Novel Biocomposites from Native Grass and Soy Based Bioplastic

Indian grass fiber reinforced soy based biocomposites were fabricated by using twin-screw extrusion and injection molding technology. The thermal and ...
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Ind. Eng. Chem. Res. 2005, 44, 7105-7112

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Novel Biocomposites from Native Grass and Soy Based Bioplastic: Processing and Properties Evaluation Wanjun Liu,† Amar K. Mohanty,‡ Lawrence T. Drzal,† and Manjusri Misra*,† Composite Materials & Structures Center, Michigan State University, East Lansing, Michigan 48824, and School of Packaging, 130 Packaging Building, Michigan State University, East Lansing, Michigan 48824

Indian grass fiber reinforced soy based biocomposites were fabricated by using twin-screw extrusion and injection molding technology. The thermal and mechanical properties and the morphology of the biocomposites were evaluated using a dynamic mechanical analyzer (DMA), a universal testing system (UTS), and an environmental scanning electron microscope (ESEM). Raw Indian grass fiber improved the tensile and flexural properties as well as the heat deflection temperature (HDT), but did not improve the impact strength of the biocomposites. The impact fracture of the raw Indian grass fiber reinforced biocomposites was found to occur on the outer surface of the fiber, due to intrinsic differences in the morphological structure between the outer and inner surfaces of the grass fiber. Treatment of the Indian grass fiber with an alkali solution significantly improved the tensile, impact, and flexural strengths of its reinforced soy based biocomposites, presumably due to the homogeneous dispersion of fibers in the matrix and the enhanced aspect ratio of the fibers. Introduction Over the past 10 years, there has been an increased interest in the production and use of biodegradable polymers, due to the serious environmental pollution arising from consumed plastics with time. Although commercially available biodegradable polymers such as polycaprolactone (PCL), polyhydroxybutyrate (PHB), poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), and aliphatic and aromatic copolyesters possess the desired characteristics for producing blown film and injectionmolded materials, these polymers are not routinely used because of their high costs.1 Therefore, biopolymers from natural resources, such as starch and proteins, are fast becoming attractive alternatives, due to their abundance, availability, renewability, and relatively low cost. Soy proteins in particular are of immense interest since these complex amino acid polymers contain 20 amino acids, each of which can accommodate a functional group on its side chains or at the end of the main chain.2,3 These functional groups, such as amide, hydroxyl, and carboxyl, can be hydrophilic in nature and thus are able to interact with various plasticizers. Using extrusion technology, soy protein can be masterfully converted to soy protein plastics.4-6 Unfortunately, soy protein plastic products tend to have lower strength and higher moisture absorption.7 In this research, we have found that an effective way to overcome these drawbacks is to combine soy protein with biodegradable polymers. Currently, the biodegradable polymers being used to blend soy protein plastic include polyester amide and polycaprolactone,8,9 whose processing windows match that of soy protein plastic. To get higher strength and modulus materials from soy * To whom correspondence should be addressed. Tel.: (517) 353-5466. Fax: (517) 432-1634. E-mail: misraman@ egr.msu.edu. † Composite Materials & Structures Center. ‡ School of Packaging.

based bioplastics, it is best to reinforce them with natural fibers, since natural fibers have the advantages of low density, acceptable specific strength properties, easy separation, and biodegradability.9-11 There are very few references or reports on natural fiber reinforced soy based biocomposites.12,13 Tummala et al.12 studied the effect of hemp fiber loading on mechanical properties of soy based biocomposites. Lodha and Netravali13 investigated the interfacial and mechanical properties of ramie fiber reinforced soy protein isolate composites and found that the fracture stress increased with an increase in fiber length and fiber content. Agricultural byproducts such as corn stalk, rice straw, wheat straw, and grass are becoming a potential resource for natural fibers since they are commercially viable and environmentally acceptable. However, very little research about switch grass reinforced composites has been published in the area of composite materials.14 Indian grass belonging to the Poaceae family is a native grass of USA and grows throughout most of North America; thus it is a good choice as a reinforcing material in polymer-based composites. The effect of surface treatment on the morphology, structure, and thermal properties of Indian grass fiber and its reinforced biocomposites has been reported previously.15,16 In the present paper, Eastar Bio GP copolyester, an aliphatic-aromatic copolyester17 from Eastman Chemical Company, was incorporated with the soy protein polymer to form a soy based bioplastic. Eastar Bio has a melting temperature of 108 °C and wide processing windows. Its chemical structure is shown in Figure 1. Raw and alkali-treated Indian grass fiber reinforced soy based biocomposites were prepared with a twin-screw extruder and an injection molder. The physical properties of the biocomposites have been investigated using thermomechanical analysis and morphological structure characterization. Experimental Section Materials. Soy flour (defatted soy flour no. 063-130) with 52% protein was obtained from Archer Daniels

