CHAPER 5

OYNAMICAUY VULCANISED BLENDS

Nitrile rubber (NBR) and

isotactic polypropylene (PP)

blends (TPV)

Contents of this seetion convnunicated to J. Po+ Sci : P ~ J ? B: Pdym. Phys

5.1.1 Introduction

Dynamic vulcanisation is a technique of preparing Thermoplastic elastomers

from thermoplastic polymers and elastomers by carrying out both blending and

cross-linking reaction simultaneously in an internal mixer. Such materials are

generally composed of a crystalline thermoplastic and an elastomer in which the

elastomer phase is predominately crosslinked. After blending and curing operations

the resulting blend has the rheological properties suitable for thermoplastic

processing. The morphology of the dynamic cured blend is controlled by extent and

mechanism of c ros s - l i ng reaction apart from the nature of the blend before

cross-linking. Dynamic vulcanisation improves the morphology of the

thermoplastic rubber plastic blends. The domain size of the rubber phase gets

modified, that is, gets reduced depending on the intensity of crosslinking. A

uniform and fine distribution of rubber particles in thermoplastic matrix is obtained

as a result of dynamic curing. In the case of blends having continuous rubber

matrix, phase inversion is expected.

Dynamic vulcanisation allows loading maximum of rubbery phase to impart more

and more elastomeric properties while still retaining the thermoplastic

processibility

Dynamic mechanical properties of dynamically vulcanised blend of isotactic

polypropylene (PP) and nitrile rubber (NBR) are investigated in order to assess the

applicability of models. Blends of 70130 PP/NBR vulcanised with three different

curative systems are considered in this study. These are sulphur (S), a combination

of sulphur and dicumyl peroxide (S+DICUP) and dicumyl peroxide (DCP). The

crosslinking scheme with different cure systems may be represented as (Fig 5.1):

Mixed cure : mixed linkages

Sulphur cure: mono, d or pdysulphidic linkages

Fig 5.1 Schematic pesentation of the type of noss linkages obhed

with the three cure systems.

The development of morphology in this case of 70130 PPINBR with the three

different curative systems can be schematically represented as (Fig 5.2):

CONTINUOUS PP PHASE \Fgy&"P,

CROSSLINKED NBR PARTICLE CONTINUOUS PP PHASE

DCP SYSTEM

CROSSLINKED CONTlXUOUS NBR PARTICLE?\ rpp

DYNAMIC c R o s s L m m a

N O PHASE INVERSION

PP INBR MIXED 10130 SYSTEM

CROSSLINKED NBR PARTICLE CONTNUOUS PP PHASE

SULPHUR SYSTEM

Fig 5.2 schematic representatioll of dyxumc vulcanised 70130 PP/NElR blend morphology

5.1.2 Experimental

The detals of raw materials, preparation of dynamic vul~anised blend and

dynamic and morphological evaluation have been described earlier '64. The chapter

on PP/NBR Physical blends (chapter 3, section 3.2) may also be referred. Dynamic

vulcanisation was done while melt blending in a Brabender plasticorder (PLE-330)

at a temperature of 180 "C. Dynamic mechanical measurements were carried out on

a Rheovibron DDV viscoelastometer. The experiments were conducted in uniaxial

tension mode fiom 20 to 150 O C at a frequency of 35 Hz for the crosslinked systems.

SEM scans were taken using a JEOL scanning microscope

5.1.3 Results and discussion

George. et a1 '64 reported the effect of different dynamic vulcanising systems

on dynamic properties vis-a-vis blend morphology development. In this section, the

predictions of 70130 P P m R blend (based on unvulcanised components) are

compared with the dynamic response of dynamically vulcanised 70130 P P M R

blend. These are shown as storage modulus versus temperature (Figure 5.3) and

t a d versus temperature (Figure 5.4) plots for the sulphur, mixed and peroxide cure

systems, upto temperatures beyond the glass transition temperature of PP. From the

figures, it is observed that different cure systems give different degree of cure as

manifested in the resulting dynamic response behaviour of the blends. It is

observed that peroxide system increases considerably the storage modulus while

the S- -ern gives marginally inferior values compared to the unmodified blend.

