Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood

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1 Modern Environmental Science and Engineering (ISSN ) October 2016, Volume 2, No. 10, pp Doi: /mese( )/ /007 Academic Star Publishing Company, Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood Jacob M. Owoyemi, Henry H. Adebayo, and John T. Aladejana Department of Forestry and Wood Technology, Federal University of Technology, Nigeria Abstract: The major problem of wood in service is dimensional instability caused by varying relative humidity of the surrounding environment. This study investigated the effect of thermal modification on the physical and mechanical properties of Gmelina arborea wood. Freshly felled Gmelina arborea trees were machined and trimmed to standard size of mm for the determination of physical properties (colour, volumetric shrinkage, swelling and water absorption) and mm for mechanical properties (Modulus of Rupture and Modulus of Elasticity) assessment. Thermal modifications were performed in several batches using 160, 180 and 200 o C at 1, 2 and 3 hours. Visual observation of treated samples showed that wood colour changed from light yellowish to very dark brown with increasing treatment temperature. Water absorption, shrinkage and swelling values decreased with increase in treatment duration and temperature. At a treatment time of 3hrs, compared with the control, mean values of the thermally treated samples for volumetric swelling ranged from 2.65% at 160 C to 1.94% at 200 C. Mean values of the thermally treated samples for volumetric shrinkage ranged from 6.58% at 160 o C to 3.65% at 200 C. Mean values of the thermally treated samples for water absorption ranged from 33.40% at 160 C to 26.8% at 200 C. The MOE of heat treated Gmelina arborea wood varied from N/mm 2 at 160 C to N/mm 2 at 180 C. The MOR varied from N/mm 2 at 200 C to N/mm 2 at 160 C. MOR was significantly reduced while there were no significant effects on the MOE as a result of heat treatment. The result showed reduction in the hygroscopic properties of Gmelina arborea wood making it suitable for use in high moisture prone areas in construction. Key words: dimensional stability, hygroscopicity, colour change, moisture content 1. Introduction Wood is a ligno-cellulosic material and natural resources. It is the most versatile raw material the world has ever known. Throughout history, man relied on wood for needs varying from farming tools to building materials. Wood remained virtually the most predominant material used for construction and energy generation until the last half of the 19th century [1]. Being a hygroscopic and biodegradable material, wood in service shrinks and swells due to varying humidity and its service life can be shortened due to decay fungi when exposed to humid or wet conditions, and insects such as powder post beetles and termites [2, 3]. An Corresponding author: Jacob M. Owoyemi, Ph.D., Lecturer I, research areas/interests: wood utilization and protection. jacobmayowa@yahoo.com. alternative way of protecting wood from decay and insects without the use of toxic preservatives is by improving its dimensional stability through thermal modification [4-6]. Thermal modification is the process of subjecting solid wood to temperatures close to or above 200 C for several hours in an atmosphere with low oxygen content [4]. This process results to the modification of cell wall components. Portions of hemicelluloses are hydrolyzed into their monosaccharide components such as glucose, galactose, mannose, arabinose, and xylose, while the amorphous regions of cellulose are hydrolyzed, breaking cellulose into shorter chains. The degradation of these two major components of the cell wall lead to a reduction of free hydroxyl groups in their chemical structures. In contrast, lignin s cross-linking is believed to increase [6-8].

