Znd. Eng. Chem. Res. 1995,34, 1889-1896
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Renewable Agricultural Fibers as Reinforcing Fillers in Plastics: Mechanical Properties of Kenaf Fiber-Polypropylene Composites Anand R. Sanadi,*gt Daniel F. Caulfield,' Rodney E. Jacobson: and Roger M. Rowelltg' Department of Forestry, University of Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706, and Forest Products Laboratory, USDA, 1 Gifford Pinchot Drive, Madison, Wisconsin 53705
Kenaf (Hibiscus cannabinus) is a fast growing annual growth plant that is harvested for its bast fibers. These fibers have excellent specific properties and have potential to be outstanding reinforcing fillers in plastics. In our experiments, the fibers and polypropylene (PP)were blended in a thermokinetic mixer and then injection molded, with the fiber weight fractions varying t o 60%. A maleated polypropylene was used to improve the interaction and adhesion between the nonpolar matrix and the polar lignocellulosic fibers. The specific tensile and flexural moduli of a 50% by weight (39% by volume) of kenaf-PP composite compare favorably with a 40% by weight of glass fiber-PP injection-molded composite. These results suggest that kenaf fibers are a viable alternative to inorganidmineral-based reinforcing fibers as long as the right processing conditions are used and they are used in applications where the higher water absorption is not critical.
Introduction Newer materials and composites that have both economic and environmental benefits are being considered for applications in the automotive, building, furniture, and packaging industries. Mineral fillers and fibers are used frequently in the plastics industry to achieve desired properties or to reduce the cost of the finished article. For example, glass fiber is used to improve the stiffness and strength of plastics although there are several disadvantages associated with the use of the fiber. Glass fibers require a great deal of energy to produce since processing temperatures can exceed 1200 "C.They tend t o abrade processing equipment and also increase the density of the plastic system. Lignocellulosic fibers have received a lot of interest for use with thermoplastics such as polyolefins,in high-volume, lower cost applications. The inherent polar and hydrophilic nature of the lignocellulosic fibers and the nonpolar characteristics of the polyolefins create difficulties in compounding and result in inefficient composites. Proper selection of additives is necessary to improve the interaction and adhesion between the fiber and matrix phases. Most of the prior work on lignocellosic fibers in thermoplastics has concentrated on wood-based flour or fibers, and significant advances have been made by a number of researchers (Woodhamset al., 1984; Klason and Kubat, 1986; Myers et al., 1992; Kokta et al., 1989; Yam et al., 1990; Bataille et al., 1989; Sanadi et al., 1994a). Recent research on the use of annual growth lignocellulosic fibers suggests that these fibers have the potential for use as reinforcing fillers in thermoplastics, and a brief preliminary account was published earlier (Sanadi et al., 1994b). The use of annual growth agricultural crop fibers such as kenaf has resulted in significant property advantages as compared to typical wood-based fillerdfibers such as wood flour, wood fibers, and recycled newspaper. The low cost and densities and the nonabrasive nature of the fibers allow high filling levels, thereby resulting in significant material cost
* Corresponding author. E-mail:
[email protected]. + University
of Wisconsin.
* Forest Products Laboratory, USDA.
