied the effect of reactions onto the different types of PE and the

modifications of them. They reported that chemical modification of PE is more

effective on secondary carbon atoms than the other types of carbon atoms such as at

branch points; vinyl end groups. Also from their results, they concluded that the

crystallisation property of PE decreased during graft modification with a polar

monomer.

Δ

Chapter 1

10

1.5 Compatibilisation process

The compatibilisation process in reactive extrusion can either be a one-step or two-

step process. During two-step processing, a polymer is chemically functionalised in a

separate extruder and then mixed and blended with a second polymer in another

extruder. The most common reaction for two-step processing is graft

copolymerisation. With one-step processing, functionalising and blending are

performed in the same extruder. Both initiation and grafting reactions are carried out

in one extruder. In contrast with two-step processing, one-step reactive extrusion

compatibilisation is economically preferred, due to consumption of fewer polymers

and reactants. Also, reaction condition control in one-step processing is easier than

two-step processing.59,68,110-112

During two-step processing, when a pre-made copolymer is used as a compatibiliser,

physical compatibilisation occurs. In contrast, one-step processing produces

chemical compatibilisation. Numerous research groups have investigated

improvements of polymer blend properties by reactive extrusion.18,21-23,56,59,68-70,113,114

Yu et al21 investigated interfacial reaction kinetics and the properties of reactive

extrusion compatibilisation. Wang and co-workers68 studied the compatibilisation of

thermoplastic starch/PE blends by one step processing, using MAH as a

compatibiliser. They found that the blends showed good intermolecular dispersion.

As well, when DCP was used as an initiator, the degree of grafting into the blend’s

structure was improved. Sun et al113 described compatibilisation of polyolefin and

polystyrene by using Friedel-Crafts alkylation by reactive extrusion and

demonstrated that the blends displayed improved mechanical properties. Tselios and

co-workers79 investigated the compatibilisation of PP/PE blends by reactive

extrusion. They described that the addition of a compatibiliser into this blend

improved its mechanical properties. This was due to improved adhesion between the

two polymer phases.

1.5.1 Example of compatibilisation extrusion

An example of reactive extrusion is the compatibilisation of starch and polyester

with MAH. Starch is a hydrophilic biodegradable polymer composed of amylose and

amylopectin. Due to its low cost and availability, starch is often used for increasing

Chapter 1

11

the biodegradability of some polymer blends.68,110,115-117 In spite of its benefits, it has

the disadvantage that it is immiscible with most synthetic polymers, including

polyesters. By increasing the concentration of starch in a polymer blend, mechanical

properties are decreased.117 There are two main techniques used for improving starch

miscibility. The first is chemical modification of starch involving a grafting reaction.

In this technique, starch is reacted to form functional groups such as esters, ethers,

anhydride and isocyanate. The second is physical modification of starch, which

involves either linking hydrophobic agents onto the starch particles or cross-linking

of starch granules that increase its hydrophobic properties.118 In reactive extrusion,

compatibilisation by chemical modification is preferred because the reaction has a

higher yield and greater economic benefits.114-116 There have been several reports on

improving the properties of polyester/starch blends. In these studies, several types of

polyesters, such as polycaprolactone (PCL), poly (hydroxy butyrate-co-valerate)

(PHBV), poly ε-caprolactone, polybutylene succinate, and EnPol® (poly(butylene

succinate adipate (PBSA)) have been considered.118-121 According to Maliger and co-

workers119 the main reaction mechanism of compatibilisation reaction of PBSA and

starch in the presence of MAH as a compatibiliser in reactive extrusion, is as

described below:

O (CH2)2 O C

O

(CH2)4 C

O

EnPol 1.9

1. Initiation of PBSA macro-radical

O (CH2)2 O C

O

CH2 CH2 CH2 CH2 C

O+ R'O

-R'OH

O (CH2)2 O C

O

CH2 CH2 CH2 CH C

O

EnPol microradical 1.10

2. PBSA will undergo a β-scission reaction at temperatures higher than 150 ºC,

and 2 compounds will produce (as shown in equasion 1.11). An alkoxy

β-scission

PBSA microradical

Chapter 1

12

radical can react with compound (2) to produce another alkyl radical. In

addition, grafting of MAH onto the PBSA macro-radical will take place:

O (CH2)2 O C

O

CH2 CH2+ 2HC CH C

O

(1)

Free radical associated chain

(2)

+ R'O

CH2 CHR'O

CO

OO O

O (CH2)2 O C

O

CH2 CH2

O

C

O O

+

1.11

The first possibility for propagation of the reaction is attaching the macro-molecule

of compound (2) in the graft processing and formation of a stable complex.

Compound (2) Compound (1)

Chapter 1

13

3. In this stage the C-6 atom of starch will attack the anhydride group, creating

a MAH link between the starch and the polyester.

CHR'O

+O (CH2)2 O C

O

CH2 CH2

OO O

CH2O

CH2OH

CO OO

CHR'O

O (CH2)2 O C

O

CH2 CH2

C C

CH2 CO

CH2

O

O

OO

OHO

Starch-MAH-polyester complex (I) 1.12

The second possibility for propagation of the reaction is attaching another PBSA

macro-radical in the graft processing which leads to the construction of a stable

complex (II).

Chapter 1

14

O (CH2)2 O C

O

CH2 CH2

OO O

+

O (CH2)2 O C

O

CH2 CH2

OO O

CH2 CH2 C O

O

(CH2)2 O

+

OCH2OH

OO

CH2 CH2 C OO

(CH2)2 O

CH2O (CH2)2 O C

O

CH2 CH2

C C

CH2 O

CH2

O

O

OO

OHO

Starch-MAH-polyester complex (II)

(CH2)2 O

1.13

Chapter 1

15

1.6 Process conditions in reactive extrusion

There are numerous factors which affect the process of reaction extrusion. The most

significant of these is temperature, which has a direct influence on polymer

degradation, reaction rate or residence time and initiator half-life.8 Based on the

Arrhenius equation, temperature has an inverse relationship with viscosity of the

polymer.

In this equation:

η =⎛⎝⎜

⎞⎠⎟A

ERT

exp 1.14

where

T= absolute temperature

R= gas constant

E= activation energy

A= function of pressure and shear rate

During reactive extrusion for each polymer, the optimum processing temperature

must be determined. If the material’s temperature inside the extruder is below this

temperature, the products will not be homogeneous. If the temperature is too high,

then the products degrade. As a result, control of the barrel, screw, die and in-put

material temperature is very important.6,84

Another important factor is mixing. Like temperature, mixing conditions directly

affect the process rate and output amount; moreover, it controls heat transfer

between screws and materials.6 Hence, mixing and temperature are directly related to

each other. These factors determine the system pressure, solubility of initiator, and

solubility of monomer in the polymer.13,46,51,60 Homogeneity of the polymer can be

controlled by extruder mixing. Hence, mixing is an important factor which is used to

achieve an homogeneous product especially in modification or compatibilisation

processes.122

Pressure is another important factor, because of its relationship with reaction rate.

Pressure is directly related to viscosity.13 According to the thermodynamic-based

Arrhenius equation, it can be explained that:

Chapter 1

16

( )η α= B Pexp . 1.15 where

P= pressure

α= pressure coefficient of viscosity

B= a function of temperature and shear rate.84

The type of polymer affects the reactive extrusion process conditions. Physical and

chemical properties of polymers such as molecular weight and melting point must

also be considered when choosing processing conditions. The nature of the

monomers or compatibilisers, their concentrations and their solubility in the polymer

are major factors. Also, properties of the initiator, such as half-life, solubility in the

polymer and monomer and concentration are important when determining reaction

conditions. Important factors influencing extruder design include diameter and screw

length, as well as screw and die geometry. These can change the residence time and

process conditions in reactive extrusion.6,123

1.7 Features of reactive extrusion

The use of extruders as chemical reactors (i.e. reactive extrusion) has increased

rapidly in recent years.10,16,54 This is because of:

- improved temperature control

- high pressure capability

- improved mixing of reactant materials

- modification of screw design

- improvement of twin screw extruders’ design

- no reaction between monomers and by-products and

- capability to vary the residence time by changing the screw rate.8,10,122,123

In extruders several factors such as mass flow, pressure rheology, mixing, time and

thermal behaviours have to be controlled. This is because these factors can affect the

reaction conditions and residence time for the process.49

Chapter 1

17

An extruder consists of a screw, which is placed into and rotates within a barrel. An

extruder barrel can be divided into three parts or zones (see Figure 1.2). The first is

the feed section. The second is the transition or compression section. The last

incorporates a die, which is placed at the end of the extruder and screw.50,98

Figure 1.2 Schematic representation of an extruder showing the different zones within the barrel (picture was modified from reference 98)98

As the barrels must resist high internal pressures (between 10000 and 20000 psi),

they are usually made of thick-walled steel. Barrel size is commonly expressed as the

ratio of the length to the inside diameter of the barrel, L/D.

The first section of the extruder, the feeder, allows reactants to be fed into the

extruder. Many factors influence the selection of the feed system, such as:

- type of extrusion process and aim of the processing

- extruder and barrel size

- type of screw used (single or twin screw)

- physical form of the feed material (liquid, powder, pellets)

- physical space available between barrel and screw or other parts and

- time required for cleaning.

There are two types of feed systems. The first is a hopper, which is shaped like a

funnel. The second type is a feed opening for introducing reactants into the extruder.

A hopper is placed directly over the barrel and is connected to extruder by a feed

throat. Hoppers and feed throats are designed according to the type and size of the

feeds to permit materials to flow freely within the extruder. Usually the feed throat is

die

perforated breaker plate screw

hopper

Feed section

Compression section

Metering section

barrel

Chapter 1

18

cooled by a cold water-jacket which keeps the temperature of the feed system lower

than the sticking temperature of the polymer or resin. If a water-jacket is not used the

pellets and feeds can stick together and can block the hopper and feed throat.124

Another important part of the barrel is the vent, which is situated at an intermediate

point. The vent removes air and volatile matter from the processed materials during

heating. The heater system covers the whole external surface of the barrel and is

composed of a series of heater systems which are separately controlled. To maintain

the barrel at an appropriate temperature, the heater systems are equipped with

thermocouples which are usually placed within the metal wall of the barrel. In some

new extruder designs, cooling systems can also be supplied. This is to prevent the

polymer from over-heating by mechanical working. The barrel cooler usually lowers

the temperature by air or water flow.6,124

The screw is an important component within all extruders. A screw is described

using several parameters such as channel flight, pushing flight, helix angle, pitch,

channel width and root which are shown in Figure 1.3.

Figure 1.3 The parameters of an extruder screw (picture was modifieded from reference 124) 124

In addition, a screw can be divided into three sections: feed; transition or

compression; and metering (seeFigure 1.4). The feed section is specifically designed

to push feed from the hopper and feed throat further into the barrel. This section of

the screw has a constant and deep channel. The transition section of the screw starts

where the channel is shallower and it is here that the polymerisation reaction takes

place during reactive extrusion. The last section of the screw is called the metering

section and has a constant but shallow channel depth.

Channel Flight

Pushing Flight

Helix Angle

Pitch Channel Width

Root

Chapter 1

19

Figure 1.4 The sections of a screw (picture was modifieded from reference 124) 124

Extruders are divided into two groups: single; or twin screw extruders, the main

difference between them being material transport mechanisms. Fundamentally, in a

single screw extruder, the screw has been closely fitted to the barrel, leaving only a

narrow distance between screw and barrel wall. This is necessary and important to

prevent materials from sticking to the screw or barrel wall. As a result, mixing in

single screw extruder is poor. In a twin screw extruder, two screws are placed into a

barrel. Screws can be co-rotating or counter-rotating. In a co-rotating extruder, both

screws rotate in the same direction, either left-handed or right-handed. However, in

counter-rotating extruders, screws rotate in opposite directions. Co-rotating generally

gives longer residence times (because of longer path length) whereas counter

rotating allows for more shear (because of more mixing in small spaces in between

the two extruder shafts).125 When using single screw extruders, it has been shown

that increasing the length of the extruder improves mixing and homogeneity. As a

result, the ratio of length to diameter of single screw extruders is higher than twin

screw extruders.126 Twin screw extruders have numerous benefits over their single

screw counterparts, such as reducing the possibility of un-reacted monomer and by-

pass products, good distribution and dispersion mixing, excellent heat transfer and a

self-wiping operation. However, twin screw extruders have multi-operational

parameters, for example temperature, pressure, speed and configuration of screws

and rate of feeding, and matching all of these parameters is therefore difficult and

complicated.17,127 Despite this, according to numerous reports, twin screw extruders,

especially co-rotating screw extruders, are preferred by many researchers and

industries.10,16,17,46,50,54,55,127

Feed Transition Metering

Chapter 1

20

The final section of the extruder contains the die. Different types of die have been

designed depending on the rheology of the melt, product shape and extrusion

process. Generally a die is used to shape the melt or processed polymer as it exits

from the extruder. For example, to produce film or sheet products, flat dies are used.

Thus, the types of dies are categorised by the shape of the final product: blown film;

pipe and tubing; sheet; extrusion coating; and monofilament extrusion dies. In

addition to this, dies are used to modify the physical properties of the polymer such

as molecular orientation in the product.128

Dies are usually attached to the extruder directly or sometimes a break plate is placed

between the die and the extruder. A break plate is a round plate which contains many

holes. This reduces and in some cases stops the “spiralling action” of the molten

polymer as it exits from the extruder. The break plate can act as a seal between

materials inside the barrel and the die. Because of the holes in the screen of the

breaking plate, it can filter the material and molten polymer through the die, and

increase the pressure at the extruder head. A diagram of two different types of break

plates with different hole sizes is presented in Figure 1.5.

Figure 1.5 Diagram of different dies showing hole configuration and size within the break plate (picture was modifieded from reference 54)54

R= 3mm 19 holes

R= 4mm 7 holes

Chapter 1

21

1.8 Rheological modelling of extruders

There are numerous factors which should be considered when modelling an extruder.

These are explained below.

1.8.1 Flow

By understanding the flow it is possible to determine how some variables such as

screw speed and temperature can change other parameters such as volumetric

production. Polymers (solution or melts) are a type of non-Newtonian fluid.

Therefore, it is necessary to determine the relationship between strain and stress, as

this is important for measuring flows in extruders.98

According to Todd,129 one type of flow is drag flow (Qd). In any type of extruder, the

molten polymer flows along the barrel. Pushing and flowing of the melted polymer

occurs by rotating the screw. As noted, at the end of the extruder, there is a die. So,

during extrusion another force or flow occurs in the opposite direction to the drag

flow. This opposite flow is called back flow (Fd). Drag flow can be calculated by the

following equation:

Q F D w h Nd d= . . . . . .cosπ Φ2

1.16

where Fd = back flow

N = speed of screw

D = diameter

w = flight width

h = flight depth and

Ф = helix angle.

Another important flow in an extrusion is pressure flow (Qp).

Q F W h P Lp p= ⎛⎝⎜

⎞⎠⎟

. . . .sin . .3

12Δ

Φ η 1.17

where η = viscosity

L = length of channel

W = width of channel

Chapter 1

22

Ф = helix angle and

ΔP = difference pressure, (caused by the flow and the resistance to flow).

The difference between drag flow and pressure flow results in volumetric or net flow

(Q):129,130

Q Q Qd p= − 1.18

Determination of the pressure inside the extruder is possible using net flow. As a result:122

( )P N Q= −ηλ

χ. 1.19

where:

λ and χ are constants depending on the screw geometry

N = screw rotating speed

P = pressure and

η = viscosity.

1.8.2 Extruder size

The choice of extruder size depends on several parameters such as:

- nature of the feed (powder, granules or molten polymer)

- the number of steps for grafting/blending of the polymer

- type of reaction (e.g. free radical reaction)

- pressure

- type of screws (single or twin)

- residence time and

- feed rate.

These main parameters can change the requirements for the size and length of the

extruder.98,129

1.8.3 Extruder volume

In reactive extrusion, the required volume for the extruder (V) depends on other

necessary stages such as feeding, melting and pressure and can be calculated using

the following formula:

Chapter 1

23

V W f=θς

1.20

where

W = weight rate of flow

θ = residence time

ς = melt density and

f = volumetric fraction.

Volumetric fraction (f) can be obtained by:

fQQd

= 1.21

Residence time or the required time (θ) can be calculated using:

θ =2L

Z N. 1.22

where

N = screw speed

L = barrel length and

Z = lead length.

Generally Z ranges between 0.25 and 1.5D. Besides these factors, other less

important factors such as feeding, melting and pressure can have an effect on the

required volume of the extruder.98,129

1.8.4 Shear rate

Natov et al54 observed that shear rate is another important factor which can have a

direct effect on reactive extrusion processes. In the first zone of an extruder, the

melted polymer flows along the extruder towards the end of the screw and the die.

The flowing polymer has a higher shear rate at the beginning of the extruder. The

shear rate (γ) can be determined by the following equation:54

γ πδ

= .DN

1.23

where

D = diameter of screw

Chapter 1

24

N = screw speed and

δ = distance between barrel and zero line (screw).

It is possible to calculate polymer viscosity (η) using sheer rate via the following

formula:10

( )η γ= −⎛⎝⎜

⎞⎠⎟

⎡

⎣⎢

⎤

⎦⎥ + −

−m b

T Tb C C

nm A A0

1

00

1 1.exp 1.24

where

γ = shear rate

T = temperature

CA= moisture concentration

m0 = power law constant

n = power law index and

b and bm = constants.

The hole patterns of the break plate within the die has a direct effect on the pressure

and shear rate at the head of the extruder (Figure 1.5). Therefore, the shear rate in

each hole can be calculated by the following equation:60

γπ

=4

3

QR.

1.25

where

Q = net or volumetric flow and

R = radius of holes.

1.9 Degradation of polymers

Polymer degradation results in changes to the physical and chemical properties of a

polymer. Chemical and physical reactions may involve bond scission reactions in the

polymer backbone leading to break-down of the polymer macromolecules. Polymer

degradation may manifest itself as a reduction in molecular weight, discolouration,

production of volatile compounds or creation of oxygen containing functional

groups. Alternatively, degradation may involve cross-linking which tends to increase

the molecular weight of the polymer.

Chapter 1

25

There are numerous causes of polymer degradation. Chemical degradation is one of

these and is caused by contact between polymers and some types of chemicals such

as acids, bases, solvents and reactive agents. Thermal degradation is another type of

degradation which occurs at high temperatures. Shear forces can cause another type

of degradation known as mechanical degradation. The degree of thermal and

mechanical degradation under given conditions is dependent upon polymer structure.

Another type of degradation is biological degradation due to microbial attack which

is also known as biodegradation. This can take place over a wide range of

temperatures. Light-induced polymer degradation, also known as photodegradation,

is a common cause of degradation. The main wavelengths that cause this degradation

are ultraviolet (UV) radiation. Finally, oxidative degradation can occur which

proceeds by free-radical mechanisms. In most cases, more than one type of

degradation occurs at the same time, such as thermo-oxidative and thermo-

mechanical degradations.96,131

Degradation is one of the most important problems for polymers and consequently

has been well-studied.20,37,96,97,131-136

1.9.1 Degradation during reactive extrusion

Degradation is a significant limitation of reactive extrusion processes and is a

complex phenomenon.96 Degradation during melt-state processing is a short-term

process occurring under several micro-environmental attacks. The common types of

degradation occurring during reactive extrusion are:

- Thermal

- Mechanical

- Oxidative or

- A mixture of two or all of the above.20,84,97,137,138

According to Gonzalez-Gonzalez and co-workers,20 polymer properties such as

molecular weight and viscosity, and reactive extrusion conditions such as shear

stress, extruder and die geometries and screw speed affect the type and degree of

degradation. Pinheiro et al,136 da Costa et al,96 Gao et al43 and Epacher et al139

reported that the degradation rate of a polymer is also determined by residence time

and residence time distribution of the polymer. These factors are determined by the

Chapter 1

26

velocity profile of the extruder and screw speed. Other factors include the presence

of degradation agents such as peroxide, deformation rate and feed rate. In the case of

polymer blends, degradation is influenced by degradation products which can lead to

production of new compounds, as well as the structure of blended polymers and the

reactivity between the produced components.112

In reactive extrusion processing a combination of processing conditions such as high

shear rate and high temperature can affect the molecular structure of the polymer.

These conditions have a direct influence on rheological and mechanical properties of

polymers. Any changes in mechanical and rheological properties of the polymer

during the process can change its melt oxidation and results in cross-linking or chain

scission reactions.140 In comparison with high temperature, high shear rate has more

influence on polymer degradation.107,112 Mechanical degradation is increased by

increasing the viscosity of the polymer or increasing the screw speed.112

As mentioned previously thermal and mechanical degradation are common

phenomena in reactive extrusion. Additionally, oxidative degradation is another

reaction which can lead to the production of polymer macro-radicals, which

depending on the nature of the polymer, can undergo cross-linking or chain scission

reactions. For example, PE is prone to cross-linking while PP tends to show chain

scission. In the presence of oxygen, especially during reactive extrusion under

industrial process conditions, chain scission leads to the formation of vinylidene or

vinyl groups. As a result, oxidative degradation can be detected by identification of a

wide range of oxygen-containing groups such as aldehydes, ethers, ketones and

carboxylic acids.112,136,137,141,142 Thermal and mechanical degradation is exacerbated

in the presence of oxygen which is either captured from the environment or

dissolved in the polymer.97,138,142 All types of polymer degradation that occur during

reactive processing involve free radicals, ions, ion pairs, or low molecular weight

species. To date, numerous studies have been performed to investigate the kinetics of

this type of polymer degradation.37,84,131,136

Although polymer degradation during reactive extrusion is very complicated, many

researchers are interested in understanding and modelling these

pathways.36,37,43,44,112,139,143,144 A kinetic model for thermal degradation of PP during

Chapter 1

27

reactive processing has been proposed by Tzoganakis et al.145,146 Reactions included

in this kinetic model are thermal degradation of PP macro-radicals. Pn R•

r + R•n-r 1.26 Thermal degradation

where:

Pn = polymer molecules length “n” and

R•r and R•

n-r = macro-radicals of length “r” and “n-r”, respectively.

This mechanism is followed by transformation of produced macro-radicals to

polymer molecules:

R•s + Pr Ps + Pn + R•

r-n

1.27

where:

R•s and R•

r-n = macro-radicals of length “s” and “r-n”, respectively and

Pr ,Ps, and Pn = polymer molecules length “r”, “s”, and “n”.

There are two possibilities for the termination reaction. One of these, which is

presented in equation 1.28, is related to combination by primary radicals.

R•n + R•

c Pn + RcH

1.28

where:

R•n and R•

c= macro-radicals of length “n” and “c”, respectively

Pn = polymer molecules length “n” and

RcH = dead primary free radicals.

The second possible termination reaction is presented in Equation 1.29, which is

termination by combination.

R•r + R•

s Pr.s

1.29

where:

R•r = macro-radicals of length “r”

R•s = macro-radicals of length “s” and

Chapter 1

28

Pr.s = polymer molecules length “r” + “s”.

