Abstract
The efficiency of surface pretreatment with natural essential oils relative to the weathering performance of Norway spruce wood was examined. This study investigated the combination of this pretreatment increasing biological resistance and oil-based coating with protection against ultraviolet light and hydrophobic topcoat with silicon nanoparticles during natural and artificial weathering. The coating systems were exposed to 24 months of natural weathering in climatic conditions of Central Europe and 2,016 hours of artificial weathering. The synergistic effect of a coating system based on safflower with essential oils and commercial oil-based coating was the most efficient. The application of one layer of an oil-based coating in a wet state of surface pretreatment exhibited results comparable to the application of two layers in a dry state. For all coating systems, increasing changes of colour, roughness, and surface wettability were observed, which differed according to the weathering method. Weathering performance of transparent coating systems was evaluated during both weathering tests by exact measurements, laser scanning microscopy and visual evaluation as well. Total colour difference did not prove to be a sufficient evaluation criterion to indicate coating performance during weathering.
Download PDF
Full Article
Effect of Surface Pretreatment with Natural Essential Oils on the Weathering Performance of Spruce Wood
Eliška Oberhofnerová,* Miloš Pánek, and Martin Böhm
The efficiency of surface pretreatment with natural essential oils relative to the weathering performance of Norway spruce wood was examined. This study investigated the combination of this pretreatment increasing biological resistance and oil-based coating with protection against ultraviolet light and hydrophobic topcoat with silicon nanoparticles during natural and artificial weathering. The coating systems were exposed to 24 months of natural weathering in climatic conditions of Central Europe and 2,016 hours of artificial weathering. The synergistic effect of a coating system based on safflower with essential oils and commercial oil-based coating was the most efficient. The application of one layer of an oil-based coating in a wet state of surface pretreatment exhibited results comparable to the application of two layers in a dry state. For all coating systems, increasing changes of colour, roughness, and surface wettability were observed, which differed according to the weathering method. Weathering performance of transparent coating systems was evaluated during both weathering tests by exact measurements, laser scanning microscopy and visual evaluation as well. Total colour difference did not prove to be a sufficient evaluation criterion to indicate coating performance during weathering.
Keywords: Essential oil; Hydrophobicity; Pretreatment; Spruce wood; Weathering
Contact information: Department of Wood Products and Wood Constructions, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences in Prague, Kamýcká 129, Prague 165 00, Czech Republic; *Corresponding author: oberhofnerova@fld.czu.cz
INTRODUCTION
Norway spruce is traditionally the wood species most often used for construction applications in Central Europe. However, its lower natural durability against bio-attack (EN 350 2016) makes it unsuitable (Meyer-Veltrup et al. 2017) for application in severe environmental conditions (Van Acker et al. 2003) without any protection, as defined for Class 3-5 in EN 460 (1994) standard. When it is left unprotected and used as an exterior material, it loses its original properties and rapidly degrades via weather, fungi, bacteria, or insects. The degradation process caused by abiotic factors, such as solar radiation, water in all its states, temperature, wind, dust, or pollution (Temiz et al. 2005; Evans 2008), is called weathering. The surface is rapidly changed as well by moulds and wood-staining fungi action (Gobakken and Lebow 2010). Unlike decay or insect attack, weathering is typically not a significant factor in the failure of wood components and the collapse of a structure (Yildiz et al. 2011), but it can be a starting point for a decay (Buchner et al. 2018). Generally, weathering is mainly considered as an aesthetic phenomenon (Rüther 2011). It affects surface properties quickly, and as a result, unprotected wood changes colour to grey and acquires a typical rough structure (Feist 1992; Evans 2008; Gupta et al. 2011; Oberhofnerová et al. 2017). All wooden elements should respect the basic principles of construction design. Wood species with higher natural durability can be left unprotected in exterior applications, but species with lower natural durability, such as spruce, should be protected using the proper surface finish (Jones and Brischke 2017). The protective function of an exterior coating generally includes protection against moisture uptake and related dimensional changes, protection against photochemical degradation, and prevention of microbiological degradation (De Meijer 2001; Evans et al. 2015). However, coating systems themselves also undergo degradation caused by several weathering factors (Gobakken and Lebow 2010; Gaylarde et al. 2011; Grüll et al. 2011).
