A Study on Biocomposites from Recycled ... - ACS Publications

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

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A Study on Biocomposites from Recycled Newspaper Fiber and Poly(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 were fabricated by a microcompounding and molding system. RNCF-reinforced polypropylene (PP) composites were also processed with a recycled newspaper fiber content of 30 wt % and were compared to PLA/RNCF composites. The mechanical and thermal-mechanical properties of these composites have been studied and compared to PLA/talc and PP/talc composites. These composites possess similar mechanical properties to talc-filled composites as a result of reinforcement by RNCF. The tensile and flexural modulus of the biocomposites was significantly higher when compared with the virgin resin. The tensile modulus (6.3 GPa) of the PLA/RNCF composite (30 wt % 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 to increase, whereas the mechanical loss factor (tan δ) was found to decrease. Differential scanning calorimetry (DSC) thermograms of neat PLA and of the composites exhibit nearly the same glass transition temperatures and melting temperatures. The morphology evaluated by scanning electron microscopy (SEM) indicated good dispersion of RNCF in the PLA matrix. Thermogravimetric analysis (TGA) thermograms reveal the thermal stability of the biocomposites to nearly 350 °C. These findings illustrate that RNCF possesses good thermal properties, compares favorably with talc filler in mechanical properties, and could be a good alternative reinforcement fiber for biopolymer composites. Introduction The use of renewable sources for both polymer matrixes and reinforcement material offers an answer to maintaining sustainable development of economically and ecologically attractive structural composite technology. Significant environmental advantages include preservation of fossil-based raw materials, complete biological 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 renewable resources, can be synthesized from petrobased chemicals or can also be microbially synthesized in the laboratory.1 One of the most promising biodegradable polymers is poly(lactic acid) (PLA), the matrix resin used in this study. PLA is a thermoplastic that has high strength and modulus and can be manufactured from renewable resources, most commonly from corn. PLA is currently used in industrial packaging and in the production of biocompatible/bioabsorbable medical devices.1 Although PLA is a relatively stiff polymer characterized by good mechanical strength, it is considered too brittle for many commercial applications. * To whom correspondence should be addressed. Tel. 1-517353-5466. Fax: 1-517-432-1634. E mail: misraman@ egr.msu.edu.

Reinforcing PLA with fibers offers one possibility to enhance its mechanical and thermal stability.2 The physical and mechanical properties of a polymeric material are strongly dependent on its structure, relaxation processes, and morphology.3 The properties of composite materials are determined by the characteristics of the polymer matrixes, the content and properties of the reinforcements, as well as by fiber-matrix adhesion. Composite mechanical properties are also dependent on good fiber dispersion and minimization of voids. The interfacial adhesion depends on the bonding strength at the interface.4-6 Good dispersion of fibers in a polymeric matrix has been reportedly difficult to achieve.7 Cellulose is the most abundant renewable material resource in the world. It is estimated that 830 million tons of cellulose are produced each year through photosynthesis.8 If an average plant contains 40% cellulose (on a dry weight base), the annual biobased resource would be approximately 200 million dry tons.8 The world market for newsprint is growing over 2% a year and is forecast to be worth almost US$25 billion by 2004.8 The United States is one of the world leaders in the recovery and recycling of newspapers, recycling 71.2% of the newsprint consumed in 2002. Virgin paper is made from highly compressed and heated cellulose fibers from soft woods, mainly grown and harvested as “paper pulp

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

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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.

