A Study on Biocomposites from Recycled Newspaper Fiber and Poly(lactic acid)

A Study on Biocomposites from Recycled Newspaper Fiber andPoly(lactic acid)

Masud S. Huda, Lawrence T. Drzal, and Manjusri Misra*

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

Amar K. Mohanty

School of Packaging, 130 Packaging Building, Michigan State University, East Lansing, Michigan 48824

Kelly Williams and Deborah F. Mielewski

Materials Science Department, Ford Research and Advanced Engineering Laboratory, Ford Motor Company,Dearborn, Michigan 48121

Recycled newspaper cellulose fiber (RNCF) reinforced poly(lactic acid) (PLA) biocomposites werefabricated by a microcompounding and molding system. RNCF-reinforced polypropylene (PP)composites were also processed with a recycled newspaper fiber content of 30 wt % and werecompared to PLA/RNCF composites. The mechanical and thermal-mechanical properties of thesecomposites have been studied and compared to PLA/talc and PP/talc composites. These compositespossess similar mechanical properties to talc-filled composites as a result of reinforcement byRNCF. The tensile and flexural modulus of the biocomposites was significantly higher whencompared with the virgin resin. The tensile modulus (6.3 GPa) of the PLA/RNCF composite (30wt % fiber content) was comparable to that of traditional (i.e. polypropylene/talc) composites.The DMA storage modulus and the loss modulus of the RNCF-PLA composites were found toincrease, whereas the mechanical loss factor (tan δ) was found to decrease. Differential scanningcalorimetry (DSC) thermograms of neat PLA and of the composites exhibit nearly the sameglass transition temperatures and melting temperatures. The morphology evaluated by scanningelectron microscopy (SEM) indicated good dispersion of RNCF in the PLA matrix. Thermogravi-metric analysis (TGA) thermograms reveal the thermal stability of the biocomposites to nearly350 °C. These findings illustrate that RNCF possesses good thermal properties, comparesfavorably with talc filler in mechanical properties, and could be a good alternative reinforcementfiber for biopolymer composites.

Introduction

The use of renewable sources for both polymer ma-trixes and reinforcement material offers an answer tomaintaining sustainable development of economicallyand ecologically attractive structural composite technol-ogy. Significant environmental advantages include pres-ervation of fossil-based raw materials, complete biologi-cal degradability, reduction in the volume of refuse,reduction of carbon dioxide released to the atmosphere,as well as increased utilization of agricultural resources.Biodegradable polymers may be obtained from renew-able resources, can be synthesized from petrobasedchemicals or can also be microbially synthesized in thelaboratory.1 One of the most promising biodegradablepolymers is poly(lactic acid) (PLA), the matrix resin usedin this study. PLA is a thermoplastic that has highstrength and modulus and can be manufactured fromrenewable resources, most commonly from corn. PLAis currently used in industrial packaging and in theproduction of biocompatible/bioabsorbable medical de-vices.1 Although PLA is a relatively stiff polymercharacterized by good mechanical strength, it is con-sidered too brittle for many commercial applications.

Reinforcing PLA with fibers offers one possibility toenhance its mechanical and thermal stability.2

The physical and mechanical properties of a polymericmaterial are strongly dependent on its structure, re-laxation processes, and morphology.3 The properties ofcomposite materials are determined by the character-istics of the polymer matrixes, the content and proper-ties of the reinforcements, as well as by fiber-matrixadhesion. Composite mechanical properties are alsodependent on good fiber dispersion and minimizationof voids. The interfacial adhesion depends on the bond-ing strength at the interface.4-6 Good dispersion of fibersin a polymeric matrix has been reportedly difficult toachieve.7

Cellulose is the most abundant renewable materialresource in the world. It is estimated that 830 milliontons of cellulose are produced each year through pho-tosynthesis.8 If an average plant contains 40% cellulose(on a dry weight base), the annual biobased resourcewould be approximately 200 million dry tons.8 The worldmarket for newsprint is growing over 2% a year and isforecast to be worth almost US$25 billion by 2004.8 TheUnited States is one of the world leaders in the recoveryand recycling of newspapers, recycling 71.2% of thenewsprint consumed in 2002. Virgin paper is made fromhighly compressed and heated cellulose fibers from softwoods, mainly grown and harvested as “paper pulp

* To whom correspondence should be addressed. Tel. 1-517-353-5466. Fax: 1-517-432-1634. E mail: [email protected].

