INTRODUCTION

Wood materials have always been one of the most preferred products some sectors such as furniture industry, buildings, fibreboard, particleboard, plywood, paper industry etc., although there are alternatives. However, limited resources of forests and increased consumption depend on developments in technology cause a decrease in the forest resources day by day. To overcome this issue, these materials should have been coated by varnishes and paints in order to ensure long-term use. Finishing applications, especially in furniture and decoration industry were made to prevent the wooden materials from wearing out over time, to provide aesthetic properties to wooden materials and to obtain easily cleanable surfaces (^{Ulay; Budakçı, 2015}). Moreover, wood materials may be exposed to mechanical effects such as abrasion and shock, physical effects such as dust, light, temperature, dirt and oil, chemical effects such as acids, bases, oxidation of air and cleaning products. For coping with all these problems, it is stated that applying surface treatments such as varnish and paint against these external influences are crucial to provide partial durability and protection (Budakçı et al., 2009; ^{Malkoçoğlu et al., 2012}). Also, almost all physical and mechanical properties of wood materials decreases when exposed to outdoor conditions without any protection measures (^{Pelit; Korkmaz, 2017}).

Most of the varnishes produced are solvent based in Turkey. However, the importance of water-based paints and varnishes has increased since the Clean Air Agreement, signed in 1970, contains substances that restrict the release of volatile organic compounds into the atmosphere. In addition, the use of binding resins such as polyurethanes in the production of water-based varnish systems has not only led to an increase production of water-based paints and varnishes, increased the use of water-based paints and varnishes as well. These binding resins gain properties such as elasticity, resistance to chemical and mechanical effects to paints and varnishes. In this context, the hardness value observed in the surface layers is indicative of the durability of the product against external influences. A decrease in the hardness of the paint and varnish layers will result in a fall in the strength of these layers to some mechanical effects such as shock, scratch and friction ^{(Budakçı et al., 2009}).

One of the factors affecting the durability of surface treatment such as varnish and paint is the cohesion force between the molecules of the surface treatment materials and adhesion force between the wood and surface treatment materials (^{Sönmez et al., 2002}; Budakçı et al., 2009). But, the adhesion strength is one of the unstable factors used to determine the durability of protective layers among these (^{Budakçı; Sönmez, 2010}). In addition, ^{Sonmez et al. (2009}) states that the adhesion strength in hardwoods is higher than softwoods. There are several methods used to determine the adhesion strength. It has also been found that the most preferred of these are the pull-off test and the cross-cut test (Budakçı; Sönmez, 2010).

According to place of use of wood products, protective surface applications show changes. For instance, polyurethane, acrylic or water-based varnishes are generally used for varnishing of solid wood parquets. Water-based varnishes are used instead of solvent-based varnishes especially in large halls in order not to cause health problems. Laminated flooring is usually varnished with 5-7 times using by acrylic or polyurethane varnishes. Then, the varnished surface is dried by using UV drying system (^{Döngel et al., 2008}). Hence, determining the performance of polyurethane alkyd varnish (PAV) and water-based acrylic varnish (WBAV) applications for the Crimean juniper wood will provide guidance in detecting which types of varnishes will be better for finishing of juniper wood.

^{Sönmez et al. (2004}), in their study, one and two components water-based varnishes were applied on Scotch pine (Pinus silvestris L.), beech (Fagus orientalis L.), and oak (Qercus petreae L.) wood sample surfaces with brush, sponge roller and spray gun. As a result, they found that the application method was not effective on the hardness and adhesion strength values of the water-based varnish layers, whereas the spraying method was effective on the glossiness. In a study conducted by ^{Salca et al. (2016}), it was shown that black alder wood (Alnus glutinosa Gaertn. L.) samples varnished by using water-based and UV varnishes by two application methods. As compared to spraying, the application of roller method caused to a higher adhesion strength and glossiness for UV coated samples. On the other hand, water-based coating by using spraying presented a better adhesion than those of UV coating.

