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Effect of thermal-treatment of wood fibres on properties of flat-pressed wood plastic composites
Polymer Degradation and Stability 96 (2011) 818e822
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Effect of thermal-treatment of wood fibres on properties of flat-pressed wood plastic composites Nadir Ayrilmis a, *, Songklod Jarusombuti b,1, Vallayuth Fueangvivat c, 2, Piyawade Bauchongkol c, 2 a
Istanbul University, Forestry Faculty, Department of Wood Mechanics and Technology, Bahcekoy, Sariyer, 34473 Istanbul, Turkey Kasetsart University, Forestry Faculty, Department of Forest Products, Chatuchak, 10903 Bangkok, Thailand c Wood Research and Development Office, Royal Forest Department, 61 Phaholyothin Rd., Lad-Yao, Chatuchak, 10903 Bangkok, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2010 Received in revised form 24 January 2011 Accepted 6 February 2011 Available online 15 February 2011
This study aimed to enhance the dimensional stability of flat-pressed wood plastic composites (WPCs) containing fast growing wood fibres by a thermal-treatment method. The wood fibres were treated at three different temperatures (120, 150, or 180 C) for 20 or 40 min in a laboratory autoclave. The WPC panels were made from dry-blended Eucalyptus camaldulensis wood fibres and polypropylene (PP) powder (50:50 by weight) using a conventional flat-press process under laboratory conditions. Thickness swelling and water absorption of the WPC panels significantly decreased with increasing the treatment temperature and time. The thermal-treatment of eucalyptus wood fibres slightly decreased the screw withdrawal resistance of the WPC panels as compared to the reference panels while the flexural properties and internal bond strength were more seriously affected by the treatment. The present study revealed that the thermal-treatment of the wood fibres significantly improved the dimensional stability of the WPC panels. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Eucalyptus fibre Hot press moulding Mechanical properties Physical properties Thermal-treatment Wood plastic composite
1. Introduction Wood plastic composites (WPCs) are roughly 50:50 mixtures of thermoplastic polymers and small wood particles. The presence of wood in a plastic matrix can result in a stiffer and lower-cost material than if plastic alone was used. Also, the compression properties (resistance to crushing) for most WPCs are superior to that of wood loaded perpendicular to the grain [1]. However, the inherent problems with wood such as moisture sorption and thickness swelling (TS) remain. Wood is a hydrophilic porous composite of cellulose, lignin, and hemicellulose polymers that are rich in functional groups such as hydroxyls, which readily interact with water molecules by hydrogen bonding. For this reason, WPCs have potential to take up water under humid conditions due to the presence of numerous hydroxyls. Eucalyptus has been one of the most researched species to be used as raw material to manufacture pulp and paper. Eucalyptus is
* Corresponding author. Tel.: þ90 212 226 1100/25083; fax: þ90 212 226 1113. E-mail addresses: [email protected] (N. Ayrilmis), [email protected] (S. Jarusombuti), [email protected] (V. Fueangvivat), bauchongkol@yahoo. com (P. Bauchongkol). 1 Tel.: þ66 0 2942 8109; fax: þ66 0 2942 8371. 2 Tel.: þ66 0 2561 4292; fax: þ66 0 2579 5412. 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.02.005
among the fastest growing hardwood trees in the world, is grown in more than 90 countries and represents 8 percent of all planted forests [2]. Eucalyptus is native to Australia with over 600 species and is a fast growing tree with a high level of resistance against diseases [3]. Some species can reach 35 m high within seven years. They are planted on a large scale in tropical, subtropical and temperate locations where they grow with high productivity. Government agencies and private companies in many countries are in the process of taking long-term care of renewing forest resources [3]. The advantage of eucalyptus wood fibres is short as compared to softwoods. They do not flocculate as readily as long fibres. Consequently, boards made with eucalyptus fibre have better surface properties and are preferred worldwide as the substrate for pre-finished hardboard products. Despite many advantages, eucalyptus wood has some disadvantages such as high swelling, low dimensional stability, and several drying problems limiting its use [4]. It may not be suitable for use in those parts of places that have large seasonal changes in atmospheric moisture because of high dimensional changes of wood (14%) [5]. These disadvantages can also cause a loss of flat-pressed WPC’s dimensional stability containing eucalyptus wood fibres. Heating wood modifies the cell wall components in all mass of the wood sample. This chemical modification is accompanied by an increase of dimensional stability, at the expenses of some chemical
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degradation of wood. The changes are mainly caused by thermal degradation of hemicelluloses and result in the following advantageous modifications: swelling and shrinkage due to moisture is decreased; biological durability is improved; several extractives flow from the wood; the wood becomes lighter in weight; the equilibrium moisture content decreases; pH decreases; and thermal insulation properties are improved [6]. However, some strength properties became altered, namely hardness and abrasion resistance that are reduced [7,8]. Dehydration due to reduction of free hydroxyl groups leads to decreased moisture uptake, additionally supplied by the formation of hydrophobic substances due to cross linkage reactions of the wood polymers [9]. When wood-based panels such as medium density fibreboard (MDF) and particleboard have contact with water, the wood fibres swells and some of that residual stress created within the fibre mat during hot-pressing is released, causing an increase in the thickness of the panel. Excessive TS not only causes a poor appearance, but also markedly weakens panel products. Previous studies reported that dimensional stability of the wood-based panels is improved by thermal-treatment of wood fibres or particles, although mechanical properties dramatically decreased [10e13]. For example, Ayrilmis et al. [12] investigated dimensional stability of MDF panels made from thermally treated rubberwood fibres. They reported that the average TS value of the MDF panels made from untreated fibres was 23.8% while it was found 16.2% for the panels made from fibres treated at 180 C for 30 min. Similar results were also found for oriented strand board and particleboard [13,14]. But it has not been studied whether similar changes in the flat-pressed WPCs. Wood fibres are primarily responsible for the TS and water absorption (WA) of WPCs. The application of wood fillers is limited mainly because of the changes in geometry due to moisture sorption and swelling. The hydroxyl groups in the cellulose, the hemicellulose, and the lignin build a large amount of hydrogen bonds between the macromolecules of the wood polymers. The hydroxyl groups then form new hydrogen bonds with water molecules which induce the swelling. When the hygroscopicity of the wood fibres is decreased by thermal-treatment method, the WPCs can be used in severity conditions. An extensive literature search did not reveal any information about the effects of the thermal-treatment of the wood fibres on the properties of the WPC panels. The aim of the research reported here was to investigate the physical and mechanical properties of the WPC panels made from polypropylene (PP) powder and eucalyptus wood fibres (50:50 by weight (wt)) treated at different levels of temperature and time.
