Critical Issues in Extrusion Foaming of Plastic/ Woodfiber Composites (Review)

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1 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites Critical Issues in Extrusion Foaming of Plastic/ Woodfiber Composites (Review) G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8 Faculty of Engineering and Applied Sciences, University of Ontario Institute of Technology Received: 21 April 2005 Accepted: 15 November 2005 ABSTRACT Foaming of wood-fiber/ plastic composites (WPC) with a fine-celled structure can offer benefi ts such as improved ductility and impact strength, lowered material cost, and lowered weight, which can improve their utility in many applications. However, foaming of WPC is still a poorly understood art. This paper presents a review of material published, which address the various critical issues particularly in extrusion foaming of WPC, and the proposed processing techniques and strategies, for producing artifi cial wood with enhanced properties. INTRODUCTION For the last two decades, interest in wood-fiber/plastic composites (WPC) has been driven by the lowered cost, environmental regulations and advances in processing technology. However, the shortcomings of WPC as an artificial wood, such as heavy weight, low ductility, low impact strength, poor nailingability and screwing-ability, and high flammability have limited their utility in many applications. These shortcomings can be effectively compensated for by incorporating a fine-celled structure and by using proper additives in these composites. In addition, foaming of WPC results in lowered material cost, better surface definition, and sharper contours and corners than un-foamed profiles (1). During production, the foamed composites run at lower temperatures and at faster speeds than their un-foamed counterparts, due to the plasticizing effects of gas, thus the production cost is reduced too (1). If the cell morphology of the foamed WPC consists of a large number of uniformly distributed small cells, the specific mechanical properties are significantly improved (2,3). Extrusion is the most commercially viable plastic processing option and hence is the focus of this paper. However, in order to fabricate wood like structure with uniformly distributed fine cells, it is essential to understand the underlying Cellular Polymers, Vol. 24, No. 6,

2 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim issues involved in extrusion processing of WPC. Therefore, this paper presents a review of the published material, highlighting the critical issues in foaming of WPC, and discusses processing strategies for producing high-performance artificial wood in a cost effective way. CRITICAL ISSUES AND PROCESSING STRATEGIES Dispersion of Wood-fiber and Fiber-Matrix Bonding Wood-fiber (WF) is hydrophilic in nature due to its polar structure, and tends to adhere to each other by inter-fiber hydrogen bonding, rather than disperse in the generally hydrophobic thermoplastic matrix. Using treatment of fibers and/or use of external processing aids can facilitate the dispersion process. Coupling agents (CAs) are usually used to improve fiber-matrix bonding. Coupling agents act to form chemical bridges between resin and fiber (or filler). Most commonly used CAs are organotrialkoxysilanes, titanates, zirconates, organic acid-chromium chloride coordination complexes, stearic acid and maleated polymers. Lu et al. (4) investigated a number of coupling agents and found that coupling agents with a high molecular weight, moderate acid number, and low concentration level performed better at improving interfacial adhesion in WPC which facilitates fiber dispersion. Maldas et al. (5-6), Kokta et al. (7), and Raj et al. (8) have studied various coupling agents and described the improvement fiber dispersion and the consequent enhancement in properties due to their utilization. Processing equipment such as K-mixers (9) and twin-screw compounders can also improve the dispersion process. The use of coupling agents for improving composite properties, and the use of high shear mixing devices for improving fiber dispersion, has been extensively studied in context of traditional WPC fabrication and is well documented (10-20) and will not be further discussed in this paper. Processing Difficulties Due to Increased Viscosity The addition of the solid WF particles to the plastic resin increases the apparent viscosity of the molten mixture during the extrusion processing. Guo et al. studied the viscosity of molten composite as a function of WF content (17) and their results, shown in Figure 1, highlight the non-newtonian behavior of WPC. With an increase of the WF content, the viscosity increases significantly. This causes an increase in the processing pressure with the consequent processing difficulties. The greater the amount of WF added, the greater are the problems 348 Cellular Polymers, Vol. 24, No. 6, 2005

