ppg plasticizer mech. & thermal pprties of green pla comp. with natural fillers 2008

Mechanical and ThermaPolylactide Composites w

t Msk

Introduction

In recent years a considerable research effort has been

devoted to the formulation and characterization of new

ecologically friendly plastic materials. It has been reported

that thermoplastic polymers can be compounded with

natural fibers to reduce costs of production while

maintaining their properties.[1] In particular, composites

of biodegradable polymers, such as polycaprolactone,

poly(hydroxybutyrate), polylactides, and copolymers, with

natural fillers appear very promising for the production of

intrinsic characteristics of a biodegradable thermoplastic

polyester.[36] PLA is rigid and brittle below the glass

transition temperature (Tg 5060 8C) but plasticizationimproves its tensile behavior and impact properties.

Several plasticizers for PLA are known and among those

are citrate esters,[7,8] poly(ethylene glycol) (PEG),[8,9] and

poly(propylene glycol) (PPG);[1012] the last one was found

to also be effective in the case of semicrystalline PLA.[11,12]

Biodegradable composites of PLA with flax fibers, micro-

crystalline cellulose, wood fibers, wood flour, cellulose

fibers, cellulose nanowhiskers, and hemp fibers have also

kenaf fibers[21] and bamboo fibers[22] were also recently

applied. Usually the modulus of the composite was

-

Full Paper

E. Lezak, Z. Kulinski, R. Masirek, E. Piorkowska, K. Gadzinowska

eraitiepucys

temperature. They also affected the mechanical propertiesof the compositions, increasing the modulus of elasticity

nsile impactstrength although with few exceptions. Plasticization ofthe PLA matrix improved the ductility of the composites.

1190improved, whereas the elongation at break decreased as

compared to neat PLA. To modify the mechanical proper

ties of PLA-based composites, plasticization has been

employed. Oksman et al.[14] reported a decrease of tensile

stress and increase in the elongation at break of PLA/flax

Centre of Molecular and Macromolecular Studies, PAS, Sienkie-wicza 112, Lodz 90-363, PolandE-mail: [email protected]. PracellaInstitute for Composite and Biomedical Materials, IMCB-CNR,Sect. of Pisa, Via Diotisalvi 2, Pisa 56126, Italymaterials with low environmental impact.[2] Polylactide

(PLA) has recently gained growing attention because of its

been examined.[1319] Methods of reactive compatibiliza-

tion of composites of PLA with cellulose fibers were

described by Braun et al.[20] To reinforce PLA, plant-derivedbut decreasing the elongation at break and teEmil Lezak, Zbigniew Kulinski, RoberMariano Pracella, Krystyna Gadzinow

Green composites of PLAwithmicropowders dhusks, cocoa shells, and apple solids that remmixing. The thermal and mechanical propermatrix crystallization and plasticization withene glycol), have been studied. All fillers ncrystallization and decreased the cold-crMacromol. Biosci. 2008, 8, 11901200

2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheiml Properties of Greenith Natural Fillers

asirek, Ewa Piorkowska,*a

ived from agricultural by-products such as oatn after pressing have been prepared by melts of the composites, including the effect ofoly(propyl-leated PLAtallizationDOI: 10.1002/mabi.200800040

with hydrophobized ground chalk. Samples of all composi-

tions with either an amorphous or crystalline matrix were

the supplier web page (www.microfood.pl). Cocoa shells are

known to contain about 32 wt.-% of cell wall polysaccharides,

with a plasticized matrix, PLA was first blended for few minutes

with 10 wt.-% of PPG. Next, an appropriate amount of filler was

added and mixed with the plasticized PLA. In the composite

notation employed here, for instance PLA/CC10 or PLA/P/CC10, the

number stands for a filler weight fraction while the P letter

indicates plasticizationwith PPG. Allmaterialswere then stored in

a dry atmosphere in desiccators at ambient temperature.

Measurements

The molar mass of neat PLA and PLA in the PLA/P blend and in the

composites was determined by means of SEC. To determine the

molecular mass of PLA, SEC traces were recorded with an Agilent

(Santa Clara, CA, USA) series 1100 isocratic pump, a degasser, an

autosampler thermostatic box for the columns, and a set of TSKgel

Mechanical and Thermal Properties of Green Polylactide Composites with . . .prepared by varying thermal history. Cold crystallization

was chosen as the crystallization method because it leads

to more intense spherulite nucleation, which results in a

shorter crystallization time and smaller spherulites.[25]

PPG was used as a PLA matrix plasticizer to enhance the

drawability of the compositions. Although the paper is

mainly focused on the mechanical properties of the

composites, their thermal behavior and morphology are

also investigated.

