Manufacture of High-Performance Rice-Straw Fiberboards

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Ind. Eng. Chem. Res. 2010, 49, 1428–1435

Manufacture of High-Performance Rice-Straw Fiberboards So¨ren Halvarsson,*,†,‡ Håkan Edlund,† and Magnus Norgren† Department of Natural Sciences, Engineering and Mathematics, Fibre Science and Communication Network (FSCN), Mid Sweden UniVersity, SE-851 70 SundsVall, Sweden, and Mechanical Fiber Unit, Department of Research, Technology and DeVelopment (RTD), Metso Corporation, SE-851 50 SundsVall, Sweden

Rice straw, a waste agriculture material grown and harvested in Willows, CA, was used as a raw material in the production of thin medium- and high-density fiberboards (MDFs and HDFs). The rice straw was cleaned, size-reduced, and soaked in water before being refined. Defibration was performed in a pressurized pilotplant single-disk refiner, OHP 20”. The fiber production capacity reached a level of 63 kg/h, and the proper fiber quality for MDF/HDF production was established. Analysis of the produced fiber showed an average fiber length of approximately 0.9 mm, an average fiber width of 31 µm, a shive weight of below 24%, and a dust content of less than 30%. Production of fiberboards was performed by addition of 3%, 4%, and 5% methylene diphenyl diisocyanate (MDI). The flexural properties, internal bond strength, and thickness swelling of the produced fiberboards were evaluated according to ASTM methods. The finished fiberboards based on rice straw and MDI resin showed excellent properties. The internal bond (IB) reached levels of 2.6 MPa, and the modulus of rupture (MOR) and modulus of elasticity (MOE) showed levels comparable to those of woodbased fiberboards and were acceptable according to the requirements of medium-density fiberboard (MDF) for interior applications (American National Standards Institute, ANSI A208.2-2002). The water-repelling properties of the 3-mm rice-straw fiberboards were encouraging; the thickness swelling (TS) was in the range of 15-30%. Two different methods to avoid adhesion between the press plates and the resinated fiber material during hot pressing were investigated: protective paper sheets were placed between the fiber mat and press plates, or a press-release agent was sprayed on steel plates that were then placed in the press before pressing. Satisfactory results were obtained with both methods, and no adhesion was observed between the fiberboard and the steel plates. The method of using press-release agent during pressing had no notable negative effects on the fiberboard properties. 1. Introduction Wood-based fiber resources for the production of fiberboards, medium-density fiberboard (MDF), and high-density fiberboard (HDF) will become increasingly restricted in the near future. Forresting regulations, cost-effective pulp and paper products, lumber, and new wood-based bioenergy applications will probably result in increased competition for wood-based raw materials. Alternative nonwood raw materials are therefore of high priority. Several encouraging investigations of fiberboard and particleboard production based on agricultural waste materials have been reported.1-11 Moreover, cereal grains are grown in large quantities in urban regions, and the lignocellulosic fiber components of straw are produced in quantities of several hundred million tons annually. Development of commercial composite products from agricultural waste materials has a promising future and provides environmental benefits. Most of the literature describing the manufacture of straw MDF/HDF is related to wheat straw as the lignocellulosic raw material. The fiberboard properties of the straw MDF/HDF produced thus far have given moderate results, and in particular, the water resistance has been of poor quality. However, it has been concluded that wheat-straw fiberboards can be manufactured by conventional or slightly modified wood-based dry fiberboard processes. Reported investigations of fiberboard production based on rice straw are less frequent than those based on wheat straw, and the rice-straw materials are often combined with other raw materials12 or used for the production of * To whom correspondence should be addressed. Tel.: +46 60148983. Fax: +46 60148820. E-mail: [email protected]. † Mid Sweden University. ‡ Metso Corporation.