3 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites 1.0E E+05 η* (Pa s) 1.0E E E E E E E E+03 ω (rad/s) Figure 1.Complex viscosity vs. frequency for PWC with different wood fibre contents (0%, 10%, 30%, 50%) associated with the increased processing pressures. The processing pressure can be reduced by either increasing the process temperature, or by using a higher melt index (MI) material. But the volatile emissions from WF increase at higher temperatures. Moreover, the temperature of the melt cannot be increased higher than 200 C to minimize the degradation of the WF. Thus using a higher MI resin may be the only viable strategy available for increasing the WF loading, while keeping the processing temperature low. It is known that the high melt strength of the resin prevents cell coalescence so that the final product retains most of the nucleated cells ensuring uniform cell morphology (10). But the high MI resins have lower melt strength, therefore, are less able to maintain a finecelled structure. Hence, the resins used should have the lowest possible MI at which it can be processed with the desired WF loading. Volatiles from Woodfiber and Thermo-Gravimetric Analysis (TGA) Studies Wood or WF is composed of four basic constituents, which are cellulose, hemicelluloses, lignin and extractives (11-12). Apart from these four constituents, wood also contains water. During the high extrusion foam processing temperatures, WF releases moisture and other volatiles and it deteriorates the Cellular Polymers, Vol. 24, No. 6,

4 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim cell structure of WPC foams, by causing cell coalescence and cell collapse (10). Rizvi et al. investigated the devolatilization behavior of moisture/volatiles from the WF using TGA, at 110 C, a typical drying temperature, and at 200 o C, a typical processing temperature and their findings shown in Figure 2, demonstrate that even after WF is considered oven-dry, it still releases about 3% volatiles when the temperature is raised from 110 o C to 200 o C (18). Therefore, it can be deduced that in extrusion foam processing, whenever the processing temperature is raised to a higher level in any zone of the barrel, additional moisture and volatiles will be generated and will affect the foaming process significantly. In order to ensure fine-celled morphology, the volatile contents of WF need to be reduced to a bare minimum, using any of the standard drying techniques, such as online devolatilization (13,16), oven drying, hot air convective drying, drying in K-mixer (9) and the like, and during subsequent processing stages the temperature should be maintained below the drying temperature (13,16,19). Critical Processing Temperature in Extrusion Foaming of WPC In order to minimize the volatile emissions from the WF during extrusion processing, the processing temperature should be kept to a minimum. But lowering the temperature causes increase in the apparent viscosity of the molten extrudate and thereby increased processing difficulty. Therefore, the determination of an optimum processing temperature becomes crucial to ensure the formation of acceptable cellular structure, while maintaining satisfactory processing conditions. Guo et al. (16) showed that the highest processing temperature after the drying stage primarily governs the emissions Figure 2. TGA thermogram for the devolatilisation of WF 350 Cellular Polymers, Vol. 24, No. 6, 2005

5 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites from WF which affect the foam morphology. The material used was equal amounts of HDPE and WF by weight mixed with 3% coupling agent. They used a tandem extrusion system, shown in Figure 3a (16), in which the WPC melt is devolatilized in the first extruder, and processed in the second extruder, and determined a critical processing temperature above which the excessive volatile emissions prevent the formation of a uniformly distributed fine-celled structure. The drying method used was on line devolatilization through a (a) (b) (c) Figure 3. Schematic of PWC processing systems: (a) tandem extrusion system; (b) twin-screw compounding system; (c) single-screw extrusion system Cellular Polymers, Vol. 24, No. 6,

6 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim vent and no blowing agent, chemical or physical, was used in the processing, so that, the resultant foam structure was caused mainly by the emissions, from WF, generated during processing. The typical cell morphologies of the composite foam samples (Figure 4 (16) ) indicate that optimum composites were produced with the barrel temperature of 160 C which exhibited small-sized bubbles and voids with uniform distribution. The cell morphology above this temperature was visibly irregular and at lower temperatures, the foaming effect was insignificant. This study demonstrates that maintaining the processing temperature below a critical value, ensures reduced volatile emissions, which reduces the deteriorative effects on cell morphology. Control of Residence Time in Extrusion Foaming of WPC Guo et al. (16) proposed that as the TGA gives the time-dependant rate of weight loss at each condition, it can also be used, referring to (Figure 3a), to predict the amount of volatiles that may be released, from the devolatilized WF, upon further processing in the second extruder and thereby contribute to the foaming process. They assumed that the same processing temperature is used in the entire tandem system, and the volatiles liberated in the first extruder, in addition to the adsorbed moisture, are all purged out at the devolatilizing vent or through the hopper inlet. They further assumed that most of the volatiles liberated in the second extruder are trapped in the melt and contribute to the formation of the foam structure. The amount of volatiles liberated in each extruder depends on the time spent by the composite melt in that extruder. Figure 5 (16) shows the estimated amount of volatiles used in foaming, based on the TGA results, which is essentially the same as the weight loss by WF Figure 4. Effects of barrel temperature on the cell morphology of HDPE/WF foams. T v = 160 C, T d = 140 C: (a) T b = 145 C; (b) T b = 165 C; (c) T b = 175 C 352 Cellular Polymers, Vol. 24, No. 6, 2005

