wictiv

Aa Composite Materials and Structures Center, 2100 Engineering Building, Michigan State University, East Lansing, MI 48824, USA

b

mentally friendly composites from biodegradable polymers. 2005 Elsevier Ltd. All rights reserved.

high specic strength and modulus, low density, ease ofber surface modication, relative non-abrasiveness, andlow cost [1,2]. Furthermore, rising oil prices and increased

have several disadvantages compared to natural/bio-bers(Table 1) [37]. Vollenberg and Hinkens [8] reported thatthe specic moduli (the ratio of the composite modulusto the composite specic gravity) of high ber volume frac-tion bast bers/polypropylene (PP) composites are in thesame range as glass ber/PP composites. The specic ten-

* Corresponding author. Tel.: +1 517 353 3969; fax: +1 517 432 1634.E-mail address: misraman@egr.msu.edu (M. Misra).

Composites Science and Technolog

COMPOSITESKeywords: A. Glass bres; B. Thermomechanical properties; C. Stress transfer; D. Scanning electron microscopy (SEM); E. Extrusion

1. Introduction

There is growing interest in the use of natural/bio-bersas reinforcements for biodegradable polymers because nat-ural/bio-bers have the functional capability to substituteglass bers that are currently being used in the industrytoday and also have advantages such as wide availability,

activity with regards to environmental pollution preventionhave also catalyzed the research and development of biode-gradable polymers. Fiber reinforced composites havegained importance from automotive to geotextiles sectors,where high mechanical properties and dimensional stabilitymust be coupled with low weight. Although glass bers arewidely used commercially in the composite industry, theySchool of Packaging, 130 Packaging Building, Michigan State University, East Lansing, MI 48824, USA

Received 3 March 2005; received in revised form 21 July 2005; accepted 19 October 2005Available online 6 December 2005

Abstract

Natural/bio-bers are replacing synthetic reinforcements traditionally used for the preparation of the environmentally friendly com-posites. Composite materials are also replacing conventional materials in various elds due to their ease of processability. Chopped glassber- and recycled newspaper cellulose ber (RNCF)- reinforced poly(lactic acid) (PLA) composites were processed using a full size twin-screw extruder and an injection molder. Additionally, a glass-reinforced polypropylene (PP) composite was compounded and molded,and compared to PLA/RNCF and PLA/glass ber composites. The tensile and exural moduli of RNCF- reinforced composites weresignicantly higher when compared to the virgin resin. The morphology, evaluated by scanning electron microscopy, indicated uniformdispersion of both bers in the PLA matrix. The mechanical and thermo-physical properties of PLA/RNCF, PLA/glass and PP/glassber composite were studied and compared using dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA).DMA results conrmed that the storage and loss moduli of the PLA/RNCF composites increased with respect to the pure polymer,whereas the mechanical loss factor (tan delta) decreased. The results of the TGA experiments indicated that the addition of bersincreased the thermal stability of the biocomposites compared to neat PLA. The heat defection temperature of PLA/RNCF was foundto be comparable to that of the glass ber-reinforced PLA composites. Such studies are of great interest in the development of environ-Chopped glass and recycled nein injection molded poly(lact

compara

Masud S. Huda a, Lawrence T. Drzal a,0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2005.10.015spaper as reinforcement bersacid) (PLA) composites: Ae study

mar K. Mohanty b, Manjusri Misra a,*

www.elsevier.com/locate/compscitech

y 66 (2006) 18131824

SCIENCE ANDTECHNOLOGY

Table 1Comparison between glass bers and natural/bio-bers

Glass bers

Health risk when inhaled YesCost [5] US $1.20$1.50/kgRecyclability NoEnergy consumption HighCarbon dioxide neutrality NoDensity [6] High (2.5 g/cm3)Renewability NoDisposal Not biodegradableDistribution Wide

ergy

1814 M.S. Huda et al. / Composites Science asile and exural moduli of the 50% (w/w) kenaf coupled[2% (w/w) maleic anhydride grafted polypropylene] com-posites were marginally higher than typical values reportedfor the 40% (w/w) coupled glass/PP injection-molded com-posites (Table 2) [9]. In addition, natural bers have highspecic strength and stiness, which is also comparableto that of glass ber [2,3]. Besides their low density, theyreduce the abrasion of screw and barrel in the extruderand injection molds, as well as the energy input for distrib-uting them in the polymer melt. Also, the biodegradabilityof this renewable raw material is important.

It is increasingly recognized that recycled newspapersand used paper products constitute a valuable source ofbers [7]. Newspapers are one of the most collected mate-rials in most community recycling programs. Recyclednewspaper materials consist of lignocellulosic materialand other inorganic llers, which invariably contain print-ing inks and other process aid materials. Recycled newspa-per cellulose ber (RNCF)-reinforced plastic compositesmay nd applications as structural materials for the hous-ing industry, such as load bearing roof systems, framing

Fiber emissions [7] Glass ber (/kg): enSeparation DicultShatter resistance LowWeight reduction [7] 2.52.8 g/cm3

Sound absorbing properties Lowcomponents, and non-structural products such as doors,furniture, and automotive interior parts that might be sim-ilar to wood-based composites [6,7,11]. Therefore, recyclednewspapers are an ideal source of bers and could be con-verted into high value composite materials using dry com-mon techniques such as compression- or injection-molding.