Experimental Part

Materials

PLA manufactured by Cargill-Dow Inc. (Minnetonka, MN, USA)

with a D-lactide content of 4.1% and a residual lactide content of

0.1%, was used. The weight-average molar mass Mw and

polydispersity Mw=Mn of PLA, determined in dichloromethane

by size exclusion chromatography (SEC) with a multi-angle laser

light-scattering (MALLS) detector, were 126 kg mol1 and 1.48,respectively. PPG with a nominal Mw of 425 g mol1 waspurchased from SigmaAldrich, Inc. (USA). Matrix-assisted laser

desorption/ionization (MALDI) time-of-flight (TOF) techniques

helped to establish that theMw andMw=Mn of PPG were equal to

530 g mol1 and 1.03, respectively.[10]Ground fillers derived from oat (Avena sativa) husks (OC), cocoa

(Theobroma cacao) shells (CC), and solids that remained after

pressing juice out of apples (AC) were supplied by Microfood

Poland (Warsaw, Poland). Ground chalk (CH) coated with stearic

acid was obtained from the same supplier. Figure 1 shows particle

size distributions determined for the micropowders by means of a

Coulter LS230 particle size analyzer (Beckman Coulter, Inc.composition plasticizedwith triacetine. On the contrary, in

our previous studies[18] plasticization of a PLA matrix with

PEG did not improve the tensile properties of PLA/hemp

composites.

Recently, agricultural by-products, for instance corn

husks,[23] have gained attention as a source of cellulose

fibers, as such they can save the land and other natural

resources required to grow fiber crops. The enormous

production of agricultural by-products worldwidemakes it

attractive to utilize them as components of biodegradable

composites. Recently, Mohamed et al.[24] studied the

thermal properties of PLA composites with apple fibers

and sugar beets.

In the present work we prepared and studied compo-

sites of PLA with natural micropowders that contain

cellulose derived from agricultural by-products: i.e., oat

husks, cocoa shells, and solids that remain after pressing

the juice out of apples that are used as dietary supple-

ments because of the high dietary fiber content. For

comparison, we also examined composites of the same PLAFullerton, CA, USA) based on a light scattering technique. The

Macromol. Biosci. 2008, 8, 11901200

2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimcomposed of 35 wt.-% of cellulose, 20 wt.-% of hemicellulose, and

45 wt.-% of pectin.[26] A protein content determined in ref.[26] was

approx. 20 wt.-%. The chalk contains 95 wt.-% of CaCO3.

Composite Preparation

Before use the polymers and the fillers were dried under vacuum

at 100 8C for 4 h. Compositeswith 5, 10, and 20wt.-% of fillerswereprepared in a Brabender (Duisburg, Germany) internal mixer,

operating at 190 8C and 60 rpm for 20min, under a flow of gaseousnitrogen. Also neat PLAwas processed in the sameway to obtain a

referencematerial. Amelt-blend of PLAwith 10wt.-% of PPG (PLA/

P) was obtained under the same conditions. To prepare compositesvalues of the particle size when the percentage of a cumulative

distribution reaches 50 (D50) and 97% (D97) were: 68 and 251 mm

for AC, 24 and 157 mm for OC, 16 and 76 mm for CC, and 4.7 and

20.3 mm for chalk micropowder, respectively. The OC filler

contains 50 wt.-% of cellulose, 25 wt.-% of hemicellulose,

35 wt.-% of lignin, 4 wt.-% of proteins, 1.8 wt.-% of fat, and 3.2

wt.-% of digestible carbohydrates. The respective numbers for the

AC filler are: 15, 10, 16, 1.6, 6.4, and 24 wt.-%. Further details of the

chemical composition of AC and OC micropowders are given on

Figure 1. Size distributions of OC, AC, CC, and chalk (CH) fillerparticles.columns (G 2000 HXL and G 6400 HXL) at 25 8C. An Optilab rEX

www.mbs-journal.de 1191

gauge length of 40 and width of 5mm) at the rate of 2 mm min1using an extensometer. Specimens for tensile impact tests had

shapes that conformed to ISO 8256: total length of 80mm, narrow

section length and width of 30 and 10 mm, respectively. The tests

were performed on 810 samples of each material using an

instrumented impact tester CEAST (Charlotte, NC, USA) Resil 5.5,

with maximum hammer energy of 1 J and velocity of 2.9 m s1.The 0.7 mm thick specimens were used in order to fit the absorbed

energy level to the measurement range of the hammer.

Results and Discussion

Molar MassThe Mw and Mw=Mn of neat PLA, PLA/P, and the

compositions with 20 wt.-% of fillers are listed in

Table 1. Processing decreased the Mw of the neat PLA to

102 kg mol1, whereas the polydispersity changed to 1.27.

E. Lezak, Z. Kulinski, R. Masirek, E. Piorkowska, M. Pracella, K. Gadzinowska

PLA/OC20 85 64 1.33

1192interferometric refractometer and a MALLS Dawn Eos laser

photometer (Wyatt Technology Corp., Santa Barbara, CA, USA)

were applied as detectors. Dichloromethanewas used as an eluent

at a flow rate of 0.8 mL min1. Mn and Mw=Mn were calculatedfrom the experimental traces with the Astra program (version

4.90.07) from Wyatt Technology.