savings. The primary advantages of using annual growth lignocellulosic fibers as additives in plastics are listed as follows: (1) low densities, (2) low cost, (3) nonabrasive nature, (4) high filling levels possible, ( 5 ) low energy consumption, (6) high specific properties, (7) biodegradability, (8) wide variety of fibers available throughout the world, and (9) generation of a rural/ agricultural-based economy. The two main disadvantages of using lignocellulosic fibers in thermoplastics are the high moisture absorption of the fibers and composites (Sanadi et al., 1994b) and the low processing temperatures permissible. The moisture absorbed by the composite and the corresponding dimensional changes can be reduced dramatically if the fibers are thoroughly encapsulated in the plastic and there is good adhesion between the fiber and the matrix. If necessary, moisture absorption of the fibers can be significantly reduced by the acetylation of the hydroxyl groups present in the fiber (Rowel1et al., 19861, although a t some increase in cost. The disadvantage of the high moisture absorption of the composite can be minimized by selecting applications where the high moisture absorption is not a major drawback. For example, polyamide and its composites absorb water, but applications are such that this deficiency is not of prime importance. The processing temperature of the lignocellosic fibers in thermoplastics is limited due to potential fiber degradation a t higher temperatures. The plastics that can be used are limited to low melting temperature plastics. In general, we have not experienced any problems of deterioration of properties due to fiber degradation when processing temperatures are maintained below about 200 "C, except for short periods. Kenaf fibers are extracted from the bast of the plant Hibiscus cannabinus, and filament lengths longer than 1 m are common. These filaments consist of discrete individual fibers, generally 2-6 mm long, which are themselves composites of predominantly cellulose, lignin, and hemicelluloses. Filament and individual fiber properties can vary depending on the source, age, separating techniques, and history of the fiber. Furthermore, the properties of the fibers are difficult to measure, and we have made no attempt to measure the properties of kenaf. Earlier work on a similar natural
0888-588519512634-1889$09.00/0 0 1995 American Chemical Society
1890 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
bast filament, sunhemp (Crotalariajuncea), suggested that the filament properties ranged widely. The tensile strengths of the filaments of sunhemp varied from about 325 to 450 MPa, while the tensile modulus ranged from 27 to 48 MPa (Sanadi et al., 1985). Kenaf was chosen for the study because it is now a fiber crop grown commercially in the United States. Several other fibers isolated from annual growth crops have potential as reinforcing fillers in plastics. The choice of the fiber for plastics applications depends on the availability of the fiber in the region and also on the ultimate composite properties needed for the specific application. In this study, the mechanical properties of kenafPP composites have been evaluated. The effect of the fiber content on the tensile, flexural, and impact properties of the composites was determined. The use of a maleated PP to improve the fiber-matrix interaction and adhesion has been discussed in regards to why and how they function t o improve properties.
Experimental Methods Kenaf filaments, about 15-20 cm long, harvested from mature plants, were obtained from AgFibers Inc., Bakersfield, CA, and cut into lengths of about 1cm. The fibers were not dried to remove any of the moisture present, and the moisture content of the fibers varied from 6% to about 9% by weight. In all our experiments the weight and volume percent reported is the amount of dry fiber present in the blend. The homopolymer was Fortilene-1602 (Solvay Polymers, TX)with a melt flow index of 12 gI10 min as measured by ASTM D-1238. A maleic anhydride grafted polypropylene (MAPP),Epolene G-3002, from Eastman Chemical Products, TN, was used as a coupling agent to improve the compatibility and adhesion between the fibers and matrix. The G-3002 had a number-average molecular weight of 20 000, a weight-average molecular weight of 40 000, and an acid number of 60. An acid number of 60 is about equivalent t o a 6% by weight of maleic anhydride in the G-3002 (Eastman Chemical Company, 1992). The filaments were not pulped prior t o compounding as the procedure can consume a significant amount of energy. The short fibers, MAPP, and PP (the latter two in pellet form) were compounded in a high-intensity kinetic mixer (Synergistics Industries Ltd., Canada) where the only source of heat is generated through the kinetic energy of rotating blades. The blending was accomplished at 4600 rpm, which resulted in a blade tip speed of about 30 d s . The blend was automatically discharged from the mixer when the temperature reached 190 "C. A total weight (fibers, PP, and MAPP) of 150 g was used for each batch, and about 1.5 kg of blended material was prepared for each set of experiments. The total residence time of the blending operation depended on the proportions of fiber and PP present and averaged about 2 min. Experiments were designed t o study the effect of the amount of coupling agent used in the blend on composite mechanical properties. In all these experiments the blends contained 50% by weight of kenaf and an equivalent 50% by weight of the matrix material (PP and MAPP). The amount of MAPP varied from 0% to 3%, and the amount of PP used in the blend was changed so that the total matrix amount was 50% by weight of the composite. In another series of experiments, the amount of coupling agent was kept constant at 2% by weight of the composite and the amount of
fibers was varied from 20% to 60% by weight of the composite; the amount of PP correspondingly changed depending on the amount of fiber used. The mixed blends were then granulated and dried at 105 "C for 4 h. Test specimen were injection molded a t 190 "C using a Cincinnati Milacron Molder, and injection pressures varied from 2.75 to 8.3 MPa depending on the constituents of the blend. Specimen dimensions were according to the respective ASTM standards. The specimens were stored under controlled conditions (20% relative humidity and 32 "C)for 3 days before testing. Tensile tests were conducted according t o ASTM 63890, Izod impact strength tests according to ASTM D 25690, and flexural testing according to the ASTM 790-90 standard. The crosshead speed during the tension and flexural testing was 5 m d m i n . Although all the experiments were designed around the weight percent of kenaf in the composites, fiber volume fractions can be estimated from composite density measurements and the weights of dry kenaf fibers and matrix in the composite. The density of the kenaf present in the composite was estimated to be 1.4 g/cm3.