7 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites Amount of volatililes (%) Amount of volatililes (%) Residence time in 2nd extruder (min) (a) after 2 min in 1st extruder Residence time in 2nd extruder (min) (b) after 4 min in 1st extruder Amount of volatililes (%) Amount of volatililes (%) Residence time in 2nd extruder (min) (c) after 8 min in 1st extruder Residence time in 2nd extruder (min) (d) after 12 min in 1st extruder Figure 5. Estimated amount of volatiles used for foaming based on TGA data. The time in minutes indicates the residence time spent in the second extruder: (a) after a residence time of 2 minutes in 1st extruder; (b) after a residence time of 4 minutes in 1st extruder; (c) after a residence time of 8 minutes in 1st extruder; (d) after a residence time of 12 mintues in 1st extruder (or volatiles released) at the temperature maintained in the second extruder. Comparing Figures 5 (a) through (d), it can be seen that as the residence time in the first extruder is increased, the volatile emissions in the second extruder are decreased. Secondly, the lower the residence time in the second extruder, the lesser are the emissions from the WF. This indicates that if the WF is exposed to a high temperature for a long time in the first extruder, the amount of the volatiles generated from WF in the second extruder can be very small even at higher temperatures like above 175 C. This strategy may possibly be used to produce fine-celled foam structure in WF composites with a high melting-temperature polymeric material. However, this would mean more Cellular Polymers, Vol. 24, No. 6,

8 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim degradation of WF in the first extruder. Therefore, it would be desirable to process the foamed WF composite materials at a temperature as low as possible, preferably below 160 C as discussed earlier. Extrusion Foaming of WPC with Moisture Wood-fiber inherently contains moisture that can potentially be used as a blowing agent. Rizvi et al. (18) conducted an experimental study on foam processing of polystyrene (PS) and high-impact polystyrene HIPS/wood-fiber composites in extrusion using moisture as a blowing agent. Undried wood-fiber was processed together with PS and HIPS materials in extrusion and wood-fiber composite foams were produced. Because of the high stiffness of styrenic materials, moisture condensation during cooling after expansion at high temperature did not cause much contraction of the foamed composite and a high volume expansion ratio up to 20 was successfully obtained. The experimental results showed that the expansion ratio could be controlled by varying the processing temperature and the moisture content in the wood fiber. Matuana et al. (20) also demonstrated that wood flour moisture could be used effectively as foaming agent in the production of rigid PVC/wood-flour composite foams. They investigated the relationships between the density of foamed rigid PVC/wood-flour composites and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all-acrylic foam modifier, and the extruder die temperature, by using a response surface model based on a four-factor central composite design. Their results indicated that there is no synergistic effect between the CFA content and the moisture content of the wood flour. The foam density of rigid PVC/wood-flour composites as low as 0.4 g/cm 3 was produced without the use of chemical foaming agents. However, successful foaming of the composites with the moisture strongly depends upon the presence of all-acrylic foam modifier in the formulation and the extrusion die temperature. Extrusion Foaming of WPC with CBAs When WPCs are foamed with chemical blowing agents (CBAs), the decomposition temperature of CBA used dictates the processing temperatures. Rizvi et al. (21) used CBAs with lower decomposition temperature in order to keep the processing temperatures low so as to suppress the volatiles generated from woodfiber during processsing. They successfully produced WPC foams with various CBAs (decomposition temperatures are lower than 165 C) and identified the optimal processing window (21). The WPC were processed in two stages. In the initial stage the equal amounts of HDPE and WF by weight were mixed with 3% 354 Cellular Polymers, Vol. 24, No. 6, 2005

