EFFECT OF HEAT TREATMENT ON THE MECHANICAL PROPERTIES, AND DIMENSIONAL STABILITY OF FIR WOOD

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1 EFFECT OF HEAT TREATMENT ON THE MECHANICAL PROPERTIES, AND DIMENSIONAL STABILITY OF FIR WOOD Hamiyet ŞAHİN KOL 1, Yusuf SEFİL 2, Sema AYSAL KESKIN 1 1 Karabuk University Faculty of Forestry Forest Industrial Engineering Department, Karabuk, Turkey 2 İzzet Baysal Vocational and Technical High School, Bolu, Turkey Key words Heat treatment, Thermowood, Physical properties, Mechanical Properties, Fir Wood Abstract Heat treatment alters chemical, physical, and mechanical properties of wood. In this study, some mechanical and physical properties of heat treated fir (Abies nordmanniana subsp.) wood at temperatures 17, 18, 19, and 212 o C for 2 h with ThermoWood method were determined. The results were compared with kiln-dried reference samples. As a result, due to the increasing of heat treatment temperature, as the bending strength (MOR) was decreasing, the compression strength parallel to the grain and modulus of elasticity increased. Also when the treatment temperature increased, equilibrium moisture content decreased. It was seen that a significant increase of dimensional stability. Corresponding author:semaaysal@karabuk.edu.tr, S. Aysal, Karabuk University Faculty of Forestry Forest Industrial Engineering Department, Karabük/TURKEY. 1. INTRODUCTION Wood as a raw material has been used for centuries for indoor and outdoor applications. Recently, with an increasing environmental awareness, use of heat treated wood material, that produced in order to increase the lifetime of the scarce resource of wood products with an environmentally friendly method, has been increased. Heat treatment of wood is an effective method to improve dimensional stability and durability of wood without any toxic chemicals. There are four different heat treatment methods of wood that are common: The Finnish process (ThermoWood) uses steam, German method (OHT-Oil Heat Treatment) heated oil, the Dutch method (Plato Wood) uses a combination of steam and warm air and the French processes (Rectification and Bois Perdure) an inert gas (Esteves et al., 28; Rapp, 21). The thermowood process is known as the most successful method in Europe (Boonstra, 28). Heat treated wood has several application fields such as exterior cladding, window and door joinery, paneling, garden furniture, sauna furniture, flooring and decking etc. (Yıldız et al., 26; Özçifçi et al., 29; Viitaniemi, 2). The mechanical properties of heat treated wood are essential for the performance of wood products in these kind of applications. The mechanical properties of wood such as modulus of rupture (MOR) and modulus of elasticity (MOE), compression resistance parallel to the grain are important where the load-bearing capacity is needed. Besides all these in the humid conditions like saunas, bathrooms and garden furniture dimensional stability of wood is significant. Therefore these properties of wood have been mostly studied by the researchers. Kol et al. (215). Effect of heat treatment on the mechanical properties, and dimensional stability of fir wood