5593Ind. Eng. Chem. Res. 2005, 44, 5593-5601

10.1021/ie0488849 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 06/03/2005

trees.” Some virgin materials are made from hardwoodand some are from a mix of hard and soft woods.Newsprint contains mostly mechanical (ground soft-wood) pulp, produced by reducing pulpwood logs andchips into their fiber components by the use of mechan-ical energy, via grinding stones, refiners, etc. Softwoodshave a mixture of entangled and elongated fibers, butthey do not have great strength. Among the mostcommon woods in this category are pine, spruce, andpoplar.

Cellulose-based polymer composites are characterizedby their low cost, low density, high specific stiffness andstrength, biodegradability, and good mechanical prop-erties.3,9-12 Cellulosic fibers are also nonabrasive andreduce wear on machinery.12,13 However, cellulose fibersare not extensively used in reinforcing thermoplastics,because of their low thermal stability during processingand poor dispersion in the polymer melt.12,14 Theproperties of the interface between the fiber and matrixare critical to many properties of a composite material,which are the result of many influences, such as fiberroughness, chemistry of the fiber surface and/or coating,and properties of the matrix.12,15,16 Much attention hasbeen given to the modification of the fibers and/orpolymer by physical and chemical methods.5,7,12,17,18

Traditionally, the addition of fillers to polymers is aninexpensive way to stiffen the properties of the basematerial.19 For example, polypropylene has been modi-fied by many fillers and elastomers to improve itstoughness, stiffness, and strength balance, dependingon the particular application.20,21 So the incorporationof the filler (e.g, talc, a typical filler in the market) inthermoplastics is a common practice in the plasticsindustry with the purpose of improving properties andreducing the production cost of molded products.19

The objective of this work is to evaluate the mechan-ical and thermomechanical properties of recycled news-paper cellulose fiber (RNCF)-reinforced PLA biocom-posite materials that were processed by a microcom-pounding molding system. RNCF-reinforced PP com-posites were also microcompounded and molded with a

cellulose content of 30 wt % and compared to PLA/RNCF(70 wt %/30 wt %) composites. PLA/talc (70 wt%/30 wt%)composites, processed using the microcompoundingmolding system, were compared to the PLA/cellulosefiber (70 wt%/30 wt%) composite as well.

Experimental Section

Materials. PLA (Biomer L 9000; Mw 20 kDa, Mn 10.1kDa) was obtained from Biomer (Krailling, Germany).Polypropylene (ProFax 6523) was supplied by BasellPolyolefins (Elkton, MD). Talc was obtained from R.T.Vanderbilt Co. (NYTAL 200, hydrous calcium magne-sium silicate mineral mixture). CreaFill Fibers Corp.(Chestertown, MD) supplied the RNCF (CreaMix TC1004). The TC 1004 fibers are reclaimed from newspaper/magazine or kraft paper stock.22 TC 1004 fibers are soldat less than $0.20/lb. The average length of the recycledcellulose fibers was 850 µm and the average width ofthe fibers was 20 µm. The high cellulose content (75%minimum) indicates that this is an R-cellulose with amaximum ash content of 23%. “Ash” is a combinationof the carbon left after burning and any other organics/nonorganics (clays, inks, lignins, tannins, extractives,etc.) that are not volatilized after ignition. The moisturecontent of TC 1004 was less than 5%.

