The Effect of Heat Modification and Densification on Physical Properties of Poplar Wood

In this study, density, volumetric swelling, mass loss, volume weight, fi ber saturation point and water absorption of the poplar wood (Populus usbekistanica) were investigated with the effect of heat modifi cation (HM) and heat-modifi ed densifi cation (HMD). Poplar samples were modifi ed with steam at 120 °C, 160 °C and 200 °C for 1 and 3 h. After heat modifi cation, the samples were compressed in hot press at a temperature of 120 °C, press pressure of 5 MPa and press time of 30 minutes for densifi cation. Physical properties of the samples were determined according to Turkish standards. The results showed that heat modifi cation affected densifi cation and increased density. Densifi cation had a positive effect on oven dry density (D0), oven dry density after soaking (D0S), volume weight (R) and fi ber saturation point (FSP), except for volumetric swelling. Similarly, the densifi cation process had an increasing effect on the water absorption, excluding 360 hours.

The aim of this research is to determine the interactive effect of heat modifi cation and post-heat densification on the poplar wood.The specifi c objective of the study was to analyze volumetric swelling and other chosen properties of heat threated and densifi ed samples compared with samples densifi ed without heat modifi cation.These properties are: density, mass loss, volume weight, fi ber saturation point and water uptake.

MATERIJALI I METODE
Populus trees were obtained from Kahramanmaraş province in Turkey.Wood samples used in the study were prepared in dimensions 20×20×30 mm.For each group of modifi cation, 27 samples were prepared from logs.Before testing, each sample was conditioned at 20 °C temperature and at 65 % relative humidity until reaching a moisture content of 12 %.
The samples were fi rst heat modifi ed by using laboratory drying oven at a temperature of 120 °C, 160 °C and 200 °C and modifi cation time of either 60 or 180 min (Table 1).During heat modifi cation, 100 ml water vapor was heated up to 100 °C degrees in the oven, and then the vapor was removed and kept still until the desired temperatures reached 120 °C, 160 °C and 200 °C.After the heat modifi cation, samples were hot pressed by using a laboratory hot press at 120 °C temperature and 5 MPa press pressure for 30 minutes (Table 1).
The obtained results were statistically analyzed by using one-way ANOVA and Duncan's mean separa-

INTRODUCTION 1. UVOD
There have been many studies on heat modifi cation recently, due to its many advantages such as improving the dimensional stability and durability of wood, without using chemicals (Tjeerdsma et al.;1998;Weiland et al.;2003;Garcia et al., 2012).This technology was applied to wood at a temperature of about 200 º C for several hours at low oxygen level, a non-infl ammable gas like oil or nitrogen.The main aim of this technique is to convert the chemical composition of wood using heat, which results in a change of physical properties.The process of thermal modifi cation is based on high temperatures, which cause the decomposition of basic wood component like hemicelluloses and celluloses (Tjeerdsma and Militz, 2005;Rowell et al., 2009).Schneider (1973) noted that treatment, at temperatures above 200 ºC and of longer duration, decreased swelling and shrinkage up to 50 %.Besides, he stated that sorption and desorption characteristics also changed in heat modifi ed wood.
In 1980s, densifi ed wood products were produced from low density wood species, especially for utilization of some fast growing trees (Wang et al., 2000).The utilization of hardwood species for fl ooring and furniture reduced their supply all over the world.Various efforts have been made to modify the surface of low-quality softwood and planted fast-growing species.The surface densifi cation technology has been developed and several researches have studied this technology (Rautkari et al., 2008;Gong and Lamason, 2007;Diouf et al., 2011).The thermal compression process might affect the drying duration, dimensional The effect of heat modifi cation on density is given in Table 2.According to this Table, it can be seen that oven dry density of densifi ed samples differs from that of undensifi ed samples.Heat modifi cation has not shown signifi cant effect on undensifi ed poplar and has caused 1 % decrease in density.The highest decrease in density among undensifi ed poplar was observed at 200 ºC heat modifi cation (HT31 and HT33).Means of samples densifi ed with heat modification (897 kg/m 3 ) were compared to density of densifi ed control samples (668 kg/m 3 ), and the results clearly showed that heat modifi cation increased density by 34 %.The density of samples densifi ed with and without heat modifi cation was increased by 102 % and 174 % on average, respectively.Gong et al. (2010) studied the effect of heat treatment on density between undensifi ed and densifi ed poplar (Populus tremuloides) and concluded that wood density in densifi ed poplar decreased to 11 % during the heat treatment, while density decreased to 2 % in undensifi ed poplar wood.In another words, they concluded that the heat treatment of densifi ed poplar increased density.Wang and Cooper (2005) found that the density of densifi ed wood was affected by compress temperature, press duration time, press closing time and humidity of samples before compressing The average volumetric swelling is given in Figure 1 for undensifi ed and densifi ed poplar samples with and without heat modifi cation.The undensifi ed volumetric swelling values at 120 and 160 ºC are higher than those of control samples but at 200 ºC they are lower.These differences are insignifi cant (p<0.062).Besides, the duration of heat modifi cation did not indicate signifi cant differences in all parameters, except for control samples.On the other hand, in densifi ed samples, all temperatures and durations had a signifi cant effect (p<0.001) on respective values.Moreover, as the duration of heat modifi cation rose, volumetric swelling decreased.The mean volumetric swelling of undensifi ed and modifi ed samples (10.1 %) decreased by 3.7 % in comparison with the undensifi ed control group (10.49%).The value of modifi ed and densifi ed samples (180.35%) increased by 85.68 % compared to their control samples (97.13 %).tion test to populate homogeneity groups that showed signifi cant differences at the 95 % confi dence level.
Oven dry density (D 0 ): This term expresses the amount of substance in the full dry unit volume and it is calculated according to TS 2472.
Volume weight value (R): This value describes the amount of substance in the full wet unit volume and it is determined according to TS 2472.
Fiber saturation point (FSP) was calculated according to Turkish standards TS 2371 and Equation 1given below: (1) where V S is volumetric swelling (%).
Oven dry density after soaking (D 0S ): After soaking for 15 days in water, samples were dried to 0 % moisture content and oven dry densities were determined according to TS 2472.Mass loss (ML), volumetric swelling (%) and water absorption (W A ) were calculated by equations ( 2), ( 3) and ( 4), respectively, (2) ML -mass loss (%), M o -mass of oven-dry sample before heat modifi cation (gr), M -mass of the sample after heat modifi cation (gr).
V S -volumetric swelling (radial, tangential and longitudinal), L -wet dimension after immersion in water (mm), L o -oven-dry dimension (mm), W A -water absorption, W -weight after immersion in water (gr), W o -oven-dry weight before immersion in water (gr).