10.1021/ie050257b CCC: $30.25 © 2005 American Chemical Society Published on Web 08/02/2005

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Figure 1. Chemical structure of Eastar Bio, a copolyester produced by Eastman Chemical Company. Table 1. Details of Processing Conditions in Injection Molding Fabrication of Grass Fiber Reinforced Soy Based Biocomposites grass fiber content (wt %)

shot size (in.)

fill press. (psi)

pack press. (psi)

hold press. (psi)

0 15 30 40

1.00 1.05 1.12 1.15

900 1100 1800 2400

900 1100 1800 2400

700 900 1600 2300

Midland Company (ADM) (Decatur, IL). Glycerol and sodium hydroxide were supplied by J. T. Baker (Phillipsburg, NJ). Eastar Bio (21154 GP copolyester) (poly(tetramethylene adipate-co-terephthalate) (PTAT)) was supplied by Eastman Chemical Company (Kingsport, TN). Indian grass was used as received from Smith, Adams & Associates LLC (Okemos, MI). Alkali Treatment of Grass Fiber. Chopped grass stems with 20 mm length were treated in 10 wt % sodium hydroxide solution in water. The weight ratio of alkali solution to grass fiber was 20:1, and after 4 h of soaking, the fibers were rinsed with distilled water until the pH of the rinse solution stabilized at 7. After drying at room temperature for 4 days, the alkalitreated and raw fibers were dried under vacuum at 80 °C for 16 h prior to use. Sample Preparation. Extrusion. The soy flour, the fibers, and the biodegradable polymer were dried at 80 °C under vacuum for 16 h before processing. After drying, soy flour was blended with glycerol according to the weight ratio of 70/30 using a blender. The materials were equilibrated in a sealed plastic bag for at least 1/2 h and then fed into the extruder (ZSK-30 Werner and Pfliderer twin-screw extruder [length-todiameter ratio (L/D) ) 30] with six zone barrels). The processing temperatures were 95, 105, 115, 125, 130, and 130 °C, and the screw speed was set to 100 rpm. The plasticized soy flour was reextruded with PTAT at a weight ratio of 1:1, at a processing temperature of 130 °C, and at a screw speed of 100 rpm. The pelletized soy based bioplastic was extruded with grass fiber under conditions similar to those described before. The soy based bioplastic was fed at a rate of 30 g/min. The feeding rates of grass fiber were 5.3, 13, and 20 g/min for 15, 30, and 40 wt % fiber content reinforced biocomposites, respectively. Sample Preparation. Injection Molding. A Cincinnati Milacron injection molder (with screw L/D ) 17) with a capacity of 85 tons was used to get specimens for measurement. Soy flour based bioplastic and biocomposites were injection molded with a barrel temperature of 130 °C and a mold temperature of 20 °C. The processing cycle time was 112 s. Table 1 provides more detailed information of the injection molding processing. Dynamic Mechanical Properties. The dynamic mechanical properties of bioplastic and biocomposites were studied with a dynamic mechanical analyzer (2980 DMA, TA Instruments, USA) under DMA multifrequency and three-point bending modes, at a frequency of 1 Hz. The DMA samples with size of 2.15 in. × 0.5 in. × 0.125 in. was cut from injection-molded tensile

Figure 2. DMA curves of Indian grass fiber reinforced biocomposites for (A) PTAT, (B) soy based bioplastic, (C) 15 wt % raw grass fiber reinforced soy plastic composites, (D) 30 wt % raw grass fiber reinforced soy plastic composites, and (E) 40 wt % raw grass fiber reinforced soy plastic composites.

