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Ozgenc, O., Okan, O. T., Yıldız, U. C., and Deniz, I. (2013). "Wood surface protection against artificial weathering with vegetable seed oils," BioRes. 8(4), 6242-6262.

Abstract

Effects of UV-light irradiation and water spray on the mechanical strength and surface characteristics of untreated and pretreated Scots pine sapwood samples were studied. The specimens were treated with parsley seed oil, pomegranate seed oil, linseed seed oil, nigella seed oil, canola oil, sesame seed oil, and soybean oil. The compositional changes and surface properties of the weathered samples were characterized by Fourier transform infrared (FTIR-ATR) spectroscopy and color and surface roughness measurements. The results showed that all vegetable oils provided lower color changes than the control group after 600 h of exposure in a weathering test cycle. The least color change was found on the Scots pine surface pretreated with pomegranate seed oil. The vegetable oil treatment retarded the surface lignin degradation during weathering, indicating that the surface roughness values of pine wood treated with vegetable oils decreased with irradiation over time compared with those of control samples. The effect of artificial weathering on mechanical strength was determined with a compression strength test. It was observed that the compression strength values of Scots pine samples treated with vegetable oils was higher than that of untreated samples after 600 h of weathering exposure.


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Wood Surface Protection against Artificial Weathering with Vegetable Seed Oils

Ozlem Ozgenc,a,* Onur Tolga Okan,b Umit C. Yıldız,b and Ilhan Deniz b

Effects of UV-light irradiation and water spray on the mechanical strength and surface characteristics of untreated and pretreated Scots pine sapwood samples were studied. The specimens were treated with parsley seed oil, pomegranate seed oil, linseed seed oil, nigella seed oil, canola oil, sesame seed oil, and soybean oil. The compositional changes and surface properties of the weathered samples were characterized by Fourier transform infrared (FTIR-ATR) spectroscopy and color and surface roughness measurements. The results showed that all vegetable oils provided lower color changes than the control group after 600 h of exposure in a weathering test cycle. The least color change was found on the Scots pine surface pretreated with pomegranate seed oil. The vegetable oil treatment retarded the surface lignin degradation during weathering, indicating that the surface roughness values of pine wood treated with vegetable oils decreased with irradiation over time compared with those of control samples. The effect of artificial weathering on mechanical strength was determined with a compression strength test. It was observed that the compression strength values of Scots pine samples treated with vegetable oils was higher than that of untreated samples after 600 h of weathering exposure.

Keywords: Artificial weathering; Color change; Compression strength; FTIR-ATR spectroscopy; Surface roughness; Vegetable oil

Contact information: a: Faculty of Technology, Department of Woodworking Industry Engineering, Karadeniz Technical University, 61830 Trabzon, Turkey; b: Faculty of Forestry, Department of Forest Products Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey;

* Corresponding author: ozlem_ozgenc@hotmail.com

INTRODUCTION

Weathering leads to surface degradation of wood that is initiated primarily by solar radiation, but other factors are also important. The wetting and drying of wood through precipitation, diurnal and seasonal changes in relative humidity (RH), abrasion by windblown particulates, temperature changes, atmospheric pollution, oxygen, and human activities such as walking on decks and cleaning surfaces with cleaners and brighteners, sanding, and power-washing all contribute to the degradation of wood surfaces (Williams 2005). Some changes to the wood surface include grain loosening, roughened surfaces changing color, and checks and splinters and fragments breaking off the surface. The photon energy in solar radiation is the most damaging, initiating a wide variety of chemical changes at the wood’s surface (Feist 1988). Color change on the wood surface is also related to the rate of formation of carbonyl groups and the degradation of lignin (Pandey 2005).