Results and Discussion Molecular Interactions at the Fiber Surface. The dispersion and adhesion between the nonpolar PP matrix and the polar lignocellulosic fibers are critical factors in determining the properties of the composite. Several different types of functionalized additives have been used to improve the dispersion and the interaction (Dalvag et al., 1985; Kokta et al., 1989) between cellulose-based fibers and polyolefins. Maleic anydride (MA) grafted polypropylene (MAF'P)has been reported t o function efficiently for lignocellulosic-PP systems. Earlier results suggest that the amount of MA grafted and the molecular weight are both important parameters that determine the efficiency of the additive (Felix et al., 1993; Sanadi et al., 1993). The maleic anhydride present in the MAF'P not only provides polar interactions but can covalently link to the hydroxyl groups on the lignocellulosic fiber. The formation of covalent linkages between the MA and the OH of cellulose has been indicated through IR and ESCA analysis by Gatenholm et al. (1992). The interactions between the PP matrix chains and the MAPP are predominantly that of chain entanglement. When polymer chains are very short, there is little chance of entanglements between chains and they can easily slide past one another (Neilsen, 1977). When the polymer chains are longer, entanglement between chains can occur and the viscosity of the polymer becomes high. Stresses applied to one chain can be transmitted to other entangled chains, and stress is distributed among many chains. These entanglements function like physical cross-links that provide some mechanical integrity up to and above the glass transition temperature, Tg, but become ineffective at much higher temperatures (Neilson, 1974). A minimum molecular weight (MW,) is necessary to develop these entanglements, and a typical polymer has a chain length between entanglements equivalent to a MW varying from 10 000 to about 40 000. The M W , varies depending on the structure of a polymer, and for example, linear polyethylene has a corresponding minimum molecular weight for entanglements of about 4000 (Neilsen, 1977). Factors such as the presence of hydrogen bonding or the presence of side chains that affect the glass transition temperature of the polymer will also effect
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the MW, of the polymer melt. It is important t o note that the presence of the fiber surface is likely to restrict the mobility of the polymer molecules and that the minimum entanglement lengths will vary according the fiber surface characteristics. To develop sufficient stress transfer properties between the matrix and the fiber, two factors need to be considered. Firstly, the MAPP present near the fiber surface should be strongly interacting with the fiber surface through covalent bonding a n d o r acid-base interactions. This means that sufficient MA groups should be present in the MAPP so that interactions can occur with the OH groups on the fiber surface. This is to ensure that that the fiber surface-interphase interaction is strong. Secondly, the polymer chains of the MAPP should be long enough to permit entanglements
with the PP in the interphase. Polar polymers that can develop hydrogen bonding between chains tend to reach mechanical integrity a t lower molecular weights. In the case of MAPP interaction with the fiber surface there is a possibility that two or more MA groups, from the same MAPP molecule, interact with different OH groups on the fiber surface to form a tightly bound MAPP molecule. Segmented loops are then formed between sites of MA and OH interactions. It must be pointed out that, although loop formation can and may occur, there will also be cases where a tail section of the MAPP “sticks out” into the interface region. This section we believe has a greater possibility of interaction with the PP molecules because of greater entanglement possibilities as compared to any MAPP that has formed loops. It must be noted that a minimum segmented
1892 Ind. Eng. Chem. Res., Vol. 34, No. 5 , 1995 200
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the surface energetics of the fiber surface. MAPPs with a very high molecular weight but low MA content do not perform as well as the G3002 (Sanadi et al., 1994a). Theoretically, extremely long chains of MAPP with a lot of MA grafted would be an ideal additive in lignocellulosic-PP composites, creating both covalent bonding to the fiber surface and extensive molecular entanglement to improve the properties of the interphase. However, extremely long chains may reduce the possibility of migration of the MAPP to the fiber surface because of the short processing times. If the MW of the MAPP is too high, the MAPP may entangle with the PP molecules so that the polar groups on the MAPP have difficulty "finding" the OH groups on the fiber surface. Transcrystallinity around the fiber surface complicates the understanding since crystallites can act like cross-links by tying many molecules together. Crystallites have much higher moduli as compared to the amorphous regions and can increase the modulus contribution of the polymer matrix to the composite modulus. Furthermore, the influence of the molecular weight on the interphase morphology and transcrystallinity are important considerations. Previous