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2.2. Thermal-treatment The wood fibres were treated with the saturated steam under pressure at 120, 150, or 180 C for 20 or 40 min in the laboratory autoclave. The treated fibres were then dried to a moisture content of 2e3% based on the weight of the oven-dried fibres in the oven prior to the WPC panel manufacture. 2.3. Flat-pressed WPC panel manufacture Flat-pressed WPC panels were manufactured using standardized procedures that simulated industrial production at the laboratory. After mixing wood fibres and PP powder (50:50 by wt), the mixture was placed in a rotary drum blender. Following the blending treatment for 10 min, the mixture was weighed and then formed into a mat on an aluminium caul plate, using a forming box. Teflon sheet was used to avoid direct contact of PP powder with the hot press metal platens during heating and pressing. To reduce the mat height and to densify the mat, the mat was cold pressed. This procedure allowed for easy insertion of the mats into the hot press. The mats were then subjected to hot-pressing, using a manually controlled electrical-heated press. The maximum press pressure, pressing temperature, and total press cycle were 5 N/mm2, 180 C, and 6 min, respectively. At the end of the press cycle, the WPC panel was removed from the press for cooling. The nominal panel size was 350 mm 350 mm 10 mm after the cooling process. A total of 14 experimental panels, two for each type of panel, were manufactured. The average density of the WPC panels was 0.80 g/cm3. 2.4. Determination of water resistance Water resistance of the panels, TS and WA, was evaluated according to EN 317 [16]. Sixteen specimens, 50 mm 50 mm 10 mm, from each type of panel were used for the TS and WA properties. Prior to tests, the samples were conditioned in a climate-controlled room at 20 C and 65% relative humidity (RH). The weights and thicknesses of the specimens were measured at different time intervals during the long period of immersion. At the end of 1-, 7-, 14-, and 28-days of submersion, the specimens were removed from the water, all surface water were wiped off with a dry cloth, and weighed to the nearest 0.001 g and measured to the nearest 0.01 mm immediately. The specimen thickness was determined by taking a measurement at a specific location, the diagonal crosspoint, on the sample. Density of the specimens was evaluated according to the test method specified in EN 323 [17]. 2.5. Determination of mechanical properties
2. Experimental 2.1. Materials The cellulosic fibres used in this study, Eucalyptus camaldulensis wood fibres with an average length of 0.8 mm, were obtained from a commercial MDF plant in Thailand. The average fibre diameter, wall thickness, and fibre lumen width of E. camaldulensis wood are 17.2 m, 3.3 m, and 10.7 m, respectively [15]. The fibres were produced using a thermo-mechanical refining process without any chemical and resin. The moisture content of the fibres, as determined by oven-dry weight, was found to be 2e3% prior to treatment. Virgin PP (Tm ¼ 160 C, r ¼ 0.9 g/cm3, MFI/230 C/2.16 kg ¼ 6.5 g/10 min) produced by Petkim Petrochemical Co., Izmir, Turkey, was used as a polymeric material. The PP granules were then processed by a rotary grinder to pass through a US 40-mesh screen and retained on a US 80-mesh screen.
Flexural properties (modulus of rupture (MOR) and modulus of elasticity (MOE)) of the specimens conditioned to equilibrium at a temperature of at 20 C and 65% RH were conducted according to EN 310 [18]. A total of twelve specimens with dimensions of 250 mm 50 mm 10 mm, six parallel and six perpendicular to the panel surface, were tested for each type of panel to determine MOR and MOE. The flexural specimens were tested on an Instron testing system Model-4466 equipped with a load cell with a capacity of 10 kN. Internal bond (IB) tests were conducted on the specimens cut from the experimental WPC panels according to EN 319 [19]. Sixteen specimens in 50 mm 50 mm 10 mm from each type of panel were used to determine the IB strength. For screw withdrawal resistance (SWR) perpendicular to the plane of the panel, sixteen specimens with dimensions of 75 mm 75 mm 10 mm from each type of panel were tested. Embedment depth for the screws was 10 mm, and measurement of SWR in N was undertaken
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according to EN 320 [20]. Specification of the screw is described in the Standard.
2.6. Statistical analysis An analysis of variance, ANOVA, was conducted (p < 0.01) to evaluate the effect of the thermal-treatment on the physical and mechanical properties of the WPC panels. Significant differences among the average values of the WPC groups were determined using Duncan’s multiple range test.
3. Results and discussion 3.1. Physical properties The thermal-treatment of the wood fibres showed a highly significant effect on the TS and WA of the WPC panels, especially above 150 C for 40 min (Table 1). For example, the average TS and WA values of the panels containing wood fibres treated at 180 C and 40 min after 28-days of submersion were 3.25% and 17.84% as compared to the reference panels containing untreated wood fibres which were 8.15% and 25.94%, respectively. The thermal-treatment of the wood fibres at 120 C for 20 min showed a reduction in the TS after 28-days of submersion of 36%, while the treatment at 180 C for 40 min resulted in a reduction of 60%. Significant reductions also were found for the WA. In general, the treatment temperature had a stronger effect on the TS and WA than the treatment time (Table 1). Statistical analysis found significant differences at a probability level of 0.01 between some group averages for the TS and WA values. Significant differences between groups were determined individually for these tests by Duncan’s multiplecomparison tests (Table 1). The TS values of the WPC panels increased with increasing water exposure time. The majority of TS and WA occurred during the first 14 days. After that, the percentage of the changes was negligible (Table 1). No significant differences in density resulted from the thermal-treatment levels when compared to reference specimens. Wood-based panel standards were used here for comparison of the TS and the WA values since there was no established minimum property for the WPC panel. 24-h TS values of all WPC panel types met maximum requirements of EN 312 [21] and EN 622-5 [22] with respect to particleboard Type 7 (9%) and MDF Type HLS (10%). The TS and WA values of the WPC specimens were also much less than those of commercial particleboard, oriented strand board, and MDF panels [23,24]. This was mainly attributed to the hydrophobic character of the PP because of its being devoid of functional polar groups such as hydroxyls in the molecular and thus chemically inactive.
Water resistance of the WPC panels containing thermally treated wood fibres were higher than in previous studies [25e28]. For example, Ayrilmis and Jarusombuti [28] found that average TS and WA values of 10 mm thick (0.80 g/cm3) WPC panels made from rubberwood fibres and PP powder (50:50 by wt) were 5.03% and 7.14%, respectively. Higher water resistance of the WPC panels containing thermally treated wood fibres was mainly attributed to hemicelluloses were hydrolyzed during the thermal-treatment. Due to their amount of hydrophilic groups they have excellent swelling properties. The water absorption of composites was due to the hydrogen bonding of the water molecules to the free hydroxyl groups present in the cellulosic cell wall materials and the diffusion of water molecules into the filler-matrix interface [29]. Hemicelluloses are hydrolyzed during thermal-treatment, and this decreases the hygroscopity of the lignocellulosic materials. Del Menezzi and Tomaselli [8] and Winandy and Krzysik [30] each reported that the reduction in moisture absorption could happen because of the hemicelluloses, one of the more hygroscopic polymers within the cell wall and also generally the most heat sensitive polymers of the wood components. Exposure time and temperature are two important factors affecting hemicelluloses degradation. Compared to other tree species, eucalyptus wood possesses the higher content of extractives, which have greater effect on wood properties. Under the action of heat and moisture, the phenolic compounds in wood may flow and spread out between the microcapillary systems, such as pit membrane, microfibril gap etc., which would occupy the positions in which should be the combination of water with the main components in wood, such as hemicellulose, cellulose, and lignin [31]. The phenolic compounds in the eucalyptus wood could decrease the TS and WA of the WPC panels with the increase of the treatment temperature. The TS of the WPC panels made from treated wood fibres could also be suppressed due to increased cellulose crystallinity, degradation of the amorphous regions in the cellulose microfibrils or because fewer hydroxyl groups exist after the hydrothermaltreatment. Cross linking between the cell wall polymers, especially lignin, esterification between the cellulose microfibrils, and the formation of ether linkage by the splitting of two adjacent hydroxyl groups are other viable reasons for the swelling loss [9]. 3.2. Mechanical properties The thermal-treatment of eucalyptus wood fibres slightly decreased the screw withdrawal resistance of the WPC panels as compared to the reference panels while the flexural properties and IB strength were more seriously affected by the treatment (Table 2). The thermal-treatment of the wood fibres led to reductions of 5e19% and 7e22% in MOR and MOE, respectively, depending on the
Table 1 Physical properties of the WPC panels as a function of thermal-treatment of the wood fibres (the ratio of wood fibre to polypropylene: 50:50 by wt). Thermal-treatment level
Untreated reference 120 Ce20 min 120 Ce40 min 150 Ce20 min 150 Ce40 min 180 Ce20 min 180 Ce40 min
Physical properties Density (g/cm3)
Thickness swelling (%)
0.82 0.80 0.82 0.81 0.82 0.83 0.81
3.98 Aa 2.94 B 2.64 BC 2.28 C 2.05 CD 1.84 D 1.72 D
(0.03) (0.02) (0.04) (0.02) (0.03) (0.04) (0.03)
1-day
Water absorption (%)
7-days (0.28) (0.20) (0.18) (0.25) (0.22) (0.19) (0.15)
5.06 A 3.82 B 3.48 BC 3.15 CD 3.05 D 2.28 E 2.05 E
(0.39) (0.42) (0.41) (0.38) (0.37) (0.34) (0.30)
14-days
28-days
1-day
7.82 A 4.97 B 4.59 B 4.27 B 3.81 C 3.17 D 3.06 D
8.15 A 5.20 B 4.84 B 4.51 C 4.02 C 3.39 D 3.25 D
6.45 A 4.82 B 4.39 B 3.85 C 3.47 CD 3.15 D 2.91 E
(0.54) (0.48) (0.51) (0.44) (0.48) (0.40) (0.35)
(0.47) (0.52) (0.44) (0.49) (0.51) (0.46) (0.39)
7-days (0.52) (0.49) (0.43) (0.53) (0.44) (0.37) (0.42)
16.53 A 14.72 B 14.09 B 13.37 BC 12.96 C 10.88 D 9.79 E
14-days (1.54) (1.43) (1.35) (1.47) (1.24) (1.32) (0.97)
24.12 A 21.59 B 20.96 B 19.77 C 19.25 C 17.92 D 16.95 E
28-days (1.78) (1.56) (1.67) (1.58) (1.48) (1.44) (1.51)
25.94 A 23.18 B 21.46 B 21.12 C 20.58 D 18.64 E 17.84 E
Values in parentheses are standard deviations. a Groups with same letters in column indicate that there is no statistical difference (p < 0.01) between the specimens according Duncan’s multiply range test.
