The Effect of Artificial Weathering on Surface Properties of Thermally Modified Oriental Beech Wood

• The study aimed to determine some surface parameters such as surface roughness and color changes of thermally modi ﬁ ed Oriental beech (Fagus orientalis L.) wood samples after 750 hours of arti ﬁ cial weathering. The results of the study showed that arti ﬁ cial weathering led to an increase in surface roughness of Oriental beech wood. Thermal modi ﬁ cation interval of 210 °C - 230 °C gave a smoother surface than unmodi ﬁ ed samples after arti ﬁ cial weathering. Arti ﬁ cial weathering caused darker, reddish, and yellowish tone of unmodi-ﬁ ed and thermally modi ﬁ ed Oriental beech wood. According to the study ﬁ ndings, surface properties of thermally modi ﬁ ed Oriental beech wood were better than those of unmodi ﬁ ed Oriental beech.


UVOD
Wood has been preferred in the fi eld of construction sector and furniture manufacture since ancient times, due to its natural beauty, thermal insulation, high strength, ease of use and processing (Su, 1997). However, wood is readily decomposed by environmental factors, including water, solar rays, bacteria, fungi and insects, fi re, etc. (Kiguchi and Evans, 1998). This problem may be decreased by using chemical or thermal treatments. There has been a rise in the chemical treatment of wooden materials to enhance its mechanical, physical, and other properties for building proposes (Su, 1997;Yalinkilic et al., 1999;Brelid et al., 2000). However, the harms of chemicals used in treatment to increase the durability of wood are discussed. The demand for treating wood with environmentally friendly substances is increasing.
Methods called heat treatment or thermal modification are made to improve the properties of wood without using chemicals (Johansson, 2008). Heat treatment at high temperatures causes serious changes in the properties of wood. Therefore, it can be said that a new material has been formed as a result of heat treatment (Sundqvist, 2004). Thermal modifi cation can be made in order to ensure the dimensional stability of the wood by reducing the water absorption rate, as well as to protect against biological organisms (Akgul and Korkut, 2012). However, since thermal modifi cation negatively affects the mechanical and physical properties of wood, it has some commercial disadvantages (Gunduz and Aydemir, 2009).
It is common to use wood outdoors in architecture. However, wooden surfaces exposed to outdoor weather conditions can quickly deteriorate. Since the lignin in wood easily absorbs ultraviolet (UV) light, the fact that lignin absorbs UV light causes signifi cant depolymerization in the main structural components of wood (Evans et al., 2002). Some adverse conditions arising from weather conditions occur in the physical, mechanical, chemical and biological properties of wood and wood-based materials (Grelier et al., 2000;Zhang and Kamdem, 2000;Evans et al., 2005;Williams, 2005). It has been revealed as a result of scientifi c studies that the heat-treated wood material shows better resistance against weather conditions than the untreated wood (Temiz et al., 2006;Nuopponen et al., 2004;Ayadi et al., 2003). Due to the chromophoric lignin structure, the heat-treated wood material can interfere with the light absorption process and induce photostability (Srinivas and Pandey, 2012).
Turkoglu et al. (2015) exposed heat-treated wood to natural weathering for six months. After six months of natural weathering, they experimented with some surface properties of wood such as surface roughness, gloss, and color changes. After the natural weathering process, the experimental results showed that the surface properties of wood have improved after the heat treatment. They also concluded that, after heat treatment at high temperatures, natural weathering affects the surface properties of wood to a lesser extent. Kucuktuvek et al. (2017) investigated surface roughness characteristics of heat-treated Scots pine wood after weathering. They reported that heat-treated Scots pine wood had smooth surface after weathering. Toker et al. (2016) studied total color changes of heat-treated Oriental beech (Fagus orientalis L.) and Scots pine (Pinus sylvestris L.) woods after weathering. They found that total color changes of heat-treated Oriental beech (Fagus orientalis L.) and Scots pine (Pinus sylvestris L.) woods were lower than those of un-heated wood after weathering. Baysal et al. (2014), in an experimental study, investigated the gloss, surface roughness, and color change of the wooden test specimens after 500 hours of artifi cial weathering of heat-treated Scots pine (Pinus sylvestris). They stated that the surface properties of thermally modifi ed Scots pine showed better surface properties as a result of artifi cial weathering than unmodifi ed control samples. Garcia et al. (2014) reported that heat-treated teak sapwood (Tectona grandis) showed higher performance against color change after artifi cial weathering compared to control samples. Thus, wood material can become even more valuable as a result of heat treatment. The advantages of heat treatment can be listed as improved dimensional stability due to reduced hygroscopy, higher performance against degradation by microorganisms and insects, and dark color which is attractive to some users (Huang et al., 2012).
The number of scientifi c studies examining the change in the surface properties of wood materials treated above 200 °C after artifi cial weathering is limited. For this reason, in this study, Oriental beech wood (Fagus orientalis L.), which was heat-treated at temperatures above 200 °C, is aimed to investigate color and surface roughness after artifi cial weathering.