thermoplastic composites.13 Finally, the production of hard boards (HBs) from rice-straw pulp has also been reported.14 The more exposed and popular wheat-straw processing and defibration process differ from rice-straw processing. Fundamental differences in straw morphology and structure exist between rice and wheat. Perhaps the most noticeable differences are the shape of the typical annular wheat-straw tube (internode) and the silicon contents, as rice straw contains a distinct inner core in addition to the typical annular straw tube and exhibits ash contents up to 20%.15 In 1991, the California legislature passed the ConnellyAreias-Chandler Rice Straw Burning Reduction Act of 1991 to constrain and eliminate the burning of rice straw. A suggested alternative was disposal of the waste straw directly into the rice fields. Disposal of the rice straw created new problems such as diseases, new water demands, and methane generation.16 However, a hopeful method to overcome this type of problem is to convert rice-straw fibers into high-performance MDF/HDF. One of the pioneers in this area was CalAg LLC (CalAg). CalAg developed and patented a new type of fiberboard product based on rice straw.17 In pilot-plant and small-scale laboratory investigations, fiberboards based on annual plant materials in combination with conventional urea-formaldehyde (UF) resins have displayed straw MDF properties that are comparable with those of woodbased fiberboards. However, the main disadvantage of nonwood fiberboards is the poorer water resistance.1,4-6,8 To improve the poor water resistance of straw MDF, paraffin, wax, or wax emulsions can be added in small quantities, approximately 0.5-1.0%. The most promising method of producing wheatstraw fiberboards reported so far involves the addition of

10.1021/ie901272q  2010 American Chemical Society Published on Web 12/14/2009

Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010

adhesives that are more hydrophobic than conventional UF resins. Methylene diphenyl diisocyanate (MDI) and melaminemodified UF resins are typical examples of adhesives that improve water resistance. The elevated interest in formaldehyde-free or low-emission adhesives in buildings, furniture, and laminate fiberboard products have resulted in an increased pressure on the board producers to use formaldehyde-free adhesives as binders in the production of fiberboards. Formaldehyde emissions in fullscale production units will almost be eliminated if adhesives such as MDI are used. However, a small amount of formaldehyde will be generated from the wood during defibration. The general view of using MDI is that it provides excellent mechanical board properties and lower levels of consumed resin but perhaps at a slightly higher resin cost compared to UF resins. In general, a troublesome adhesion between MDI-resinated fibers and press plates and other metal surfaces has been reported.18 The pressing of wood-based MDF/HDF on an industrial basis is performed in a continuous process, in which the MDIresinated fibers are in direct contact with a hot steel belt during pressing. It is necessary to reduce the strong adhesion of MDI to the steel belt to avoid costly interruptions of operation and damage to the press steel belts. Press-release chemicals are sprayed on the press steel belt or on the fiber mat feeding into the press. Alternatively, use of intermediate paper sheets to avoid direct contact with the MDI-resinated fibers and the steel belts is also a possibility. However, the handling of protective paper is not recommended for full-scale applications. A more attractive alternative is therefore to use press-release agents. The use of press-release agents is necessary for the successful production of MDI-resinated wood-based fiberboards, but so far, the same has not been demonstrated for rice-straw-based MDF/HDF. This article reports an investigation of a method to obtain high-quality lignocellulosic fiber and high-performance MDF/ HDF based on rice straw and a commercial MDI resin. The interest was to develop a sustainable and environmentally friendly straw fiberboard process capable of full-scale MDF/ HDF production. The complete method of converting rice straw from raw material to finished MDF/HDF on the pilot-plant scale is demonstrated. The fiber defibration process of rice straw, the fiber quality parameters, and the effects of different press-release methods of the MDF/HDF quality are related to the fiberboard properties. Use of agricultural waste materials in full-scale MDF/ HDF applications is an attractive alternative for board producers to show an economically sound straw MDF process, in combination with a great opportunity for creating better environmental conditions and add economic benefits for the involved rice-straw farmers. 2. Materials and Methods 2.1. Rice-Straw Substrate. Rice straw (Oryzae satiVa L.) was harvested; baled; and stored in Willows, CA. Management was supervised by CalAg. The moisture content of the baled straw was approximately 15%. The dried and baled rice straw was cleaned; chopped; and hammer-milled before being transported to Metso Paper Technology Center in Sundsvall, Sweden, where defibration and production of rice-straw fibers were performed. 2.2. Added Chemicals. A commercial polymeric methylene diphenyl diisocyanate (MDI) resin was supplied from Huntsman (Rubinate 1840). The addition of MDI resin was set to 3%, 4%, and 5%, on a dry fiber basis. A wax emulsion from Borden (Cascowax EW-58A) was added at a level of 1%, on a dry fiber