Composites Processing. Prior to processing, theRNCF and PLA were dried under vacuum at 80 °C for24 h, resulting in it to a moisture content of 1-2% forthe RNCF, and then stored over desiccant in sealedcontainers. The PP matrix, however, was not dried. Thepolymer and the cellulose fibers were extruded at 100rpm with a Micro 15 cm3 compounding system (DSMResearch; Geleen, The Netherlands) at 183 °C for 10min.23 A photograph of the instrument is shown inFigure 1. The extruder has a screw length 150 mm, aL/D of 18, and a net capacity of 15 cm3. To obtain thedesired specimen samples for various measurementsand analysis, the molten composite materials weretransferred after extrusion through a preheated cylinderto a mini-injection-molder, which was preset with the

Figure 1. Microcompounding molding equipment: (a) DSM microcompounding and molding system (inside DSM mini-twin-extruder,open), (b) injection molder, (C) injection molding cylinder, and (d) tensile and flex molds.

5594 Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

desired temperature (injection temperature at 183 °C)and cooling system (mold temperature at 40 °C). Injec-tion-molded samples were placed in sealed polyethylenebags in order to prevent moisture absorption.

Measurements. (1) Mechanical Properties. Amechanical testing machine, United Calibration CorpSFM 20, was used to measure the tensile properties,(according to the ASTM D 638 standard) and flexuralproperties (according to ASTM D 790).16 System controland data analysis were preformed using Datum soft-ware. The notched Izod impact strength was measuredwith a Testing Machines Inc. (TMI) 43-02-01 monitor/impact machine according to ASTM D256. All resultspresented are the average values of five measurements.

(2) Differential Scanning Calorimeter (DSC). Themelting and crystallization behavior of the compositeswas studied using a TA 2920 DSC equipped with acooling attachment, under a nitrogen atmosphere.24

Each sample was heated from 25 to 200 °C at a heatingrate of 5 °C/min, held for 4 min to erase the thermo-mechanical prehistory, and then cooled at 5 °C/min to-50 °C, maintained at -50 °C for 2 min, and thenreheated to 200 °C at a rate of 5 °C/min. Scans werecollected and recorded during cooling and throughoutthe second heating. Both thermal and crystallizationparameters were obtained from the heating and coolingscans.

(3) Dynamic Mechanical Analysis (DMA). Thestorage modulus, loss modulus, and loss factor (tan δ)of the composite specimen were measured as a functionof temperature (20-100 °C for PLA-based compositesand -50 to 150 °C for PP-based composites) using a TA2980 DMA equipped with a dual-cantilever bendingfixture at a frequency of 1 Hz and a heating constantrate of 5 °C/min.24 In addition, material dampingproperties were determined using a Rheometrics Sci-entific DMTA 3E system. Samples were prepared frominjection-molded disks to the following dimensions: 1mm × 25 mm × 10 mm. Samples were mounted insingle cantilever bending geometry. All tests were runat a strain amplitude of 0.01%. Frequency/temperaturesweeps had a frequency range from 0.01 to 100 Hz anda temperature range from 25 to 85 °C.

(4) Heat Defection Temperature (HDT). HDTmeasurements were obtained on injection-molded flexbars at 66 psi load according to ASTM Standard D 648deflection test using a TA 2980 DMA equipped with adual-cantilever bending fixture with a heating rate of2 °C/min.24

(5) Thermogravimetric Analysis (TGA). The ther-mogravimetric analysis was carried out in a TA 2950TGA. The samples were scanned from 25 to 500 °C at aheating rate of 10 °C/min, in the presence of nitrogen.24

(6) Scanning Electron Microscopy (SEM). Themorphology of the composites’ impact fracture surfaceswas observed by a JEOL JSM-6300F scanning electronmicroscope (SEM) with field emission filament.25 Anaccelerating voltage of 10 kV was used to collect theSEM images of the composite specimens. A gold coating,a few nanometers in thickness, was applied on theimpact fracture surfaces. The samples were viewedperpendicular to the fractured surfaces.