studies are somewhat conflicting regarding the effects of surface characteristics on transcrystalline formation. Quillin et al. (1993)found that improvement in surface energetics between the cellulosic fibers and the PP resulted in the prevention of transcrystalline zones. Transcrystallinity was abundant when there was no surface modification of the polar fiber and the nonpolar matrix. On the other hand, Gatenholm et al. (1992) found that the higher the molecular weight of the MAPP, the greater were the number of nuclei formed at the fiber surface. It should be pointed out that both the above experiments were model single-fiber (cellulose) tests with controlled cooling cycles. Vollenberg and Heikens' (1989a) studies on small particle size glass- or alumina-filled PP composites suggest that any sizing or coupling agent on the glass or alumina eliminated the formation of high-modulus material near the fiber surface. This hypothesis was suggested for the annealed composite based on eliminating other pos-
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length is necessary for good interaction, whether through a tail or loop section of the MAPP. An ideal situation would be to create a molecule with a "head-tail" configuration, where one or more MA groups are grafted onto one end of the MAPP molecule while the other end has no grafted MA molecules. This would maximize the length of entaglement with the PP molecules and also permit the MA groups to interact with the OH groups on the fiber surface. Using G-3002, a maleic anhydride grafted PP, has resulted in efficient composites, and this is probably due to the higher molecular weight coupled with a high MA content (Sanadi et al., 1994a). However, there is some free anhydride present in the G-3002, and this complicates the understanding of the characteristics and function of the MAPP on the properties of the fibermatrix interphase. The free MA may preferentially bond to available OH sites on the fiber and reduce the interaction between the MAPP and the fiber. Furthermore, free MA bonded to the fiber surface can change 7s
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0 40 50 40 40 40 40 40 0 30 39 30 18 18 19 18 tensile modulus, GPa D638 1.7 6 7.7 4.4 4 3.5 9 7.6 5.9 7.2 4.5 3.1 2.8 7.3 6.0 specific tensile modulus, GPa 1.9 tensile strength, MPa D638 33 56 62 53 35 25 110 39 specific tensile strength, MPa 37 55 58 54 28 20 89 31 elongation at break, % D638 >>lo 1.9 2.2 3 X X 2.5 2.3 flexural strength, MPa D 790 41 82 91 80 63 48 131 62 specific flexural strength, MPa 46 80 107 49 85 82 50 38 flexural modulus, GPa D 790 1.4 5.9 7.8 3.9 4.3 3.1 6.2 6.9 specific flexural modulus, GPa 1.6 5.8 7.3 4.0 3.4 2.5 5.0 5.5 notched Izod impact, J/m D256A 24 28 32 21 32 32 107 27 specific gravity 0.9 1.02 1.07 0.98 1.27 1.25 1.23 1.26 water absorption %, 24 h D570 0.02 1.05 0.95 0.02 0.02 0.06 0.03 mold (linear) shrinkage cm/cm 0.028 0.004 0.003 0.01 0.01 0.004 a Data for commercial systems from various sources: Modern Plastics Encyclopedia, 1993, and Machine Design, 1994, Materials Guide Issue.
sibilities. Solid state nuclear magnetic resonance experiments (Vollenberg and Heikens, 1989b)support this explanation. Effect of MAPP on Composite Properties. Figures 1-3 show the effect of the amount of G-3002 on composite properties of 50% by weight (about 39% by volume) of kenaf fibers in PP. A small amount of the MAPP (0.5% by weight) improved the flexural and tensile strength, tensile energy absorption, failure strain, and un-notched Izod impact strength. The anhydride groups present in the MAPP can covalently bond to the hydroxyl groups of the fiber surface (Gatenholm et al., 1992). A n y MA that is in acid form can interact with the fiber surface through acid-base interactions. The improved interaction and adhesion between the fibers and matrix, either through covalent bonding or acid-base interaction (such as H-bonding), or a combination of both, leads t o better matrix to fiber stress transfer. There was little difference in the properties obtained between the 2%and 3%(by weight) G-3002 systems. The drop in tensile modulus with the addition of the MAPP is probably due t o the molecular
morphology of the polymer near the fiber surface or in the bulk of the plastic phase. Transcrystallization and changes in the apparent modulus of the bulk matrix can result in changes in the contribution of the matrix to the composite modulus and will be discussed later. There is little change in the notched impact strength with the addition of the G-3002, while the improvement in the un-notched impact strength is significant. In the notched test, the predominant mechanism of energy absorption is through crack propagation as the notch is already present in the sample. Addition of the coupling agent has little effect on the amount of energy absorbed during crack propagation. On the other hand, in the un-notched test, energy absorption is through a combination of crack initiation and propagation. Cracks are initiated at places of high stress concentrations such as the fiber ends, defects, or at the interface region where the adhesion between the two phases is very poor. The use of the additives increases the energy needed t o initiate cracks in the system and thereby results in improved un-notched impact strength values with the addition of the G-3002.