In comprehensive study of adhesion strength, ^{Darmawan et al. (2018}) employed the crosscut tape method at the determination of the adhesion strength. They found that as the surface roughness of the jabon (Anthocephalus cadamba) and sengon (Paraserianthes falcataria) wood samples varnished with water-based acrylic and oil-based alkyd decreased, the adhesion strength of the both varnished wood samples went down. Similarly, in an investigation into adhesion strength, ^{Vitosytė et al. (2012}) found that the ash (Fraxinus excelsior L.) and birch (Betula L.) wood samples coated with 3 different acrylic-polyurethane varnishes presented a better adhesion strength with the increasing of surface roughness. A recent study by ^{Bekhta et al. (2018}) showed that adhesion strength values of veneer samples varnished after thermally pressing process exhibited higher values than control samples. Moreover, ^{Altun and Esmer (2017}) stated in their research that ash (Fraxinus excelsior L.), Scotch pine (Pinus sylvestris L.) and iroko (Chlorophora excelsa) wood samples were heat treated by using ThermoWood™ method. Then, they applied various varnishes to the surfaces of the samples and determined not only the effects of surface roughness, but also varnish types on varnish adhesion strength. As a result of their research, it was determined in heat treated samples that the varnish adhesion strength was generally lower. The increase in surface roughness up to the certain limits caused an increase in varnish adhesion strength. The highest adhesion strength was found in the samples coated with polyurethane varnish. They also expressed that the water-based varnish had the lowest adhesion strength.

Budakçı et al. (2009) prepared water-based varnish by adding the boric acid solution in different ratios and they investigated that the impact of that modified varnish on the hardness of varnish layer. They showed that boric acid modification increased the hardness value of the varnish layer. ^{Anwar et al. (2018}) investigated mechanical properties of samples coated with nanoclay added water-based varnish. They observed that adding of nanoclay at the percentage 2, 4, and 6 was not significantly effect on the adhesion strength. Also, they stated that the increasing amount of nanoclay percentage of varnish could cause an increase the mechanical properties of water based coated samples up to a certain percentage.

^{Saygin and Budakçı (2017}) aimed in their study that determining the impact of resin and resin cleaning methods on the hardness of water-based varnish layer. Consequently, those cleaning applications had not any effect on layer hardness of water-based varnishes. One study by ^{Özçiftçi et al. (1999}) examined wood samples varnished with acrylic, synthetic, polyurethane and acid-catalysed varnishes after than bleaching with sodium hydroxide + hydrogen peroxide, sodium hydroxide + calcium hydroxide + hydrogen peroxide, hypochlorite and hydrochloric acid. They demonstrated the effect of bleaching on the glossiness of varnish layer and adhesion strength of varnish. Consequently, they observed that there was no effect of bleaching process on the glossiness of the varnish layer. Additionally, they found that varnish type and wood species had effect on the glossiness of the varnish layer. Moreover, they showed that the HCl solution, one of the bleaching chemicals, reduced the adhesion strength of the varnishes.

^{Palija et al. (2013}) investigated that the impact of the staining of spruce wood (Picea abies Karst.) on the adhesion strength of different types of varnishes (nitrocellulose, polyurethane and water based varnishes). They found that water-based varnish had the highest adhesion strength value (4.50 MPa) within the used varnishes for unstained (control) samples, while polyurethane varnish had the highest adhesion strength value (4.87 MPa) within the used varnishes for stained (test) samples. In a study of ^{Ghofrani et al. (2016}), wood extractives was removed from alder (Alnus subcordata) and Ironwood (Zelkova carpinifolia) samples. Then these samples were coated with polyurethane alkyd and polyester varnishes. They found that the extractive-free samples exhibited stronger adhesion strength than untreated (control) samples.

In an analysing of colour changes, ^{Söğütlü and Döngel (2009}) observed that when cellulosic, synthetic, polyurethane and acid-catalysed varnishes were applied to oak, east beech, chestnut and Scotch pine woods, the most colour changes occurred in synthetic varnished samples. ^{Liu et al. (2012}) stated that poplar and scots pine (Pinus sylvestris L.) veneer samples coated with water-based acrylic varnish. They found that the lightness aberration of veneer (ΔL*) and chromatism (ΔE* ab) significantly reduced. Moreover, detailed examination of colour changes by ^{Yalınkılıç and Sönmez (2015)} showed that only polyurethane and synthetic varnishes caused an increase in the red colour tone in the wood samples obtained from Scots pine, oriental beech and chestnut, which were applied cellulosic, synthetic, polyurethane and acid-catalysed varnishes. In addition, they stated that water-based varnishes caused colour changing effect on mostly in tannin-rich woods. A recent study by ^{Moya et al. (2017}) involved that less colour change was observed in 9 kinds of tropical wood samples coated by water-based varnish modified with titanium dioxide nanoparticles after one year of natural weathering process. Detailed examination of high gloss polyurethane and traditional shellac varnishes by ^{Šimůnková et al. (2018}) showed that polyurethane varnish was much more durable than shellac varnish. Furthermore, at the beginning of the test, shellac varnish demonstrated better strength to artificial ageing process, while this considerably decreased end of the process.