Results and Discussions

Tensile Properties of the Composites. The tensileproperties of PLA/RNCF composites were compared toPP/RNCF composites. Table 1 shows the tensile strength

and modulus of the tested materials, respectively. NeatPLA has a higher tensile strength and modulus (62 MPaand 2.7 GPa) than neat PP (36 MPa and 1.2 GPa). Inaddition, though the tensile strength of PLA/TC 1004(70/30) composite did not improve, the use of cellulosefibers as reinforcement improved the tensile modulusfor both PLA and PP matrixes. This indicates that thestress is expected to transfer from the matrix polymerto the stronger fiber,6 indicating good interfacial adhe-sion and improved mechanical properties. In the caseof PLA/TC 1004 (70/30) composite, the tensile strengthdecreased and the tensile modulus increased with theaddition of TC 1004 fibers. Both PLA and PP matrixeswere also reinforced with traditional talc filler com-monly used in the automotive industry. As seen in Table1, the addition of talc gave a 92% increase in tensilemodulus for PLA and 75% for PP. The RNCF reinforcedPLA resulted in a greater tensile modulus but weakertensile strength than the talc-reinforced composite. ThePP with the addition of RNCF resulted in a comparabletensile modulus and a greater tensile strength than thetalc-filled material. These results indicate that a PLA/RNCF composite could act as a replacement for someapplications currently using talc-filled PP.

Flexural Properties of the Composites. The flex-ural results for tested materials are shown in Table 2.The modulus and strength of PLA and PP increasesignificantly with the addition of cellulose fibers. Al-though the strength of the cellulose composites is lowerthan that of typical talc-filled composites,21,26 the modu-lus of the 30% cellulose PLA/TC 1004 composite iscomparable to that of talc. As seen in Table 2, theaddition of talc improved both the flexural strength andmodulus of the PLA significantly, where high modulussuggests an efficient stress transfer between PLA andfiller as well as the good dispersion.27 Table 2 also showsthe mechanical properties of the PP, RNCF-filled PPcomposites, and talc-filled PP composites. The flexuralmodulus and strength of PP increased significantly withthe addition of the talc, and PP/TC 1004 is comparablein properties.

Notched Izod Impact Strength of the Compos-ites. The notched Izod impact strengths of both PLAand PP matrixes and their composites are shown inTable 3. Neat PLA has impact strength of 25 J/m. Theimpact strength of the RNCF-reinforced compositesample was lower than the virgin matrix (Table 3). The

Table 1. Tensile Properties of the Composites

polymer/RNCFor talc (wt %)

tensilestrength(MPa)

tensilemodulus

(GPa)

improvement(modulus)

(%)

neat PLA 62.8 ( 4.9 2.7 ( 0.4 -PLA/TC 1004 (70/30) 47.7 ( 2.5 6.3 ( 0.4 132PLA/talc (70/30) 58.4 ( 1.8 5.2 ( 0.3 92neat PP 36.4 ( 3.6 1.2 ( 0.1 -PP/TC 1004 (70/30) 38.9 ( 0.9 2.0 ( 0.3 64PP/talc (70/30) 35.7 ( 1.2 2.1 ( 0.4 75

Table 2. Flexural Properties of the Composites


flexuralstrength(MPa)

flexuralmodulus

(GPa)

improvement(modulus)

(%)

neat PLA 98.8 ( 1.0 3.3 ( 0.1 -PLA/TC 1004 (70/30) 77.7 ( 4.6 6.7 ( 0.1 103PLA/talc (70/30) 113.4 ( 2.4 9.7 ( 0.2 196neat PP 32.9 ( 1.8 1.5 ( 0.2 -PP/TC 1004 (70/30) 39.8 ( 0.7 2.1 ( 0.2 46PP/talc (70/30) 45.4 ( 2.2 2.8 ( 0.4 87

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impact strength of fiber-reinforced polymeric compositesis dependent on the fiber, the polymer matrix, and thefiber/matrix interface.28 The addition of high fibercontent increases the probability of fiber agglomeration,which creates regions of stress concentrations thatrequire less energy to elongate the crack propagation.29

Optimizing the interface between the fibers and thematrix through the use of compatibilizers or couplingagents can improve the toughness of these composites.The addition of 30% talc to neat PLA has almost noeffect on the impact strength (Table 3). As seen in Table3, the impact strength of PP/TC 1004 fiber (70/30)composite was an improvement over neat PP. In addi-tion, the impact strength of the PP/talc (70/30) compositewas significantly higher than that of both the neat andcellulose-reinforced PP. Usually, good filler/matrix in-terfacial adhesion provides an effective resistance tocrack propagation during impact tests.30