coupons. The heating rate was 4 °C/min, and the heating range was -50 to 90 °C. Heat Deflection Temperature. The dynamic mechanical analyzer (2980 DMA, TA Instruments) was also used to measure the heat deflection temperature (HDT) of the biocomposites under a DMA control force and three-point bending modes with a load of 66 psi, according to ASTM D 648. The heating rate was 2 °C/ min. Mechanical Properties Testing. The tensile and flexural properties of the injection-molded specimens were measured with a United Testing System SFM-20 according to ASTM D 638 and ASTM D 790, respectively. System control and data analysis were performed using Datum software. The notch Izod impact strength was measured with a Testing Machines Inc. 43-02-01 Monitor/Impact machine according to ASTM D 256. In all mechanical properties measurements, five specimens were measured for each sample to ensure reproducibility. X-ray Photoelectron Spectroscopy (XPS). A Perkin-Elmer PHI 5400 ESCA system was used to collect XPS spectra of the raw fiber, alkali solution treated

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Figure 3. Tensile and flexural properties of grass fiber reinforced biocomposites of (A) soy plastic, (B) 15 wt % raw grass fiber reinforced soy plastic composites, (C) 30 wt % raw grass fiber reinforced soy plastic composites, and (D) 40 wt % raw grass fiber reinforced soy plastic composites.

grass fiber, and soy plastic. The instrument was equipped with a nonmonochromatic X-ray source (Mg anode) operating at 15 kV and 300 W. The analyzed area was approximately 250 µm2, and the base pressure was less than 10-7 Torr. The angle between the electron analyzer and the sample surface was 45°. Quantification of elements was accomplished using MultiPak software and sensitivity factors supplied by the manufacturer. Environmental Scanning Electron Microscopy. The morphologies of the grass fiber and its reinforced soy based biocomposites were observed with a Phillips Electroscan 2020 environmental scanning electron microscope (ESEM) with an accelerating voltage of 20 kV. Results and Discussion Raw Fiber Reinforced Soy Based Biocomposites. Dynamic Mechanical Properties. The plots of storage modulus as a function of temperature for grass fiber reinforced soy based biocomposites are shown in Figure 2 (top). It is observed that the storage modulus of PTAT increased after the addition of plasticized soy flour. Once they were reinforced with grass fiber, the storage modulus of the biocomposites increased with increasing amounts of grass fiber. These results are consistent with those obtained for tensile modulus and flexural modulus. The curves of tan delta versus temperature of grass fiber reinforced soy based biocomposites are shown in Figure 2 (bottom). It was found that the peak value of tan delta in the glass transition region of soy based bioplastic (curve B) was much lower than that of PTAT (curve A), and the tan delta peak values of 15, 30, and 40 wt % grass fiber reinforced biocomposites (curves C, D, and E, respectively) were also lower than that of soy based bioplastic. The damping in the transition zone measures the imperfection in the elasticity, and much

Figure 4. Impact strength and HDT behavior of grass fiber reinforced biocomposites of (A) soy plastic, (B) 15 wt % raw grass fiber reinforced soy plastic composites, (C) 30 wt % raw grass fiber reinforced soy plastic composites, and (D) 40 wt % raw grass fiber reinforced soy plastic composites.

of the energy used to deform a material under DMA condition is dissipated directly into heat.18 That is, the more viscous the nature of polymeric materials, the higher the energy used to overcome friction of molecular motion and hence the higher the damping value in the transition region. The above results indicate that, after the addition of grass fiber, the molecular mobility of polymer segments in the composites decreased and the mechanical loss to overcome friction between intermolecular chains was reduced. The lower peak value of soy based bioplastic compared with PTAT was caused by the nature of plasticized soy flour. The damping of plasticized soy flour is dependent on its composition, namely the content and nature of the plasticizer. The plasticized soy flour used here contained 30 wt % glycerol and 70 wt % soy flour, and had a lower tan delta peak value in the glass transition region (about 0.2). Generally, the damping of the polymer is much greater than that of fibers. Therefore, adding fiber to polymeric materials will undoubtedly cause increases in elasticity and decreases in viscosity, which leads to smaller amounts of energy being used to overcome the friction force between molecular chains than to decrease mechanical loss. Also, blending with plasticized soy flour and subsequent reinforcement with grass fiber resulted in the composition of the materials being changed. Since plasticized soy flour and fiber are more rigid than the PTAT, less damping appeared in the glass transition region. Tensile Properties. The importance of fiber reinforced composites mainly comes from the significant improvement in strength and modulus, which provide a good chance for application of composites. The tensile properties of grass fiber reinforced soy based biocomposites with different contents of grass fiber are shown in Figure 3 (top). It was found that the tensile modulus

