Abstract
Heat treatment in wood materials and the use of water-based varnishes (WBV) in furniture made from heat-treated wood materials come to the fore in terms of supporting environmentally friendly production in the wood sector. In this study, the total color changes (ΔΕ*) and surface roughness (SR) were investigated after heat-treated tree species were exposed to natural and accelerated weathering with and without varnish. In this context, test samples were first divided into two groups. Surface treatment was applied to the samples in the first group by applying modified WBV (A1) and normal WBV (A2), and the second group was left unvarnished. Then, one group of the varnished samples was subjected to natural weathering (NAT) for 90 days and the other group was subjected to accelerated weathering (QUV) for 720 h with VAS samples, and then the ΔΕ* and SR values of the samples were calculated. The results revealed that ΔΕ* was the lowest in the QUV WBV group, and NAT was more effective than QUV in ΔΕ*. The highest SR values among all groups were measured in the VAS group, and among the varnished samples, there was no statistical difference between A1 and A2 in SR values, except for Rq.
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Weathering’s Effect on Color and Roughness in Some Heat-treated Wood Species with Modified Water-based Varnish
Ayhan Aytin,a,* and Nevzat Çakıcıer b
Heat treatment in wood materials and the use of water-based varnishes (WBV) in furniture made from heat-treated wood materials come to the fore in terms of supporting environmentally friendly production in the wood sector. In this study, the total color changes (ΔΕ*) and surface roughness (SR) were investigated after heat-treated tree species were exposed to natural and accelerated weathering with and without varnish. In this context, test samples were first divided into two groups. Surface treatment was applied to the samples in the first group by applying modified WBV (A1) and normal WBV (A2), and the second group was left unvarnished. Then, one group of the varnished samples was subjected to natural weathering (NAT) for 90 days and the other group was subjected to accelerated weathering (QUV) for 720 h with VAS samples, and then the ΔΕ* and SR values of the samples were calculated. The results revealed that ΔΕ* was the lowest in the QUV WBV group, and NAT was more effective than QUV in ΔΕ*. The highest SR values among all groups were measured in the VAS group, and among the varnished samples, there was no statistical difference between A1 and A2 in SR values, except for Rq.
DOI: 10.15376/biores.17.4.6358-6376
Keywords: Total color change; Heat treatment; Weathering; Modified water-based varnish; Surface roughness
Contact information: a: Interior Design, Department of Design, Duzce Vocational School, Duzce University, Duzce 81100 Turkey; b: Faculty of Forestry, Forest Industrial Engineering Department, Duzce University, Duzce 81100 Turkey; *Corresponding author: ayhanaytin@duzce.edu.tr
INTRODUCTION
Wood has many advantages, for example, it is natural; environmentally friendly; aesthetic; renewable; reusable; sustainable; and non-waste-forming engineering material. It is easily available as a raw material, has a high durability, and is light and easy to shape by processing (Bozkurt and Erdin 1997; Çolak and Değirmentepe 2020). However, due to the variability in dimensional stability and mostly due to a species with low natural durability, their areas of use can be limited (Ali et al. 2021). In contrast, color changes may occur within short periods of use. Today, basic processes, such as drying, impregnation, and surface treatments, are applied to minimize the problems that may arise from the structure of the wood material and that may occur at the place of use (Yalınkılıç et al. 2021). However, these processes may be insufficient in many areas where wood materials are used. For this reason, a number of new studies have been conducted to make more rational use of wood. These studies aimed to both eliminate the adverse attributes in the wood material and produce a more useful material. Wood modification is one of the important studies in this context (Tomak and Yıldız 2010). As a result of modification studies, significant improvements have been achieved in some wood-based materials, water exchange in the wood has been reduced with an effective modification process, and dimensional stability has been increased with resistance to rot fungi and external weather conditions. It is known that some successful and useful methods, such as acetylation (Rowell 2004), furfurylation, and dimethylol dihydroxyethylene urea specifically heat treatment (HT) of wood, have been transferred from laboratories to the industry and put on the market since the 1990s, mostly through the academic studies conducted within the scope of wood modification (Demirel and Temiz 2015).
Due to increasing environmental awareness, the demand for materials produced with environmentally friendly production techniques is increasing daily. Heat-treated wood material is a raw material that is produced with an environmentally friendly production technology. The heat treatment technology does not contain environmentally harmful substances in the material structures produced for alternative uses, and can be used in both indoor and outdoor conditions. Its use is becoming increasingly widespread. The amount of energy required for its processing and the CO2 emissions are lower compared to the energy and CO2 release required in the processing of alternative materials for similar places of use, which makes heat-treated wood materials attractive in terms of environmental sensitivity (Korkut and Kocaefe 2009; Aytin 2013).