(1.87) (1.72) (1.83) (1.50) (1.58) (1.45) (1.33)
N. Ayrilmis et al. / Polymer Degradation and Stability 96 (2011) 818e822 Table 2 Mechanical properties of the WPC panels as a function of thermal-treatment of the wood fibres (the ratio of wood fibre to polypropylene: 50:50 by wt). Thermal-treatment Mechanical properties level Modulus of Modulus of elasticity rupture (N/mm2) (N/mm2) Untreated reference 120 Ce20 min 120 Ce40 min 150 Ce20 min 150 Ce40 min 180 Ce20 min 180 Ce40 min
25.9 Aa (1.3) 2682.4 A (135) 24.7 B 24.2 BC 23.7 CD 23.0 D 21.7 F 21.0 F
(1.2) (1.1) (1.2) (1.2) (0.9) (1.0)
2483.8 B 2434.5 B 2358.3 C 2264.2 D 2225.8 D 2102.5 E
Internal bond Surface screw strength withdrawal (N/mm2) resistance (N) 0.95 A (0.14) 1010 A (58)
(113) 0.86 B (0.10) 982 AB (128) 0.83 BC (0.12) 976 AB (108) 0.77 CD (0.09) 951 BC (97) 0.75 D (0.08) 924 CD (111) 0.71 DE (0.09) 915 CD (89) 0.68 E (0.07) 894 D
(51) (55) (48) (42) (50) (38)
a Groups with same letters in column indicate that there is no statistical difference (p < 0.01) between the specimens according Duncan’s multiply range test. Values in parentheses are standard deviations.
temperature and time conditions. The treatment groups showing significant differences (p < 0.01) were determined according to the Duncan’s multiply range test (Table 2). The reference panels had the highest MOR and MOE values while the lowest values were found for the panels made from the fibres treated at 180 C and 40 min (Table 2). Although the thermal-treatment decreased the MOR and MOE of all panel types, they met the minimum requirements (15 and 2050 N/mm2) of EN 312 [21] Type P III particleboards for nonstructural applications in humid conditions. The loss of the mechanical properties of the WPC panels containing thermally treated wood fibres could be related to formation of soluble acidic chemicals; such as formic acid and acetic acid, from the hemicellulose degradation [32]. These acids accelerate depolymerization of the cellulose by breaking down the long chain cellulose (crystalline structure) to shorter chains. In addition, CeC and CeO linkages cleavage at the intra-polymer level with increasing temperature and time of the treatment. This leads to the separation of the lignin-hemicellulose copolymer system and depolymerization of hemicelluloses and amorphous cellulose [33]. Depolymerization and shortening of the cellulose polymer could affect MOE and MOR of the wood [34]. Loss in MOR and MOE of wood [35,36] and wood-based panels [12,37] treated at high temperatures was reported by different authors. Similar results were also found the WPC panels in the present study. The IB values of the WPC panels were significantly decreased by the thermal-treatment of the wood fibres. The thermal-treatment of the wood fibres reduced the IB strength of the WPC panels from 9% to 28% as compared to the reference panels, depending on the temperature and time conditions. The IB results indicated that the mechanical interlocking between the wood fibres and polymer matrix was negatively affected by increasing severity of the thermaltreatment. The WPC panel containing the fibres treated at 180 C for 40 min had the lowest IB value with 0.68 N/mm2 while the highest IB value with 0.95 N/mm2 was found for the reference panel (Table 2). Low IB strength of the WPC panels containing thermally treated wood fibres was mainly attributed surface roughness of the wood fibres. Mechanical bonding can occur due to the physical interlocking of two surfaces [38]. The surface roughness positively affects mechanical interlocking of two surfaces because it increases the total interfacial area between wood and polymer matrix. It could also provide mechanical interlocking effect that could trap the polymer in the cavities and act like an anchor to each other. Previous studies reported that surface roughness of wood was improved by increasing thermal-treatment temperature and time [39e42]. For example, Unsal and Ayrilmis [42] reported that average surface roughness (Ra) of E. camaldulensis wood samples treated at 180 C for 10 h had was 7.2 mm while it was found as
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10.0 mm for reference wood samples. This could be one of the mechanisms responsible for poor mechanical interlocking between thermally treated eucalyptus wood fibre and the polymer matrix. Wettability of the wood fibres also affects mechanical interlocking between the wood fibres and polymer matrix. Wetting is of great importance in achieving good adhesive interaction between two phases [38]. Wettability is defined as a condition of a surface that determines that how fast a liquid will wet and spread on the surface or whether it will be repelled and not spread on the surface [43]. In WPCs it is characterised by the degree of direct interfacial contact between the wood and polymer surface. Adhesion is sticking together two surfaces so that stress can be transmitted between them and can be quantified by the amount of work is required to pull the two surface apart. Previous studies reported that wettability of wood was decreased by increasing temperature and time [44e46]. Heat-induced chemical modification results in surface inactivation of fibres in layers near the surface. The surface of wood exposed to high temperatures is less polar, resulting in a lower wettability than in the case of untreated wood [47]. Sernek et al. [43] found in their work about the inactivation of wood surfaces in consequence of exposure to heat up to 180 C. The IB results can be rationalised in terms of the decreased wettability of the fibre surfaces, which reduces mechanical interlocking between the wood fibre and polymer matrix. The SWR of the WPC panels was slightly decreased by the thermal-treatment of the wood fibres (Table 2). The thermaltreatment of the wood fibres reduced the SWR from 3% to 11% as compared to the reference panels, depending on the temperature and time conditions. These results were in agreement with previous studies in wood-based panels [12,48]. For example, Ayrilmis et al. [12] reported that thermal-treatment of wood fibres had a negative effect on SWR of MDF panels. They found that the average SWR value of the MDF panels made from thermally treated rubberwood fibres at 180 C for 30 min decreased from 1750 N to 1450 N compared to the reference panels. Although the SWR of the wood-based panels was seriously affected by the thermal-treatment of the wood fibres, this situation was not observed for the WPC panels in the present study. Low SWR of the WPC panels containing thermally treated wood fibres was mainly attributed to depolymerization of the cellulose by breaking down the long chain cellulose (crystalline structure) to shorter chains. It was concluded that the depolymerization of the cellulose decreased the ability of the wood fibres to conform around the thread of the screw, allowing continuous load transfer along the thread. All WPC panel types met minimum surface SWR (875 N) requirement for grade 120 MDF (panel thickness 21 mm) specified in ANSI A208.2 [49]. In addition, the SWR values of the WPC panels containing thermally treated wood fibres were slightly lower than in a previous study, which was 1199 N for 10 mm thick (0.80 g/cm3) WPC panels made from untreated rubberwood fibres and PP powder (50:50 by wt) [28]. 4. Conclusions The water resistance of the WPC panels significantly improved with increasing treatment temperature and time of eucalyptus wood fibres. The thermal-treatment of the wood fibres led to reductions of 60% and 31% in the TS and WA after 28-days of water soaking, respectively, depending on the temperature and time conditions. This was mainly attributed to the fact that hemicelluloses were hydrolyzed during the thermal-treatment. The thermal-treatment of the wood fibres slightly decreased the SWR of the WPC panels while the flexural properties were more seriously affected by the treatment. The thermal-treatment of the wood fibres led to reductions of 5e19%, 7e22%, and 9e28% in