To determine the rate of degradation within a polymer, the change in concentration

of products as a function of time can be calculated. In reactive extruders, the

degradation rate is calculated using a similar equation, where time is substituted with

extruder length.

It should be noted that some degradation during reactive extrusion is caused by the

initiator.131 As mentioned before one of the most important applications of reactive

extrusion is grafting modification of polymers such as PP and PE for which a

peroxide is often used as an initiator. Because of this, the mechanism of the polymer

modification is influenced by the effect of initiator, both on the formation of the

polymer structure and degradation of the formed polymer.19,38,40,42 Several

investigations have exploited initiator-based degradation to produce modified

polymers with particular molecular weights and desirable physical and mechanical

properties or to recycle polymer waste.38-40,81,96,107,140,141,147 El’darov et al132 developed kinetic models for oxygen-initiated polymer degradation

during extrusion processing. They demonstrated that oxygen usage at the extruder

entry is very low. However, the consumption of oxygen increased within the

extruder until all oxygen had been consumed which occurred at seven tenths of the

extruder length. Pinhero and co-workers136 explained the role of chain scission and

chain-branching in LDPE during thermo-mechanical degradation and Iedema et al131

described the use of molecular weight distribution for the identification of kinetic

models of peroxide degradation. Al-Malaika and Peng140 explained that the

mechanism of thermal oxidation in reactive extrusion processing of LLDPE was not

only affected by processing conditions but also the structural characteristics of the

polymer. For example, co-monomers or any unsaturated structure present in the

polymer as defects can change the mechanism of the thermo-oxidative degradation.

They demonstrated that the presence of some groups such as vinyl, vinylidene and

trans-vinylene has an effect on the mechanism of LLDPE degradation under

different processing conditions. Da Costa and co-workers96 investigated PP

degradation during multiple extrusion conditions. They established that by a

combination of thermal, mechanical and chemical degradation, the mechanical and

Chapter 1

29

physical properties of PP can change dramatically. Not only the viscosity of the

polymer decreased, but the elasticity was also greatly reduced. Pinheiro et al136

studied the mechanism of thermo-mechanical degradation of linear PE in reactive

extrusion and found that during processing the longer polymer chains suffered from

entanglements. Dissolved oxygen in the melt polymer reacted with the polymer

chain causing chain scission and produced a stable carbonyl end group such as an

aldehyde or ketone. However, shorter chain macro-molecules were not as influenced

by friction as longer polymer chains. As a result, they do not undergo chain scission

reactions.

1.10 Monitoring reactive extrusion

Because of customer demand for improved product quality, optimising the

polymerisation process by decreasing production costs, developing real-time

processing and controlling the reaction during polymerisation have become

important. Any method used for monitoring the polymerisation process has to be

fast, accurate and reliable.34,48,54,88,90,148-159 From an engineering point of view,

polymer reactions are monitored for process analysis and polymer analysis. Process

analysis mostly concerns monitoring the polymerisation process, whilst polymer

analysis involves characterisation of the final polymer.160 Real-time monitoring for a

polymerisation process can be categorised into four techniques:152,161

- Off-line

- At-line

- On-line and

- In-line.

Laboratory experiments are the basis of off-line monitoring techniques. Moreover

sampling is done manually during the reaction from the reaction mixture or after

completion of the process.55,130,149,150 The sampling process of at-line monitoring is

similar to off-line monitoring; however, the measurements are collected near the

reactor.150 On-line monitoring sampling is performed automatically and the sample is

transferred by a sample line or “by-pass” through the automatic

analyser.48,149,150,161,162 For in-line monitoring processes, a measurement system such

Chapter 1

30

as a probe or sensor is placed in the process line and connected to an automatic

analyser.48,130,149,150,163 This type of monitoring involves a non-contact controlled

process as the measurement equipment is located in the reactor and does not come

into contact with the materials.62,150,163 Generally, on-line, in-line and at-line

monitoring are performed during process analysis and off-line monitoring is carried

out for polymer characterisation.160

A large amount of information can be obtained by monitoring. It is mainly used for

indicating the structure of products, determining the kinetics of the reaction,

concentration of monomers or products, and morphology and rheology of the

polymer.55,130,149,151,162,164-167 Rheometry, optical microscopy, ultrasonics, X-ray

diffraction, light scattering, and spectroscopies such as infrared (IR), UV, dielectric,

nuclear magnetic resonance (NMR), and Raman are some measurement approaches

which can be applied to monitoring during polymerisation or modification in

reactive extrusion processing.130,148,150,161 Some of these techniques such as Raman,

ultrasonic and optical microscopy have recently been the subject of much

research.130,150,168,169

Because many studies have focused on off-line and on-line monitoring,48,148-

153,161,162,164-175 these two techniques will now be discussed briefly.

1.10.1 Off-line monitoring

As mentioned before, in off-line or laboratory monitoring techniques, samples which

are taken from the reactor during the reaction or of the finished product are

transferred to the laboratory for analysis.55,150 Off-line monitoring techniques are

usually applied for measuring the melt flow index (MFI), moisture content of

products or reactants, mechanical and rheological properties, composition and

molecular structure.149

This type of monitoring has many limitations, one of which is lag time. This is

mainly because preparing the equipment for experiments in the laboratory can be

very time-consuming and for some tests several hours are required before the results

are obtained. Hence, because there is a large time gap between sampling and

obtaining the results, and because the reaction is continuous, the sample and end

Chapter 1

31

product do not have the same properties, which can result in large amounts of wasted

material.98,149

Another problem when using off-line monitoring is that the sample analysed is small

and as a result might not have all the properties that the bulk possessed.130 Moreover,

in some experiments, some properties of the sample change during analysis. For

example, during measurements of melt-polymer shear by rheometry. The melt

residence time in the extruder during reactive extrusion is one minute; however, the

minimum melt residence time in a rheometer is five minutes. As a result, the

extended time at high temperatures could change the sample properties. Because of

these limitations and problems for off-line monitoring, on-line monitoring is

preferred.

1.10.2 On-line monitoring

Over the last decade, the use of on-line monitoring techniques in the field of polymer

research and in the general polymer industry has increased.48,49,161,176 This technique

can provide accurate real-time feedback for identifying aspects of the process, as

well as, decreasing the total time required for analysis.55,154

Coates et al130 and Fischer and Eichhorn150 have shown that on-line techniques can

be performed directly in the extruder to monitor the processing of materials. The

measurement equipment was situated in the processing line and sampling was

performed in the processing stream where a sample was continuously transferred to

the analyser. However, the sample was small compared with the total material being

processed. Although on-line monitoring can be a very fast, robust and accurate

technique, it has a few limitations. One of these, particularly for on-line monitoring

in reactive extrusion, is time lag. This problem arises because the melt-polymer or

melt-sample is transferred from the “process stream” by a heated by-pass to a cell for

analysis. As a result, this process causes a time gap between taking the sample and

passing it into the measurement system.

Recently, numerous developments have occurred in the field of on-line monitoring

of reactive extrusion processes. These developments are improving fibre optic

technology, especially when using spectroscopic instruments Raman, near-IR (NIR),

Chapter 1

32

UV-Vis and ultrasonic spectroscopy, as well as, robust probes for

monitoring.100,130,153,158,175,177-181 Fibre optic probes are capable of collecting data and

information during processing from adverse environments such as high temperature,

high pressure and in the presence of harsh chemicals. Fibre optic techniques do not

require any sample preparation and can be multipurpose and multipoint, making

them desirable and convenient for monitoring polymerisation processes involving

reactive extrusion.158

Several methods have been successfully applied for on-line monitoring of reactive

extrusion processes.180 Methods such as mid-IR, NIR, Raman, UV-Vis, NMR and

ultrasonic spectroscopy have all been used to investigate the molecular and

morphological properties of polymers during this process.157 Although reactive

extrusion has only been a recent focus of research, a number of studies have

concentrated on the field of on-line monitoring of reactive extrusion using

spectroscopic and rheological techniques simultaneously.96 Some highlights of their

findings are discussed below.

George et al11 studied spectroscopic probes to measure the real-time modification of

polymers, as well as their degradation. Śaśić et al153 have discussed on-line

monitoring by NIR spectroscopy of the copolymerisation of ethylene and vinyl

acetate. They determined that this technique was very fast and reliable for the real-

time identification, of the reactive extrusion process. Furthermore, Fischer et al150

used Raman and NIR for the on-line monitoring of polymer blend processing via

reactive extrusions and Mijovic et al173 used remote mid-IR spectroscopy for the

same purpose. Wateri and Ozaki182 used NIR spectroscopy for on-line monitoring of

ethylene-vinyl acetate copolymerisation. They determined that this technique can be

used for the construction of calibration models for this copolymerisation process.

Fischer and co-workers176 also applied NIR spectroscopy for identification of real-

time processing of PE/ethylene vinyl acetate copolymer. They determined that NIR

spectroscopy was an appropriate technique for qualitative and quantitative analysis

of the polymers. Rodriguez-Guadarrama183 used on-line monitoring by Fourier

transform (FT) NIR spectroscopy to monitor conversion during the anionic

polymerisation of butadiene and to investigate the experimental kinetics.

Chapter 1

33

1.11 Vibrational spectroscopy

One of the most significant tools for on-line and in-process monitoring is vibrational

spectroscopy. There are numerous advantages of using vibrational spectroscopy for

monitoring. One of these is the high information content obtained relating to the

sample’s molecular structure. Additionally, the sampling techniques are simple, non-

destructive and do not require high vacuum or additional sample preparation such as

solvent extraction.62-64,148,153,156,163,184

Because of its capability to obtain significant and accurate information in complex

macro-molecules, vibrational spectroscopy has been useful for monitoring in

polymer research and industry.184 Recent developments in vibrational spectroscopic

instrumentation such as sensors, fibre optics and detectors, as well as coupling with

chemometric software, have led to accurate information being obtained by these

methods.34,62,88,90,130,156,163 Vibrational spectroscopy involves two different

techniques: IR; and Raman spectroscopy.184 These will be briefly discussed below.

1.11.1 Infrared spectroscopy

IR spectroscopy is a commonly used, yet powerful method not only used for the

analysis of polymers, but is also used for a broad range of molecules and

compounds. This spectroscopy is based on absorption, reflection or emission of

electromagnetic radiation. IR spectroscopy probes the vibrational states of molecules

and there are specific absorption bands which can be related to particular chemical

bonds or groups. As a result, these vibrations can be used for identification of the

chemical bonds. Molecules are IR active when they have permanent dipole moments

or if dipoles are generated during vibrations. Conversely, molecules which do not

have any dipole moments are IR inactive.130,184,185 In recent years, IR techniques

have been improved so there are numerous types of sampling methods: transmission,

reflection, attenuated total reflection (ATR), microscopic FT-IR, FT-IR imaging, and

fibre optic-IR are some examples. One of the techniques used for in situ monitoring

of the reaction during the processing is React IR®. This technique facilitates the user

to detect initiation and end point of the reaction. This technique is a powerful

techniques to investigation of the unstable intermediate product during the

processing. 184 The IR spectral region is divided into three ranges: far-IR; mid-IR;

Chapter 1

34

and NIR. Monitoring and analysing in the polymer field mainly uses the mid-IR and

NIR ranges.34,62,63,88,90,130,156,163

The mid-IR region ranges from 4000 – 400 cm-1. In this region, fundamental

vibrations occur.130,150,152,173,186 Significant and accurate information about the

concentration and composition of materials can be determined using this region.150 In

spite of these advantages, there are several problems when using these frequencies

for monitoring purposes. The common problem is that only expensive materials such

as chalcogenide and metal halides can be used for optical fibres in this region.150,152

The next most powerful range for in situ measurements is NIR spectroscopy which is

utilised broadly, particular in polymer research. This region lies between

approximately 12000 and 4000 cm-1 and overtone and combination bands occur here,

rather than fundamental absorptions.130,149,150,152,181,186 The most intense bands which

can be detected are C-H, O-H, and N-H. Because most polymers incorporate these

functional groups, NIR spectroscopy is a powerful technique for investigation and

monitoring of polymerisation processes.149,159,186

By introducing fibre optic probes and the development of chemometrics methods,

the application of NIR spectroscopy to in situ qualitative and quantitative

investigations has increased and it has been applied to several areas of research such

as oil, clinical, biological and medical analyses, as well as polymer investigations.186

Compared with other IR regions, NIR has some advantages, particularly within the

polymer field. One of these is that this technique is suitable for several types of

polymers and polymerisation process monitoring. For example, it can be used for

kinetics studies for solution polymerisation and reactive extrusion processing

including real-time monitoring.149,151,181 Another advantage in using NIR

spectroscopy is the utilisation of quartz fibre optics which are relatively inexpensive

and these can be coupled with sensitive short response time detectors.97,149,181 In

addition, probes utilised for NIR monitoring have a very good response, can easily

be coupled with the reactor and do not necessarily require expensive software for

manipulation of the information.149,152

Chapter 1

35

In spite of the many advantages in applying NIR for monitoring, there are a few

disadvantages. The most important disadvantages are that peaks in this region have a

weak intensity, are usually broad and overlapped and are not always specific to a

particular chemical group.148,150,152,181 Because of this, NIR can have a lower

information content compared with mid-IR.150 These disadvantages can be overcome

by using multivariate data analysis.34,62,63,156,163

Recently, a number of studies for successful monitoring have been carried out in the

field of reactive extrusion by NIR and mid-IR spectroscopy.11,55,130,149,150,153,161,169,173

1.11.1.1 Attenuated total reflectance

ATR, also known as internal reflection spectroscopy (IRS) or multiple internal

reflectance (MIR), is one of the more powerful sampling techniques of IR. This

technique is non-destructive and is based on total reflection of the IR beam.187-189 As

a result the sample must be placed in contact with an internal reflection element

(IRE), through which the IR light is guided. Figure 1.6 shows single and multiple

reflection systems. The IRE must have a high refractive index and be IR transparent.

The most common materials utilised as IREs are: zinc selenide (ZnSe); silicon (Si);

germanium (Ge); and diamond.188,190

Figure 1.6 Total internal reflection elements. Schematic A shows single TIR and schematic B

shows multi-TIR (picture was modifieded from reference 188).188

n1

n2

Sample

Ө

IRE

Sample

Sample

n1 IRE n2

n2

A)

B)

Chapter 1

36

As the ATR technique is based on internal reflection, the sample’s refractive index

must be lower than the refractive index of the element or crystal. According to

Snell’s law:

SinnnCθ = 2

1

1.30

where:

θC =Critical angle

n2 = Refractive index of sample and

n1 = Refractive index of crystal.

The angle of incidence of the beam has to be larger than the critical angle (θC). One

of the advantages of ATR for thin samples is penetration depth of the IR beam.

Depth of penetration (dp) is defined as the distance which is “required for the

electrical field amplitude to fall to e-1 of its value at the interface”.188

The depth of penetration is defined as:187,190

( )d

n Sin np =

−

λ

π θ2 12 2

22 1 2

./

1.31

where: θ = incident angle

λ =wavelength of the incident beam

n1 = refractive index of crystal and

n2 = refractive index of sample.

One of the important advantages of ATR is that this technique often does not require

sample preparation. Because there is no need for sample preparation through ATR

investigations, sampling is fast and easy and spectra can be collected from samples

in their natural state. Although there are a lot of advantages for this technique, there

are some limitations. Contact between the sample and the ATR crystal is very

important. If the sample does not have a good contact with the crystal the spectra and

data obtained will not be accurate. Because of the advantages for ATR technique, it

has been used in many areas of research, such as polymer, biology and

biochemistry.187

Chapter 1

37

1.11.2 Raman spectroscopy

Raman spectroscopy is a powerful technique of vibrational spectroscopy which has

produced very successful results in monitoring, particularly in on-line monitoring of

polymerisation processes.130,148,168-172,177 Raman spectroscopy is a light scattering

technique171,184,185 which leads to a change in the frequency of the monochromatic

light irradiated onto the sample.171,185 Raman spectroscopy has a number of

advantages. One of these is that in contrast with IR spectroscopy non-polar or

symmetric molecules often show intense Raman peaks. As well as this, unsaturated

and aromatic compounds often have sharp and intense peaks in Raman

spectroscopy.171 Another benefit is that, because water has a very weak Raman

intensity, the technique is suitable for the study of emulsion

polymerisation.168,171,177,185 Additionally, Raman can be a fast, robust technique184

and fibre optic probes can be utilised for monitoring allowing spectra to be collected

through the glass walls of reactors.172 However, Raman spectroscopic applications

are sometimes limited by fluorescence.171,184,185 This limitation can be minimised by

using a longer excitation wavelength between 700 and 1064 nm to decrease the

fluorescence effect.171,184

1.12 Project outline

With increasing demands for improving product quality in the polymer industry,

several methods have been developed for monitoring polymerisation reactions,

especially in the case of reactive extrusion processing.

The approaches used in this study represent the application of vibrational

spectroscopy for the characterisation and monitoring of product quality during and

after processing. In this project, microscopic ATR/FTIR and NIR have been applied

as two powerful vibrational techniques for investigation of the polymer melt

processing and products. Simultaneously, polymer viscosity was measured and used

to examine the degree of degradation during processing.

This study involves four experimental sections. Chapter 3 focuses on the

experimental application of microscopic-ATR/FTIR spectroscopy to directly assess

the quality of the blend product of polyester-starch produced by reactive extrusion

Chapter 1

38

processing. This chapter includes investigations into the effect of initiator, DCP, and

monomer, MAH, concentration on the quality of the polymer blends.

As mentioned previously, PP is one of the desirable polymers used in industry. To

increase the physical and mechanical properties of PP, it must be blended with other

polymers. Due to the lack of functional groups in the PP chains it is immiscible with

most other polymers and hence, has to be functionalised before blending. Chapter 4

focuses on the on-line monitoring of the functionalisation of PP with MAH in the

presence and absence of initiator (DCP) using the laboratory scale Minilab extruder

and following the process using a NIR fibre optic probe. At the same time viscosity

changes were also studied. In this chapter the effect of DCP and MAH concentration

on viscosity, rate of the reaction and activation energy were considered. In addition,

by understanding the effect of DCP and MAH, proposed kinetics and a mechanism

for this reaction have been presented.

Degradation is one of the limitations in the reactive extrusion technique, due to the

high temperature and nature of this technique. A novel technique for identification of

polymer degradation by application of profluorescent nitroxide as probe, has been

developed at the Queensland University of Technology. Chapter 5 focuses on the use

of a nitroxide to examine PP degradation during reactive extrusion processing.

In Chapter 6 the on-line monitoring by NIR-spectroscopy using fibre optic detection

of a TPU-clay nano-composite mix processing was investigated. In this chapter the

effect of the size of the clay nano-particle and temperature on the process was

examined.

Chapter 7 provides the overall conclusions from these studies and proposed future

work.

Chapter 2

39

Chapter 2 Materials and experimental techniques

2.1 Introduction

This chapter discusses the general materials and techniques used for the work

reported in this thesis. The specific materials and experimental techniques are

explained under separate subheadings in the related chapters.

2.2 Materials

2.2.1 Polypropylene

The PP used was minimally stabilised PP of Moplen HF500N grade which is

appropriate for extrusion applications (produced by Basell Polyolefin and supplied

by Ciba, Austria). The melt flow rate of PP was 14 g/10 minutes at 200 ºC and

accordingly, the volume flow rate was 16 cm3/10 minutes. The softening temperature

was 156 °C. This type of PP contains 70 ppm Irganox 1076 as stabiliser.

2.2.2 Maleic anhydride

MAH was purchased from Fluka (Sigma-Aldrich) company and its assay was ≥

99%. The melting point was 52 – 54 ºC, and it was used without further purification.

2.2.3 Dicumyl peroxide, xylene and acetone

Dicumyl peroxide (DCP) was supplied by Sigma-Aldrich. Its assay was 98% and it

was used without further purification. Xylene was also supplied by Sigma-Aldrich,

with an assay of 98.5%, and it was used for dissolving PP grafted with MAH.

Acetone AR grade (Merck) was used to dissolve un-grafted MAH and precipitate PP

grafted with MAH during purification.

2.2.4 Nitroxide

1,1,3,3-tetramethyl-2,3-dibenzo[e,g]-isoindoline-2-yloxyl (TMDBIO) was supplied

by James Blinco, a PhD candidate from the group of A/Professor Steven Bottle at

QUT.191,192

Chapter 2

40

2.2.5 Diethyl ether

Diethyl ether was purchased from Ajax Finchem. Its assay was 98% and it contained

2% ethanol and 5% water as stabilisers. To coat PP with the nitroxide, diethyl ether

was applied as a solvent.

2.3 Experiments

2.3.1 Laboratory scale melt processing: Minilab extruder

Melt processing was performed using a Thermo Haake Minilab Rheomex CTW5

laboratory scale extruder, with a conical twin screw of diameters of 5/14 mm and

length of 109.5 mm (see Figure 2.1 and Figure 2.2). As this instrument contains a

twin screw, it can simulate the performance of a co-rotating or a counter-rotating

extruder. In all experiments, the co-rotating operation was applied. The Minilab

operates at speeds between 10 to 360 min-1 and its heating capacity and maximum

temperature are 800 W and 450 °C, respectively.

A backflow channel in the Minilab also allows it to be utilised as a mixing reactor in

that the material can be recirculated rather than exiting through the die. In addition to

mixing operations, the Minilab can also be used as a rheometer to measure

rheological characteristics, by the use of a pair of pressure transducers in the

backflow channel. An approximate value for viscosity of the melt is measured

automatically by the Minilab by using the difference between these two sensors. The

Minilab has a maximum feed capacity of 6 grams.

As there was no sensor or probe port in the Minilab, the instrument had to be

modified to allow for in situ NIR monitoring (see Figure 2.3). A hole, 8 mm in

diameter, was bored into the top barrel chamber near the union nut of the swivel arm,

in line with the second pressure transducer (200 bar). The hole was sealed, flush with

the inside surface, with a clear sapphire window which was fixed in place with a

Superflex-red high temperature, silicone adhesive sealant (RTV). Recommended

windows for the NIR region are glass, quartz, and sapphire. Here, a sapphire window

was chosen for its strength, NIR transparency and stability at high temperatures. To

Chapter 2

41

increase the NIR beam reflection within this region the inside wall of the hole was

polished to a high shine.

Figure 2.1 Haake Minilab Laboratory Scale Extruder: External view

Figure 2.2 Inside the barrel showing twin screws

Screws

Pressure sensors

Backflow channel

Chapter 2

42

Figure 2.3 Photographs of (a) The NIR spectrometer linked to the Haake Minilab extruder by a

fibre optic cable. (b) close-up of fibre optic probe inserted into Minilab.

For clarity, a representation of the modified Minilab connected to the NIR fibre optic

and the way in which the light beam passes through to the polymer have been

presented in Figure 2.4 and Figure 2.5.

For PP grafted MAH processing, a total of 4 g material was fed into the Minilab

extruder. The operating temperature was set at 200, 210, or 220 °C. In all

experiments, the screw speed was 30 rpm and the experiment was completed after 90

minutes.