Heat treatment is a physical modification process that causes permanent changes in the chemical structure of polymer compounds in the wood material cell wall. As an environmentally friendly production method, heat treatment improves stability without completely destroying the wood structure (Zivkovic et al. 2008; Kačíková et al. 2020; Bytner et al. 2021) and provides homogeneity and darkening in color (Johansson 2005; Aytin 2013; Gürleyen et al. 2018; Torniainen et al. 2021) together with resistance to biological degradation (Shukla 2019). When considered industrially, it is possible to cite the main HT methods that provide modification in different physical ways applied in different countries such as the ThermoWood® process (Finland), Plato-Process (Netherlands), Oil-Heat Treatment process (Germany), Rectification process, and Perdure Bois (France) (Torniainen et al. 2021).
The main problem with the use of ThermoWood® materials, especially those exposed to external weather conditions, is that the dark color obtained with HT fades after a short time depending on the conditions of use. This situation leads to loss of value in terms of aesthetics (Jirouš-Rajković and Miklečić 2019). It is known that this change in the HT wood material is caused by the outdoor conditions that have a high destructive mechanism on the material. This destruction mechanism affects the protective layer on the surface of HT wood materials. Many factors, such as temperature differences between day and night, dew, snow, air pollution, acid rain caused by industrial chimneys and exhaust gases, humidity, and sunlight (ultraviolet-UV), reduce the resistance of the protective layer and cause color changes (Çakıcıer and Korkut 2009).
The deteriorating effects of factors arising from different environmental conditions necessitate the creation of a protective surface layer on HT wood materials, and varnish is the most widely used material in creating a protective surface layer. Varnish is a transparent solution consisting of two-element systems in which generally solvents are used. Varnishes make the natural color and aesthetic appearance of the wood material more prominent and are used mainly for aesthetic, protection, economic, and cleaning purposes (Kurtoğlu 2000). However, due to the negative effects of the solvent present in varnish on human and environmental health, the use of solvent-containing varnish systems has been reduced in recent years. Organic-based volatile compounds (VOC) are emitted to the atmosphere from the surface of the wood material on which solvent-containing varnish films have been applied, which adversely affects human and environmental health. In addition to environmental sensitivity, human health and other factors has drawn attention to water-based varnish (WBV) systems in recent years, and studies on reducing the use of solvents in varnishes have been accelerated. With the use of some polymers in varnish production, WBV systems have been developed from different synthetic resins, such as alkyds, polyesters, acrylics, and polyurethanes, containing very low levels of VOC. Various studies with WBV have shown that it can provide protection and brightness close to the natural color of the wood material; however, it has been understood that the hardness and adhesion of the surface film of WBV are weaker, and therefore its resistance to mechanical effects is lower than solvent-containing varnishes (Ulay and Budakçı 2015).
Weathering (WEA) simulates the environmental conditions to which materials to be used in areas with different climatic characteristics will be exposed, in natural or laboratory environment, and can imitate degradation. After WEA, the properties of materials arising from their structures are tested and some information about their end-use areas can be obtained (Çakıcıer and Korkut 2009).
With WEA tests, it is possible to have information about the natural structure of the wood material and the properties of the surface protective layers such as color, hardness, adhesion, and surface roughness (SR). A review of the literature shows that in recent years HT wood materials are exposed to natural weathering (NAT) and accelerated weathering (QUV), in varnished/unvarnished state, and color and SR changes have been investigated. A meta-analysis study examining the results of the studies performed so far reported that the results obtained from different studies are both similar to and different from each other (Jirouš-Rajković and Miklečić 2019). For example, Turkay Turkoglu et al. (2017) conducted a study with Fagus orientalis that the color and SR values obtained after QUV from the HT and polyurethane varnish (PUV) applied samples were better than the samples to which only the varnish had been applied. In another study, Kart et al. (2018) exposed Scotch pine and Fagus orientalis samples to NAT for 3 months. The samples were applied PUV and cellulosic varnish (CV) after HT. As a result, after HT, HT + PUV varnished Scotch pine and Fagus orientalis surfaces had a reddish and yellowish color, while HT + CV varnished surfaces had a reddish and bluish color. They stated that after HT + varnish, ΔΕ of both wood samples was lower than that of the only varnished control samples. Ayata and Çakıcıer (2017) reported that ΔΕ* increased after 432 h of QUV in the TM samples to which they applied WBV. Esteves et al. (2020) found that the color of TM samples remained approximately constant up to 750 h in the WEA treatment and that the HT initially became more reddish and turned greener and slightly more yellow as the time extended. Aytin et al. (2021) stated that WBV with color pigments can be used against color change in outdoor use of HT wood materials. In another study, it was reported that TM samples were better than SR control samples (Aytin et al. (2020).