9 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites coupling agent, and processed through a twin-screw compounder in which the temperatures of all the zones were raised to 175 ºC to effectively devolatilize the WF (Figure 3b) (21). The extrudate was pelletized and then processed in a single screw extruder (Figure 3c) (21) after mixing an appropriate amount of CBA. Rizvi et al. showed that when the CBA was employed, nearly all the blowing agents displayed similar foaming behaviors (Figure 6) (21). There was little change in foam density as the die temperature was reduced, till it reached about 135 C, whereas it was expected that the density would start to decrease as the die temperature was lowered (22). The authors explain this as a possible result of a competition between a density decrease due to lowered diffusivity, and a higher density due to less evolution of gases from the WF. Apparently the two competing effects approximately balanced each other and the density remained nearly unchanged from 180 to 135 ºC, after which it dropped suddenly (21). Their explanation is that at lower temperatures, the diffusivity of the dissolved gas decreases, so that more gas is retained in the extrudate as the temperature of the foam skin approaches the crystallization temperature. The dissolved gas continues to diffuse into the already nucleated bubbles, increasing their internal pressure and thus causing them to grow, which reduces the density. However when the die temperature was lowered to decrease the density, the temperature of the heat exchanger which was used as a diffusion-enhancing device was also lowered. This in turn reduced the volatiles generated from the WF, and the two effects balanced out (21). The results show that the density of the WPC can be controlled by varying the amount of CBA used and the processing window is reasonably wide Density kg/m Die temperature ( C) Figure 6. PWC produced with CBAs Cellular Polymers, Vol. 24, No. 6,

10 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim Extrusion Foaming of WPC with Physical Blowing Agents (PBAs) Compared to CBA-based foaming process, PBA-based foam processing (such as with environmentally friendly CO 2 and N 2 ) has no decomposition temperature requirements, reduces cost, and generally produces better cell morphology. Therefore, it is more logical to use PBA for WPC foaming, although it is technologically more challenging and necessitates proper capital investment (i.e., a gas injection system and the associated system modifications). The main feature of PBA processing is that PBAs (such as N 2 or CO 2 ) are injected into the extruder and uniformly dispersed into the plastic matrix under high pressure using high shear which facilitates in dissolution of the gas into the polymer matrix. The WF/plastic-gas solution is then homogeneously cooled down in a heat exchanger that has a static mixer in order to increase the melt strength, which is necessary to prevent cell coalescence (10). Finally, the WF/plastic-gas solution passes through the die, where foaming occurs (10). The cell size in the extrudate can be made smaller by inducing the nucleation of a large number of cell and preventing their collapse and coalescence. Large nucleation is achieved by properly designing the die to provide a large pressure drop rate and cell deterioration is prevented by increasing the melt strength which is done by cooling the extrudate as described earlier (10). It turns out that the PBA-based processing window is larger than the CBA-based one (Figure 7). WPC with a fine-cell structure (i.e., cell sizes less than 100 μm) have been produced in extrusion foaming with N 2 (23) Foam density (g/cc) Die temperature ( C) Figure 7. WPC produced with a PBA 356 Cellular Polymers, Vol. 24, No. 6, 2005

11 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites WPC Foams in Injection Molding Injection molding is one of the most widely used plastic processing methods for mass producing complex parts which cannot be produced by extrusion. Bledzki et al. (24-28) have been reporting on foaming of WPCs in injection molding. The injection-molded parts are made with wood-flour, PP and CBAs, with density reduction of about 24% and the cell size variation from 10 to 50 μm for the WF content ranging from 30% to 50% (25). It has also been observed that the mechanical properties (specific tensile strength and specific flexural strength) of injection molded WF/PP composite foams improve up to 50% when the adhesion between the WF and the plastic matrix is aided by the use of a maleic anhydride PP-based coupling agent (5%). In addition, the injection molded composite foams without the coupling agent show remarkably higher water absorption than the composite foams with the coupling agent. This indicates that coupling agents would improve the dimensional stability of WPCs by reducing the water absorption. Bledzki et al. have also examined the effects of different CBAs and CBA contents, as well as the consequences of adding a coupling agent to the wood-fiber-pp foamed composites in injection molding and extrusion processing (28). In their studies, injection molding yields better results than extrusion does, and the exothermic foaming agents perform better than the endothermic ones with respect to cell morphology and density reduction. Stretching-Induced Foaming of WPC Apart from chemically making the surfaces of the WF and the plastic more compatible by using coupling agents, molecular orientation is another way to improve the mechanical properties of WPC. In most cases, it leads to an increase in material s toughness and strength as a result of plastic deformation of polymeric materials (29-32). Kim et al. (33-34) studied this innovative stretching technology of orienting the polymer molecules and using it to induce cell nucleation (32) in WPC. There are many advantages of the oriented WPC, including the almost wood-like texture; similar density range as natural wood; excellent nailability and screwability; and better properties than the unoriented WPC (33). Stretching of WPC causes unidirectional orientation of the polymer molecules and enhances the mechanical properties significantly along the stretching direction (33). However, there are still some problems associated with the oriented WPC. As no coupling agent is used, the interface between WF and plastic is weak. But Kim et al. (34) demonstrates that use of coupling agent in stretching WPC can improve mechanical properties by strengthening the interfacial bonding between the polymer and the WF. However, the degree of void generation at the interface of WF and polymer matrix by stretching decreases Cellular Polymers, Vol. 24, No. 6,