REZULTATI I RASPRAVA
Average oven density values of densifi ed and undensifi ed poplar wood samples with and without heat modifi cation are shown in Table 2. Comparable results were recorded in other studies.For instance, Gong et al. ( 2010) studied the effect of heat modifi cation on densifi cation and stated that heat modifi ed wood is more resistant to swelling than densifi ed wood.Also, Bal and Bektaş (2012) noted that heat treatment without steam decreased thickness swelling specially above 180 °C.In some studies, it was noted that the relationship between volumetric swelling and oven dried density was potently positive (Table 3) (Kord et al., 2010;Kurt, 2010).Candan et al. (2013) noted that all thermally compressed boards increased thickness swelling values.This result could be explained by springback behavior of wood due to densification (Abraham et al., 2010).
The results of physical properties of undensifi ed and densifi ed poplar samples without and with heat modifi cation are given in Table 3.
Densifi cation process with and without heat modifi cation increased density values according to Table 3.The highest density for undensifi ed poplar was observed for heat modifi cation temperature of 120 °C compared to control group.Table 3 shows that the densities measured at 200 °C in undensifi ed samples seem to be lower, but this decrease is not signifi cant.As seen from statistical results in Table 3, while heat temperature duration had no effect on density in undensifi ed samples, a signifi cant increase in density was observed when the duration of heat modifi cation decreased from  4 shows that the effect of heat modifi cation on water absorption (%) differed between undensifi ed and densifi ed samples.It can be seen from Table 4 that, in the densifi ed samples, the heat modifi cation time had no statistically signifi cant effect on water absorption at 120 ºC (HM11, HM13), whereas at 200 ºC (HM31, HM33) the respective effect was signifi cant (p<0.000).As shown in Table 4, the modifi cation temperature and duration have no statistically signifi cant effect in the fi rst 48 hours only in heat modifi ed (not densifi ed) wood.The water absorption values of heat modifi ed samples decreased after 48 hours compared to control group.However, in 72 h and 96 h there was no statistically sig-3 to 1h for densifi ed poplar wood.As for densifi ed samples, density decreased depending on the increase of the process duration (from 1 to 3h), except for the temperature of 200 °C.No signifi cant difference was observed.This is in accordance with literature data.For example, Metsä-Kortelainen and Viitanen (2011) determined that 120 °C heat modifi cation increased density compared to control samples, while 200 °C heat modifi cation decreased density due to degradation of hemicellulose and cellulose.In another study, Cao and Huang (2012) noted that steam-heat-modifi cation increased wood thermal conductivity and density.
Comparing the mean oven dry density after soaking (D 0S ) with oven dry density (D 0 ), the density decreased 1 % in undensifi ed samples and decreased 55 % in densifi ed samples as seen in Table 3.This reduction occurred due to the permanent springback effect of the compressed wood.Densifi ed and undensifi ed poplar oven-dry densities after soaking (D os ) decreased compared to D 0 .However, D 0S of densifi ed poplar wood was still higher after soaking compared to undensifi ed samples.
Values given in Table 3 demonstrate that densification process did not cause mass loss but heat modification increased mass loss with increased temperature.ML values of poplar at 200 °C were greater than at 120 and 160 °C.The average mass loss at 200, 160 and 120 °C was 6 %, 1.33 % and 0.45 %, respectively for all samples.A notably increased dimensional stability following thermal modifi cation above 180 °C was noted in many studies (Welzbacher et al., 2008).This could be explained by the decreased hygroscopicity of components on wood cell walls (Fang et al., 2012).Decomposition of hemicelluloses and lignin at elevated temperatures and stress relaxation in samples caused a reduction in the hygroscopicity of wood (Cai et al., 2013).The value of volume weight of densifi ed poplar