In recent years, many different methods have been discovered in the development of protective systems for wood to prevent photodegradation during outdoor weathering. Several approaches have been developed to prevent the photodegradation of wooden surfaces during outdoor weathering. One of the approaches considered is the application of clear-coating, which is thought to be the easiest and most common method (Yang et al. 2001; Chang and Chou 2000; Decker et al. 2004; Chou et al. 2008; Dawson et al. 2008; Forsthuber and Grüll 2010; Saha et al. 2011; Ozgenc et al. 2012; Corcione and Frigione 2012; Forsthuber et al. 2013). The other main approach considered for enhancement of weathering resistance is wood treatment (Temiz et al. 2005; Zhang et al. 2009; Ozgenc et al. 2012). Another method is the chemical modification of the molecular structure of polymers as a fundamental approach to improving the resistance of materials to photodegradation (Evans et al. 2000; Pandey and Chandrashekar 2006; Evans 2009; Hill 2011). New trends in wood preservation focus on products and processes that utilize environmentally friendly technologies and sustainable resources using recycled materials or byproducts from other industries (Temiz et al. 2007). There is growing interest within Europe in the use of oils, which is usually known as the Royal process, and water repellents for wood preservation at both industrial and research levels. This interest focuses on the screening of different natural and synthetic oils and on the process development of wood preservation technology (Palanti and Susco 2004). Linseed oil, tall oil, orange oil, soybean oil, nut oil, and hemp oil have been used either commercially or on a laboratory scale for wood preservation (Van Acker et al. 1999; Treu et al. 2001; Nakayama and Osbrink 2010). Traditional wood protection methods are highly effective against weathering, but in recent years, their use has been restricted due to the toxicity of traditionally used agents, which results in environmental hazards. With regard to developing environmentally benign wood preservatives without any toxicity to humans, the activities of various essential oils and extracts from plants against wood decay and termites have been investigated in recent studies (Yamaguchi et al. 1999; Kartal et al. 2006; Chang et al. 2008; Sen and Yalҫin 2010). Another study investigated the water repellent efficiency of crude tall oil and crude tall oil emulsions, and the possibilities of reducing the amount of oil needed with the emulsion technique. Natural oils (e.g. tall oil, linseed oil) appear to be capable of preventing water uptake by wood. Tall oil treatments reduce the water uptake into sapwood. With tall oil emulsion treatments almost equal water repellent efficiencies are reached as with pure tall oil, even when the oil retentions are considerably lowers (Hyvönen et al. 2006).

The effects of novel organic product on compression properties of fir (Abies alba), beech (Fagus sylvatica), and deciduous oak (Quercus sp.) were investigated. The effectiveness of the chemical treatment changes from one botanical species to another morphological structure, which leads to different mechanical strength of each species to be impregnated and to retain the consolidation product (Lionetto and Frigion 2012). Effects of treatment of southern pine with some organic wood preservation system on mechanical properties are not deleterious when compared with untreated controls (Barnes and Lindsey 2009). The effects of vegetable oil treatments on mechanical properties have been found to be directly related to several key wood material factors and pretreatment, treatment, and post-treatment processing factors (Tomak 2011; Yıldız et al. 2011a).

The objective of this study was to investigate the influence of artificial weather-ing on color stability and chemical change occurring in the surface structure of vegetable oil-treated Scots pine wood (Pinus sylvestris L.). The effect on compression strength of artificial weathering in the case of Scots pine wood treated with vegetable oil also was investigated.

EXPERIMENTAL

Materials

Preparing wood samples

Scots pine sapwood (Pinus sylvestris L.) was prepared at 20 mm thick, 76 mm width, and 120 mm length (3 tests and 3 controls for each variation). The samples were planed and equilibrated at 65% relative humidity and 20 °C. Before applying the vegetable oil on the wood surface, the surfaces of the samples were sanded with 120-grit sandpaper. The vegetable oils were applied in three layers to only one surface of each sample by brush. Later, the specimens were kept at room temperature for a week. The retentions values of vegetable oils are shown in Table 1.

The seeds were washed several times with water, then cleaned for removal of sugars, adhering material, and foreign matter. Then seeds were dried under outdoor conditions. Their moisture content was 8 to 9 wt% (dry basis). Finally, the dried, clean seeds were extracted to obtain the oil, using the commercial cold press method.

The vegetable oil and control sample groups tested in the study included the following: A = Parsley seed oil; B = Pomegranate seed oil; C = Nigella sativa oil; D = Linseed oil; G = Canola oil; V = Sesame oil; M = Soybean oil; and K = Control.

The retention for each treatment solution was calculated following formula (1):

Retention (kg/m3) = (MysM0) / V0 (1)

where Mys is the mass of sample after application of vegetable oil, Mis the mass of sample containing 0% moisture, and Vis the volume of sample in cubic centimeters at 0% moisture content.

Table 1. Retention Values of Vegetable Oils for Scots Pine (Pinus sylvestris L.)

Methods

Accelerated weathering test (QUV/spray)

Artificial weathering was performed in a QUV/spray accelerated weathering tester (Q-Panel Lab Products, Cleveland, OH, USA) equipped with UVA 340 lamps; the temperature in the chamber was approximately 50 °C (ASTM G 53-96). The weathering experiment was carried out in cycles of UV-light irradiation for 2 h followed by a water spray for 18 min in an accelerated weathering test cycle chamber over 25 d (600 h). Four replicate samples for each oil-treated system were prepared for each artificial weathering test condition.