1894 Ind. Eng. Chem. Res., Vol. 34,No. 5 , 1995
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to failure of the composite. There is a plateau after which further addition of coupling agent results in no further increase in ultimate failure strain. The number average M w of the G-3002 is about 20 000, and the amount of entanglements between the PP molecules and MAPP is limited; molecules flow past one another at a critical strain. Any further increase in the amount of G-3002 does not increase the failure strain past the critical amount. However, a minimum amount of entanglements is necessary through the addition of about 1.5%by weight (Figure 2) for the critical strain to be reached.
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Use of the MAPP increases the failure strain and the tensile energy absorption. Thermodynamic segregation of the MAPP toward the interphase can result in covalent bonding t o the OH groups on the fiber surface. Entanglement between the PP and MAPP molecules results in improved interphase properties and the strain
There is little difference in the tensile strength of uncoupled composites compared with the unfilled PP, irrespective of the amount of fiber present (Figure 4). This suggests that there is little stress transfer from the matrix to the fibers due to incompatibilities between the different surface properties of the polar fibers and nonpolar PP. The tensile strengths of the coupled systems increase with the amount of fiber present and strengths of up to 74 MPa were achieved with the higher fiber loading of 60%by weight or about 49% by volume. As in case of tensile strength, the flexural strength of the uncoupled composites was approximately equal for all fiber loading levels, although there was a small improvement as compared to the unfilled PP. The high shear mixing using the thermokinetic mixer causes a great deal of fiber attrition. Preliminary measurements of the length of fibers present in the composite after injection molding show that few fibers are longer than 0.2mm. The strengths obtained in our composites were thus limited by the short fiber lengths. Higher strengths are likely if alternate processing techniques are developed that reduce the amount of fiber attrition while at the same time achieving good fiber dispersion. The tensile modulus of the kenaf composites showed significant improvements with the addition of the fibers (Figure 5). The uncoupled composites showed some very interesting behavior, with moduli higher than the
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1896
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coupled systems at identical fiber loading. The possibility of a high-stifmess transcrystalline zone forming around the fiber in the unmodified systems could lead t o the high moduli observed. Although the possibility of different fiber orientations contributing to the higher modulus cannot be ruled out, preliminary studies on fiber orientation suggest that the difference in moduli cannot be explained exclusively by the difference in fiber orientations. Several studies suggest that the morphology of the polymer chains is affected by the presence of filler particles. The specific tensile and flexural moduli of the 50% by weight of the kenaf-coupled composites were about equivalent to or higher than typical reported values of 40% by weight coupled glass-PP injection-molded composites (Table 1). The specific flexural moduli of the kenaf composites with fiber contents greater than 40% were extremely high and even stiffer than a 40% micaPP composite.
Failure Strain and Tensile Energy of Absorption The failure strain decreases with the addition of the fibers (Figure 6). Addition of a rigid filledfiber restricts the mobility of the polymer molecules to flow freely past one another and thus causes premature failure. Addition of MAPP followed a similar trend to that of the uncoupled system, although the drop in failure strain with increasing fiber amounts was not as severe. Figure 7 shows typical stress-strain curves of pure PP, uncoupled 50% by weight of kenaf-PP, and coupled systems with increasing amounts of kenaf in the composite. The decrease in the failure strain with increasing amounts of kenaf for the coupled systems is apparent. The nonlinearity of the curves is mainly due t o plastic deformation of the matrix. The distribution of the fiber lengths present in the composite can also influence the shape of the curve since the load taken up by the fibers decreases as the strain increases, and detailed explanations are available elsewhere (Hull, 1981). The tensile energy absorption, the integrated
area under the stress-strain curve up t o failure, behaves in roughly the same manner as the tensile failure strain. The difference between the coupled and uncoupled composites increases with the amount of fibers present, although the drop in energy absorbed for the coupled composites levels off after the addition of about 35 vol % of fiber.