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Figure 5. Impact fracture surfaces of grass fiber reinforced biocomposites of (a) 15 wt % raw grass fiber reinforced soy plastic composites (100× with scale bar of 450 µm), (b) 30 wt % raw grass fiber reinforced soy plastic composites (200× with scale bar of 250 µm), and (c, d) 40 wt % raw grass fiber reinforced soy plastic composites (200× with scale bar of 250 µm).

Figure 6. Morphology of raw Indian grass fiber for (a) outer surface (550× with scale bar of 100 µm) and (b) inner surface (110× with scale bar of 400 µm).

of the composites increased markedly with increasing amounts of grass fiber, whereas the tensile strengths increased only marginally. When the weight fraction of grass fiber reached 40 wt %, the strength of the biocomposites increased by only 40%, while the modulus was enhanced 1800%, compared to those of soy based bioplastic, respectively. Flexural Properties. The flexural properties of grass fiber reinforced soy based biocomposites with different contents of grass fiber are shown in Figure 3 (bottom). It was found that the flexural strength and the modulus of grass fiber reinforced biocomposites increased with increasing content of grass fiber. When the fiber content reached 40 wt %, the flexural strength and the modulus improved 140% and 1000%, respectively. These results indicate that flexural properties

of grass fiber reinforced soy based composites followed the same trends as the tensile properties of the biocomposites. Impact Strength. The value of the impact strength reflects the ability of a material to resist impact. Notched Izod impact strength indicates the energy to propagate crack under impact load. The impact strength of fiber reinforced polymeric composites is more complex than that of the polymer because of the part played by the fiber and the interface in addition to the polymer.18 The notched Izod impact strengths of grass fiber reinforced soy based biocomposite with different contents of grass fiber are shown in Figure 4 (top). These results show that the impact strength of grass fiber reinforced composites did not change with increases in grass fiber content in the composites, which indicates that grass

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fibers have almost no contribution to the impact strength of soy based biocomposites. Generally, there are two factors that affect the impact strength of fiber reinforced composites.18 First, the fibers effectively reduce the impact strength of the composites by decreasing the break elongation, thereby reducing the area under the stress-strain curves since a new stress concentrator forms around the fiber ends. Additionally, the fibers increase the impact strength by reducing the crack propagation rate and bridging the crack through fiber pullout.19,20 The practical effect of fiber on impact strength of fiber reinforced composites is dependent on the competition of the above two factors. In this raw grass fiber reinforced soy based biocomposite system, it is obvious that the hindrance effect of grass fiber on crack propagation is almost the same as that of the effect of fiber on crack initiation. Therefore, raw grass fiber had no contribution in this soy based biocomposite. Heat Deflection Temperature. The heat deflection temperature (HDT) refers to the maximum temperature at which a polymer can be used as a rigid material. Specifically, according to ASTM D 648, HDT is defined as the temperature at which the deflection of the sample reaches 250 µm under an applied load of 66 psi. Fiber reinforced composites will experience great increases in HDT because of the changes in modulus. The HDT behavior of grass fiber reinforced soy based biocomposites is shown in Figure 4 (bottom). It was found that the HDT gradually increased with increasing content of grass fiber. With the addition of 40 wt % grass fiber, the HDT temperature improved by 80%. This improvement in HDT is caused by the increases in modulus of biocomposites after the addition of fiber. This result indicates that grass fiber reinforced soy based biocomposites have a higher usage temperature range. Morphology. The fracture surface of raw Indian grass fiber reinforced soy based PTAT biocomposites are shown in Figure 5. It was found that the fibers were not very well dispersed. On careful examination, it was observed that the composites always fractured on the outer surface of the grass fiber, indicating that the outer surface became an obvious stress concentrator during deformation. This is related to the surface morphology and properties of raw grass fiber. The surface morphology of raw grass is shown in Figure 6. It was found that the outer surface morphology was totally different from the inner surface, in that the outer surface was smooth with relatively lower fluctuation and the inner surface was relatively rough with higher fluctuation. Cell wall structure is also obvious in the inside surface. XPS data showing the oxygen and carbon composition ratios of the inner and outer surfaces of the raw grass fibers, the soy protein plastic, and the inner and outer surfaces of the alkali-treated soy protein plastic are shown in Figure 7. The inner surface of the raw grass fiber had a higher ratio of oxygen to carbon compared to the outer surface. The difference of surface properties is caused by structure and composition. Taking into consideration the structure of the grass fiber, the ratio of oxygen to carbon reflects the relative content of lignin on the surface. The lower ratio of oxygen to carbon on the outer surface of the raw grass fiber compared to that of soy plastic suggests that the outer surface had a relatively higher lignin content, namely, a lower hydrophilic nature and a higher hydrophobic nature, and hence led to weak interaction with soy plastic. However, the

