Effect of adhesives and particle sizes on properties of composite materials from sawdust

Effect of adhesives and particle sizes on properties of composite materials from sawdust Chanakan Charoenwong 1* and Supachai Pisuchpen 1 1 Department of Material Product Technology, Faculty of Agro-Industry, Prince of Songkla University. * Corresponding author: e-mail: chanaonk@hotmail.com, mobile phone: +668-5965-55, phone: +66-7428-6355, fax: +66-7421-2889 Abstract The increased demand in using wood composites in particular applications results in a need for a better understanding of the effect of adhesives and particle sizes on the characteristics of wood composites. In this research, medium density particleboards from four levels of rubber wood sawdust particle sizes (>1, 1-2, 2-35 and <35 meshes) were prepared with polymeric 4, 4'- diphenylmethane diisocyanate (PMDI) or melamine-urea formaldehyde (MUF) or phenol formaldehyde (PF) adhesives. The modulus of rupture (MOR), modulus of elasticity (MOE), thickness swelling (TH) and water absorption (WA) were investigated. It was found that the mechanical properties of boards were positively affected by the particles sizes but the water resistance properties of boards were adversely affected by the particle sizes. It suggested that the courser the particles sizes, the stronger and the lower water resistance boards were obtained. Only composite boards made from 2-35 mesh with 15% PMDI content passed the EN 312 standards and TIS 876-24 standard. In addition, PMDI-bonded boards not only gave superior mechanical properties compared with MUF and PF-bonded boards but the water resistance was increased considerably as well. The significant increased in thickness swelling and water absorption was higher in MUF and PF bonded boards. Only the composite boards made from 2-35 mesh with 9% PMDI content fully satisfied the minimum requirements set by the standards. Overall, PMDI-bonded boards gave superior mechanical performance, water resistance and thickness swelling than MUF and PF-bonded composite boards. Keywords: Composite, sawdust, mechanical properties, physical properties, adhesives, and particle sizes Introduction For decades a development of wood-based panels has attributed to the economic advantage of low cost wood, other lingo-cellulosic fibrous materials and inexpensive processing with various types of binders (Ashori and Nourbakhsh, 28). Demand for composite wood products such as particleboard, plywood, oriented strandboard (OSB), hardboard, medium-density fiberboard and veneer board products has recently increased substantially throughout the world (Sellers, 2). Particleboard accounts for 57% of total consumption of wood-based panels consumed and it is continuously growing at 2 5% annually (Ashori and Nourbakhsh, 28). According to a report from Food and Agricultural Organization (FAO) of the United Nations, during 1998 world and Thailand consumption of particleboard were approximately 56.2 1 6 m 3 and.15 1 6 m 3 respectively and in 28 it had risen to nearly 14 1 6 m 3 and 2.6 1 6 m 3 respectively (Zheng et al., 26; FAO, 21). The shortage of wood due to deforestation and forest degradation, and the increasing demand of wood-based panels have encouraged manufacturers to research alternative sources of fibers (Çolak et al., 27). Researchers have been carried out on a variety of plant biomass. Through various studies, composite panels made from plant biomass in the form of wood such as particleboard has been demonstrated capable of serving as a cost-effective source of fiber. It is the largest potential 18