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MOR, MOE, and IB strength, respectively, while the SWR decreased from 3% to 11%, depending on the temperature and time conditions. Despite the fact that fast growing wood species like eucalyptus has many advantages, they have some disadvantages such as high swelling and low dimensional stability. These disadvantages can also cause a loss of flat-pressed WPC’s dimensional stability. It appears that the thermal-treatment of the fast growing wood fibres, above 150 C for 40 min, in the manufacture of the flatpressed WPC can be a practical choice for applications requiring a high degree of water resistance. Acknowledgements The authors gratefully acknowledge Department of Forest Products, Forestry Faculty, Kasetsart University, and Royal Forest Department, Thailand, for laboratory equipment used in the manufacture of the WPC panels. Further acknowledgement goes to Research Fund of Istanbul University for the financial support for this study. The authors would also like to thank Chaiyun Pangwong Pitak Hangam, and Somwung Chaichoom, Forest Products Laboratory Technicians, Royal Forest Department, Thailand, for their valuable assistance with WPC panel manufacture. References [1] Taylor A, Yadama V, Englund KR, Harper D. Wood plastic composites e a primer. UT Exension PB 1779. The University of Tennessee, Institute of Agriculture; 2011. p. 1. [2] Anonymous. Eucalyptus is part of the U.S. energy security equation. Eucalyptus facts. Summerville, SC, USA: Arborgen Inc., http://www.eucalyptusfacts.org/; 2010. [3] Nacar M, Hiziroglu S, Kalaycioglu H. Some of the properties of particleboard panels made from eucalyptus. Am J Appl Sci; 2005:5e8 [special issue]. [4] Unsal O, Korkut S, Atik C. The effect of heat treatment on some properties and colour in eucalyptus (Eucalyptus camaldulensis Dehn.) wood. Maderas Cienc Tecnol 2003;5:145e52. [5] Cetin NS, Gultekin G, Ozmen N. Dimensional stabilisation of Eucalyptus grandis w. Hill ex maiden sapwood with succinic anhyride. In: Proceedings of 3rd National Blacksea Forestry Congress National Conference, Artvin, Turkey; 20e22 May, 2010. p. 1649e1657. [6] Anonymous. Thermowood handbook. Helsinki: Finnish ThermoWood Association; 2003. p. 7. [7] Kamdem DP, Pizzi A, Jermannaud A. Durability of heat treated wood. Holz Roh Werkst 2002;60:1e6. [8] Del Menezzi CHS, Tomaselli I. Contact thermal post-treatment of oriented strandboard to improve dimensional stability: a preliminary study, vol. 64; 2006. p. 212e217. [9] Tjeerdsma BF, Militz H. Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood. Holz Roh Werkst 2005;63:102e11. [10] Garcia RA, Cloutier A, Riedl B. Dimensional stability of MDF panels produced from heat-treated fibres. Holzforschung 2006;60:278e84. [11] Mohebby B, Ilbeighi F, Najafi SK. Influence of hydrothermal modification of fibres on some physical and mechanical properties of medium density fibreboard (MDF). Holz Roh Werkst 2008;66:213e8. [12] Ayrilmis N, Jarusombuti S, Fuengvivat V, Bauchongkol P. Effect of thermal treatment of rubber wood fibres on physical and mechanical properties of medium density fibreboard. J Trop Forest Sci 2011;23:10e6. [13] Paul W, Ohlmeyer M, Leithoff H, Boonstra MJ, Pizzi A. Optimising the properties of OSB by a one-step heat pre-treatment process. Holz Roh Werkst 2006;64:227e34. [14] Tomek A. p. 157e160. Die heißvergu “tung von holzspa” nen, ein neues verfahren zum hydrophobieren von spanplatten Holztechnologie, vol. 7; 1966. [15] Nasir GM. Fibre morphology in relation to suitability for pulp and paper. Research Report. Peshawar, Pakistan: Forest Products Research Pakistan Forest Institute; 2009. p. 2. [16] European Norm (EN) 317. Particleboards and fiberboards-determination of swelling in thickness after immersion in water. Brussel, Belgium: European Committee for Standardization; 1993. [17] EN 323. Determination of density; 1993. [18] EN 310. Wood based panels: determination of bending strength and modulus of elasticity; 1993.
[19] EN 319. Particleboards and fiberboards. Determination of tensile strength perpendicular to the plane of the board; 1993. [20] EN 320. Fibreboards. Determination of resistance to axial withdrawal of screw; 1993. [21] EN 312. Particleboards-specifications; 1993. [22] EN 622-5. Fibreboards. Specifications. Requirements for dry process boards (MDF); 2009. [23] Ayrilmis. Effect of tree species on technological properties of MDF. MSc thesis, Institute of Natural Science, Istanbul University, Istanbul, Turkey; 2000. p. 25. [24] Akbulut T, Goker Y, Ayrilmis N. Using OSB panels instead of plywood. In: Review of the faculty of forestry: part A. Istanbul University; 2002. p. 65e9. vol. 52, 1. [25] Wechsler A, Hiziroglu S. Some of the properties of wood plastic composites. Build Environ 2007;42:2637e44. [26] Chen HC, Fann JD, Chen TY, Hsu CH, Chen YP. Interior flooring material made of wood particle-plastic composites by flat-platen pressing. Forest Prod J 2007;57(9):42e5. [27] Chaharmahali M, Mirbagheri J, Tajvidi M, Najafi SK, Mirbagheri Y. Mechanical and physical properties of wood-plastic composite panels. J Reinforc Plast Comp 2010;29:310e9. [28] Ayrilmis N, Jarusombuti S. Flat-pressed wood plastic composite as an alternative to conventional wood-based panels. J Compos Mater 2011;45:103e12 [29] Ashori A, Sheshmani S. Hybrid composites made from recycled materials: moisture absorption and thickness swelling behavior. Bioresour Technol 2010;101:4717e20. [30] Winandy JE, Krzysik A. Thermal degradation of wood fibres during hotpressing of MDF composites: part I. Relative effects and benefits of thermal exposure. Wood Fiber Sci 2007;39:450e61. [31] Renmei G, Xiaohui Z. Effect of steam aporizing time on the performance of wood humidity sorption. J Forest Sci Technol 1995;20(4):464e7. [32] Garrote G, Dominiguez H, Parajo JC. Study on the deactylation of hemicelluloses during the hydrothermal processing of Eucalyptus wood. Holz Roh Werkst 2001;59:53e9. [33] Colak S, Colakoglu G, Aydin I, Kalaycioglu H. Build Environ 2007;42:304e9. [34] Rowell R, editor. Handbook of wood chemistry and wood composites, vol. 1. New York: CRC Press; 2005. p. 330. [35] Yildiz S, Gezer ED, Yildiz UC. Mechanical and chemical behavior of spruce wood modified by heat. Build Environ 2006;41:1762e6. [36] Kubojima Y, Okano T, Ohta M. Bending strength and toughness of heat-treated wood. J Wood Sci 2000;46:8e15. [37] Ohlmeyer M, Lukowsky D. Wood-based panels produced from thermaltreated materials: properties and perspectives. In: Proceedings of international conference on wood frame housing durability and disaster, Las Vegas, USA; October 4e6, 2004. p. 127e131. [38] Niska KO, Sain M. Wood-polymers composites. New York: CRC Press; 2008. p. 47. [39] Gunduz G, Korkut S, Korkut DS. The effects of heat treatment on physical and technological properties and surface roughness of camiyani black pine (pinus nigra Arn. subsp. pallasiana var. pallasiana) wood. Bioresour Technol 2008;99:2275e80. [40] Korkut DS, Guller B. The effects of heat treatment on physical properties and surface roughness of red-bud maple (Acer trautvetteri Medw.) wood. Bioresour Technol 2008;99:2846e51. [41] Korkut S, Alma MH, Elyildirim YK. The effects of heat treatment on physical and technological properties and surface roughness of European Hophornbeam (Ostrya carpinifolia Scop.) wood. Afr J Biotechnol 2009;8:5316e27. [42] Unsal O, Ayrilmis N. Variations in compression strength and surface roughness of heat-treated Turkish river red gum (Eucalyptus camaldulensis) wood. J Wood Sci 2005;51:405e9. [43] Sernek M, Kamke AF, Glasser WG. Comparative analysis of inactivated wood surface. Holzforschung 2004;58:22e31. [44] Kocaefe D, Poncsak S, Dore G, Younsi R. Effect of heat treatment on the wettability of white ash and soft maple by water. Holz Roh Werkst 2008;66:355e61. [45] Petrissans M, Gerardin P, El Bakali I, Serraj M. Wettability of heat-treated wood. Holzforschung 2003;57:301e7. [46] Hakkou M, Pétrissans M, Zoulalian A, Gérardin P. Investigation of wood wettability changes during heat treatment on the basis of chemical analysis. Polymer Degrad Stabil 2005;89:1e5. [47] Christiansen AW. How overdrying wood reduces its bonding to phenol formaldehyde adhesives: a critical review of the literature. Part I. Physical responses. Wood Fiber Sci 1990;22:441e59. [48] Okino EYA, Teixeira DE, Del Menezzi CHS. Post-thermal treatment of oriented strandboard (OSB) made from cypress (Cupressus glauca Lam). Maderas Cienc Tecnol 2007;9:199e210. [49] ANSI A208.2. Medium density fibreboard (MDF) for interior applications. Gaithersburg, USA: American National Standards Institute; 2002.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Effect of thermal-treatment of wood fibres on properties of flat-pressed wood plastic composites Nadir Ayrilmis a, *, Songklod Jarusombuti b,1, Vallayuth Fueangvivat c, 2, Piyawade Bauchongkol c, 2 a