Kropat et al. (2020) stated that because there is no need to compare WEA conditions, the most reliable WEA tests are the natural environment tests in which the material will be used. However, WEA time is long in natural environment tests. However, with QUV, whose programs consist of sections simulating the effects of sun rays, temperature differences, humidity condensation, and rain, faster results can be obtained in a short time. The performance of the wood material can be determined quickly with varnish and similar materials used in the surface treatment. In accelerated WEA, the UV effect coming from the sun and reaching the earth is simulated by UV rays with a wavelength of 280 to 400 nm, and two UV rays with different wavelengths (UVA, 315 to 400 nm and UVB, 280 to 315 nm) are included in the WEA device program. Spraying and conditioning can be added to the programs if the usage environment conditions are desired to be more similar (Aytin 2013).
This study investigated the interactions of some wood and surface treatment materials obtained with environmentally friendly production technologies in terms of color and roughness. Within the scope of the study, ThermoWood® test samples prepared from some tree species that grow in Turkey and which have economic use in industry were used. The test samples with normal varnish, water-based varnish (A1), modified water-based varnish (A2), and no varnish were subjected to NAT and QUV, and the total color difference and surface roughness change were examined. The study aimed to contribute to environmental awareness, and focused on the opportunities that heat treatment and WBV interaction may create in making more effective use of wood material.
EXPERIMENTAL
Materials
Aspen (Populus tremula L.), narrow-leaved ash (Fraxinus angustifolia Vahl), and Eastern spruce (Picea orientalis) growing naturally in Turkey were used in the study. The trees were selected according to TS 4176 (1984), and the trunks taken from the forest were cut into 60 –mm- thick planks with the sharp cut method as per TS 2470 (1976). Then, the planks were dried to an average of 12% moisture content with the classical drying method, and 20 × 100 × 500 (mm × mm × mm) drafts were prepared from the planks.
Methods
Heat treatment application and creation of a trial pattern
Tree samples prepared with dimensions of 26 mm × 100 mm × 600 mm by giving dimensional tolerances from trial trees were subjected to heat treatment at a temperature of 190 °C for 1 h (TW1) and 212 °C for 1 h (TW2) using the ThermoWood® method at the Novawood Forest Products Factory (Gerede/Bolu, Turkey). The ThermoWood® trees obtained were kept in an air-conditione room with control samples at an average temperature of 20 °C ± 2 °C and a relative humidity of 60% ± 5% until they reached a constant weight.
Table 1. Classification of Working Samples and Abbreviations
Varnish properties and application
The working samples prepared from TM products that were produced within the scope of the study were divided into two groups as varnished and unvarnished to be subjected to the aging process. After the surfaces of the working samples to be varnished were divided into two with a line in the middle. One side of the surfaces was finished with primer barrier + varnish (AQUACOOLFX1707 Wood color protection barrier (primer) + AQUACOOL AG 4850 Parquet Varnish, given as A1 in the text), and the other side was treated only with varnish (AQUACOOL AG 4850 Parquet Varnish, given as A2 in the text). In order to bring the varnish layer thicknesses closer to each other in both groups, the application was completed as four layers for the A1 group and three layers for the A2 group. Varnish properties and application principles are presented in Table 2, the varnish was made by Dual Boya (İstanbul, Turkey).
Table 2. Varnish Properties and Application Principles
Varnish properties and application
Within the scope of the study, one group of unvarnished and varnished samples was subjected to QUV for 720 h using UVB EL 313 lamps, and the other group of varnished samples was subjected to NAT for 3 months. The principles of the WEA program are presented in Table 3.
Table 3. Weathering Program
Determining color values
Measurements were made based on the CIELab color system to determine the color values of the samples. The CIELab color system has three coordinates as L*, a*, and b* calculated from X, Y, and Z tristimulus values and is widely used for this purpose.
Fig. 1. Determination of L*, a*, and b* values according to CIELab color system (Anonymous 2022a)
The * sign written with letters is used to distinguish CIE formulas from similar formulas in different color systems that have been developed before (Yeşil 2010). In the CIELab system, L* is on the black-white (L* = 0 for black, L* = 100 for white) axis; a* is on the red-green (positive value red, negative value green) axis; and b* is on the yellow-blue (positive value yellow, negative value blue) axis. The L angle also expresses the color change in the wood. The narrowing of this angle shows that the color of the wood approaches the red color (a), and its widening shows that the color of the wood approaches the yellow color (b).