trees.” Some virgin materials are made from hardwood and some are from a mix of hard and soft woods. Newsprint contains mostly mechanical (ground softwood) pulp, produced by reducing pulpwood logs and chips into their fiber components by the use of mechanical energy, via grinding stones, refiners, etc. Softwoods have a mixture of entangled and elongated fibers, but they do not have great strength. Among the most common woods in this category are pine, spruce, and poplar. Cellulose-based polymer composites are characterized by their low cost, low density, high specific stiffness and strength, biodegradability, and good mechanical properties.3,9-12 Cellulosic fibers are also nonabrasive and reduce wear on machinery.12,13 However, cellulose fibers are not extensively used in reinforcing thermoplastics, because of their low thermal stability during processing and poor dispersion in the polymer melt.12,14 The properties of the interface between the fiber and matrix are critical to many properties of a composite material, which are the result of many influences, such as fiber roughness, chemistry of the fiber surface and/or coating, and properties of the matrix.12,15,16 Much attention has been given to the modification of the fibers and/or polymer by physical and chemical methods.5,7,12,17,18 Traditionally, the addition of fillers to polymers is an inexpensive way to stiffen the properties of the base material.19 For example, polypropylene has been modified by many fillers and elastomers to improve its toughness, stiffness, and strength balance, depending on the particular application.20,21 So the incorporation of the filler (e.g, talc, a typical filler in the market) in thermoplastics is a common practice in the plastics industry with the purpose of improving properties and reducing the production cost of molded products.19 The objective of this work is to evaluate the mechanical and thermomechanical properties of recycled newspaper cellulose fiber (RNCF)-reinforced PLA biocomposite materials that were processed by a microcompounding molding system. RNCF-reinforced PP composites 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 microcompounding molding system, were compared to the PLA/cellulose fiber (70 wt%/30 wt%) composite as well. Experimental Section Materials. PLA (Biomer L 9000; Mw 20 kDa, Mn 10.1 kDa) was obtained from Biomer (Krailling, Germany). Polypropylene (ProFax 6523) was supplied by Basell Polyolefins (Elkton, MD). Talc was obtained from R.T. Vanderbilt Co. (NYTAL 200, hydrous calcium magnesium silicate mineral mixture). CreaFill Fibers Corp. (Chestertown, MD) supplied the RNCF (CreaMix TC 1004). The TC 1004 fibers are reclaimed from newspaper/ magazine or kraft paper stock.22 TC 1004 fibers are sold at less than $0.20/lb. The average length of the recycled cellulose fibers was 850 µm and the average width of the fibers was 20 µm. The high cellulose content (75% minimum) indicates that this is an R-cellulose with a maximum ash content of 23%. “Ash” is a combination of the carbon left after burning and any other organics/ nonorganics (clays, inks, lignins, tannins, extractives, etc.) that are not volatilized after ignition. The moisture content of TC 1004 was less than 5%. Composites Processing. Prior to processing, the RNCF and PLA were dried under vacuum at 80 °C for 24 h, resulting in it to a moisture content of 1-2% for the RNCF, and then stored over desiccant in sealed containers. The PP matrix, however, was not dried. The polymer and the cellulose fibers were extruded at 100 rpm with a Micro 15 cm3 compounding system (DSM Research; Geleen, The Netherlands) at 183 °C for 10 min.23 A photograph of the instrument is shown in Figure 1. The extruder has a screw length 150 mm, a L/D of 18, and a net capacity of 15 cm3. To obtain the desired specimen samples for various measurements and analysis, the molten composite materials were transferred after extrusion through a preheated cylinder to a mini-injection-molder, which was preset with the