Impact Properties The impact strength of the composite depends on the amount of fiber and the type of testing, i.e., whether the samples were notched or un-notched (Figure 8). In the case of notched samples, the impact strength increases with the amount of fibers added until a plateau is reached at about 45% fiber weight, irrespective of whether MAPP was used or not. The fiber bridges crack and increase the resistance of the propagation of the crack. Contribution from fiber pullout is limited since the aspect ratio of the fibers in the system is well below the estimated critical aspect ratio of about 0.4 mm (Sanadi, et al., 1993). In the case of the unnotched samples, impact values of the uncoupled composites and the presence of the fibers decrease the energy absorbed by the specimens. Addition of the fibers creates regions of stress concentrations that require less energy to initiate a crack. Improving the fiber-matrix adhesion through the use of MAPP increases the resistance t o crack initiation a t the fibermatrix interface, and the fall in impact strength with the addition of fibers is not as dramatic.
Conclusions Table 1 shows some typical data for commercially available injection-molded PP composites and the comparison with typical kenaf-PP composites. Data on the talc, mica, calcium carbonate, and glass composites were compiled from the Resins and Compounds, Modern Plastics Encyclopedia (1993)and Thermoplastic Molding Compounds, Material Design (1994).The properties of kenaf-based fiber composites have properties superior
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to typical wood (newspaper) fiber-PP composites. The specific tensile and flexural moduli of a 50% by weight of kenaf-PP composites compares favorably with the stiffest of the systems shown, that of glass-PP and mica-PP. The properties of 40% by weight of kenafPP is far superior to fillers such as calcium carbonate, newspaper fibers, and talc. The two main disadvantages of using kenaf-PP as compared t o glass-PP are the lower impact strength and the higher water absorption. The lower notched impact strength can be improved by using impactmodified PP copolymers and flexible maleated copolymers, albeit with some loss in tensile strength and modulus, and this will be discussed in a later paper. Care needs to be taken when using these fibers in applications where water absorption and the dimensional stability of the composites are of critical importance. Judicious use of these fibers will make it possible for agrobased fibers to define their own niche in the plastics industry for the manufacture of low-cost, highvolume composites using commodity plastics. An interesting point to note is the higher fiber volume fractions of the agrobased composites compared to those of the inorganic-filled systems. This can result in significant material cost savings as the agricultural fibers are cheaper than the pure PP resin and far less expensive than glass fibers. Environmental and energy savings by using a agriculturally grown fiber instead of the high energy utilizing glass fibers or mined inorganic fillers are benefits that cannot be ignored, although a thorough study needs to be conducted to evaluate the benefits. Acknowledgment The authors acknowledge partial support of this work from the USDA, Enhancing Value/Use of Agricultural Products Program (NRI Competitive Grants P r o g r a d USDA, Award No. 9303555). One of the authors (A.R.S.) would like to acknowledge partial support by the Forest Products Laboratory, Madison, WI. The authors thank Gordon Fisher of AgFibers, Inc., of Bakersfield, CA, J. M. Killough of Solvay Polymers, TX, and David Olsen of Eastman Chemical Products, TN, for their generous donation of materials. The authors acknowledge the diligent work of L. Wieloch and K. Walz throughout the course of this study. Testing was conducted by the st& of EML, Forest Products Laboratory, and help in this regard from M. Doran, D. Cuevas, and J. Murphy is much appreciated. The assistance of J. Schneider and C. Clemons during the processing of the composites was invaluable. Literature Cited Bataille, P.; Ricard, L.; Sappieha, S. Effect of Cellulose in Polypropylene Composites. Polym. Compos. 1989, 10, 103. Dalvag, H.; Klason, C.; Stromvall, H. E. The Efficiency of Cellulosic Fillers in Common Thermoplastics. Part I1 Filling with Processing Aids and Coupling Agents. Int. J. Polym. Mater. 1985, 11, 9. Eastman Chemical Company, Publication AP-31, Aug., 1992.
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Received for review August 23, 1994 Revised manuscript received February 21, 1995 Accepted March 13, 1995@ IE940507G Abstract published in Advance A C S Abstracts, April 15, 1995. @