Extending the service life of wood and wood products using natural compounds as bioactives is proving to be an attractive approach for wood protection from the point of view of human health and environmental protection (Tomak et al. 2018). Not every natural compound can be regarded as eco-friendly. The use of all biocidal products is controlled by BPR regulation 528/2012 and requires an authorisation. To extend the service life of wood and maintain its natural look, transparent eco-friendly coating systems with minimal use of harmful chemicals are still at the center of attention (Pospíšil and Nešpurek 2000; Evans et al. 2015). Transparent finishes often contain ultraviolet light (UV) absorbers, stabilizers, biocides, and other additives, but their ability to protect wood from weathering is still limited (MacLeod et al. 1995; Pánek and Reinprecht 2014). Most of them fail within two years of exterior exposure (Evans et al. 2015).
Traditional wood protection methods employ chemicals that are considered toxic and can affect human health and environment, and there is a continuing dilemma concerning how to manage them at the end of their service life (Singh and Singh 2012). Although there are many ways to treat wood surfaces (Evans et al. 2015), multilayer coating systems using primers with commercial fungicides are currently the most-used variants of wood coating systems against weathering (Grüll et al. 2014b). Generally, there is an effort to use ecological surface treatments (Saha et al. 2011) with low volatile organic compound (VOC) content (De Meijer 2001; Miklečić et al. 2017) and to improve the performance of transparent coatings with improving modification agents. The effect of UV absorbers, stabilizers and nanoparticles is investigated in several studies (Evans 2008; Forsthuber and Grüll 2010; Ozgenc et al. 2012; Miklečić et al. 2017; Tomak et al. 2018). Hydrophobic coatings have been tested as a possible method to decrease the degradation of wood (Liu et al. 2013; Samyn et al. 2014; Žlahtič and Humar 2016). One way to increase durability of wood against biodegradation is the use of plant extractives or essential oils (Kartal et al. 2006; Nzokou and Kamdem 2006; Saha et al. 2011; Pánek et al. 2014) for surface pretreatment. The best efficiency has been found for those essential oils that contain phenolic compounds such as oregano, thyme, or clove (Lambert et al. 2001; Chittenden and Singh 2011; Pánek and Reinprecht 2014). They show promising protection against mould growth, but their interactions with coatings during the weathering process need to be investigated (Pánek et al. 2014; Reinprecht and Hulla 2015).
More qualitative parameters of wood coatings during exterior exposure can be evaluated (Grüll et al. 2011, 2014b; Žlahtič and Humar 2016). The change of colour during weathering serves as a basic indicator of the rate of weathering (Van den Bulcke et al. 2008; De Windt et al. 2014; Moya et al. 2017). The most effective protection against photoirradiation and discolouration is given by completely opaque pigmented coatings (De Meijer 2001; Reinprecht and Hulla 2015). In those applications, where wood remains visible, solar radiation will reach the wood surface and induce photochemical degradation of wood underneath the coating (MacLeod et al. 1995; De Meijer 2001).
A large percentage of coating degradation (paint defects, peeling, cracking, etc.) results from moisture changes in wood (Van Acker et al. 2014) and resulting effects on dimensional stability (Williams and Feist 1999; Gaylarde et al. 2011; Evans et al. 2015). The contact angle, which indicates the wettability by water on the exposed surfaces of coated wood, is an important indicator of the rate of weathering (Van den Bulcke et al. 2011; Žlahtič and Humar 2016). The permeability can both decrease (leaching of hydrophilic components) and increase in the initial stadium (formation of micro-cracks) during weathering (Derbyshire and Miller 1996; Oberhofnerová and Pánek 2016). The change in surface roughness and wettability during weathering has been discussed by previous researchers (Gardner et al. 1991; Gupta et al. 2011; Žlahtič and Humar 2016), but there is still a need to explore new protective treatments and test their durability. The achieved improvements of transparent coatings have not yet led to the development of a commercially affordable treatment that meets expectations of exterior durability (Evans et al. 2015; Miklečić et al. 2017).