Crystallization and Melting Behavior of theComposites. The thermal characteristics of the com-posites were investigated via DSC. The glass transitiontemperature (Tg), crystallization temperature (Tc), melt-ing temperature (Tm), crystallization enthalpy (∆Hc) andmelting enthalpy (∆Hm) obtained from the DSC studiesare summarized in Table 4. Using literature referencevalues for the PLA and PP melting enthalpies, underthe assumption that the polymer is purely crystalline,it was possible to obtain the degree of crystallinity (ø,%) in the composite, ø ) ∆Hm/∆Hm

0 × 100%, where ∆Hm) experimental melting enthalpy (J/g) and ∆Hm

0 )melting enthalpy of a pure crystalline matrix, PLA (93.7J/g)31 and PP (137.9 J/g).32

Table 4 indicates that the Tg and Tm of the compositesdo not change significantly with the addition of celluloseto the PLA matrix. The ∆Hm, ∆Hc, and Tc of the PLAcomposites decreased in the presence of RNCF in thecase of the PLA/TC 1004 composite. These resultssuggest that RNCF does not significantly affect thecrystallization properties of the PLA matrix. There aretwo main factors controlling the crystallization of poly-meric composite systems.3,33,34 First, the additives havea nucleating effect that results in an increase in crystal-lization temperature, which has a positive effect on thedegree of crystallization. Second, additives hinder themigration and diffusion of polymer molecular chains tothe surface of the growing polymer crystal in the

composites, resulting in a decrease in the crystallizationtemperature, which has a negative effect on crystalliza-tion. In this study, the crystallization temperature ofthe RNCF-reinforced composite decreases by up to 6 °C,which signifies that the cellulose fibers hinder themigration and diffusion of PLA molecular chains to thesurface of the nucleus in the composites. Similar resultswere obtained in the case of PLA/talc (70/30) composite.The crystallinity was found to decrease as a result ofthe addition of talc. When talc was added, the crystal-lization temperature of PLA decreased by approximately14 °C.

The effect of the fibers on the thermal properties ofPP has also been analyzed in DSC experiments. Theresults are reported in Table 4. The dynamic crystal-lization behavior shows a positive effect from the fiberson the crystallization behavior of PP. A marked increaseof the crystallization peak temperature can be observedwhen the fibers are incorporated in the homopolymermatrix. The ∆Hm and ∆Hc decreased with the additionof RNCF. The composites Tg, Tm, and Tc remainedconsistent with neat PP. These results suggest thatcellulose fibers significantly affect the crystallizationbehaviors of the PP matrix. The obtained data are inagreement with the results of Lopez-Manchado et al.,35

where the nucleating effect of cellulose fibers on thecrystallization rate of polypropylene was demonstrated.Similar results were obtained in the case of PP/talc (70/30) composite. Talc-filled PP composite shows an in-crease in Tg compared to neat PP. These observationsindicate that a higher Tg consequently promotes achange from soft and flexible properties to hard andtough.36

Dynamic Mechanical Properties. Figure 2 showsthe dynamic storage modulus, loss modulus, and tan δof the composites, as a function of temperature. As seenin Figure 2A, the moduli increase in the presence ofRNCF in the composite, i.e., the storage modulus ofPLA-based composites is higher than that of the unfilledPLA matrix, which indicates that stress transfers fromthe matrix to the cellulose fiber.6,37 The storage modulusplots show a sharp decrease in the temperature rangearound 55-65 °C, which correlates with the glasstransition temperature. In the glassy zone, the contri-bution of fiber stiffness to the material modulus isminimal. Generally, the major factors that govern theproperties of short fiber composites are fiber dispersion,fiber-matrix adhesion, fiber length distribution, andfiber orientation. So mixing the hydrophilic cellulosefibers with a hydrophobic matrix can result in difficul-ties associated with the dispersion of fibers in thematrix. The storage moduli of the RNCF-reinforced PLAcomposites were comparable to the storage modulus oftalc-filled PLA composite.