2 692 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood These modifications in the cell wall result in several changes in wood properties. Regardless of the process used, common results are colour change from white or yellowish into brown or dark brown; decreased shrinking and swelling by 50-90% due to reduced equilibrium moisture content of the wood; improvement of biological durability; decreased heat conductivity by 10-30%; reduction in weight by 5-15%; and extractives migration onto the wood surface [4, 5, 9]. Physical properties are the quantitative characteristics of wood and its behaviour to external influences other than applied forces. Familiarity with physical properties is important because they can significantly influence the performance and strength of wood used in structural applications [10]. Mechanical properties are the characteristics of a material in response to externally applied forces. They include elastic properties, which characterize resistance to deformation and distortion, and strength properties which characterize resistance to applied loads. Mechanical property values are given in terms of stress and strain The mechanical property values of wood are obtained from laboratory tests of lumber of straight-grain clear wood samples (without natural defects that would reduce strength, such as knots, checks, splits, etc. [11]. There is need to investigate the effects of thermal modification on physical and mechanical properties of wood. 2. Study Area This research was conducted at the Central Research Laboratory, Federal University of Technology, Akure (FUTA), Nigeria. It is located on the longitude of 70291North and latitude of 50131East. The climate is the humid sub-tropical indicating that it is basically within the tropical rainforest zone which is dominated by broad leaved hardwood trees that form hard layered stands [12]. The mean annual temperature is about 260 C (min. 190 C and max C) and rainfall of 1500 mm with bimodal rainfall pattern [13]. 3. Materials Used Materials used for this study were freshly cut trees of Gmelina arborea at the plantation of The Federal University of Technology Akure, Ondo State, Nigeria. Two trees were felled and Samples were cut at the top, middle and base portions along the grain to specified dimensions. Equipment for this study were chain saw and circular sawing machine for felling and cutting of the samples, weighing balance, Vernier calliper, electric oven-dryer, water, plastic bowls desiccators with silica gel and the Universal Testing Machine. 4. Preparation of Wood Samples The Gmelina arborea logs were flitched into lumber and the outer and inner portions of the sampling height (top, middle and base portions) along the grain pattern were used in preparation of samples. The sawn planks were machined and trimmed to standard size of mm for determination of physical properties and mm for mechanical properties and the samples were replicated three times. 5. Thermal Modification Process A large-sized electric oven-dryer with temperature control at the Central Research Laboratory, FUTA was used as the treating chamber for thermal modification. The thermal modification process was performed in several batches using temperature and time interval of 160, 180 and 200 C at 1hr, 2 hrs and 3 hrs. The treatment time was recorded once the inside temperature of the oven reached target treatment temperature. 6. Wood Colour After thermal treatment, the treated samples were characterized and compared with untreated control specimens based on colour and texture. The presence of colour changes due to treatments was also noted. 7. Hygroscopicity and Dimensional Stability The hygroscopicity (measured by water absorption) and dimensional stability (measured by volumetric

3 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood 693 swelling and shrinkage) were determined following ASTM Vol. D.09 [11] with some modifications. Sample specimens measuring mm were used to carry out this experiment. Three replicates per treatment were prepared. Prior and after submersion in water, the dimensions and weight were measured using a digital weighing balance, the samples were submerged in water. After 24 hours, the samples were removed and air-dried to drain for 10 min then the excess surface water was wiped off with dry cloth. The amount of water absorbed and volumetric swelling were calculated by using the difference of the weight and dimensions before and after submersion expressed as percentage of the initial weight or dimensions. The samples were thereafter re-oven dried to a constant weight for determination of the volumetric shrinkage. The physical properties test was calculated using the following formulae: VS= Where: D D i D o o 100 [%] (1) VS = volumetric shrinkage Di = Initial (wet) volume of the wood Do = Final (oven dry) volume of the wood T2 T1 VS = 100[%] (2) T Where: VS = Volumetric Swelling T1 = Volume of Samples after soaking T2 = Volume of Samples before soaking Wwet W WA Wdry 1 dry 100 [%] (3) Where: WA = Water Absorption W wet = Weight of the samples after soaking in water (kg) W dry = Weight of the oven dried samples (kg) 8. Mechanical Properties Mechanical tests conducted on the thermally modified Gmelina arborea wood include: Modulus of Rupture and Elasticity (MOR and MOE) in accordance with ASTM Vol. D.09 [11]. Samples dimension of mm with a total of three replicates for each treatment combinations were tested using a Universal Testing Machine. The actual width and thickness of each sample were measured prior to testing. Center loading of the 230 mm span of the sample was used. Speed of test was 20 mm/min. At the point of failure the force exerted on the sample that caused the failure was recorded and the MOR and MOE were calculated using the formula: 3 PL MOE 3 4BD H [N/mm 2 ] (4) Where: MOE = Modulus of Elasticity L= Length of sample (mm) B= Width of the samples (mm) D= Thickness of the samples (mm) P= Ultimate failure load (N) H= Increase in deflection (mm) 3PL MOR 2 2BH [N/mm 2 ] (5) Where: MOR = Modulus of Rupture L = Length of samples (mm) B = Width of samples (mm) P = Ultimate failure load (N) H = Thickness of the samples (mm) 9. Statistical Analysis Data obtained were analysed using analysis of variance (ANOVA). Test of significance of the different treatment variables was estimated using factorial experiment in a Randomized Complete Block Design (RCBD). Treatment means were separated using Duncan Multiple Range Test.