(a)

(b)

Chapter 2

43

Figure 2.4 A diagrammatic representation of the modified Minilab connected to the NIR

spectrometer with a fibre optic.

Figure 2.5 A diagrammatic representation of the modified upper barrel and sapphire window,

showing how the light beam passes through to the polymer

NIR fibre optic External surface

Light beam

Polymer

Sapphire Window

Chapter 2

44

2.3.2 Near-IR spectroscopy

A 360N SABIR fibre optic probe which contains 28 fibres of up to 1000 μm in

diameter with a length of approximately 2 m was connected to a Nicolet Nexus

spectrometer equipped with an un-cooled TEC NIR InGaAs detector, quartz beam-

splitter and tungsten halogen source. NIR spectra were collected in the range 10000

– 4000 cm-1. In all experiments, 190 scans were collected at a resolution of 16 cm-1,

which required 90 seconds. The gain and aperture size used were 8 and 100 μm,

respectively.

A background spectrum was taken from the Minilab and sapphire window at the

beginning of each experiment before monitoring began. Spectra were manipulated

and plotted with the use of the GRAMS32 AI software package (Galactic Corp.,

Salem, NH).

2.3.3 ATR/FTIR spectroscopy

IR spectra were collected using a Nicolet 870 Nexus Fourier transform infrared

(FTIR) spectrometer equipped with a Smart Endurance single bounce diamond ATR

accessory (Nicolet Instrument Corp., Madison, WI). Spectra were manipulated and

plotted with the use of the GRAMS/32 software package (Galactic Corp., Salem,

NH). The spectrometer incorporated a KBr beamsplitter and a deuterated triglycine

sulphate room temperature detector. Spectra were collected in the spectral range

4000 to 525 cm-1, using 64 scans at 4 cm-1 resolution with a mirror velocity of

0.6329 cm/s. The measurement time for each spectrum was around 60 seconds.

2.3.4 Principal Components Analysis

Principal Components Analysis (PCA) was performed on the NIR spectra using the

chemometrics module of the GRAMS/32.60 software package (Galactic Corp.

Salem, NH).

2.3.5 Fluorescence spectroscopy

Fluorescence spectra were collected using a Varian Cary Eclipse fluorescence

spectrometer equipped with a 15 W pulsed xenon lamp with pulse width of 2 – 3 μs.

It was also equipped with a photomultiplier tube detector and a diffraction grating of

30 × 35 mm with 1200 lines/mm. This instrument was able to record the spectra in a

Chapter 2

45

wavelength range between 190 nm and 900 nm. In this investigation, the

fluorescence spectrometer was equipped with a standard multicell Peltier

thermostatted sample holder and plaques were irradiated at an angle of 45° to the

surface. Samples of approximately 60 μm thick were held between two plates and

fixed with two screws. To reduce the fluorescence produced by the plates, the

samples were held by a silicon slide and aluminium plate, with a sample irradiation

hole approximately 1 mm in diameter. The samples were excited at 294 nm and

spectra were collected from 310 to 450 nm.

2.3.6 Raman spectroscopy

A Renishaw System 1000 Raman microprobe spectrometer (Renishaw plc, Wotton-

under-Edge, UK), equipped with a Renishaw He-Ne laser emitting at 632.8 nm, and

a 1200 lines/mm grating, was used for collecting Raman spectra. An Olympus

MDPlan 50× objective with a numerical aperture of 0.75 was employed to focus the

laser onto the sample. Each Raman spectrum was collected in extended mode using

the laser at maximum power for 120 seconds and two accumulations in the spectral

range of 300 – 3200 Raman shift (cm-1).

2.3.7 Thermogravimetric Analysis

Thermal decomposition of the PP-MAH and PP-DCP-MAH samples was carried out

in a TA® Instruments Inc. high-tesolution thermogravimetric analyser (Q500 model

analyser) in a flowing air atmosphere (60 cm3/min). Approximately 30 mg of sample

underwent thermal analysis with a heating rate of 5 °C/min, resolution of 7, to

1000 °C. Low resolution ramp methodology was used as the precise mechanism of

thermal decomposition was not required.

Chapter 3

46

Chapter 3 Investigation of homogeneity of extruded starch/polyester blends by using infrared

microspectroscopic mapping

3.1 Introduction

In recent years, there has been much interest in the development of biodegradable

plastics in order to reduce the amount of long-lived petroleum-derived plastic in

community waste. One approach to the synthesis of biodegradable plastics is to

utilise the naturally occurring biodegradable polymer, starch. However, starch alone

typically has very poor properties for direct application and therefore is often

blended with a high performance synthetic polymer such as a polyester.119,193 As

starch is a highly polar material it is immiscible with typical hydrophobic synthetic

polymers. This problem may be alleviated to some extent by grafting a

compatibiliser molecule, such as MAH, onto the synthetic polymer (see Section

1.5.1). This process adds polar groups to the synthetic polymer, aiding miscibility,

and also may form direct cross-links between the synthetic polymer and starch

chains.194 An initiator such as DCP is necessary to initiate the grafting reaction

between MAH and the synthetic polymer. In modern polymer processing the whole

process described above may be carried out via reactive extrusion by passing all the

components together through a suitable extruder for an appropriate residence time at

an appropriate temperature profile.8,13,123 In the extruder, both thorough mixing of

the components as well as the compatibiliser/cross-linking reaction involving MAH

occurs. The final product then exits from the extruder die in a form immediately

suitable for further applications. There is a significant gain in efficiency to combine

the three processes of mixing, graft reaction and extrusion into a continuous process

involving one piece of equipment.

The quality of the extruded material and its suitability for particular applications may

be assessed by various means such as mechanical testing. Dynamic mechanical

analysis will also give useful information about the quality of the blend, but the

information given by these and similar tests is essentially macroscopic in nature. To

assess the blended material at the microscopic level both optical and electron

Chapter 3

47

microscopy have been used. However, these do not give molecular structural

information and may not distinguish points or areas, which are chemically different.

An alternative approach is to use vibrational microspectroscopy, which is capable of

distinguishing different chemical environments. Both Raman195,196 and IR

microspectroscopy197-199 have been used to map on polymer surfaces. Raman

microscopy has by far the better lateral resolution, generally around 1 μm2, but often

suffers from a fluorescence background which can swamp the Raman signal. IR

microspectroscopy realistically has a spatial resolution of around 20 μm × 20 μm,

governed by diffraction and signal-to-noise issues. For extruded materials, the cross-

sectional area is sufficiently large that the spatial resolution of IR microspectroscopy

is quite appropriate while that of Raman microspectroscopy is too small.

IR microspectroscopy has the disadvantage that sample preparation is often required

in that transmission measurements must be made through thin sections (~20 μm).

Reflection measurements are possible but may lead to distorted spectra depending on

whether the reflection regime is specular or diffuse or, as is often the case, a mixture

of both. These problems can be overcome by the use of an ATR objective. The ATR

experiment is described in detail in Section 1.11.1.1. For ATR the surface needs to

be flat, but a thin section is not required. The objective incorporates a small crystal

(the IRE), which is brought into contact with the surface in order to collect the

spectrum. In recent years, ATR microspectroscopy has been applied to such diverse

materials as coal,200 rubber museum artefacts,201 and filled rubbers.202

As mentioned in Section 1.10, one type of monitoring is off-line monitoring, which

is more focused on the characterisation of the products. This chapter describes the

application of microscopic ATR spectroscopy, as an off-line monitoring technique,

to directly assess the quality of extruded blends by measuring the blend composition

at a number of material cross-sections. The quality of the blend is visualised by the

use of IR images. While IR imaging has been used before to characterise an extruded

product,196 it has not previously been reported for the assessment of polyester/starch

blend quality, particularly with respect to variation of composition.

Chapter 3

48

3.2 Experimental

3.2.1 Materials

The polymer blends used in this investigation were polyester-starch, produced by a

two-stage extrusion in a laboratory scale PRISM co-rotating twin screw extruder,

with a length to diameter ratio of 40:1 and screw diameter of 16 mm (Thermo

Electron Corporation). Four different samples of blended polyester-starch were

prepared by the Centre of High Performance Polymers, Division of Chemical

Engineering, University of Queensland. The polyester was EnpolTM (G4460 film

blowing grade), which is a fully biodegradable aliphatic polyester resin (Ire

Chemical Ltd, Seoul, Korea). The starch was obtained from Penford Australia Ltd.

In these samples, DCP and MAH were used as an initiator and

compatibiliser/crosslinker, respectively. The blend mechanism of polyester-starch

has been discussed in Section 1.5.1. Each sample consisted of 20 – 30 extruded rods,

each approximately 20 – 25 cm in length and 2.5 – 3 mm in diameter. In all samples,

the ratio of starch to polyester was maintained at 40:60 (wt %); however, the amount

of DCP and MAH varied, as shown in Table 3.1. Additionally, a photograph of the

received samples is presented in Figure 3.1.

Table 3.1 The composition ratio of materials in blends (parts by weight %) Sample number Starch

(part) Polyester (part)

MAH (part)

DCP (part)

1 40 60 1 1.2 2 40 60 0.5 0.8 3 40 60 1 0.8 4 40 60 1.5 0.5

Chapter 3

49

Figure 3.1 The four received polyester/starch blend samples

3.2.2 Sample preparation

Prior to examination by micro-ATR/FTIR, a sample approximately 3 cm long was

cut from the extruded material with a sharp knife. This sample was embedded in

epoxy resin (Araldite M, B/No 40044600 with hardener LC956, B/No AM 4002080:

composition ratio 5:1 vol %), so that one circular section was exposed. The resin was

cured overnight at 60 °C and the sectioned face polished with P600 sandpaper. For

each sample, ten randomly selected sections were analysed.

3.2.3 Micro-ATR/FTIR

FTIR spectra were collected using a Nicolet 860 Nexus FTIR system, coupled to a

ContiμumTM IR microscope equipped with a liquid nitrogen-cooled MCT detector and

an ATR objective, incorporating a silicon IRE (Nicolet Instrument Crop., Madison,

WI). OMNIC and Atlμs software packages (Nicolet Instrument Crop., Madison, WI)

were used for instrument control, data collection and mapping. The sample contact

area was circular, with an approximate diameter of 100 μm. The area of sample to be

measured was visually selected and then the stage was raised until contact was made

between the sample surface and the ATR crystal. Reproducible contact pressure was

achieved with the use of an electronic sensor plate, incorporating a weight cell

attached to the microscope stage. Background spectra were obtained through the

Chapter 3

50

ATR element when it was not in contact with the sample and a new background

spectrum collected after every five sample spectra. Spectra were manipulated and

plotted with the use of GRAMS/32 AI software package (Galactic Corp., Salem,

NH). An image of the ATR/FTIR objective used for mapping is shown in Figure 3.2.

Figure 3.2 The ATR/FTIR objective used for mapping To obtain maps of the sample surface, spectra were collected in a square grid pattern

covering a section of the sample. The aperture was set at 150 μm × 150 μm, with a

step size of 150 μm, so that for typical samples, 20 × 20 spectra were required.

Under computer control spectral collection was performed in the following way.

Initially, the microscope stage was moved downwards, moving the sample away

from the ATR objective. The stage was then moved to the next XY measurement

position and the sample brought into contact with the ATR objective using the

electronic pressure sensor. This process was repeated in a step wise fashion until all

desired spectra were collected. Spectra were obtained in the spectral range of 4000 –

700 cm-1, using 32 scans and an 8 cm-1 resolution. The collection time for each

spectrum was approximately 30 seconds and a complete map typically took 2 – 3

hours.

3.2.4 Construction of images

For each spectrum, the ratio of the peak area of the O-H stretching band (3680 –

3030 cm-1) to that of the C=O stretching band (1770 – 1670 cm-1) was determined

using local baselines. These bands correspond to starch and polyester components,

Chapter 3

51

respectively. Spectral images were generated from these data using the Atlμs

software package (Thermo-Nicolet, Madison, WI).

3.3 Results and discussion

3.3.1 ATR spectra

Starch and polyester, the two components of the polymer blends, have very different

IR spectra. The most obvious distinguishing features are that the starch spectrum

contains an intense broad O–H stretching absorption around 3500 cm−1, while the

polyester has an intense sharp band at 1730 cm−1 assigned to the ester carbonyl

stretching vibration. Pure samples of the blend components were not available, but

the spectral differences can be clearly seen in Figure 3.3 which shows blend spectra

from a high starch point and from a high polyester point of a sample.

Micro-ATR spectra were collected in a grid pattern across the whole of the sectioned

polished faces of the samples. Using an aperture of 150 μm x 150 μm about 400

spectra were collected for each section which was circular in shape with a diameter

of around 3 mm. For each spectrum the ratio of the area of the O-H band

(attributable to starch) to the C=O band (attributable to polyester) was calculated.

For each of the samples a number of sections, ranging from 7 to 10 and chosen

randomly, were studied. The results of the area ratio calculation for all sections are

shown in Table 3.2. In all, 36 sections were studied and approximately 14,000

spectra were collected. As mentioned above, pure blend components were not

available so spectra of pure starch and polyester could not be measured. However,

even if pure materials had been available the measurement of useful spectra would

have been difficult due to the nature of the ATR experiment. For ATR the intensity

of the spectrum depends on the degree of contact of the crystal with the sample, as

well as the density of the sample in the measurement region. As the starch was a

powdery material, the intensities of the ATR spectral bands would not have been

closely related to the intensities of the ATR spectra of the hard plastic extruded

blends.

Chapter 3

52

Figure 3.3 Example ATR/FTIR spectra of the extruded material. (A) A high starch

point of the sample; (B) a high polyester point of the sample. (The spectra have been offset for clarity.)

In the absence of useful spectra of the component polymers, the mean area ratio (O-

H/C=O) for all spectra was determined to be a reasonable representation of the true

blend proportions (40 wt% starch, 60 wt% polyester). The value of this mean was

4.1. Hence, for any point in the sample where the value of the band area ratio was

greater than the mean there is likely to be an excess of starch, and similarly where

the value was lower than 4.1 there was an excess of polyester. Examination of Table

3.2 shows that samples 1 and 3 were reasonably consistent since the value of the

band area ratio remained within relatively small ranges. However, sample 3 did

appear to show a slight excess of polyester. Samples 2 and 4 were more variable with

sections that showed large excesses of one of the components thus demonstrating

that the blending/mixing had not been as consistent. The more consistent samples (1

and 3) both had MAH contents of 1 wt%, whereas the less consistent samples (2 and

4) had MAH contents of 0.5 wt% and 1.5 wt%, respectively. As only 4 samples were

studied it is not possible to say from these data if this correlation with MAH content

was significant.

Chapter 3

53

3.3.2 IR images

The mean value of the band area ratio for each section did not give any information

about the distribution of the components within that section. This was best examined

by constructing IR images for each section, where a different colour was assigned to

each range of the area ratio value. Examples of these images are shown in Figure 3.4

(A–J). Pixels showing spectra of the epoxy embedding material surrounding the

sample were removed from the image. Occasionally, an indentation (hole) was found

which produced either no spectrum or a distorted spectrum. These areas are shown in

white within the coloured image.

Table 3.2 Infrared band area ratios for O–H/C=O stretching bands from randomly selected sections of the four samples of extruded

material Mean area ratio for

each section Sample number

1 2 3 4

1 3.375 3.957 3.378 3.335 2 4.490 5.251 3.333 3.469 3 3.302 3.756 3.834 3.591 4 4.031 4.864 3.461 5.017 5 3.339 4.476 4.120 3.919 6 4.804 4.979 4.007 6.113 7 3.752 6.853 3.235 4.097 8 4.172 3.424 3.231 4.215 9 4.941 3.143 3.802 2.813 10 4.658 3.448 7.942 Mean area ratio for sample 4.086 4.523 3.585 4.451

Chapter 3

54

Figure 3.4 Examples of infrared images from various sections of the extruded material. The colours show different ranges of the infrared band area ratio for the O–H/C=O stretching bands, as shown in the scale. Blue indicates high starch, while red indicates high polyester.

White areas indicate a defect (hole) in the sample.

Sample 2

Sample 3

Sample 1

Sample 3

Sample 4 Sample 2

Sample 2 Sample 4

Sample 1 Sample 3

Chapter 3

55

Examination of the images clearly showed not only the quality of the blend at each

section, but also the distribution of the components. For example, Images A, F and G

showed that there was an excess of starch at these sections. However, Image F also

showed that there had been poor mixing because the excess starch was found on one

side of the section, whereas for Image A the starch was well distributed across the

section. Image G showed a different distribution of the excess starch in that it was

concentrated towards the centre of the section. Image J also showed excess starch,

but although it was not well distributed there was no obvious pattern to the

distribution.

Images D, E, H and I showed sections where the polyester dominated although all

showed smaller regions of high starch as well. Images B and C indicated sections

where there was good blending because the amounts of the components was as

expected and the distribution was reasonably consistent across the whole section.

The images therefore give a measure of the component distribution at every point

(150 μm × 150 μm) on the sectioned face. Despite the fact that it is only possible to

measure a relatively small number of sections of the whole extruded material, this

would nevertheless still contribute useful information with which to judge both the

efficacy of the compatibilisation reactions and the performance of the extruder.

3.4 Conclusions

The micro-ATR/FTIR technique has been used for off-line monitoring of the

starch/polyester blend produced by reactive extrusion processing. IR

microspectroscopy has been shown to be a useful technique for the study of extruded

starch/polyester blends as these components have significantly different spectra. The

micro-ATR technique allowed good quality spectra to be collected from sectioned

faces of the extruded material. IR images constructed by taking the area ratio of the

O–H stretching absorption to the C=O stretching absorption for each spectrum

clearly showed both the proportion of each component and the distribution of those

components across the section. At the microscopic scale of the IR measurements,

these samples obtained from a laboratory scale extruder showed many sections

where the components were not well mixed or well distributed across the section.

Chapter 4

56

Chapter 4 Grafting of maleic anhydride onto polypropylene by

reactive extrusion: a laboratory scale study

4.1 Introduction

PP is one of the polyolefins which, because of its flexibility, low cost and desirable

physical and chemical properties such as stiffness, low specific gravity, non-toxicity,

and resistance to corrosive chemicals, is widely used.28,70,73,79 However, it has some

limitations, such as poor miscibility with polar polymers, because of a lack of

reactive sites, poor hydrophilicity, low melting point, poor mechanical properties and

high susceptibility to degradation.9,35,53,71,72,85

Functionalisation of PP is a common technique used to overcome these limitations.

This usually involves linking or grafting some small polar functional groups onto the

PP chain.7,20,28,60,85 One of the best methods for introducing the functional groups

onto PP is free radical grafting.9,28,29,31,35,53,58,87 PP functionalisation can be carried

out under four experimental conditions: solution state; melt-state or reactive

extrusion; solid-state; and vapour phase surface modification.9,14,29,30,58,70,86-88 The

melt-state or reactive extrusion technique is preferred because of low cost, the ability

to control the reaction, as well as high production rates.9,28,29,31,86

The main reactants in this method are an initiator which typically is an organic

peroxide; an unsaturated monomer or macro-monomer and polymer (PP in this

study).9,13,14,18,29,32,53,56,60 The reactive extrusion technique involves the synthesis of

polymers using a heated extruder as a chemical reactor and is based on a melt-phase

reaction. This technique involves mixing monomers or other reactants such as

additives with a molten polymer in an extruder at high temperature.5,13,78,127

Although a wide range of monomers have been applied for functionalisation of PP

via reactive extrusion, significant work has been focused on MAH.9,13,35,53,57,58,85,87-95

This is due to the complication of MAH homo-polymerisation during graft

functionalisation at high temperatures.13,53,89,91,92 Functionalisation of PP by MAH

improves product properties, such as chemical, mechanical or rheological properties,

Chapter 4

57

polymerisation of monomers, synthesis of new polymers and modification of

polymers, which can be carried out simultaneously.17,20,21,28,29,78

A large amount of information can be obtained by monitoring the progress of a

reaction. Monitoring is mainly used for indicating the structure of products, the

kinetics of the reaction, concentration of the monomers or products, morphology and

rheology of the polymer such as viscosity and molecular weight, flow properties,

concentration of additives and colour of the products.55,130,149,151,162,164-167 Rheometry,

optical microscopy, ultrasonics, X-ray diffraction, light scattering, and

spectroscopies such as IR, UV, dielectric, NMR, and Raman, are some measurement

approaches which can be applied for monitoring during polymerisation or

modification in reactive extrusion processing.130,148,150,161 Some of these techniques

such as Raman, ultrasonics and optical microscopy have been the subject of much

current research.130,150,168,169

One of the most significant tools for on-line and in-process monitoring is vibrational

spectroscopy, such as IR (mid and near regions) and Raman spectroscopy, which are

able to provide information for both qualitative and quantitative analysis. There are

numerous advantages which cause vibrational spectroscopy to be widely employed

for monitoring. One of these advantages is that they easily and quickly can provide

information about the molecular structure of the sample. Additionally, these

sampling techniques are simple, non-destructive and do not require high vacuum or

additional sample preparation such as solvent extraction. As a result these techniques

have received attention for in situ monitoring in polymer processing, especially in

reactive extrusion.62-64,148,153,156,163,184 Recent advances in vibrational spectroscopy

instrumentation especially the NIR region such as optical light fibres, detectors and

software, as well as the application of multivariate data analysis techniques, such as

PCA, have led to useful information being obtained by these methods.34,62-

64,88,90,130,156,163

NIR has some advantages, compared to the mid-IR spectral region, particularly in

the polymer field. One of these is that this region is suitable for monitoring several

types of polymers and polymerisation processes. For example, it can be used for

kinetic studies of solution polymerisation including real-time monitoring.94,96

Chapter 4

58

Another advantage in using NIR spectroscopy is that the relatively long wavelengths

allow the utilisation of quartz fibre optics which are inexpensive and can reduce the

cost of the monitoring instrument. Moreover, detectors used in this region have short

response times.99,149,152

This chapter reports the interfacing of an NIR spectrometer to a laboratory scale

mini-extruder using a fibre optic link. This system allows the study of the melt-state

reaction of MAH and PP over a range of temperatures and in the presence or absence

of an initiator. The NIR spectra obtained during the reaction were analysed to

investigate the kinetics and mechanism of the process. In addition, pressure sensors

in the mini-extruder simultaneously estimated the viscosity during the process.

Further information was obtained by collecting mid-IR spectra of samples of product

material after different stages of reaction.

4.2 Experimental

4.2.1 Laboratory scale melt state processing and NIR spectroscopy

In order to follow the reaction between MAH and PP, the Minilab was interfaced to

an NIR spectrometer (Nicolet Nexus NIR spectrometer, Thermo Nicolet, Madison

WS) via a hole 8 mm in diameter which was bored into the top barrel chamber of the

Minilab in line with one of the pressure transducers. The hole was fitted internally

with a clear sapphire window set flush with the internal surface of the barrel. The

NIR fibre optic probe was held vertically to point directly into the hole. The inside

wall of the hole was polished to increase reflection of the NIR light. In all

experiments spectra were collected in the 10000 – 4000 cm-1 range with 190 scans at

16 cm-1 resolution. The measurement time was 90 seconds. Photographs of the

instrumentation are shown in Figure 2.2 and Figure 2.3.