For the analysis of color difference, the color values of the samples during and after aging were determined according to the CIELab color system, and the difference between control-start and control-end values was examined.
Color values were determined by Konica Minolta CD-600 color measuring device (Konica Minolta, Inc., Osaka, Japan) in accordance with the ISO 7724-2 (1984) standards, and ΔE* was calculated according to ISO 7724-3 (1984) with the following formula. Here, Δ indicates the difference and the letter E is the initial letter of the German word Empfindung, which means feeling (Yeşil 2010).
ΔE* is the total color difference in the samples after heat treatment, ΔL* is the black-white color change, Δa* is the red-green color change, and Δb* is the yellow-blue color change.
Surface roughness (SR)
The Mitutoyo surface test measuring device was used for the SR measurements of the samples (Mitutoyo Surftest SJ-301; Mitutoyo Corporation, Kanagawa, Japan). The device measures at a 90º angle to 4 (μm) needle diameter and the longitudinal measuring angle with fiber direction, at a measuring speed of 10 mm/min. To determine the surface roughness values, five measurements were made on each of the working samples. The measurements were made from pre-marked points perpendicular to the fiber direction according to the ISO 4287 (1997) and DIN 4768 (1990) standards. Ra, Ry, Rz, and Rq roughness parameters were determined in the SR measurements of the working samples. Here:
Ra is the average roughness value, which is the arithmetic mean of all values for deviations from the profile mean line along the roughness profile;
Rz is the ten point roughness average value, which is the sum of the average values of the five highest peaks and five deepest dips along the roughness profile;
Ry is the maximum roughness value, which is the sum of the highest peak and the deepest dip according to the mean profile line, along the roughness profile (Çakıcıer 2007).
Rq is the root mean square (RMS) value, which is the root mean square of the sum of the squares of the roughness profile heights (Anonymous 2022b).
Statistical evaluation
The SPSS package program (IBM Corp., SPSS 15.0 for Windows, Armonk, NY, USA) was used for the statistical evaluation of the data. An analysis of variance was used to determine whether the factors were effective for the results obtained, and Duncan’s test was used to determine the size of the difference on the factors that were significant. The level of significance was set at P ≤ 0.05 in the analysis of variance.
RESULTS AND DISCUSSION
Findings on Color Values
Weathering (WEA), heat treatment (HT), and wood type (WT) multiple analysis of variance (MANOVA) results regarding the total color difference are presented in Table 4.
Table 4. Weathering, HT, and WT Variance Analysis Results Regarding the ΔE*
Table 5. Weathering, HT, and WT Mean Values for the ΔE* and Duncan Test Results
According to Table 4 each of, the factors of WEA, HT, and WT had a significant effect on ΔE* at p ≤ 0.05, and the interaction between WEA and HT was the factor with a significant effect of 0.891 on ΔE*. At the same time, when the factors were evaluated separately, it was observed that WEA had the highest effect (0.607), and WT had the lowest effect (0.135). Table 5 presents the ΔE* mean values and Duncan test results to better understand the effect of WEA, HT, and WT on the ΔE*.
According to Table 5, the lowest total color change was determined as 9.38 in A1-QUV-720 samples, and the highest total color change was in A2-QUV-720 samples with 14.95. It is seen that the lowest ΔE* was obtained in A1 varnishes in both NAT and QUV. According to the heat treatment and natural factor comparison, the highest ΔE* change was obtained in TW2 working sample. The amount of change in the ΔE* according to the stages of WEA is given in Fig. 2. The visuals of the working samples after 720 h of QUV and 90 days of NAT are given in Fig. 3.
Fig. 2. Average ΔE* amounts obtained during the stages of the WEA process
The graph in Fig. 2 and the visuals in Fig. 3 reveal that ΔE* changes across variations in VAS, A1, and A2 samples. With WEA, lightening-graying and yellowing mostly occurred in the colors of varnished working samples; lightening-graying is mostly seen in the A1 group and the surface appearance after WEA was relatively close to the pre-WEA appearance in this group. It is understood that yellowing is seen in the A2 group. However, in varnished working samples in the A1 group, PO turned to a whitish-yellowish structure rather than graying in QUV WEA compared to PT and FA, while the color of the same group was close to the initial color in NAT. In contrast, in VAS working samples, it was observed that there was yellowing in CON and lightenings in TW1 and TW2 after QUV. On the other hand, it is noteworthy that the yellowing of the surface layers was more intense during the three-month NAT period in the varnished working samples. In addition, as shown in Fig. 2, ΔE* was seen to be 100% or even higher in heat-treated TW1 and TW2 working samples compared to the CON samples. Here, it is also understood that the highest ΔE* was obtained in the A2-TW2 group in QUV-720, and in the A1-TW2 and A2-TW2 groups in NAT, and therefore, as the color darkness obtained with HT increased, the surface color had less stability against WEA conditions. The changes in the color appearance with NAT were due to the values of L*, a*, and b* parameters, which are the three basic components of the CIELab* color system and are used in the calculation of ΔE*. Graphs displaying the findings related to color parameters L*, a*, and b* are given in Figs. 4 through 6.