On the other hand, the mixed cure system gives intermediate values of storage

properties which are almost in the range of unmodified system.

Fig 5.3 Comparison e I computed data with c x p e r i m m ~ l rrsultr far Sbnp modulor .I 7O/SO PP/NBP oa~ulcanircd and dynamic ralranirrd blends

Referring to the loss tangent (Figure 5.4) we find that peroxide vulcanised system

imparts lowest loss tangent values, sulphur cured system the highest, and mixed

curative system possesses intermediate values. S E M ' ~ ~ (Figure 5.5 -5.7) studies on

the blends revealed better dispersion and reduction of domain dimension of NBR

phase compared to the control blend @lease see Figure 3.22 in Chapter 3 section

3.2) The domain size is finer in peroxide cured than in mixed and S-cured systems.

Scured system: S cures only NBR phase of the polyblend. The system imparts

similar or lower storage modulus values compared to the control and piedictions for

the unmodified blend. The experimental tan6 values are comparatively much higher

compared to the control and prediction. This indicates that although the

morphology is improved, there may not be improvement in the interface adhesion

(as only one phase is cross-linked) and overall stability of the morphology. This

manifests in the poor or similar mechanical response compared to the unvulcanised

blend and the prediction with the PP as matrix. It may be remarked that mere

reduction in domain sue does not improve the predictions.

l.4W

I.361

8.321

1.281

- . . . PP/NBl7O/SO(rtslc) . . . PP/NII lO/f0(nix-ralr) . . . PPINII 70/SO(diru~rulc) ... + + PPIXIIP 7O/aO(mrulc) - PP as matrix n - Pel~aggregale

d

t.121

1.141

0.111 -70 -41 21 '{I ' 7i " 62 i d 101 1st

?emperature(~I

Fig 5.4 Comprisoa af computed data with eqerimrnhl mdtt far lmr b n p n t of 70/51 PP/NlI unrmlaaired and dpamic vnlcanistd blends

Fig 5.5 Scaning clcctron micrograph of dynannically

cured 70130 PPlNRR with S

Peroxide cure system: Dicup system crosslinks both the PP anti NBR phases. This is

cvident in he higher storage modulus values of the cured system compared Lo the

unvulcanised blend. As observed in Figure 5.3, the storagc ~nodulus data predicted by

discrete particle model are lower than the experimental data for the dicup cured 70130

PPINBR blend.

It is rcported 'O that peroxide curing of EPDM in the presence of PP could result in

occasional crosslinking between the two polymer molecules. SEM photomicrograph

(Fig 5.6) of the dicup cured blend shows fine and uniform dispersion . The

enhanccment in modulus of dicup cured 70130 PPINBR blend may thus be attributed

to the cure reaction resulting in (i) development of stable morphology and (ii) better

interface adhesion by the so called occasionaI cross linking between NBR and PP.

These aspects are not considered in Kcrner discrete particle model anci hence the

lower predicted values for storage modulus.

Fig 5.6 Sca~~ilig electron micrograph of dynamically

cured 70130 PP/NBR with DICUP

Fig 5.7 Scaning electron micrograph of dynamically

cured 70130 PP/NBR with S+DICUP

Mixed cure system: It is interesting to note from Figures 5.3 and 5.4 that the mixed

cure system agrees with the prediction for both storage modulus and loss tangent

much better than the other systems. In this system, the NBR phase is crosslinked by

suIphur and Dicup; also occasional crosslinking between NBR and YP is possible. As

noted earlier, when sulphur alone is the curative, it reduces the storage modulus of the

blcnd whereas Dicup alone enhances the same property. One linds that the net result

of these opposing effects un the storage modulus of the mixed cure blend is predicted

correctly by Kerner Discrete particle model.