Figure 2B shows the variation of the loss moduluswith temperature. The Tg of all the composites shiftedto higher temperatures due to the fiber present in thePLA matrix. This can be associated with the decreasedmobility of the matrix chains, due to the addition offibers. Furthermore, the stress field surrounding theparticles induces the shift in Tg. The loss factors arevery sensitive to molecular motions, since the lossmodulus is a measure of the energy dissipated or lostas heat per cycle of sinusoidal deformation, whendifferent systems are compared at the same strainamplitude. It can be also seen from Figure 2B that theloss modulus peak values increase with 30% fiber

Table 3. Notched Izod Impact Strength of theComposites


notched Izodimpact strength

(J/m)improvement

(%)

neat PLA 25.7 ( 1.3 -PLA/TC 1004 (70/30) 13.1 ( 1.1 noPLA/talc (70/30) 25.5 ( 4.4 noneat PP 29.7 ( 3.1 -PP/TC 1004 (80/20) 31.2 ( 1.4 5PP/talc (70/30) 35.1 ( 2.9 18

Table 4. Thermal Properties of Neat Polymer andPolymer/Fiber or Filler Composites


Tg(°C)

Tc(°C)

∆Hc(J/g)

∆Hm(J/g)

ø(%)

Tm(°C)

neat PLA 54 96 27.8 47.9 51.1 172PLA/TC 1004 (70/30) 55 90 23.6 41.2 44.0 172PLA/talc (70/30) 56 82 19.7 45.5 48.5 169neat PP -8 115 84.1 77.7 56.4 153PP/TC 1004 (70/30) -7 119 56.0 40.0 29.0 156PP/talc (70/30) 9 125 62.7 57.8 41.9 166


content. The most pronounced effect of the fiber hasbeen the broadening of the transition region of the PLAcomposite with 30% fiber as well as talc contents.

Figure 2C shows that the height of the tan δ peakdecreased with the presence of cellulose fibers. Onepossible explanation is that there is no restriction to thechain motion in the neat PLA matrix, while the presenceof the cellulose fibers hinders the chain mobility, result-ing in the reduction of the sharpness and height of thetan δ peak.38 Moreover, the damping in the transitionregion measures the imperfection in the elasticity andmuch of the energy used to deform a material duringDMA testing is dissipated directly into heat.39 Hence,the molecular mobility of the composites decreased andthe mechanical loss to overcome intermolecular chainfriction was reduced after adding the cellulose fibers.According to Fay et al.,40 the reduction in the tan δ alsodenotes an improvement in the hysteresis of the systemand a reduction in the internal friction.

The effects of temperature on the thermomechanicalproperties of PP/RNCF-based composites were alsostudied by DMA. In Figure 3A, the storage modulus ofthe PP matrix dropped with increasing temperature dueto an increase in the segmental mobility. In the PPmatrix, only the amorphous part undergoes segmentalmotion during transition; the crystalline region remainsa solid until its crystalline melting temperature isreached.28 The PP/TC 1004 resulted in a greater storagemodulus than neat PP, due to the reinforcement im-parted by the cellulose fibers that allows stress transferfrom the matrix to the cellulose fiber.37 Storage modulusvalues of PP matrix and its composite are not the sameat low temperature, because the fibers impart stiffnessto the composite.28 Figure 3A shows that the storagemodulus of the PP/TC 1004 composite decreased withincreasing temperature. The reduction of modulus isassociated with softening of the matrix at higher tem-peratures.32 It is evident from Figure 3B that, after theaddition of TC 1004 fibers to the PP matrix, the loss

Figure 2. Temperature dependence of (A) storage modulus, (B)loss modulus, and (C) tan δ of PLA and PLA-based composites:(a) neat PLA (s), (b) PLA/TC 1004 (70/30) (∇), and (c) PLA/talc(70/30) (O).