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Table 1. Major Process Steps in the Making of Rice-Straw MDF step

description

1 2 3 4 5 6 7

size reduction and screening wetting (soaking) defibration resination (MDI) and addition of wax mat forming prepressing pressing

basis. The investigated press-release agent (Chemrelase RCTW9495) was supplied by Chem-Trend L.P., Howell, MI. 2.3. Manufacture of Rice-Straw MDF and HDF. The manufacturing of MDF/HDF based on rice straw and MDI resin includes several process steps (see Table 1). The rice-straw fiberboard processing is similar to the wood-based MDF/HDF dry process. However, the main difference in the manufacturing of straw MDF as compared to wood-based MDF is the preparation of the straw-based materials before refining. Debaling, chopping to suitable straw lengths, dedusting, cleaning, and wetting of the rice straw are new components in the ricestraw MDF/HDF process. Furthermore, the generated rice-straw dust and small particles in the preparation process are removed for a better adhesive efficiency, resulting in a reduced silica content in the finished straw MDF. 2.4. Production of Rice-Straw Fiber. The rice-straw material was cleaned before size reduction in an air density separator to remove dust and foreign material. Approximately 0.3% (dry basis) of the mass was separated from the rice straw as dust. The full-length straw material, 250-350 mm, was chopped to a range of 12-100 mm. After transportation to Metso, Sundsvall, Sweden, the size-reduced rice straw was prepared for refining, and the first step was to increase the moisture content to levels above 100%. The rice straw was soaked in water for 24 h before defibration. Figure 1 illustrates the straw-processing steps in the pilot plant for the production of rice straw fiber. The wetted rice-straw material was fed into the defibrator system and refined in a pressurized single-disk refiner (type OHP 20” Defibrator, Metso Paper AB, Technology Center, Sundsvall, Sweden) provided with a horizontal preheater. Refining was done at a rotational speed of 1500 rpm and a defibrator housing pressure of 0.5 or 0.6 MPa. The retention time in the defibrator system was set to 1 min. Refined fibers were vented from the refining house into the blowline and dried in a flash tube dryer without addition of wax or adhesive. The most significant refining process parameters are listed in Table 2. The fiber was collected after the dryer in plastic sacks, packed in wooden boxes, and transported to Washington State University (Wood Materials & Engineering Laboratory, Pullman, WA). The dry content of the refined fiber was approximately 10%. 2.5. MDI Resin and Wax Addition. Addition of MDI and process agents, formation of mats, and pressing of MDF were performed in a laboratory at Washington State University, Pullman, WA. The rice-straw fiber was received as processed MDF fiber at approximately 10% moisture content. The fiber was dried to approximately 8% moisture content and stored until resin blending. For that step, the rice-straw fiber was placed into a rotary drum blender, and MDI and wax emulsion were added to the specified levels by spraying (see Table 3). The resinated fiber was later processed through a Nelmor chopper, with a 15.7-mm round-hole screen for homogenization of the fiber and elimination of any large furnish agglomerates formed during the blending process. 2.6. Fibermat Forming and Prepressing. The resinated fiber material was then homogenously placed into a forming