12 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim as the amount of coupling agent increases (Figure 8 (34) ). A density reduction of approximately 30% was successfully obtained. The stretching resulted in a unidirectional orientation of the polymer molecules and enhanced the mechanical properties of WPC. Although the tensile strength and the elongation at break of the stretched PP/WF 30 wt% composites were significantly increased, about 5 times, but the tensile modulus (stiffness) was lowered by 25%. As the content of CA increased, the tensile strength and modulus increased because of enhanced interfacial bonding. However, the elongation at break decreased steadily as the CA content increased. A schematic of the WPC processing system used for this study is shown in Figure 9 (34). The three-stages of WPC processing consisted of melt blending, extrusion and sizing, and stretching. Improvement of Flame Retardancy with Nanocomposites The effects of nano-sized clay particles on foaming and flame retardancy of WPC have been studied (35-36). Adding nano-particles to WPC increases foam expansion and improves cell morphology in general with respect to the cell size and the cell density. The foamed material containing clay showed good char formation when it was simply burned, and the flame retardancy tests (according to the ASTM D635) showed the burning rate for the WPC with nano-clay is reduced by 25% (Figure 10) (35). It turned out that the degree of exfoliation of nano-particles as well as the nano-particle content are important in determining the flame retardancy of WPC (36). Figure 8. Effects of coupling agent on density reduction of stretched WPC with 30wt% woodfibre 358 Cellular Polymers, Vol. 24, No. 6, 2005

13 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites 1. Melt-blended PWC processing Vacuum dried WF CA Plastic Dry blending K-mixer Granulator Twin-screw compounding Pelletiser 2. Extrusion/sizing PWC processing Vacuum dried PWC pellets Cutting rod 3. Stretched PWC processing Stretching die Single-screw extruder Sizing/cooling bath Puller Oven Air cooling Puller Figure 9. Schematic stretching of PWC processing Figure 10. Burning rates of WPC and WPC with nanoclay CONCLUSIONS A number of factors influence the WPC processing due to use of WF. The inclusion of large amounts of WF necessitates the use of higher MI resins. Coupling agents are used to enhance the dispersion and bonding of hygroscopic WF with the hydrophobic resins. The emissions of volatiles from WF have Cellular Polymers, Vol. 24, No. 6,

14 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim to be suppressed by drying the WF, by placing restrictions on the highest allowable processing temperatures, and by controlling the residence times at the highest temperatures. With the proper control of these factors, a uniform fine-celled structure can be obtained in WPC in both extrusion and injection molding processes. Active research is being carried out for producing WPC foams in a stretching process, which generates voids in WPC and improves the mechanical properties of WPC by orienting the molecular chains and the WF. The addition of nano-clay particles to WPC increases foam expansion, improves the cell morphology, and enhances mechanical properties, as well as reduces the flammability of WPC. REFERENCES 1. J.H. Schut, Plastics Technology, July, (2001). 2. L.M. Matuana, C.B. Park and J.J. Balatinecz, Polym. Eng. Sci., 37, No.7, 1137 (1997). 3. L.M. Matuana, C.B. Park and J.J. Balatinecz, Cellular Polymers, 17, No.1, 1 (1998). 4. John Z. Lu, Qinglin Wu and Ioan I. Negulescu, J.of Appl.Polym.Sci., 96, No.1, 93 (2005). 5. D. Maldas and B.V. Kokta, Polym. Eng. Sci., 31, No.18, 1351(1991). 6. D. Maldas and B.V. Kokta, J.of Appl.Polym.Sci., 41, No.1-2, 185 (1990). 7. B.V. Kokta, D. Maldas, C. Daneault and P. Béland, J. of Vinyl and Additive Technol., 12, No.3, 146 (1990). 8. R.G. Raj, B.V. Kokta, D. Maldas and C. Daneault, J.of Appl.Polym.Sci., 37, No. 4, 1089 (1989). 9. G.E. Myers, P.C. Kolosic, I. Chahyadi, C.A. Coberly, J.A. Koutsky, and D.S. Ermer, Proceedings of Material Research Society Symposium, 197, 67 (1990). 10. C.B. Park, A.H. Behravesh and R.D. Venter, Polym. Eng. Sci., 38, No. 11, 1812 (1998). 11. J.G. Haygreen and J.L. Bowyer, Forest Products and Wood Science: An Introduction Ames: Iowa State University Press (1996). 12. A.A. Marra, Technology of Wood Bounding: Principles in Practice Van Nostrand Reinhold (1992). 13. G.M. Rizvi, C.B. Park, W.S. Lin, G. Guo and R. Pop-Iliev, Polym.Eng.Sci., 43, No. 7, 1347 (2003). 360 Cellular Polymers, Vol. 24, No. 6, 2005