Natural colour of wood exists within a range from almost white to black. Furthermore, since most of the diffuse light is absorbed by the varnish layer and then transformed into thermal energy, varnished surface reflects less diffuse light to the observer. Thus, coated wood samples seem darker than uncoated samples. Besides, the moisture content of wood impresses the colour appearance (^{Meints et al., 2017}). Some consumers prefer lighter tones in choosing of wood products, while the others prefer darker tonnes. Hence, it is preferred by the manufacturers to have colour integrity especially in the production of furniture and decoration industries (^{Fan et al., 2010}; ^{Yalınkılıç and Sönmez, 2015}). However, different colours can be seen in the same wood species. If a colour is desired except that the natural colour of the wood colouring treatment can be apply (Yalınkılıç; Sönmez, 2015).

Discolouration in the wood depends on both the environmental condition and the chemical composition of the tree species (^{Fan et al., 2010}). Also, It was determined that the colour changes of wood originated electromagnetic radiation causing photochemical reactions in wood material have all the light wavelengths in its structure (^{Döngel et al., 2008}). However, water-based varnishes could cause darkening of wood because of tannins oxidation (^{Çakicier et al., 2011}).

In this study, it was aimed to determine the adhesion strength and the hardness of varnish layer of the Crimean juniper (Juniperus excelsa M. Bieb.) wood which grows naturally in the Kastamonu region. Besides, it was focused on the colour measurements of the Crimean juniper wood. For the purpose of this study, the adhesion strength, hardness, and colour measurements tests were conducted. Next, obtained data were analysed statistically by using SPSS 22. In the last part, the obtained results were interpreted and also some suggestions were given for the future research.

MATERIAL AND METHODS

Adhesion strength of varnish layer

The test of the varnish adhesion strength was carried out using a 5 ton-capacity Shimadzu brand AG-IC 20/50 KN STD universal testing machine. Test cylinders with a diameter of 20 mm were adhered to the test specimens with Bison ™ epoxy metal adhesive at normal room temperature (~ 20 °C). The glued test cylinders were weighed in weight by two kilograms in order to provide the better adhesion during drying. The samples were left to dry for 48 hours for drying of the glue on the samples. Before the tests, the samples were conditioned for 24 hours at 50 ± 3% relative humidity and 23 ± 2 °C temperature according to ^{TS EN ISO 4624 (2016)}. Then, the bonded test cylinder on the varnish layer was cut from the around of the test cylinder to the surface of the wood sample through a cutter. Finally, in the universal tester, the adhesion of the varnishes were tested in such a way that the speed of the drawing head would be 5 mm per minute.

The test process and test samples are shown in Figure 1 and Figure 2 respectively.

FIGURE 1 Varnish Adhesion Strength Tests. 

FIGURE 2 Varnish Adhesion Strength Test Samples and Test Cylinders. 

Adhesion strength was calculated according to Equation 1 in MPa (MPa = 1 N.mm^-2), Where: X denotes the adhesion strength (MPa), F denotes the rupture force (N), d represents test cylinders with a diameter of 20 mm, and π denotes Number of Pi.

X=4F·π·d2−1 (1)

Hardness of varnish layer

Varnish layer hardness of test samples were measured using a König hardness tester in accordance with ^{ASTM D4366 (2014)}. The test samples were stored for 24 hours at 23 ± 2 ° C temperature and 50 ± 5% relative humidity on the basis of ^{TS EN ISO 1513 (2010}), before the hardness tests. After this process, the samples were placed on the device and the number of oscillations was observed by damping the pendulum. The test process are shown in Figure 3.

FIGURE 3 Varnish Layer Hardness Tests. 

Colour measurement

Model CM2500d Konica® Minolta spectrophotometer was used for colour measurements. The L*a*b* colour space was used for colour test, in which a* and b* represented the chromaticity coordinates; +a* for red (+60), -a* for green (-60), +b* for yellow (+60), and -b* (-60) for blue L* and represented the lightness in the range from black (0) to white (100), (^{Özkan, Temiz and Vurdu, 2017}). For each test samples, four measurement points were marked. Specimens colour coordinates L*, a*, and b* values were determined both before and after varnishing. These values were used to calculate the colour change value (ΔE*) among the varnished samples using two different varnishes according to the ^{ASTM D-2244 (2015}) standard. Colour measurement test equipment are shown in Figure 4.

