Composites: Part A 41 (2010) 1628–1635

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Composites: Part A

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Agrofibre reinforced poly(lactic acid) composites: Effect of moistureon degradation and mechanical properties

M.J.A. van den Oever a,⇑, B. Beck b, J. Müssig b

a Wageningen UR – Food & Biobased Research, PO Box 17, 6700 AA Wageningen, The Netherlandsb Hochschule Bremen – University of Applied Sciences, Faculty 5/Biomimetics – Biological Materials, Neustadtswall 30, 28199 Bremen, Germany

a r t i c l e i n f o

Article history:Received 1 April 2010Received in revised form 9 July 2010Accepted 25 July 2010

Keywords:A. Polymer–matrix compositesB. Mechanical propertiesE. Injection mouldingPLA degradation

1359-835X/$ - see front matter � 2010 Elsevier Ltd.doi:10.1016/j.compositesa.2010.07.011

⇑ Corresponding author. Tel.: +31 317 480105; fax:E-mail address: martien.vandenoever@wur.nl (M.J

a b s t r a c t

Natural fibre reinforced PLA composites are a 100% biobased material with a promising mechanical prop-erties profile. However, natural fibres are hygroscopic whereas PLA is sensitive to hydrolytic degradationunder melt processing conditions in the presence of small amounts of water. Here, we determine theeffect of water content in undried and dried natural fibres on semi crystalline grade PLA degradation dur-ing processing as well as on the composite’s mechanical performance. The fibres evaluated are ramie, flaxand cotton, containing 6–9 mass% moisture in the undried state and 0.2–0.4 mass% in the dried state.Intrinsic viscosity and melt flow index analysis indicate that the effect of the different levels of moisturein the fibres have a similar and small effect on PLA degradation, PLA hydrolysis appears rather affected byfibre diameter. Morphology, flexural strength and stiffness and Charpy impact of the composites are notsignificantly affected by the water present in the undried fibres.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The continuously increasing environmental awareness duringthe past decades has resulted in the introduction of natural fibrereinforced polymer composites as common materials in e.g. auto-motive industry. NMT materials have been used since the 1990sand currently agrofibre based injection moulding compounds arefinding applications [1]. With the introduction of large scale pro-duction of polylactic acid (PLA), a biobased thermoplastic polymerhas become available that has properties that may provide a basisfor materials that are competitive with composites based on poly-olefins and other bulk polymers. At the same time, PLA promises tobe nearly CO2 neutral and reduces dependency on oil for the pro-duction of composite materials [2,3]. In fact, NEC and Unitika havealready started the marketing of kenaf–PLA composite mobilephone housing [4].

Similar to other polyester polymers, PLA is sensitive to hydroly-sis under melt processing conditions in the presence of smallamounts of water [5]. NatureWorks recommends that moisturecontent in the PLA during melt processing is lower than0.025 mass% [2]. This poses the question how PLA behaves duringprocessing into natural fibre polymer composites, since naturalfibres are hygroscopic and contain 6–10 mass% of moisture at50–70% relative humidity conditions. Several researchers of natu-ral fibre–PLA composites have dried the PLA as well as the fibres

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+31 317 483011..A. van den Oever).

before processing [6–17], others only refer to drying of the fibres[18]. Some do not mention drying of the material prior to process-ing at all, without further addressing potential PLA hydrolysis[19,20]. Nishino et al. and Petersson et al. only refer to drying ofthe fibres, however they produce PLA composites from solution,thus avoiding PLA to enter the molten state [21,22]. Wong et al.and Shanks et al. have dried their solution casted composites undervacuum at 50 and 60 �C for 2 h and 3 h, respectively, prior to con-solidation at 175 �C [23,24]. Due to the high fibre content of50 vol%, it may be expected that also residual water will be re-moved under these drying conditions.