Sample preparation 2.1. Priprema uzoraka
Wood samples of 6 mm × 75 mm × 150 mm (radial by tangential by longitudinal) sized Oriental beech wood (Fagus orientalis L.) were prepared from air-dried wood. Wood samples were kept at 20 °C and 65 % relative humidity for 2 weeks before the tests started.

Thermal modifi cation 2.2. Toplinska modifi kacija
The thermal modifi cation was done in a laboratory oven with temperature control. Three different temperatures (210 °C, 220 °C and 230 °C) were applied to Oriental beech samples under atmospheric pressure and heat treatment was performed in three different times (0.5, 1, and 1.5 hours). Five replications were made for each treatment group.  lamps. After the wooden specimens were exposed to 8-hour UV-ray irradiation cycles, 4 hours of condensation was applied on the QUV device for a total of 750 hours. The average radiation is at 340 nm wavelength (λ max = 340 nm) and its maximum intensity is 0.89 W/ m 2 . The temperature during the light irradiation period and the condensation period was around 60 °C and 50 °C, respectively.

Surface roughness 2.4. Hrapavost površine
According to DIN 4768 (1990) standard, roughness measurements were made with Mitutoyo Surftest SJ-301 device. In this study, the surface roughness parameter, such as average peak-valley height (Rz), was calculated. The surface roughness profi le was scrutinized using the stylus with a diamond tip of 5 μm radius and 90° conical angle. The feed speed of the stylus was 0.5 mm/s1 along 8 mm sampling length (Zhong et al., 2013). Five replications were made for each treatment group. Surface roughness measurements were made parallel to the fi bers.

Color test 2.5. Određivanje boje
The three-dimensional CIELAB color space was used to quantify the color ( Figure 1). Here, while the L* axis represents lightness, a*, and b* denote color coordinates. The parameters +a* and -a* represent red and green, respectively, while the parameter +b* represents yellow and -b* represents blue. The L* parameter can range from 100 (white) to zero (black) (Zhang 2003). The colors of the specimens were measured with measurement geometry of d/8° and a D65 illuminant by a colorimeter (X-Rite SP Series Spectrophotometer, X-ride Pantone, MI, USA). The color coordinates were measured on the surface of samples at the same fi ve points before and after artifi cial weathering. The measuring spot was adjusted to be equal or not more than one-third of the distance from the center of this area to the receptor fi eld stops. Five replications were made for each treatment group. Color measurements were made parallel to the fi bers.

Statistička analiza ispitnih rezultata
Experimental results were examined statistically. This examination was made with variance analysis and Duncan test. Duncan test with a 95 % confi dence level of surface roughness and total color change test results were evaluated with a computerized statistical program. Homogeneity groups (HG) revealed whether the results were statistically signifi cant. The exponential letters (x a,b,c,d ) are given as homogeneity groups in Table 1 and 2.