Figure 7. Oxygen-to-carbon ratio from XPS of grass fiber and soy protein plastic for (A) outer surface of raw grass fiber, (B) inner surface of raw grass fiber, (C) soy protein plastic, (D) outer surface of 10% alkali solution treated for 4 h grass fiber, and (E) inner surface of 10% alkali solution treated for 4 h grass fiber.

similar ratio of oxygen to carbon compared with that of soy plastic, suggests that the inner surface had a lower lignin content than the outer surface and a chemical polarity similar to soy plastic, and hence better interaction with soy plastic. This result also indicates that the ratio of oxygen to carbon from XPS can give the surface information of fiber, which can be used to detect the relative interaction between the fiber and the matrix. Alkali-Treated Fiber Reinforced Soy Based Biocomposites. Although raw Indian grass improved the modulus of the soy based biocomposites, the improvements in tensile and impact strengths were modest. Therefore, other fiber surface treatment seems necessary for increasing the efficiency of fiber reinforcement. Surface treatment is expected to change the surface properties of the natural fiber and hence improve the adhesion between the fiber and the matrix.21 Alkali treatment changed the structure and properties of Indian grass fiber and hence enhanced the physical properties of grass fiber reinforced soy based polyester amide biocomposites.15,16 The optimized alkali solution treatment condition for Indian grass is 10% alkali solution for 4 h. Therefore, this alkali treatment condition was used for Indian grass fiber in the present study. Dispersion of Fiber in the Matrix. The dispersion of 30% raw and alkali-treated grass fiber reinforced composites is shown in Figure 8. It was found that the dispersion of the raw grass fiber in the matrix was not uniform, and most of the fibers were bunched. However, the dispersion of alkali-treated grass fiber in matrix was improved and the fiber size was reduced. In addition, alkali-treated fiber reinforced composites became separated fibril reinforced composites. Thus, the aspect ratio of the fiber in the matrix was improved and so was the contacting area between the fiber and the matrix. How did the bunched grass fiber become separated fiber in the composites after fiber alkali treatment? The main components of natural fiber are lignin, hemicellulose, and cellulose. Lignin and hemicellulose lie on the surface of fiber and provide a cementing force in the interfibrillar region, which keeps the three components tightly bonding together. After alkali treatment, most hemicellulose and lignin were removed from grass fibers. Therefore, the materials in the interfibrillar region became less, which reduced the cementing force between fibrils.15 This indicates that the microstructure of grass fiber changed after alkali treatment. The detailed changes in surface structure detected by XPS are shown in Figure 7, which shows the increased O/C atom ratio after alkali solution treatment, suggesting

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Figure 8. Fracture surfaces in liquid nitrogen of (a, 280× with scale bar of 150 µm) and (b, 550× with scale bar of 100 µm) 30 wt % raw grass fiber and (c, 280× with scale bar of 150 µm) and (d, 550× with scale bar of 100 µm) 30 wt % alkali-treated grass fiber reinforced composites.