4 694 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood 10. Results and Discussions 10.1 Physical Properties Colour The colour of thermally treated Gmelina arborea wood varied from pale white to pale yellowish at 160ºC for 1 hr and to very dark brown at 200 for 3 hrs (Figs. 1 and 2). It was evident that as the treatment temperature and duration was increased from 160 C for 1 hr to 200 C for 3hrs, the colour change intensified from pale yellowish to chocolate brown. This colour change in the wood can be attributed to some chemical reactions that took place during heat process. During the thermal modification of Gmelina arborea wood samples, aldehydes and phenols may have been formed from degraded carbohydrates, and this could be responsible for the formation of coloured compounds after chemical reactions as this observation is similar to the study reported by McDonald et al. [14]. For a given treatment time, the degree of colour change was greater for specimens treated at 3hrs than for specimens treated at 1 hr. Similarly, specimens treated at a given temperature of 200 C were darker than those treated at the same temperature for 160 C. However, the treatment temperature has a more profound influence than the treatment time according to Mitsui et al. [15]. Also, according to Sundqvist [16], the changes in colour of thermally modified wood are attributed to oxidative changes, which predominate over hydrolysis reactions. He concluded that both extractives and structural components (hemicelluloses and lignin) took part in colour change of heat-treated wood. Also, colour of the wood becomes darker due to the thermal degradation of lignin Hygroscopicity and Dimensional Stability of Treated and Untreated Gmelina Arborea Wood The mean values of volumetric swelling, shrinkage and water absorption are presented in Table 1. The result on Tables 2-4 showed the analysis of variance for thermally treated and untreated samples for Volumetric 160 C (1 hr) 180 C (1 hr) 200 C (1 hr) 160 C (2 hrs) 180 C (2 hrs) 200 C (2 hrs) 160 C (3 hrs) 180 C (3 hrs) 200 C (3 hrs) Fig. 1 Comparison of the colour of thermally modified Gmelina arborea wood at different temperature and duration. Fig. 2 Colour of the untreated Gmelina arborea wood. Swelling, Volumetric Swelling and Water Absorption coefficient for 24hrs. Duncan Multiple Range Test was used to separate the mean of Water Absorption due to significant in the levels of the treatment factors (Table 5). Thermally treated samples showed volumetric swelling values ranging from 1.94% (200 C for 3 hrs) to 3.62% (160 C for 1hr) as presented in Table 1 and this ranges were relative to untreated which swelled by 5.21%. It can be noted that volumetric swelling following thermal treatment decreased with increase in treatment time and temperature. Analysis of variance

5 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood 695 Table 1 Mean values of volumetric swelling, volumetric shrinkage and water absorption of thermally modified and untreated Gmelina arborea wood. ( C) (hrs.) Swelling (%) Shrinkage (%) Water Absorption (%) Untreated ± ± ± Mean ± standard deviation ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±10.60 Table 2 Analysis of variance for volumetric swelling coefficient of thermally modified Gmelina arborea wood. Source of Sum of Mean Df F-cal variation Squares Square ns ns * ns Error Total ns = no significant difference Table 3 Analysis of variance for volumetric shrinkage of thermally modified and untreated Gmelina arborea wood Sum of Mean Source Df F-cal Squares Square ns ns * ns Error Total ns = no significant difference showed that there was no significant difference in treatment time and temperature (Table 2). Thermally treated volumetric shrinkage samples had its values ranged from 3.65% (200 C for 3hrs) to 6.86% (160 C for 1 hr) (Table 1). The mean volumetric shrinkage of the untreated samples which had a value of 5.21% was greater. Analysis of variance showed that there was no significant difference in treatment