Table 2Properties of the ber reinforced PP composites

Reinforcement in PP Unit None Glass ber Kenaf ber

Fiber by volume % 0 40 50Fiber by weight % 0 19 39Specic gravity 0.9 1.23 1.07Flexural modulus GPa 1.4 6.2 7.3Specic exural modulus GPa 1.6 5.0 6.8Tensile modulus GPa 1.7 9.0 8.3Specic exural modulus GPa 1.9 7.3 7.8Notched izod impact J/m 24 107 32Elongation at break % 10 2.5 2.2PLA is a well-behaved thermoplastic, made from 100%renewable resources like corn, sugar beets, wheat and otherstarch-rich products [1,1214]. It belongs to the family ofaliphatic polyesters made from a-hydroxy acids. Thesepolymers are biodegradable and compostable [1,4,1214]and several studies on the physical and mechanical proper-ties of the PLA and other biocomposites have beenreported [1216]. Fang and Hanna [17] reported that theshear viscosity of PLA decreased as the temperatureincreased, which made ow easier. Moreover, as the shearrates increased, the viscosities of the PLA melts decreasedsignicantly. This was caused mainly by the breaking ofthe chains of the PLA molecules due to the strong shearforces. Teramoto et al. [18] reported that although noweight loss was observed for neat PLA and PLA/aceticanhydride-treated-abaca composite, the PLA/untreatedabaca composite showed an approximate 10% weight lossat 60 days, which was caused by the preferential degrada-tion of the ber. Generally, PLA polymers are made intouseful items using thermal processes, such as extrusionand injection molding [15,19]. PLA can be melt-processed

Natural/bio-bers

No$0.30$0.55/kgYesLowYesLow (1.21.6 g/cm3)YesBiodegradableWide

48.3 MJ China reed ber (/kg): energy 3.4 MJEaseHigh1.21.5 g/cm3

High

nd Technology 66 (2006) 18131824with standard processing equipment at temperatures belowthose at which natural bers start to degrade and at a rel-atively low cost. Hence, PLA is a versatile material withapplications in the medical, textile and packaging indus-tries [14,19,20]. PLA exhibits many properties that areequal to or better than many petroleum-based plastics,which makes it suitable for a variety of applications.Despite its high modulus and tensile strength, the lowdeformation at break and quite elevated price of PLA incomparison to the traditional thermoplastics may limit itsapplications. Since it is possible to overcome brittlenessand poor processability of sti and hard polymers by acombination of materials, composites are a necessity inthe evolution of engineering materials. Most research onPLA composite ultimately seeks to improve the mechanicalproperties to a level that satises a particular application.Some authors consider the enhanced toughness, which isthe main advantage of biobers in composites. Thestrength of a material, like PLA for instance, may be

e aimproved by processing with biobers. These compositesmay be easily processed by common techniques such ascompression- and injection-molding [19]. The propertiesof composite materials can be determined by the character-istics of the polymer matrices, together with reinforce-ments, and the adhesion of matrix/ber interface and thebonding strength at the interface [21]. As a consequenceof these characteristics, sensitive techniques must be usedfor this aim, such as DMA which monitors changes inthe mechanical properties, and serves as an important ther-mal analysis technique for characterizing the bermatrixinterface [2224]. It is essential to understand the thermalbehavior and phase morphology of the cellulose ber-rein-forced composites, which may aect the mechanical prop-erties and biodegradation behaviors.

Though interest in biober composites for industrialapplications in advanced countries has increased signi-cantly, the lack of availability of extensive property datais an important contributing factor limiting the wide spreadapplication of biobers in composites. Considerableresearch eort is needed to develop and exploit fully thepotential of these biober materials. The objective of thiswork is to evaluate the mechanical and thermo-physicalproperties of the RNCF-reinforced PLA as well as glassber reinforced PLA composite material that were pro-cessed using a full size twin-screw extruder and injectionmolder. It was possible to prepare cellulose ber reinforcedPLA composites by extrusion in nearly the same way aspolypropylene (PP). The glass-reinforced PP compositewas compounded and molded with a ber content of30 wt% and compared to PLA/RNCF composite. PLA/glass ber (70 wt%/30 wt%) composites were compared tothe PLA/RNCF (70 wt%/30 wt%) composite as well.

2. Experimental

2.1. Materials

Poly(lactic acid) (PLA; Mw: 20 kDa; Mn: 10.1 kDa)(product name Biomer L 9000) was obtained from Bio-mer, Krailling, Germany. Polypropylene (Pro-Fax 6523)was supplied by Basell Polyolens, Elkton, MD. Glassbers (Chopped Stand 735: ber glass chopped stands forpolypropylene) (currently priced at $0.95/lb) were providedby Johns Manville, Toledo, OH. Average glass berlengths were 413 and 387 lm after injection for PLA/glass(70 wt%/30 wt%) and PP/glass (70 wt%/30 wt%), respec-tively, from an original length of 3170 lm, which is in con-formity with the fact that the injection process aects theber length [25,26]. Usually, if it is to be eective in thecomposite, a ber of 350 mm in length and 918 mm indiameter should have a critical length [27,28]. The recyclednewspaper cellulose bers (CreaMix TC 1004) (RNCF)were supplied by CreaFill Fibers Corp., Chestertown,MD. The RNCF are reclaimed from newspaper/magazine

M.S. Huda et al. / Composites Sciencor Kraft paper stock [29]. TC 1004 bers are sold at lessthan $0.20/lb, a substantial cost savings over traditionalreinforcements such as glass. The average length of therecycled cellulose bers was 850 lm and the average widthof bers was 20 lm. The high cellulose content (75% min-imum) indicates that this is an alpha cellulose with maxi-mum ash content of 23%. Ash is a combination ofcarbon left after burning and any other organics/non-organics (clays, inks lignins, tannins, extractives, etc.) thatare not volatilized after ignition. The moisture content ofthe bers was less than 5%.