relatively increased interaction between the fiber and the matrix. Therefore, the cementing force between microfibrils can be overcome during processing by the polar interaction between the fiber and the matrix, as well as by shear force. Thus, the alkali-treated fiber reinforced composites became separated fiber reinforced composites. Thermal and Mechanical Properties. The mechanical and thermal properties of 30 wt % raw and alkali-treated grass fiber reinforced composites are shown in Figure 9. Those results show that 30 wt % alkali-treated grass fiber reinforced composites improved the tensile strength by 60%, the flexural strength by 40%, and the impact strength by 30%, compared to the 30 wt % raw fiber reinforced composites. In addition, the tensile modulus, the flexural modulus, and the HDT increased. The mechanical strength of the resulting composites was increased as a result of the aboveexplained increases in the aspect ratio of the alkalitreated fibers. An additional reason is that the homogeneous dispersion of fiber in matrix reduced the extent of stress concentration of the fiber in the matrix. The increased HDT of alkali-treated fiber reinforced biocomposites is caused by the improvement in modulus. Morphology. The tensile fracture surface of 30 wt % raw and alkali-treated grass fiber reinforced composites is shown in Figure 10. Raw fiber reinforced composites showed that lots of fibers bunched together. However, alkali-treated fiber reinforced composites had reduced size and became separated fiber reinforced composites. Actually, the reduced fiber size after alkali treatment in the matrix is easily oriented along the flow direction during injection molding processing. This is proved by Figure 10, due to the fact that most of the

Figure 9. Physical properties of (A) 30 wt % raw grass fiber reinforced soy plastic composites and (B) 30 wt % alkali-treated grass fiber (10% alkali solution 4 h) reinforced composites.

raw fibers are not in the injection direction of the sample and all the alkali-treated fibers are in the injection direction of the sample. This increased the fiber orientation factor and hence improved the mechanical proper-

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Figure 10. Tensile fracture surfaces of (a, 150× with scale bar of 300 µm) and (b, 300× with scale bar of 150 µm) 30 wt % raw grass fiber and (c, 200× with scale bar of 250 µm) and (d, 550× with scale bar of 100 µm) 30 wt % alkali-treated grass fiber reinforced composites.

ties of the composites. In addition, alkali treatment increased the content of hydroxyl group on the surface of fiber and hence increased the O/C atom ratio on the surface of the fiber (Figure 7), due to the removal of hemicellulose and lignin. Thus, alkali-treated fibers have an O/C atom ratio similar to that of soy plastic. This increased the chemical similarity and hence the interaction between fiber and soy plastic due to the hydrogen bonding or polar interaction between hydroxyl group or carboxyl group between them. Based on the above reasons, alkali-treated fiber reinforced biocomposites had improved mechanical strength. Conclusion Indian grass fiber reinforced soy based biocomposites were prepared using a twin-screw extruder and an injection molder. UTS, DMA, and ESEM were used to evaluate the mechanical properties, thermal properties, and morphology of these composites. Indian grass fiber improved the tensile properties and flexural properties of soy based biocomposites. However, Indian grass did not change the impact strength of soy based biocomposites. Compared to soy based bioplastic, the tensile strength and modulus, the flexural strength and the modulus, and the heat deflection temperature of the composites with 40 wt % Indian Grass fiber were enhanced 40%, 1800%, 140%, 1000%, and 80%, respectively. Fracture surface morphology of biocomposites indicated that the fracture of grass fiber reinforced biocomposites usually occurred in the outer surface of the raw fiber, which shows that the interaction between the matrix and the inner surface of the grass fiber was better than that between the outer surface of the fiber and the matrix. The alkali solution treated Indian grass