time and temperature (Table 3). The thermally treated samples after undergoing water absorption had its values ranged from 26.78% (200 C for 3hr) to 38.01% (160 C for 1 hrs) as showed in Table 1. This observed range means that water absorption decreased with increase in treatment temperature and time and this was proved by higher value for untreated samples (39.86%). Analysis of variance result revealed significant difference in the levels of the treatment factors and also in the interaction between the factors. Duncan Multiple Range Test carried out on treatment time and temperature separated the untreated samples from the treated ones. It further revealed a difference in treatment temperature (160 C, 180 C and 200 C) and time (1 hr, 2 hrs and 3 hrs). In general, it can be said that the volumetric swelling of all treated samples was similar in trend or behaviour to that of volumetric shrinkage except that it was lower in percentage value due to its different cell arrangement [17]. Heat treatment was effective enough to decrease the volumetric swelling and shrinkage of Gmelina arborea at 160 C at 1hr until 200 C at 3hrs. The maximum decrease was noted at treatment combination of 200 C at 3hrs. In general, it showed that as the treatment temperature and duration was increased, the percentage swelling and shrinkage decreased. The decrease in volumetric swelling and shrinkage and increase in dimensional stability of heat treated wood is attributed to a decrease in moisture absorption. It is also a very interesting finding that treated specimens have a significant sharp reduction in moisture uptake when the heat treatment conditions were at 160 C (3 hrs), 180 C (3 hrs) and 200 C (3 hrs). According to the report of Yildiz et al. [18], the reason may be attributed to material losses in the cell lumen and hemicelluloses degradation with high applied temperature. It is known that the weight of wood material and its swelling decreased when heat treatment was applied. Heat treatment reduces water uptake and wood cell wall gain less water due to

6 696 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood decrease in the amount of hydroxyl groups in the wood. As a consequence of the reduced number of hydroxyl groups, the swelling and shrinking were reduced with the same proportion. The decrease in water absorption may be attributed to the reduction of available bonding sites of hydroxyl groups in the hemicelluloses and cellulose. Thermal degradation and the increased cross-linking of lignin could also limit sorption sites for water [6, 7, 19, 20]. Table 4 Analysis of variance for water absorption of thermally modified and untreated Gmelina arborea wood. Source Sum of Df Mean F-cal Squares Square * * * * Error Total The degrees of significance are represented by significant symbol (*) in superscript. Table 5 Duncan multiple range test result for the physical properties of thermally modified and untreated Gmelina arborea wood. Water Absorption Water ( C) (%) Absorption (%) Untreated c Untreated c b 1 (hr.) a b 2 (hrs.) a a 3 (hrs.) a Means in the same column having the same superscripts are not significantly different at 5% probability level Mechanical Properties Following Thermal Treatment of the Wood Samples The results of the three-point flexural test for Modulus of Elasticity (MOE) and Modulus of Rupture (MOR) of thermally modified Gmelina arborea wood are presented in Tables 6-8. The statistical analysis of variance (ANOVA) carried out in respect of MOE and MOR are presented in Tables 9 and 10 respectively while their Duncan Multiple Range Test results were included in Tables 11 and 12. Table 6 Mean values of MOE and MOR of top part of thermally modified and untreated Gmelina arborea wood. Portion Treatment ( C-HR) MOE (N/mm 2 ) MOR (N/mm 2 ) Outer Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±26.46 Inner Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±11.87 Mean ± standard deviation Table 7 Mean values of MOE and MOR of middle part of thermally modified and untreated Gmelina arborea wood. Portion Treatment ( C-HR) MOE (N/mm 2 ) MOR (N/mm 2 ) Outer Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±2.74 Inner Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±1.21 Mean ± standard deviation