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Figure 1. Schematic of rice-straw fiber preparation system in an MDF pilot plant: (1) hammer mill, (2) dry screen, (3) soaking in water, (4) conveyer, (5) infeed screw, (6) preheater (digester), (7) defibrator (refiner), (8) blowline (no addition of adhesive), (9) dryer, and (10) fiber outlet (cyclone). Table 2. Defibration Process Parameters (Average) of Rice Straw from Willows, CA parameter

units

level

specific energy excl. idle load true disk clearance production rate preheating time preheating temperature defibrator housing pressure dryer inlet temperature dryer outlet temperature

kWh/t mm kg/h min °C MPa °C °C

185 0.2 63 1.0 157 0.5 139 70

Table 3. Target Amounts of MDI Resin and Wax, Average Fiberboard Density, and Press-Release Method trial MDI ) 3% MDI ) 4% MDI ) 5% MDI ) 3% (PR) MDI ) 5% (PR)

MDI (%) wax (%) 3 4 5 3 5

1 1 1 1 1

MDF/HDF density (kg/m3)

press-release methoda

870 960 1160 910 910

paper paper paper press-release agent press-release agent

a Intermediate papers sheets (paper) or sprayed press-release agent on press plates (press-release agent).

box with base dimensions of 610 mm × 610 mm. Then, prepressing and compression of the fibers were performed by pressing a matching wooden plate on the fiber mat in the forming box with manpower. The formed rice-straw fiber mat showed no tendency for dusting. Generally, dusting is a problem for refined annual plant fiber materials. Moreover, no disintegration or separation of small particles (dust) from the upper portion of the fiber mat through to the lower part was observed. 2.7. Pressing of Rice-Straw Fiberboards. The pressing setup was defined for thin panels of approximately 3.9-mm

thickness. The pressing cycle was guided by in-house experience of pressing annual plant fiber materials. MDFs were pressed in a 910 mm × 910 mm computer-controlled hydraulic press with oil-heated platens. Before pressing, paper sheets were applied to the fiber mat to protect the fiber mat from direct contact with the press plates. Alternatively, when press-release agent was used, the release agent was sprayed directly onto thin steel plates, the fiber mat was placed between the steel plates, and the fiber mat and steel plates were then inserted into the press. Pressing time and pressing temperature are two critical production capacity parameters for the manufacturing of fiberboards. The final fiberboard thickness was set to 3.9 mm, and the pressing time was approximately 80 s. The calculated press factor was approximately 20 s/mm, and twice the amount of time necessary to cure the MDI resin was employed. The press-plate temperature was set to 180 °C, and the resulting maximum temperature in the core of the fiber mat during pressing was at least above 120 °C. The temperature in the core could be followed by measurement of the formed gas pressure in the fiberboard during pressing. The applied pressure in the beginning of the pressing was set to 5.0 MPa for 20-25 s to create a high surface density of the fiberboard. The core density was then formed by reducing the applied pressure to approximately 1.5 MPa. Finally, the pressure was set to zero for a shorter time to release generated steam and gases from the fiberboards before opening the press. After pressing, the panels were cooled to ambient temperature. After conditioning of the pressed fiberboards, specimens for the measurement of material properties were machined and sanded to approximately 3-mm thickness. 2.8. Straw and Fiber Properties. The fiber length and size of the refined and dried fibers were measured by image analysis using a laser-based PQM1000 pulp-quality monitoring system (Metso Paper, Sundsvall, Sweden). The average fiber length, the fiber length distribution, and various different properties of the fiber quality were determined. The amounts of short fibers and dust particles were evaluated for each trial. Additionally, the shive content was measured in a Pulmac shive analyzer with 0.15-mm slots (Pulmac Instruments International, Montpelier, VT). 2.9. Mechanical Properties of Fiberboards. Rice-straw MDF panels were cut into 50 mm × 50 mm pieces for the determination of internal bond (IB) and the measurement of density profiles. Bending strength, modulus of rupture (MOR), and modulus of elasticity (MOE) were measured on 290 mm × 76 mm samples. Thickness swelling (TS) and water adsorption (WABS) properties were determined from 102 mm × 102 mm test pieces. The fiberboard specimens were immersed horizontally in water for 24 h to determine thickness and weight. The mechanical properties, namely, IB, MOR, MOE, and TS, were determined according to the standard methods of ASTM International.19 Before testing, the specimens were conditioned in a room for 48 h at 65% relative humidity and a temperature of approximately 20 °C. 3. Results and Discussion 3.1. Properties of the Rice-Straw Fiber. One of the main steps in the manufacturing of composite MDF and HDF is the production of high-quality fibers. Pressurized defibration of annual plant materials and wood-based materials is the most powerful method to generate lignocellulosic fibers. However, each type of raw material requires unique processing conditions for desirable fiber quality.20 Straw and agricultural waste materials are different from wood and have the slight disadvantage of higher proportions of nonfibrous cell elements, wax,

Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010 Table 4. Rice-Straw Fiber Samples from a Pretrial (PT) and Main Trial (T), Measured in PQM1000 Fiber Classifier and Pulmac Instruments, 0.15-mm Slota quantity number of fibers average length average width curl index coarseness number of shives shive weight average shive length average shive width shives >0.15 mm wide short fibers 0.6 MPa for grade 130. Furthermore, the average MDF density is typically between 500-1000 kg/m3. Conventional wood-based MDFs with thicknesses above 5 mm have a core density of roughly 650 kg/m3 and a high surface density, typically above 1000 kg/m3. The denser the surface material, the better the bending properties generally are. However, this type of density profile can be difficult to achieve when pressing thin MDF and HDF. Pressing fiberboards at thicknesses of approximately 4.0 mm or less can generate a rather flat or inverse density profile (high density in the core). The bending properties of the rice-straw MDF/HDF included acceptable bending strengths. Figures 4 and 5 present the modulus of rupture (MOR) and modulus of elasticity (MOE), respectively, as a function of average density. The numerical values of MOR and MOE of the rice-straw MDF were generally above the requirements in the MDF standard (ANSI, grades 120 and 130). The different press-release methods had minor effects on MOR and MOE. The bending properties of conventional wood-based particleboards and fiberboards are strongly dependent on the average density.34 The rice-straw fiberboards also

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Figure 5. Modulus of elasticity (MOE) vs fiberboard density of rice-straw MDF/HDF at different MDI contents. MDI ) 3%, MDI ) 4%, and MDI ) 5% represent paper sheets applied as protective layers in the press. MDI ) 3% (PR) and MDI ) 5% (PR) represent spraying of a press-release (PR) agent on the steel plates before pressing. The ANSI, grade 120, and grade 130 requirements are represented by lines.