2 Several studies showed that heat treatment of wood improves physical properties of wood by reducing hygroscopicity and improving dimensional stability (Yıldız et al., 26; Viitaniemi, 1997; Santos, 2). But in addition to these desired properties, heat treatment also causes negative effects like reduced mechanical properties (Cao et al., 212; Yıldız et al., 26; Esteves et al., 27a,b; Shi et al., 27). The amount of the change in the mechanical properties of wood during heat treatment depends on some factors such as process type, the maximum temperature reached in the process, and the holding time at that temperature (Shi et al., 27). In addition to, in all cases of thermal treatment, the changes on the mechanical properties of wood depends on the chemical composition of the material used (Windeisen et al., 27). Page 27 Fir (Abies bornmülleriana) is the main wood species naturally grown and commonly used in the forest products industry in Turkey. Therefore it is the potential wood species for industrial-scale heat treatment. The main objective of this study is to provide some information about the mechanical and physical properties of heat-treated Abies bornmülleriana using the ThermoWood process in various temperatures. 2. MATERIAL AND METHOD Wood species The data used in this study have been collected within a larger project to assess the physical and mechanical properties of heat-treated Fir (Abies bornmulleriana Mattf.). The sample trees used for the present study were obtained from the Bolu Forestry Departments. With the aim of avoiding from errors during sampling, extreme cases were taken into account such as excessively knotty trees and containing reaction wood or slope grain. Sections with 2 m length were cut between 1.3 and 3.3 m height of trees to obtain samples for tests. The planks chosen for experiments were cut from the sapwood region of the sections with 2 m length and planed on four sides to form a cross section of mm2. Prior to heat treatment process, the material was dried using a conventional warm air kiln drying approximately at a temperature of 7 C to a moisture content of 11 15%. The second selection of the raw material was performed at this stage. The density and moisture content of the planks, conditioned at room temperature and 65% relative humidity (RH), were measured and 2 planks with a small variation in density from each species were selected for further experiments. Then these 2 m long planks were split from the middle and cut into five 4 cm long pieces and the other halves of these 2 test planks were left as a reference material (later also called untreated control which was dried at conventional warm air kiln drying temperature of 7 C) and the other halves were heat-treated under steam at five different temperatures according to Figure 1. Thus, the kiln-dried materials were divided into six subgroups, five of which were to be heat-treated under steam at five different temperatures (later also called treated samples) and one of which was left untreated (later also called untreated control).

3 Page 271 Figure 1. Descriptions of the wood materials used in tests. Heat treatment Kiln-dried planks (2,5 cm thickness, 7 cm width, and 4 cm long) were subject to heat treatment using various schedules. Heat treatment was carried out under accurate conditions under steam with a laboratory kiln from Nova ThermoWood in Gerede, Turkey. Steam is used during the drying and heat treatment as a protective vapor. Protective gas prevents the wood from burning and also affects the chemical changes taking place in wood. According to desired end-use of the material, the heating temperature can vary between 17 C and 215 C with treatment time 2-3 h. The heat treatment was applied according to the method described in the Finnish ThermoWood Handbook. At first, the temperature of the kiln was raised near to 1 C. When the temperature inside the wood had risen to near the same temperature, the raising of the kiln temperature was carefully continued to the actual treating temperature. The target temperatures were 17, 18, 19, 2, and 212 C. The time of thermal modification at the target temperature was 2 h in every test run. After the heat-treatment phase, the temperature was lowered to 8 to 9 C using water spray system. Conditioning was carried out to moisten the heat treated wood and bring its moisture content to 4 7%. After heat treatment, only the planks that were free of defects were selected for further testing. Determination of the degree of thermal modification The weight loss that occurred by heat treatment was determined. Firstly, the planks were conditioned to a constant weight then weighed before and after the heat treatment. Weight loss (%), WL was calculated according to equation below; WL = Wut Wt 1 Wut (1) where, Wu is the weight of the sample before the heat treatment (g); Wt is the dry weight of the sample after the heat treatment (g). Dry weight (g), Wdry, was calculated according to the equation below;