Istanbul University, Forestry Faculty, Department of Wood Mechanics and Technology, Bahcekoy, Sariyer, 34473 Istanbul, Turkey Kasetsart University, Forestry Faculty, Department of Forest Products, Chatuchak, 10903 Bangkok, Thailand c Wood Research and Development Office, Royal Forest Department, 61 Phaholyothin Rd., Lad-Yao, Chatuchak, 10903 Bangkok, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2010 Received in revised form 24 January 2011 Accepted 6 February 2011 Available online 15 February 2011
This study aimed to enhance the dimensional stability of flat-pressed wood plastic composites (WPCs) containing fast growing wood fibres by a thermal-treatment method. The wood fibres were treated at three different temperatures (120, 150, or 180 C) for 20 or 40 min in a laboratory autoclave. The WPC panels were made from dry-blended Eucalyptus camaldulensis wood fibres and polypropylene (PP) powder (50:50 by weight) using a conventional flat-press process under laboratory conditions. Thickness swelling and water absorption of the WPC panels significantly decreased with increasing the treatment temperature and time. The thermal-treatment of eucalyptus wood fibres slightly decreased the screw withdrawal resistance of the WPC panels as compared to the reference panels while the flexural properties and internal bond strength were more seriously affected by the treatment. The present study revealed that the thermal-treatment of the wood fibres significantly improved the dimensional stability of the WPC panels. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Eucalyptus fibre Hot press moulding Mechanical properties Physical properties Thermal-treatment Wood plastic composite
1. Introduction Wood plastic composites (WPCs) are roughly 50:50 mixtures of thermoplastic polymers and small wood particles. The presence of wood in a plastic matrix can result in a stiffer and lower-cost material than if plastic alone was used. Also, the compression properties (resistance to crushing) for most WPCs are superior to that of wood loaded perpendicular to the grain [1]. However, the inherent problems with wood such as moisture sorption and thickness swelling (TS) remain. Wood is a hydrophilic porous composite of cellulose, lignin, and hemicellulose polymers that are rich in functional groups such as hydroxyls, which readily interact with water molecules by hydrogen bonding. For this reason, WPCs have potential to take up water under humid conditions due to the presence of numerous hydroxyls. Eucalyptus has been one of the most researched species to be used as raw material to manufacture pulp and paper. Eucalyptus is
* Corresponding author. Tel.: þ90 212 226 1100/25083; fax: þ90 212 226 1113. E-mail addresses: [email protected] (N. Ayrilmis), [email protected] (S. Jarusombuti), [email protected] (V. Fueangvivat), bauchongkol@yahoo. com (P. Bauchongkol). 1 Tel.: þ66 0 2942 8109; fax: þ66 0 2942 8371. 2 Tel.: þ66 0 2561 4292; fax: þ66 0 2579 5412. 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.02.005
among the fastest growing hardwood trees in the world, is grown in more than 90 countries and represents 8 percent of all planted forests [2]. Eucalyptus is native to Australia with over 600 species and is a fast growing tree with a high level of resistance against diseases [3]. Some species can reach 35 m high within seven years. They are planted on a large scale in tropical, subtropical and temperate locations where they grow with high productivity. Government agencies and private companies in many countries are in the process of taking long-term care of renewing forest resources [3]. The advantage of eucalyptus wood fibres is short as compared to softwoods. They do not flocculate as readily as long fibres. Consequently, boards made with eucalyptus fibre have better surface properties and are preferred worldwide as the substrate for pre-finished hardboard products. Despite many advantages, eucalyptus wood has some disadvantages such as high swelling, low dimensional stability, and several drying problems limiting its use [4]. It may not be suitable for use in those parts of places that have large seasonal changes in atmospheric moisture because of high dimensional changes of wood (14%) [5]. These disadvantages can also cause a loss of flat-pressed WPC’s dimensional stability containing eucalyptus wood fibres. Heating wood modifies the cell wall components in all mass of the wood sample. This chemical modification is accompanied by an increase of dimensional stability, at the expenses of some chemical
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degradation of wood. The changes are mainly caused by thermal degradation of hemicelluloses and result in the following advantageous modifications: swelling and shrinkage due to moisture is decreased; biological durability is improved; several extractives flow from the wood; the wood becomes lighter in weight; the equilibrium moisture content decreases; pH decreases; and thermal insulation properties are improved [6]. However, some strength properties became altered, namely hardness and abrasion resistance that are reduced [7,8]. Dehydration due to reduction of free hydroxyl groups leads to decreased moisture uptake, additionally supplied by the formation of hydrophobic substances due to cross linkage reactions of the wood polymers [9]. When wood-based panels such as medium density fibreboard (MDF) and particleboard have contact with water, the wood fibres swells and some of that residual stress created within the fibre mat during hot-pressing is released, causing an increase in the thickness of the panel. Excessive TS not only causes a poor appearance, but also markedly weakens panel products. Previous studies reported that dimensional stability of the wood-based panels is improved by thermal-treatment of wood fibres or particles, although mechanical properties dramatically decreased [10e13]. For example, Ayrilmis et al. [12] investigated dimensional stability of MDF panels made from thermally treated rubberwood fibres. They reported that the average TS value of the MDF panels made from untreated fibres was 23.8% while it was found 16.2% for the panels made from fibres treated at 180 C for 30 min. Similar results were also found for oriented strand board and particleboard [13,14]. But it has not been studied whether similar changes in the flat-pressed WPCs. Wood fibres are primarily responsible for the TS and water absorption (WA) of WPCs. The application of wood fillers is limited mainly because of the changes in geometry due to moisture sorption and swelling. The hydroxyl groups in the cellulose, the hemicellulose, and the lignin build a large amount of hydrogen bonds between the macromolecules of the wood polymers. The hydroxyl groups then form new hydrogen bonds with water molecules which induce the swelling. When the hygroscopicity of the wood fibres is decreased by thermal-treatment method, the WPCs can be used in severity conditions. An extensive literature search did not reveal any information about the effects of the thermal-treatment of the wood fibres on the properties of the WPC panels. The aim of the research reported here was to investigate the physical and mechanical properties of the WPC panels made from polypropylene (PP) powder and eucalyptus wood fibres (50:50 by weight (wt)) treated at different levels of temperature and time.
