EFFECT OF HEAT TREATMENT ON SOME PHYSICAL AND MECHANICAL PROPERTIES OF LAMINATED WOOD

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1 EFFECT OF HEAT TREATMENT ON SOME PHYSICAL AND MECHANICAL PROPERTIES OF LAMINATED WOOD Osman Percin 1, Sait Dündar Sofuoglu 1, Mustafa Altunok 2 1 Dumlupinar University, Faculty of Simav Technology, Department of Wood Works Industrial Engineering, Kutahya, Turkey 2 Gazi University, Faculty of Technology, Department of Wood Works Industrial Engineering, Ankara, Turkey Key words Heat treatment, Lamination, Adhesives, Mechanical properties, Physical properties Abstract The objective of this study was to investigate the effects of heat treatment on volumetric swelling, bending strength, and compression strength parallel to grain of laminated veneer lumbers (LVLs) with four-layered produced from Anatolian black pine (Pinus nigra J.F. Arnold subsp. nigra var. caramenica (Loudon) Rehder) wood. For this aim, veneers were heat treated at 140, 160, 180, and 200 C for 150 min and bonded with melamine formaldehyde () and D- VTKA polyurethane-based adhesives. The volumetric swelling, bending strength, and compression strength parallel to grain were determined. The results indicated that physical and mechanical properties decreased with increasing heat treatment temperature and these properties of the laminated wood are better than that of the solid wood. Also, adhesive improved the overall properties of the laminated specimens better than. Corresponding author: osman.percin@dpu.edu.tr, O.Percin, Dumlupinar University, Simav Technology Faculty, Wood Works Industrial Engineering, 43500, Simav,Kütahya/TURKEY,. 1. INTRODUCTION Wood and wood based materials have long been commonly used for many applications because of its many excellent features (such as esthetic appearance, reasonable cost, ease of use, low density, high mechanical strength, etc). However, wood also suffers a number of disadvantages because wood is a hygroscopic material. Many of studies have been done in order to improve the disadvantageous properties of wood. Heat treatment of wood has an effect on wood s chemical composition and through that on the properties of wood. For several decades, different thermal treatment methods have been used to significantly improve and enhance some the properties of wood without the use of chemical additives. Heat treatment is an effective method to improve the dimensional stability of wood and resistance against biodegradation. However, there is a noticeable reduction of mechanical properties after heat treatment, mainly due to the high temperatures involved (Esteves and Pereira (2009); Bekhta and Niemz (2003); Korkut (2008). Heat treatment of woods over 150 C alters the physical and chemical properties significantly Mitchell (1988). It was speculated that the higher the treatment temperature, the better the wood s biologicaldurability. However, the strength properties begin to deteriorate at temperatures over 150 C. The wood becomes more brittle, and bending and tension strength decrease by 10 30%. Therefore, the use of heat-treated wood in load-bearing constructions is restricted Rapp (2001). Unsal and Ayrilmis (2005) found that the maximum compression