4 Wdry = 1 Wu u+1 (2) where, Wu is the weight of the sample at moisture content u (g); u is the moisture content of the sample (%). The tests were carried out according to Turkish Standards (TSE) and 2 replicates were used in each test for treated and untreated fir. The properties were determined based on the specimens with dimension of 2x2x3 cm for equilibrium moisture content (TS 2471), 3x3x1,5 cm for anti-shrink efficiency (ASE1) and anti-swelling efficiency (ASE2) (radial, tangential and volumetric according to TS 486), 2x2x3 cm for compression strength parallel to grain (TS 2595), 2x2x3 cm for bending strength (TS 2474), 2x2x3 cm for modulus of elasticity in bending (TS 2478). The differences in properties of heat treated fir wood was provided by calculating the property difference between heat treated and untreated wood samples of the same species as a percentage of the untreated wood property according to the equation below: Page 272 D(%) = Mut Mt Mut 1 (3) Where, D (%) is difference of property (MOE, MOR, CS, Anti-Swelling and Anti-Shrinkage); Mut is the mean value of untreated samples; and Mt is the mean value of the heat treated samples. 3. RESULTS AND DISCUSSIONS Generally, heat treatment improved some physical properties such as dimensional stability, equilibrium moisture content of wood (shown at Table 1) but reduces some mechanical properties such as bending strength (shown at Table 2). Table 1. Changes on the some physical properties of fir wood with increasing temperature Heat Treatment Temperatu re ( C) Weight Loss after heattreatment (%) Equilibrium moisture content (%) Volumetric swelling (%) Volumetric shrinkage (%) Tangential Swelling (%) Radial (%) Swelling Tangential Shrinkage (%) Radial Shrinkage (%) Cont 11,18 ±,4 14,3 ±,72 12,85 ±,47 9,95 ±,98 4,34 ±,62 9,4 ±,54 3,8 ±, ,7 ±,2 9,3 ±,41 13,13 ±,69 12,33 ±,6 9,13 ±,6 4, ±,36 8,86 ±,54 3,47 ±,6 18 1,8 ±,2 8,2 ±,65 11,26 ±,6 12,7 ±,38 7,98 ±,5 3,27 ±,23 8,83 ±,32 3,24 ±, ,5 ±,19 7,2 ± 66 1,57 ±,42 11,16 ±,48 7,48 ±,33 3,9 ±,25 8,16 ±,37 3, ±,48 2 2,8 ±,2 6,5 ±,49 9,87 ±,65 9,78 ±,29 6,94 ±,46 2,93 ±,31 6,96 ±,28 2,82 ±, ,4 ±,19 6 ±,55 9,56 ±,57 8,9 ±,26 6,73 ±,37 2,82 ±,3 6,21 ±,26 2,69 ±,26 Weight loss The weight losses and some physical properties of the samples used in the tests are presented in Table I. The results showed that after the heat treatment the weight losses on the wood materials increased and the higher heat treatment temperature caused greater weight losses in the samples. It is clear that when the treatment temperature increased, the weight losses also increased correspondingly. It is stated that the minimum weight losses (1,7%) in Fir wood were in treated samples at 17 o C, maximum weight losses (5,4%) were in treated samples at 212 o C treatment temperature (Table I).

5 ASE 1 (%) Difference of property (%) Proceedings of the 27 th International Conference Similar results have been indicated by several authors (Seborg et al., 1953; Stamm, 1956; Rusche, 1973; Fung et al., 1974). Feist and Sell (1987) indicated that weight loss of beech wood was between 1-15% at temperature 18-2 C. Then again in another study, spruce wood has,8% weight loss at 17 C and at temperature 2 C this rate became 15,5% (Fengel, 1966). Özçifçi et al. (29) determined that heat treated yellow pine wood at various temperatures (15, 17, 19 o C) for 4 hours has a weight loss of about 1.22%, 2,14%, 3,43%. In the same study they found that the weight loss is increased with increased treatment time. Page 273 Equilibrium Moisture Content (EMC) In the treated samples equilibrium moisture content decreased comparing with the control samples. With increasing temperature, the equilibrium moisture content was reduced. When the minimum decrease of the equilibrium moisture content was 19,5 % at 17 C, the maximum decrease of the equilibrium moisture content was 49,2% at 212 C (Table I). The reasons of the decreasing equilibrium moisture content could be degradation of hemicelluloses and amorphous zone of cellulose (Bhuiyan and Hirai, 25; Tjeerdsma et al., 1998; Tjeerdsma and Militz, 25; Esteves et al., 27). Anti-swelling efficiency (ASE 1 ) ASE 1 of treated samples were increased compared with the control samples and with the increasing temperature, the ASE 1 of wood was also increased. When the minimum increase of 4 33,1 3,9 the ASE rate was 8,1% at 17 C, the maximum 21,2 increase of the ASE 1 was 33,1% at 212 C 2 (Figure 2). Anti-shrink efficiency (ASE 2 ) ASE 2 of treated samples were increased compared with the control samples and with the increasing temperature, the ASE 2 of wood also increased. The minimum increase of ASE 2 was 4,1 % at 17 C, while the maximum increase of the ASE 2 was 3,7% at 212 C ,5 8,1 31, ,9 Treatment Temperature ( C) 49, Treatment Temperature (ºC) Figure 2. Increase rate of EMC of heat treated fir wood. Figure 3. Increase rate of ASE 1 of heat treated fir wood.