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desired temperature (injection temperature at 183 °C) and cooling system (mold temperature at 40 °C). Injection-molded samples were placed in sealed polyethylene bags in order to prevent moisture absorption. Measurements. (1) Mechanical Properties. A mechanical testing machine, United Calibration Corp SFM 20, was used to measure the tensile properties, (according to the ASTM D 638 standard) and flexural properties (according to ASTM D 790).16 System control and data analysis were preformed using Datum software. The notched Izod impact strength was measured with a Testing Machines Inc. (TMI) 43-02-01 monitor/ impact machine according to ASTM D256. All results presented are the average values of five measurements. (2) Differential Scanning Calorimeter (DSC). The melting and crystallization behavior of the composites was studied using a TA 2920 DSC equipped with a cooling attachment, under a nitrogen atmosphere.24 Each sample was heated from 25 to 200 °C at a heating rate of 5 °C/min, held for 4 min to erase the thermomechanical prehistory, and then cooled at 5 °C/min to -50 °C, maintained at -50 °C for 2 min, and then reheated to 200 °C at a rate of 5 °C/min. Scans were collected and recorded during cooling and throughout the second heating. Both thermal and crystallization parameters were obtained from the heating and cooling scans. (3) Dynamic Mechanical Analysis (DMA). The storage modulus, loss modulus, and loss factor (tan δ) of the composite specimen were measured as a function of temperature (20-100 °C for PLA-based composites and -50 to 150 °C for PP-based composites) using a TA 2980 DMA equipped with a dual-cantilever bending fixture at a frequency of 1 Hz and a heating constant rate of 5 °C/min.24 In addition, material damping properties were determined using a Rheometrics Scientific DMTA 3E system. Samples were prepared from injection-molded disks to the following dimensions: 1 mm × 25 mm × 10 mm. Samples were mounted in single cantilever bending geometry. All tests were run at a strain amplitude of 0.01%. Frequency/temperature sweeps had a frequency range from 0.01 to 100 Hz and a temperature range from 25 to 85 °C. (4) Heat Defection Temperature (HDT). HDT measurements were obtained on injection-molded flex bars at 66 psi load according to ASTM Standard D 648 deflection test using a TA 2980 DMA equipped with a dual-cantilever bending fixture with a heating rate of 2 °C/min.24 (5) Thermogravimetric Analysis (TGA). The thermogravimetric analysis was carried out in a TA 2950 TGA. The samples were scanned from 25 to 500 °C at a heating rate of 10 °C/min, in the presence of nitrogen.24 (6) Scanning Electron Microscopy (SEM). The morphology of the composites’ impact fracture surfaces was observed by a JEOL JSM-6300F scanning electron microscope (SEM) with field emission filament.25 An accelerating voltage of 10 kV was used to collect the SEM images of the composite specimens. A gold coating, a few nanometers in thickness, was applied on the impact fracture surfaces. The samples were viewed perpendicular to the fractured surfaces. Results and Discussions Tensile Properties of the Composites. The tensile properties of PLA/RNCF composites were compared to PP/RNCF composites. Table 1 shows the tensile strength

Table 1. Tensile Properties of the Composites polymer/RNCF or talc (wt %)

tensile strength (MPa)

tensile modulus (GPa)

improvement (modulus) (%)

neat PLA PLA/TC 1004 (70/30) PLA/talc (70/30) neat PP PP/TC 1004 (70/30) PP/talc (70/30)

62.8 ( 4.9 47.7 ( 2.5 58.4 ( 1.8 36.4 ( 3.6 38.9 ( 0.9 35.7 ( 1.2

2.7 ( 0.4 6.3 ( 0.4 5.2 ( 0.3 1.2 ( 0.1 2.0 ( 0.3 2.1 ( 0.4

132 92 64 75

Table 2. Flexural Properties of the Composites polymer/RNCF or talc (wt %)

flexural strength (MPa)

flexural modulus (GPa)

improvement (modulus) (%)


98.8 ( 1.0 77.7 ( 4.6 113.4 ( 2.4 32.9 ( 1.8 39.8 ( 0.7 45.4 ( 2.2

3.3 ( 0.1 6.7 ( 0.1 9.7 ( 0.2 1.5 ( 0.2 2.1 ( 0.2 2.8 ( 0.4

103 196 46 87

and modulus of the tested materials, respectively. Neat PLA has a higher tensile strength and modulus (62 MPa and 2.7 GPa) than neat PP (36 MPa and 1.2 GPa). In addition, though the tensile strength of PLA/TC 1004 (70/30) composite did not improve, the use of cellulose fibers as reinforcement improved the tensile modulus for both PLA and PP matrixes. This indicates that the stress is expected to transfer from the matrix polymer to the stronger fiber,6 indicating good interfacial adhesion and improved mechanical properties. In the case of PLA/TC 1004 (70/30) composite, the tensile strength decreased and the tensile modulus increased with the addition of TC 1004 fibers. Both PLA and PP matrixes were also reinforced with traditional talc filler commonly used in the automotive industry. As seen in Table 1, the addition of talc gave a 92% increase in tensile modulus for PLA and 75% for PP. The RNCF reinforced PLA resulted in a greater tensile modulus but weaker tensile strength than the talc-reinforced composite. The PP with the addition of RNCF resulted in a comparable tensile modulus and a greater tensile strength than the talc-filled material. These results indicate that a PLA/ RNCF composite could act as a replacement for some applications currently using talc-filled PP. Flexural Properties of the Composites. The flexural results for tested materials are shown in Table 2. The modulus and strength of PLA and PP increase significantly with the addition of cellulose fibers. Although the strength of the cellulose composites is lower than that of typical talc-filled composites,21,26 the modulus of the 30% cellulose PLA/TC 1004 composite is comparable to that of talc. As seen in Table 2, the addition of talc improved both the flexural strength and modulus of the PLA significantly, where high modulus suggests an efficient stress transfer between PLA and filler as well as the good dispersion.27 Table 2 also shows the mechanical properties of the PP, RNCF-filled PP composites, and talc-filled PP composites. The flexural modulus and strength of PP increased significantly with the addition of the talc, and PP/TC 1004 is comparable in properties. Notched Izod Impact Strength of the Composites. The notched Izod impact strengths of both PLA and PP matrixes and their composites are shown in Table 3. Neat PLA has impact strength of 25 J/m. The impact strength of the RNCF-reinforced composite sample was lower than the virgin matrix (Table 3). The