Oksman et al. [25] show that during extrusion compounding offlax–PLA, the Mw of PLA only reduced from 97,000 to 80,000 g/molas determined with gel permeation chromatography (GPC) and thatmolecular mass distribution was unimodal. Liu et al. [8] state thatGPC indicates no PLA scission after a 4 min compression mouldingprocess at 180–200 �C after vacuum drying prior to moulding. Plack-ett et al. [26] have evaluated the effect of jute fibre drying prior toprocessing on the molar mass of PLA, however, they have not re-ported it’s effect on composite mechanical performance.

Despite no mentioning of drying of fibres and/or PLA prior tomelt processing by Bodros et al. they still find an improvementof strength of PLA composites with increasing fibre content up tothe level where fibre agglomeration takes place [20]. Shibataet al. have observed a constant strength with increasing fibrecontent [18]. Mathew et al. have not presented the strengthperformance of their cellulose fibre and wood fibre–PLA compos-ites [19].

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In order to particularly address the effect of natural fibre mois-ture content on PLA composite properties, in this paper a worstcase scenario for natural fibre–PLA composite production will bestudied by incorporation of undried agrofibres in pre-dried PLA.A best practice reference will consist of drying the fibres as wellas possible on lab scale directly prior to compounding. The effectof the drying of the natural fibres on the composites flexural andimpact properties has been discussed in short in a recent publica-tion [27]. In the present paper a more detailed study of the effect ofwater in the fibres on PLA chain scission has been evaluated byanalysis of intrinsic viscosity (IV) of the PLA. IV analysis of PLA mo-lar mass was preferred over GPC analysis because within several ofour PLA project we have been confronted with GPC results whichturned out to be misleading. The IV of various PLA grades relatesto the molar mass according to the Mark–Houwink equation[28–34]. The Mark–Houwink constants vary a lot between theseauthors. However, together with many other researchers [35–45],we believe that the IV of PLA may provide a good indication ofPLA molar mass reduction, caused by PLA hydrolysis. From allMark–Houwink values found in literature [28–34], those deter-mined by Dorgan et al. [34] yield the most median molar mass val-ues, Mv,

½g� ¼ 0:0153�M0:759v ð1Þ

where [g] represents the IV. This relation will be used in the presentstudy to obtain an indication of the number of scissions in the PLAmolecular chain due to hydrolysis.

The degradation of PLA during melt processing can also be eval-uated by determination of the melt flow index (MFI) values of thepolymer composites, if the contribution of the fibres to the MFIdoes not change at the same time. An advantage of this route isthat the MFI of a polymer composite can be easily determined.From the only small reduction in flexural strength of jute–PP com-posites upon 30 min heating at 200 �C [46] it may be concludedthat such natural fibre structure retains its integrity, and as a con-sequence retains its structure and effect on flow. Further, from thelow shear rate in MFI analysis it is not expected that fibre orienta-tion will take place during such analysis [47].

The added value of a composite most often includes it’smechanical performance. Therefore, the effect of natural fibres,processed by both the best practice and the worst case approach,on the flexural and impact performance of PLA will be addressed.The analysis includes PLA composites based on ramie, flax and cot-ton fibres.

Table 1Codes for compound composition produced.

Fibre content Flax Ramie Cotton

2. Materials and methods

2.1. Materials

PLA polymer (NatureWorks� 4042D, having a Tg of 52–58 �Cand Tm of 150 �C) was purchased from NatureWorks LLC (Minne-sota, USA). Ramie, flax and cotton fibre samples were used in sliverform, which is an intermediate product from the textile spinningprocess. The ramie was chemically separated, and was suppliedby Buckmann company (Bremen, Germany) in 2001. The cottonwas delivered by Trützschler GmbH & Co. KG (Mönchengladbach,Germany) in 2006. Flax bast fibres originated from Holstein Flachs(Mielsdorf, Germany) and were purchased in 2005.

Dried Undried Dried Undried Dried Undried

0 C110 C2 C7 C1220 C3 C8 C1330 C4 C6 C9 C11 C14 C1650 C5 C10 C15

2.2. Compounding and Injection moulding

PLA was dried in a Gerco granules dryer (Gerco ApparatenbauGmbH & Co KG, Sassenberg, Germany) at 80 �C with pre-dried airbefore melt compounding for at least 4 h, but no longer than 8 h.