819
2.2. Thermal-treatment The wood fibres were treated with the saturated steam under pressure at 120, 150, or 180 C for 20 or 40 min in the laboratory autoclave. The treated fibres were then dried to a moisture content of 2e3% based on the weight of the oven-dried fibres in the oven prior to the WPC panel manufacture. 2.3. Flat-pressed WPC panel manufacture Flat-pressed WPC panels were manufactured using standardized procedures that simulated industrial production at the laboratory. After mixing wood fibres and PP powder (50:50 by wt), the mixture was placed in a rotary drum blender. Following the blending treatment for 10 min, the mixture was weighed and then formed into a mat on an aluminium caul plate, using a forming box. Teflon sheet was used to avoid direct contact of PP powder with the hot press metal platens during heating and pressing. To reduce the mat height and to densify the mat, the mat was cold pressed. This procedure allowed for easy insertion of the mats into the hot press. The mats were then subjected to hot-pressing, using a manually controlled electrical-heated press. The maximum press pressure, pressing temperature, and total press cycle were 5 N/mm2, 180 C, and 6 min, respectively. At the end of the press cycle, the WPC panel was removed from the press for cooling. The nominal panel size was 350 mm 350 mm 10 mm after the cooling process. A total of 14 experimental panels, two for each type of panel, were manufactured. The average density of the WPC panels was 0.80 g/cm3. 2.4. Determination of water resistance Water resistance of the panels, TS and WA, was evaluated according to EN 317 [16]. Sixteen specimens, 50 mm 50 mm 10 mm, from each type of panel were used for the TS and WA properties. Prior to tests, the samples were conditioned in a climate-controlled room at 20 C and 65% relative humidity (RH). The weights and thicknesses of the specimens were measured at different time intervals during the long period of immersion. At the end of 1-, 7-, 14-, and 28-days of submersion, the specimens were removed from the water, all surface water were wiped off with a dry cloth, and weighed to the nearest 0.001 g and measured to the nearest 0.01 mm immediately. The specimen thickness was determined by taking a measurement at a specific location, the diagonal crosspoint, on the sample. Density of the specimens was evaluated according to the test method specified in EN 323 [17]. 2.5. Determination of mechanical properties
2. Experimental 2.1. Materials The cellulosic fibres used in this study, Eucalyptus camaldulensis wood fibres with an average length of 0.8 mm, were obtained from a commercial MDF plant in Thailand. The average fibre diameter, wall thickness, and fibre lumen width of E. camaldulensis wood are 17.2 m, 3.3 m, and 10.7 m, respectively [15]. The fibres were produced using a thermo-mechanical refining process without any chemical and resin. The moisture content of the fibres, as determined by oven-dry weight, was found to be 2e3% prior to treatment. Virgin PP (Tm ¼ 160 C, r ¼ 0.9 g/cm3, MFI/230 C/2.16 kg ¼ 6.5 g/10 min) produced by Petkim Petrochemical Co., Izmir, Turkey, was used as a polymeric material. The PP granules were then processed by a rotary grinder to pass through a US 40-mesh screen and retained on a US 80-mesh screen.
Flexural properties (modulus of rupture (MOR) and modulus of elasticity (MOE)) of the specimens conditioned to equilibrium at a temperature of at 20 C and 65% RH were conducted according to EN 310 [18]. A total of twelve specimens with dimensions of 250 mm 50 mm 10 mm, six parallel and six perpendicular to the panel surface, were tested for each type of panel to determine MOR and MOE. The flexural specimens were tested on an Instron testing system Model-4466 equipped with a load cell with a capacity of 10 kN. Internal bond (IB) tests were conducted on the specimens cut from the experimental WPC panels according to EN 319 [19]. Sixteen specimens in 50 mm 50 mm 10 mm from each type of panel were used to determine the IB strength. For screw withdrawal resistance (SWR) perpendicular to the plane of the panel, sixteen specimens with dimensions of 75 mm 75 mm 10 mm from each type of panel were tested. Embedment depth for the screws was 10 mm, and measurement of SWR in N was undertaken
820
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according to EN 320 [20]. Specification of the screw is described in the Standard.
2.6. Statistical analysis An analysis of variance, ANOVA, was conducted (p < 0.01) to evaluate the effect of the thermal-treatment on the physical and mechanical properties of the WPC panels. Significant differences among the average values of the WPC groups were determined using Duncan’s multiple range test.
3. Results and discussion 3.1. Physical properties The thermal-treatment of the wood fibres showed a highly significant effect on the TS and WA of the WPC panels, especially above 150 C for 40 min (Table 1). For example, the average TS and WA values of the panels containing wood fibres treated at 180 C and 40 min after 28-days of submersion were 3.25% and 17.84% as compared to the reference panels containing untreated wood fibres which were 8.15% and 25.94%, respectively. The thermal-treatment of the wood fibres at 120 C for 20 min showed a reduction in the TS after 28-days of submersion of 36%, while the treatment at 180 C for 40 min resulted in a reduction of 60%. Significant reductions also were found for the WA. In general, the treatment temperature had a stronger effect on the TS and WA than the treatment time (Table 1). Statistical analysis found significant differences at a probability level of 0.01 between some group averages for the TS and WA values. Significant differences between groups were determined individually for these tests by Duncan’s multiplecomparison tests (Table 1). The TS values of the WPC panels increased with increasing water exposure time. The majority of TS and WA occurred during the first 14 days. After that, the percentage of the changes was negligible (Table 1). No significant differences in density resulted from the thermal-treatment levels when compared to reference specimens. Wood-based panel standards were used here for comparison of the TS and the WA values since there was no established minimum property for the WPC panel. 24-h TS values of all WPC panel types met maximum requirements of EN 312 [21] and EN 622-5 [22] with respect to particleboard Type 7 (9%) and MDF Type HLS (10%). The TS and WA values of the WPC specimens were also much less than those of commercial particleboard, oriented strand board, and MDF panels [23,24]. This was mainly attributed to the hydrophobic character of the PP because of its being devoid of functional polar groups such as hydroxyls in the molecular and thus chemically inactive.