2.2. Composites processing: fabrication of polymer/ber

composites by WP (Werner and Pider) twin-screw

extruder

In order to insure that all absorbed and adsorbed mois-ture were removed and to prevent void formation, the cel-lulose bers and the PLA matrix resin were dried at 80 Cunder vacuum for 18 h before processing. This resulted in amoisture content of 12% in the case of RNCF, and theproducts were then stored over desiccant in sealed contain-ers. The bers and polymer matrix resin, mixed at a ratio of70 wt% or 30 wt%, were fed into a ZSK-30 WP twin-screwextruder with an L/D of 30 [30]. A uniform temperature of183 C was maintained for all the six zones of the extruder.The PLA matrix resin was fed at 46.5 g/min, while thebers were fed through a side feeder at a rate of 20 g/min. The screw speed was set at 100 rpm. The productextruded over the rst three minutes of each run was dis-carded and then the strands of the extrudate were collectedand chopped to form pellets.

In the case of the PP based composite, compoundingwas carried out at a screw speed of 100 rpm and extrudertemperatures were set at 180 C (zones 13), 183 C (zone4), 184 C (zone 5) and 185 C (zone 5). The feedingspeed of glass bers was 15 g/min for 30 wt% ber rein-forced composite. The extruded material was granulatedinto pellets having lengths of 11 mm. A Conair-Jetrodivision model 304 pelletizer was used to chop thestrands into pellets. The pelletizer has rotating knives,and the rotor has 36 teeth, which means that there are36 knives at the end of the rotor. The pelletized compos-ites were then molded into tensile coupons using theinjection molding technique, which is widely employedin industry.

The pelletized composites were dried in a convectionoven at 80 C for 2 h prior to injection molding. Theinjection molding was carried out on a Cincinatti-Milla-cron injection molding machine (85 ton capacity) withfour temperature zones [31]. The nozzle temperaturewas 185 C and the zones were kept at 183 C. The cool-ing time was 50 s and the hold, pack and ll pressureswere maintained at 6.89, 8.96, and 6.89 MPa, respec-tively. The glass ber reinforced PP pelletized compositewas also formed into testing specimens using the Cincin-nati-Milacron injection molder, where the cylinder heat-

nd Technology 66 (2006) 18131824 1815ing zones had set point temperatures of 178 C (rear),185 C (front) and 195 C (center), and the nozzle

e atemperature was set at 185 C and the mould tempera-ture was set at 65 C. The rst ve tensile coupons werediscarded and the subsequent coupons were used for test-ing. Samples were molded for exural and tensile testsaccording to ASTM D 790M and ASTM D 638 stan-dards, respectively.

2.3. Measurements

2.3.1. Mechanical testing

A mechanical testing machine, United CalibrationCorp SFM 20, was used to measure the tensile proper-ties, according to ASTM D 638 standard and exuralproperties, according to ASTM D 790. System controland data analysis were preformed using Datum software.A 1000 lbs load-cell was used for measuring the tensilestrength and tensile modulus of the injection molded ten-sile coupons. The strain was measured using a modelEXT62LOE laser extensometer. Crosshead speeds of 0.2and 0.05 in./min were used for testing the plastics andthe composites, respectively. The injection molded tensilecoupons are cut so that they could be accommodated ina three point bending set up. In order to determine theexural strength and exural modulus, the size of theexural testing samples used was 3.0 in. 0.5in. 0.125 in. Specimens were placed on a 2 in. spanand loaded with a crosshead speed of 0.05 in./min. Loadwas measured using a 1000 lbs load-cell. For determiningthe notched izod impact strength, the injection moldedtensile coupons were cut so that the samples were2.5 in. 0.5 in. 0.125 in. Notches 0.1 in. deep were cutinto sample beams using a TMI notch cutter. Notchedizod impact testing was performed on a TestingMachines Inc. 43-02-01 Monitor/Impact machine as perthe ASTM D 256 standard. A 5 ft-lb pendulum was usedto impact the samples. All results presented are the aver-age values of ve measurements.

2.3.2. Dierential scanning calorimeter (DSC)

The melting and crystallization behavior of the matrixpolymer and the composites were studied using a TAInstruments 2920 modulated dierential scanning calorim-eter (DSC) equipped with a cooling attachment, under anitrogen atmosphere [32]. The data were collected by heat-ing the composite specimen from 25 to 200 C at a constantheating rate of 5 C/min. A sample weight of approxi-mately 10 mg was used. The samples were sealed in alumi-num pans and the sealed samples were placed on a heatingsurface in the furnace along with an empty reference alumi-num pan. The heat ow and energy changes in and out ofthe samples in the sealed aluminum pans were recordedwith reference to an empty aluminum pan. Melting temper-ature was obtained from the peak in the heating curve. Thedetermination of melting temperature of the PLA-RNCFsystem helped to evaluate the processing temperatures that

1816 M.S. Huda et al. / Composites Sciencwere needed on the extruder when natural ber was addedto the system to fabricate the composite.2.3.3. Dynamic mechanical analysis (DMA)

The storage modulus, loss modulus, and loss factor (tandelta) of the composite specimen were measured as a func-tion of temperature (20100 C for PLA based compositesand 50150 C for PP based composites) using a TA 2980DMA equipped with a dual-cantilever bending xture at afrequency of 1 Hz and a heating constant rate of 5 C/min[32]. The injection molded tensile coupons were cut so thatthe samples were 2.15 in. 0.5 in. 0.125 in. to accommo-date the DMA.