15 Critical Issues in Extrusion Foaming of Plastic/Woodfi ber Composites 14. S.M. Yushkova, A.A. Tager, S.N. Potepalova V.B. Mozzhukhin and S.K. Bogatkinn, Plast.Massy, 12, 38 (1987). 15. C. Clemons, Forest Products Journal, 52, 6 (2000). 16. G. Guo, G.M. Rizvi, C.B. Park and W.S. Lin, J.of Appl.Polym.Sci., 91, 621(2004). 17. G. Guo, K.H. Wang, G.M. Rizvi, Y.S. Kim and C.B. Park, PPS-20, Paper #105, Akron, Ohio, June 20-24, G.M. Rizvi, L.M. Matuana and C.B. Park, Polym. Eng. Sci., 40, No.10, 2124(2000). 19. H. Zhang, G.M. Rizvi, W.S. Lin, G. Guo and C. B. Park, SPE, ANTEC, Technical Papers, 47, 1746 (2001). 20. L.M. Matuana and F. Mengeloglu, J. of Vinyl and Additive Technol., 8, 264 (2002). 21. G.M. Rizvi, C.B. Park, G. Guo and K.H. Wang, SPE, ANTEC, Technical Papers, Paper #1041, 2003 (2003). 22. H.E. Naguib, C.B. Park and N. Reichelt, J.of Appl.Polym.Sci., 91, No. 4, 2661 (2004). 23. G. Guo, Y.H. Lee, G.M. Rizvi and C.B. Park, SPE, ANTEC, Technical Papers, Paper #102038, Boston, MA, May 1-5, A.K. Bledzki and O. Faruk, SPE, ANTEC, Technical Papers, Paper #154 (2002). 25. A.K. Bledzki and O. Faruk, Cellular Polymers, 21, 417(2002). 26. A.K. Bledzki, Zhang, W. and O. Faruk, Wood Science and Technology, 63(1), 30 (2005). 27. A.K. Bledzki, J. Gassan and W. Zhang, J of Cellular Plastics, 35, 550 (1990). 28. A.K. Bledzki and O. Faruk, SPE, ANTEC, Technical Papers, , A. Ciferri and I.M. Ward, Ultra-High Modulus Polymers, Applied Science Publishers, London (1979). 30. A.E. Zachariades and R.S. Porter, The Strength and Stiffness of Polymers, Marcel Dekker, Inc. New York (1983). 31. P.D. Coates and I.M. Ward, Polym. Eng. Sci., 21, 612 (1981). 32. F.W. Maine, Patent WO 01/45915 A Y.S. Kim, G. Guo, K.H. Wang, C.B. Park and F.W. Maine, SPE, ANTEC, Technical Papers, Paper #809, May (2004). Cellular Polymers, Vol. 24, No. 6,

16 G.M. Rizvi, G. Guo, C.B. Park and Y.S. Kim 34. Y.S. Kim, G. Guo and C.B. Park, Foams 2004, Wilmington, DE, October 5-6, G. Guo, K.H. Wang, C.B. Park, Y.S. Kim and G. Li, SPE, ANTEC, Technical Papers, Paper #520, May 16-19, (2004). 36. G. Guo, Y.H. Lee, C.B. Park, Y.S. Kim and M. Sain, Natural Fiber and Wood Composites 2004, New Orleans, Louisiana, December 8-10, Cellular Polymers, Vol. 24, No. 6, 2005