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Table 3. Notched Izod Impact Strength of the Composites polymer/RNCF or talc (wt %)

notched Izod impact strength (J/m)

improvement (%)


25.7 ( 1.3 13.1 ( 1.1 25.5 ( 4.4 29.7 ( 3.1 31.2 ( 1.4 35.1 ( 2.9

no no 5 18

Table 4. Thermal Properties of Neat Polymer and Polymer/Fiber or Filler Composites polymer/RNCF or talc (wt %)

Tg (°C)

Tc (°C)

∆Hc (J/g)

∆Hm (J/g)

χ (%)

Tm (°C)


54 55 56 -8 -7 9

96 90 82 115 119 125

27.8 23.6 19.7 84.1 56.0 62.7

47.9 41.2 45.5 77.7 40.0 57.8

51.1 44.0 48.5 56.4 29.0 41.9

172 172 169 153 156 166

impact strength of fiber-reinforced polymeric composites is dependent on the fiber, the polymer matrix, and the fiber/matrix interface.28 The addition of high fiber content increases the probability of fiber agglomeration, which creates regions of stress concentrations that require less energy to elongate the crack propagation.29 Optimizing the interface between the fibers and the matrix through the use of compatibilizers or coupling agents can improve the toughness of these composites. The addition of 30% talc to neat PLA has almost no effect on the impact strength (Table 3). As seen in Table 3, the impact strength of PP/TC 1004 fiber (70/30) composite was an improvement over neat PP. In addition, the impact strength of the PP/talc (70/30) composite was significantly higher than that of both the neat and cellulose-reinforced PP. Usually, good filler/matrix interfacial adhesion provides an effective resistance to crack propagation during impact tests.30 Crystallization and Melting Behavior of the Composites. The thermal characteristics of the composites were investigated via DSC. The glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), crystallization enthalpy (∆Hc) and melting enthalpy (∆Hm) obtained from the DSC studies are summarized in Table 4. Using literature reference values for the PLA and PP melting enthalpies, under the assumption that the polymer is purely crystalline, it was possible to obtain the degree of crystallinity (χ, %) in the composite, χ ) ∆Hm/∆Hm0 × 100%, where ∆Hm ) experimental melting enthalpy (J/g) and ∆Hm0 ) melting enthalpy of a pure crystalline matrix, PLA (93.7 J/g)31 and PP (137.9 J/g).32 Table 4 indicates that the Tg and Tm of the composites do not change significantly with the addition of cellulose to the PLA matrix. The ∆Hm, ∆Hc, and Tc of the PLA composites decreased in the presence of RNCF in the case of the PLA/TC 1004 composite. These results suggest that RNCF does not significantly affect the crystallization properties of the PLA matrix. There are two main factors controlling the crystallization of polymeric composite systems.3,33,34 First, the additives have a nucleating effect that results in an increase in crystallization temperature, which has a positive effect on the degree of crystallization. Second, additives hinder the migration and diffusion of polymer molecular chains to the surface of the growing polymer crystal in the