2 strength parallel to grain decrease in Turkish river red gum (Eucalyptus camaldulensis Dehn.) wood samples was 19.0% at 180 C for 10 h treatment. In a study on the effect of heat treatment, with pine sapwood heated at 110, 130, 150 and 180 C it was found that compression strength decreased by 5% Schnerder (1973). The decrease of equilibrium moisture of wood due to heat treatments leads to an improvement of wood dimensional stability Esteves and Pereira (2009). Tjeerdsma et al. (1998a) reported that the heat treatment allowed the decrease of swelling from 7.3% to 5.7 for Fagus sylvatica wood and from 4.7% to 2.8 for Pinus sylvestris wood corresponding to efficiencies of 22% and 40%, respectively. Sahin Kol (2010) reported that volumetric swelling and shrinkage of heat treated pine were decreased by 63.9 and 51.8% with the heat treatment. For heat treated fir, the reductions were determined as 40.6% in volumetric swelling and 34.6% in volumetric shrinkage. Page 150 Laminated veneer lumber (LVL) is more economic and aesthetic than solid woods. It can be used for production of furniture, cupboard, desk, chair, table, etc. building materials, column and beams Eckelman (1993). The bending strength of laminated beech and poplar combination was found N mm 2 Keskin and Togay (2003). It can be used for structural and non-structural applications Keskin and Atar (2005). This study was done to determine the impact of heat treatment on the volumetric swelling, bending strength (MOR), compression strength (CS) of the laminated veneer lumber (LVL) prepared from heat treated Anatolian black pine veneers at 140, 160, 180, and 200 C for 150 min laminated with and adhesives. 2. MATERIALS AND METHODS Wood material Anatolian black pine (Pinus nigra J.F. Arnold subsp. nigra var. caramenica (Loudon) Rehder) wood was selected as a test material because of usage in industry. Adhesives Melamine formaldehyde () and Desmodur-VTKA ( ) (Desmodur-Vinyl Trie Ketonol Acetate) adhesives were used for the bonding of the laminations. These adhesives are usually preferable for the assembly or furniture production in the woodworking industry. is generally preferred as melamine-urea or melamine-phenol formaldehyde mixtures in hardwood plywood: end-jointing and edge-gluing of lumber, and scarf joining softwood plywood. Before the bonding process, the veneers were stored in a climatization chamber until they reached a moisture content of about 12 %. The and adhesives were applied on a single bonding surface of veneers at approximately 200 g/m 2 by using a roller coater. After the gluing process, veneers were pressed under 0.8 N/mm 2 at 110 C for 4 min. Ozcifci et al. (2007); Ors et al. (2004). Some physical properties of adhesives used in this study are given in Table 1.

3 Table 1. Some physical properties of adhesives Adhesive Density Viscosity Time to solidify Gel time Spread amount (g/cm 3 ph ) (MPa s) at C at 20 C (g/m 2 ) D-VTKA ± min 5-10 min ± min min 200 Preparation of test samples Anatolian black pine (Pinus nigra J.F. Arnold subsp. nigra var. caramenica (Loudon) Rehder) wood was cut in parallel to grain directions and sawn into specimens measuring 50 x 50 x 560 (tangential, radial, longitudinal) mm long. Wood materials were conditioned at 20 ± 2 C and at 65 ± 5% relative humidity in the conditioning cabinet until their weights became stable. Page 151 Then specimens were cut parallel to grain direction and cut into specimens measuring 5 x 50 x 560 (tangential, radial, longitudinal) mm long. The samples were heat-treated at 140, and 200 C for 150 min. Heat treatment was carried out under atmospheric pressure, with water vapour as a shielding gas. After the heat treatment, specimens dimensions were remeasured and weights recorded. After the heat treatment, the test samples were conditioned at 20 ± 2 C and at 65 ± 5% relative humidity in the conditioning cabinet until their weights became stable. Afterwards the samples were bonded with and adhesives. Volumetric swelling (VS) test samples were prepared from the laminated veneer lumbers and solid wood according to TS 4086 (1983). Ten samples were prepared for the LVL tests, and ten samples were prepared for solid wood tests, for each test group. The all samples were dried in an oven at 103 ± 2 C until their weights were stable. The dimensions and weights of the samples were measured to calculate volumetric swelling. To calculate volumetric swelling, the samples were immersed in distilled water for three weeks. At the end of the immersion period, the dimensions and weights of the samples were measured again. Bending strength (MOR) of test samples, with a dimension of 20 x 20 x 400 mm, perpendicular to the fiber and glue line were cut from the drafts having an average humidity of 12% according to TS EN 408 (1997), and compression strength (CS) of test samples with a dimension 20 x 20 x 30 mm, parallel to grain according to TS 2595 (1976). The air-dry densities (δ 12 ) of laminated wood samples were determined according to TS 2472 (1976). Data analyses The impacts of heat treatment on physical and mechanical properties of the laminated Anatolian black pine (Pinus nigra J.F. Arnold subsp. nigra var. caramenica (Loudon) Rehder) wood material bonded and was analysed by ANOVA (Analysis of Variance). When the differences between groups were found to be significant, Duncan s Multiple Range Test was used to determine the differences between means at prescribed level of a = 0.05.