Water resistance of the WPC panels containing thermally treated wood fibres were higher than in previous studies [25e28]. For example, Ayrilmis and Jarusombuti [28] found that average TS and WA values of 10 mm thick (0.80 g/cm3) WPC panels made from rubberwood fibres and PP powder (50:50 by wt) were 5.03% and 7.14%, respectively. Higher water resistance of the WPC panels containing thermally treated wood fibres was mainly attributed to hemicelluloses were hydrolyzed during the thermal-treatment. Due to their amount of hydrophilic groups they have excellent swelling properties. The water absorption of composites was due to the hydrogen bonding of the water molecules to the free hydroxyl groups present in the cellulosic cell wall materials and the diffusion of water molecules into the filler-matrix interface [29]. Hemicelluloses are hydrolyzed during thermal-treatment, and this decreases the hygroscopity of the lignocellulosic materials. Del Menezzi and Tomaselli [8] and Winandy and Krzysik [30] each reported that the reduction in moisture absorption could happen because of the hemicelluloses, one of the more hygroscopic polymers within the cell wall and also generally the most heat sensitive polymers of the wood components. Exposure time and temperature are two important factors affecting hemicelluloses degradation. Compared to other tree species, eucalyptus wood possesses the higher content of extractives, which have greater effect on wood properties. Under the action of heat and moisture, the phenolic compounds in wood may flow and spread out between the microcapillary systems, such as pit membrane, microfibril gap etc., which would occupy the positions in which should be the combination of water with the main components in wood, such as hemicellulose, cellulose, and lignin [31]. The phenolic compounds in the eucalyptus wood could decrease the TS and WA of the WPC panels with the increase of the treatment temperature. The TS of the WPC panels made from treated wood fibres could also be suppressed due to increased cellulose crystallinity, degradation of the amorphous regions in the cellulose microfibrils or because fewer hydroxyl groups exist after the hydrothermaltreatment. Cross linking between the cell wall polymers, especially lignin, esterification between the cellulose microfibrils, and the formation of ether linkage by the splitting of two adjacent hydroxyl groups are other viable reasons for the swelling loss [9]. 3.2. Mechanical properties The thermal-treatment of eucalyptus wood fibres slightly decreased the screw withdrawal resistance of the WPC panels as compared to the reference panels while the flexural properties and IB strength were more seriously affected by the treatment (Table 2). The thermal-treatment of the wood fibres led to reductions of 5e19% and 7e22% in MOR and MOE, respectively, depending on the
Table 1 Physical properties of the WPC panels as a function of thermal-treatment of the wood fibres (the ratio of wood fibre to polypropylene: 50:50 by wt). Thermal-treatment level
Untreated reference 120 Ce20 min 120 Ce40 min 150 Ce20 min 150 Ce40 min 180 Ce20 min 180 Ce40 min
Physical properties Density (g/cm3)
Thickness swelling (%)
0.82 0.80 0.82 0.81 0.82 0.83 0.81
3.98 Aa 2.94 B 2.64 BC 2.28 C 2.05 CD 1.84 D 1.72 D
(0.03) (0.02) (0.04) (0.02) (0.03) (0.04) (0.03)
1-day
Water absorption (%)
7-days (0.28) (0.20) (0.18) (0.25) (0.22) (0.19) (0.15)
5.06 A 3.82 B 3.48 BC 3.15 CD 3.05 D 2.28 E 2.05 E
(0.39) (0.42) (0.41) (0.38) (0.37) (0.34) (0.30)
14-days
28-days
1-day
7.82 A 4.97 B 4.59 B 4.27 B 3.81 C 3.17 D 3.06 D
8.15 A 5.20 B 4.84 B 4.51 C 4.02 C 3.39 D 3.25 D
6.45 A 4.82 B 4.39 B 3.85 C 3.47 CD 3.15 D 2.91 E
(0.54) (0.48) (0.51) (0.44) (0.48) (0.40) (0.35)
(0.47) (0.52) (0.44) (0.49) (0.51) (0.46) (0.39)
7-days (0.52) (0.49) (0.43) (0.53) (0.44) (0.37) (0.42)
16.53 A 14.72 B 14.09 B 13.37 BC 12.96 C 10.88 D 9.79 E
14-days (1.54) (1.43) (1.35) (1.47) (1.24) (1.32) (0.97)
24.12 A 21.59 B 20.96 B 19.77 C 19.25 C 17.92 D 16.95 E
28-days (1.78) (1.56) (1.67) (1.58) (1.48) (1.44) (1.51)
25.94 A 23.18 B 21.46 B 21.12 C 20.58 D 18.64 E 17.84 E
Values in parentheses are standard deviations. a Groups with same letters in column indicate that there is no statistical difference (p < 0.01) between the specimens according Duncan’s multiply range test.
(1.87) (1.72) (1.83) (1.50) (1.58) (1.45) (1.33)
N. Ayrilmis et al. / Polymer Degradation and Stability 96 (2011) 818e822 Table 2 Mechanical properties of the WPC panels as a function of thermal-treatment of the wood fibres (the ratio of wood fibre to polypropylene: 50:50 by wt). Thermal-treatment Mechanical properties level Modulus of Modulus of elasticity rupture (N/mm2) (N/mm2) Untreated reference 120 Ce20 min 120 Ce40 min 150 Ce20 min 150 Ce40 min 180 Ce20 min 180 Ce40 min
25.9 Aa (1.3) 2682.4 A (135) 24.7 B 24.2 BC 23.7 CD 23.0 D 21.7 F 21.0 F
(1.2) (1.1) (1.2) (1.2) (0.9) (1.0)
2483.8 B 2434.5 B 2358.3 C 2264.2 D 2225.8 D 2102.5 E
Internal bond Surface screw strength withdrawal (N/mm2) resistance (N) 0.95 A (0.14) 1010 A (58)
(113) 0.86 B (0.10) 982 AB (128) 0.83 BC (0.12) 976 AB (108) 0.77 CD (0.09) 951 BC (97) 0.75 D (0.08) 924 CD (111) 0.71 DE (0.09) 915 CD (89) 0.68 E (0.07) 894 D
(51) (55) (48) (42) (50) (38)
a Groups with same letters in column indicate that there is no statistical difference (p < 0.01) between the specimens according Duncan’s multiply range test. Values in parentheses are standard deviations.