6 Difference of property (%) ASE 2 (%) Proceedings of the 27 th International Conference Many studies show that the dimensional stability generally develops with the increasing treatment temperature and the treatment time. In addition that the treatment technique also effects the dimensional stability (Yıldız, 22; Stamm et al., 1946; Kaygın et al., 29; Akyıldız et al., 29; Esteves et al., 27b). Usually, degradation of wood starts apparently at a temperature of about 165 C (Stamm and Hansen, 1937) and hemicelluloses are more sensitive than the other components against to high temperature (Fengel and Wegener, 1984; Figure 4. Increase rate of ASE 2 of heat treated fir wood. Stamm, 1964). When wood is heated, the hemicelluloses begin to degrade, and this degradation process ends with the production of methanol, acetic acid and various volatile heterocyclic compounds, such as furans, γ -valerolactone, etc. (Hill, 26). Chang and Keith (1978) reported that there is a relationship between volumetric shrinkage and weight loss of wood. The presence of free hydroxyl groups of wood polysaccharides has a significant effect on the absorption and desorption (Boonstra and Tjeerdsma, 26). Heat treatment helps the reducing free hydroxyl groups (Pizzi et al., 1994). The reasons of the improvement of the dimensional stability may be carbohydrates and especially depolymerization of hemicelluloses that cause reducing of total amount of hydroxyl groups (Burmester, 1975; Kollman and Schneider, 1963). After heat treatment a large number of hydrophilic hydroxyl groups are reduced due to the replacement of hydrophilic oxygen-acetyl groups hence the dimensional stability is increased considerably (Cao et al., 212) ,1 6,1 13,1 23,9 3, Treatment Temperature ( C) Page 274 Compression strength parallel to grain (CS) The CS increased in heat treated Fir wood samples comparing to the control samples. When the minimum CS value of samples was 51,95 N/mm2 at 17 C, the maximum value was 56,46 N/mm2 at 212 C (Table 2). According to test results CS was enhanced 4,4% at 17 C and this increase continued to the temperature of 212 C. The similar test results are indicated in ThermoWood Handbook 23 and also Şahin Kol (21) indicated that CS increased by 4,2% for treated at 212 C pine wood and 17% for treated at 19 C fir wood. Boonstra et al. 27 exposed the Scots pine samples 2 stages (first step hydrolysis at 165 C for 3 min and second step curing at 18 C for 6 h) heat treatment. The CS was increased by 28% after treatment. The reasons of increase of the compressive strength in longitudinal direction might be: 16 12,6 13,4 14 1, ,6 8 4, Treatment Temperature ( C) Figure 5. Increase rate of CS of heat treated fir wood. 1. Decrease of amount of bound water in the heat treated wood - Reducing hydrogen bonding between organic polymers (cell wall constitutes) depends on the increased amount of bound water and it is concluded with decreasing of the strength properties of wood because the strength is not only connected with covalent bounds but also hydrogen intramolecular bounds (Fengel and Wegener, 1984).