MAH and DCP are the most commonly used monomer and initiator for modification

of PP under reactive extrusion conditions. The maximum concentration of MAH and

DCP used for most graft processing was 7% and 0.5%, respectively.28,29,31,35,87,104

The concentrations of MAH and DCP used in this study were 3.5 – 7 wt% and 0.25 –

0.5 wt%, respectively. Before extrusion, powdered PP was premixed with the

appropriate amount of MAH and DCP and then the materials were introduced to the

Chapter 4

59

Minilab extruder. The screw speed was set to 30 rpm, and the temperature was set

between 200 – 220 °C. To identify the effect of an initiator on the grafting

mechanism, experiments were performed either with or without addition of DCP.

The experimental design and concentration of MAH and DCP for each set of

experiments are summarised in Table 4.1.

Table 4.1 Reactive graft processing settings used for PP,

MAH, and DCP

Experiment No.

PP (% w/w)

MAH (% w/w)

DCP (% w/w)

T (ºC)

1 100 - - 200 2 99.5 - 0.5 200 3 93 7 - 200 4 92.5 7 0.5 200 5 93 7 - 210 6 92.5 7 0.5 210 7 93 7 - 220 8 92.5 7 0.5 220 9 92.25 7.5 0.25 200 10 96 3.5 0.5 200

Experiment 1 was a test run without MAH and DCP. Experiments 1 and 2 were

performed to check for system efficiency and to check for possible changes and

interferences. Each set of experiments was performed in triplicate and the average

viscosity and NIR result for each trial was calculated.

Each melt state processing experiment was carried out for 90 minutes and during this

time an NIR spectrum and a viscosity measurement was collected every 2 minutes.

4.2.2 ATR/FTIR

IR spectra were collected using a Nicolet 870 Nexus FTIR spectrometer equipped

with a Smart Endurance single bounce diamond ATR. Spectra were collected using

OMNIC (Thermo-Nicolet, Madison, WI) software. Spectra were manipulated and

plotted with the use of the GRAMS/32 software package (Galactic Corp., Salem,

NH). Spectra were collected in the spectral range 4000 to 525 cm-1, using 64 scans, 8

cm-1 resolution and a mirror velocity of 0.6329 cm/s. The measurement time for each

spectrum was about 60 seconds.

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60

4.2.3 Determination of grafting efficiency

There was a possibility that after completion of processing some ungrafted MAH

remained in the matrix of the final product. To quantify the grafted MAH that

reacted, and to confirm the accuracy of results obtained from on-line NIR

monitoring, ATR/FTIR spectra were recorded from the final products. Before

collecting ATR/FTIR spectra from samples they were purified by two methods to

remove unreacted MAH from the final products.

In the first method,28 a 20 – 30 mg of the sample was placed in an oven under high

vacuum, at 110 ºC for 24 hours whereby un-reacted MAH was vaporised from the

sample matrix. Also because of the high temperature used carboxylic acids were

converted to anhydrides. In the second method,29,31,53,58,72 a small amount of sample

was dissolved in xylene at 140 ºC and refluxed for 4 hours and then extracted with

acetone and filtered at room temperature. By using this method, ungrafted MAH

remained in the acetone. The extracted PP-grafted-MAH was washed several times

with fresh acetone. In order to evaporate any volatile compounds from the surface of

the sample and to transform any carboxylic acids present into their anhydride form

the sample was dried in a vacuum oven at 110 ºC for 24 hours.

4.2.4 Data analysis

The GRAMS/32.60 software package (Galactic Corp. Salem, NH) was used to

manipulate the collected NIR spectra. A calibration data set for the investigation of

structural changes was developed by use of PCA. All data was mean-centred and

normalised to the 6500 – 7400 cm-1 band, which is related to the CH3 combination

band, as well as CH stretching and CH2 deformation vibrations. In order to remove

baseline effects, the second derivative of the spectra was used in further multivariate

data analysis.

4.3 Results and discussion

4.3.1 In situ NIR Spectroscopy

NIR spectra of PP, MAH and DCP in solid (powder) forms are presented in Figure

4.1 and Figure 4.2. The bands in the NIR spectra of melt-state PP have been

previously assigned90 (see Table 4.2). Referring to the band assignments, the peaks

Chapter 4

61

observed between 8200 and 8700 cm-1 correspond to the second overtone of CH2 and

CH3 stretching vibrations, while the bands assigned at the region of 5000 – 6100 cm-

1 are associated with the first overtone of CH2 and CH3 stretching modes. The peaks

observed between 6600 and 7400 cm-1 are assigned to combination bands and the

first overtones of CH2 and CH3 stretching vibration with fundamental C-H

deformation modes respectively.

Figure 4.1 An NIR spectrum of pure PP at 200 °C: assignments shown in Table 4.2

Table 4.2 Peak assignments of PP NIR spectrum90

Chapter 4

62

DCP bands were not observed in the NIR spectra of grafting reactions owing to its

low concentration and the fact that its major bands overlapped with PP and MAH

peaks. For MAH (Figure 4.2 (a)) the strong band situated at 6117 cm-1 was assigned

to the first overtone of the =C-H stretching band. Because this band did not overlap

with PP bands, it was clearly observed in the NIR spectra of the PP-MAH mixture in

Figure 4.3. Another observable band for MAH was the combination of =C-H and

C=O stretching bands at 4966 cm-1. The 6117 and 4966 cm-1 bands were good

references for investigating MAH grafting onto the PP backbone.

Figure 4.2 NIR spectra of (a) MAH and (b) DCP in powder form.

(a)

(b)

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63

Figure 4.3 NIR spectrum of the PP-MAH grafting process after 2 minutes showing the presence

of MAH vibrational bands

4.3.2 NIR monitoring and viscosity investigation of PP processing

In order to determine the stability of PP during melt-state processing for up to 90

minutes, experiments were performed on PP alone (i.e. without DCP and MAH).

Figure 4.4 shows NIR spectra collected after 2 and 90 minutes of melt-state

processing at 200 ºC. No significant changes in the spectra can be seen. This set of

NIR spectra was further investigated by PCA. The PC1 factor loadings and scores

plots are presented in Figure 4.5. The lack of structure in the factor loadings plot

(Figure 4.5(a)), together with the random nature of the scores plot (Figure 4.5(b)),

confirmed that there was no significant change in the molecular structure of PP

during this experiment.

In order to establish the effects of DCP on PP and its molecular structure, the

experiments were repeated on PP and DCP processing in the absence of MAH. The

PCA analysis of the obtained NIR spectra gave similar results showing that the PP

molecular structure did not have significant changes for up to 90 minutes processing

Chapter 4

64

at elevated temperatures even in the presence of DCP. Hence it could be understood

that DCP did not change the molecular structure of PP.

Figure 4.4 On-line NIR spectra of the PP reactive extrusion at 200 °C without MAH and DCP.

Spectra were collected at (A) 2 minutes and (B) 90 minutes of processing.

Figure 4.5 PC1 (a) factor loadings and (b) scores plot of PP on-line monitoring at 200 °C at the range of 4700 – 8700 cm-1, normalized at 6500 – 7400 cm-1.

As mentioned previously, viscosity changes were measured simultaneously with

NIR spectral collection. The data obtained for PP alone (i.e. without DCP and

MAH) showed that the viscosity of PP started to change slowly at the beginning of

processing and these changes continued until the end of the processing. Because

viscosity is related to molecular weight, it is most likely that the molecular weight of

PP decreased due to chain breakage, which may be due to mechanical and/or

(a) (b)

Chapter 4

65

thermal degradation. These types of degradation are probably due to the high

temperatures and shear rate during the melt-state processing.

Investigation of the viscosity data for PP shows that the viscosity change in the

presence of DCP was very large immediately on commencement of processing

(Figure 4.6). Before commencement of the experiment, PP and the PP mixed with

DCP had same molecular weight. However, when the experiment was started the

viscosity of the PP mixed with DCP rapidly lowered to approximatly 0.65 kPa.s, 10

times lower than the starting viscosity, within 2 minutes of processing.

Figure 4.6 Viscosity changes for PP and PP in the presence of 0.5% DCP at 200 °C

Because viscosity is related to molecular weight, it is most likely that the molecular

weight of PP decreased due to chain breakage. Because of the presence of DCP, the

PP molecular chains underwent sudden radical-initiated degradation reactions

leading to chain breakage. As well as chain breakage due to radical-initiated

degradation, thermal and mechanical degradation can have effects on decreasing the

molecular weight and viscosity. It is interesting to note that the viscosity changes

indicated significant degradation of the polymer while the NIR spectra appeared to

show that no change in molecular structure occurred. This is because reactions which

cause chain breaking can markedly change the molecular weight (and hence the

Chapter 4

66

viscosity), but have little effect on NIR or mid-IR spectra because the relative

proportions of chemical groups present has changed very little.

From the viscosity data, the number of chain scissions per molecule of processed PP

and processed PP in the presence of DCP were calculated. As mentioned before,

viscosity and molecular weight have a direct relationship, so the molecular weight

can be calculated from the viscosity using Equation 4.1.203 4.3

wMK=η 4.1

where:

η = viscosity

Mw = molecular weight and

K = constant.204

After calculating the molecular weight, the number of chain scissions per molecule

can be obtained from Equation 4.2:

1−=oM

MS w 4.2

where:

S = the number of chain scissions per molecule

Mw = molecular weight after n minutes and

Mº = molecular weight at the beginning of the processing.

The number of chain scissions per molecule of PP after 2 and 90 minutes processing

were approximately 0.0134 and 0.452, respectively. In the presence of DCP the

number was 1.003 after 2 minutes and increased to 3.575 chain scissions per

molecule after 90 minutes of processing. This shows that chain scission was higher

in the presence of DCP. In fact, after 2 minutes of processing the chain scission in

the presence of DCP was approximately 100 times higher in the absence of DCP.

Furthermore, the amount of chain scission remaining at the end of processing was

still approximately 10 times higher in the presence of DCP compared to PP in the

absence of DCP. It can be concluded that DCP increases the amount of chain

scissions which may lead to higher amounts of PP degradation during processing.

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67

4.3.3 Investigation of final product by ATR/FTIR

After completing the processing for each set of experiments, an ATR spectrum was

collected from each of the products. ATR spectra of the components before

processing were also collected and displayed two MAH bands which did not overlap

with PP: 1715 and 1784 cm-1. After processing, there was a possibility that some un-

grafted MAH remained within the product matrix. To remove this the collected

products were purified by the two techniques described in section 4.2.3. Figure 4.7

shows the ATR/FTIR spectra of PP, PP-g-MAH before purification, PP-g-MAH

heated in a vacuum oven for 24 hr at 110 °C and PP-g-MAH purified by xylene and

acetone and heated in a vacuum oven for 24 hr at 110 °C. In the case of PP-g-MAH

without purification, two obvious bands were observed at 1784 and 1715 cm-1 which

can be assigned to the carbonyl stretching vibration (-C=O) of anhydride and

carboxylic acid, respectively. Carboxylic acid is present because the grafted

anhydride absorbs moisture from the environment and hydrolyses, consequently the

anhydride ring opens and the carboxylic acid is formed. The intensity of the band

observed at 1715 cm-1 was greater than that at 1784 cm-1 which showed that the

concentration of the carboxylic acid group was higher than the anhydride. Figure 4.8

presents the hydrolysis reaction scheme of MAH grafted onto the PP backbone.

Chapter 4

68

Figure 4.7 ATR/FTIR spectra of (a) pure PP, (b) PP-g-MAH without any purification, (c) PP-g-MAH after 24 hr heating in vacuum oven at 110 °C, and (d) PP-g-MAH purified by xylene and

heated in vacuum oven for 24 hr at 110 °C. (Spectra are offset for clarity.)

C

O OO

+ H2O

C

OOOHHO

Figure 4.8 Hydrolysis reaction scheme of MAH grafted onto the PP backbone

After purification, examination of the carbonyl stretching absorption bands at 1784

and 1715 cm-1 indicated that a certain amount of MAH had been grafted onto the PP

molecular chain. However, after purification by either technique the intensity of

1715 cm-1 band decreased. This established that some of the carboxylic acid was

converted back to its corresponding anhydride. As there was no difference between

the spectra obtained by the two purification techniques, it can be concluded that no

ungrafted MAH was left in the final product. Hence all of the MAH either reacted

with the PP or was removed from the product by the purification step. The amount of

MAH which had been grafted on to the PP backbone will be discussed and identified

in section 4.3.9.

Chapter 4

69

4.3.4 Investigation into the effect of initiator on final product

As summarised in Table 4.1, except for the first two experiments, two main groups

of experiments were carried out: one in the absence of DCP (initiator); and one in the

presence of DCP.

The ATR spectrum of PP grafted with MAH in the absence of DCP (Figure 4.10)

after purification, shows two peaks at 1792 and 1784 cm-1 as well as a band at 1715

cm-1. The spectra obtained from samples formed in the presence of DCP; however,

show only one band at 1784 cm-1 (Figure 4.11). Several researchers, such as Roover

et al,89 Li et al,53 Sclavons et al,57,85 Severini80 and Zhu et al78 have assigned the band

near 1790 cm-1 to a single MAH anhydride attached to the end of the PP chain.

Furthermore, it has been reported that the 1784 cm-1 band can be assigned to MAH

attached to a tertiary carbon which affects other nearby MAH groups attached to the

PP backbone.85,92-94 As a result in this study it can be concluded that in the absence

of DCP both tertiary and primary carbon radicals were produced. However, in the

presence of DCP, a majority of tertiary carbon radicals were produced and the bulk

of MAH reacted with these radicals.

Figure 4.9 ATR/FTIR spectra of PP-MAH processed at 200 °C in the absence of DCP after

purification of final product

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70

Figure 4.10 ATR/FTIR spectra of PP-MAH processed at 200 °C in the presence of DCP after

purification of final product

4.3.5 Effect of temperature on PP-MAH graft processing

The effect of temperature on the rate and mechanism of the reaction in absence of

DCP was studied. Figure 4.11 shows NIR spectra of PP-MAH mixtures after

processing at 200 ºC for different processing times. Figure 4.2, Figure 4.3 and Figure

4.11 also showed two bands at 6117 and 4966 cm-1 assigned to the MAH first

overtone of the =C-H stretch and the combination of =C-H and C=O stretching

bands, respectively. Both of these bands diminished in intensity during melt

processing indicating that MAH was grafted into the PP.

Chapter 4

71

Figure 4.11 In situ monitoring NIR spectra of PP-MAH grafting process after (a) 2, (b)30, (c) 60

and (d) 90 minutes at 200 °C. (Spectra are offset for clarity.) In order to understand the pattern of MAH consumption during processing, PCA was

performed on the NIR data. By employing PCA, the major changes in the spectra

were determined and plotted. Figure 4.12 shows the PC1 scores plot (obtained from

second derivative spectra) and the factor loadings plot for PP-MAH grafting in the

absence of DCP at 200 ºC. From the factor loadings plot it can be seen that the major

spectral changes occurred as expected at 6117 and 4966 cm-1.

Figure 4.12 Second derivative PC1 (a) scores and (b) factor loadings plots for PP and MAH grafting without DCP at 200 °C

(a) (b)

Chapter 4

72

Figure 4.13 presents scores plots of PP-MAH at three different temperatures in the

absence of DCP. These curves can be fitted to the first order rate equation of the

type: ktAey −= 4.3

where

y = the number in scores plot

A= a constant number relate to the plot

k = rate constant and

t = time.

Figure 4.13 Second derivative PC1 scores plots for PP and MAH grafting without DCP at 200,

210 and 220 °C

Since the factor loadings plot shows that the majority of changes were associated

with the reaction of MAH it can be concluded that the reaction follows first order

kinetics with respect to the MAH concentration. Details of the fitted equations are

given in Table 4.3 for both the PCA data and also for the simple area ratio data for

which the area of the MAH band was ratioed to the area of the unchanged band at

7100 cm-1.

Chapter 4

73

Table 4.3 PC1 scores and peak area ratio plot equations related to PP-g-MAH in the absence of DCP at three

different temperatures. PC1 scores plots

equations Peak area ratio plots equations

T (ºC) tey 026.033.0 −= tey 029.0062.0 −= 200 tey 030.037.0 −= tey 040.0081.0 −= 210 tey 038.032.0 −= tey 047.0065.0 −= 220

As an alternative technique for analysing the data, peak area ratios for the NIR

spectra were collected. The peak area ratios were calculated by taking the ratio of the

area of the MAH band at 6117cm-1 to the area of the band near 7100 cm-1, which

varied very little during processing. Figure 4.14 shows the subtraction spectrum

obtained by subtracting the spectra taken at 90 minutes from that taken at 2 minutes.

It is clear that the band near 7100 cm-1 was hardly affected by the processing and

hence makes a suitable basis of measure for the change in MAH bands areas.

Figure 4.14 Spectral subtract of PP-MAH after 2 and 90 minutes processing at 200 °C

Peak area ratio data for NIR data collected in PP and MAH grafting in the absence of

DCP at 200, 210 and 220 °C are shown in Figure 4.15. All three plots can be fitted to

first order reaction equations (see Table 4.3). As expected, the rate constant

increased with temperature.

Chapter 4

74

Figure 4.15 Peak area ratio measurements for PP-MAH at 200, 210 and 220 °C

Peak area ratios and PCA give approximately similar results; however, one would

expect the PCA results to be more accurate since this method utilised the whole

spectrum whereas peak area ratio only used a small amount of data around specific

peaks. The PCA method therefore would be less affected by spectral noise or

artefacts such as baseline variation.

4.3.6 Effect of DCP on PP-MAH graft processing

The experiments carried out in the presence of DCP can be divided into two groups.

In the first group, concentrations of DCP or MAH were constant and the only

variable was temperature: 200, 210 and 220 ºC. In the second group, temperature

was constant and the concentrations of reactants were varied.

To determine if there was any difference between the NIR spectra of graft processing

in the absence or presence of DCP, the NIR spectra collected during processing were

compared to those collected before processing. Figure 4.16 shows NIR spectra of

PP-MAH and PP-MAH-DCP at 200 ºC. As noted before there are two bands in these

spectra (6117 and 4966 cm-1) which are related to MAH. Bands related to DCP were

Chapter 4

75

not seen in the spectra, probably because of its low concentration in comparison to

PP and MAH.

Figure 4.16 Spectra of PP-MAH and PP-MAH-DCP at 200 °C. (Spectra are offset for clarity.)

Figure 4.17 presents PC1 factor loadings plots from NIR spectra of PP-MAH and

PP-MAH-DCP during 90 minutes processing in the Minilab at 200 °C. In both cases,

the only changes in the spectra occurred in the 6117 and 4966 cm-1 regions. This

provides evidence that DCP does not affect the molecular structure of PP and in both

reactions MAH had possibly been consumed.

Chapter 4

76

Figure 4.17 PC1 factor loadings plots of (a) PP-MAH and (b) PP-MAH-DCP

The NIR spectra obtained for graft processing of MAH onto the PP backbone in the

presence of DCP were investigated by both PCA and also the simple peak area ratio

method. Figure 4.18 shows the PC1 scores plots for MAH grafting onto the PP

backbone in the presence of DCP at 200, 210 and 220 ºC. As can be seen, there were

only small changes in the plots at different temperatures which suggested that the

reaction behaved similarly at different temperatures. Furthermore, the plots followed

first order reaction equations which are summarised in Table 4.4.

Table 4.4 PC1 scores and peak area ratio plot equations for PP-g-MAH in the presence of DCP at three different temperatures.

PC1 score plot equation

Peak area ratio plots equation

T (ºC)

tey 024.037.0 −= tey 025.0058.0 −= 200 tey 031.035.0 −= tey 033.0056.0 −= 210 tey 036.020.0 −= tey 040.0038.0 −= 220

Chapter 4

77

Figure 4.18 PC1 scores plots for PP-MAH-DCP graft processing at 200, 210 and 220 °C

Figure 4.19 Peak area ratio measurements for PP-MAH-DCP graft processing at 200, 210 and

220 °C

Chapter 4

78

Figure 4.19 shows the peak area ratio plots for PP-MAH-DCP at 200, 210 and 220

ºC. In the presence of DCP, first order kinetics were followed for all three

temperatures. As expected, the rate coefficient increased with temperature.

In the presence of DCP, it was anticipated that an increased processing temperature

leads to macro-radicals generated not only by DCP but also via mechanical shear

effects and thermal processing leading to a higher overall concentration of macro-

radicals compared with the processing in absence of DCP. Because of the higher

concentration of microradicals in the presence of DCP, it was expected that the

MAH would be consumed more quickly in comparision with graft processing in the

absence of DCP and the 6117 cm-1 band would decrease at a faster rate. However,

the rate equations for data obtained in the presence of DCP shows that the rate

constants for all temperatures were approximately similar compared with the rate

constants obtained in the absence of DCP at related temperatures (see Table 4.3 and

Table 4.4). Hence, in the presence of DCP the rate of consumption of MAH did not

change.

The similarity of the equations within both tables shows that the presence of DCP

probably does not change the rate of the reaction. However, it was possible that

there were different mechanisms for grafting MAH in absence and presence of DCP

due to the different peaks observed in ATR/FTIR spectra (see Figure 4.10 and

Figure 4.11).

4.3.7 Investigation into the effect of MAH and DCP concentration on

graft processing

The second group of experiments (experiments 9 and 10 in Table 4.1) was carried

out to investigate the effect of reactant concentration. All experiments in this section

were carried out at 200 ºC, while the concentrations of MAH and DCP were altered.

The NIR spectra obtained were investigated by both PCA and simple peak area

ratios.

Figure 4.20 and Figure 4.21 present the PC1 score and peak area ratio plots of PP-

MAH (3.5 wt%)-DCP (0.5 wt%) and PP-MAH (7 wt%) - DCP (0.25 wt%) and PP-

MAH (7 wt%) - DCP (0.5 wt%) at 200 ºC. All plots follow a first order reaction

Chapter 4

79

equation; however, there were some obvious differences between them. The first

order equations for these experiments are summarised in Table 4.5. The results

obtained for these three experiments show that the rate coefficients from the PC1

analysis were approximately similar, showing that decreasing DCP concentration to

0.25wt% does not significantly alter the reaction rate. Furthermore, changing the

MAH concentration also did not alter the rate constant. It was concluded that the

graft processing was possibly controlled by concentration of MAH.

Figure 4.20 PC1 scores plots for PP-MAH (3.5 wt% )-DCP (0.5 wt%), PP-MAH (7 wt%)-DCP

(0.25 wt %) and PP-MAH (7 wt%)-DCP (0.5 wt%) graft processing at 200 ºC The peak area ratios for each of these concentrations were also investigated.

Comparing the kinetic equations for the peak area ratio plots (Figure 4.21) gave

similar results to the PCA analysis. These are also summarised in Table 4.5.