temperature and time conditions. The treatment groups showing significant differences (p < 0.01) were determined according to the Duncan’s multiply range test (Table 2). The reference panels had the highest MOR and MOE values while the lowest values were found for the panels made from the fibres treated at 180 C and 40 min (Table 2). Although the thermal-treatment decreased the MOR and MOE of all panel types, they met the minimum requirements (15 and 2050 N/mm2) of EN 312 [21] Type P III particleboards for nonstructural applications in humid conditions. The loss of the mechanical properties of the WPC panels containing thermally treated wood fibres could be related to formation of soluble acidic chemicals; such as formic acid and acetic acid, from the hemicellulose degradation [32]. These acids accelerate depolymerization of the cellulose by breaking down the long chain cellulose (crystalline structure) to shorter chains. In addition, CeC and CeO linkages cleavage at the intra-polymer level with increasing temperature and time of the treatment. This leads to the separation of the lignin-hemicellulose copolymer system and depolymerization of hemicelluloses and amorphous cellulose [33]. Depolymerization and shortening of the cellulose polymer could affect MOE and MOR of the wood [34]. Loss in MOR and MOE of wood [35,36] and wood-based panels [12,37] treated at high temperatures was reported by different authors. Similar results were also found the WPC panels in the present study. The IB values of the WPC panels were significantly decreased by the thermal-treatment of the wood fibres. The thermal-treatment of the wood fibres reduced the IB strength of the WPC panels from 9% to 28% as compared to the reference panels, depending on the temperature and time conditions. The IB results indicated that the mechanical interlocking between the wood fibres and polymer matrix was negatively affected by increasing severity of the thermaltreatment. The WPC panel containing the fibres treated at 180 C for 40 min had the lowest IB value with 0.68 N/mm2 while the highest IB value with 0.95 N/mm2 was found for the reference panel (Table 2). Low IB strength of the WPC panels containing thermally treated wood fibres was mainly attributed surface roughness of the wood fibres. Mechanical bonding can occur due to the physical interlocking of two surfaces [38]. The surface roughness positively affects mechanical interlocking of two surfaces because it increases the total interfacial area between wood and polymer matrix. It could also provide mechanical interlocking effect that could trap the polymer in the cavities and act like an anchor to each other. Previous studies reported that surface roughness of wood was improved by increasing thermal-treatment temperature and time [39e42]. For example, Unsal and Ayrilmis [42] reported that average surface roughness (Ra) of E. camaldulensis wood samples treated at 180 C for 10 h had was 7.2 mm while it was found as
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10.0 mm for reference wood samples. This could be one of the mechanisms responsible for poor mechanical interlocking between thermally treated eucalyptus wood fibre and the polymer matrix. Wettability of the wood fibres also affects mechanical interlocking between the wood fibres and polymer matrix. Wetting is of great importance in achieving good adhesive interaction between two phases [38]. Wettability is defined as a condition of a surface that determines that how fast a liquid will wet and spread on the surface or whether it will be repelled and not spread on the surface [43]. In WPCs it is characterised by the degree of direct interfacial contact between the wood and polymer surface. Adhesion is sticking together two surfaces so that stress can be transmitted between them and can be quantified by the amount of work is required to pull the two surface apart. Previous studies reported that wettability of wood was decreased by increasing temperature and time [44e46]. Heat-induced chemical modification results in surface inactivation of fibres in layers near the surface. The surface of wood exposed to high temperatures is less polar, resulting in a lower wettability than in the case of untreated wood [47]. Sernek et al. [43] found in their work about the inactivation of wood surfaces in consequence of exposure to heat up to 180 C. The IB results can be rationalised in terms of the decreased wettability of the fibre surfaces, which reduces mechanical interlocking between the wood fibre and polymer matrix. The SWR of the WPC panels was slightly decreased by the thermal-treatment of the wood fibres (Table 2). The thermaltreatment of the wood fibres reduced the SWR from 3% to 11% as compared to the reference panels, depending on the temperature and time conditions. These results were in agreement with previous studies in wood-based panels [12,48]. For example, Ayrilmis et al. [12] reported that thermal-treatment of wood fibres had a negative effect on SWR of MDF panels. They found that the average SWR value of the MDF panels made from thermally treated rubberwood fibres at 180 C for 30 min decreased from 1750 N to 1450 N compared to the reference panels. Although the SWR of the wood-based panels was seriously affected by the thermal-treatment of the wood fibres, this situation was not observed for the WPC panels in the present study. Low SWR of the WPC panels containing thermally treated wood fibres was mainly attributed to depolymerization of the cellulose by breaking down the long chain cellulose (crystalline structure) to shorter chains. It was concluded that the depolymerization of the cellulose decreased the ability of the wood fibres to conform around the thread of the screw, allowing continuous load transfer along the thread. All WPC panel types met minimum surface SWR (875 N) requirement for grade 120 MDF (panel thickness 21 mm) specified in ANSI A208.2 [49]. In addition, the SWR values of the WPC panels containing thermally treated wood fibres were slightly lower than in a previous study, which was 1199 N for 10 mm thick (0.80 g/cm3) WPC panels made from untreated rubberwood fibres and PP powder (50:50 by wt) [28]. 4. Conclusions The water resistance of the WPC panels significantly improved with increasing treatment temperature and time of eucalyptus wood fibres. The thermal-treatment of the wood fibres led to reductions of 60% and 31% in the TS and WA after 28-days of water soaking, respectively, depending on the temperature and time conditions. This was mainly attributed to the fact that hemicelluloses were hydrolyzed during the thermal-treatment. The thermal-treatment of the wood fibres slightly decreased the SWR of the WPC panels while the flexural properties were more seriously affected by the treatment. The thermal-treatment of the wood fibres led to reductions of 5e19%, 7e22%, and 9e28% in
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MOR, MOE, and IB strength, respectively, while the SWR decreased from 3% to 11%, depending on the temperature and time conditions. Despite the fact that fast growing wood species like eucalyptus has many advantages, they have some disadvantages such as high swelling and low dimensional stability. These disadvantages can also cause a loss of flat-pressed WPC’s dimensional stability. It appears that the thermal-treatment of the fast growing wood fibres, above 150 C for 40 min, in the manufacture of the flatpressed WPC can be a practical choice for applications requiring a high degree of water resistance. Acknowledgements The authors gratefully acknowledge Department of Forest Products, Forestry Faculty, Kasetsart University, and Royal Forest Department, Thailand, for laboratory equipment used in the manufacture of the WPC panels. Further acknowledgement goes to Research Fund of Istanbul University for the financial support for this study. The authors would also like to thank Chaiyun Pangwong Pitak Hangam, and Somwung Chaichoom, Forest Products Laboratory Technicians, Royal Forest Department, Thailand, for their valuable assistance with WPC panel manufacture. References [1] Taylor A, Yadama V, Englund KR, Harper D. Wood plastic composites e a primer. UT Exension PB 1779. The University of Tennessee, Institute of Agriculture; 2011. p. 1. [2] Anonymous. Eucalyptus is part of the U.S. energy security equation. Eucalyptus facts. Summerville, SC, USA: Arborgen Inc., http://www.eucalyptusfacts.org/; 2010. [3] Nacar M, Hiziroglu S, Kalaycioglu H. Some of the properties of particleboard panels made from eucalyptus. Am J Appl Sci; 2005:5e8 [special issue]. [4] Unsal O, Korkut S, Atik C. The effect of heat treatment on some properties and colour in eucalyptus (Eucalyptus camaldulensis Dehn.) wood. Maderas Cienc Tecnol 2003;5:145e52. [5] Cetin NS, Gultekin G, Ozmen N. Dimensional stabilisation of Eucalyptus grandis w. Hill ex maiden sapwood with succinic anhyride. In: Proceedings of 3rd National Blacksea Forestry Congress National Conference, Artvin, Turkey; 20e22 May, 2010. p. 1649e1657. [6] Anonymous. Thermowood handbook. Helsinki: Finnish ThermoWood Association; 2003. p. 7. [7] Kamdem DP, Pizzi A, Jermannaud A. Durability of heat treated wood. Holz Roh Werkst 2002;60:1e6. [8] Del Menezzi CHS, Tomaselli I. Contact thermal post-treatment of oriented strandboard to improve dimensional stability: a preliminary study, vol. 64; 2006. p. 212e217. [9] Tjeerdsma BF, Militz H. Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood. Holz Roh Werkst 2005;63:102e11. [10] Garcia RA, Cloutier A, Riedl B. Dimensional stability of MDF panels produced from heat-treated fibres. Holzforschung 2006;60:278e84. [11] Mohebby B, Ilbeighi F, Najafi SK. Influence of hydrothermal modification of fibres on some physical and mechanical properties of medium density fibreboard (MDF). Holz Roh Werkst 2008;66:213e8. [12] Ayrilmis N, Jarusombuti S, Fuengvivat V, Bauchongkol P. Effect of thermal treatment of rubber wood fibres on physical and mechanical properties of medium density fibreboard. J Trop Forest Sci 2011;23:10e6. [13] Paul W, Ohlmeyer M, Leithoff H, Boonstra MJ, Pizzi A. Optimising the properties of OSB by a one-step heat pre-treatment process. Holz Roh Werkst 2006;64:227e34. [14] Tomek A. p. 157e160. Die heißvergu “tung von holzspa” nen, ein neues verfahren zum hydrophobieren von spanplatten Holztechnologie, vol. 7; 1966. [15] Nasir GM. Fibre morphology in relation to suitability for pulp and paper. Research Report. Peshawar, Pakistan: Forest Products Research Pakistan Forest Institute; 2009. p. 2. [16] European Norm (EN) 317. Particleboards and fiberboards-determination of swelling in thickness after immersion in water. Brussel, Belgium: European Committee for Standardization; 1993. [17] EN 323. Determination of density; 1993. [18] EN 310. Wood based panels: determination of bending strength and modulus of elasticity; 1993.
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