source of biomass when compared to others. Most of these potentials are found in wood processing byproducts known as sawdust which is most readily available. The presence of sawdust in large quantity primarily creates disposal challenges for the wood processing industries. However, sawdust residue can be converted to a value added composite panel by mixing a resin and forming the mix into a sheet (Kuti and Adegoke, 28). Composite panel made from sawdust is generally very prone to expansion and discoloration due to moisture. The most common types of resins or adhesives used for composite board are formaldehydebased resins for example urea formaldehyde (UF), melamine-urea formaldehyde (MUF) and phenol formaldehyde (PF) (Dunky, 23). Melamine-urea formaldehyde (MUF) resin is among the most used adhesives for exterior and semi-exterior wood panels. It is much higher resistance to water attack which is the main distinguishing characteristic from urea formaldehyde (UF) resins (Pizzi and Mittal, 23). However, the use of with formaldehyde-based resins in composite board brings a concern in formaldehyde emissions which are a probable human carcinogen. Polymeric 4, 4 - diphenylmethane diisocyanate (PMDI) is a relatively new adhesive on the wood composite industry in Thailand. Its main advantages include lower resin content, shorter time and lower temperature for hot pressing, high strength, moisture resistance and no formaldehyde emission of finished board. PMDI is a cross-linking thermoset and non polar. The resin cures by reacting with the moisture in wood and creating a rigid polar network of urethane linkages. This is an important contributor to the properties of isocyanate (Umemura and Kawai, 22). The present work was conducted to characterize the properties of composite boards made from MUF, PF and PMDI resins and particle sizes (>1, 1-2, 2-35 and <35 meshes) of sawdust. An understanding of adhesives how it reacts to different particle sizes will help us understand the behavior of wood composite materials and design a composite wood product to have material properties suitably for a desired function. Materials and Methods Materials Sawdust was collected from the rubber wood toys production process. Subsequently, sawdust was screened to four levels (>1, 1-2, 2-35 and <35 meshes) by sieving with screens having 2.,.84, and.5 mm openings and oven-dried to less than 5% moisture content. Polymeric 4, 4'- diphenylmethane diisocyanate (PMDI) (1% solid content) was supplied by Huntsman (Thailand) Co., Ltd. Phenol formaldehyde (PF) (MD-54, 41.42% solid content) was obtained from Siam Chemical Industry Co., Ltd. Melamine-urea formaldehyde (MUF) (EM76, 62.67% solid content) and 25% (w/v) ammonium chloride were obtained from Eternal Resin Co., Ltd. MUF and PF adhesives were added with 1% of hardener (25% (w/v) ammonium chloride). The characteristics of the MUF, PF and PMDI are given in Table 1. Methods Composite materials panels preparation To fabricate a single-layer board, dried particles were then blended uniformly with MUF or PF or PMDI in a rotating drum-type mixer fitted with a pneumatic spray gun. Adhesives were applied according to oven dried solid composition basis. The blended materials were evenly distributed in a molding block; the hand-formed block was then cold pressed at pressure of 7 kg/cm 2 (1, psi). A cold pressing step was applied to reduce a mat to about 2.5 cm high for ease of introducing into the hot press. The adhesive-coated mat was then followed by hot pressing maintained at 125±5ºC using pressure of 21 kg/cm 2 (3, psi) for 5 min. Bar stoppers were used to control formed panel at a given thickness. After hot pressing, the panel was trimmed to a final size of 23 cm by 27 cm. The target density was control at.8 g/cm 3 and target thickness was 6 mm for all composite panels. Three replicates of each board were made. 19

Properties testing of composite materials The samples were conditioned in a chamber at 25ºC and 55±2%RH for 48 hours before cutting into testing specimens. The mechanical and physical properties of composite panel were determined in accordance with modified European Union (EN) and Thai Industrial Standards (TIS): Density (TIS 876-24), modulus of rupture (MOR) and modulus of elasticity (MOE) (EN 312 2 and EN 312 3, and TIS 876-24 respectively). In addition, water resistance properties including thickness swelling (TH) was evaluated by using modified EN and TIS standards (EN 312 4 and TIS 876-24 respectively) and water absorption (WA) was evaluated according to ASTM D57. Data for each test were statistically analyzed. The effects of adhesive types and contents on the composite panels properties were evaluated by analysis of variance (ANOVA) and Duncan Multiple Range Classification at 95% confidence level. Experimental design Effect of particle sizes on properties of composite materials The sawdust collected from wood converting processes of rubber wood toy was dried to less than 5% moisture content. High strength, smooth surface, and equal swelling of particleboards are normally obtained by using a homogeneous material with high degree of slenderness (long and thin particles) (Youngquist, 1999; Pan, 27). However, a major effect to a quality of composite panels is particle sizes of fibrous material. To study the effects of particle sizes on the properties of composite material from sawdust, a Completely Randomized Design (CRD) was employed. The particle size distribution of >1, 1 2, 2 35, and <35 mesh was analyzed using a modified ASTM standard method with a sieve shaker (ASTM E828-81, 1997). Composite panels were prepared by using 15% (w/w) of each an adhesive and the properties of the composite panels were determined by methods specified in Section 2.2.2. The proper particle size was then selected for a following study. Effect of adhesives on properties of composite materials To study the effects of adhesive types and contents on the qualities of particleboards, the particles size selected from a former study was conducted with study factors in a Completely Randomized Design (CRD). The tested levels of MUF, PF, and PMDI adhesives contents were 3%, 6%, 9%, 12%, and 15% based on the total dried weight of composition. The qualities of the composite panels were fully evaluated with the methods specified in the following section 2.2.2. Result and Discussions Effect of particle sizes on properties of composite materials The size distribution of sawdust collected from the rubber wood toy production process is shown in Table 2. The majority sizes were in the range of.84-2. mm diameter which represented 38.2% of sawdust. The sawdust sizes tended to distribute in courser fiber sizes; only 11.3% of sawdust was classified in sawdust flour. The density, MOR, MOE, thickness swelling (TH) and water absorption (WA) for all composite boards prepared from various particle sizes and adhesives are summarized in Table 3. The average density of boards varied in a narrow range from.76-.8 g/cm 3 which still met the TIS standard. The mechanical properties of board were significantly affected by the particle sizes. It can be seen from Fig 1 and 2 that the MOR and MOE differed significantly with the particle sizes. The courser particle offered better mechanical properties. Furthermore, the adhesive types show significant effect on the mechanical properties of board as well and the best results were found in the board prepared from PMDI. Even though the course particle improved the mechanical properties, the water resistance properties were adversely affected. The highest thickness swelling and water absorption were observed in the >1 mesh particle size as shown in Fig 3 to 6. The thickness swelling and water absorption decreased significantly in PMDI-bonded board; they varied from 4.76-13.2% thickness swelling and 18.9-11

