Abstract
This paper utilized 12 coating systems, based on an acrylate and a hydrophobic polymer, with the addition of light pigments, nano-sized polyvalent metal (AsS-chelate complex) for ultraviolet protection, and iodopropynyl butylcarbamate fungicide. This study deals with the impact of the number of coats on the color stability and the surface defects of painted black locust (Robinia pseudoacacia L.) and Norway spruce (Picea abies Karst L.) woods after up to three years of natural weathering, at a slope of 45°. The best coating system was created from three coats, which consisted of two pigmented acrylates (PerlColor) and one transparent hydrophobic water-repellent (AquaStop). The total color change, ΔE*, of the weathered surfaces was approximately two times lower when the application involved a pigmented coating system compared with a transparent one. The color stability of the surfaces and their resistance to defects was better when the coating system was applied to black locust wood compared with spruce wood. Smoother surfaces of wood before painting resulted in a higher resistance against cracking and other defects caused by natural weathering; however, the effect of the initial wood roughness on the color stability of painted woods during natural weathering was usually negligible.
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Effect of the Number of UV-Protective Coats on the Color Stability and Surface Defects of Painted Black Locust and Norway Spruce Woods Subjected to Natural Weathering
Miloš Pánek,a and Ladislav Reinprecht b
This paper utilized 12 coating systems, based on an acrylate and a hydrophobic polymer, with the addition of light pigments, nano-sized polyvalent metal (AsS-chelate complex) for ultraviolet protection, and iodopropynyl butylcarbamate fungicide. This study deals with the impact of the number of coats on the color stability and the surface defects of painted black locust (Robinia pseudoacacia L.) and Norway spruce (Picea abies Karst L.) woods after up to three years of natural weathering, at a slope of 45°. The best coating system was created from three coats, which consisted of two pigmented acrylates (PerlColor) and one transparent hydrophobic water-repellent (AquaStop). The total color change, ΔE*, of the weathered surfaces was approximately two times lower when the application involved a pigmented coating system compared with a transparent one. The color stability of the surfaces and their resistance to defects was better when the coating system was applied to black locust wood compared with spruce wood. Smoother surfaces of wood before painting resulted in a higher resistance against cracking and other defects caused by natural weathering; however, the effect of the initial wood roughness on the color stability of painted woods during natural weathering was usually negligible.
Keywords: Coatings; Wood; Weathering; UV-protections; Color stability; Surface defects
Contact information: a: Czech University of Life Sciences Prague, Faculty of Forestry and Wood Sciences, Kamýcká 129, 165 21 Prague 6 – Suchdol, Czech Republic; b: Technical University in Zvolen, Faculty of Wood Sciences and Technology, Masarykova 24, 960 53 Zvolen, Slovak Republic;
* Corresponding author: panekmilos@fld.czu.cz
INTRODUCTION
Wood for exterior use is degradable by biotic and abiotic agents (Hon and Feist 1986; Van Acker et al. 2003; Alfredsen et al. 2007). These agents may result in a reduction of wood’s mechanical and physical properties, as well as micro-cracks, discoloration, and other aesthetical defects in the wood’s surface (Feist 1982, 1990; Evans 2008). Suitable types of pigments (Reinprecht and Pánek 2013) or UV-absorbing additives in transparent coating systems can stabilize the color of painted solid wood and are known to decrease or slow down the degradation on its surface (De Meier 2001; Gobakken and Lebow 2010; Syvrikaya et al. 2011; Forsthuber et al. 2013; Pánek and Reinprecht 2014). Currently, discovering and testing the best coating system for individual applications is of interest in the field (Oltean et al. 2010; Grüll et al. 2011, 2014; Sandak et al.2013). Additionally, the health and ecological aspects of a color coating system with a low emission of volatile organic compounds (VOC) is important (De Meier 2001; Tesařová et al.2010).