The phase transitions were studied by means of differential

scanning calorimetry (DSC) with a TA Instruments (New Castle,

DE, USA) 2920 DSC. Samples of 10 mg were rapidly heated to

180 8C, annealed at the molten state for 3 min, cooled at10 8C min1 to 0 8C, and subsequently heated at 10 8C min1.The entire treatment was carried out under a flow of dry gaseous

nitrogen. The glass transition temperature, Tg, was measured as

the temperature that corresponded to the midpoint of the heat

capacity increment. A crystallinity level was determined from the

crystallization enthalpy,DHc, and the melting enthalpy, DHm,

assuming the PLA enthalpy of fusion of 106 J g1.[27]To have a direct insight into crystallization, compression

molded 1030 mm thick films of composites with 10 wt.-% of

fillers, either plasticized or unplasticized were rapidly heated to

180 8C, melt annealed for 3 min and cooled at 30 8C min1 to thetemperature of 116 and 112 8C, respectively, and crystallizedisothermally. The entire procedure was conducted under a flow of

dry gaseous nitrogen, in a hot stage Linkam (Waterfield, United

Kingdom) THMS 600mounted in a polarized lightmicroscope, that

allowed direct observation of the crystallization.

For examination of the mechanical properties 0.71 mm thick

films of all materials were prepared. The films were compression

molded at 180 8C for 3 min in a hydraulic hot press and quenchedbetween thickmetal blocks kept at room temperature; theywill be

referred to throughout the paper as quenched films. In order to

preparefilmswith a crystallinematrix, someof the quenchedfilms

were cold-crystallized upon heating at 10 8C min1 from roomtemperature to a selected final temperature, at which they were

annealed to allow the crystallization to complete. The thermal

treatment was carried out between two metal blocks equipped

with heaters and Pt resistance thermometers connected to a

temperature controller; details of the method are given else-

where.[9] The ranges of annealing temperature and timewere 130

140 8C and 1 h for unplasticized materials, respectively, and 120130 8C and 30 min for plasticized materials, respectively. Thesefilms will be referred to throughout the paper as annealed films.

Both types of films, quenched and annealed, were characterized by

a DSC technique during heating at a rate of 10 8C min1.The crystal structure of the annealed filmswas probed bywide-

angle X-ray diffraction (WAXD) in the reflection mode while the

long period was determined by two-dimensional (2D) small-angle

X-ray scattering (SAXS) utilizing the equipment and methods

described in ref.[9].

All mechanical tests were performed at constant room

temperature on specimens cut out from the films. Uniaxial

tensile tests were carried out with an Instron 5582 tensile

machine (Lancombe, United Kingdom) following the ASTM D638

procedure, on oar-shaped specimens, with a gauge length of

10 mm and width of 5 mm. Three to five samples of each material

were drawn at a rate of 0.5 mm min1. The fracture surfaces of allmaterials were coated with gold and analyzed in a Jeol 5500LV

(Tokyo, Japan) scanning electron microscope. The modulus ofelasticity was measured on larger oar-shaped samples (with a


2008 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimPLA/AC20 78 64 1.22

PLA/CC20 76 61 1.25

PLA/CH20 99 72 1.38

PLA/P 93 71 1.31

PLA/P/OC20 73 57 1.3

PLA/P/AC20 69 63 1.1

PLA/P/CC20 75 58 1.29

PLA/P/CH20 66 50 1.32Blending with PPG lowered the Mw and Mw=Mn to

93 kg mol1 and 1.31, respectively. The Mw and Mn ofPLA also diminished in all other materials, however, more

so in the plasticized than in the unplasticized composites.

Mn decreased less than Mw, which resulted in polydis-

persity in the range of 1.101.38. Molecular characteristics

of the PLA in the compositeswithOC fillerwas less affected

than in the compositeswith AC and CC fillers. On themolar

mass distributions recorded for the plasticized materials

appeared small separate peaks that originated from PPG.

Degradation of PLA under the processing conditions was

obviously enhanced by PPG. Others reported a noticeable

decrease ofmolecularmass of PLA upon blendingwith PEG

plasticizer.[28] Many other studies on the thermal degrada-

tion and thermal stability of PLA have been reported.[29]

The chemical impurity of PLA, e.g., the presence of residual

Table 1. Molar mass of neat PLA, PLA in PLA/P blend and in thecomposites with natural fillers and chalk.