Table 4.5 PC1 scores and peak area ratio plot equations related to differing concentrations of MAH and DCP at 200 ºC

PC1 score plot equation

Peak area ratio plots equation

Concentration

tey 027.015.0 −= tey 03.0022.0 −= PP-MAH (3.5 wt%)-DCP(0.5 wt%)

tey 018.040.0 −= tey 024.0069.0 −= PP- MAH (7 wt%)-DCP (0.25 wt%)

tey 024.037.0 −= tey 025.0058.0 −= PP-MAH (7 wt%)-DCP (0.5 wt%)

Chapter 4

80

Comparing Table 4.3, Table 4.4, and Table 4.5 it can be seen that rate constants in

the absence and presence of DCP were approximately similar. Hence, the rate of

reaction was not affected by DCP concentration, but was controlled by the

concentration of MAH. Furthermore, it is of note that the reaction rate was not

significantly affected by the absence of DCP.

Figure 4.21 Peak area ratio measurements for PP-MAH (3.5 wt% )-DCP (0.5 wt%), PP-MAH (7

wt%)-DCP (0.25 wt %) and PP-MAH (7 wt%)-DCP (0.5 wt%) graft processing at 200 ºC

4.3.8 Viscosity

Viscosity measurements were carried out simultaneously with NIR spectral

collection. Figure 4.22 shows the viscosity measurements for PP-MAH (graft

processing in the absence of DCP) at three temperatures. Also shown for comparison

are the PP viscosity measurements at 200 ºC. In the absence of DCP, all the macro-

radicals produced are due to degradation of PP or β-scission reactions. Because of

the high temperature and entanglements between the macro-radicals, the most

probable cause of degradation is by thermal and mechanical means. At the

commencement of the experiments, the order of the viscosity was as expected, i.e.

lower viscosity at higher temperatures. Although the viscosity dropped between 2

and 4 minutes, it was fairly constant for about the first 15 minutes in each reaction.

After 15 minutes, the viscosity dropped with time in all cases, but it decreased more

Chapter 4

81

rapidly at lower temperatures. A possible explanation for this was that there is a

higher degree of entanglement between macro-radicals at higher viscosity causing

breakage of the macro-radicals. This decreased the molecular weight leading to a

more rapid drop in viscosity. When comparing 210 ºC with 220 ºC the viscosity

decrease with time for the latter was higher. This was due to higher temperatures

leading to higher thermal degradation; however, at these temperatures entanglements

between macromolecule chains are lower. From this it can be concluded that

possibly thermal degradation led to less overall degradation due to the production of

fewer macro-radicals compared with mechanical degradation.

Figure 4.22 Viscosity measurements for PP-MAH at 200, 210 and 220 °C and viscosity changes

for PP at 200 °C

Figure 4.22 clearly showed that the viscosity of PP without added MAH changed

rapidly from the beginning of processing, while the viscosity of PP-MAH showed an

initial drop over the first 4 minutes and then remained approximately steady for the

following 11 minutes. To explain this phenomenon it is proposed that at the

beginning of the experiment a small fraction of PP chains undergo β-scission

reactions due to mechanical degradation. After production of primary macro-radicals

due to β-scission reactions any MAH present reacts with the macro-radicals. There is

a possibility that one MAH reacts with 2 primary macro-radicals and because of this

the molecular weight does not change and hence changes in viscosity were not

Chapter 4

82

detected between 2 and 15 minutes. The structure of the possible product is

presented in Figure 4.23. After 15 minutes, possibly because (42% - 48%) of the

MAH was grafted, the possibility of an MAH linkage between two macro-radicals is

much lower and hence β-scission reactions lead to a reduction in molecular weight

and a decrease in viscosity was observed. This might be the reason that a band at

1784 cm-1 assigned to anhydride linked to tertiary carbon can be detected in the

ATR/FTIR spectra (Figure 4.9) of the related purified products (the mechanism of

PP-MAH graft processing in the absence of DCP is explained in section 4.3.10).

O OO

Figure 4.23 MAH linkage between two macro-radicals.

The viscosity changes of PP during processing in the presence of DCP were

completely different. Figure 4.24 shows the viscosity measurements for PP-MAH-

DCP at 200, 210 and 220 °C with the same y-axis scale as shown in Figure 4.22.

This shows that in the presence of DCP, viscosity was very low at all temperatures

throughout processing. This was due to the high concentration of macro-radicals

generated immediately by DCP which causes rapid PP degradation and reduction of

molecular weight.

Chapter 4

83

Figure 4.24 Viscosity measurements for PP-MAH-DCP at 200, 210 and 220 °C

When the concentration of DCP was reduced from 0.5% to 0.25% the viscosity was

significantly higher (Figure 4.25); remaining around 2 kPa.s for the first 30 minutes

before falling to the same level as 0.5% DCP after about 75 minutes. Nevertheless,

the presence of even low levels of DCP (0.25%) led to a much lower viscosity than

when DCP was absent. The drop in viscosity was explained by DCP radicals causing

PP degradation by chain scission which lowered the molecular weight and hence the

viscosity (see section 4.3.10 for the reaction mechanism). Chain scission also

explained why the NIR spectra were essentially unchanged during the reaction, as

scissioning of the molecule has almost no effect on the overall molecular structure.

As mentioned previously, presence of DCP increased the chain scission possibility

(section 4.3.2), as a result it can be said that the degradation of PP is more probable.

Chapter 4

84

Figure 4.25 Viscosity changes for PP-MAH (3.5% of PP)-DCP, PP-MAH-DCP (0.25% of PP)

and PP-MAH-DCP graft processing at 200 °C

4.3.9 TGA investigation of products

An important parameter for MAH grafting is the quantity of MAH which was

grafted onto the PP after processing. For this reason, TGA was used to determine the

amount of MAH un-grafted remaining after 6 and 90 minutes processing.

The TGA thermogram for the PP-MAH product after 6 minutes at 200 °C is

presented in Figure 4.26. Two decomposition steps were observed: the first which

occurred between 116 °C and 163°C was related to decomposition of un-reacted

MAH; and the second was related to PP grafted with MAH. The first step was

calculated to be a mass loss of 5.78%. As MAH represented only 7% of total

ingredients before processing, the grafting degree (Gd) was calculated as follows:

%171007

78.57=×

−=dG 4.4

Hence, 17% of MAH or 0.12 g was grafted onto PP (3.3 - 3.6 g) after 6 minutes.

Chapter 4

85

Figure 4.26 TGA thermogram of PP grafted with MAH in the absence of DCP at 200 °C after

(a) 6 and (b) 90 minutes processing. Also shown is (c) pure PP (d) and MAH.

The TGA of PP-MAH at 200 °C after 90 minutes processing showed a one step

decomposition related to PP-MAH (also shown in Figure 4.26). As a result, it was

assumed that all of the MAH was grafted onto the PP backbone.

If it is assumed that there was a linear relationship for the consumption of MAH

during processing, then after 15 minutes approximately 64 wt% of MAH would have

been grafted onto the PP. The peak area ratio graphs of PP grafted using MAH in

absence and presence of DCP (Figure 4.15 and Figure 4.19) showed that between

42% and 48% of MAH was grafted to PP. As these results do not correlate, it was

thought that the consumption of MAH was not linear.

The TGA results for PP grafted with MAH in the presence of DCP have

approximately the same degree of grafting in comparison with processing in the

absence of DCP. Hence, The TGA results showed that DCP did not have a

significant effect on the amount of MAH grafted onto the PP backbone, as the same

degree of grafting occurred when DCP was present compared to when it was absent.

Chapter 4

86

Also shown in Figure 4.26 is the TGA thermogram for pure MAH. This was

interesting in that it showed that pure MAH volatilised at a low temperature (56 ºC);

however, this was not evident in the TGA thermogram for PP-MAH. The reason for

this was thought to be because a required higher temperature is required for MAH to

evaporate from a solid polymer.

4.3.10 Mechanism of the grafting reaction

Although functionalisation of PP by MAH is a well-known process, its mechanism is

not entirely clear.13,28,29,31,35,53,57,58,71,72,77,78,80,85,88,89,91-95 Figure 4.27 shows the

proposed mechanism for MAH grafted onto PP.

The PP-MAH grafting process is a free radical mechanism. The half-life of the DCP

initiator at 200 °C is approximately 0.5 min and upon decomposition creates two

primary radicals.28,29,31,72,77,78,85 After this, two reaction paths could occur. In one,

some of the initiator free radicals (R-O• or CH•3)13 could react with MAH monomer,

as shown in reaction Figure 4.27, reaction (A). This results in some saturated

peroxide MAH radical species (Figure 4.28).

Chapter 4

87

R O

O OO

OO O

RO C

OO O

RO

C C

O OO

C

O OO

O OO

O OOC

O OO

O OO

O OO

C

O OO

C

O OO

O OO

O OO

O OO

n

O OO

O OO

O OO

n

C

A

B Disproportionation

C D

ERecombination

(2)

F

(3)

G

(4)

I

P

(7)M

B-scission

+

(5)

(6)

K

B-scission

OR(8)

(6)

L

(9)

+

+

H

Disproportionation

Disproportionation

Disproportionation

N

(10)

Figure 4.27 Proposed mechanisms for MAH grafted onto PP backbone13,29,71,72,80,89

OO O

RO C

Figure 4.28 Peroxide MAH radical species

Chapter 4

88

If this radical reacts with another MAH monomer, poly-MAH will be produced. Shi

et al58 reported that this reaction is favoured when the concentration of MAH is

lower than the concentration of DCP. In the current study the concentration of MAH

was much higher than DCP and as a result, there is little likelihood of poly-MAH

formation. Furthermore, because the temperature used for the PP graft processing

was higher than the ceiling temperature of MAH,17,35,58,85,92,95 homo-polymerisation

of MAH was not possible.

In the second reaction path (Figure 4.29), the radical initiator abstracts hydrogen

atoms from the PP and forms a PP macro-radical. Such macro-radicals can either be

primary, secondary or tertiary. The concentration of tertiary macro-radicals is 50

times higher than secondary and primary carbon-centred radicals and the

concentration of secondary macro-radicals is 10 times higher than primary macro-

radicals.13,29,71,72,80,89

C

CC

(a) (b) (c)

Figure 4.29 Produced PP macroradical, (a) tertiary, (b) secondary and (c) primary carbon

radical13

According to Shi et al77 this mechanism can follow two proposed path ways. The

macro-radicals undergo β-scission reactions and form two shorter segments (reaction

H in Figure 4.27). In general, the PP molecular weight will drop if the β-scission

reaction occurs and a corresponding drop in viscosity will occur. Alternatively,

macro-radicals can react with MAH and create PP-g-MAH.

In the present study, no change in viscosity occurred during 15 minutes graft

processing in the absence or presence of low concentrations of DCP. From these

results and from other researchers’ investigations28,29,31,35,71,85,92,93 it was expected

Chapter 4

89

that reactions between macro-radicals and MAH monomers were more probable and

hence under these conditions the grafting reaction was favoured over the β-scission

reaction. Therefore, initially PP macro-radicals reacted with MAH and the monomer

attached and distributed along the PP backbone. Then two possible reactions might

occur. One of these is that β-scission reactions occurred along the PP macro-radical

grafted with MAH (reaction L in Figure 4.27). The second possibility is that after

approximatly 15 minutes of processing, because the concentration of MAH dropped,

the probability for the reaction between one MAH and two macro-radicals decreased.

As a result, the concentration of macro-radicals with lower molecular weight

increased. This was concluded from the data shown in Figure 4.22 and Figure 4.25

where a further decrease in viscosity occurred after 15 minutes.

Due to the higher concentration of tertiary carbon-centred radicals compared with

secondary or primary radicals and according to results obtained in this study and

those reported by other investigators,35,53,58,71,91,92 most of the MAH monomers were

attached to the tertiary carbon-centred radicals along the PP backbone.

In addition to the viscosity measurements, further evidence to support the hypothesis

of initial MAH grafting followed by an increase in β-scission reactions after about 15

min was seen in the ATR/FTIR data where a peak was observed at 1792 cm-1. This

band has been assigned to carbonyl groups from five-membered cyclic anhydride

groups.29,92 According to Zhang et al,35 the concentration of MAH grafted to radical

chains produced by β-scission reactions is very low. This report also discussed two

possibilities for segments produced by β-scission reactions of MAH grafted onto PP

(Figure 4.27, reaction L). As stated earlier, because the temperature of PP graft

processing was higher than the ceiling temperature of MAH, homo-polymerisation

of MAH was not possible. As a result, there was no possibility for reactions M and N

(Figure 4.27) to occur, hence products 9 and 10 were not formed. As a result the

observed band at 1784 cm-1 in ATR/FTIR spectra was not related to poly-MAH.

This band in fact arose from MAH species attached along the PP backbone. Because

these species are closely linked to PP and the distance between them is very short,

there are some interactions between the grafted MAH groups. As a result the 1792

cm-1 band is more likely to arise from the carbonyl group of the MAH shifted

slightly because of interactions.85 Therefore, analysis of the results obtained in this

Chapter 4

90

study indicated that MAH-grafted PP was an in-chain graft process and was not

related to homo-polymerisation of MAH.

4.3.11 Kinetics of the MAH-grafted PP process

The functionalisation reaction of PP by MAH is a free radical reaction. In the

presence of DCP the reaction is initiated by the thermal decomposition of initiator

DCP as shown in Equation 4.5.

ROOR 2 ROT kd 4.5

Alkoxyl radicals then attack the PP resulting in hydrogen abstraction from tertiary,

secondary and primary carbon atoms. As discussed in section 4.3.10 the

concentration of tertiary carbon macro-radicals is higher than the secondary and

primary carbon macro-radicals.

CRO + PP

P tr

•

ktr 4.6

RO + PP

C

P sd

•

ksd 4.7

RO + PP

C

P pr

•

kpr 4.8

As discussed previously there was a possibility that some of the tertiary carbon

radicals underwent a β-scission reaction and resulted in a lowered viscosity product.

A possible β-scission reaction and its products are presented in Equation 4.9.

C C +

P tr

• P sd•

kβs

4.9

It was thought that competition between secondary carbon radical grafting and/or

end-chain grafting and tertiary carbon radical grafting had occurred. After

examination of the spectra, it was proposed that tertiary carbon radical grafting was

Chapter 4

91

favoured as no peak at 1792 cm-1 was detected during graft processing in the

presence of DCP. Therefore, Equation 4.9 can be ignored.

In the next step, the produced tertiary and secondary carbon macro-radicals will react

with MAH as shown in Equations 4.10 to 4.12.

CO OO

+C

C

O OO

P tr

• M PM •

kgtr 4.10

C O OO+

C

O OO

P sd

• M PM •

kgsd 4.11

C +

O OO C

O OO

P sd

• M PM •

kgβs 4.12

As mentioned previously because in the ATR/FTIR spectra of the product produced

in presence of DCP showed no peak at 1792 cm-1, it was likely that reaction 4.12 did

not occur or only occurred at a very low concentration. As a result this equation can

be ignored.

During the graft processing, because of the high temperature and shear rate, thermal

and mechanical degradations can occur which manifest as β-scission reactions. The

possible β-scission reaction and possible degradation products are presented in

Equations 4.13 and 4.14:

C

O OO O OO

+

C

PM • PM

kS1 4.13

C

O OO

C

O OO

+

PM • PM •

kS2 4.14

Chapter 4

92

Another possible reaction is cross-linking between 2 macro-radicals (Equation 4.15):

C

C

C

CCombination

kc 4.15

The last part of the mechanism is the termination reactions which are presented in Equations 4.16 and 4.17:

CC

O OO

+ P tr

• PM •

O OO

+

P PM

kt1 4.16

C

O OO

+

C

O OO

PM • PM •

O OO

+

O OO

PM PM

kt2 4.17

Chapter 4

93

Rate equations for each step in the mechanism are summarised as follows:

At this stage the rate equation for each species can be written as follows: d RO

dtk ROOR k RO PP k RO PP

d ROdt

k ROOR RO PP k k

d tr sd

d tr sd

[ ] [ ] [ ][ ] [ ][ ]

[ ] [ ] [ ][ ]( )

•• •

••

= − −

⇒ = − +

2

2

4.30

[ ]d Pdt

k RO PP k P k P MAH k P PMtrtr s tr gtr tr t tr

[ ] [ ][ ] [ ][ ] [ ][ ]•

• • • • •= − − −β 1

4.31

d Pdt

k RO PP k P MAH k Psdsd gsd sd c sd

[ ] [ ][ ] [ ][ ] [ ]•

• • •= − − 2

4.32

d PMdt

k P MAH k P MAH k PM k PM

k P PM k PM

gtr tr gsd sd S S

t tr t

[ ] [ ][ ] [ ][ ] [ ] [ ]

[ ][ ] [ ]

•• • • •

• • •

= + − −

− −

1 2

1 22

4.33

Because the concentration of secondary macro-radical was 10 times less than the

concentration of tertiary macro-radical, [ ]dtPd sd

•

and Equations 4.20, 4.24 and 4.27 can

be ignored.

As a result: [ ] [ ]

dtPd

dtPd tr

••

=

4.34

[ ]ROORka d21 = 4.18

a k PP ROtr2 =•[ ][ ] 4.19

a k PP ROsd3 =•[ ][ ] 4.20

a k PP ROpr4 =•[ ][ ] 4.21Because of low concentration of the primary

carbon radical, this equation can be ignored. [ ]•= trs Pka β5 4.22

a k M Pgtr tr6 =•[ ][ ] 4.23

a k M Pgsd sd7 =•[ ][ ] 4.24

a k PMS8 1= •[ ] 4.25

a k PMS9 2= •[ ] 4.26

a k Pc sd102= [ ] 4.27

[ ][ ]•= PMPka trt111 4.28

[ ]2212•= PMka t

4.29

Chapter 4

94

If for various radical species one presumes the classical steady state hypothesis, it

can be said that:

[ ] [ ] [ ] 0===

•••

dtPMd

dtPd

dtROd

4.35

Therefore:

[ ] [ ][ ]PPkROORkRO

r

d2=•

4.36

In addition

d Pdt

k RO PP k P MAH k P PMtrtr gtr tr t tr

[ ][ ][ ] [ ][ ] [ ][ ]

•• • • •= − − 1

4.37

Based on Equations 4.34 and 4.35, for simplification of equation 4.37 it can be said

that:

[ ] [ ][ ] [ ] [ ][ ] [ ][ ] 01 =−−−= ••••••

PMPkMAHPkPkPPROkdtPd

tgsr β 4.38

As a result of the equations 4.34 and 4.36:

[ ] [ ][ ] [ ] sgt

d

kMAHkPMkROORk

Pβ++

= ••

1

2

4.39

The concentration of PM• can be calculated by using equations 4.33and 4.35:

d PMdt

k P MAH k P MAH k PM k PM

k P PM k PM

gtr tr gsd sd S S

t tr t

[ ] [ ][ ] [ ][ ] [ ] [ ]

[ ][ ] [ ]

•• • • •

• • •

= + − −

− − =

1 2

2 32 0

4.40

For simplification of the equation 4.33, it was assumed that

sss kkk =+ 21 4.41

And

[ ] [ ]PPtr =• 4.42

Then after the simplification of the equation 4.40 it can be said that:

[ ] [ ] [ ]( ) [ ]2

22

12

2

4

t

gttsts

k

MAHkkPkkPkkPM

−++−−=

•••

4.43

In the next step, it was interesting to know what is the concentration of PP grafted by

MAH, PM. The rate equation of PM production is summarised as follows:

Chapter 4

95

[ ] [ ][ ] [ ]221••• += PMkPMPk

dtPMd

tt 4.44

Equation 4.44 is used to determine the concentration of PM. For this it was

necessary to calculate [P•] and [PM•] from Equations 4.39 and 4.43. According to the

results obtained from PCA and peak area ratio analyses graft processing of MAH

onto PP backbone in the presence of DCP possibly followed first order reaction

kinetics or kinetics similar to first order. It was thought that the concentration of

DCP did not have an effect on the rate of the reaction. To prove this, Equations 4.39

and 4.43 needed to be solved and therefore simplified. However, there are some

unknown parameters which need to be determined and for that, it is necessary to

perform additional studies. These will be discussed further in the future work section

of this document.

The kinetic equations for graft processing of PP using MAH without an initiator

have not been studied by other researchers and it was the first time that the kinetics

of this reaction had been investigated.

As discussed previously, the functionalisation reaction of PP by MAH is a free

radical reaction. In the absence of DCP the reaction is initiated by chain-scission and

possibly macro-radicals. Because of the high temperature and shear rate, thermal and

mechanical degradation can occur which manifest as β-scission reactions. Hence

primary macro-radicals can be produced. The possible macro-radical production

reaction due to β-scission is shown in Equation 4.45.

C

+

PP P•

k´S

4.45

Next, the produced primary carbon macro-radicals will react with MAH as shown in

Equation 4.46:

Chapter 4

96

C

+

O OO C

O OO

P• MAH PM •

k´gp 4.46

As discussed in section 4.3.7, during graft processing of PP with MAH in absence of

DCP, the viscosity of the PP did not change for the first 15 minutes of processing.

This was possibly due to one MAH reacting with two primary macro-radicals and

because of this the molecular weight did not change and hence changes in viscosity

were not detected between 2 and 15 minutes. The possible reaction is shown in

Equation 4.47:

C

O OO

+

C

O OO

PM • P• PM

keg

4.47

The other possible termination reactions are as follows:

C C

+

P• P• PP

k´c 4.48

C

O OO

+

C

O OO

+

PM • P• PM P

k´t

4.49

Rate equations for each step in the mechanism are summarised as follows:

At this stage, the rate equation for each species can be written as follows:

[ ]′ = ′a k Ps1 4.50

[ ][ ]′ = ′ •a k P MAHgp2 4.51

[ ][ ]′ = ′ • •a k P PMgt3 4.52

[ ]′ = ′ •a k Pc4

2

4.53

[ ][ ]′ = ′ • •a k P M Pt5 4.54

Chapter 4

97

[ ] [ ] [ ][ ] [ ][ ] [ ] [ ][ ]•••••••

′−′−′−′−′= PPMkPkPMPkMAHPkPkdtPd

tcgtgps2

4.55

[ ][ ] [ ][ ] [ ][ ]d PMdt

k P MAH k P PM k PM Pgp gt t[ ]• • • • • •= ′ − ′ − ′

4.56

[ ] [ ][ ]MAHPkdt

MAHdgp

•′−= 4.57

If for various radical species the classical steady state hypothesis is assumed, it can

be said that:

[ ] [ ] [ ] 0===••

dtPMd

dtPMd

dtPd

4.58

Based on Equation 4.58 it can be said that:

[ ] [ ] [ ][ ] [ ][ ] [ ] [ ][ ]

[ ] [ ] [ ] [ ]( ) [ ] [ ] [ ]( ) [ ]c

sctgtgptgtgp

tcgtgps

kPkkPMkPMkMAHkPMkPMkMAHk

P

PPMkPkPMPkMAHPkPkdtPd

′

′′−′+′+′+′+′+′=

=′−′−′−′−′=

•••••

•••••••

2

4

0

2

2

4.59

In addition [PM•] can be calculated from the following equation:

[ ] [ ]tgt

gp

kkMAHk

PM′+′

′=•

4.60

Also, [PM] can be obtained as Equation 4.61:

[ ] [ ][ ]( )tgt kkPPMPM ′+′= •• 4.61

This is the first time that the kinetics of the modification of PP by MAH in absence

of DCP has been suggested. Based on the equations 4.59 and 4.60 the concentration

of [PM•] produced only depends on the [MAH] and the rate of the graft modification

is dependant on the [MAH]. Because the data in Tables 4.4 and 4.5 showed that the

rate of PP-MAH grafting in the absence and presence of DCP had similar rate

coefficients and that the mechanism of graft processing was controlled by MAH

concentration, the rate of consumption of MAH in absence and presence of DCP

should be similar.