The degradation of coatings on wood is revealed during service life (Grüll et al. 2011) or through natural (NW) and artificial weathering (AW) tests (Dawson et al. 2008; Van den Bulcke et al. 2008; Grüll et al. 2014b; Žlahtič and Humar 2016; Miklečić et al. 2017). Yet the methods differ greatly in achieved values of surface property changes (Grüll et al. 2014b; Reinprecht and Pánek 2015). Some studies have found no significant correlation between results obtained by natural and artificial weathering (Deflorian et al. 2008; Van den Bulcke et al. 2008; Moya et al. 2017). Laboratory artificial tests can accurately and rapidly predict the durability of coatings (Van den Bulcke et al. 2008; Reinprecht and Pánek 2015), but not all of the factors of weathering and moulding and staining can be recreated in an accelerated chamber (Grüll et al. 2014b). Thus the natural weathering should be used to verify the laboratory testing (Crewdson 2008; Grüll et al. 2014b).
The aim of this study was to evaluate the durability of different transparent coating systems applied on native or pretreated spruce wood during natural and artificial weathering. The combination of safflower oil, clove, and oregano essential oils was used as a surface pretreatment. Other coatings tested were the commercial oil-based coating with UV protection and the hydrophobic top coat on the base of silicon nanoparticles. Different combinations and layerings of these coatings were evaluated in this study. The efficiency of the coatings on the performance of wood was evaluated by confocal laser scanning microscopy and by measuring the discolouration, wettability, and roughness change.
EXPERIMENTAL
Materials
The experiment was conducted using Norway spruce wood (Picea abies L.) from the Central Bohemian Region with a mean density of 510 kg/m3. Each group of coating systems was represented by 14 heartwood samples with the dimensions 375 mm × 78 mm × 20 mm (longitudinal x radial x tangential) for the natural weathering test and by 4 samples with the dimensions of 310 mm × 78 mm × 20 mm (longitudinal × radial × tangential) for the artificial weathering test. The samples were sanded with the sandpaper grit of 120, and the top ends were sealed by silicon to prevent water uptake. The samples were conditioned in laboratory conditions (20 ± 2 °C and 65% RH) before coating application and subsequently before each measurement.
Surface pretreatment and coating application
The surface pretreatment (the compound of essential oils – clove and oregano – in 5% solution of safflower oil), commercial oil-based paint UV OSMO 420 (Osmo Holz und Color GmbH & Co. KG, Warendorf, Germany), and the hydrophobic treatment based on silicon nanoparticles PMO 62 (HF Servis Ltd., Plešnice, Czech Republic) were applied by brush on the exposed radial surfaces of samples. The specifications and spreading rates based on producer recommendation are listed in Table 1. The list of transparent coating systems (A-F) with initial properties before weathering is given in Table 2.
Table 1. Specification and Spreading Rates of Used Treatments
Table 2. Coatings Systems (A-F) and their Initial Properties Before Weathering
Natural Weathering (NW)
The NW test was performed at Suchdol, Prague (50° 07′ 49.68″ N, 14° 22′ 13.87″ E, elevation above sea level 285 m) and lasted 24 months from December 15, 2014, to December 15, 2016. The samples were exposed outdoors, at a 45 ° inclination, with radial surface facing south and placed approximately 1 m above the ground according to EN 927-3 (2006). At the 17thmonth of outdoor exposure, the samples were hit by hail approximately 3 cm in size. Overview of the climatic conditions during 24 months of weathering can be seen in Table 3.