4 3. RESULTS AND DISCUSSION Table 2 shows the values of the air-dry density, volumetric swelling, bending strength, and compression strength of LVL manufactured from veneers treated under different heat treatment temperatures. According to the averages, all the parameters decreased with increasing heat treatment temperature. It is evident from the test results that these values were all lower in heat treated samples than in control samples. Page 152 Table 2. The effect of heat treatment on physical and mechanical properties of the LVL Heat treatment Unheated 140 C 160 C 180 C 200 C Adhesives Air-dry Volumetric Bending Compression Statistical density swelling strength strength Values (g/cm 3 )* a (%)** a (N/mm 2 )*** a (N/mm 2 )**** a Avg D A C BCD ± sd 0, Avg A B A A ± sd 0, Avg B AB AB B ± sd 0, Avg F C CD CDE ± sd 0, Avg C C A A ± sd 0, Avg E C BC BC ± sd 0, Avg J D E F ± sd 0, Avg G DE C BCD ± sd 0, Avg H D DE EF ± sd 0, Avg L 9.86 E F G ± sd 0, Avg I 8.97 F DE DEF ± sd 0, Avg K 9.57 EF F G ± sd 0, Avg O 8.11 G H I ± sd 0, Avg M 7.26 H F F ± sd 0, Avg N 7.71 GH G H ± sd 0, Avg. = average; ± sd = standard deviation, a: Homogenous groups: letters in each column indicate groups that are Statistically different according to Duncan s multiple range test at P < *LSD: , **LSD: , ***LSD: 2.768, **** LSD: Heat treatment results indeed in a decrease of density. The highest density was obtained in unheated and laminated with adhesive (0,572 g/cm 3 ) and the lowest in heat-treated solid wood samples at 200 C (0,499 g/cm 3 ). Boonstra et al. (2007) reported that the decrease of the

5 density is caused by; degradation of wood components into volatile products which evaporate during heat treatment; evaporation of extractives; and a lower equilibrium moisture content. Density of the laminated wood materials have more superior values than the solid wood materials which were representing their kinds. The increase is assumed to be due to glue usage and layered structure. Similar results have been reported by Keskin (2003). Page 153 Volumetric swelling was decreased for Anatolian black pine after heat treatment. The result of ANOVA proved that the effect of temperature on volumetric swelling of LVL samples was significant with 0.05 probabilities. The highest volumetric swelling was obtained in unheated solid wood (13.72 %) and the lowest in heat-treated at 200 C and laminated with adhesive samples (7.26 %). Volumetric swellings of solid wood and heat treated and laminated with and adhesives Anatolian black pine were decreased by 40.88, 43.81, and % with the heat treatment at 200 C, respectively. Theoretically, the available OH groups in hemicellulose have the most significant effect on the physical properties of wood. Heat treatment slows water uptake and wood cell wall absorbs less water because of the decrease of the amount of wood s hydroxyl groups. As a consequence of the reduced number of hydroxyl groups the swelling and shrinking are lower (Gunduz et al. 2008; Inoue et al. 1993). The improvement in dimensional stability of heat treated wood is because of the reduction in relative proportion of hemicellulose when the wood material is exposed to high heat treatment temperature Tjeerdsma et al. 1998b. The increase in dimensional stability for heat treated wood is mainly because of the decrease of wood hygroscopicity in view of the chemical changes at high temperatures (Esteves and Pereira 2009). According to the test results obtained in this study, all the MOR values were decreased with increasing heat treatment temperature. Heat treatment caused a decrease in MOR for solid wood and laminated with and adhesives by 18.97, 14.78, and % with the heat treatment at 200 C, respectively. ANOVA and Duncan s Multiple Range Tests show that all the differences were significant. The greatest changes were realized at 200 C. This level of treatments was also give the lowest bending strength value of 78.36, 87.56, and N/mm 2 for solid wood and laminated samples with and adhesive, respectively. The decrease on the bending strength (MOR) is mainly due to the degradation of hemicelluloses (Esteves et al. 2008). Laminated samples with gave better results than solid wood laminated with samples adhesive. This may be due to the resistive property of against bending strength. Inoue et al. (1993) reported that decrease on bending strength (MOR) for Cryptomeria japonica of 80, 45, and 20 % with heat treatment during 8 hours at 220, 200, and 180 C, respectively. Poncsak et al. (2006) reported heat treated birch and showed a reduction of bending strength (MOR) with increasing heat treatment temperature, especially above 200 C. The largest decrease in compression strength occurred at treatment conditions of 200 ºC where the compression strength was measured about 60.21, 69.82, and N/mm 2 for solid wood and laminated samples with and adhesive, respectively. The compression strength losses for 200 ºC was 17.70, 9.80, and %. Gunduz et al. (2008) reported that the maximum decreases for all parameters were recorded heat treated at 180 ºC and 10 h. The lowest compression strength value obtained was N/mm 2, total loss of compression strength 27.2 % compared to the control. Korkut et al. (2008) determined that, for the heat treatment of Scots pine (Pinus sylvestris L.) at 120, 150, and 180 C for 2, 6, and 10 h, the