The degradation of the coating system on wood’s surface differs depending on the climate conditions (Creemers et al. 2002; Valverde and Moya 2014), wood species, and other factors. Therefore, it is necessary to test selected coatings on various woods in different regions. A summary of such results can provide general information about the stability, aesthetics, preservation effects, and component additives that improve the specific properties of the coating. The weathering index rate (Wind from 0 to 1000) predicted on the base of chemometric models of mid infrared spectra can be useful method for evaluation of climate effects at modelling the weathering kinetics of wood or coatings (Sandak et al. 2015).
Previous work has shown that the stability and durability of coatings on painted wood can be improved by increasing their total thickness, e.g., by using a primer layer, a top-coat layer, and a final, water-repellent layer, or using more layers of the same coating (Masaryková et al. 2010). The stability of the coatings on a treated wood surface can be improved by the addition of hydrophobic agents, UV-additives, or other specific compounds (De Meier 2001; Ghosch et al. 2009; Grüll et al. 2011; Samyn et al. 2014). From an aesthetic and functional durability point of view, the effect of different wood species (Ozgenc et al. 2012; De Windt et al. 2014) and the effect of the initial surface roughness of the wood before application of coatings (Ozdemir and Hiziroglu 2009; Scrinzi et al. 2011; Reinprecht and Pánek 2015) have been previously documented. These effects depend on other specific conditions of the coating system, i.e., the type and number of coating layers, pigments and UV-additives, and climatic conditions. The aforementioned conditions have been continuously studied to investigate and validate current systems.
Spruce is traditionally and the most often used wood species for houses, bridges, and other exterior constructions in the Central Europe. However, its lower natural durability (class 4 for decaying fungi according to the standard EN 350-2 (1997)) and poor permeability to liquid preservatives (Usta 2005; Pánek and Reinprecht 2008) make it unsuitable for application in severe environmental conditions. Black locust wood also has poor permeability; however, in addition it is significantly more durable against biological agents and has a pleasant color and surface appearance. Currently black locust is commonly chosen for exterior construction projects in the Central Europe. It is often used as a substitute for tropical wood species for decking or garden furniture. In this view, it is necessary to test different modern coating systems applied on surfaces of Norway spruce and black locust wood and compare obtained results from various climatic regions.
The aim of this work was to investigate the effect of the number of coats on the color change and other aesthetic defects on the surface of Norway spruce and black locust woods painted with a UV-protected coating system. The coating system contained nano-sized polyvalent metal (AsS-chelate) complex or also pigments, both as UV-protection. The wood was subjected to outdoor weathering lasting 36 months. In contrast to the study of Reinprecht and Pánek (2015), the coating systems used in this work did not contain a transparent primer layer; so wood was immediately treated with differently pigmented top-coat layers. The following kinds of layering were used: (1) one top-coat layer, (2) two top-coat layers, (3) one top-coat layer and one final water-repellent layer, or (4) two top-coat layers and one final water-repellent layer. The effects of the initial wood roughness on the surface quality of painted woods after weathering were also analyzed.
EXPERIMENTAL
Wood
Wood samples with dimensions of 375 × 78 × 20 mm (axial × radial × tangential) were prepared from black locust (Robinia pseudoacacia L.) and Norway spruce (Picea abies Karst L.) naturally dried boards, in accordance with the standard EN 927-3 (2006). The top surface of each sample had two different areas of roughness, with lengths of 187.5 mm. The surface area ground with 60-grit sandpaper was referred to as rough (R), and the surface area ground also with 120-grit sandpaper was referred to as smooth (S). Both transverse sections (78 x 20 mm) of the sample were treated with silicone for water-resistance and its other non-exposed sides with transparent UV-protective coating.
Coating Systems and their Application
Wood samples were painted with one of 12 coating systems (Table 1). They did not contain a primer layer usually recommended by producers, because the aim of this study was to search stability of coatings without this anchoring paint. The samples were immediately painted with one or two layers of a top-coat (PerlColor) in three pigmentations (T = transparent, P = pine, and L = larch). Then, one-half of the samples were treated with a single water-repellent (AquaStop) layer. All of these coatings were manufactured by the Böhme AG Farben & Lacke Co. (Switzerland).