Table 3. Overview of Climatic Conditions at the Test Site during the Exposure
Artificial Weathering (AW)
The AW test was performed in laboratory using the QUV chamber with UVA fluorescent lamps (Q-Lab, Westlake, USA), which simulated exterior conditions according to EN 927-6 (2006). One cycle held for one week, which consisted of a condensation phase with a temperature of 45 °C (for 24 h) and the phases of UV radiation with the wavelength of λ = 340 nm and intensity of radiation 0.89 W/(m2/nm) in the temperature of 60 °C (for 2.5 h) and water spraying with intensity of 6.5 L/min (for 0.5 h). The samples were cycled for 12 cycles (2016 h) under these conditions, which approximately corresponded to conditions during 24 months of natural weathering test (see above) based on the total UV irradiance during both weathering methods, as in the study of Fedor and Brennan (1996).
Methods
Colour Measurements
Colour parameters L*a*b* were determined according to the Commission International de l´Eclairage (CIE 1986) before weathering, and after 3 months, 6 months, 9 months, 12 months,18 months, and 24 months of NW, as well as after 168 h, 504 h, 1008 h, 1512 h, and 2016 h of AW. The colour characteristics were measured at the same measuring points (8 per sample) using a spectrophotometer 600d (Konica Minolta, Tokyo, Japan). The device was set to an observation angle of 10 °, d/8 geometry, and D65 light source. The total colour difference (E) was calculated according using Eq. 1,
(1)
where L* is the lightness from 100 (white) to 0 (black), a* is the chromaticity coordinate from -60 (green) to + 60 (red), b* is the other chromaticity coordinate from -60 (blue) to +60 (yellow), and ΔL*, Δa*, and Δb* represent the differences between L*, a*, and b* values before and after weathering.
Surface Wetting Measurements
The sessile drop method with static contact angle measurement (without external interference) was performed using the methodology of Bastani et al. (2015). The wettability measurements were taken using a goniometer Krüss DSA 30E device (Krüss, Hamburg, Germany) on radial surfaces of wood samples before weathering and after 3 months, 6 months, 9 months, 12 months, 18 months, and 24 months of NW as well as 2016 h of AW. Twenty measurements were taken for each sample, with distilled water drops with a dosing volume of 5 μL. The contact angle values were determined after 5 s of drop deposition. The phenomena of spreading and absorption of drops on the wood surface was investigated via variations of the weathering method, time, and coating system.
Surface Roughness Measurements
The surface roughness (arithmetic average of roughness value Ra) was determined according to ISO 4287 (1997) and ISO 4288 (1996) using a profilometer Talysurf Form Intra (Taylor-Hobson, Berwyn, USA). The measurements were taken in four traversing lengths oriented perpendicularly to the length of the samples over the radial surface before weathering and after 3 months, 6 months, 9 months, 12 months, 18 months, and 24 months of NW and 2016 h of AW.
Confocal Laser Scanning Microscopy and Visual Evaluation
To evaluate the surface degradation of coatings, the samples were regularly scanned using a Canon 2520 MFP scanner with 300 DPI resolution (Canon, Tokyo, Japan) and analysed using a confocal laser microscope Lext OLS4100 (Olympus, Tokyo, Japan). The cracks and degree of surface degradation were evaluated before and after weathering using both laser scanning microscopy and visual evaluation on the base of EN ISO 4628 (2003).
Statistical analysis
The statistical evaluation was performed with Statistica 12 software (Statsoft, Palo Alto, CA, USA) and MS Excel 2013 (Microsoft, Redmond, WA, USA) using mean values, standard deviations, analysis of variance (ANOVA), and the Tukey Unequal N HSD multiple comparison test.
RESULTS AND DISCUSSION
Colour Changes
In the ANOVA, the effect of coating system, weathering time, and their interaction on investigated properties (total colour difference, colour parameters, roughness, and contact angle value) was evaluated as statistically significant after both weathering tests (p < 0.01). Total colour difference (∆E*) indicated the coating durability during weathering, as stated in other studies (Van den Bulcke et al. 2008; De Windt et al. 2014; Moya et al. 2017). Using the multiple comparison test, the effect of surface pretreatment on the total colour difference value was evaluated as statistically insignificant after NW (p = 0.98), but as statistically significant after AW (p < 0.01) by comparing the coating systems (B) and (E).