Fibres were dried at 105 �C for 24 h and subsequently at 80 �Cusing pre-dried air for 12–20 h.

Dried PLA granules were melted in a Haake Rheomix batchkneader (Thermo Fischer Scientific, Karlsruhe, Germany) at a settemperature of 175 �C and at a rotor speed of 100 RPM during2.5 min under a constant flow of pre-dried nitrogen. Subsequently,pre-dried or undried fibres were fed to the PLA melt and com-pounded up to a total time of 13 min. The temperature of the poly-mer melt was determined using a thermocouple. The compositionsof the produced composites and related codes are indicated inTable 1.

The composites were granulated and dried overnight in a Gercogranules dryer at 80 �C with pre-dried air and injection moulded toflexural/impact test bars with dimensions 80 � 10 � 4 mm3 usinga Demag ERGOtech 25–80 (Mannesmann Demag Kunststofftech-nik, Wiehe, Germany). All composite specimens were conditionedfor at least 7 d at 23 �C and 50% RH before further analysis.

2.3. PLA extraction

Injection moulded specimens of kneaded PLA (C1) and all30 mass% fibre–PLA compounds (C4, C6, C9, C11, C14, C16) wereput each in 160 ml of chloroform and heated under reflux in a250 ml flask until the PLA was dissolved in the boiling chloroform.The PLA–chloroform solution was filtered over a P2 filter with poresize 40–100 lm to remove fibres and collected. Subsequently, thefilter was washed with boiling pure chloroform and the filtrateswere combined. The combined filtrate was heated again and fil-tered over a Whatman GF/A glass microfibre paper with particleretention of 1.6 lm. The obtained filtrate was dried using a Rotav-ap R-124 (Büchi Labortechnik, Flawil, Switzerland) to obtain thePLA.

2.4. Intrinsic viscosity (IV)

Intrinsic viscosities [g] (ml/g), of all extracted PLA grades wereeach determined from four solutions of PLA in chloroform in theconcentration range of 0.001–0.01 g/ml. Efflux times of fixed vol-umes of PLA solutions were measured (via stopwatch) using a Can-non–Fenske capillary (type 50) viscometer (Schott Geräte GmbH,Hofheim, Germany) at 25 �C according to ASTM D 2857-95. Molarmass, Mv, was determined from [g], using Eq. (1). Virgin PLA (here-with coded C0) was included as a reference.

2.5. Melt flow index (MFI)

Composite granules were dried at 80 �C with pre-dried air for 4and for 24 h. The MFI was determined at 190 �C and 2.16 kg massunder pre-dried nitrogen using Zwick MFI equipment (ZwickGmbH, Ulm, Germany). After 240 s pre-heating, samples were col-lected after such time intervals that ca. 0.40 g of material was col-lected per time interval. The data were calculated into amounts offlown compound per 10 min.

Table 2Temperature of PLA melt at end of compounding in �C.

Fibre mass% Flax Ramie Cotton

Dried Undried Dried Undried Dried Undried

0 19110 197 193 19630 203 202 205 205 201 20450 211 219 227

1630 M.J.A. van den Oever et al. / Composites: Part A 41 (2010) 1628–1635

2.6. Moisture analysis

Fibre samples of ramie, flax and cotton were conditioned at23 �C and 50% RH, weighed and dried overnight in an oven at105 �C in open glass bottles of known mass. Subsequently the bot-tles were sealed immediately upon removal from the oven with afilm of known mass. The mass of the dried fibres was determinedfrom the previously measured tare mass of bottle and seal andthe gross mass of the bottle, fibres and seal. The moisture contentof the conditioned fibres was determined after drying at 105 �C.The dried fibre samples were heated in a Mettler Toledo Stromboli(Mettler Toledo GmbH, Schwerzenbach, Switzerland) at 200 �C andthe released gas was analysed for water content using a Mettler To-ledo DL39 Karl Fisher Coulometer, thus obtaining a value for waterrelease at 200 �C from fibres that were pre-dried at 105 �C. Hydran-al� Coulomat AG oven (Riedel No. 34739) was used as anolyte andHydranal� Coulomat CG (Riedel No. 34840) was used as catholyte.