Material code Mw Mn Mw=Mn

kg molS1 kg molS1

PLA 102 80 1.27DOI: 10.1002/mabi.200800040

catalysts, can play a role in the reduction of molar mass

during processing. Several other factors such as mechan-

ical blending and shearing processes during compounding

of PLA with fillers and the presence of hydroxy groups of

components can contribute to the molar mass reduc-

tion.[28]

Phase Transition BehaviorThe DSC heating thermograms shown in Figure 2a and 2b

exhibit three main transitions successively: a glass

transition, a cold crystallization exotherm, and a melting

endotherm. The measured values of the phase-transition

parameters are summarized in Table 2. The Tgs of the

unplasticized compositions was in the range of 5855 8C,with a tendency to decrease with an increase of natural

filler content. The temperature of the cold crystallization

Mechanical and Thermal Properties of Green Polylactide Composites with . . .Figure 2. The second DSC heating thermograms for: a) PLA andunplasticized PLA composites with natural fillers and chalk, andb) PLA/P and PLA composites with natural fillers and chalk, plas-

ticized with PPG. Filler type and content indicated in the figures.


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimpeak, Tc, of the unplasticized composites with 10 wt.-% of

fillers was slightly higher than that of neat PLA, but an

increase of the filler content to 20 wt.-% diminished it

markedly, especially in PLA/AC20. The melting peak

temperature was in the range of 152154 8C. The decreaseof Tc was accompanied by a slight decrease of melting

temperature. The lowest Tc, observed for PLA/AC20,

resulted in double peak melting behavior, which suggests

the occurrence of reorganization phenomena of the

crystals during the heating run.[9,10] The lower Tc was

accompanied by a markedly enlarged DHc, which resulted

from a longer time available for completion of crystal-

lization. The DHc increased from 13 to 35 J g1 of PLA,which corresponded to a crystallinity level in the range 12

33%. DHc and DHm were equal, which indicated that neat

PLA and the unplasticized composites were amorphous

after cooling.

It can be noticed that the addition of PPG causedmarked

changes in the phase transition behavior of the polymer

matrix. Tg was reduced to values in the range of 3641 8C;the lowest values were measured for plasticized compo-

sites with 20 wt.-% of fillers. The Tc shifted to 111 8C in thecase of PLA/P, and to 94104 8C for the plasticizedcomposites. Moreover, the crystallization peaks appeared

very sharp and intense in comparison with the unplas-

ticized samples because of the faster crystallization. Both

effects are related to acceleration of spherulite growth rate

as a result of the plasticization with PPG, as observed in

ref.[11] An increase of a filler content caused a Tc shift to

lower values, larger in the case of natural fillers than in the

case of chalk. The melting endotherm of the plasticized

materials exhibited two peaks, the first in the range of

139146 8C (Tm1) and the second in the range of 152154 8C(Tm2). The decrease of Tc was accompanied by a lower Tm1and Tm2. The presence of the two melting peaks has been

attributed in the past to lamellar reorganization likely a

result of less stable crystals formed at a relatively low

temperature.[9,10,30] DHc corresponded to DHm, except for

PLA/P/OC20, PLA/P/AC20, and PLA/P/CC20 for which

DHm DHc ranged from 1.5 to 3 J g1, which indicatesthat a low crystallinity level, approx 1.53 wt.-%, was

developed during cooling. The other plasticized composites

and PLA/P blends were entirely amorphous prior to

heating in the DSC. DHm was in the range of 2733 J g1of the polymer matrix, which corresponded to a crystal-

linity level in the range of 2531%; the largest values were

measured for PLA/P/AC20 and PLA/P/CC20. These compo-

sites crystallized at the lowest temperature, about 94 8C. Itis possible that the crystallinity of the matrix increased as

a result of annealing during post-crystallization heating,

that was facilitated by a longer time interval between the

crystallization and the melting. A change of the molecular

characteristics of PLA during processing could also[31]contribute to the decrease of Tc and increase of DHm.



th n

of P

.0)

.4)

.3)

.6)

.1)

.8)

.0)

.1)

1194Table 2. Calorimetric data for PLA, PLA/P, and PLA composites wi

Material code Tg Tc DHc of PLA (DHc

-C -C J gS1

PLA 59.2 124.7 25.3

PLA/OC10 58.3 129.8 12.6

PLA/OC20 57.9 122.5 26.6

PLA/AC10 58.2 127.7 18.4

PLA/AC20 55.3 116.1 35.2

PLA/CC10 58.3 125.2 20.9

PLA/CC20 57.7 122.6 29.6

PLA/CH10 59.4 127.1 19.2

PLA/CH20 59.2 122.3 25.8

PLA/P 38.9 111.1 31.0 (28

PLA/P/OC10 36.6 102.4 31.6 (28

PLA/P/OC20 36.2 96.9 31.4 (28

PLA/P/AC10 40.7 105.0 32.8 (29

PLA/P/AC20 36.7 94.1 33.5 (30

PLA/P/CC10 38.5 101.8 29.7 (26

PLA/P/CC20 36.0 93.6 31.1 (28

PLA/P/CH10 40.7 105.2 33.4 (30Others have reported that the spherulite growth rate of

poly(L-lactide) increased with a decreasing molecular

mass.[32] However, we did not observe a clear correlation

between the Mw of the polymer matrix and the Tc of the

composites. Moreover, the marked drop of Tc with the

increasing filler content is suggestive of the nucleation

activity of fillers. Indeed, the nucleation activity of the

fillers was evidenced by light microscopy observations of

the crystallization of the composites. On the micrographs

shown in Figure 3 and 4 for the unplasticized and

plasticized composites, respectively, small filler particles

are discernible in some spherulite centers, whereas large

filler particles are frequently surrounded by transcrystal-

line layers. Therefore, it seems that the nucleation of PLA

crystals on filler particles played an important role in

decreasing Tc during non-isothermal cold crystallization of

the composites.