7 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood 697 Table 8 Mean values of MOE and MOR of base part of thermally modified and untreated Gmelina arborea wood. Portion Treatment MOE (N/mm 2 ) MOR (N/mm 2 ) ( C-HR) Outer Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±13.33 Inner Control ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ± HR ± ±5.67 Mean ± standard deviation Table 9 Analysis of variance for MOE of thermally modified and untreated Gmelina arborea wood. Source of Variation Sum of Squares Df Mean Square F-cal Location * Portion ns * * Location * Portion * Location * Temp * Location * ns Portion * ns Portion * * * Location * Portion * Temp ns Location * Portion * * Location * Temp * * Portion * Temp. * ns Location * Portion ns * Temp. * Error Total Table 10 Analysis of variance for MOR thermally modified and untreated Gmelina arborea wood. Sum of Source Df Mean Square F-cal Square Location * Portion ns ns ns Location * ns Portion Location * * Location * ns Portion * ns Portion * * * * Location * ns Portion * Temp. Location * * Portion * Location * Temp * * Portion * Temp. * ns Location* Portion ns * Temp. * Error Total *= Significant; ns=no significant difference Table 11 Duncan multiple range test result for the MOE and MOR at different location settings. Location MOE (N/mm 2 ) MOR (N/mm 2 ) Top b b Middle a b Base c a Means in the same column having the same superscripts are not significantly different at 5% probability level. Table 12 Duncan multiple range test result for the MOE at different temperature and time settings. ( C) MOE (N/mm 2 ) MOE (N/mm 2 ) Control b Control b a 1HR ab a 2HR a b 3HR a Means in the same column having the same superscripts are not significantly different at 5% probability level.

8 698 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood Modulus of Elasticity Thermally treated samples for MOE for the top to bottom varied from to N/mm 2 respectively at 160 C to 200 C for 1 hr, 2 hrs and 3 hrs (Tables 6-8). The mean values for outer wood portion and inner portion ranged from to N/mm 2 and to N/mm 2 respectively at 160 C to 200 C for 1 hr, 2 hrs and 3 hrs. However, ANOVA showed the change in the MOE was not significantly different to the untreated. Also, the locations (top middle and base) were significantly different while the portions (outer and inner) are not significantly different. The temperature regimes (160 C, 180 C and 200 C) were not significantly different while time intervals (1 hr, 2 hrs and 3 hrs) were significantly different (Table 9). From the results in Table 9, ANOVA showed that the change in the MOE of the treated samples was not significantly different to the control as a result of temperature and time combinations. Similar result was found by Rapp and Sailer [21] with the oil heat treatment and dry-air heat treatment of pine (Pinus sylvestris L.) treated at 180, 200, and 220ºC. The heat-treated pine s MOE was not significantly different with the untreated although at 200ºC the highest MOE of 11,000 N/mm 2 was achieved. For the treated samples, it was deduced from the Analysis of Variance that temperature factor contributed to the variation in MOE. Among the three wood locations, the highest temperature setting of 200 C does not significantly vary with 160 and 180 C in Table 12. This is confirmed with the work done by Finnish ThermoWood Association [22]. Duncan Multiple Range Test carried out in separation of wood portion, treatment temperature and time showed that the three wood locations were significantly different in response to increased temperature and time which revealed reduction in MOE to the range of MOE obtained in the untreated wood samples (Tables 11 and 12) Modulus of Rupture The variation of the thermally treated samples for Modulus of