2.7. Fibre width analysis

The image analysis system Fibreshape (IST AG, VIlters, Switzer-land) was used to determine the width distribution of the fibresand the fibre bundles. The samples were prepared between twoglass slides for colour positives (Company Gepe, Zug, Switzerland,type 69 01) after 24 h conditioning at 20 �C and 65% RH of air. Theslides of the cotton and ramie fibres were scanned by a CanoScanFS 4000 US (Canon Gmbh, Krefeld, Germany) at 4000 dpi. The flaxfibre bundles were scanned with a Epson Perfection V700 scanner(Epson Deutschland GmbH, Meerbusch, Germany) at 2400 dpi. TheSoftware GIMP was used for the scanning operation. To measurethe width of the prepared fibres and fibre bundles with the systemFibreshape (version 5.03) an adapted setup based on a standardlong fibre measuring mask was used. The system was calibratedusing 8 IWTO wool standards.

2.8. Scanning electron microscopy (SEM)

SEM micrographs were made from fracture surfaces obtainedby Charpy impacting of injection moulded 30 mass% fibre–PLA

Fig. 1. SEM of fracture surface of 30 m

compounds. Analysis was performed using a Jeol JSM-6300 F scan-ning electron microscope (Jeol Ltd., Tokyo, Japan) after covering thefracture surfaces with a 10 nm layer of platinum using an OxfordCT1500 sputter coater (Oxford Instruments, Abingdon, England).

2.9. Mechanical performance tests

The flexural properties were measured on a Zwick 1445 (ZwickGmbH & Co. KG, Ulm, Germany) according to ISO 178 at a supportlength of 64 mm and a crosshead speed of 2 mm/min for the mod-ulus and 10 mm/min for the strength. The flexural strength andmodulus were determined from five specimens per batch. TheCharpy unnotched impact strength was determined using a CeastResil 50 pendulum impact tester (Ceast S.p.A., Pianezza, Italy)according to ISO 179/1fU using an impact hammer of 4 J at a speedof 2.9 m/s. The Charpy impact strength was determined from eightspecimens per composition.

3. Results and discussion

3.1. Processing

A total feeding time of 12 min has been required for the50 mass% fibre–PLA compounds. In order to enable all fibres tomix well with the PLA, a total kneading time of 13 min has been se-lected for all compositions. The long feeding time was a result ofthe manual feeding of the fibres into the batch compounder. Forindustrial processing, fibre in sliver form could be automaticallyfed to an extruder [46]. The highest temperatures of the melt ofthe natural fibre–PLA compounds during processing are presentedin Table 2. Obviously, the higher the fibre content, the more pro-nounced the melt temperature rises. No difference in odour andcolour could be observed between the dried and undried fibrecomposites.

3.2. Morphology

SEM micrographs of impact fracture surfaces of 30 mass% fibre–PLA composites are presented in Figs. 1–4. The far majority of thefibres are well embedded by the polymer matrix, which is ofimportance for the strength performance of the composites. Forthe flax and ramie composites even several split single fibres canbe observed in the close up micrographs, indicating that fibre–ma-trix bonding must be stronger than the internal fibre lateralstrength.

All three fibre grades have a typical fibre diameter in the rangeof 10–20 lm. Based on SEM pictures (not presented), it is con-cluded that the diameter of ramie and cotton fibres was in this

ass% dried flax–PLA composite.

Fig. 2. SEM of fracture surface of 30 mass% undried flax–PLA composite.

Fig. 3. SEM of fracture surface of 30 mass% dried ramie–PLA composite.