Chapter 4

98

4.3.12 Activation energy of MAH graft modification of PP

The activation energy for MAH graft modification of PP with and without initiator

was also investigated. Based on the rate constant coefficient obtained for the three

temperatures examined (200; 210; and 220 ºC) and by using an Arrhenius plot the

activation energies for PP graft processing with and without DCP were calculated

and are shown in Figure 4.30 and Figure 4.31.

These plots also show the linear fitting of these results. The linear regression value

(r2) determined using the peak area ratio method was calculated to be 0.98 and 0.85

for results with and without DCP, respectively. The r2 values for the line of best fits

of the activation energies calculated based on the PCA analyses with and without

DCP were 0.80 and 0.84, respectively. The activation energies for each measurement

are summarised in Table 4.6 .

Figure 4.30 Arrhenius plots for the apparent rate constant (k) where T is absolute temperature determined according to peak area ratio equations for PP-MAH in presence and absence DCP.

Chapter 4

99

Figure 4.31 Arrhenius plots for the apparent rate constant (k) where T is absolute temperature

determined according to PC1 score equations for PP-MAH in presence and absence DCP. From Table 4.6, the activation energy obtained from the PC1 and peak area ratio

equations were approximately similar. However, because chemometric techniques,

especially PCA, are more accurate than peak area ratio estimations, the calculated

activation energies based on PC1 equations are more acceptable.

Table 4.6 Calculated activation energy based on Arrhenius plots for each sets of equations

Ea (kJ/mol.°K)

PC1 equations for PP-MAH without DCP

Peak area ratio equations for PP-MAH without DCP

PC1 equations for PP-MAH with DCP

Peak area ratio equations for PP-MAH with DCP

39.5 ± 1.25 48.9 ± 0.94 44.7 ± 0.88 49.4 ± 0.65 Since the activation energies obtained for reactions with and without DCP were

approximately similar it suggested that essentially the same mechanism applied in

both cases.

4.3.13 Conclusions

The graft modification of PP with MAH was carried out via laboratory scale reactive

extrusion. The process was monitored in situ by NIR spectroscopy utilising a fibre

Chapter 4

100

optic link and possible changes in molecular structure were investigated. Changes in

the polymer viscosity were measured simultaneously during the graft processing.

The graft modification of PP by MAH was carried out at three different temperatures

and different concentrations of MAH and DCP, including experiments where DCP

was not added. ATR/FTIR studies showed that during graft processing without

initiator two bands were detected at 1792 and 1784 cm-1; however, only the peak at

1784 cm-1 was observed in the presence of DCP. The band detected at 1792 cm-1 was

related to a single MAH attached to the end of the PP chain and that one at 1784 cm-1

assigned to MAH attached to a tertiary carbon which was influenced by other nearby

MAH groups attached to the PP backbone. This demonstrated that in the presence of

DCP most macro-radicals produced were tertiary carbon radicals and in the absence

of DCP both tertiary and primary macro-radicals were produced. In these

experiments increasing temperature resulted in the rate of concomption of MAH

during the graft processing also increasing, as expected.

PCA and peak area analysis of the NIR spectra obtained during processing showed

similar results regardless of the whether initiator was included in the reaction

mixture. The similarity between these results demonstrated that the presence of DCP

does not change the mechanism of the reaction. Additionally, it was determined that

the reaction rate was not affected by DCP concentration, but was controlled by the

concentration of MAH.

Viscosity measurements showed that in the absence of DCP all the macro-radicals

produced were due to PP degradation or β-scission reactions. Because of the high

temperature used and shear forces between the macro-radicals, the most probable

mechanisms of degradation were thermal and mechanical. Due to the higher overall

viscosities measured in the absence of DCP compared to when DCP was used, it was

concluded that a high concentration of macro-radicals were immediately generated

by DCP which caused rapid PP degradation and reduction of molecular weight.

Although it was found that the presence of DCP increased the concentration of

macro-radicals, it did not have an affect on the grafting rate.

Chapter 5

101

Chapter 5 Investigation of PP degradation using a novel nitroxide probe during melt processing in reactive extrusion: A

laboratory scale study

5.1 Introduction

It is well established that photo- and thermo-oxidation are common phenomena in

polymeric materials. One of the most effective ways for minimising polymer

degradation is to incorporate additives such as stabilisers and antioxidants into the

polymer. There are many varieties of stabilisers, some of which are only used for one

type of polymer.205 In this respect, Malík et al206 and Kaci et al207 demonstrated that

an appropriate polymer stabiliser must have minimal diffusion, high solubility and

compatibility in the polymer matrix. In addition, an effective stabiliser requires a

high degree of distribution within the polymer matrix.

Polymer degradation, particularly in polyolefin systems, has been shown to be

mediated by free radicals. Studies have indicated that reducing the number of free

radicals is the most successful approach for increasing the lifetime of the polymer.

This can be facilitated by using antioxidants and stabilisers which extend the lifetime

of the polymer by decomposing the hydroperoxide and scavenging free radicals

attached to macromolecules. According to the literature,205-211 hindered amine

stabilisers (HAS) are one of the most effective stabilisers developed for protecting

polymers from degradation. This type of stabiliser was originally developed as an

additive to limit photo oxidation and as a result is also known as a hindered amine

light stabiliser. This type of stabiliser is particularly effective for polyolefins.205-211

Due to the protection mechanism of HASs, they are not only used as light stabilisers,

but they can also be applied as thermal stabilisers. These compounds are often used

as inhibitors for the control of photo-oxidative and thermo-oxidative degradation of

polymers. Because of their multifunctional mechanism they are more effective than

other varieties of stabilisers.207

The main stabilisation mechanism of HAS involves interaction with the peroxide

molecule or macro-radicals produced during degradation. The HAS amine group

Chapter 5

102

decomposes hydroperoxide bonds, to form a complex with hydroperoxide, oxygen or

transition metal ions. The hindered amine then converts to the corresponding

nitroxide. The nitroxide is able to scavenge free radicals to form the alkoxyl amine

and so stabilise the polymer. Moreover, HASs diminish the photo-reactivity of α-β

unsaturated carbonyl groups in polyolefins.207,212

Physical properties of HASs such as thermal stability, compatibility with the

polymer, mobility and solvent extraction resistance affect their stabilisation

properties.209,211 The molecular weight of the HAS can also have an effect on these

properties. A HAS with a very low molecular weight can be extremely volatile and

sublime from the polymer. Additionally, they have a very low resistance to solvent

extraction and, as they are bases, are ineffective in acidic environments. As a result,

HASs with low molecular weights are not appropriate for high temperature

processing.205,208,213

There have been many studies on the identification and monitoring of polymer

degradation, but a particular challenge is measuring free radical reactions in the

earliest stages of polymer degradation. One group of compounds that have been

applied to this issue is nitroxides and as noted above, these are formed in the

stabilising reactions of HASs. In the early stages of degradation, the polymer alkyl

radical, (R•) reacts with oxygen to form a peroxyl radical (RO2•), the main chain

carrier in oxidation. If the alkyl radical can be trapped by a nitroxide, this can

prevent peroxyl radical formation, the polymer is stabilised. Within the large

nitroxide group, isoindoline nitroxides such as 1,1,3,3-

tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO),191 1,1,3,3-

tetramethylisoindolin-2-yloxyl (TMIO),214 1,1,3,3-tetramethyl-1,3-dihydroisoindol-

2-yloxyl and 1,1,3,3-tetramethylisoindolin-2-yloxyl215 have been preferred. The

advantage of the isoindoline ring system in comparison with other systems is in their

oxidative stability due to the fused aromatic moiety.214

One type of isoindoline nitroxide is the hindered-fluorophore nitroxide which has

profluorescent characteristics. This means that in the nitroxide radical form there is

no fluorescence. However, when the radical is removed (by radical trapping or

reduction) fluorescence is immediately observed. Because of this, nitroxides can not

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only can be used as stabilisers, but they can also be used as probes for determining

degradation onset, as well as, carbon-centred radical or macro-radical concentrations

produced during degradation.191,214

Moad et al216 have demonstrated that the concentration of the trapped carbon-centred

radicals by a fluorophore nitroxide is directly related to the intensity of the measured

emission. As a result, by monitoring the fluorescence it is possible to measure the

concentration of free radicals caused by degradation.

This chapter investigates the use of TMDBIO as a probe for the identification of PP

degradation during melt processing. TMDBIO is a profluorescent nitroxide, which

contains a phenanthrene fluorophore incorporated within the nitroxide ring system

(the structure of which is shown in Figure 5.1). Compared with the other types of

fluorophore nitroxides, TMDBIO has advantages in its chemical and physical

properties. For example, unlike other nitroxides it has no labile linkages and does not

suffer from hydrolysis. By using this nitroxide any carbon-centred free radicals

formed on the polymer chain of PP can be detected at the primary stages of

degradation.

N O R N OR

Figure 5.1 Carbon-centred free-radical scavenging by TMDBIO191

5.1.1 Degradation of PP

As discussed in section 1.4, PP is the most commonly used commercial polymer. In

spite of several advantages of using PP industrially, it easily undergoes photo-

oxidation and mechanical and thermal degradation. In this section, PP degradation

during melt processing was investigated. As well as using a profluorescent nitroxide,

IR and Raman spectroscopy were also used to monitor any PP degradation.

TMDBIO Profluorescent

TMDBIO-R Fluorescent

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IR140,147,217-222 and Raman223-225 spectroscopy are the most widely used techniques in

the recognition of the degradation of PP and are potent techniques for kinetics

studies. According to Andreassen,226 the degradation mechanism is affected by the

cause of degradation and the PP type and morphology. Although there are different

types of mechanisms, similar chemical species are produced: usually hydroxyl (O-

H), carbonyl (C=O) and unsaturated (C=C) groups.

The common degradation products which contain hydroxyl and carbonyl groups are

alcohols, carboxylic acids, ketones and esters. Hydroxyl groups usually appear as a

broad band around 3400 cm-1, while carbonyl groups commonly occur at

approximately 1720 cm-1. Unsaturated groups, on the other hand, are produced in the

absence of oxygen. Raman spectroscopy is sensitive to unsaturated groups. Two

peaks arising from these groups occur: one around 889 cm-1 and the other at 1648

cm-1. As a result, IR and Raman spectroscopy are appropriate and powerful

techniques for the identification and characterisation of PP degradation and species

produced from this process.

The aim of this work was the investigation of PP degradation at an early stage of

processing by using a profluoresent nitroxide, TMDBIO. The extent of degradation

was investigated by measuring the intensity of the nitroxide fluorescence which

correlates with the alkyl radical concentration formed on processing. Viscosity

changes were also measured simultaneously. Modifications to industrial experiments

will be able to be completed to reduce degradation during melt processing or reactive

extrusion, based on these results.

5.2 Experimental

5.2.1 Sample preparation

Because the nitroxide was used as probe, it was thought that it would be best to

introduce this in a homogeneous manner. Therefore, it was decided that the PP

should be pre-coated with nitroxide. Two types of samples were prepared: PP coated

with nitroxide of 0.05% (w/w); and 0.1% (w/w). Although the maximum Minilab

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capacity was 6 grams, the optimum volume for material mixing was between 4.5 and

5.0 grams. In this investigation, it was determined that approximately 4.5 g of PP

was optimum, so either 2.25 mg or 4.5 mg of TMDBIO was added. The process was

performed by first dissolving the TMDBIO in 20 mL of diethyl ether. After the

nitroxide was completely dissolved, the appropriate amount of PP powder was added

and stirred for 5 minutes. The solvent was then removed under vacuum using a

rotary evaporator. This resulted in nitroxide coated PP which could be introduced

into the Minilab.

5.2.2 Minilab extruder conditions

Two kinds of experiments were performed to establish degradation during melt

processing. In the first kind the material, either pure PP or nitroxide coated PP, was

circulated for about 76 minutes in the Minilab at a temperature of either 190, 200,

210, 230, or 260 °C. The pressure difference was measured continuously during the

experiment and an approximate viscosity was calculated automatically by the

Minilab. In the second kind of experiment the material was circulated in the Minilab

in the same way, but small samples (approx. 50 mg) were removed via the die every

4 minutes and studied further. Both experiments could not be performed

simultaneously because removal of material affects the viscosity measurements.

As the samples were rod-shaped, three cross-sections approximately 60 μm thick

were obtained using a microtome. All the samples were stored at 4 °C to reduce the

probability of further degradation. The degree of sample degradation was examined

using micro-ATR/FTIR, Raman and fluorescence spectroscopy as off-line

monitoring techniques. Viscosity measurements were applied as an on-line

monitoring technique.

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5.3 Results and discussion

5.3.1 Polypropylene

5.3.1.1 Raman and ATR/FTIR spectroscopy

The first section of the spectroscopic studies focused on FTIR and Raman

spectroscopy. Recognition of PP degradation via IR spectroscopy was carried out by

observing the hydroxyl and carbonyl group peaks. These bands appeared around

3400 cm-1 and 1720 cm-1, respectively.

Investigation of the blank samples (pure PP) by ATR/FTIR and Raman spectrometry

over 76 minutes of processing showed no observable degradation. Figure 5.2 and

Figure 5.3 present ATR/FTIR and Raman spectra of PP at the beginning and the end

of the experiments at 260 ºC. Examining Figure 5.2 showed there was no difference

between the spectra collected after 4 minutes compared with those obtained after 76

minutes of processing. Most importantly, no peaks corresponding to hydroxyl or

carbonyl groups were observed. There was no difference between the 4 minute

sample Raman spectra and the 76 minutes sample spectra, from Figure 5.3,

particularly no bands related to unsaturated groups were detected.

Figure 5.2 PP ATR/FT-IR spectra at (a) 4 minutes and (b) 76 minutes after processing at 260

ºC. (Spectra have been offset for clarity.)

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Figure 5.3 PP Raman spectra at (a) 4 minutes and (b) 76 minutes of processing at 260 ºC. (Spectra have been offset for clarity.)

5.3.2 Fluorescence spectrometry

As TMDBIO was used to identify degradation through fluorescence spectroscopy,

pure PP samples were examined to determine their extent of fluorescence under

similar conditions. These samples were excited at 294 nm, which is the excitation

wavelength of the alkoxyl amine from TMDBIO; however, the pure PP did not show

any fluorescence bands in the region of interest. Fluorescence emission of PP after

20 minutes processing at 200 ºC is presented in Figure 5.4. Clearly, there was no

band between 310 nm and 450 nm which could interfere with the observation of

peaks related to the alkoxyl amine from TMDBIO (Figure 5.6).

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Figure 5.4 Fluorescence emission of PP at 200 ºC after 20 minutes

5.3.2.1 Viscosity investigation

The change in viscosity over the time of the experiment was measured twice for each

temperature and the graph of the averaged results is presented in Figure 5.5.

Figure 5.5 PP viscosity changes during the processing at 190, 200, 210 and 260 ºC

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As mentioned previously, temperature and viscosity have a negative correlation. As

expected, by increasing the temperature, the initial PP viscosity (4 minutes after

introduction of PP to the Minilab) was lowered.

These results also show that the viscosity decreased throughout PP processing, for all

temperatures examined, but decreased more steeply for the lower temperatures of

190 and 200 ºC. This decrease resulted from the thermal and mechanical degradation

of PP and, as discussed below, suggested mechano-scission was dominant. During

degradation of this type, macro-molecular chain breakage occurs, reducing the PP’s

molecular weight. Because viscosity and molecular weight have a direct correlation,

decreasing a macro-molecule’s molecular weight results in lowered viscosity. These

results showed that viscosity was a sensitive measurement tool to identify this type

of degradation while IR and Raman were not sensitive to chain scission and were

most effective in detecting oxidation-based degradation.

The viscosity of PP at 190 ºC was initially much higher than the other temperatures

examined; however, the viscosity decrease with processing time was also much

larger than the other temperatures examined, dropping lower than the viscosities for

200 and 210 ºC by the end of the experiment. This was thought to be due to

mechanical degradation. As discussed in section 1.9 in comparison with high

temperature, shear rate and hence mechanical degradation has more influence than

temperature on polymer degradation.107,112

At 190 ºC, entanglements between the macro-molecule chains were more extensive

than in samples held at higher temperatures. This caused the chains to break and

form molecules with lower molecular weights which may or may not be macro-

radicals and led to a lowering of viscosity with respect to time. At a processing

temperature of 200 ºC, the entanglements between the macromolecule chains were

lower. As a result, the changes in viscosity with time should be lower than what

occurs at 190 ºC.

When compared with higher processing temperatures (210 and 260 ºC), the

reduction in viscosity with time at 200 ºC occurred at a greater rate. Again, this is

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due to lower entanglements between macromolecule chains resulting in lower

mechanical degradation. So, it was thought that the effect of thermal degradation on

PP was lower than mechanical degradation, because the viscosity changes with

respect to time at 210 and 260 ºC were smaller than the changes at 190 and 200 ºC.

In other words, at lower temperatures the thermal degradation was probably lower

but mechanical degradation was higher; however, at elevated temperatures thermal

degradation became more important.

When examining Figure 5.5 it was apparent that the 190, 200 and 210 ºC graphs

cross over each other, and so possess an iso-viscosity point (35 - 40 minutes after

processing commenced). The viscosity at this time was approximately 4 kPa.s. It

was thought at this point the quantity of chain breakage and degradation (thermal

and mechanical) were approximately similar for these three temperatures.

Furthermore, the molecular weight of PP was approximately similar during this

period of processing. To further understand this phenomenon more experiments are

required.

In the next step of this study it was important to understand how the presence of the

nitroxide would affect the PP degradation. So, an investigation of PP degradation at

the early stage of processing was performed. TMDBIO (0.05% w/w) was mixed with

PP and an investigation was carried out at 190, 200, 210, 230 and 260 ºC.

5.3.3 Raman and ATR/FTIR spectrometry

As the concentration of TMDBIO was very low, no peaks related to the nitroxide

could be observed by ATR/FT-IR or Raman spectrometry. As mentioned previously,

carbonyl and hydroxyl groups are only formed during oxidative degradation. As

there was a lack of oxygen in the processing system, it was thought that this was the

reason behind the absence of PP degradation products in the Raman and ATR/FTIR

spectra.

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5.4 Fluorescence spectrometry

5.4.1 PP mixed with TMDBIO (0.05% w/w)

According to Micallef et al,191 the fluorescent emission of TMDBIO-doped PP after

degradation should be observed as four peaks between 350 – 425 nm. Samples taken

from the experiment utilising PP coated with nitroxide at 190 °C showed

fluorescence peaks within this range (Figure 5.6) matching those obtained in

previous studies of PP thermo-oxidative degradation probed by pro-fluorescent

nitroxide.191,192 Since the fluorescence originates from the reaction between a

carbon-centred radical and a nitroxide molecule, an increase in the radical

concentration (caused by degradation of the PP) would be expected to lead to an

increase in fluorescence peak intensity.191,192 In the current study, the peak intensity

increased with increasing processing time, showing thermal and mechanical

degradation of PP increased during processing.

Figure 5.6 Fluorescence emission of TMDBIO-PP at 190 ºC after 8 minutes processing in the Minilab

In order to quantify the concentration of carbon-centred radicals produced, the area

of the TMDBIO-PP fluorescence emission was measured. The peak area between

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345 and 425 nm, which corresponds to the TMDBIO emission, was measured for

each spectrum collected. As mentioned previously, the samples obtained from the

extrusion process were microtomed to a thickness of 60 μm and then the

fluorescence spectrum was collected. To increase the accuracy of the data, three

slices were measured for each sample and the fluorescence data averaged.

Figure 5.7 shows the change in fluorescence peak area over processing time at 190

°C for each of the three slices of each sample, as well as the average. There is some

variation in the data for each slice which might be due to small changes in the

thickness of the sliced samples, or to heterogeneous PP degradation phenomena

occurring during processing. Also, the sample being measured was quite small (2

mm) which could reduce the reproducibility of the measurement.

Figure 5.7 The peak area of each sample slice and the average of the slices at 190 °C

Broadly, this figure shows that the fluorescence signal increased for about the first

50 minutes of processing after which it remained fairly constant throughout the

remainder of the processing time. From this, it can be concluded that PP degradation

was observed as the change in viscosity in Figure 5.5 was closely linked to the

formation of alkyl radicals in the polymer and this commenced at the beginning of

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processing (i.e. there is no induction period). There were two possible reasons for the

stable peak area observed after 50 minutes of processing. One of these was that the

nitroxide started to decompose after this time to non- or less-fluorescent products.

The second possibility was that by this stage all the TMDBIO had reacted with

radicals and although PP degradation was proceeding there was no further increase

in fluorescence. To test this, the investigation of PP degradation during melt

processing was studied with higher concentration of nitroxide (0.1% w/w). The

results for this further investigation will be presented in section 5.4.2.

The results obtained from the remaining processing temperatures (200, 210, 230 and

260 °C) are summarised in Figure 5.8. It must be noted that during the mixing

process no air or oxygen was injected into the Minilab. As a result, oxidative

degradation could only be caused by any oxygen present before commencing the

experiment.

Figure 5.8 Calculated fluorescence peak area graphs related to PP-TMDBIO (0.05% w/w) at

190, 200, 210, 230, 260 °C When examining Figure 5.8, it was clear that graphs related to the TMDBIO-PP melt

processing at 190, 200 and 210 ºC had similar patterns in the peak area ratio, whilst,

the pattern observed during processing at 230 and 260 ºC were completely different .

Based on this it was evident that processing at 200 ºC produced the highest overall

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peak area and that higher treatment temperatures (210, 230 and 260 oC) resulted in

lower overall peak areas. Based on the information obtained from the PP viscosity

changes during melt processing (see Figure 5.5) mechanical degradation effects were

greater than thermal degradation. However Figure 5.8 showed different information.

A possible explanation of why the peak areas obtained at 190 ºC were lower than

200 ºC was that the viscosity of PP at 190 ºC was higher and as a result the

TMDBIO radicals could not freely move to scavenge PP macro-radicals. Therefore,

the amount of carbon-centred radicals which could be quenched by nitroxide was

lower.