Rupture (MOR) for the top to bottom varied from to N/mm 2 respectively at 160 C C to 200 C C for 1 hr, 2 hrs and 3 hrs (Tables 6-8). The mean values for outer and inner portion ranged from to N/mm 2 and to N/mm 2 respectively at 160 C C to 200 C C for 1 hr, 2 hrs and 3 hrs. However, ANOVA (Table 10) showed that the treatments used in the study had significant effects on the modulus of rupture (MOR) of the treated samples in comparison to the untreated. The locations (top middle and base) are significantly different while the portions (outer and inner) are not significantly different, the temperatures (160 C, 180 C and 200 C) were not significantly different and time (1 hr, 2 hrs and 3 hrs) were significantly different. The general trend showed that as the treatment temperature increases, the MOR of thermally treated samples decreases. From the results in Table 10, ANOVA showed that the treatments used in the study had significant effects on the Modulus of Rupture (MOR) of the treated samples in comparison to the untreated. In Plato-treatment, Beech wood s MOR was reduced by only 5 to 18% [23]. In Thermo Wood, MOR was reduced from 10 to 30% for the Finnish pine, spruce and birch. The different MOR values obtained can be attributed to various parameters such as atmosphere, temperature, duration, rate of heating, species, weight and dimensions of the pieces treated and original moisture content [24]. The Duncan Multiple Range Test carried out on wood locations showed that the basal portion of the wood was significantly different from the top and middle parts (Table 11). This may be as a result of fibre concentration at the lower part of the wood, Hence, the reason for the highest MOR at the basal portion of the wood. 11. Conclusion Thermal treatment of wood is the most commercially developed strategy to modify wood characteristics. The thermal modification of Gmelina arborea wood affected both its physical and

9 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood 699 mechanical properties. The degree of modification varied with temperature and duration of treatment. In general, physical and mechanical properties of Gmelina arborea wood showed significant modifications at 200 C. The colour of the wood changed from light yellowish to very dark brown with increasing treatment temperature and duration. The thermally modified wood had a smoky smell and no defects were observed. The hygroscopicity and dimensional stability was improved as measured by reduced volumetric swelling, shrinkage and water absorption. The major disadvantage of thermal modification according to this research is that there was significant reduction in the mechanical properties of the wood most especially the Modulus of Rupture under a long period of heat treatment at 200 C. From the study conducted, it can be suggested that desired wood properties can be defined from the various treatment combinations employed. For instance, if only improvement of dimensional stability and wood colour change are desired with no change in wood strength, the 180ºC-1hr treatment combination would suffice since it would result in 50 % improvement. However, if the desired properties are dimensional stability with acceptable reduction in wood strength, the optimum treatment combination of 200ºC-3 hrs is recommended. The type of use for the thermally modified Gmelina arborea wood can be based on the desired properties. Hence thermal modification of wood reduced the hygroscopic properties and improved the dimensional stability. References [1] W. M. Douglas, America s Forest: A History of resiliency and recovery, (10) (1995) [2] Joseph B. White, Self-driving cars are still in park, Wall Street Journal (Nov 20, 2013) 133. [3] W. T. Simpson, Drying and control of moisture content and dimensional changes, in: Wood Handbook Wood as an Engineering Material, Madison, Winconsin: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 1999, p [4] T. L. Highley, Bio-deterioration of wood, in: Wood Handbook Wood as an Engineering