Hrapavost površine
The Rz values used for surface roughness parameter before and after artifi cial weathering are presented in Table 1.
Before artifi cial weathering, the average Rz value of the unmodifi ed control sample was 13.6. The test results indicated that, except for 0.5 h at 220 °C heat treatment, surface roughness values of all thermally modifi ed Oriental beech wood samples were increased before weathering. Thermal modifi cation gave a smoother surface than unmodifi ed Oriental beech after artifi cial weathering. The increase in smoothness or reduction in roughness can be crucial for the use of solid wood. Wooden materials with rough surfaces require much more sanding than those with a smooth surface, which leads to a decrease in thickness of the material and, therefore, increases the losses due to the sanding process (Dundar et al., 2008). Besides, it ensures that losses in the planer are reduced, and high-quality surfaces are obtained (Sevim Korkut et al., 2008).
Surface roughness is considered to be a significant parameter to determine the wood surface quality, and many parameters affect the wood surface quality (Yildiz et al., 2013). The weathering exposure increased the roughness of the un-heated and thermal treated Oriental beech. After artifi cial weathering, the increasing range of Rz was from 26.03 % to 75.26 % for thermal modifi ed Oriental beech wood samples, while the increase of Rz by 106.63 % was observed for unmodifi ed Oriental beech. This is because artifi cial  Table 2 shows the color changes of thermally modifi ed Oriental beech before and after artifi cial weathering.

Color changes 3.2. Promjena boje
As could be expected, L* values of heat-treated Oriental beech wood were lower than those of un-heated (control) wood. This is because thermal modifi cation caused the decrease of L* values of Oriental beech wood. Therefore, Oriental beech wood becomes darker after thermal modifi cation. This tonality is generally developed with increasing temperature and durations. These fi ndings are generally compatible with previous   (Aksoy et al., 2011;Baysal et al., 2014;Militz, 2002;Yildiz et al., 2011;Toker et al., 2016). The decreases of L* values of thermally modifi ed wood can be attributed to changes in lignin structure and noncellulosic polysaccharides (Grelier et al., 2000;Petric et al., 2004;Hon and Chang, 1985). While the (+) values of a* show that Oriental beech turned to red, (+) values of b* show that Oriental beech turned to yellow. According to our results, the thermally modifi ed wood surface turned to red and yellow after thermal modification. Baysal et al. (2014) found that, while a* values decreased with the increase in temperature and time, the b* values increased initially and then decreased at temperatures used. Akgul and Korkut (2012) investigated the color characteristics of thermally treated Uludag fi r wood. According to their fi ndings, with higher treatment temperature and durations, yellow tone initially increased and then decreased. Gunduz (Grelier et al., 2000). Moreover, Δb* of unmodifi ed wood was higher compared to thermally modifi ed wood. ΔE* values of thermally modifi ed Oriental beech were lower compared to unmodifi ed wood. For example, ΔE* of unmodifi ed wood was 17.42, while it varied between 1.49 and 8.11 in thermally modifi ed wood. There was a statistically signifi cant difference between ΔE * values of the unmodifi ed (control) and thermally modifi ed Oriental beech wood (p≤0.05). In general, total color changes in wood decreased with temperature and duration after artifi cial weathering.

ZAKLJUČAK
Thermal modifi cation reduced the surface roughness and color changes of Oriental beech after artifi cial weathering compared to untreated beech wood. The surface of heated and un-heated wood was darkened due to artifi cial weathering exposure. However, thermally modifi ed wood gave a lighter surface than unmodifi ed wood after artifi cial weathering. Oriental beech wood tended to be reddish and yellowish after artifi cial weathering. Total color changes of heat treated Oriental beech wood were lower compared to those of un-heated wood. Generally, higher treatment temperature and treatment time resulted in lower surface roughness and total color changes of Oriental beech wood after artifi cial weathering.