Fig. 4. SEM of fracture surface of 30 mass% dried cotton–PLA composite.

M.J.A. van den Oever et al. / Composites: Part A 41 (2010) 1628–1635 1631

range already prior to compounding into PLA. The flax startingmaterial however contained a fraction of fibre bundles, which areknown to refine during compounding [46].

3.3. PLA chain scission

In the worst case scenario, each water molecule present in thecompound causes one hydrolytic cleavage in a PLA molecule chain.

The maximum number of cleavages per PLA molecule, #cleavages, isequal to the quotient of the moles of water molecules and the mo-les of PLA molecules present in the composite. The moles of water,molH2O, and PLA, molPLA, in the composites can be determinedfrom:

molH2O ¼ mcomp � ðXf � XH2O;f þ Xm � XH2O;mÞ=MH2O ð2ÞmolPLA ¼ mcomp � Xm=MPLA ð3Þ

Table 3Water contents present in dried and undried fibres and calculated number of maximum possible PLA chain cleaves and related average molar mass of PLA in 30 mass% agrofibre–PLA composites.

Fibre H2O content in fibres (mass%) Calculated maximum nr. of cleaves (molH2 O/molPLA) Calculated worst case molar mass (D)

Dried Undried Dried Undried Dried Undried

Ramie 0.0022 0.067 12.7 391 19,310 627Flax 0.0040 0.090 23.1 525 10,593 467Cotton 0.0029 0.066 17.1 385 14,364 636

1632 M.J.A. van den Oever et al. / Composites: Part A 41 (2010) 1628–1635

where mcomp is the composite mass, Xf and Xm are fibre and matrixmass fractions, XH2O;f and XH2O;m are the mass fractions of water inthe fibre and the matrix, and MPLA and MH2O, are the molar massesof the virgin PLA and water. Considering Xm = 1 � Xf, the number ofcleavages per PLA molecule can be expressed as:

#cleavages ¼ MPLA � ðXf � XH2O;f þ ð1� Xf Þ � XH2O;mÞ=ðMH2O � ð1� Xf ÞÞ ð4Þ

Considering the water contents of the fibres as summarized inTable 3, and assuming that the water content in the PLA after dry-ing is negligible, and assuming that the PLA molar mass is245,000 g/mol, then the maximum number of cleavages for30 mass% fibre–PLA composites can be calculated as summarizedin Table 3. The water present in a 30 mass% undried flax–PLA com-pound could result in a maximum average of 525 cleavages per vir-gin PLA chain. Even the dried fibres contain a relatively largeamount of water that is being released at 200 �C and the waterpresent in the dried flax fibre bundles could still result in 23 cleav-ages in each virgin PLA chain. The worst case PLA molar mass val-ues calculated from these number of cleavages per PLA moleculeare 10,593 and 467 g/mol for dried and undried flax, respectively.Polymers with such molar mass would rather have waxy proper-ties, not able to provide high strength and stiffness. In the nextparagraphs we will discuss the actual PLA degradation and the nat-ural fibre–PLA composite properties.

The kneading of pre-dried pure PLA during 13 min at 190 �C re-sults in a significant reduction of IV (Table 4). Following Eq. (1), thereduction of IV corresponds to a reduction of the Mv of PLA from245,000 to 149,400 (Table 4). The reduction of Mv can be explainedby hydrolysis of the polyester backbone in the molten state in thepresence of even small amounts of water [5,47]. PLA degradation,despite 4–8 h of pre-drying at 80 �C as recommended by Nature-Works [2], is confirmed by an increase of the MFI of PLA afterkneading (Table 4), as previously reported [27].

The incorporation of 30 mass% of agrofibres into PLA causes afurther decrease of IV and Mv of the PLA (Table 4). The effect of fi-bre grade appears to have a larger effect on the reduction in IV andMv than drying/not drying the fibres. The calculated Mv of PLA con-taining dried flax and cotton differs as much as a factor of 2.5,whereas the calculated Mv of the dried and undried flax com-pounds only differs 10%, and Mv of dried and undried cotton com-pounds only differs 3%. The larger reduction in Mv for cotton fibremay be caused by its larger surface area compared to ramie and

Table 4IV and Mv of pure PLA and PLA extracted from 30 mass% agrofibre–PLA compositesand MFI of pure PLA and 30 mass% agrofibre–PLA composites.