Material, Madison, Wisconsin: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 1999, p [5] A. Rapp, Review on heat treatments of wood, cost action E22-environmental optimization of wood protection, in: Proceedings of Special Seminar in Antibes, França, [6] W. J. Homan and A. J. M. Jorissen, Wood modification developments, Heron 49 (4) (2004) , available online at: [7] B. Sundqvist, Colour changes and acid formation in wood during heating, doctoral thesis, Lulea University of Technology, [8] M. Nuopponen, FT-IR and UV raman spectroscopic studies on thermal modification of scots pine wood and its extractible compounds, academic dissertation, Helsinki University of Technology, 2005, p. 40, available online at: [9] F. Sundholm, NMR studies of thermally modified wood, in: Reaction Mechanisms of Modified Wood, 2001, available online at: Old_Pdf/12_ww.pdf?from= [10] S. Jamsa and P. Viitaniemi, Heat treatment of wood Better durability without chemicals, in: Proceedings of Special Seminar Held in Antibes, France, [11] J. E. Wiandy, Effects of long-term elevated temperature on CCA-treated southern pine lumber, Forest Products Journal 44 (6) (1994) [12] American Society for Testing and Materials (ASTM), Annual Book of Standards, Vol. D.09 Wood, Philadelphia, PA, [13] V. A. J. Adekunle and A. O. Olagoke, Timber harvest in tropical ecosystem, Ondo State, Nigeria: Implication on carbon release, in: Onyekwelu, J. C., Adekunle, V. A. Oke, D. O. (Eds.), Climate Change and Forest Resources Management: The Way Forward: Proceedings of the Second National Conference of the Forests and Forest Products Society, Federal University of Technology, Akure, Nigeria, April 2010, pp [14] O. V. Oyerinde and O. V. Osanyande, Farmers adaptation strategies and perception to climate change: A case study of communities around Idanre forest reserve, Ondo State, Nigeria, in: Onyekwelu, J. C., Adekunle, V. A. J., Oke, D. O. (Eds.), Climate Change and Forest Resources Management: The Way Forward, Proceedings of the Second National Conference of the Forests and Forest Products Society, Federal University of Technology, Akure, Nigeria, April 2010, pp [15] A. G. Mcdonald, M. Fernandez, B. Kreber and F. Laytner, The chemical nature of kiln brown stain in radiata pine, Holzforschung 54 (1) (2000) [16] K. Mitsui, A. Murata and L. Tolvaj, Changes in the properties of light irradiated wood with heat treatment:

10 700 Physico-Mechanical Properties of Thermally Modified Gmelina arborea (Roxb.) Wood Part 3, Monitoring by DRIFT spectroscopy, HolzRoh-Werkst 62 (2004) [17] B. Sundqvist, Colour Response of Scots Pine (Pinus sylvestris), Norway spruce (Picea abies) and birch (Betula pubescens) subjected to heat treatment in capillary phase. HolzRoh-Werkst 60 (2) (2002) [18] M. M. Maruzzo, W. M. America and R. P. Escobin, Wood anatomical properties of big-leafed mahogany [Swieteniamacrophylla King) and malapapaya [Polysciasnodosa (Blume) Seem], FPRDI Journal 29 (2003) [19] S. Yildiz, E. D. Gezer and U. C. Yildiz, Mechanical and chemical behavior of spruce wood modified by heat, Build. Environ. 41 (2006) [20] J. P. Jimenez and R. A. Razal, Physical and chemical properties of thermally modified Yemane (Gmelina arborea R. Br.) wood, FPRDI Journal 30 (2004) [21] H. Wikberg, Advanced solid state NMR spectroscopic techniques in the study of thermally modified wood, academic dissertation, Finland: Laboratory of Polymer Chemistry, Dep t. of Chemistry, University of Helsinki, [22] Finnish Thermowood Association [FTA], ThermoWood Handbook, Helsinki, Finland, available online at: [23] H. Militz and B. Tjeerdsma, Heat treatment of wood by the plato-process, in: Rapp A. O. (Ed.), Review on Heat Treatments of Wood: Proceedings of Special Seminar of Cost Action E22, Antibes, France, February 9, 2001, pp Available online at: bitwon/retification/dgfh_hitze.eng.pdf. [24] M. Vernois, Heat treatment in France, in: Proceedings of Seminar Production and development of heat treated wood in Europe, Helsinki, Stockholm, Nov. 2000, soslo.13 (1) (2000)