Fig. 3. Appearance of the samples before and after WEA
As shown in Fig. 4, the L* values of the CON and HT samples, which were exposed to the WEA process as varnished, are different. In unvarnished working samples, L* first decreases at the beginning of QUV, starts to increase after 144 h WEA, and continues to increase until QUV is over. While the L* value in the CON group (A1-CON and A2-CON) continued to decrease during both QUV and NAT in varnished working samples, it increased from the onset of WEA in the HT groups (A1-TW1, A1-TW2, A2-TW1, and A2-TW2). This increase appears to be approximately twice as high in 90-day NAT than in 720-h QUV.
** Represents the values at the beginning of weathering
Fig. 4. L* amounts obtained from CON and HT variations in the stages of the WEA process
Fig. 5. Red green color (a*) amounts obtained from CON and HT variations in the stages of the WEA process
Fig. 6. Yellow-blue color (b*) values obtained from CON and HT variations in the stages of the WEA process
Although the red-green (a*) color value of the unvarnished samples increased rapidly in the VAS-CON group in QUV at the beginning (first 144 h), it remained almost the same in the HT groups and decreased approximately 10 to 20% in all three groups until the end of the QUV after 144 h. Although the a* value increased at the onset of aging in both QUV and NAT in the CON group (A1-CON and A2-CON) in varnished working samples (especially evident in the A2-CON group), it decreased in the HT groups (A1-TW1, A1-TW2, A2-TW1, and A2-TW2) from the beginning of aging. The change in the yellow-blue (b*) color value is almost similar to the change in a*.
Table 6. Total Color Change, L*, a*, and b* Values Measured at the Beginning and End of the WEA Process
There are studies in the literature investigating the ΔE* difference and L*, a*, and b* parameters after HT, varnishing, and WEA. For example, Turkoglu et al. (2017) found that QUV of Fagus orientalis with polyurethane varnish after HT at 210 °C for 0.5, 1, and 1.5 h caused an increase in L*, a*, and b* values and the value of ΔE* increased from 11 to 15%. In another study, Baysal et al. (2014) investigated ΔE* and L*, a*, and b* after 500 h of QUV exposure to Scotch pine samples, which were heat-treated at 140, 170, and 200 °C. They revealed that with a value ranging from 1.80 to 15.9, ΔE* was lower in the heat-treated samples in all HT groups except the samples treated at 140 °C for 2 h. In addition, they found that as HT temperature and time increased, L* decreased after QUV, a* and b* color values increased; however, the amount of increase decreased depending on the increase in HT temperature and time. Ayata and Çakıcıer (2017) applied single and double component WBV to the oak, Fagus orientalis and Scotch pine working samples. The samples were also subjected to 2 h of heat treatment at 190 °C and 1 and 2 h of heat treatment at 212 °C, and then to QUV up to 432 h, and color change was investigated. The study revealed that after 432 h of QUV, the lowest ΔE* (30.75) was found in oak with double-component WBV at 2 h of heat treatment at 212 °C, and the highest ΔE* (62.64) was found in Scotch pine. The study reports that the highest and lowest values of other color parameters (L*, a*, and b*) were 29.79, 55.56; 5.14, 13.00; and 6.29, 26.70, respectively, in double-component WBV. Esteves et al. (2020) applied 720 h of QUV and reported that in heat-treated pine species, L* remained almost the same with minimal changes during the aging period, and the heat-treated products generally turned yellowish at the end of the aging process. In their study with Abies nordmanniana and Scotch pine, Kesik et al. (2017) exposed water-based varnished and dyed test samples after heat treatment to NAT for one year and investigated ΔE*. Their study revealed that after NAT, the HT and varnished samples were more stable against color change than the untreated varnished samples.
Findings on surface roughness
The MANOVA results pertaining to the Ra, Ry, Rz, and Rq values across aging, heat treatment, and tree samples are given in Table 7.
Table 7. MANOVA Results For Surface Roughness Ra, Ry, Rz, and Rq Factors