31.22% water absorption. The reason for this behavior is attributed to the strength of course particle size and the high adhesion between course particle sizes. The course particle sizes are tightly interlocked and oriented randomly in three dimensions which cause an increase in the strength of the boards. In most cases of course particles the mechanical properties of boards are better but the water resistance properties are worse than the boards prepared from fine particle sizes. This may be related to the fact that the fine particle sizes offer denser packing, more uniform grain sizes and easier adhesive penetration compared to the course particle sizes. In addition, the void areas or pore sizes could be easily observed on the boards prepared from the course particle sizes (>1 and 1-2 mesh). With regard to the EN and TIS standards, only boards prepared from particle size of 2-35 mesh could satisfy requirements of mechanical and physical properties of the standards. As discussed previously, it can be seen in Fig 1 to 6 that PMDI considerably posed better strength and resistance to water and water absorption of boards than MUF and PF. Moreover, in order to provide equivalent board mechanical and physical properties using MUF, PF and PMDI, it is possible to use PMDI at a considerable lower dosage. This tendency remained the same at all particle sizes. Effect of adhesives on properties of composite materials A comparison of adhesives was carried out on a particle size of 2-35 mesh at various adhesive contents. The density, MOR, MOE, thickness swelling (TH) and water absorption (WA) for all composite boards prepared from various particle sizes and adhesives are summarized in Table 4. Density of prepared boards varied from.64-.8 g/cm 3 but still within the TIS standard. The density of board seemed to depend on the adhesive contents. Higher adhesive contents generally produced higher density of board and boards prepared from PMDI had more consistency of density than others. It can be seen in Fig 7 and 8 that MOR and MOE of boards differed significantly with the adhesive types and contents. The MOR and MOE increased as the adhesive contents increased and reached the maximum at 15% adhesive contents. The highest MOR and MOE resulted in boards prepared from PMDI. In addition, the strengths of PMDI-bonded boards containing 9% adhesive contents were several times higher than those of PF and MUF-bonded boards respectively. A significant water resistance in boards prepared from PMDI was observed in Fig 9 to 12. There were much lower than other adhesives but there was no significant difference compared to among levels of PMDI contents. The results indicated that only 3% PMDI contents could meet the minimum water resistance required by EN and TIS standards. The results suggested that PMDI was also best compatible to the sawdust than MUF and PF. Statistical analysis indicated that the use of 6% to 12% PMDI contents had no significant difference on mechanical and physical properties. However, only the produced board from 9%PMDI contents was economically comparable to those required by EN and TIS standards. The enhanced mechanical and physical properties of boards obtained by PMDI were obvious. The excellent adhesion properties of PMDI to the increased strength originate from the reactivity of the isocyanate groups. There groups react with compounds of sawdust that have an active hydrogen from water, alcohols and amines to form polyureas. In addition, the covalent bonds also form between hydroxyl groups in the sawdust and the isocynate. The covalent bonds act to connect the polyurea to the sawdust which help join the gap between sawdust particles. This adhesive network shows to create urethane linkages with molecules in the sawdust. This is an important donor to the mechanical and water resistance properties of PMDI-bonded boards. Conclusions Mechanical properties of boards and water resistance were strongly affected by the particle sizes, adhesive type and adhesive contents. The courser the particle sizes, the more strength and water sensitiveness of board obtained. PMDI contents of 3-15% had insignificant difference on water resistance properties of boards, whereas higher PMDI contents resulted in higher strength of board. The composite boards made from 2-35 mesh with 9% PMDI content could be satisfied fully the 111