7 Difference of property (%) Proceedings of the 27 th International Conference 2. Increase of the crystalline cellulose due to degradation and/or crystallization of amorphous cellulose (Boonstra, 27), 3. Increase of cross linking of the lignin polymer network - improves the strength of the middle lamella which effects the strength properties of the cell wall (Boonstra, 27), 4. Degradation of the hemicelluloses matrix (Boonstra, 27). There are also opposite results comparing to these results in the literature and according to the other studies the compression resistance reduced rate of 2%-32% (Schneider, 1973; Korkut, 22; Unsal and Ayrılmış, 25; Korkut et al., 28; Yıldız et al., 22). It might be also related to heat treatment method. Page 275 Table 2. Changes on the some mechanical properties of fir wood with increasing temperature Heat Treatment Temperature ( C) Compression resistance parallel to fibre (N/mm 2 ) Bending Strength (N/mm 2 ) Modulus of Elasticity (N/mm 2 ) Control 49,78 ± 4,75 88,55 ± 6, ,42 ± 144, ,95 ± 1,37 86,19 ± 8, ,7 ± 855, ,8 ± 1,4 84,66 ± 9, ,61 ± 1258, ,1 ± 1,31 82,81 ± 9, ,8 ± 947, ,4 ± 1,66 78,99 ± 9,5 9386,6 ± 987, ,46 ± 5,96 75,23 ± 16, ,19 ± 291,67 Bending strength (MOR) Comparing with control samples, bending strength of treated samples was reduced. When the treatment temperature increased, the bending 2 strength decreased, contrarily. When the minimum bending strength value of samples was 1,8 75,23 N/mm2 at 212 C, the maximum bending 1 6,5 4,4 strength of samples was 86,19 N/mm2 at 17 C 5 2,7 (Table 2). According to test results MOR was reduced 2,7% at 17 C and this decrease continued to the temperature of 212 C. The maximum reduction was 15% at temperature of 212 C (Figure 5). In many studies it is clear that the heat treatment Treatment Temperature ( C) Figure 6. Decrease rate of MOR of heat treated fir wood. reduces the bending strength with the rates of 1-72% (Yıldız, 22; Johansson and Moren, 26; Esteves et al., 27a; Esteves et al., 27b; Shi et al., 27; Korkut, 28; Korkut et al., 28). It is attributed that decrease in the bending strength is related to degradation of hemicelluloses. Because at lower treatment temperature, in cellulose and lignin neither depolymerization nor degradation are observed. Besides there is a positive relation between

8 Difference of Property (%) Proceedings of the 27 th International Conference hemicelluloses content and bending strength (Sweet and Winandy, 1999; Winandy and Lebow, 21; Winandy and Morell, 1993). Modulus of elasticity (MOE) Comparing with control samples, MOE of treated samples increased. When the treatment temperature increased, the MOE increased, simultaneously. The minimum MOE value of samples was 9294,19 at 212 C while the 8 maximum MOE of samples was 9964,7 at 17 6,2 C (Table 2). There was a significant increase 6 4,5 between control samples and the heat treated 3,6 4 samples. But this increase has continued to decrease from temperature 17 C (6,2%) to 1,4 2,4 212 C ( 4). It is also similar in literature. Shi et al. (27) found that there was an increase 17% in fir wood. Bekhta and Niemz (23) indicated that the change of modulus of elasticity value was insignificant. According to Hillis and Rozsa (1978), the effects of higher temperature and Treatment Temperature ( C) Figure 7. Increase rates of the MOE compared with the control samples longer treatment time on the MOE were explained and they indicated that because of wood consists of partially crystal micro fibrils and on a large scale hemicelluloses and lignin, when it is modified above a significant temperature, most of the amorphous polymeric components may convert their glassy structures to elastic. At the conversion temperature from glassy structure to elastic, particular polymers have enough energy that reduces the mutual gravitational forces. Thus the wood polymers can be converted to elastic or mostly plastic construction. Recognizable increase in the modulus of elasticity might be based on increasing of the relative cellulose content after heat treatment although the hemicelluloses degraded. Lower moisture content of treated wood than control also effects the modulus of elasticity (Boonstra, 27). Page CONCLUSIONS The results of this study indicated that the compression resistance values of fir wood were increased with increasing temperatures which is the similar with literature. Bending strength values were reduced after heat treatment comparing with control. By this thermal modification, some mechanical properties of wood are reduced, but the most important property of heat treated wood compared to untreated wood is reducing the equilibrium moisture content of the treated wood and as a consequence of this shrinkage and swelling of the wood is also reduced without using any water repellents. References Akyildiz, M. H., Ates, S., & Özdemir, H. (29): Technological and chemical properties of heattreated Anatolian black pine wood. African Journal of Biotechnology, 8(11). Bekhta, P., & Niemz, P. (23): Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood.holzforschung, 57(5),

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11 Corresponding author: S. Aysal Karabuk University Faculty of Forestry Forest Industrial Engineering Department, Karabük/TURKEY, Page 279 Author(s) 215. This article is published under Creative Commons Attribution (CC BY) license.