followed this density dependence. Additionally, a small improvement of the bending properties was observed for increased MDI resin content. The poor water resistance or excessive thickness swelling (TS) of fiberboards based on annual plant materials is the most challenging problem to overcome. Several investigations indicate poorer water swelling behavior compared with wood furnish.1,4-6 Generally, one would expect only small differences in TS between fiberboards produced from wood and annual plants, as almost the same process and resin are used. However, one of the fundamental differences between straw- and wood-based materials is the composition of the main chemical components in the lignocellulosic material. Hardwoods and softwoods have about the same amounts of cellulose, but they differ in the amounts of hemicellulose and lignin. Differences in swelling behavior between hardwoods and softwoods have been reported.35 Apart from high amounts of mineral components and wax, wood and nonwood fibers have different chemical compositions. For example, the presence of the hydrophobic polymer lignin is less pronounced and the amount of hemicelluloses is larger in straw than in wood-based materials.36-39 Hemicelluloses are branched polymers and are comparatively the most hydrophilic wood polymer. The hemicellulose polymers have the ability to absorb water and swell readily in contact with water. The combination of smaller amounts of lignin and greater amounts of hemicellulose components contributes to the inherently higher water swelling of most annual plant fibers compared to what is found for wood fibers. The water swelling behavior of fiberboards can be reduced by introducing hydrophobic components such as wax and lipids; acetylating the fibers; modifying the traditional UF resin with melamine; or selecting resins (adhesives) comprising components of higher hydrophobicity, such as phenol formaldehyde (PF) or MDI resin.40,41 Even the addition of divalent ions such as Ca2+ can reduce the water swelling properties of fiberboards.42,43 Consequently, the chemical composition of the fibers in woodbased fiberboards is often more favorable for obtaining improved water resistance than that in fiberboards based on most annual plant fibers. Unexpectedly low thickness swelling and water absorption were determined for the pressed MDFs/HDFs based on MDI resin and rice-straw fibers, as shown in Figure 6. Almost all tested fiberboards displayed swelling in the range of 15-30%. An increased MDI resin loading (MDI ) 5%) resulted in a TS range of 15-25%. The requirement in the wood-based MDF

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Figure 6. Thickness swelling vs density of rice-straw MDF/HDF at different MDI contents. MDI ) 3%, MDI ) 4%, and MDI ) 5% represent paper sheets applied as protective layers in the press. MDI ) 3% (PR) and MDI ) 5% (PR) represent spraying of a press-release (PR) agent on the steel plates before pressing. The ANSI, grade 120, and grade 130 requirements are represented by lines.

Figure 7. Water absorption vs density of rice-straw MDF/HDF at different MDI contents. MDI ) 3%, MDI ) 4%, and MDI ) 5% represent paper sheets applied as protective layers in the press. MDI ) 3% (PR) and MDI ) 5% (PR) represent spraying of a press-release (PR) agent on the steel plates before pressing.

standard (ANSI, grades 120 and 130)33 for approved fiberboards is a thickness swelling of 1.5 mm, or lower than 50% for sanded fiberboard thicknesses of approximately 3.0 mm. Furthermore, the different press-release methods had no effect on the water swelling or the water absorption (WABS) properties. The WABS of produced straw fiberboards was estimated to 30-60% (see Figure 7). However, a restricted water weight increase was observed for the fiberboards at high MDI loading (MDI ) 5%), and the WABS was only on the order of 20-35%. For higher densities, the water repelling properties seemed to be improved. Increased loadings of MDI resin reduced the thickness swelling and water absorption. 4. Conclusions The preferred pilot-plant process steps from rice-straw bales to finished straw MD/HD fiberboards demonstrated a capable method of producing straw MDF/HDF at a possible industrial level. Defibration of the size-reduced rice-straw waste material at a 0.5 MPa defibrator house pressure and a 1.0 min retention time generated fibers of sufficient quality for manufacture of thin (3.0-mm) rice-straw MDF/HDF. The average fiber length of the refined rice-straw fibers was 0.9 mm, the shive weight was 24%, and the amount of dust was close to 30%. The produced rice-straw fiber displayed properties similar to those