By increasing the temperature to 200 ºC, the intensity and peak area of TMDBIO

fluorescence emission increased. Due to the negative correlation between

temperature and viscosity, an increase in temperature from 190 to 200 ºC, essentially

lowered the viscosity. This could result in the nitroxide radicals moving and acting

faster than at 190 ºC. Subsequently, there were more opportunities for them to react

with PP macro-radicals. As mentioned before, the intensity and peak area of the

TMDBIO-PP fluorescence has a direct relationship with the concentration of

captured macro-radical with nitroxide.191,216 Therefore, at 200 ºC the amount of

macro-radicals which reacted and were captured by TMDBIO radicals was higher.

For processing at 210 ºC it was expected that there would be an increased

fluorescence peak area. This was because an increase in temperature results in a

higher amount of thermal degradation and more PP macro-radicals are produced.

Also, at lower viscosities the nitroxide radicals have more freedom to move and react

with carbon-centred radicals. Despite this, when comparing the 210 ºC treatment to

the 200 ºC processing, the intensity and peak area of fluorescent spectra of TMDBIO

emission was lower. A possible explanation for this was that at temperatures higher

than 200 ºC TMDBIO started to decompose. As a result the concentration of

TMDBIO was not sufficient to react with all the macro-radicals produced. However,

sufficient nitroxide radicals remained within the samples to prove the degradation

phenomena. When comparing the results obtained at 190, 200 and 210 ºC it was

found that samples at 200 °C were approximately 37.5% more intense than the 210

and 190 °C samples. Hence, at higher temperatures the decomposition of the

TMDBIO probably becomes a competitive reaction reducing the amount of available

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nitroxide leading to a reduced fluorescence signal. Also, there was another

possibility that, at temperatures higher than 200 ºC, the reaction between macro-

radicals and nitroxide radicals have been reversed. As a result, a decrease in the

overall intensity or peak area of TMDBIO emission has been observed. Figure 5.9

presents the possible reversible reaction between macro-radical and nitroxide radical.

N O N O R N O R R

+T>200ºC

Figure 5.9 Possible reversible reaction which occurred between nitroxide radical and carbon

centred-radicals at temperatures higher than 200 °C

When examining data obtained from processing at 230 and 260 °C, very low

fluorescence intensities were observed and showed a linear trend throughout

processing. In addition to these observations, it was evident that treatment at 260 °C

resulted in the lowest peak areas. It was thought that the linear relationship seen for

both of these treatment times was due to the decomposition of a large amount of

TMDBIO. However, because the fluorescence did not fall to zero, it was thought that

possibly a small proportion of nitroxide remained and reacted with some amount of

macro-radicals or the decomposition products were fluorescent to some extent.

Compared with the 200 °C samples, the intensities of the 210 and 230 °C samples

dropped. For example the intensity of the samples after 50 minutes processing at 210

and 230 °C dropped by 37.5% and 68.75% respectively, so it was thought that

possibly, the TMDBIO started to decompose at a temperature range of about 200 –

210 °C. Consequently, it can be concluded that under these processing conditions

TMDBIO was most effective at determining PP degradation around 200 ºC.

Another possible explanation is at higher temperature the viscosity is lower and the

entanglement between the macro-molecules is lower. As a result, the amount of

macro-radicals produced by mechanical degradation is also lower. As mentioned

previously, the amount of macro-radicals produced by thermal degradation in

comparison with mechanical degradation are very low. Therefore, by increasing the

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temperature the amount of macro-radicals produced decreased and resulted in a

decrease in the overall intensity or peak area of TMDBIO emission was observed.

For demonstration of this explanation further experiments and investigations are

required.

5.4.2 PP mixed with TMDBIO (0.1% w/w)

After investigation of PP degradation with TMDBIO at 0.05% w/w, the effect of

increased nitroxide concentration on its ability to quench the PP macro-radicals was

investigated. Because it was found that 200 ºC provided the best response for PP

degradation, the experiments were performed at this temperature

Figure 5.10 displays the results obtained from the averaged fluorescence peak areas

calculated from PP-TMDBIO with a concentration of 0.05% w/w and 0.1% w/w at

200 ºC. The PP-TMDBIO (0.05% w/w) data presented in Figure 5.10 is a

reproduction of the data from Figure 5.8. When comparing the first data point for

each concentration, doubling the concentration of nitroxide, doubled the

fluorescence. However, subsequent samples obtained over longer treatment times did

not show such a marked increase in fluorescence emission. This meant that, doubling

the concentration of nitroxide did not show a proportional increase in fluorescence

emission. For example, double the nitroxide concentration after 50 minutes of

processing only increased the overall response by 9.5%. An explanation for this

occurrence is that the amount of TMDBIO at 0.05% w/w was high enough to quench

and scavenge most of the macro-radicals. Therefore, while fluorescence was affected

by increasing concentration, this was not a proportional increase.

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Figure 5.10 Average fluorescence peak areas of PP-TMDBIO at a concentration of 0.05% w/w

and 0.1% w/w, processed at 200 ºC

5.5 Viscosity

5.5.1 PP-TMDBIO (0.05% w/w)

Figure 5.11 presents the average viscosity changes for three experiments during the

extrusion process for PP coated with 0.05% w/w TMDBIO. As expected, at 190 ºC,

the lowest temperature examined in this study, the initial viscosity measurement was

high (7.3 kPa.s). However, at this temperature the viscosity by the end of processing

was approximately 2.3 kPa.s, which was lower than the final viscosity measurements

at the higher temperatures. This was similar to results obtained when using pure PP.

As mentioned previously, the possibility of degradation by shear stress was higher

and the entanglements between the PP chains were higher at higher viscosities. This

resulted in an increased resistance between macromolecular chains and significant

mechanical degradation. Over time, this caused chain breakages and a drop in

molecular weight and viscosity.

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Figure 5.11 PP-TMDBIO (0.05% w/w) viscosity changes during processing at 190, 200, 210 and

260 ºC When examining the results obtained at 200 ºC, the viscosity changes at this

temperature were approximately similar to those at 190 ºC. The viscosity of the PP at

the beginning of the processing (4 minutes after starting) was lower than 190 ºC

because of the higher temperature. However, at the end of the processing (76

minutes) the viscosity was higher than that observed at 190 ºC. There were two

possible reasons for this phenomenon. One of these was at 200 ºC the mechanical

degradation, shear rate and shear stress was lower in comparison with processing at

190 ºC. As a result degradation of PP which causes the change in molecular weight

of the PP was lower in comparison with processing at 190 ºC. The second

explanation was that because viscosity was lower at 200 ºC, the nitroxide radicals

had more opportunity to mix and react with carbon-centred radicals and behave as

stabilisers, leading to lower degradation and ultimately a slightly higher viscosity. At

210 ºC the changes in viscosity were much lower than 190 and 200 ºC. Although the

possibility of mechanical degradation in this temperature was lower than at 190 and

200 ºC, the probability of thermal degradation was higher. The results obtained for

viscosity changes at 260 ºC showed an approximate trend similar to 210 ºC.

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When examining Figure 5.11, it can be observed that the viscosity changes were

similar to that of pure PP (see Figure 5.5). As with pure PP, the PP-TMDBIO (0.05%

w/w) data set obtained at 190, 200 and 210 ºC had an iso-viscosity point of around 4

kPa.s. This point, however, occurred around 45 - 50 minutes after processing. It was

thought that the presence of TMDBIO controlled the degradation of PP and lowered

the molecular weight of PP. To understand this in greater detail, further studies must

be undertaken.

5.5.2 Comparison between viscosity changes for PP and PP with 0.05%

(w/w) TMDBIO

Because TMDBIO can act as a stabiliser, it was expected that there would be some

differences when comparing viscosity changes (Figure 5.12) between pure PP and

PP-TMDBIO (0.05% w/w) when these materials were melt processed. It should be

mentioned that the data presented in this figure is a reproduction of the data from

Figure 5.5 and Figure 5.11. For temperatures of 190, 200 and 210 °C the viscosity

was generally higher during processing indicating that the TMDBIO was indeed

trapping radicals and reducing degradation. However, these changes were not as

dramatic as expected. This might be because the concentration of TMDBIO was not

high enough at 0.05% w/w to have a large effect as a stabiliser. Despite this, it was

possible to demonstrate that degradation occurred at the start of the processing.

Therefore, 0.05% TMDBIO was probably not enough to act as a stabiliser but it was

enough to act as a probe to identify carbon-centred radicals.

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(a)

(b)

Chapter 5

121

Figure 5.12 Comparison between viscosity changes of PP and PP-TMDBIO (0.05%w/w) at (a)

190, (b) 200, (c) 210 and (d) 260 ºC

(c)

(d)

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122

At a processing temperature of 260 ºC (Figure 5.12(d)) the presence of TMDBIO did

not affect the viscosity during processing compared with pure PP. As noted before,

this was most likely due to decomposition of TMDBIO at this temperature.

5.5.3 Comparison between viscosity changes for PP and PP with 0.05%

and 0.1% (w/w) TMDBIO

Figure 5.13 presents the viscosity changes for PP, PP-TMDBIO (0.05% w/w) and

PP-TMDBIO (0.1% w/w) at 200 ºC. For example, it was observed that increasing the

concentration of TMDBIO to 0.1% w/w resulted in a much smaller viscosity drop

during processing (50% smaller change at 50 minutes after processing in comparison

with material containing 0.05% TMDBIO). This shows that at higher concentrations

of TMDBIO, its behaviour as stabiliser was more probable.

Figure 5.13 Viscosity changes of PP and PP-TMDBIO with a concentration of 0.05% w/w and

0.1% w/w at 200 ºC When comparing the viscosity results in Figure 5.13 with those presented in Figure

5.10, an interesting observation can be made. Doubling the concentration of

TMDBIO resulted in only a 9.5% increase in fluorescence emission; however, the

viscosity change over time was approximately 50% lower at 50 minutes after

processing. As a result it can be said that, although increasing the concentration of

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123

TMDBIO had an effect on the stabilisation of the PP, it did not lead to a remarkable

increase in fluorescence emission. This means that, the effect of nitroxide in

fluorescence emission is not as marked as it is on viscosity at long times.

5.6 Conclusions

Profluorescent nitroxides have the potential to be powerful for the identification of

PP degradation at a very early stage. Because of the nature of nitroxide, they act as

both a probe for degradation and as stabilisers during polymer processing. Most of

the macro-radicals produced during the mix processing are caused by chain scisson,

mechanical and thermal degradation of PP.

Although IR and Raman spectroscopy are two powerful techniques to illustrate the

PP degradation during reactive extrusion, they were not sensitive enough to detect

early stages of polymer degradation because most of the degradation that occurred

changed the polymer chain length, but not the molecular structure.

Investigation of the emission intensity of PP-TMDBIO spectra showed that

increasing the nitroxide concentration did not have an obvious effect on the obtained

result during the processing at 200 ºC. From this it was likely that a TMDBIO

concentration of 0.05% was enough for the identification of PP degradation at the

early stage.

The rheology study, in this case viscosity measurements, showed that the viscosity

changes started at the beginning of the reaction; however, these changes were

reduced in the presence of TMDBIO.

The fluorescence spectra showed that at higher temperatures than 200 ºC the

intensity of the fluorescence emission was lowered. This is may be due to the

nitroxide decomposing or reversible reactions of trapped macro-radicals with the

nitroxide. As a result using TMDBIO at temperatures higher than 200 °C was not

appropriate. Another explanation for this phenomenon, is that at higher temperatures

the mechanical degradation of PP is lower, and as a result, the amount of macro-

radicals produced is lowered; correspondingly, the emission intensity of the

PP_TMDBIO is smaller.

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Chapter 6 Segmented polyurethane nano-composites: laboratory

scale melt processing

6.1 Introduction

One of the oldest and most important segmented block copolymers that has received

significant attention from many researchers is thermoplastic polyurethane (TPU).227-

234 TPU is a linear copolymer generally synthesised by step-growth

polymerisation.233 Polyurethanes usually involve a soft and a hard segment. In this

type of polymer, the soft segments are usually composed of a linear, long chain diol,

which provides flexibility and plasticity to the polymer.229-233 The properties of the

TPU at low temperatures, i.e. the solvent resistance and the weather resistance, are

influenced by the soft segments. Hydroxyl terminated polyesters and polyethers are

the most common soft segments in TPU. However, in some applications hydroxyl

terminated saturated and unsaturated hydrocarbons, polycarbonates and

polydimethylsiloxanes can be used as soft segments.235,236 A hard segment, which

acts as an effective cross-linker, is formed by a diisocyanate and small molecules

such as diols and diamines, which act as chain extenders.230,232 Although there are a

large number of polyisocyanates available for use as hard segments, only a few of

them are used industrially. 4-4´-Diphenylmethane diisocyanate (MDI) and toluene

diisocyanate (TDI) are two important types of isocyanate. The hard segments largely

control the mechanical properties of TPU, as well as its stability at higher

temperatures. This means that by increasing the percentage of hard segments within

TPU, the temperature stability can also be increased.235 The relative proportions of

hard or soft segments of polyurethane influence the hardness, tensile strength and

clarity of the polymer.231

Hard and soft segments; however, are thermodynamically incompatible. Increasing

the molecular weight of TPU will cause the material to undergo phase separation.

Control of relative hard and soft segment length and length distribution, as well as

relative Tg and solubility parameter, will have a significant bearing on the

morphology and properties of the resulting TPU. Also, if one of the segments can

crystallise, the probability of phase separation is higher.235-237 Phase separation of the

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125

polyurethane depends on the thermal history of the polymer. This phenomenon is

especially important in reactive extrusion processing.237

An effective technique to enhance TPU’s mechanical, thermal and barrier properties

is to incorporate organically-modified nano-sized layered clay or silicate particles

into the polymer, the concentration of which is typically between 1 wt% and 10 wt%.

Even very low organo-clay concentrations have an obvious effect on the mechanical

properties of TPU. Different types of clay can modify and improve the polymer

properties.229-231 In addition, Finnigan et al229 have reported that not only is the type

of clay significant for improving the polymer properties, but clay particle size or

aspect ratio (average diameter: thickness ratio of clay platelets) also plays an

important role. The clay or silicate nanoparticle surfaces are usually modified by a

variety of organic surfactants which increase the compatibility and dispersion of the

clay into the polymer. The surfactants generally used are alkyl ammoniums, alkyl

phosphoniums and imidazoliums.238

In situ monitoring of the TPU-clay nano-composite using NIR fibre optics has not

been reported within the literature. The process for better dispersion of the clay

through the matrix of TPU, degradation of the TPU during processing, further

reaction of TPU, possible degradation of the modified clay at particular temperatures

and change in molecular structure of the nano-composite during melt and mix

processing are also unknown. The aim of the work reported in this chapter was to

investigate TPU-clay nano-composite melt processing by using a NIR fibre optic

probe. Additionally, the effect of clay particle size on the polymer melt processing

and polymer molecular structure was studied in situ. In addition to this, the effect of

temperature on the rheological properties, in this case viscosity, of the polymer was

also studied.

6.2 Experimental

6.2.1 Materials

The segmented polyurethane nano-composite used in this investigation consisted of

TPU and a synthetic fluoromica (Somasif MEE) which was treated with a surfactant.

The TPU consisted of poly(tetramethylene oxide) (PTMO) as the soft segment, and

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4,4´-methylenediphenyldiisocyanate (MDI) and 1,4-butanediol (BDO) made up the

hard segment. Figure 6.1shows the chemical structure of the employed TPU.

CH2NCHO

NH

CO

O (CH2)4 O COZ

NH

CH2 NH

C OO

(CH2)4 O

p q

MDI PTMO MDI BDO

Hard segment Hard segmentSoft segment

Figure 6.1 The chemical structure of TPU

The clay used in this study was a surface modified synthetic fluoromica, which was

called Somasif MEE (MEE) which has the elemental composition of

Na0.66Mg2.68(Si3.98All0.02)O10.02F1.96. It was surface modified by 23 wt%

dipoly(oxyethylene-coco-methyl-ammonium).229,238 The MEE concentration used

was 3 wt% of the nano-composite. The chemical structure of the MEE surfactant is

presented in Figure 6.2.

NH3C CH2

R

CH2 O H

CH2 CH2 O HX

YX+Y=2

Figure 6.2 The chemical structure of MEE surfactant

In order to investigate the effect of the nanoparticle clay on the TPU molecular

structure and viscosity, four different clay platelet particle sizes were used: 30, 75,

200, and 650 nm.

6.2.2 Sample preparation

The four different samples of TPU nano-composites and one TPU sample without

MEE (TPU-control) were prepared by Dr. Darren Martin polymer group at the

Australian Institute for Bioengineering and Nanotechnology (AIBN) at the

University of Queensland. The TPU-control was synthesised with 35 wt% of hard

segment. The preparation and characterisation of the TPU-control has been discussed

elsewhere.239 The nano-composites were prepared via a pre-cast method from

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dimethylacetamide (DMAc). Briefly, a 5 wt% solution of TPU-control in DMAc was

mixed with 5 wt% of solution of MEE powder in toluene. The mixture was then cast

onto glass plates and dried at 50 °C for 48 hours under nitrogen atmosphere. Then

the produced films were dried under vacuum conditions at 50 °C for 12 hours

followed by drying in a vacuum oven at 80 °C for 12 hours. Finally they were stored

at room temperature.229 When used for melt-processing, the dried films were cut up

before being fed into the extruder. This ensured good pre-dispersion of the organo-

clay variants into the host TPU.

6.2.3 Minilab extruder conditions

Four grams of cut-up material was fed into the Minilab extruder and experiments

were performed at 190 °C and 200 °C. For all experiments, the screw speed was 60

rpm. The maximum processing time to produce TPU-MEE samples with suitable

mechanical and physical properties was between 18 - 20 minutes. For this study,

sample processing was performed for 18 minutes.

6.2.4 Data analysis

The software used for the chemometric study in this chapter was described in section

2.3.4. The region of the NIR spectra selected for this investigation was between 7500

and 4540 cm-1. To remove baseline effects, the second derivative was applied to the

spectra.

6.3 Results and discussion

For comparative purposes, the NIR spectra of TPU with and without MEE clay were

collected. Figure 6.3 depicts the NIR transmittance spectrum of the TPU before

processing. The peak at 4621 cm-1 was assigned to a combination band of the

aromatic C-H stretching. The peak observed at 4680 cm-1 corresponds to the

isocyanate from un-reacted starting materials which appears as a shoulder of the

4621 cm-1 band. The band at 4916 cm-1 was assigned to the N-H combination band

of urethane. The peaks observed between 5308 and 6204 cm-1 were assigned to the

overlapped aromatic and aliphatic C-H first overtone stretching vibrations. The band

near 6740 cm-1 arose from the first overtone N-H stretching vibration of the urethane

group. Additionally, the peak observed between 8200 – 8600 cm-1 was associated

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with the second overtone of the aromatic C-H stretching vibration.227,228,232,233 Table

6.1 presents a summary of the vibrational assignments for TPU.

Figure 6.3 The NIR spectrum of pure TPU (TPU-control) before processing.

Table 6.1 Assignments of TPU vibrational bands227,228,232,233

Assignments Peak regions (cm-1) C-H aromatic stretching combination

band 4621

Isocyanate 4680 N-H combination (urethane) 4916

Overlapping bands of the aromatic and aliphatic C-H stretching first

overtone

5308 – 6204

N-H overtone (urethane) 6750 C-H stretching second overtone 8200 – 8600

6.3.1 NIR spectroscopy of starting materials

Initially, it was important to know if peaks related to the MEE clay could be detected

when it was mixed with TPU and if so, whether any of these bands overlapped those

of the TPU. For this reason the NIR spectra of the TPU-control and the MEE clay

were compared (Figure 6.4). It was evident that the weak bands observed at 6988

and 5237 cm-1 in spectrum (b) could be assigned to the MEE. In addition, a broad

band at 8298 cm-1 was observed in both the MEE and the TPU-MEE mixed spectra.

A band at 8334 cm-1 observed in spectrum (a) was also apparent in spectrum (b);

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however it shifted to 8313 cm-1. This shift probably occurred due to the TPU hard or

soft segments crystallising during mixing with the MEE. When comparing spectrum

(a) to (c), there was no band at 6740 cm-1 in the latter. This band arose from the N-H

first overtone and was therefore specific to the polyurethane component.

Figure 6.4 NIR spectra of (a) TPU-control (pure TPU), (b) TPU-MEE 30 and (c) MEE 30.

(Spectra have been offset for clarity.)

To identify any differences in the molecular structure of TPU-MEE when using

MEE with different particle sizes, the NIR spectra were collected and compared

before processing (Figure 6.5). The results showed that there were no significant

differences between the NIR spectra of TPU-MEE when mixed with clay particles of

different sizes. This was not suprising given the low MEE loading of only 3 wt%.

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Figure 6.5 NIR spectra of TPU containing MEE clay with a particle size of (a) 30, (b) 75, (c) 200,

and (d) 650 nm. (Spectra have been offset for clarity.)

6.3.2 NIR melt-processing investigations

In order to identify alterations caused by melt processing in samples of the

nanocomposites, the last spectrum (18 minutes) was subtracted from the first (2

minutes) (Figure 6.6). The spectra collected from melt processing at 190 and 200 °C

were examined in a similar manner. There were no significant differences between

these results when processing at 190 °C compared to 200 °C. An almost identical

difference spectrum was obtained for the processing of the TPU-control, indicating

that none of the bands should be assigned to changes in the MEE component of the

nanocomposite

Figure 6.6 presents the difference spectrum described above for the 190 oC

experiment. From this figure, three obvious changes can be seen around 5270, 6753

and 7090 cm-1. Based on Figure 6.3 and Table 6.1, it can be seen that the negative

band near 5270 cm-1 is related to urethane N-H and carbonyl vibrations. Also the

band detected in 6753 cm-1 correlates with an intense band in the NIR spectrum

(Figure 6.3) which has been assigned to the overtone of the urethane N-H strech and

the third overtone of the carbonyl group.232 It is difficult to assign the observed band

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near 7090 cm-1; however, this positive band might be related to the overtone of an O-

H stretching mode. It could indicate the generation of hydroxyl groups, or probably

water during processing. Same results were obtained for TPU-control spectra

subtraction which showed that the most significant change during melt processing

was in the molecular structure of TPU. These changes are not related to any changes

in the MEE structure since the concentration of the MEE was very low.

Figure 6.6 Difference NIR spectrum from melt processing of TPU-MEE (75 nm) at 190 ºC

obtained by subtracting the 18 minutes spectrum from the 2 minutes spectrum To examine the spectra in more detail PCA was carried out on each data set (see

section 6.3.4).

6.3.3 ATR/FTIR investigation

In order to investigate any differences in the molecular structure due to processing,

ATR/FTIR spectra of control and composite TPUs before and after processing at 190

and 200 °C were collected. It should be noted that the ATR/FTIR spectra of all TPU

samples mixed with clays of different particle sizes were similar.