minimum requirement by the EN standard and TIS 876-1993 standard. Overall, PMDI-bonded boards gave superior mechanical performance and water resistance than PF and MUF-bonded boards. Acknowledgments This research was supported by a program for graduate students of Prince of Songkla University, a grant from the Agro-Industry Practice School Program: APS by National Science and Technology Development Agency: NSTDA and Thai government and the authors is thankful to Plan Creations Co., Ltd. for Sawdust, Eternal Resin Co., Ltd. for Melamine-Urea formaldehyde resin and 25% (w/v) Ammonium chloride, Siam Chemical Industry Co., Ltd. for Phenol formaldehyde resin and Huntsman (Thailand) Co., Ltd. for Polymeric 4, 4 methylene diphenyl isocyanate (PMDI). References American Society of Testing and Materials. 1997. Standard Test Method for Designating the Size of RDF-3 from its Sieve Analysis. Designation: ASTM E828-81 (Reapproved 1997). ASTM, West Conshohocken, PA, pp. 699 76. Ashori, A. and Nourbakhsh, A. 28. Effect of press cycle time and resin content on physical and mechanical properties of particleboard panels made from the underutilized low-quality raw materials. Industrial crops and products. 28, 225 23. Çolak, S., Çolakoğlu, G., Aydın, I., and Kalaycioğlu, H.. 27. Effects of steaming process on some properties of eucalyptus particleboard bonded with UF and MUF adhesives. Journal of Building and Environment. 42 (1), 34 39. Dunky, M. 23. Adhesives in the Wood Industry. Handbook of Adhesive Technology, 2 nd Edition. FAO, 21. FAO Year books of Forest Products. http://faostat.fao.org/site/626/desktopdefault.aspx?pageid=626#ancor. [April 1, 21] Kuti, O. A., and Adegoke, C. O. 28. Comparative performance of composite sawdust briquette with kerosene fuel under domestic cooking conditions. Department of mechanical system. Pan, Z., Zheng, Y., Zhang, R., and Jenkins, B. M. 27. Physical properties of thin particleboard made from saline eucalyptus. Journal of Industrial crops and products 26 (2): 185-194. Pizzi, A., and Mittal, K. L. 23. Melamine-formaldehyde adhesives. Handbook of Adhesive Technology, 2 nd Edition. Sellers, T., 2. Growing markets for engineered products spurs research. Journal of Wood Technology. 127 (3), 4 43. Umemura, K., and Kawai, S. 22. Effect of polyol on thermo-oxidative degradation of isocyanate resin for wood adhesives. Journal of Wood Science. 48. 25-31. Youngquist, J. A. 1999. Wood-based composites and panel products. Wood Handbook: Wood as an Engineering Material. Gen. Tech. Rept. FPL-GRT-113. USDA Forest Serv., Forest Prod. Lab, Madison, WI, pp. 1 31 (Chapter 1). Zheng, Y., Pan, Z., Zhang, R., Jenkins, B. M., Blunk, S., 26. Properties of medium-density particleboard from saline Athel wood. Journal of Industrial Crops and Products. 23 (3), 318 326. 112

Table 1. The Characteristic of Adhesives Item Unit Appearance - Milky white liquid Reddish brown liquid Dark brown liquid Viscosity (25 C) cps. 112 186 253 Non-volatile content % 62.67 41.45 1 ph - 9.9 11.7 8.2 Gel time sec. 8 83 76 Specific gravity - 1.271 1.155 1.364 Type - - Resole - Table 2. Size distribution of sawdust was collected from wood toys production process Particle size Percent by weight >1 mesh 28.8% 1-2 mesh 38.2% 2-35 mesh 21.7% <35 mesh 11.3% 45 4 35 MOR (N/mm 2 ) 3 25 2 TIS standard* EN standard* 15 1 5 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig.1 Effect of particle sizes on MOR of composite materials panels Note: Symbol (*) is minimum requirement of standard 113