FIGURE 4 Colour Measurement Test Samples and Test Equipment. 

The colour change value (ΔE*) among the test samples were calculated by using Equation2, where:𝞓E; Total colour change value, 𝞓L; Total colour change value of black (0) to white (100), 𝞓a; Total colour change value of red (+60) to green (-60), 𝞓b; Total colour change value of yellow (+60) to blue (-60).

ΔE∗=ΔL∗2+Δa∗2+Δb∗2 (2)

RESULTS AND DISCUSSION

The varnish adhesion strength of samples were given in Table 1.

Table 1 shows that the comparison and statistical analysis of adhesion strength of samples coated with WBAV and samples coated with PAV. According to the results, in terms of adhesion strength, samples coated with WBAV was found to be higher than samples coated with PAV as 2.25 MPa and 2.81 MPa, respectively. Additionally, determining the effects of coating types on adhesion strength, T test was performed. According to the T test result, it was found that coating types were statistically significant on adhesion strength (α<0.05). In a study of Budakçı and Sönmez (2010), it was shown that cellulosic, polyurethane, acrylic and water-based varnishes were applied to the scots pine (Pinus sylvestris L.), fir (Abies bornmülleriana Mattf), oriental beech (Fagus orientalis L.) and oak (Quercus petraea L.) wood sample surfaces. They found that the interaction between varnish types and wood species on the surface adhesion strength was significant. Moreover, by supporting our test results, they demonstrated that the highest adhesion strength was obtained from polyurethane and acrylic varnishes.

The varnish layer hardness values were given in Table 2.

Overall, Table 3.2 shows that the surface hardness and statistical analysis results of test samples. According to the results, samples coated with WBAV was found to higher than samples coated with PAV in terms of surface hardness as 71 oscillation and 25 oscillation, respectively. It was reported that differences between tree species had no effect on the hardness of varnish layers, while type of varnish had effect on the hardness of varnish layers (^{Kaygın and Akgün, 2008}). Türkoğlu et al. (2015) in the study, varnish layer hardness value of Scots pine wood samples coated with polyurethane varnish was found to be 23.4 oscillation. These results support our results from this study.

The colour change values between the varnished samples were given in Table 3.

As it was shown on Table 3.3 that total colour change of between unvarnished (control) samples and samples coated with WBAV (test samples) was determined to be 8.90 unit. In addition, the total colour change of between control samples and test samples coated with PAV was determined to be 16.84 unit. The L* value of the test samples showed a slight change towards the black colour as compared to the control samples. Moreover, as compared to the control samples, the a* and b* values of the test samples showed a slight change towards the red and yellow colour tone, respectively. Besides, in terms of changing towards the yellow colour, especially samples coated with PAV were much greater than samples coated with WBAV. ^{Özçifçi and Atar (2002}) studied cellulosic, synthetic, polyurethane and acid-catalysed varnishes applied to Scotch pine, oriental beech and chestnut wood and then they examined colour changes. According to their results, polyurethane varnish increased both yellow and red colour tone. Thus, it was clearly seen that their results supported our results. Specifically, when looking at the varnish types that caused colour change, the L* value of the samples coated with PAV showed a slight change towards the black colour as compared to the samples coated with WBAV. Furthermore, as compared to the samples coated with WBAV, the a* and b* values of the samples coated with PAV showed a slight change towards the red and yellow colour, respectively.

CONCLUSIONS

This paper argued that determining the varnish layer hardness and varnish adhesion strength of the varnished Crimean juniper (Juniperus excelsa M. Bieb.) wood growing naturally in the Kastamonu region of Turkey. Another aim of this study was to investigate the effects of varnish types on the colour change of coated wood samples.

The results of this research showed that the adhesion strength of samples coated with PAV was found to be higher than samples coated with WBAV. Moreover, in terms of surface hardness value, samples coated with WBAV were found to be higher than samples coated with PAV. Hence, this research supports that when surface hardness is required in the varnishing of the Crimean juniper wood, it is recommended that WBAV based should be used. When better surface adhesion is desired, PAV should be used.

For future research, the impact of artificial weathering or natural weathering applications on the adhesion strength, surface hardness value and colour change of coated samples can be studied. Additionally, after the coating of Crimean juniper wood by using wood dyes, the impact of artificial weathering or natural weathering applications on the adhesion strength, surface hardness value and colour change can be studied.