From these facts we can say that the size of the domains or its alterations may

not be the reason for the significant deviation of the properties in the NBR transition

zone as observed in the case of physical blends (chapter3, section 3.2) (Kerner's

theory says that the dispersed particles be spherical and randomly distributed). It may

be due to the much difference in relaxation behaviour between the NBR and PP

polymers in the blend leading to poor interface adhesion.

SECTION 5.2

Nylon6 and ethylene

propylene rubber (EPM)

TPV blends

5.2.1 Introduction

The development of morphology in the case of dynamic wlcanisation of

50150 EPWnylon 6 blends can be schematically represented as (fig 5.8):

Fig 5.8 Schematic rqn'esenlation of morphology dnzlopmnt of 50150 EPMInylon 6

owing to dynamic curing

Here, the continuous EPM rubber phase changes to uniform dispersed particles

while the continuous nylon 6 remains unchanged. As a result the blend exhibits the

rheological properties of thermoplastic while enjoying the desirable properties of

higher incorporation of EPM.

A number of different curator chemicals are used in this study (given in the next

section), they are expected to produce C-C crosslinks in the EPM phase of

vulcanisate as given in Figure 5.9:

Fig 5.9 C - C Linkages

In this section, we investigate the applicability of Kerner's models for predicting

dynamic properties and morphology of dynamically cured 50150 EPMInylon 6

blends

5.2.2 Experimental

Nylon 6 (molecular weight (Mw) 24,000 and density 1.14) of DSM and EPM

(ethylene propylene copolymer of molecular weight (Mw) 80,000, density 0.96

@cc and 78% ethylene mutent) of EXXON Chemical Company are used for the

present invedgation The experimental details of preparation of dynamic vulcanised

blends and characterisation have been described earlier In.

Details of arring agents used are given below (notations used are given in brackets

against each chemical).

1,6-diamino hexane(A2); 3,4- Dimethyl 3,4-diphenyl hexane, perkadox 58 paste

(PA2); cumme hydroperoxide ( CHP), 2,s-dimethyl 2,s di ( tert-butyl peroxy)

hexyne (CP2), and 2,s bis( tert - butyl peroxy) 2,S-dimethyl-3-hexyne ( CTX2)

The blends were prepared by melt blending technique using Haake Mixer under Nz

environment at a temperature of 250 'C and a rotor speed of 100 rpm for 10

minutes. All the polymers and the curaton (2 % by weight) for each blend are fed

to the mixer simultaneously so that the cure reaction takes place while being mixed.

Dynamic mechanical measurements &re carried out on a DMTA machine. The

experiments were conducted in uniaxial tension mode kom -70 to 150 O C at a

frequency 10 Hz. SEM scans were taken on a PHILLIP'S Model scanning electron

microscope operating at 10 kV with cryogenically hctured samples.

The blends used for investigation are designated as:

EPM/NYLON6: 50150 A2 (I,&diamino hexane)

EPMlNYLON6: 50150PA2 (3,4- Dimethyl 3,4-diphenyl hexane, perkadox58 paste)

EPMNYLON6: 50150CHP (cumene hydroperoxide)

EPM/NYLON6:50/50CP2 ( 2,5-dimethyl2,5 di ( tert-btrtyl peroxy) hexyne }

E P m O N 6 : 50150CTX2 (2,s bis( tert - butyl peroxy)2,5-dimethyl-3-heGne)

5.2.3 Results and discussion

It is observed from the Figure (5.10 and 5.1 1 .) for storage modulus and loss

tangent that all the cure systems give similar response. Differences in the

mechanical response of the blends lie in narrow band. However, it is noted that' the

system with PA2 has given somewhat lower data for storage modulus (which falls

bel.ow those of EPM) in the low temperature range. The lower band of the storage

modulus data range belongs to the system with A2 curator.

Best fit (polynomial) data of dynamically cured EPM and of nylon 6 for storage

modulus a d loss tangent over the temperature range study are used to calculate the

model response. The best-fit data are shown as continuous line in the corresponding

experimental data set in the Figures 5.10 and 5.1 1.