4 35 MOE (N/mm 2 ) 3 25 2 TIS standard* EN standard* 15 1 5 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig. 2 Effect of particle sizes on MOE of composite materials panels Note: Symbol (*) is minimum requirement of standard 25 TH 2 hrs (%) 2 15 1 TIS standard** EN standard** 5 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig. 3 Effect of particle sizes on TH 2 hours of composite materials panels Note: Symbol (**) is maximum requirement of standard 114

25 2 EN standard** TH 24 hrs. (%) 15 1 5 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig. 4 Effect of particle sizes on TH 24 hours of composite materials panels Note: Symbol (**) is maximum requirement of standard 6 5 WA 2 hrs. (%) 4 3 2 1 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig. 5 Effect of particle sizes on WA 2 hours of composite materials panels 115

6 5 WA 24 hrs. (%) 4 3 2 1 >1 mesh 1-2 mesh 2-35 mesh <35 mesh Particle sizes Fig. 6 Effect of particle sizes on WA 24 hours of composite materials panels 116

Table 3. Properties testing of the composite panel made from various particle sizes. Adhesive type MUF PF PMDI Particle Density MOR (N/mm 2 ) MOE (N/mm 2 TH 2 hrs. ) sizes (%) TH 24 hrs. (%) WA 2 hrs. (%) WA 24 hrs. (%) >1 mesh.8 ±.3 16.14 ± 1.39 c 2216 ± 81 f 2.24 ± 1.21 g 22.56 ±.43 h 38.57 ± 1.2 f 41.53 ±.94 gh 1-2 mesh.79 ±.2 12.95 ± 2.64 b 182 ± 37 cde 17.8 ± 1.8 f 19.68 ±.97 g 38.8 ± 1.95 f 4.95 ± 1.94 fgh 2-35 mesh.8 ±.2 9.1 ±.16 a 813 ± 39 a 14.54 ±.28 e 17.55 ±.91 f 32.59 ± 1.6 de 37.63 ± 1.58 ef <35 mesh.79 ±.2 9.39 ±.28 a 91 ± 28 a 12.62 ± 1.18 de 15.61 ± 4.1 ef 36.33 ± 1.96 ef 39.4 ± 1.47 ef >1 mesh.78 ±.3 3.95 ± 1.19 f 312 ± 112 g 12.8 ±.64 d 14.71 ±.79 e 37.37 ±.49 f 42.87 ±.61 h 1-2 mesh.79 ±.1 19.68 ± 1.18 e 199 ± 224 ef 8.94 ±.44 c 13.27 ± 1.76 de 33.71 ±.87 d 39.95 ±.55 efg 2-35 mesh.8 ±.2 17.49 ±.9 e 1424 ± 13 c 6.82 ±.22 bc 1.84 ± 1.39 cd 32.23 ± 1.81 d 38.42 ± 1.47 e <35 mesh.8 ±.1 15.3 ±.68 c 1349 ± 93 bc 2.43 ±.79 a 7.24 ±.81 b 29.4 ± 1.71 c 34.23 ±.74 d >1 mesh.8 ±.2 38.93 ± 1.7 g 319 ± 321 g 4.76 ±.68 ab 13.2 ±.67 de 18.9 ± 1.5 b 31.22 ± 1.15 c 1-2 mesh.76 ±.1 21.88 ± 1.89 de 224 ± 357 def 4.21 ±.38 a 8.71 ±.47 bc 11.53 ±.47 a 26.63 ±.69 b 2-35 mesh.78 ±.2 27.1 ±.8 cd 336 ± 15 cd 4.9 ±.71 a 6.54 ±.67 ab 1.66 ±.72 a 26.31 ±.44 ab <35 mesh.8 ±.4 16.79 ± 1.64 c 166 ± 139 b 2.43 ±.68 a 4.8 ±.63 a 9.41 ±.21 a 24.5 ± 1.61 a Note: Values within the same column followed by different letters (a-h) are significantly different at P <.5. The composite panels were made with 15% adhesives contents 117

3 25 MOR (N/mm 2 ) 2 15 1 TIS standard* EN standard* 5 3% 6% 9% 12% 15% Adhesives content Fig. 7 Effect of adhesives on MOR of composite materials panels Note: Symbol (*) is minimum requirement of standard MOE (N/mm 2 ) 3,5 3, 2,5 2, 1,5 1, 5 TIS standard* EN standard* 3% 6% 9% 12% 15% Adhesives content Fig. 8 Effect of adhesives on MOE of composite materials panels Note: Symbol (*) is minimum requirement of standard 118