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of high-performance MDF/HDF at MDI resin contents of 3%, 4%, and 5%. The finished rice-straw fiberboards were acceptable according to MDF standards33 for grades 120 and 130. The mechanical properties of the rice-straw fiberboards, including IB, MOR, and MOE, improved as the MDI resin content and fiberboard density increased. The thickness swelling and water absorption were also improved as a result of the increased density and MDI resin levels. Two different methods of press release were investigated: adding intermediate paper sheets or spraying a press-release agent on the steel press plates before pressing. No significant difference in MDF/HDF properties was observed, and consequently, the function of the press-release agent was satisfactory. Thus, agricultural waste materials such as rice straw, MDI adhesive, and press-release agent can successfully be used for the production of MDF/HDF. Acknowledgment The authors are grateful to Mid Sweden University and Fiber Science and Communication Network (FSCN) for supporting this research. They also express great gratitude to Jerry Uhland and Les Younie at CalAg LLC for their valuable contributions. Thanks are also extended to all of those involved at Metso Mechanical Fiber AB, Huntsman, Chem-Trend L.F., and Washington State University for support and advice. Literature Cited (1) Sauter, S. L. Developing composites from wheat straw. In 30th International Particleboard/Composite Materials Symposium; Wolcott, M. P., Ed.; Washington State University: Pullman, WA, 1996; pp 197214. (2) Han, G. P.; Zhang, C. W.; Zhang, D. M.; Umemura, K.; Kawai, S. Upgrading of urea formaldehyde-bonded reed and wheat straw particleboards using silane coupling agents. J. Wood Sci. 1998, 44 (4), 282–286. (3) Eroglu, H.; Istek, A. Medium Density Fibreboard (MDF) manufacturing from wheat straw (Triticum AestiVum L.). Inpaper Int. 2000, 11–14. (4) Han, G. P.; Umemura, K.; Zhang, M.; Honda, T.; Kawai, S. Development of high-performance UF-bonded reed and wheat straw medium-density fiberboard. J. Wood Sci. 2001, 47 (5), 350–355. (5) Wasylciw, W. Straw-based composite panelssAttributes, issues, and UF bonding technology. In 35th International Particleboard/Composite Materials Symposium Proceedings; Wolcott, M. P., Tichy, R., Miklosko, L. C., Eds.; Washington State University: Pullman, WA, 2001; pp 145153. (6) Mantanis, G.; Berns, J. Strawboards bonded with urea formaldehyde resins. In 35th International Particleboard/Composite Materials Symposium Proceedings; Wolcott, M. P., Tichy, R., Miklosko, L. C., Eds.; Washington State University: Pullman, WA, 2001; pp 137-144. (7) Halvarsson, S.; Norgren, M.; Edlund, H. Manufacturing of fiber composite medium density fiberboards (MDF) based on annual plant fiber and urea formaldehyde resin. In Proceedings of the First International Conference on EnVironmentally Compatible Forest Products; Jorge, F. C., Ed.; Fernando Pessoa University: Oporto, Portugal, 2004; pp 131-147. (8) Ye, X. P.; Julson, J.; Kuo, M. L.; Womac, A.; Myers, D. Properties of medium density fiberboards made from renewable biomass. Bioresour. Technol. 2007, 98 (5), 1077–1084. (9) Halvarsson, S.; Edlund, H.; Norgren, M. Properties of mediumdensity fibreboard (MDF) based on wheat straw and melamine modified urea formaldehyde (UMF) resin. Ind. Crops Prod. 2008, 28 (1), 37–46. (10) Mo, X. Q.; Cheng, E. Z.; Wang, D. H.; Sun, X. S. Physical properties of medium-density wheat straw particleboard using different adhesives. Ind. Crops Prod. 2003, 18 (1), 47–53. (11) Hague, J.; McLauchlin, A.; Quinney, R. Agri-materials for panel products: A technical assessment of their viability. In 32nd International Particleboard/Composite Materials Symposium; Tichy, R. J.; Bender, D. A.; Wolcott, M. P., Eds.; Washington State University: Pullman, WA, 1998; pp 151-159. (12) Hiziroglu, S.; Jarusombuti, S.; Bauchongkol, P.; Fueangvivat, V. Overlaying properties of fiberboard manufactured from bamboo and rice straw. Ind. Crops Prod. 2008, 28 (1), 107–111. (13) Habibi, Y.; Ei-Zawawy, W. K.; Ibrahim, M. M.; Dufresne, A. Processing and characterization of reinforced polyethylene composites made

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ReceiVed for reView August 12, 2009 ReVised manuscript receiVed November 30, 2009 Accepted November 30, 2009 IE901272Q