Composition IV (ml/g) Mv (D) MFI (g/10 min)

Dried Undried Dried Undried Dried Undried

Virgin PLA 188.4 245,000 2.26Kneaded PLA 129.4 149,400 3.53Ramie–PLA 105.6 114,300 0.51 0.31Flax–PLA 126.7 117.1 145,300 131,000 0.53 0.61Cotton–PLA 62.9 61.6 57,800 56,200 1.00 0.47

flax fibre. Before compounding the average diameters of the flax,ramie, and cotton samples were 62.3, 25.5 and 16.6 lm, respec-tively. After compounding and injection moulding the flax, ramieand cotton fibres have a diameter in the range 15–20, 10–15, and10–15 lm, respectively, as can be concluded from Figs. 1–4. Inte-grated over compounding time, the cotton fibres have the largestsurface area in contact with PLA, followed by ramie and flax. Con-sidering that the diffusion coefficient of water in PLA is far lowerthan in natural fibres, the water diffusion rate is expected to berather affected by fibre–PLA contact surface than by moisture con-tent in the fibres. As a consequence, water diffusion from cotton fi-bres to the PLA matrix is expected to be highest and leading to ahigher chance for PLA hydrolysis, followed by ramie and flax. Fu-ture research work will address natural fibre chemical compositionand a more detailed analysis of fibre surface area on PLA degrada-tion during processing.

The IV analysis is considered to be based on PLA without any fi-bre residues because the used filter had a pore size of 1.6 lm,which is expected to filter all fibres as from SEM analysis (Figs. 1–4) it can be concluded that the fibres have a diameter of about5 lm and larger.

Further, it may be noted that using the Mark–Houwink con-stants as defined by other researchers [28–33] will exhibit exactlythe same trends in PLA molar mass reduction, though beingslightly less or slightly more pronounced. These same trends areobtained despite the Mark–Houwink constants being determinedin different solvents at different temperatures for different PLAgrades.

Comparing the worst case molar mass data to the Mv of 145,300and 131,000 g/mol as calculated from actual IV data, it is concludedthat by far not all water molecules available in the melt com-pounded material cause PLA cleavage. Whereas the PLA processingguides [5] mention that even small amounts of moisture willhydrolize PLA in the melt phase, it is concluded that relatively largeamounts of moisture present in the natural fibres have a limitedadditional effect on PLA degradation during 13 min ofcompounding.

Similar observations have been reported by Placket et al. [26].Only very small differences in weight-average molar mass, Mw,as determined with size exclusion chromatography, were observedfor PLA composites based on vacuum-oven dried and undried jutefibres. Actually, the reduction in Mw of PLA in pre-dried and un-dried jute–PLA composites was very similar to that for similarlyprocessed pure PLA.

MFI values of 30 mass% fibre reinforced PLA are presented in Ta-ble 4, together with the values for virgin and kneaded PLA. The ef-fect of different fibre grade and of drying or not drying the fibresprior to compounding on MFI appears smaller than the effect of fi-bre content. Apparently, the MFI values do not provide a simplemethod to compare PLA degradation in different natural fibrecomposites.

The development of MFI of 30 mass% dried fibre reinforced PLAcomposites versus storage time at 190 �C is presented in Fig. 5. Thecomposites dried at 80 �C for 4 h show a sharp increase of the MFIafter about 500 s residence time at 190 �C, which is a typical pro-cessing temperature for PLA and its composites. The fibres are

0

1

2

3

4

5

6

7

0 500 1000 1500

Time (s)

MFI

(g/1

0min

)

Fig. 5. Melt flow index (MFI) versus storage time at 190 �C of 30 mass% agrofibre–PLA composites, based on pre-dried fibres, after 4 h (grey symbols) and 24 h drying(black symbols) at 80 �C prior to analysis: Ramie (N), Flax (j), Cotton (�). Adaptedfrom Recent advances in research on biodegradable polymers and sustainablecomposites (Vol. 1), M.J.A. van den Oever, B. Beck, J. Müssig, Natural fibre-PLAcomposites: processing and mechanical properties, 81–93. Copyright 2009, withpermission of Nova Science Publishers, Inc.