Exemplary DSC heating thermograms of the quenched

films prepared for themechanical tests, plotted in Figure 5,

are similar to those shown in Figure 2a and 2b. The differ-

ences in enthalpy relaxation at the glass transition could

result from the different times elapsed before DSC testing.

The Tg of the unplasticized and plasticized materials was

within the range of 5458 and 3539 8C, respectively,whereas Tc was in the range 112124 and 8098 8C,respectively. While the glass transition temperatures were

PLA/P/CH20 36.1 102.7 32.9 (29.6)


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimatural fillers and chalk upon the second heating at 10 8C min1.

LARPPG) Tm DHm of PLA (DHm of PLARPPG)

-C J gS1

153.6 25.3

153.6 12.6

152.3 27.0

153.7 18.6

148.8, 155.0 35.5

153.1 20.3

152.2 30.1

153.9 19.4

153.1 25.9

146.2, 152.8 31.0 (28.0)

142.3, 152.7 31.6 (28.4)

140.4, 152.5 32.6 (29.4)

144.3, 153.6 33.3 (30.0)

138.6, 152.2 36.2 (32.6)

142.4, 152.9 30.5 (27.4)

138.6, 152.1 34.6 (31.2)

144.1, 153.6 32.8 (29.6)similar, the crystallization temperatures of the quenched

composites were usually lower than those listed in Table 2

for specimens cooled at a constant rate of 10 8C min1,likely a result of the different thermal history that affected

the nucleation of crystallization. The decrease of Tc was

reflected in a lower melting temperature and double

melting peak behavior. The plasticization not only

decreased Tc but also narrowed the crystallization peaks,

which is similar to the case of specimens cooled in the DSC.

DHc of the quenched unplasticized films, within the range

of 2533 J g1 of PLA, correspond to the DHm, whichindicates that the films were essentially amorphous prior

to heating in the DSC. DHm of the quenched plasticized

films was in the range 2831 J g1 of the polymer matrix.These values correspond to the crystallinity levels of

2431 and 2629%, respectively. Moreover, the DHm of

plasticized composites exceeded the DHc slightly, by

approx. 13 J g1, which indicated a development ofcrystallinity, although below 3%, during quenching from

the molten state.

Exemplary DSC heating thermograms for the annealed

films, shown in Figure 6, exhibit a glass transition and a

melting endotherm. The Tg determined from the thermo-

grams was in the range of 5559 and 3439 8C for theunplasticized and plasticized films, respectively. It was

similar to that of the quenched films with a corresponding

141.8, 153.5 32.7 (29.5)

DOI: 10.1002/mabi.200800040

which crystallized in the a-form

Mechanical and Thermal Properties of Green Polylactide Composites with . . .composition, also in the case of the plasticizedmaterials in

spite of an increased plasticizer content in the amorphous

phase as a result of crystallization. Moreover, the glass

transition in the plasticized films was broad and hardly

recognizable. Similar phenomenon was observed and

discussed in ref.[9,11] as related to the redistribution of a

plasticizer during crystallization and a possible phase

separation. The lack of a cold-crystallization exotherm

indicated that crystallization was completed in all the

annealed films prior to heating in the DSC.

The melting peaks of the unplasticized films were

centered at 153158 8C. The peaks exhibited shoulders onascending slopes. PLA/P exhibited a single melting peak

centered at 150 8C, while the thermograms of plasticizedcomposites were featured by two melting peaks with Tm1and Tm2 in the ranges of 140144 and 151154 8C,respectively. The DHm of unplasticized materials was in

the range of 4246 J g1 of PLA, which corresponds to thecrystallinity level of 4043%.DHmof the PLA/Pmatrix in the

plasticized materials, in the range of 3740 J g1, corre-sponds to the crystallinity level of 3538%. These crystal-

linity levels in the annealed films are higher than those

developed in the quenched films during heating in the DSC.

Obviously, isothermal annealing of the films enhanced the

crystallinity of PLA by increasing the time for a completion

thickness decreased

unplasticized mater

their different meltin

Mechanical PropertiExemplary stress/stra

whereas mechanical

Table 3. Except for

unplasticized films f

elongation at break (eb) not exceeding 0.06. Although thePLA/CH5 yield stress (sy) of 47.7MPawas lower than the syofneat PLA (56.2MPa

of neat PLA (approx.

content resulted in a

at break (sb) genera

modulus of elasticity

strength (U) decrease

be noticed that the P

featured by relatively

kJ m2, exceeded thIn general, crystal

stiffness and brittlen

whichwas reflected i

andU. It can be notic

Figure 3. Polarized light micrographs of PLA spherulites during isothermal crystallizationfrom melt in: a) PLA/AC10, b) PLA/CC10, c) PLA/OC10, and d) PLA/CH10 at 116 8C.