2.3.4. Heat defection temperature (HDT)

Heat deection temperature (HDT) is an importantparameter when a material is being used for high tempera-ture structural applications. For determining the HDT, aconstant load of 0.46 MPa was applied at the center of a3-point bending injection molded tensile coupon bar sam-ple (size: 2.15 in. 0.5 in. 0.125 in.) according to ASTMStandard D 648 deection test using a TA 2980 DMA[32] and heated at the rate of 2 C/min from room temper-ature to 200 C. The sample deection was recorded as afunction of temperature.

2.3.5. Thermo gravimetric analysis (TGA)

The thermo gravimetric analysis was carried out in a TA2950 TGA. The samples were scanned from 25 to 500 C ata heating rate of 10 C/min, in the presence of nitrogen[32].

2.3.6. Scanning electron microscopy (SEM)The morphology of impact fracture surfaces of the com-

posites was observed by scanning electron microscope(SEM) at room temperature. A JEOL (model JSM-6300F) SEM with eld emission gun and acceleratingvoltage of 10 kV was used to collect SEM images for thecomposite specimen [33]. A gold coating of a few nanome-ters in thickness was coated on impact fracture surfaces.The samples were viewed perpendicular to the fracturedsurface.

3. Results and discussions

3.1. Tensile properties of the composites

The stressstrain curves of the PLA- and PP-basedcomposites are shown in Fig. 1, which shows the eectof chopped glass bers and RNCF on the tensilestrengths of virgin and reinforced PLA. The tensile prop-erties of PLA/ber composites were compared to PP/ber composites. The results of the tensile tests per-formed with the composites and the pure PLA and PPare shown in Table 3. PLA has better mechanical prop-erties than PP. The pure PLA has a tensile strength of62 MPa and a modulus of 2.7 GPa in contrast to36 MPa and 1.2 GPa of pure PP. It can be seen in Table

nd Technology 66 (2006) 181318243 that the addition of 30 wt% RNCF increased the ten-sile strength of virgin PLA from 62.9 to 67.9 MPa, which

e a0 2 4 6 80

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M.S. Huda et al. / Composites Sciencindicates that the stress is expected to transfer from thePLA polymer matrix to the stronger bers [34]. Furtherimprovement (up to 80.2 MPa) was achieved by adding30 wt% glass bers. Both tensile strength and modulusof the PP based composites were also increased, whichindicates improved adhesion between the glass bersand the PP matrix. Osswald [35] suggested that at a crit-ical ber length, stress is transferred from the matrix tothe ber, resulting in a stronger composite. Stress is e-ciently transferred only if the bond between the matrixand ber is good [35]. Generally, stress is assumed tobe transferred from the matrix to the ber by a sheartransfer mechanism when the bers are of nite length.There is a minimum ber length required in case of agiven ber to build up the shear stress between berand resin to the value of tensile fracture stress of theber [21,28]. The matrix cannot eectively grip the berto take the strain, and the bers will slip and be pulledout, instead of being broken under tension, if the berlength is less than this length. This shortest ber length(pull-out length) is called the critical ber length (orthe maximum value of load transfer length) [35]. Oksman

StraFig. 1. Stressstrain curves: (a) PP (100%) (q), (b) PP/glass (70/30) (s), (c) PL

Table 3Tensile properties of the composites

Polymer/bers (wt%) Tensilestrength (MPa)

Modulus ofelasticity (GPa)

Improvement(modulus) (%)

Neat PLA 62.9 4.9 2.7 0.4 PLA/glass (70/30) 80.2 1.6 6.7 0.4 145PLA/RNCF (70/30) 67.9 0.5 5.3 0.4 96Neat PP 32.9 3.6 1.2 0.1 PP/glass (70/30) 51.9 0.7 4.1 0.6 91et al. [36] reported that the tensile strength and modulusof PLA/ax composites are 53 MPa and 8.3 GPa, respec-tively, with a 30 wt% ax ber content. Usually, thequality of a ber-reinforced composite depends consider-ably on the bermatrix interface because only a well

10 12 14 16 18 20

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nd Technology 66 (2006) 18131824 1817formed interface allows stress transfer from the matrixto the ber. Therefore, good interfacial adhesion betweenthe matrix and bers is essential to improve the mechan-ical strength of composites. Our results indicated that theRNCF contributes more to tensile strength and modulus,whereas the PLA matrix plays the important role inimproving these properties. Glass ber reinforced PLAcomposites (with 30 wt% glass ber content) showed atensile strength of about 80 MPa [37] (in Table 3),though higher values (88 MPa) at 22 vol% of reinforce-ment bers have been reported for the glass ber mat/polypropylene composites [10]. Cyras et al. [38] reporteda tensile modulus of 0.7 GPa and a maximum strength of14.4 MPa with a 30 wt% sisal ber content in the case ofstarch/poly(e-caprolactone)/sisal ber composites. Thesevalues are low compared to our PLA/RNCF compositemodulus of 5.3 GPa and the strength of 68 MPa. Mostof the products used, for example in the automotiveindustry, have a blending ratio of 50 wt%/50 wt% fornatural bers and 30 wt%/70 wt% for glass bers; andthese ratios generate the optimal tensile strengths[39,40]. Rowell et al. [41] reported that the recyclednewspaper bers have high aspect ratios and contributeto an increase in the moduli of the composites and canalso improve the strength of the composite when suitableadditives are used to improve stress transfer between thematrix and the bers.