6 maximum Compression strength (CS) losses were observed to be 25.4 % at 180 C and 10 h of heat treatment. Yildiz et al (2006) suggested that the decrease in compression strength properties can be reduced by using a closed system with an inert gas like nitrogen or water vapor as the shielding gas instead of air. 4. CONCLUSIONS Page 154 At the end of the tests, the results showed that the density, volumetric swelling, bending strength and compression strength of the LVL manufactured from heat treated Anatolian black pine (Pinus nigra J.F. Arnold subsp. nigra var. caramenica (Loudon) Rehder) wood veneers decreased for all heat treatment temperature. The greatest losses were realized at 200 C. The volumetric swelling of heat-treated and laminated Anatolian black pine wood were improved from 40.88, to %. adhesive gave better results than adhesive. Also, properties of the laminated wood materials have more superior values than the solid wood materials which were representing their kinds. References Bekhta P., Niemz, P., (2003): Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood Holzforschung 57: Boonstra MJ, Acker J, Tjeerdsma BF, Kegel E. (2007): Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents. Annals of Forest Science 64: Eckelman CA. (1993): Potential Uses of Laminated Veneer Lumber in Furniture, Forest Products Journal 43(4): Esteves BM, Domingos IJ, Pereira HM, (2008): Pine wood modification by heat treatment in air. Biroresources, 3(1), Esteves BM, Pereira HM. (2009): Wood modification by heat treatment: A review, BioResources 4: Gunduz G, Korkut S, Korkut DS. (2008): The effects of heat treatment on physical and technological properties and surface roughness of camiyanı black pine (pinus nigra Arn.subsp. pallasiana var. pallasiana) wood. Bioresource Technol., 99: Inoue M, Norimoto M, Tanahashi M, Rowell RM. (1993): Steam or heat fixation of compressed wood. Wood and Fiber Science 25 (3), Keskin H, Atar M. (2005): Comparison of some technological properties of (5 layers) wood laminations centred with poplar. Gazi University, Journal of Science,18(1): Keskin H, Togay A. (2003): Physical and mechanical properties of laminated wood materials made up with the combination of Oriental beech wood and Lombardy poplar wood. SDU, Turkish Journal of forestry, A/2: Keskin H. (2003): Physical and mechanical properties of laminated oriental spruce wood materials, SDU, Turkish Journal of Forestry, 1(2), Korkut S, (2008): The effects of heat treatment on some technological properties in Uludag fir (Abies bornmuellerinana Mattf.) wood, Build Environ 43: Korkut S, Akgul M, Dundar T. (2008): The effects of heat treatment on some technological properties of Scots pine (Pinus sylvestris L.) wood. Bioresource Technology 99(6), Mitchell PH. (1988): Irreversible property changes of small loblolly pine specimens heated in air, nitrogen, or oxygen. Wood and Fiber Science 20(3),

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