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim), its eb of approx.0.09didnot reach that0.13). In all cases, an increase of filler

decrease of eb of the composites. Stresslly did not exceed the sb of PLA. The

(E) increased but the tensile impact

d with increasing filler content. It can

LA/CC and PLA/CH composites were

high E and U; the U of PLA/CH5 at 71

at of neat PLA (62 kJ m2).lization of the PLA matrix enhanced

ess of the neat PLA and composites,

n increased E and in diminished eb, sb,typical of calcite. The long period

determined by the SAXS for all the

materials was 20 0.5 nm. Thus, thelower crystallinity of the plasticized

materials infers that the lamellae

as compared with that in the

ials, which was indeed reflected in

g behavior.

esin curves aredrawn inFigure8aandb,

data (average values) are collected in

neat PLA and PLA/CH5 all quenched

ractured early before a yield, at andescribed as orthorhombic. [33] The

diffractograms of chalk composites

exhibited additional sharp peaks,of conversion of the melt into spher-

ulites and/or for increasing lamellae

thickness and perfection, which was

reflected in the elevation of Tm. The

crystallinity change was larger in the

unplasticized than in the plasticized

films. Moreover, in the latter the

melting started at approx. 100 8C,which indicated a presence of less

stable crystals likely formed during

post-annealing quenching to room

temperature.

X-Ray MeasurementsExemplary WAXD scans of annealed

films, shown in Figure 7, evidenced

that the fillers did not affect in any

way the crystallographic form of PLAed that only the annealed PLA/CH and


PLA/P/CC films. The plasticization

improved the U of the quenched

PLA/P blend and the PLA/P/CH com-

posites; the U of PLA/P/CH5 compo-

site at 89 kJ m2 was slightly largerthan that of PLA/P blend (84 kJ m2).

The plasticization improved the

ability to the plastic deformation in

all the annealed materials, although

to a differing extent. Crystallization

of the plasticizedmaterials decreased

sy, although less so for the compo-

sites with natural fillers than for the

PLA/P blend. However, with the

exception of PLA/P/CC and PLA/P/

CH10, sy was lower for the filled

systems than for PLA/P. In PLA/P/AC

and PLA/P/OC composites, sy deter-

mined with 2% offset due to lack of a

pronounced yield point, decreased

slightly with the increasing filler

content. As sy decreased, the sb of

the plasticized materials either

remained at the same level or

increased as a consequence of differ-

ent tensile behavior due to crystal-


sothermal crystallization

1196Figure 4. Polarized light micrographs of PLA spherulites during iPLA/CC films exhibited E larger than that of the annealed

neat PLA (4 250 MPa). In particular, E of the PLA/CH

composites increased monotonically with the increase in

chalk to the value of 5 570 MPa. U was improved only in

the PLA/CH composites, however, not enough to reach the

level of the quenched neat PLA.

The plasticization improved ductility of both the

quenched and annealed films. All of them yielded and

underwent plastic deformation. Tensile stresses were

markedly lowered whereas eb was enlarged in comparisonwith values measured for the corresponding unplasticized

composites.

The quenched PLA/P blend yielded at a sy of 38.8 MPa

and reached a eb of 3. The sy of the quenched plasticizedcompositions was lower and decreased with the increase

of filler content. The eb of the composites with naturalfillers was lower than that of the quenched PLA/P, except

for the PLA/P/CC compositewith an eb of 3.3. The PLA/P/CHcomposites reached the largest elongation at break; the ebof PLA/P/CH5 and PLA/P/CH10, both above 4, exceeded

that of the PLA/P blend. In general, the eb of the quenchedplasticized composites decreased with the increase of filler

content. In a majority of cases, E was markedly reduced

because of the plasticization. However, E of PLA/P/CH10,

PLA/P/CH20, PLA/P/AC10, and of all PLA/P/CC composites

exceeded that of the PLA/P blend, being the highest for the

from melt in: a) PLA/P/AC10, b) PLA/P/CC10, c) PLA/P/OC10, and d) PLA/P/CH10 at 112 8C.