Based on the graphs (Fig. 1), both weathering tests caused an increase of colour differences. High increases were noted during the initial part of weathering, which corresponded with other studies (Sharratt et al. 2009; Ozgenc et al. 2012; Reinprecht and Pánek 2015). However, during the next phase of NW, the discolouration due to the presence of mould, dirt, and dust in porous structure of wood was observed, corresponding with the study of Evans (2008) and Gaylarde et al. (2011). These factors caused a higher discolouration of coating systems (A) and (D) (based on safflower and essential oil mixture). The lower durability of these coatings had not been revealed during AW, for which all of the coatings reached similar total colour difference values (∆E*).
The overall changes of the total colour differences ∆E* of coatings based on safflower and essential oils only, (A) and (D), were in the range 47 to 48 after NW and 23 to 24 after AW. For the rest of the coating systems, the changes of total colour difference were in the range of 11 to 13 after NW and 17 to 24 after AW.
Fig. 1. Total colour difference of coated spruce wood during 24 months of natural weathering (a) and 2016 h of artificial weathering (b); ANOVA results representing the 95% confidence interval in vertical bars
Further investigation of colour parameters L*, a*, and b* showed the differences between the durability of the tested coating systems. The statistically significant effect of surface pretreatment on the colour parameters L*, a*, and b* had been observed after NW and AW (p < 0.01). Generally, the increase of values a* and b* shows a tendency for the surface to turn reddish and yellowish, respectively, and the increase of L* indicates a tendency to turn into light colour (Temiz et al. 2005, 2007; Tolvaj and Mitsui 2005; Reinprecht and Pánek 2015). Based on the graphs (Fig. 2), the performance of UV-protective commercial oil-based coating systems (B, C, E, F) significantly differed from those coatings without this protection (A, D). All coating systems with UV protection layers (B, C, E, F) were characterized by decreased values of lightness, increased values of parameter a*, and balanced values of parameter b* during both weathering tests (Fig. 2), which corresponded well with the study of Baysal et al. (2014).
This result indicated that decomposed lignin (Teacă et al. 2013) and wood extractives (Pandey 2005) were protected against leaching by the protective coating layers. The coating systems based only on safflower with essential oils (A and D) acted differently. Both parameters a* and b* decreased during weathering, and lightness was the only exception (Fig. 2). The difference was caused by the presence of dust and moulds in the exterior (L* value decrease), which pollute the degraded surface of wood (Evans 2008). This cannot be simulated in a UV chamber (Dawson et al. 2008; Ozgenc et al. 2012). Only the degradation of lignin and extractives, observed as initial decreased L* values (Pandey 2005; Teacă et al. 2013), and their leaching by distilled water measured as increased L* values (Sudiyani et al. 1999) was observed during AW (Fig. 2).
Fig. 2. Changes of colour parameters L*, a*, and b* of coated spruce wood during NW (a, c, and e) and AW (b, d, and f); ANOVA results representing the 95% confidence interval in vertical bars
Surface Roughness Changes
The surface roughness value Ra*, based on ISO 4287 (1997), was the second investigated parameter that indicated the performance of coating during weathering (Gardner et al. 1991; Gupta et al. 2011). The effect of coating system, weathering time, and their interaction on surface roughness was evaluated as statistically significant after NW and AW (p < 0.01). Based on the graphs (Fig. 3), the performance of the coating system based only on safflower and essential oils (A, D) differed from the rest of the coating systems (increasing roughness). There was no significant effect of surface pretreatment on the surface roughness during NW (p = 1.00) and AW (p = 0.99) when the coating systems (B) and (E) were compared using the multiple comparison test. The increased roughness indicated a lower coating durability (Yalcin and Ceylan 2017) and an advanced state of surface degradation, which became even more apparent after 18 months of NW, when the samples were hit by hail. The wood surfaces exposed to AW contained several cracks and splits, which was also observed in other studies (Temiz et al. 2005; Miklečić et al. 2017). The overall percentage changes of the surface roughness ∆Ra* of coatings (A) and (D) were in the range of 481 to 576 after NW and 292 to 427 after AW. For the rest of the coating systems, the percentage changes of surface roughness were in the range of 45 to 113 after NW and 21 to 95 after AW.