Fig5.16 Dependence of Storage modulus on temperature for EPLI/NYLONG dynamic vulcanised blends ( f = 1 0 ~ z )

10' - 2 - 0 - 10' w 1 - 3 w

o 7 Q u m L 0 + rA

10'

10'

"..- ."...m"..."..

. . . epm-dy2 - epm-dyi! f i l . . . nylon8

. .

. nylon6 f i t . . . epm/nylon 50150 A2 "\

. . ., epmlnylon 50150 PA2

. , , epm/nylon 50150 CP2 epmlnylon 50/50 CTX2 . . . . . . epmlnylon 50150 CHP

-100 -75 -50 -25 0 25 50 75 100 125 150

Temperature (OC)

. . . epm-dy2 - epm-dy2 fit . . . nylon6 -- .. nylon4 f i t . . . epm/nybo 50150 A 2 . , epm/y lon 50150 PA2 + , , epmlnylon 58\50 CPZ

e p m l n y l u ~ 50150 CTIZ . . . epm/njlon 50/5O CW

Temperature ('c) Fig 5.1 I . Dependence of loss tangent on

temprntare for EPY/NYLON6 dynamic rulcanised blends (f=tOHz)

The experimental data of the above blends are compared with predicted results in

the Figures 5.12 and 5.13.

- e5OaZrlO - epm a s matr ix - nylon8 ar matrix - p o i j a g g r e ~ a t e 1

Temperature (OC)

Fig 5.12 Comparison of experimental and computed storage modulus data s t dillerent temperatures for dynamic rulcasired epmlnylon8 blend (1-10~1)

- epm as matr ix - nylon6 a s matrix

0.80 - p o l j r ~ g r c g a t e

Temperature ('c) Fig 5.13 Comparison of experimental and computed loss

tangent data a t different temperature* fo r dynamic ruleanired rpm/nj lon6 blend (l=lOHz)

It is observed that the experimental storage modulus data agrees comparatively well

to the discrete particle model prediction with nylon 6 as matrix. However, the

prcdictions slightly overestimate the experimental results. The other two calcu~ations

i.e., discrete particle with EPM as matrix and the polyaggregate, are far from the

experimental data over the temperature range beyond the first transition.

Rcfcrring to the loss tangent calcuIations, it is observed from Figure.5.13 that the

predictions are of higher magnitude. However, the trend is similar to thc experimental

results regarding the two transitions.

SEM investigations (Figure 5.14) reveal dispersed phase morphology. Spherical

particles of EPM rubber are dispersed in the matrix of nylon 6. This supports the

predicted structure of the blend from computations of Kerner mode1 using dynamic

data of component polymers. 'The model predictions in the case of physical blends

were discussed earlicr (chapter3, section 3.5.). A co-continuous packed grain structure

using Kerner's polyaggregate model was found to suitably predict the blcnd

propcrties. In the case of dynamic vulcanisation, where thc purpose is to load

maximum of rubbery phase to impart Illore and more elastomeric propcrties (while

still retaining the thermoplastic processibility), the rubber phase gets discretisied as

revcaled in the SEM photomicrographs,

Fig: 5.14 change of tnorphology (a,a I ) co-continuous to

- (b.bl) dispersed phiisc on dynamic vulcanisation.

The blend experimental dynamic propexties (storage mbdulus and loss tangent) are

found to agree satisfactorily with the discrete par&icle predictions with nylon 6 as

matrix. The change over of the co-continuous morphology of the physical blends to

dispersed phue morphology of dynamic vulcanised blend is neatly modelled by the

Kerner theory

Since all the dynamic vulcanised blends given similar dynamic response

behaviour imspective of the curator type (individual dynamic responses lying in a

narrow band) comparison is made using data of dynamicaily cured EPM with one

water only This would not introduce any error on the nature of predictions with

respect to overall blend morphology.