Figure 5.DSC heatingPLA composites withline) and plasticizedcontent indicated in


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimlization, as illustrated in Figure 8a

thermograms for quenched films of PLA andnatural fillers and chalk, unplasticized (solid(dashed line) with PPG. Filler type andthe figure

DOI: 10.1002/mabi.200800040

Mechanical and Thermal Properties of Green Polylactide Composites with . . .and 8b. Crystallization of the plasticized materials

diminished eb; the eb of the annealed PLA/P was 1.2, whilethat of the filled systems was even lower and decreased

with filler content. Nevertheless, it must be emphasized

that the tensile stresseswere reducedwhereas eb remainedmarkedly improved as compared to those of the annealed

films without the plasticizer. Crystallization diminished E

of the PLA/P blend by approx. 40%. E of the annealed

Figure 6.DSC heating thermograms for annealed films of PLA andPLA composites with natural fillers and chalk: unplasticized (solidline) and plasticized with PPG (dashed line). Filler type andcontent indicated in the figure.

Figure 7. WAXD diffractograms of PLA/P blend and PLA/CC10,PLA/AC20, PLA/P/CC10, and PLA/P/CH20 composites.


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimplasticized composites was generally higher than that of

the annealed PLA/P blend but remained below 2.4 GPa

being, therefore, lower than E of the other materials. With

the exception of PLA/P/AC composites, E increased with

the increasing filler content.

Crystallization decreased the U of the plasticized

composites with natural fillers. On the contrary, U was

improved in the PLA/P/CH composites. As a consequence,

the U of the annealed plasticized composites with natural

fillers was less, whereas the U of the annealed PLA/P/CH

composites was larger than that of the annealed PLA/P

blend (60 kJ m2). However, in general, the plasticizedannealed composites exhibited a higher U than the

respective unplasticized materials.

Figure 8. Stress/strain plots for PLA and PLA composites withnatural fillers and chalk, unplasticized and plasticized with PPG:a) quenched and b) annealed.



erseris

1198Table 3. Mechanical properties of quenched and annealed (numbbreak, modulus of elasticity, E, and tensile impact strength, U. Astof a sy value indicates fracture prior to yield.

Material code sy sb

MPa MPa

PLA 56.2 () 51.0 (42.8)

PLA/OC10 () 48.0 (43.3)

PLA/OC20 () 43.1 (43.8)

PLA/AC10 () 43.2 (39.2)

PLA/AC20 () 37.4 (32.5)

PLA/CC5 () 57.5 (45.3)

PLA/CC10 () 54.0 (48.3)

PLA/CC20 () 42.8 (48.5)

PLA/CH5 47.7 () 49.2 (48.9)

PLA/CH10 () 52.2 (52.8)Scanning Electron Microscopy (SEM)Exemplary SEMmicrographs of fracture surfaces of tensile

specimens are shown in Figure 9. SEM revealed features of

a rather brittle fracture of the tensile specimens of both

quenched and annealed PLA, identical to those shown in

ref.[9] for the same PLA drawn under similar conditions. On

the contrary, more plastic deformation is visible on the

fracture surfaces of the plasticized materials, especially

those with the amorphous matrix.

AC particles and a majority of OC particles, visible on

fracture surfaces, had irregular shapes but without a

significant shape anisotropy. Occasionally, fiber-like par-

ticles, with lengths greater than 100 mm were visible on

the fracture surfaces of the PLA composites with OC filler,

as shown in Figure 9d. On the contrary, a significant

fraction of CC particles had a plate-like shape as shown in

Figure 9e. Large particles, greater than 100 mm in size, were

absent in the PLA composites with CCmicropowder, unlike

in the composites with other natural fillers, which was

PLA/CH20 () 44.0 (45.3)

PLA/P 38.8 (32.4) 22.3 (28.6)

PLA/P/OC10 30.1 (29.1) 16.1 (27.1)

PLA/P/OC20 25.2 (25.2) 9.7 (24.1)

PLA/P/AC10 28.4 (25.5) 17.0 (25.7)

PLA/P/AC20 27.6 (23.6) 20.0 (22.8)

PLA/P/CC5 36.1 (33.9) 24.2 (28.3)

PLA/P/CC10 34.8 (32.2) 19.1 (30.2)

PLA/P/CC20 33.7 (33.3) 22.8 (30.5)

PLA/P/CH5 27.6 (27.7) 21.8 (20.2)

PLA/P/CH10 30.3 (32.4) 25.1 (28.0)

PLA/P/CH20 20.0 (25.5) 8.6 (22.9)


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim0.054 (0.031) 3 450 (3 930) 31 (30)

0.042 (0.031) 3 830 (4 110) 24 (29)

0.050 (0.022) 3 780 (4 180) 31 (28)

0.032 (0.018) 4 070 (3 990) 22 (26)

0.060 (0.035) 3 950 (4 570) 47 (35)

0.049 (0.041) 3 840 (5 100) 52 (38)

0.036 (0.032) 4 230 (4 640) 33 (24)

0.092 (0.066) 3 720 (4 480) 71 (53)

0.056 (0.031) 4 020 (4 760) 46 (30)in brackets) films: yield stress, sy, stress, sb, and elongation, eb, atk denotes the yield stress determined with 2% offset. A dash instead

eb E U

m mS1 MPa kJ mS2

0.130 (0.059) 3 740 (4 250) 62 (36)consistent with the particle size distributions shown in

Figure 1.