6 5 TH 2 hrs. (%) 4 3 2 EN standard** TIS standard** 1 3% 6% 9% 12% 15% Adhesives content Fig. 9 Effect of adhesives on TH 2 hours of composite materials panels Note: Symbol (**) is maximum requirement of standard TH 24 hrs. (%) 6 5 4 3 2 EN standard** 1 3% 6% 9% 12% 15% Adhesives content Fig. 1 Effect of adhesives on TH 24 hours of composite materials panels Note: Symbol (**) is maximum requirement of standard 119

8 7 6 WA 2 hrs. (%) 5 4 3 2 1 3% 6% 9% 12% 15% Adhesives content Fig. 11 Effect of adhesives on WA 2 hours of composite materials panels 8 7 6 WA 24 hrs. (%) 5 4 3 2 1 3% 6% 9% 12% 15% Adhesives content Fig. 12 Effect of adhesives on WA 24 hours of composite materials panels 12

Table 4. Properties testing of the composite panel made from various adhesives. Adhesive Types MUF PF PMDI Contents Density MOR (N/mm 2 ) MOE (N/mm 2 ) TH 2 hrs. (%) TH 24 hrs. (%) WA 2 hrs. (%) WA 24 hrs. (%) 3%.64 ±.2.79 ±.5 a 65 ± 16 a 36.7 ±.83 h 38.93 ± 1.33 g 58.37 ±.42 f 61. ±.82 g 6%.71 ±.4 3.59 ±.41 b 41 ± 7 b 27.66 ± 1.4 g 3.85 ± 1.2 f 49.16 ± 1.66 e 52.7 ± 1.25 f 9%.74 ±.3 4.75 ±.3 b 546 ± 111 bc 22.65 ± 1.63 e 25.62 ± 1.48 e 42.83 ±.57 d 46.59 ±.5 e 12%.79 ±.6 8.1 ±.15 c 712 ± 9 bcd 18.58 ± 1.66 d 21.85 ± 1.62 d 37.9 ± 2.65 c 41.76 ± 2.4 cd 15%.8 ±.2 9.1 ±.16 cd 813 ± 39 de 14.54 ±.28 c 17.55 ±.91 c 32.59 ± 1.6 b 37.63 ± 1.58 e 3%.64 ±.3 3.46 ±.5 b 549 ± 69 bc 45.65 ± 1.95 i 47.46 ± 2.18 h 65.92 ± 2.29 g 67.95 ± 2.6 h 6%.67 ±.4 9.5 ±.58 cd 814 ± 52 cd 41.59 ± 1.22 f 43.13 ± 1.29 e 6.59 ± 3.87 f 63.83 ± 2.61 g 9%.74 ±.1 12.98 ±.56 ef 1122 ± 142 ef 26.18 ± 1.67 d 29.28 ± 3.62 d 5.29 ± 3.64 e 53.94 ± 3.84 f 12%.81 ±.1 14.99 ±.65 f 127 ± 16 fg 17.86 ± 3.74 c 2.93 ± 4.75 c 38.79 ±.77 c 43.58 ± 1.37 d 15%.8 ±.2 17.49 ±.9 g 1424 ± 13 g 6.82 ±.22 b 1.84 ± 1.39 b 32.23 ± 1.81 b 38.42 ± 1.47 e 3%.7 ±.2 7.66 ±.59 c 675 ± 72 bcd 6.3 ±.79 ab 1.19 ±.41 b 12.34 ±.66 a 29.27 ±.84 a 6%.74 ±.1 11.12 ±.58 de 1793 ± 52 h 4.92 ±.43 ab 9.25 ±.5 b 11.3 ±.11 a 27.46 ±.23 a 9%.75 ±.3 18.1 ±.56 g 2,27 ± 142 h 4.25 ±.22 a 6.58 ±.23 a 1.47 ±.99 a 26.2 ± 1.7 a 12%.76 ±.3 18.4 ±.73 g 2,533 ± 16 i 4.11 ±.75 a 6.59 ±.93 a 1.68 ±.86 a 26.71 ± 1.21 a 15%.78 ±.2 27.1 ±.8 h 336 ± 15 j 4.9 ±.71 a 6.54 ±.67 a 1.66 ±.72 a 26.31 ±.44 a Note: Values within the same column followed by different letters (a-h) are significantly different at P <.5. The composite panels were made with 2-35 mesh particles. 121