Figure 3. Temperature dependence of (A) storage modulus, (B)loss modulus, and (C) tan δ of PP and PP-based composites: (a)neat PP (s), (b) PP/TC 1004 (70/30) (∇), and (c) PP/talc (70/30)(O).


modulus increases with fiber loading, reaching a maxi-mum at 6 °C and then decreasing. This increase in lossmodulus with talc content is also prominent at highertemperature. The maximum heat dissipation occurs atthe temperature where the loss modulus is maximum,indicating the glass transition temperature of thesystem.33

Figure 3C shows that the tan δ values of the PP/TC1004 composite are slightly lower than those for the PPmatrix at low temperatures. As reported by Muraya-ma,41 damping is affected through the incorporation offibers in a composite system due to shear stress con-centrations at the fiber ends in association with theadditional viscoelastic energy dissipation in the matrixmaterial. Here, the tan δ peak can be related to theimpact resistance of a material. As seen in Figure 3C,incorporation of fibers as well as talc reduces the tan δpeak height by restricting the movement of the PPpolymer molecules. Amash et al.42 reported the ef-fectiveness of cellulose fiber in improving the stiffnessand reducing the damping in polypropylene/cellulosecomposites.

Damping characteristics were also determined usingthe frequency/temperature sweep test on the DMA.Temperature ranges were determined from the glasstransition temperature found in earlier temperatureramp experiments. During the test, data is collected ateach temperature for the full range of frequencies; thisis repeated for each increasing temperature. Using atime temperature superposition technique, the materialbehavior can be determined for higher frequencies. Theglass transition temperature is the reference point fromwhich the data is shifted. The time temperature shiftfactors, aT, were determined empirically for each mate-rial. It should be noted that lower temperatures cor-respond to higher frequencies. Figure 4 shows theshifted tan δ data for PLA, PLA/TC1004, and PLA/talc.The neat PLA has higher damping characteristics at lowfrequencies, whereas the talc-reinforced composite per-forms better at high frequencies. The RNCF-reinforcedcomposite has slightly higher damping properties thenthe neat PLA at high frequencies and is comparable tothe talc filler in the upper ranges.

Heat Deflection Temperature (HDT). The HDTof the RNCF-reinforced composites was higher than the

HDT of the neat resin, where HDT indicates thetemperature at which the deflection of the specimenreaches 0.25 mm under an applied load of 4.6 × 10-1

MPa according to ASTM D 648. As seen in Table 5,though it is difficult to achieve high HDT enhancementwithout strong interaction between the matrix andcellulose fibers, the HDT of PLA/talc (70/30) is relativelyhigh compared to the other PLA-based composite, apossible result of better dispersion during compoundingas well as due to the talc generating a stiffer interfacein the matrix.19 In this context, the HDT of the PP basedcomposite was higher than that of the PP resin.

Thermogravimetry. The TGA curves given in Fig-ure 5 show the thermal stability of the RNCF- and talc-reinforced composites. Approximately 0.4% and 0.5%weight loss was observed at 150 °C for the compositesof PLA/talc (70/30) and PLA/TC 1004 (70/30), respec-tively. In addition, 3.3% and 4.2% weight loss wasobserved at 300 °C for the composites of PLA/talc (70/30) and PLA/TC 1004 (70/30), respectively. TGA wasperformed on the TC 1004 fibers and they degraded inthree stages. The first stage from 40 to 130 °C was dueto the release of absorbed moisture in the fibers, evenafter the 24 h of drying was conducted to eliminatemoisture. The second transition (the temperature rangeof the decomposition was from 195 to 360 °C) wasrelated to the degradation of cellulosic substances, suchas hemicellulose and cellulose. The third stage (360-469 °C) of the decomposition was due to the degradationof noncellulosic materials in the fibers.

Morphology of the Composites. The morphologyof the TC 1004 fibers investigated by SEM (Figure 6a)showed evidence of fiber breakage for the TC 1004fibers. Figure 6b shows the morphology of the talcinvestigated by SEM. SEM micrographs of the impactfracture surfaces of the PLA/TC 1004 composites arerepresented in Figure 7. SEM micrographs of thecomposite sample illustrate its roughness. Figure 7 alsoshows the aggregation of the cellulose fiber like materi-als in the PLA/TC 1004 composite sample surface. Somefibers are tightly connected with the matrix, and somecellulose fibers are broken and/or torn up. It is probable

Figure 4. Damping factors for PLA-based materials versusshifted frequencies at Tref ) 55 °C.