fiber significantly increased the tensile strength (60%) and impact strength (30%) as well as the flexural strength (40%) due to the improved dispersion of the fiber in the matrix and the enhanced aspect ratio of the fiber. Acknowledgment The financial support from USDA-NRI (Grant 200135504-10734) is gratefully acknowledged for this research. We are grateful to GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) 2002 Award No. GR02-066 for partial financial support. We also thank ADM (Decatur, IL), Eastman Chemical Company (Kingsport, TN), and Smith, Adams & Associates LLC (Okemos, MI) for their generosity in supplying soy flour, Eastar Bio GP copolyester, and grass samples, respectively. Literature Cited (1) Liu, W. J.; Yang, H. L.; Wang, Z.; Dong, L. S.; Liu, J. J. Effect of nucleating agents on the crystallization of poly(3hydroxybutyrate-co-3-hydroxyvalerate). J. Appl. Polym. Sci. 2002, 86, 2145. (2) Phillips, L. G.; Whitehead, D. M.; Kinsella, J. Structurefunction properties of food proteins; Academic Press: London, 1994; Chapter 1, pp 1-23. (3) Utsumi, S.; Matsumura, Y.; Mori, T. Structure-function relationships of soy protein. In Food proteins and their applications; Damodaran S., Paraf A., Eds.; Marcel Dekker: New York, 1997; pp 257-291. (4) Mo, X.; Sun, X. Plasticization of soy protein polymer by polyol-based plasticizers. J. Am. Oil Chem. Soc. 2002, 79 (2), 197. (5) Wang, S.; Sue, H. J.; Jane, J. Effects of polyhydric alcohols on the mechanical properties of soy protein plastics. J. Macromol. Sci., Pure Appl. Chem. 1996, A33 (5), 557.

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(6) Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein sheets. Polymer 2001, 42, 2569. (7) Liang, F.; Wang, Y.; Sun, X. S. Curing process and mechanical properties of protein-based polymers. J. Polym. Eng. 1999, 19 (6), 383. (8) John, J.; Bhattacharya, M. Properties of reactively blended soy protein and modified polyesters. Polym. Int. 1999, 48 (11), 1165. (9) Drzal, L. T.; Mohanty, A. K.; Tummala, P.; Misra, M. Environmentally friendly bio-composites from soy-based bio-plastic and natural fiber. Polym. Mater. Sci. Eng. 2002, 87, 117. (10) Mohanty, A. K.; Misra, M.; Hinrichsen, G. Biofibres, biodegradable polymers and biocomposites. An overview. Macromol. Mater. Eng. 2000, 276/277, 1. (11) Mohanty, A. K.; Misra, M.; Drzal, L. T. Sustainable Biocomposites from Renewable Resources: Opportunities and Challenges in the Green Materials World. J. Polym. Environ. 2002, 10 (1/2), 19. (12) Tummala, P.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Ecocomposite materials from novel soy protein-based bioplastics and natural Fibers. Proceedings of the International Conference on Composite Materials (ICCM-14), 2003; No. 1759. (CD-ROM.) (13) Lodha, P.; Netravali, A. N. Characterization of interfacial and mechanical properties of “green” composites with soy protein isolate and ramie fiber. J. Mater. Sci. 2002, 37 (17), 3657. (14) Stokke, D. D.; Kuo, M.; Curry, D. G.; Gieselman, H. H. Grassland flour/polyethylene composites. Proceedings of the 6th International Conference on Woodfiber-Plastic Composites, Madison, 2001; Forest Products Society: Madison, WI, 2002; pp 4353.

(15) Liu, W.; Mohanty, A. K.; Drzal, L. T.; Askeland, P.; Misra, M. Effects of alkali treatment on the structure, morphology and thermal properties of native grass fibers as reinforcements for polymer matrix composites. J. Mater. Sci. 2004, 39, 1051. (16) Liu, W.; Mohanty, A. K.; Askeland, P.; Drzal, L. T.; Misra, M. Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protein based biocomposites. Polymer 2004, 45, 2247. (17) Haile, W. A.; Bhat, G. S.; Williams, F. W. Biodegradable copolyester for fibers and Nonwovens. Int. Nonwovens J. 2002, 40, 39. (18) Nielsen, L. E.; Landel, R. F. Mechanical properties of polymers and composites, 2nd ed., revised and expanded; Marcel Dekker: New York, 1994. (19) Crosby, J. M.; Drye, T. R. How fibers affect fracture behavior of nylon-66 composites. Mod. Plast. 1986, 63 (11), 74. (20) Kim, H. C. Toughening mechanisms of long-fiber-reinforced thermoplastics. Soc. Automot. Eng., [Spec. Publ.] SP 1998, SP1340, 167. (21) Bledzki, A. K.; Gassan, J. Composites reinforced with cellulose based fibres. Prog. Polym. Sci. 1999, 24 (2), 221.

Received for review February 27, 2005 Revised manuscript received June 16, 2005 Accepted June 20, 2005 IE050257B