Color measurements

Color measurements were performed with a Konica Minolta CM-600d (Canada). The reflection spectrum was acquired from a measuring area of 8 mm in the 400- to 700-nm wavelength range, where four measurements at precisely defined points on the weathered surfaces of each sample were carried out periodically (ISO 7724-1). Thus, color changes during weathering were always monitored on the same spot of wood. The CIE (Commission Internationale de l’Eclairage) color parameters L* (lightness), a* (along the X axis red (+) to green (-)), and b* (along the Y axis yellow (+) to blue (-)) were calculated using the Konica Minolta Color Data Software CM-S100w SpectraMagic NX Lite (ISO 7724-2), from which the color differences ΔE* were calculated according to Eq. 2:

ΔE* = (ΔL*2 + Δa*2 + Δb*2)1/2 (2)

Measurements were always performed at the end of a UV irradiation step to provide consistent specimen conditions during color measurements. Five replicates were used for each sample to evaluate color change.

Surface roughness

A Mitutoyo SurfTest SJ-301 instrument was employed for surface roughness measurements. The Ra and Rz roughness parameters were measured to evaluate the surface roughness of the surfaces of unweathered and weathered treated and untreated samples according to DIN 4768. Ra is the arithmetic mean of the absolute values of the profile departures within the reference length, and Rz is the arithmetic mean of the 4-point height of irregularities (DIN 4768). The cut-off length was 2.5 mm, the sampling length was 12.5 mm, and the detector tip radius was 10 µm in the surface roughness measure-ments.

Fourier transform infrared spectroscopy

The FTIR spectra were obtained using a Perkin-Elmer Spectrum One FTIR instrument with a Universal ATR Diamond/ZnSe crystal with one reflection. Five accum-ulated spectra for each sample with a resolution of 16 cm−1 were obtained. Spectra were displayed in transmittance mode and limited to the region of interest: 1850 cm−1 to 900 cm−1.

Compression strength

Some wood samples to which aromatic oil was applied, and their controls, were tested at the following compression strength: Compression strength parallel to grain with samples milled to 20 × 20 × 30 mm taken from twenty pine and beech specimens each. The compression test was performed in accordance with American Society for Testing and Materials 143 (1996). Samples of the velocity of crosshead speed in the compression test at the time of breaking the machine is set up to break the force measured at 1.5 to 2 minutes.

RESULTS AND DISCUSSION

Color Change

The color changes of untreated and vegetable seed oil-treated Scots pine (Pinus sylvestris L.) samples for different time periods are showed in Fig. 4. The lowest values of ∆L*, which is the most sensitive parameter of wood surface quality, were obtained for the untreated wood samples after 600 h of exposure. The negative lightness stability (∆L*) values occur during weathering because the surface becomes darker (Temiz et al. 2007). Other vegetable oil treatments caused fewer changes in the lightness (∆L*) than the control samples. The test samples composed of linseed oil-treated wood samples showed positive ∆L* values, thus indicating that the wood surface became lighter (Fig.1). Except for control and linseed oil-treated samples, the lightness values for all samples decreased during first 200 h and increased afterwards to values comparable or higher than the initial L* values (Nzokou 2004).

Fig. 1. Lightness change of weathered Scots pine (Pinus sylvestris L.) samples (A = Parsley seed oil; B = Pomegranate seed oil; C = Nigella sativa oil; D = Linseed oil; G = Canola oil; V = Sesame oil; M = Soybean oil; and K = Control)

Positive values of ∆a* and ∆b* indicate an increase in yellow color and a tendency of the wood surface to turn a reddish color. Negative values of ∆a* and ∆b* indicate an increase in the color blue, causing the surface to be perceived as a greenish color, as can be seen in Fig. 2 and 3. (Temiz et al. 2007). The change in chromacity coordinates ∆a* and ∆b* (Fig. 2 and 3) shows an increase during 100 hours and a decrease afterwards for all wood samples except for control sample. This is due to the sample surface becoming reddish and yellowish during the first phase of weathering process, and progressively greenish and bluish with extended exposure to artificial weathering (Nzokou 2004).

Fig. 2. Color parameter change (Δa*) of weathered Scots pine (Pinus sylvestris L.) samples (A = Parsley seed oil; B = Pomegranate seed oil; C = Nigella sativa oil; D = Linseed oil; G = Canola oil; V = Sesame oil; M = Soybean oil; and K = Control)

Fig. 3. Color parameter change (Δb*) of weathered Scots pine (Pinus sylvestris L.) samples (A = Parsley seed oil; B = Pomegranate seed oil; C = Nigella sativa oil; D = Linseed oil; G = Canola oil; V = Sesame oil; M = Soybean oil; and K = Control)

The smallest change in color (∆E*) was determined for the pomegranate seed oil-treated wood samples (B group), probably because of the chemical breakdown of lignin and wood extractives (Feist 1988). All vegetable oil-treated sample groups exhibited lower color changes than the control group after 600 h of exposure.