M.J.A. van den Oever et al. / Composites: Part A 41 (2010) 1628–1635 1633

not expected to affect composite flow during indicated residencetimes and temperatures, because they are likely to retain theirintegrity and structure as may be concluded from only a smallreduction in flexural strength of jute–PP composites observed after30 min heating at 200 �C [46]. The sharp increase of MFI after 500 sis therefore attributed to an onset point for accelerated degrada-tion of PLA. These results indicate that the drying time for naturalfibre–PLA composites prior to further processing should exceed the4 h drying time recommended for pure PLA [2] in order to avoidaccelerated PLA degradation in case processing of the compositetakes more than 10 min. After 24 h of drying, degradation of thePLA in the composite granules proceeds more steadily withincreasing residence time at 190 �C.

3.4. Mechanical properties

The injection moulded pure NatureWorks PLA 4042D has a flex-ural strength of 103(1) MPa, a flexural modulus of 3.16(0.05) GPaand a Charpy impact strength of 25(4) kJ/m2; between bracketsthe standard deviation is provided. Fig. 6 shows the normalized ef-fect of the incorporation of ramie, flax and cotton in PLA on thesemechanical properties. The flexural stiffness increases linearly withfibre content and the increase is most pronounced for ramie fibre.Absence of fibre drying before compounding had a small, yet sig-nificant, negative effect on stiffness of the ramie composites, no

0

50

100

150

200

250

300

350

0 10 20 30 40 50

Fibre content (mass %)

Nor

mal

ized

pro

perti

es (%

)

Fig. 6. Mechanical properties of undried (open symbols) and pre-dried (closedsymbols) fibre reinforced PLA composites versus fibre content: Ramie (N), Flax (j),Cotton (�), flexural modulus (black symbols), flexural strength (dark grey symbols),Charpy impact strength (light grey symbols).

significant effect was found for flax and cotton though. The levelsof flexural stiffness are significantly larger than the maximum val-ues of flax–PP presented by Bos et al. [48], being 3.8 GPa and5.7 GPa for 30 and 50 mass% flax–PP composites versus 7.3 and10.0 GPa for 30 and 50 mass% flax–PLA, respectively. These flax–PP compounds have been produced with the same batch kneadingequipment under fairly similar conditions. The higher stiffness forthe PLA compounds is partly due to the higher stiffness of PLAcompared to the stiffness of PP at room temperature, being 3.2and 1.2 GPa, respectively. However, also the fibre stiffness effi-ciency, A, as defined in Eq. (5) is larger in the PLA composites [27].

A ¼ ðEcomp � EPLAÞ=Xf ð5Þ

where Ecomp, EPLA, and Xf are the stiffness of the composite and thePLA and the fibre mass fraction respectively.

Further increase of the flexural strength of PLA, which is high al-ready, was marginally with increasing fibre content. The slight in-crease however confirms the observations made by SEM that fibresand PLA matrix exhibit good interaction. Also, the increase in com-posite strength confirms that the fibres have not fatally sufferedfrom the temperatures of the melt being over 200 �C. The absenceof fibre drying before compounding had rather a small positive ef-fect on flexural strength of flax and ramie composites. From theSEM micrographs it has been concluded that fibre–PLA compatibil-ity was in between that of flax–PP composites and flax–PP compos-ites in which MAPP coupling agent has been used. The use ofsuitable coupling agents may further improve the PLA compositestrength. Nishino et al. [49] report the application of silane cou-pling agent on kenaf fibre nonwoven before PLLA composite pro-duction by solution casting method. This increases the tensilestrength of the PLLA grade composites from 50 to 65 MPa.