Figure 6.7 (I) shows the ATR/FTIR spectra of the TPU-control and TPU-MEE (75

nm) before processing. Features common to both spectra are the band observed at

3300 – 3350 cm-1 which was assigned to the N-H stretching band of the urethane

7090

5270 6750

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132

group. Additionally, the sharp doublets at 2850 and 2928 cm-1 corresponded to

aliphatic C-H stretching. Two bands appearing at 1704 and 1732 cm-1 were related to

the hydrogen-bonded and non-hydrogen-bonded urethane (hard segment),

respectively.240-244 These two peaks can provide information about degradation and

degree of phase separation of the soft and hard segments. This means that the

increased intensity of the band at 1704 cm-1 showed that phase separation had

occurred due to processing (disordered urethane group). Examination of the ATR

spectra from the TPU control before and after processing (Figure 6.7 (II)) did not

show any changes in the intensity of 1704 and 1732 cm-1 band. As a result it can be

said that the degree of phase separation and the amount of hydrogen-bonded and

non-hydrogen-bonded urethane did not change before and after processing. The band

observed at 1604 cm-1 was assigned to the C=C aromatic stretching vibration. Two

sharp bands which were observed at 1533 and 1220 cm-1 arose from the N-H

bending vibrations related to the amide II and CH3 bending vibrations,

respectively.240-242,245-248 For identification of differences between the two spectra

(Figure 6.7 (Ia) and (Ib)) spectral subtraction was performed. The difference

spectrum is presented in Figure 6.7 (II). There were some obvious positive bands

observed at 3300, 2928, 2852, 1638 and 1563 cm-1, as well as an intense negative

band at 1007 cm-1. As mentioned previously in section 6.2.2, during preparation of

the nano-composite polyurethane, the MEE powder and polyurethane were mixed,

pre-cast and heated at 80 °C. Therefore, the nano-composites had been pre-heated

before melt processing. Hence, there was a possibility that some hard or soft

segments degraded, or physically changed in some way, prior to the melt processing.

There was also a possibility that, the compounds trapped in to pores of the material

volatised during the heat and vacuum drying steps during pre-cast processing. The

TPU-control, which had not previously been heated, showed a more intense band at

3300 cm-1 compared with the TPU-MEE (75nm) (see Figure 6.7). The sharp bands at

3300, 1638 and 1563 cm-1 which disappear from the spectrum after the TPU was

heat-treated were best explained by postulating the presence of a urea group.242,247,249

This was unexpected; however, the TPU had been synthesised by using industrial

raw material, which may have contained some impurities. Hence it was plausible that

the TPU-control contained a small amount of a volatile urea contaminant. The

differences observed at 2928 and 2852 cm-1 which correspond to aliphatic anti-

symmetric and symmetric CH2 stretching vibrations might be related to degradation

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133

of the hard segment as well as phase separation. The sharp band detected at 1007 cm-

1 maight be related to MEE. A full analysis of these spectral changes was difficult

due to limited product details disclosed for the commercial sample of TPU.

Figure 6.7 (I) ATR/FTIR spectra of (a) TPU-control and (b) TPU-MEE (75 nm) before

processing and (II) difference spectrum (a – b) As mentioned previously, one aim of this study was to understand the effect of

temperature on the molecular structure of TPU. So, one of the first steps was to

(II)

(I)

3300

1638

1563

2852 2928

1007

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134

investigate how processing and temperature can change the structure of the base

polymer (TPU-control).

Figure 6.8 displays the ATR/FTIR spectra of the TPU-control before and after

processing at 190 °C. In this figure, one can observe how processing affects the

molecular structure of TPU. The difference ATR/FTIR spectrum obtained by

subtracting the ATR/FTIR spectrum of TPU-control before and after processing is

also shown in Figure 6.8. Clearly, most of the band changes are similar to those seen

in Figure 6.7. Figure 6.8 obviously substantiates the effect of heat-treatment on the

TPU-control. The spectra obtained for the TPU-control processed at 200 °C showed

a similar response. By this stage it was understood that temperature and processing

can change the molecular structure of TPU. However it was not clear when and how

these changes occurred.

It was also important to determine how the presence of the MEE clay and its particle

size affected the nano-composite molecular structure during processing. Figure 6.9

displays the ATR/FTIR spectra of TPU-MEE (200 nm) before and after processing

at 190 °C as well as the difference spectrum.

Clearly Figure 6.9 shows that, some changes occurred at 1109 and 1077 cm-1 which

correspond to stretching of C-C bonds within benzene rings, CH2 stretching

vibrations and C-O-C stretching vibrations in the hard segment. When comparing

spectra collected at 200 ºC with those already discussed, all spectral changes were

similar to those reported for processing at 190 ºC. These bands might be related to

crystallinity changes of the TPU segments and/or degradation of the TPU-MEE.

However, at this stage, it was not clear when these changes occurred at 190 ºC and

200 ºC. More information about changes in molecular structure of the TPU nano-

composite could be obtained by PCA analysis of these spectra.

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135

Figure 6.8 (I) ATR/FTIR spectra of TPU-control (a) before processing and (b) after processing at 190 °C and (II) difference spectrum (a – b)

(I)

(II)

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136

Figure 6.9 (I) ATR/FTIR spectra of TPU-MEE (200 nm) (a) before processing and (b) after

processing at 190 °C and (II) difference spectrum (a – b)

(I)

(II)

29283300

1007

1109

1638

1077

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137

6.3.4 PCA of NIR spectra from TPU clay nano-composites

In order to extract further information from the NIR spectra sets obtained during

melt processing they were subjected to PCA data analysis.

6.3.4.1 TPU-control

Figure 6.10 shows a PC loadings plot of factor 1 from the TPU-control processed at

200 ºC. The loadings plot shows a strong negative peak near 6753 cm-1 due to

urethane’s first overtone of the N-H stretch and the third overtone of the carbonyl

group. It also shows another strong negative band at 5270 cm-1, which as mentioned

before, relates to urethane N-H and carbonyl vibrations. Also present was the

positive band observed at 7090 cm-1 which might be arise from the overtone of an O-

H stretching band. These results are similar to what were obtained from the spectral

diffraction presented in Figure 6.6. This information confirmed that the detected

changes are related to TPU molecular structure.

Figure 6.10 Factor loadings plot for TPU-control processed at 200 ºC

The PC1 scores plot versus time of TPU processed at 190 and 200 ºC is shown in

Figure 6.11. It was apparent from this that two different phenomena occurred during

processing. The first may be related to melting of the hard segment and the second

possibly corresponded to the degradation of TPU. There is some possibility that

these two phenomena occurred at the same time; however, there may be competition

between them. At the beginning of processing, melting of the TPU hard segment was

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138

more probable and after some time (dependant on the temperature) degradation and

phase separation was more likely. When operating at higher temperature, the first

phenomenon occurs after a shorter processing time which was thought to be related

to melting of the hard segment. Degradation of TPU manifests as phase separation of

the hard and soft segments. Chain breakage of the hard segment was also possible,

but continued processing remixed the hard and soft segments. As a result there was a

possibility that TPU with a lower molecular weight had been produced, which might

be more thermally stable. The second phenomena detected again could be related to

degradation of TPU. The plateau seen when processing at 200 ºC might be

associated with polyurethane degradation and associated formation and separation of

more thermally stable, longer hard segment sequences in the polyurethane.237 Figure

6.11 shows the probable degradation of the TPU-control was faster at 200 °C.

Hence, it was thought that at higher temperature the TPU-control started to degrade

at an earlier stage during melt processing.

Figure 6.11 PC1 scores plot versus time for NIR spectra obtained from TPU-control at 190 and

200 °C

6.3.4.2 TPU mixed with MEE

The effect of MEE particle size on the NIR spectra of TPU-MEE was also examined

using PCA analysis. The factor loadings plots from TPU-MEE generally had the

same pattern as factor loadings plots of the TPU-control; however, there were some

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139

distinct differences. Figure 6.12 shows a factor loadings plot of TPU-MEE (200 nm)

at 200 °C, which was representative of the factor loadings plots obtained using the

other MEE particle sizes. This plot is similar to the factor loadings plot of the TPU

control (Figure 6.10) and shows two negative bands at 6750 and 5270 cm-1 ,as well

as, a positive band at 7090 cm-1 which was similar to the detected differences which

were discussed above. This plot displays negative bands at 6073, 5787 and 5594 cm-

1, which might be a consequence of signal-to-noise ratio differences.There was also a

negative change at 4916 cm-1, which corresponds to the N-H combination band of

urethane.

Figure 6.12 Factor loadings plot of TPU-MEE (200 nm) which was melt processed at 200 ºC

Although all of the factor loading plots for TPU-MEE, regardless of particle size,

showed the same result, the results obtained for the scores plots of different particle

sizes showed some variation.

Figure 6.13 shows the PC1 results of TPU-MEE (30 nm) at 190 and 200 ºC. This

graph showed phenomena similar to those seen for the TPU-control (Figure 6.11).

For both temperatures the points of inflection were at 4 minutes after processing and

the plots show a sharp negative trend for the first 4 minutes of processing followed

by a weakly negative trend until the end of the mix processing. One possible

explanation for these results is that melting of the hard segment occurred in the early

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140

stage of processing and then possible degradation of hard segment occurred. The rate

of degradation at 190 ºC was approximately similar to that at 200 ºC. For this

experiment it appeared that a change in temperature had an approximately similar

effect on the molecular structure of TPU. It was also possible that competition

between hard segment melting and degradation of the TPU had the same pattern for

both temperatures.

Figure 6.13 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE (30 nm) at

190 and 200 °C

Figure 6.14 shows the PC1 results from NIR spectra obtained during 18 minutes of

TPU-MEE (75 nm) melt processing. The plot for the processing at 200 ºC clearly

showed a positive trend. Because of the high temperatures this phenomenon might

be related to the degradation and phase separation of TPU. The first phenomenon

which was thought to be due to melting of the hard segments within the TPU nano-

composite might have occurred at an earlier stage of processing. It was possible that

melting of the hard segment at 200 ºC occurred within the first 2 minutes which

could not be detected within the NIR spectra and therefore PC1 data processing.

Inflection point

Possible TPU degradation

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141

During mix processing at 190 ºC, the graphs showed a negative trend for the first 4

minutes after which time there was an increase in intensity until 12 minutes and then

an approximate steady-state was reached. Again, it was thought that hard segment

melting occurred during the first 4 minutes and changes in the graph at later

processing times were thought to be due to hard and soft segment phase separation

and degradation of the TPU.

Figure 6.14 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE (75 nm) at

190 and 200 °C Figure 6.15 and Figure 6.16 present the PC1 scores plots for TPU-MEE (200 nm)

and TPU-MEE (650 nm), respectively. During the melt processing of TPU-MEE

(200 nm) and TPU-MEE (650 nm) at 190 ºC the point of inflection occurred after

processing for 6 minutes. After this, the graphs reached a steady-state.

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142

Figure 6.15 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE (200 nm) at

190 and 200 °C

Figure 6.16 PC1 scores plot versus time for NIR spectra obtained from TPU-MEE (650 nm) at

190 and 200 °C

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143

The 200 ºC results were different from this as there was a positive trend observed

throughout the entire experiment. This was thought to be related to the TPU hard

segment melting which happened very quickly at the higher temperature, even before

a spectrum was collected. By comparing Figure 6.15 and Figure 6.16, the rate of

hard segment melting at 190 ºC appeared faster for the smaller particle sized clay

(200 nm). Also observed was a slower rate of TPU-MEE degradation when using the

650 nm MEE clay compared to the 200 nm clay.

Therefore, analysis of the PCA results indicated that higher temperatures increased

the polymer degradation. Also, by increasing the clay particle size the rate of the

segments’ phase separation and degradation of TPU appeared to decrease. This

implies that decreasing the particle size results in a better dispersion of the clay into

the polymer.

6.3.5 Viscosity measurements

Viscosity was also monitored in situ for melt processing at 190 and 200 °C. The

results obtained for each set of experiments are presented in Figure 6.17 and Figure

6.19.

Figure 6.17 Viscosity changes for TPU-MEE with different particle sizes at 190 °C

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144

These results show that the TPU-control had the highest viscosity over the examined

processing time, despite the viscosity decreasing with processing time. The reason

for the decrease in viscosity, as mentioned previously in the PCA section, probably

depends on the phase separation of the segments and degradation of the TPU. During

the degradation of TPU, the phase separation and remixing of soft and hard segments

produces lower molecular weight TPU, which has a shorter chain length and is

probably more stable.240-242 Results for 200 oC show a similar pattern to those at 190 oC except that overall the viscosity was lower because of the higher temperature. At

both temperatures, the mixture of TPU-MEE (30 nm) had the lowest viscosity in

comparison with the other particle sizes used. It was thought that because the smaller

particle size has better dispersion within the TPU matrix, it was expected that the

TPU-MEE with a smaller particle size, for example TPU-MEE (30 nm), would have

a lower viscosity in comparison with the TPU-MEE mixed with the larger particle

sizes, for example TPU-MEE (650 nm). At high temperatures the access of the TPU

chains to the “inter-gallery space” of MEE or interlayer spacing of the fluoromica is

higher. Therefore the chance of TPU chains reacting with the O-H on the surfactant

(especially after phase separation) is higher. As a result, a much lower molecular

weight for TPU-MEE (30 nm) would have been expected in comparison with the

TPU-MEE (650 nm) (see Figure 6.18).229

Figure 6.18 TPU nano-composite with (a) MEE (650nm) with the inter-gallery space size of 3.5 nm (b) and MEE (30nm) with inter-gallery space of 10 nm.

These results suggested that during the mixing process the MEE and surfactant, in

addition to the expected degradation and reformation of TPU, might have had an

effect on the viscosity. This effect was thought to be amplified when using smaller

particle sizes.

(a) (b)

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145

Figure 6.19 Viscosity changes for TPU-MEE with different particle sizes at 200 °C

The results from the viscosity investigations showed that the pattern of changes over

the range of particle sizes used and two different temperatures were similar. It was

also thought that during processing some amount of surfactant bleeding from the

MEE occurred and this reacted with TPU or one of the separated phases and acted as

a plasticiser. Additionally, it was possible the bleeding was higher for the smaller

particle sizes compared to the larger particle sizes resulting in the lower viscosity

observed in the TPU-MEE (30 nm). This may well be the reason that the TPU-

control displayed a higher viscosity compared to the TPU-MEE nano-composites.

Surfactants can degrade at temperatures higher than 160 °C,238 so these degradation

products might affect the properties of the final nano-composite. One possible effect

was that the surfactant degradation products reacted with separated or reformed

segments and form a polymer with a lower molecular weight. The surfactant

degradation products could also act as plasticisers and decrease the viscosity of the

polymer nano-composite.

Because the smaller particle size has a higher surface-to-volume ratio, the quantity of

surfactant which could have decomposed or degraded was higher. Consequently,

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146

TPU mixed with 30 nm particle sized clay might have resulted in a higher amount of

surfactant degradation products. In turn, if these products acted as plasticisers, the

viscosity change with time would have been lower in comparison with the larger

particle sizes. However, more studies must to carried out for verify this explanation.

Another possible explanation is that the organo-clays have O-H groups which can

take part in reactions, such as transurethanisation, at 190 and 200 ºC. These O-H

groups are more likely to be more accessible for the lower aspect ratio clays. The

higher aspect ratio clays also have less intercalation of TPU and this probably affects

the reaction kinetics.

It was thought that the lower viscosity evidence by the smaller particle sized organo-

clays were due to a better degree of dispersion within the polymer. In addition,

changes in viscosity were not only caused by degradation and phase separation of

TPU, but may also have been influenced by surfactant degradation products.

6.4 Conclusions

In this chapter the effect of temperature and the particle size of MEE were

investigated during melt-processing in a Minilab extruder. The results showed that

during processing, phase separation and degradation of TPU occurred. It was also

shown that increasing the particle size increased the observed effects, except for

viscosity where smaller particle size had a larger effect.

A PCA study of the NIR results detected two phenomena occurring during melt

processing at 190 ºC. One of these was related to melting of the hard segment and

the other due to degradation and phase separation of TPU. By increasing the

temperature the first phenomenon was not detected. This showed that hard segment

melting occurred at the early stage of processing. From these results it was

understood that by increasing temperature, melting of the hard segment occurred at

an earlier stage of the mix processing (before 2 minutes after processing). It was also

thought that increasing the temperature to 200 ºC resulting in the degradation of TPU

and its phase separation commenced at an earlier processing time.

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147

From a rheology point of view, when using a larger MEE particle size, the viscosity

was initially higher and remained so throughout processing. This may be due to

smaller particle sizes being better dispersed through the TPU matrix and after hard

segment degradation TPU chains have better access through the inter-gallery space

of MEE. It was also possible that the smaller particle sized MEE surfactants can

degrade faster and the degradation products may behave as plasticisers further

affecting viscosity.

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148

Chapter 7 Conclusions and future work

7.1 Conclusions

Because of the poor physical, chemical and mechanical properties of the polymers

examined, they are rarely used as pure materials. As most of these polymers are

immiscible, to improve their properties, one must increase the polymers’ miscibility.

Reactive extrusion is one of the most appropriate techniques for polymer

compatibilisation and modification. Although, there are many benefits for using

reactive extrusion for polymer processing, there are some limitations such as

increasing the probability of polymer degradation. The aim of this project was to

investigate polymer processing and polymer degradation during reactive extrusion

conditions.

Microscopic ATR/FTIR spectroscopy was used to investigate the polymer blend

quality under reactive extrusion conditions. This technique was used to study the

homogeneity of dispersions of starch throughout a polyester matrix. The quality of

the blend was visualised by the use of IR images. This technique was shown to be

appropriate for identification of the starch/polyester blend homogeneity. This

technique also showed that reactive extrusion was suitable to use for modifying the

processing conditions in order to improve the homogeneity of polymer blend

products.

Graft modification of PP with MAH in the absence and presence of DCP was also

investigated. The process was carried out in a laboratory scale reactive extruder and

then monitored in real-time by simultaneous NIR spectroscopy, using a fibre optic

probe, and viscosity measurements. ATR spectra of product material showed that

during graft processing in the absence of DCP, MAH reacted with tertiary and

primary carbon radicals. In the presence of DCP most of the MAH reacted with

tertiary carbon radicals. Therefore, it was inferred that in the presence of DCP

mostly tertiary carbon radicals were produced and in the absence of DCP, both

tertiary and primary carbon radicals were produced.

Chapter 7

149

The NIR spectra obtained were analysed using PCA and peak area techniques.

Similar results were obtained from both techniques in the absence and presence of

DCP. The similarity between the results showed that the presence of DCP did not

change the reaction mechanism. The viscosity data showed that in the absence of

DCP all the macro-radicals produced were due to PP degradation or β-scission

reactions. In the presence of DCP, viscosity was low throughout the processing. This

was due to the high concentration of macro-radicals immediately generated by DCP

which led to a substantial reduction in average molecular weight. NIR and viscosity

measurements showed that, although the presence of DCP can produce a high

concentration of macro-radicals, it did not affect the rate of graft processing. Also,

the mechanism of graft processing in the absence or presence of DCP was

investigated and the activation energy for both sets of experiments was calculated.

PP degradation was investigated in laboratory scale reactive extrusion by application

of a pro-fluorescent nitroxide, TMDBIO, as a probe for the presence of carbon-

centred radicals, with simultaneous viscosity measurements. Although Raman and

IR did not show any degradation during mix processing, fluorescence investigations

of TMDBIO showed that radicals were present at an early stage of the processing

indicating an early onset of degradation. TMDBIO was not stable at temperatures

higher than 220 °C which was one of the limitations of this nitroxide.

In addition to the in situ monitoring of polymer processing, TPU-MEE nano-

composite melt processing was also examined. During mix processing NIR and

viscosity were measured simultaneously. PCA analysis of the spectra showed that

during mix processing of TPU-MEE two phenomena occurred which were possibly

related to melting and then degradation of the hard segment of the polyurethane. By

increasing the temperature, melting of the hard segment was not detected. This

showed that hard segment melting occurred at the early stage of processing. The

viscosity investigation showed that, when using a larger MEE particle size, the

viscosity was initially higher and remained higher throughout processing. Thus, the

smaller particle sizes had better dispersion throughout the MEE matrix. Another

possibility for these observations was that smaller particle sizes caused the MEE’s

surfactant to degrade faster and the degradation products may have behaved as

plasticisers, affecting the viscosity. This work also showed that the TPU studied in

Chapter 7

150

this project underwent some thermal changes even after a short time at elevated

temperature. In fact, the preparation of the nano-composites themselves caused a

change in the TPU molecular structure.

7.2 Future work

Overall, in situ monitoring of the graft processing of PP using MAH has provided

suitable information about the effect of ingredients and the mechanism of the

reaction. However, further work must be done to understand the effect of DCP on the

reaction. The kinetics obtained were complex and to clarify this, some unnecessary

parameters must be removed and further study should be done. As an example, by

using one type of compound which can compete with MAH to react with primary or

tertiary carbon-centred radicals. By this method, one of the parameters which are the

amount of primary or tertiary carbon-centred radicals can be removed and the

formula can be simplified. As another example, by using two types of nitroxide with

different excitation wavelengths, which are able to react with tertiary and primary

carbon-centred radicals the concentration of these two type of macro-radicals can be

detected.

The in situ monitoring of PP graft processing using MAH was carried out under

Minilab extruder conditions. In future it would be useful if the processing and

monitoring could be done in a pilot plant or in industrial conditions. This experiment

can show if the results obtained from this project is near to real industrial processing.

For understanding the effect of initiator and shear rate on the rate of the reaction,

furture investigation into different screw speeds and different types of initiators and

their concentration should be performed. For example, it is suggested that

investigations into the effect of dibenzoyl peroxide and peroxy dicarbonates on the

reaction rate and the type of grafting, in-chain or end-chain, be performed. The effect

of other types of monomers used for modification of PP should also be examined.

The application of this on-line monitoring technique to obtain better information

about the mechanism of graft processing should be performed, possibly by

examining the consumption of other monomers such as styrene, which can decrease

the rate of chain scission. Also, it is suggested that the rate of decomposition of DCP

at different temperature over time be investigated.

Chapter 7

151

The profluorescent nitroxide TMDBIO was a suitable compound to understand and

investigate PP degradation during reactive extrusion at the early stage of the

processing. However, this compound is not stable at high temperatures. Further

study using more stable nitroxides during industrial processes should be undertaken.

These results might help to optimise the process conditions to avoid degradation of

the polymer products. In future, it would be useful if the concentration of macro-

radicals produced by thermal and mechanical degradation can be separated. By

measuring the amount of PP macro-radicals produced through thermal degradation

and differentiating this amount from the total amount of macro-radicals measured in

this sample, the concentration of macro-radicals created from mechanical

degradation can be determined. Further use of nitroxides to investigate the effect of

shear rate on polymer structure and the amount of thermal degradation at high

temperatures could be performed.

In the polymer industry, there is some doubt about homogenous mixing in extruders.

This could be further investigated using appropriate nitroxides and in situ monitoring

by application of fluorescent fibre optics.

For further investigation of TPU nano-composite clays, it would be interesting if the

effect of the moisture contamination in the clay and its effect on physical and

mechanical properties and degradation of the polymer be examined. Also, the effect

of additional surfactant on the TPU behaviour in different processing conditions and

its degradation can be investigated.

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