Fig. 4. Color change of weathered Scots pine (Pinus sylvestris L.) samples (A = Parsley seed oil; B = Pomegranate seed oil; C = Nigella sativa oil; D = Linseed oil; G = Canola oil; V = Sesame oil; M = Soybean oil; and K = Control)

The color changes (∆E*) increased quickly within the first 100 h of exposure and had a slight increase after 100 h of weathering (Table 1). When the control samples are compared with all the test samples, it is shown that all vegetable oil treatments prevented color change. When the samples exposed to continuous artificial weathering are compared within the group itself, it is determined that the pomegranate seed oil-treated wood samples (B group) had the lowest color change, followed by the nigella seed oil- (C group) and linseed oil- (D group) treated wood samples. The color change performances of parsley seed, canola, sesame, and soy oil-treated wood samples (A, G, V, and M groups) were very similar after 600 h of artificial weathering (Fig. 4).

The changes in the IR spectra of the wood surfaces demonstrate that UV light modified the chemical structure of wood. The absorption of UV light induced lignin degradation and the photooxidation of –CH2– or –CH (OH)- groups. These reactions are combined with the color changing of wood surfaces (Müller et al. 2003). The color of the wood changes to yellow or brown because of the chemical breakdown (photooxida-tion) of lignin and wood extractives. In addition, the water spray in artificial weathering will leach the water-soluble chemicals from the wood surface during exposure (George et al. 2005; Williams 2005). It was already determined that vegetable oil treatments would show less discoloration compared with control samples after artificial weathering. This can be attributed to the formation of complexes between oils and the guaiacyl unit of lignin. It is notable that the vegetable oil treated wood samples differed from control samples with respect to the FTIR-ATR spectra vibration in the region of 1220 to 1270 cm−1 (Figs. 6-13). The vegetable oil pretreatments for wood were effective at photostabilizing lignin, the component of wood that is most susceptible to photodegradation (Evans et al. 2002; Temiz et al. 2007).

Surface Roughness

Results for the surface roughness of pine wood are shown in Table 2. Oxidized surfaces exhibit higher oxygen content than carbon content, compared to untreated surfaces, thus indicating that weathered wood surfaces contain cellulose with carbonyl groups, whereas the former lignin content was degraded and leached away by water (Feist and Hon 1984; Temiz et al. 2005; Nzokou et al. 2011). As a result of weathering, most of the solubilized lignin degradation products on the untreated wood (control) are washed out by water spray (Fig. 13). But as can be seen in IR spectra figures, vegetable oil-treated wood (especially the pomegranate seed and linseed oil-treated) samples are protected from lignin degradation during weathering time. The control wood surfaces after UV irradiation and water spray contained several checks, splits, and cracks (Fig. 5). The surface roughness values (Ra and Rz) of pretreated and untreated samples, which indicate the weathering effects, are listed in Table 2. Generally, the surface roughness values of vegetable oil-treated pine wood samples decreased over the irradiation time when compared with the surface roughness values of the control samples. In addition, it was concluded that the surfaces of the pomegranate seed and linseed oil-treated wood samples (B and D groups) were rougher than those of other test groups. However, the roughness of wood is a complex phenomenon because wood is an anisotropic and heterogeneous material, and several factors, such as anatomical differences, growing characteristics, and the machining properties, should be considered in evaluating the surface roughness of wood (Aydin and Colakoglu 2005).

Table 2. Changes in the Surface Roughness after 600 h of Artificial Weathering

The data were statistically evaluated by multiple-way ANOVA to demonstrate the effect of vegetable oil-treated and untreated wood samples on surface roughness (SR). Differences between before and after weathering time for each group were not statistically significant at the 0.05 confidence level except for control group. As can be seen from Table 2, the SR parameters after QUV test was found a little bit higher on vegetable oil treated wood than untreated wood (control).

Untreated control samples displayed a high density of large cracks on the surfaces after outdoor weathering. In combination with the erosion of earlywood cells, these cracks produced a tangible surface roughness (Xie et al. 2008). In constrast, exposed front surfaces of Scots pine treated with vegetable oils developed fewer and smaller cracks, and this group of wood samples appeared smoother than the untreated ones, as can be seen in Fig. 5. Less cracking also occured on the surface of the vegetable oil-treated wood samples compared with the untreated control samples; moreover, the vegetable oil-treated wood samples were less deformed, especially with regard to cupping, than the untreated samples (Fig. 5).