mechanical properties near by glass ber mat reinforced

Table 5Notched izod impact strength of the composites

Polymer/bers (wt%) Notched izodimpact strength (J/m)

Improvement (%)

Neat PLA 25.7 1.3 PLA/glass (70/30) 39.4 1.1 53PLA/RNCF (70/30) 23.5 0.4 No

Table 4Flexural properties of the composites

Polymer/ber (wt%) Flexuralstrength (MPa)

Flexuralmodulus (GPa)

Improvement(modulus) (%)

Neat PLA 98.8 1.0 3.3 0.1 PLA/glass (70/30) 108.9 1.2 8.2 0.3 152PLA/RNCF (70/30) 106.2 1.8 5.4 0.0 63

1818 M.S. Huda et al. / Composites Science and Technology 66 (2006) 181318243.2. Flexural properties of the composites

The exural strength and modulus of the PLA- and PP-based composites are summarized in Table 4. The exuralmodulus results were comparatively higher than the corre-sponding tensile modulus ones. The exural modulus andstrength of both PLA- and PP-based composites increasedsignicantly with the addition of the glass bers as well aswith the addition of the RNCF. It should be mentionedthat the PLA composite with the ber content of 30 wt%has a exural strength and modulus of 106.2 MPa and5.4 GPa, respectively, which are much higher than that ofneat PLA. Although the strength of the composites arelower than typical glass composites, the moduli of thehighly loaded RNCF composites might be comparable tothose of glass ber composites. The RNCF composites alsohave the added advantage of being reprocessed without sig-nicant loss in properties, which is unlikely in the case ofglass composites [42]. The insucient wetting of reinforc-ing material by the matrix resin could be one cause forthe decrease in the exural properties in the case of highber content [5]. Table 4 shows that the glass ber rein-forced PLA composite has a exural strength of106.2 MPa (17.4 vol%) in contrast to 79.7 MPa(13.3 vol%) for the glass ber reinforced PP composite.Here, ber volume fraction (vf) was calculated using Eq.(1):

vf 100wf=qf=fwm=qm wf=qfg; 1where wm is the weight fraction of matrix in composite, qmis the density of matrix, wm is the weight fraction of ber incomposite, and qm is the density of ber. The densities ofPLA matrix, PP matrix, and glass bers are 1.25 g/cm3,0.91 g/cm3, and 2.54 g/cm3, respectively. Wambua et al.[43] described that hemp-reinforced PP composites showed

Neat PP 36.4 1.8 1.5 0.2 PP/glass (70/30) 79.7 0.8 5.3 0.4 124the high exural strength properties (54 MPa) and com-pared well with glass mat composites (60 MPa). It seemsthat the mechanical properties of the RNCF-reinforced

Table 6Thermal properties of neat PLA and PLA-based composites

Polymer/RNCF or Talc (wt%) Tg (C) Tc (C)

Neat PLA 54 96PLA/glass (70/30) 57 91PLA/RNCF (70/30) 56 90PLA composites compared favorably with the correspond-ing properties of glass ber-reinforced PLA composites.

3.3. Notched izod impact strength of the composites

Table 5 represents the results of the notched izod impactstrength measurements of the PLA- and PP-based compos-ites. The pure PLA has impact strength of 25 J/m and afteraddition of RNCF, the impact strength of the compositedecreased slightly (Table 5). The RNCF composite showedlow impact strengths compared to glass ber reinforcedcomposites. Pure PP has an impact strength of 29.7 J/m(or 7.5 kJ/m2) (Table 5). The standard deviation obtainedfor the impact strength results was not considered highfor this type of test, as discussed earlier by Fejeskozma[44]. According to the Herrmann et al. [45], natural bercomposites appear to compare well with glass mat compos-ites in terms of specic impact strength properties (impactstrength divided by density). In this study, it is obvious thatthe low value of impact strength at ber content might bedue to the presence of too many ber ends within the bodyof cellulose ber-reinforced composites, which may causecrack initiation and potential composite failure [46]. More-over, the addition of cellulose ber content also increasesthe probability of ber agglomeration that creates regionsof stress concentrations that require less energy to elongatethe crack propagation [6,47]. A recent study looking at howinterfacial adhesion may be a problem for cellulose bersand polymer matrix composites, pointed out that adhesionneeds to be further improved to optimize the mechanicalproperties of this RNCF-reinforced composite [11]. Riedleet al. [45,48] have shown that the mechanical properties ofnatural ber mats reinforced PLA composites reached

Neat PP 29.7 3.1 PP/glass (70/30) 44.9 1.5 51plastics. Mueller and Krobjilowski [49] suggested thatbased on an optimized manufacture (referring to thermalprocessing conditions) even the impact strength of natural

DHc (J/g) DHm (J/g) v (%) Tm (C)

27.8 47.9 51.1 17220.4 41.7 44.5 17021.2 44.3 47.4 170

ture (Tg) was been observed. The Tg is usually interpreted

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Fig. 2. Temperature dependence of (a) storage modulus, (b) loss modulus,and (c) tan delta of PLA and PLA based composites: (a) PLA (100%) (),(b) PLA/RNCF (70/30) (s), and (c) PLA/glass (70/30) (e).

e aber composites can reach values comparable to those ofglass ber composites.