In Figure 9ch, a predominant feature is a separation of

filler particles from a polymer matrix, independent of filler

type and content. The plastically deformed polymer

formed walls that surrounded hollows with filler particles

inside. In particular, the same features were visible on the

fracture surfaces of PLA composites with chalk (not

shown), although the chalk particles were smaller than

the particles of the other fillers.

General Discussion

The PLA phase transition behavior was affected by the

fillers and the addition of PPG plasticizer. All the fillers

nucleated PLA crystallization, which decreased the cold-

crystallization temperature, especially in the plasticized

systems. Although the nucleation activity of the fillers is

suggestive of their good adhesion to PLA, the separation of

0.047 (0.033) 4 960 (5 570) 45 (47)

3.00 (1.20) 2 550 (1 490) 84 (60)

0.43 (0.12) 1 850 (1 420) 29 (37)

0.097 (0.084) 2 000 (1 730) 27 (27)

0.70 (0.14) 3 610 (2 220) 37 (31)

0.11 (0.11) 2 060 (1 780) 26 (23)

3.30 (0.25) 2 690 (1 730) 43 (35)

1.20 (0.16) 3 900 (2 078) 50 (43)

0.19 (0.088) 3 330 (2 440) 36 (29)

4.70 (0.52) 2 340 (1 830) 89 (89)

4.30 (0.38) 2 910 (1 970) 63 (72)

3.00 (0.13) 3 230 (2 120) 59 (69)

DOI: 10.1002/mabi.200800040

Mechanical and Thermal Properties of Green Polylactide Composites with . . .filler particles from the PLA matrix was evidenced in SEM

micrographs of fracture surfaces of tensile specimens.

However, not all the filler particles nucleated crystal-

lization of PLA. Particles, even of the same natural filler,

may differently interact with PLA because of their

different composition and surface properties, especially

when grinding at the factory may expose different

surfaces. We note that microcrystalline cellulose and

wood flour,[17] as well as hemp fibers[18] have already been

found to nucleate PLA crystallization.

In general, filling improved the stiffness of the

composites but also resulted in a diminished eb, which

cized matrix, whic

However, with the

P/CH10, and PLA/

premature fractur

reasons for early

especially those w

large particles of

surfaces of these

The U of PLA/C

composites with

exhibited a relati

than the PLA/P b

Figure 9. SEMmicrographs of fracture surfaces of: a) quenched PLA/P, b) annealed PLA/P,c) quenched PLA/OC20, d) annealed PLA/OC20, e) quenched PLA/CC10, f) annealed PLA/CC10, g) quenched PLA/P/CC10, and h) annealed PLA/P/CC10.


2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwith natural fillers exhibited worse

drawability than the PLA/P blend, an

opposite effect was found in quenched

PLA/P/CH composites, characterized by

the large elongation at break in the

range of 34.7. They also exhibited a

relatively high U, which was improved

by crystallization.

The plasticization softened all the

materials. The lowest E values were

measured for annealed plasticized com-

posites. During crystallization of plasti-

cized materials, the PPG plasticizer was

exuded from the crystalline phase and

its concentration in the amorphous

phase increased from a nominal 10 to

15 wt.-% as can be judged from thecrystallinity level in those systems.

Obviously the further softening of the

amorphous phase overwhelmed the

effect of the formation of stiff crystals

and resulted in a marked drop of E, as it

has been similarly reported by us for

PLA and PLA/hemp composites plasti-

cized with PEG.[18] However, in the

plasticized composites, the filling com-

pensated to some extent the softening

of the polymer matrix.

Separation of particles from a matrix

during the tensile drawing facilitated

the plastic deformation of the plasti-

h is reflected in a decreased yield stress.

exception of quenched PLA/P/CH5, PLA/

P/CC5, the presence of fillers led to a

e during tensile experiments. One of the

fracture of the composites of PLA,

ith AC and OC fillers, could be incidental

a size above 100 mm, found on fracture

materials.

C and PLA/P/CC exceeded that for the

other natural fillers. PLA/P/CC also

vely good drawability and was stifferwas accompanied by a lower U, i.e.,

by decreased toughness. Crystallization

of the unplasticized films further en-

hanced the stiffness, reduced draw-

ability, and worsened tensile impact

strength.

The plasticization improved the duc-

tility of all types of composites as

reflected in the yielding and in

increased eb.Whereas all plasticized compositeslend. This might be related to smaller


particle sizes evidenced by the particle size distributions.U

of the PLA/CH and PLA/P/CH composites was relatively

high. The PLA/P/CH composites, especially those that were

quenched, also exhibited a better drawability than the

other composites. Because of the relatively high density of

chalk (2.6 g cm3), its volume was always lower than thatof the corresponding mass of the natural fillers (density of

cellulose

ppg plasticizer mech. & thermal pprties of green pla comp. with natural fillers 2008

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