When comparing to performance data presented in literature, itmust be noted that the PLA family exhibits a wide range of proper-ties. These properties depend on the (co)polymer structure, molarmass and crystallinity [50]. Furthermore, the mechanical proper-ties of fibre reinforced PLA composites depend on fibre–matrixadhesion, which may also be directly related to processing condi-tions. Plackett et al. [26] show that the tensile strength and modu-lus of jute nonwoven–PLA composites increase by 38% and 17%,respectively, when increasing the compression moulding tempera-ture from 180 �C to 210–220 �C. The possible explanations for theincreased performance are mentioned by Placket et al. to be im-proved fibre wetting, differences in crystallinity and compositeporosity. The normalized increase in flexural strength and modulusobtained in this study is similar to what has been reported forother melt blended PLA composites with good fibre–matrix adhe-sion, being concluded from SEM analysis [10]. Some other authorsreport a small reduction in flexural strength, indicating poor fibre–matrix adhesion [7,9].

The Charpy impact strength reduces with increasing fibre con-tent. The trend in reducing impact strength is similar for the threefibre sources investigated. Drying of the fibres before compoundinghad no significant effect on impact strength. The reduction in im-pact strength of PLA by incorporation of natural fibres has been re-ported before [7,9,10,15]. Only the incorporation of rayon fibres[15,17,18] and long kenaf fibres [9] have resulted in an increaseof PLA impact strength.

4. Conclusions

The analysis of the molar mass of PLA in the evaluated naturalfibre composites, which have been melt compounded for 13 min,indicates that by far not all water molecules available in the fibrescause PLA cleavage. Actually, the difference in PLA degradation islarger between different fibre grades than between ‘drying’ and

1634 M.J.A. van den Oever et al. / Composites: Part A 41 (2010) 1628–1635

‘not drying’ the fibres prior to compounding; the number of chainscissions calculated from intrinsic viscosity analysis was 0.7–3.2per PLA molecule for the dried fibre grades and 0.9–3.4 per PLAmolecule for the undried fibres, while based on the water presentin the natural fibres the maximum possible number of chain scis-sions was 12.7–23.1 for the dried fibres and 385–525 for the un-dried fibres. The different effect of the fibre grades on PLAdegradation is mainly attributed to a difference in fibre–PLA con-tact surface area, which is related to fibre or fibre bundle diameter.It is concluded that relatively large amounts of moisture present inthe fibres did not cause fatal PLA degradation during 13 min ofcompounding, whereas the PLA processing guides indicate thateven small amounts of moisture will hydrolize PLA in the meltphase. This is good news for natural fibre–PLA composite proces-sors. At industrial processing scale, the required level of fibre dry-ing may significantly influence the economic feasibility of naturalfibre–PLA composite production. If further processing of the com-posite takes more than 10 min, however, residual moisture in thecomposite appears to cause accelerated PLA degradation.

Differences in morphological and mechanical properties be-tween compounds based on dried and undried fibres were mar-ginal. SEM shows that the flax and ramie fibre bundles have beenrefined during compounding to a similar level of fibre diameter.All three fibre grades are able to improve the polymer’s flexuralproperties to a level above that of natural fibre–PP composites,independent of fibre pre-drying. Impact behaviour decreased withincreasing fibre content as expected.

Acknowledgements

This work was performed within the Ecobinders EU project,contract FP6-2005-NMP-011734. The European Commission iskindly acknowledged for partly funding this project. The authorsthank mr Martin Snijder of GreenGran BV (Wageningen, The Neth-erlands) for kindly providing the flax sliver, Buckmann company(Bremen, Germany) for providing the ramie sliver and TrützschlerGmbH & Co. KG (Mönchengladbach, Germany) for providing thecotton sliver, Mrs Jacqueline Donkers of Wageningen UR for per-forming the SEM analysis.

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