composites, resulting in a decrease in the crystallization temperature, which has a negative effect on crystallization. In this study, the crystallization temperature of the RNCF-reinforced composite decreases by up to 6 °C, which signifies that the cellulose fibers hinder the migration and diffusion of PLA molecular chains to the surface of the nucleus in the composites. Similar results were obtained in the case of PLA/talc (70/30) composite. The crystallinity was found to decrease as a result of the addition of talc. When talc was added, the crystallization temperature of PLA decreased by approximately 14 °C. The effect of the fibers on the thermal properties of PP has also been analyzed in DSC experiments. The results are reported in Table 4. The dynamic crystallization behavior shows a positive effect from the fibers on the crystallization behavior of PP. A marked increase of the crystallization peak temperature can be observed when the fibers are incorporated in the homopolymer matrix. The ∆Hm and ∆Hc decreased with the addition of RNCF. The composites Tg, Tm, and Tc remained consistent with neat PP. These results suggest that cellulose fibers significantly affect the crystallization behaviors of the PP matrix. The obtained data are in agreement with the results of Lopez-Manchado et al.,35 where the nucleating effect of cellulose fibers on the crystallization rate of polypropylene was demonstrated. Similar results were obtained in the case of PP/talc (70/ 30) composite. Talc-filled PP composite shows an increase in Tg compared to neat PP. These observations indicate that a higher Tg consequently promotes a change from soft and flexible properties to hard and tough.36 Dynamic Mechanical Properties. Figure 2 shows the dynamic storage modulus, loss modulus, and tan δ of the composites, as a function of temperature. As seen in Figure 2A, the moduli increase in the presence of RNCF in the composite, i.e., the storage modulus of PLA-based composites is higher than that of the unfilled PLA matrix, which indicates that stress transfers from the matrix to the cellulose fiber.6,37 The storage modulus plots show a sharp decrease in the temperature range around 55-65 °C, which correlates with the glass transition temperature. In the glassy zone, the contribution of fiber stiffness to the material modulus is minimal. Generally, the major factors that govern the properties of short fiber composites are fiber dispersion, fiber-matrix adhesion, fiber length distribution, and fiber orientation. So mixing the hydrophilic cellulose fibers with a hydrophobic matrix can result in difficulties associated with the dispersion of fibers in the matrix. The storage moduli of the RNCF-reinforced PLA composites were comparable to the storage modulus of talc-filled PLA composite. Figure 2B shows the variation of the loss modulus with temperature. The Tg of all the composites shifted to higher temperatures due to the fiber present in the PLA matrix. This can be associated with the decreased mobility of the matrix chains, due to the addition of fibers. Furthermore, the stress field surrounding the particles induces the shift in Tg. The loss factors are very sensitive to molecular motions, since the loss modulus is a measure of the energy dissipated or lost as heat per cycle of sinusoidal deformation, when different systems are compared at the same strain amplitude. It can be also seen from Figure 2B that the loss modulus peak values increase with 30% fiber


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).

content. The most pronounced effect of the fiber has been the broadening of the transition region of the PLA composite with 30% fiber as well as talc contents. Figure 2C shows that the height of the tan δ peak decreased with the presence of cellulose fibers. One possible explanation is that there is no restriction to the chain motion in the neat PLA matrix, while the presence of the cellulose fibers hinders the chain mobility, resulting in the reduction of the sharpness and height of the tan δ peak.38 Moreover, the damping in the transition region measures the imperfection in the elasticity and much of the energy used to deform a material during DMA testing is dissipated directly into heat.39 Hence, the molecular mobility of the composites decreased and the mechanical loss to overcome intermolecular chain friction was reduced after adding the cellulose fibers. According to Fay et al.,40 the reduction in the tan δ also denotes an improvement in the hysteresis of the system and a reduction in the internal friction.