Figure 5. Thermogravimetric curves of the PLA and PLA-basedcomposites: (a) neat PLA (---), (b) neat TC 1004 (-‚-‚), (c) PLA/talc(70/30) (‚‚‚), and (d) PLA/TC 1004 (70/30) (s).

Table 5. HDT of the Neat Polymer and Polymer/Fiber orFiller Composites

polymer/RNCF or talc (wt %) HDT (°C)

neat PLA 64.5PLA/TC 1004 (70/30) 73.1PLA/talc (70/30) 88.9neat PP 106.3PP/TC 1004 (70/30) 154.1PP/talc (70/30) 112.2


that the fiber surface has been covered with a thin layerof the matrix, as fibrils linking the fiber surface to thematrix can be seen in Figure 7, which led to better stresstransfer between the matrix and the reinforcing fibers.SEM micrographs of the impact fracture surfaces of thetalc-reinforced composites are represented in Figures 8and 9. Figures 8 and 9 show the SEM micrographs of30% talc-filled PLA and PP specimens, respectively,

which show good filler particle dispersion in the matrixand indicate that the talc has been separated duringthe extrusion process. No large aggregates are present;this morphology is optimal for toughening to occur.

ConclusionsThe mechanical and thermo-mechanical properties of

RNCF/talc-reinforced PLA composites as well as RNCF/

Figure 6. SEM micrographs of the (a) TC 1004 fibers (100 µm) and (b) talc (50 µm).

Figure 7. SEM micrographs of PLA/TC 1004 composites: (a) 100 µm and (b) 50 µm.

Figure 8. SEM micrographs of PLA/talc composites: (a) 100 µm and (b) 50 µm.

Figure 9. SEM micrographs of PP/talc composites: (a) 100 µm and (b) 50 µm.


talc-reinforced PP composites have been investigated.The mechanical and thermo-mechanical properties ofthe RNCF-reinforced PLA composites were found tocompare favorably with the corresponding properties ofPP composites. Compared to the neat resin, the tensileand flexural moduli of PLA composites were signifi-cantly higher as a result of reinforcement by the RNCF.From the DMA results, it is revealed that incorporationof the fibers gives rise to a considerable increase of thestorage modulus (stiffness) and a decrease in the tan δvalues. These results demonstrate the reinforcing effectof RNCF on both PLA and PP matrixes. The datacollected from the frequency/temperature sweep indi-cates that the RNCF-reinforced PLA composite hashigher damping characteristics than neat PLA andcomparative damping properties to the talc-reinforcedPLA, for high frequencies. The study performed by DSCrevealed the nucleation ability of the RNCF on PPcrystallization. An increase in the crystallization tem-perature with the introduction of the fibers was ob-served. The glass transition temperature and crystallinemelting point of PLA did not change after reinforcementwith RNCF. The crystallization temperature of theRNCF-reinforced PLA composites decreased as com-pared to neat PLA, which signifies that the cellulosefibers hinder the migration and diffusion of PLA mo-lecular chains to the surface of the nucleus in thecomposites. Future work will concentrate on efforts toevaluate the biodegradability of these developing andpromising composites.

AcknowledgmentThe financial support from USDA-MBI Award Num-

ber 2002-34189-12748-S4057 for the project “Biopro-cessing for Utilization of Agricultural Resources”, NSF2002 Award # DMR-0216865, under “Instrumentationfor Materials Research (IMR) Program” and NSF AwardDMI-0400296 “PREMISE-II: Design and engineeringof ‘green’ composites from biofibers and bioplastics” isgratefully acknowledged. The authors also wish toexpress their appreciation to CreaFill Fibers Corp.,Basell Polyolefins, and Biomer for supplying the RNCF,polypropylene, and poly(lactic acid), respectively.

Note Added after ASAP Publication. This articlewas released ASAP on June 3, 2005, with an error inthe estimated annual biobased resource in the Introduc-tion. The version posted on June 17, 2005, and the printversion are correct.

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Received for review November 18, 2004Revised manuscript received April 25, 2005

Accepted April 26, 2005

IE0488849


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