Fig. 3. Change of roughness of coated spruce wood during natural (a) and artificial weathering test (b); ANOVA results representing the 95% confidence interval in vertical bars
Surface Wettability Changes
The ANOVA showed the effect of the coating system, weathering time, and their interaction on the wettability as statistically significant after NW and AW (p < 0.01). The expected positive effect of the hydrophobic top layer on the surface wettability was observed only after its application, which was also recorded in studies of Liu et al. (2013) and Samyn et al.(2014). However, after NW and AW, there was no statistical significance (p = 0.91; p = 0.90, respectively), when comparing the coating systems (E) and (F) using the multiple comparison test. Based on the graphs (Fig. 4), the wettability of all the tested coating systems increased, i.e., contact angle decreased, during both NW and AW, as in the study of Žlahtič and Humar (2016). The same trend of coating systems (A) and (D) was observed – the contact angle after exposure decreased to minimal values and the surface reached full wettability. The effect of surface pretreatment on the contact angle was noted as statistically significant after NW (p = 0.02) and insignificant after AW (p = 0.39) by comparing the coating systems (B) and (E). The rest of these coating systems were characterized by the similar decrease of contact angle during both weathering procedures. The overall percentage changes of the contact angle ∆CA* value of coatings (A) and (D) were approximately -100 % after NW and in the range of -100 to -91 after AW. For the rest of the coating systems, these percentage changes were in the range of -31 to 36 after NW and -57 to -46 after AW.
Fig. 4. Surface wettability, i.e. contact angle values, of coated spruce samples during natural (a) and artificial weathering (b); ANOVA results representing the 95% confidence interval in vertical bars
Visual Evaluation
The coating performance during NW was also evaluated visually in accordance with other studies (Grüll et al. 2011; De Windt et al. 2014). An initial visual inspection confirmed that weathering caused discolouration and degradation at natural outdoor and laboratory conditions as observed in other studies (Gupta et al. 2011; Pánek and Reinprecht 2014). The disruption of coating systems prevents the degradation of underlying wood (Evans 2015). The different performance of coating systems based only on pretreatment by safflower and essential oils (A, D) was visible already after 3 months of weathering when these samples became darker (Fig. 5). After 6 months of outdoor exposure, the samples were already grey with the beginning of mould growth, as in the study of Žlahtič and Humar (2016). After 18 months of weathering, the presence of mould on these coating systems had become obvious. The efficiency of essential oils layer only, as natural additives against biologic factors, proved to be low. They were leached from wood by water during exterior exposure (Pánek et al. 2014). During the 17th month of exposure, the samples were hit by hail, which caused mechanical damage of surface coatings that led to fungal staining, which was also recorded in the study of Uiterwaal and Blom (1973). The most affected were the coatings without the surface pretreatment (E and F), which showed degradation of disrupted coating in the form of greying, peeling, and formation of mould and fungi (Fig. 5). This was further investigated by confocal laser scanning microscopy (Fig. 6 and 7). Chalking was not considered because nonpigmented coatings were used (Dawson et al. 2008). The different performance of (A) and (D) coating systems in comparison with the rest of the systems is demonstrated in Fig. 5. More cracks were revealed during AW than NW (Grüll et al. 2014b; Miklečić et al. 2017), due to the shorter reaction time of wood to adapt to climatic changes and the associated dimensional changes during AW.