3.4. Crystallization and melting behavior of the composites

The thermal properties such as glass transition tempera-ture (Tg), crystallization temperature (Tc), melting temper-ature (Tm), crystallization enthalpy (DHc) and meltingenthalpy (DHm) obtained from the DSC studies aresummarized in Table 6. Using literature reference valuesfor the PLA melting enthalpies, under the assumption thatthe polymer is purely crystalline, it was possible to obtainthe degree of crystallinity (v %) in the composite,v DHm=DH 0m 100, where DHm is the experimentalmelting enthalpy (J/g) and DH 0m is the melting enthalpyof a pure crystalline matrix, PLA (93.7 J/g) [12]. The neatPLA shows a 51.1% degree of crystallinity with a clearglass transition temperature at 54 C [50]. Table 6 showsthat with the addition of RNCF to the PLA matrix theTg and Tm of the composites do not change signicantly.Similar results were obtained in the case of PLA/glass(70/30) composite. The DHm, DHc and Tc of the PLA com-posites decreased for the presence of RNCF in the case ofPLA/RNCF composite. These results suggest that RNCFdo not signicantly aect the crystallization properties ofthe PLA matrix. In this study, the crystallization tempera-ture of the RNCF reinforced composite decreases by up to6 C, which signies that the cellulose bers hinder themigration and diusion of PLA molecular chains to thesurface of the nucleus in the composites [51,52]. The crys-tallinity was found to decrease as a result of the additionof glass in the case of PLA/glass (70/30) composite. Whenglass was added the crystallization temperature of PLAdecreased by approximately 5 C.

3.5. Dynamic mechanical properties

Fig. 2 shows the storage modulus, loss modulus, and tandelta of the PLA and its composites, as a function of tem-perature. Here, the DMA was performed to show howexposing the microcomposites to elevated temperatureswould aect the stiness of the composite material. As seenin Fig. 2(a) and Table 7, the storage modulus of PLA basedcomposites is higher than that of PLA matrix. This is dueto the reinforcement imparted by the bers, which allowedstress transfer from the matrix to the ber [34,35]. Thereare two possible reasons for this high modulus values: thepresence of intramolecular bonds, and rigid skeletal con-formation of cellulose molecule [6,42,51]. Glass ber rein-forced PLA composite showed a high storage modulus ofabout 11.9 GPa at 25 C (in Table 7), though it seems thatthe storage moduli of the RNCF-reinforced PLA compos-ites compared favorably with the corresponding storagemodulus of glass ber-reinforced PLA composite.

Fig. 2(b) shows the variation of the loss modulus of the

M.S. Huda et al. / Composites SciencPLA and its composites with temperature. From the lossmodulus curves in Fig. 2(b), the glass transition tempera-14000

nd Technology 66 (2006) 18131824 1819as the peak of either the tan delta or the loss moduluscurves obtained during the dynamic mechanical test [32].

ber-reinforced PP composite has high HDT as seen inTable 8.

Storage modulus(GPa) at 40 C

Storage modulus(GPa) at 60 C

Reinforcementimparted by the bersat 25 C (modulus) (%)

3.1 1.8 11.6 9.7 2409.9 8.6 1882.4 1.4 7.4 6.1 172

Table 8HDT of the composites

Polymer/bers (wt%) HDT (C)

Neat PLA 64.5PLA/glass (70/30) 73.9PLA/RNCF (70/30) 80.2Neat PP 106.3PP/glass (70/30) 172.9

80

100 (a)(b)

e and Technology 66 (2006) 18131824As seen in Fig. 2(b) and Table 7, due to the ber present inthe PLA matrix, the Tg of both PLA-based compositesshifted to higher temperature. The shifting of Tg to highertemperatures can be associated with the decreased mobilityof the matrix chains, due to the addition of bers. Further-more, the stress eld surrounding the particles induces theshift in Tg. Since the loss modulus is a measure of theenergy dissipated or lost as heat per cycle of sinusoidaldeformation, when dierent systems are compared at thesame strain amplitude, the loss factors are very sensitiveto molecular motions [34,53]. It can be also seen fromFig. 2(b) that the loss modulus peak values increases with30 wt% ber content. The most pronounced eect of theber has been the broadening of the transition region asthe presence of the 30 wt% ber content. Fig. 2(c) showsthat the height of the tan delta peak decreased with thepresence of bers. Generally, the damping in the transitionregion measures the imperfection in the elasticity and thatmuch of the energy used to deform a material during DMAtesting is dissipated directly into heat [53]. Hence, themolecular mobility of the composites decreased and themechanical loss to overcome inter-friction between molec-ular chains reduced after adding bers.

As seen in Table 7, the addition of 30 wt% glass bersincreased the storage modulus of the PP matrix, i.e., thestorage modulus of the PP/ber composite was higher thanthat of PP matrix due to the reinforcement imparted by thebers that allows stress transfer from the matrix to the ber[34]. The storage modulus of the PP/ber compositedecreased with the increase of temperature. The reductionof modulus is associated with softening of the matrix at

Table 7The storage modulus of the composites

Polymer/bers (wt%) Tga (C) Storage modulus

(GPa) at 25 C

Neat PLA 63 3.2PLA/glass (70/30) 65 11.9PLA/RNCF (70/30) 67 10.1Neat PP 13 2.9PP/glass (70/30) 18 7.9

a Tg obtained from the loss modulus curves.