The effects of temperature on the thermomechanical properties of PP/RNCF-based composites were also studied by DMA. In Figure 3A, the storage modulus of the PP matrix dropped with increasing temperature due to an increase in the segmental mobility. In the PP matrix, only the amorphous part undergoes segmental motion during transition; the crystalline region remains a solid until its crystalline melting temperature is reached.28 The PP/TC 1004 resulted in a greater storage modulus than neat PP, due to the reinforcement imparted by the cellulose fibers that allows stress transfer from the matrix to the cellulose fiber.37 Storage modulus values of PP matrix and its composite are not the same at low temperature, because the fibers impart stiffness to the composite.28 Figure 3A shows that the storage modulus of the PP/TC 1004 composite decreased with increasing temperature. The reduction of modulus is associated with softening of the matrix at higher temperatures.32 It is evident from Figure 3B that, after the addition of TC 1004 fibers to the PP matrix, the loss

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Figure 5. Thermogravimetric curves of the PLA and PLA-based composites: (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 or Filler Composites

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

modulus increases with fiber loading, reaching a maximum at 6 °C and then decreasing. This increase in loss modulus with talc content is also prominent at higher temperature. The maximum heat dissipation occurs at the temperature where the loss modulus is maximum, indicating the glass transition temperature of the system.33 Figure 3C shows that the tan δ values of the PP/TC 1004 composite are slightly lower than those for the PP matrix at low temperatures. As reported by Murayama,41 damping is affected through the incorporation of fibers in a composite system due to shear stress concentrations at the fiber ends in association with the additional viscoelastic energy dissipation in the matrix material. Here, the tan δ peak can be related to the impact resistance of a material. As seen in Figure 3C, incorporation of fibers as well as talc reduces the tan δ peak height by restricting the movement of the PP polymer molecules. Amash et al.42 reported the effectiveness of cellulose fiber in improving the stiffness and reducing the damping in polypropylene/cellulose composites. Damping characteristics were also determined using the frequency/temperature sweep test on the DMA. Temperature ranges were determined from the glass transition temperature found in earlier temperature ramp experiments. During the test, data is collected at each temperature for the full range of frequencies; this is repeated for each increasing temperature. Using a time temperature superposition technique, the material behavior can be determined for higher frequencies. The glass transition temperature is the reference point from which the data is shifted. The time temperature shift factors, aT, were determined empirically for each material. It should be noted that lower temperatures correspond to higher frequencies. Figure 4 shows the shifted tan δ data for PLA, PLA/TC1004, and PLA/talc. The neat PLA has higher damping characteristics at low frequencies, whereas the talc-reinforced composite performs better at high frequencies. The RNCF-reinforced composite has slightly higher damping properties then the neat PLA at high frequencies and is comparable to the talc filler in the upper ranges. Heat Deflection Temperature (HDT). The HDT of the RNCF-reinforced composites was higher than the

polymer/RNCF or talc (wt %)

HDT (°C)


64.5 73.1 88.9 106.3 154.1 112.2

HDT of the neat resin, where HDT indicates the temperature at which the deflection of the specimen reaches 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 enhancement without strong interaction between the matrix and cellulose fibers, the HDT of PLA/talc (70/30) is relatively high compared to the other PLA-based composite, a possible result of better dispersion during compounding as well as due to the talc generating a stiffer interface in the matrix.19 In this context, the HDT of the PP based composite was higher than that of the PP resin. Thermogravimetry. The TGA curves given in Figure 5 show the thermal stability of the RNCF- and talcreinforced composites. Approximately 0.4% and 0.5% weight loss was observed at 150 °C for the composites of PLA/talc (70/30) and PLA/TC 1004 (70/30), respectively. In addition, 3.3% and 4.2% weight loss was observed at 300 °C for the composites of PLA/talc (70/ 30) and PLA/TC 1004 (70/30), respectively. TGA was performed on the TC 1004 fibers and they degraded in three stages. The first stage from 40 to 130 °C was due to the release of absorbed moisture in the fibers, even after the 24 h of drying was conducted to eliminate moisture. The second transition (the temperature range of the decomposition was from 195 to 360 °C) was related to the degradation of cellulosic substances, such as hemicellulose and cellulose. The third stage (360469 °C) of the decomposition was due to the degradation of noncellulosic materials in the fibers. Morphology of the Composites. The morphology of the TC 1004 fibers investigated by SEM (Figure 6a) showed evidence of fiber breakage for the TC 1004 fibers. Figure 6b shows the morphology of the talc investigated by SEM. SEM micrographs of the impact fracture surfaces of the PLA/TC 1004 composites are represented in Figure 7. SEM micrographs of the composite sample illustrate its roughness. Figure 7 also shows the aggregation of the cellulose fiber like materials in the PLA/TC 1004 composite sample surface. Some fibers are tightly connected with the matrix, and some cellulose fibers are broken and/or torn up. It is probable