ZAKLJUČAK
The main outputs of this study are summarized below: 1. Density of undensifi ed poplar wood was minimally (1 %) affected by heat modifi cation.However, density was infl uenced by heat modifi cation before densifi cation and it increased density by 34 %. 2. Volumetric swelling of undensifi ed poplar wood decreased signifi cantly only at 200 °C heat modifi cation.Moreover, the densifi cation increased the volumetric swelling due to the springback effect.3.In comparison to oven dry density (D 0 ), the oven dry density after soaking (D 0S ) decreased in undensifi ed and densifi ed samples at the rate of 1 % and 55 %, respectively.In addition, the mass loss increased with the increase of heat modifi cation temperature in all samples, notwithstanding the densifi cation process.Densifi cation process increased the volume weight and fi ber saturation point compared to heat modifi cation.4. No signifi cant correlation was found between water absorption time and heat modifi cation process for undensifi ed samples.On the other hand, in densifi ed samples, water absorption retention improved as the heat modifi cation temperature and duration increased.Besides, the effect of the application time increased depending on temperature increase.5. Poplar trees are fast growing and have low durability.However, poplar wood has become a material with high density and lower water absorption by heat modifi cation and densifi cation process.Thus, species with low durability can be converted to wood material of high density and extended service life by heat modification and densifi cation process.
6.One of the remarkable characteristics of this study is that no chemicals have been used during the process and there have been no side effects that could adversely affect human and environmental health during and after the process.7.In order to make the above physical properties meaningful and useful at the desired level, mechanical properties should also be researched in future as a part of this study.

o , g/cm 3 D os , g/cm 3 ML, % R, g/cm 3 FSP, %
R) increased because density of densifi ed poplar was higher than that of undensifi ed poplar.The volume weight of densifi ed wood increased by 9.31 %.The value of volume weight of undensifi ed poplar (R) decreased at 200 °C with heat modifi cation compared to other temperatures.Furthermore, FSP is directly related to volumetric swelling and oven dry density according to Equation (1) mentioned in the Method section.As is known, there is a linear relationship between VS and FSP.The FSP value of the control group (undensifi ed and control) was 31.74 %.After the samples were heat modifi ed at 200 °C (the mean of HT31 and HT33), the FSP values decreased to 23.75 % in poplar wood.Furthermore, the FSP values at 120 °C and 160 °C increased by 7.3 %.The FSP value of the control group (densifi ed and control) was 146.02 %.After the densifi cation, the FSP values increased to 40.47 %.Bal and Bektas (2012) noted that heat treatment at higher temperatures (180 °C) decreased values of FSP.Additionally, the volumetric swelling is very high due to the back-spring effect of wood.Abraham et al. (2010) also noted that the densifi ed wood exhibited this behavior.For this reason, the high springback effect has caused the FPS value to increase up to 223 %.Table D 0 -Oven dry density / gustoća u apsolutno suhom stanju; D OS -Oven dry density after soaking / gustoća u apsolutno suhom stanju nakon natapanja; ML -Mass loss / gubitak mase; R -Volume weight / volumna težina; FSP -Fiber saturation point / točka zasićenosti vlakanaca.*Means with the same small letter are not signifi cantly different in Duncan's mean separation test./Srednje vrijednosti označene istim malim slovom nisu signifi kantno različite prema Duncanovu testu......Düzkale Sözbir, Bektaş: The Effect of Heat Modifi cation and Densifi cation on Physical...(