1820 M.S. Huda et al. / Composites Scienchigher temperature [54]. As seen in Table 7, the Tg ofPP/glass composite shifted to higher temperature due tothe glass bers present in the PP matrix.

3.6. Heat deection temperature (HDT)

As seen in Table 8, though it is dicult to achieve highHDT enhancement without strong interaction betweenPLA matrix and cellulose bers at a stress of 0.46 MPa,the measurements of HDT suggested that the PLA polymerappeared to have gone through a change. The HDT ofPLA/RNCF (70/30) composite was 80.2 C, relatively highcompared to PLA/glass (70/30) composite. This improve-ment in HDT might be derived from the increases in mod-ulus as well as from the good dispersion of the RNCF thathelps the reinforcement. These results on the HDT ofRNCF composites demonstrate that the RNCF reinforcedPLA composite would be useful for higher temperatureapplications than the PLA/glass composite, because HDTis a property that would provide a basis for the selectionof the material to be used at higher temperatures. In gen-eral, there are three options to increase the HDT of a poly-mer: increasing the Tg, increasing the crystallinity, andreinforcing. In this context, the increase of HDT of neatPLA by reinforcement with RNCF could be an importantdevelopment. The measurements of HDT show that glass0 100 200 300 400 500 6000

20

40

60

Wei

ght (%

)

Temperature (0C)

(c)

(d)

Fig. 3. Thermogravimetric curves of the PLA, TC 1004 ber, and PLAbased composites: (a) PLA (100%) (), (b) PLA/glass (70/30) (- -), (c) PLA/RNCF (70/30) ( ), and (d) RNCF (100%) (- -).

3.7. Thermogravimetry

The thermal stability of pure PLA and ber-reinforcedPLA composites was investigated with TGA. Fig. 3 showscomplete weight loss in a single step between 342 and414 C in the case of virgin PLA. Usually, an increase inthe decomposition temperature results in a more thermalstable product. The 5%, 25%, 50%, and 75% weight-losstemperatures (T5, T25, T50, and T75, respectively) are listedin Table 9 for all specimens shown in Fig. 3. The rstweight loss of the composite samples was between 200and 250 C. 72% and 75% weight-loss for the RNCF-rein-forced composite were observed at 400 and 432 C, respec-tively, which indicate that the presence of cellulose bersdoes aect the degradation process. The derivative of theTGA curve of 100% RNCF showed a 20% weight loss peakat 294 C followed by another 60% weight loss peak at384 C. In general, there were three stages of degradationthroughout the temperature runs, especially in the case ofRNCF. This was consistent with results reported by Vande Velde and Baetens [56].

3.8. Morphology of the composites

The morphology of the fracture surface of notched izodspecimens of the virgin PLA matrix was investigated bySEM as shown in Fig. 4. SEM micrographs of PLA illus-

trate the topography of this sample. SEM observations ofthe fracture surface of notched izod specimens of PLA/glass composite indicate that glass bers are well dispersedin the PLA matrix (in Fig. 5). The glass bers are coveredwith a thin layer of matrix linking the ber surface to thematrix, and thus better stress transfer could be expected.The state of the bermatrix interface in the PLA/RNCFcomposite was investigated by SEM. SEM micrograph ofthe impact fracture surfaces of the composite with the30% cellulose content is represented in Fig. 6. These micro-graphs illustrate the individual separation and dispersionof the RNCF in the form of single bers, which indicatesthat the cellulose ber have been separated during theextrusion process as well as well dispersed in the PLAmatrix. Some bers are tightly connected with the matrix.Raj and Kokta [57] reported that it is dicult to achievea good dispersion of the bers in the polymeric matrix.In the case of the RNCF, a considerable improvement ofthe adhesion at the interface of the composite is observed

Table 9TGA characterization of the composites

Polymer/bers (wt%) T5 (C) T25 (C) T50 (C) T75 (C)

Neat PLA 356 385 401 414PLA/glass (70/30) 360 389 407 >600PLA/RNCF (70/30) 345 370 385 432Neat RNCF 177 305 330 348

che

M.S. Huda et al. / Composites Science and Technology 66 (2006) 18131824 1821Fig. 4. SEM micrographs of the fracture surface of notFig. 5. SEM micrographs of PLA/glass ber (7d izod specimen of neat PLA: (a) 50 lm, and (b) 5 lm.0/30) composite: (a) 100 lm, and (b) 5 lm.

when compared to our previous study [58] due to thechanges in the processing using the large mixer-com-pounder WP twin-screw system. Hence, there were fewervoids on the fractured surface, which indicated that thebers were trapped by the PLA matrix. Closer observationrevealed (Figs. 5(b) and 6(b)) that some of the matrix couldbe found on the surface of the ber which was a good indi-cation of enhanced bermatrix adhesion. It is probablethat both RNCF and glass bers have been covered witha thin layer of the matrix as brils linking the ber surfaceto the matrix can be seen in micrographs. This suggeststhat the bers have some interaction with the matrix, which

The morphology of the fractured surfaces of notchedizod specimens of virgin PP were investigated by SEM(Fig. 7). The results show evidence of a relatively smoothtopography for the PP specimen. Fig. 8 shows the fracturedsurface of a glass ber-reinforced composite with manybers pulled-out but a relatively clean ber surface.Fig. 8(b) shows a more detailed micrograph of the glass-reinforced PP composite. In Fig. 8(b), a higher magnica-tion SEM micrograph of the interface shows that the glassber surfaces are clean, where lack of matrix on ber sur-face that is an indication of low adhesion between them.SEM studies of fractured surfaces of PLA/glass (in

Fig. 6. SEM micrographs of PLA/RNCF (70/30) composite: (a) 100 lm, and (b) 10 lm.