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.

that the fiber surface has been covered with a thin layer of the matrix, as fibrils linking the fiber surface to the matrix can be seen in Figure 7, which led to better stress transfer between the matrix and the reinforcing fibers. SEM micrographs of the impact fracture surfaces of the talc-reinforced composites are represented in Figures 8 and 9. Figures 8 and 9 show the SEM micrographs of 30% talc-filled PLA and PP specimens, respectively,

which show good filler particle dispersion in the matrix and indicate that the talc has been separated during the extrusion process. No large aggregates are present; this morphology is optimal for toughening to occur. Conclusions The mechanical and thermo-mechanical properties of RNCF/talc-reinforced PLA composites as well as RNCF/

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talc-reinforced PP composites have been investigated. The mechanical and thermo-mechanical properties of the RNCF-reinforced PLA composites were found to compare favorably with the corresponding properties of PP composites. Compared to the neat resin, the tensile and flexural moduli of PLA composites were significantly higher as a result of reinforcement by the RNCF. From the DMA results, it is revealed that incorporation of the fibers gives rise to a considerable increase of the storage modulus (stiffness) and a decrease in the tan δ values. These results demonstrate the reinforcing effect of RNCF on both PLA and PP matrixes. The data collected from the frequency/temperature sweep indicates that the RNCF-reinforced PLA composite has higher damping characteristics than neat PLA and comparative damping properties to the talc-reinforced PLA, for high frequencies. The study performed by DSC revealed the nucleation ability of the RNCF on PP crystallization. An increase in the crystallization temperature with the introduction of the fibers was observed. The glass transition temperature and crystalline melting point of PLA did not change after reinforcement with RNCF. The crystallization temperature of the RNCF-reinforced PLA composites decreased as compared to neat PLA, which signifies that the cellulose fibers hinder the migration and diffusion of PLA molecular chains to the surface of the nucleus in the composites. Future work will concentrate on efforts to evaluate the biodegradability of these developing and promising composites. Acknowledgment The financial support from USDA-MBI Award Number 2002-34189-12748-S4057 for the project “Bioprocessing for Utilization of Agricultural Resources”, NSF 2002 Award # DMR-0216865, under “Instrumentation for Materials Research (IMR) Program” and NSF Award DMI-0400296 “PREMISE-II: Design and engineering of ‘green’ composites from biofibers and bioplastics” is gratefully acknowledged. The authors also wish to express 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 article was released ASAP on June 3, 2005, with an error in the estimated annual biobased resource in the Introduction. The version posted on June 17, 2005, and the print version are correct. Literature Cited (1) Vink, E. T. H.; Rabago, K. R.; Glassner, D. A.; Gruber, P. R. Applications of life cycle assessment to NatureWorks polylactide (PLA) production. Polym. Degrad. Stab. 2003, 80, 403-419. (2) Shibata, M.; Shingo Oyamada, S.; Kobayashi, S.; Yaginuma, D. Mechanical Properties and Biodegradability of Green Composites Based on Biodegradable Polyesters and Lyocell Fabric. J. Appl. Polym. Sci. 2004, 92, 3857-3863. (3) Krassig, H. A. Polymer Monographs; Elsevier Press: New York, 1993; Vol. 2. (4) Sims, G. D.; Broughton, W. R. Comprehensive composite materials; Kelly, A., Zweben, C., Ed.; Elsevier Press: London, 2000; Vol. 2. (5) Folkes, M. J. Short Fibre Reinforced Thermoplastics; Devis, M. J., Ed.; Wiley: Herts, U.K., 1982. (6) Rana, A. K.; Mitra, B. C.; Banerjee, A. N. Short jute fiberreinforced polypropylene composites: Dynamic mechanical study. J Appl. Polym. Sci. 1999, 71, 531-539. (7) Raj, R. G.; Kokta, B. V.; Dembele, F.; Sanschagrain, B. Compounding of cellulose fibers with polypropylene: Effect of fiber

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Received for review November 18, 2004 Revised manuscript received April 25, 2005 Accepted April 26, 2005 IE0488849

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