1822 M.S. Huda et al. / Composites Science and Technology 66 (2006) 18131824led to better stress transfer between the matrix and the rein-forcing bers.Fig. 7. SEM micrographs of the fracture surface of notch

Fig. 8. SEM micrographs of PP/glass ber (70Fig. 5(a)) and PP/glass (in Fig. 8(a)) composites revealedless voids in the PLA/glass composite. Glass bers wereed izod specimen of neat PP: (a) 50 lm, and (b) 5 lm.

/30) composite: (a) 100 lm, and (b) 5 lm.

pullout is the most important phenomenon [59]. Themorphologies of the fractured surfaces of both PLA- and

2002;21:43342.[16] Bleach NC, Nazhat SN, Tanner KE, Kellomaki M, Tormala P. Eect

e aPP-based composites therefore show phase information,and fractured characteristics reect the reasons why themechanical properties have been changed and in turndecide the mechanical properties of these polymericcomposites.

4. Conclusions

The mechanical and thermo-physical properties of glassber- and RNCF-reinforced PLA composites as well as ofglass ber reinforced PP composites have been investigated.The mechanical properties of the ber-reinforced PLA com-posites were found to compare favorably with the corre-sponding properties of PP composites. Compared to theneat resin, the tensile and exural moduli of PLA compos-ites were signicantly higher as a result of reinforcement bythe cellulose ber. The stiness of PLA is increased from 3.3to 5.4 GPa with an addition of 30 wt% RNCF. From theDMA results, incorporation of the bers gave rise to a con-siderable increase of the storage modulus (stiness) and to adecrease of the tan delta values. These results demonstratethe reinforcing eect of RNCF on PLA matrix. The TGAthermograms reveal the thermal stability of the compositeswith respect to the pure PLA resin. The SEM investigationsconrm that the RNCF were well dispersed in the PLAmatrix and were separated into single bers using the largemixer-compounder WP twin-screw system. We have shownin this study that the composites with RNCF provide apromising way to improve the stiness of PLA consider-ably. These results indicate that natural ber-reinforcedPLA composites have mechanical and thermo-mechanicalproperties high enough for use instead of conventional ther-moplastic composites. Furthermore, both mechanical andthermo-physical properties of RNCF-reinforced PLA com-posite tested were found to compare favorably with the cor-responding properties of glass ber-reinforced PLAcomposite. This suggests that these RNCF-reinforcedPLA composites have a potential to replace glass in manyapplications that do not require very high load bearingcapabilities. Future studies should be focused on theimprovement of the interfacial bonding as well as to con-centrate on eorts to evaluate the biodegradability of thesedeveloping and promising composites. With such improve-ments, it may be possible to achieve optimum dispersion ofbers and bermatrix adhesion.

Acknowledgments

The nancial support from USDA-MBI Award Numberseen outside the fractured surfaces (in Figs. 5(a) and 8(a))indicating that during crack propagation, the glass bersare broken and pulled out of the polymer matrix, although

M.S. Huda et al. / Composites Scienc2002-34189-12748-S4057 for the project Bioprocessing forUtilization of Agricultural Resources as well as NSF-DMIof ler content on mechanical and dynamic mechanical properties ofparticulate biphasic calcium phosphatepolylactide composites. Bio-materials 2002;23:157985.

[17] Fang Q, Hanna M. Rheological properties of amorphous andsemicrystalline polylactic acid polymers. Ind Crop Prod1999;10:4753.

[18] Teramoto N, Urata K, Ozawa K, Shibata M. Biodegradation ofaliphatic polyester composites reinforced by abaca ber. PolymDegrad Stabil 2004;86:4019.

[19] Kasuga T, Ota Y, Nogami M, Abe Y. Preparation and mechanical2004 award #0400296 for the Project PREMISE II: De-sign and Engineering of Green Composites from Biobers

and Bioplastics is gratefully acknowledged. The authorsalso express their appreciation to CreaFill Fibers Corp.,Chestertown, MD, to Johns Manville, Toledo, OH, toBasell Polyolens, Elkton, MD, USA, and to Biomer, Ger-many for supplying the RNCF, glass ber, polypropylene,and poly(lactic acid), respectively.

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Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: A comparative studyIntroductionExperimentalMaterialsComposites processing: fabrication of polymer/fiber composites by WP (Werner and Pflider) twin-screw extruderMeasurementsMechanical testingDifferential scanning calorimeter (DSC)Dynamic mechanical analysis (DMA)Heat defection temperature (HDT)Thermo gravimetric analysis (TGA)Scanning electron microscopy (SEM)

Results and discussionsTensile properties of the compositesFlexural properties of the compositesNotched izod impact strength of the compositesCrystallization and melting behavior of the compositesDynamic mechanical propertiesHeat deflection temperature (HDT)ThermogravimetryMorphology of the composites

ConclusionsAcknowledgmentsReferences