Table 1. Color Components L*, a*, and b* of “Rough” (R) Reference and Painted Norway Spruce and Black Locust Samples before Natural Weathering
AS: AquaStop; pigments in the top-coat PerlColor layer: T = transparent, P = pine, L = larch; number of layers: (1x) = one layer; (2x) = two layers of PerlColor. The values represent a mean ± (SD) of 18 measurements. Wood samples with initially smooth surfaces (grinded with 120-grit sandpaper) exhibited very similar initial color components as in table present rough samples grinded with 60-grit sandpaper
PerlColor is a top-coat containing acrylate resin modified with oils, a nano-sized polyvalent metal AsS-(arsinoaryltio)-chelate complex for UV protection, hydrophobic additives, iodopropynyl butylcarbamate fungicide, and pigments on the base of iron oxides. AquaStop is a transparent, water-repellent, and sunblock coating containing the nano-sized metal AsS-chelate. Individual layers of both coatings were deposited with a manual low-pressure air spraying technique at 120 ± 10 g.m−2. Before the other layers of coatings were applied, the previously painted samples were sanded with 240-grit sandpaper. Between consecutive coatings, the samples were dried for 24 h at 20 °C and 65% relative humidity.
Natural Weathering
Natural outdoor weathering of the wood samples took place under metal stands at a slope of 45°, oriented to the South, 300 m above sea level, according to the standard EN 927-3 (2006). The exposure field is situated in a valley of the industrial town, Zvolen, with many foggy days, smog, and high temperature differences between summer (35 °C) and winter (-25 °C). The mean climatic conditions of the testing area are listed as follows: mean temperature of 9.4 °C; relative humidity of 83%; 700 mm/year of precipitation; and 1100 kWh/m2 of irradiation. Three replicates for each type of coating system for each wood species were exposed to weathering for 0, 6, 12, 24, and 36 months.
Color and Surface Defect Measurements
The color parameters of the individual wood samples before and after weathering (6, 12, 24, and 36 months) were measured with a CR-10 Tristimulus colorimeter using CIE Standard Illuminant D65 and CIE 10° Standard Observer (Konica Minolta Inc., Japan). For each type of coating system (12 total), the wood species (Norway spruce and black locust), surface roughness (rough or smooth), the color parameters were recorded from six sites, using three replicates, for a total of 18 measurements. The evaluation of color change was done using the L*a*b* color system, based on the L*, a*, and b* components (CIE 1986): where L* is the lightness from 100 (white) to 0 (black), a* is the chromaticity coordinate (+ red; – green), and b* is the other chromaticity coordinate (+ yellow; – blue). The total color difference of the samples, between weathered and initial state, E*, was calculated using Eq. 1:
(1)
The surface quality of the painted samples during weathering (after 12, 24, and 36 months) was analyzed visually using a magnifier at 10X magnification. The aesthetic defects in the surface of the painted samples were measured according to methodology by Van Acker et al. (1992) (De Windt et al. 2014; Table 4).
Statistical Analyses
The data were analyzed using the software program, STATISTICA 12 (StatSoft CR, Czech Republic). Duncan’s tests were used to compare differences in the means, and the data was tabulated using Microsoft® Excel 2013 (Redmond, WA, USA). The data are represented as the mean value and the associated standard deviation (SD).
RESULTS AND DISCUSSION
Color Stability of Painted Woods after Weathering
The color stability of painted black locust and Norway spruce was increased to a statistically significant degree because of the presence of light pine and light larch pigments in the PerlColortop-coat layer (Tables 2 and 3). On average, the effect of natural weathering on the total color difference, ΔE*, of wood surfaces painted with the pigmented coating systems was approximately two times lower than those painted with transparent coats. However, it should be noted that the effect of the pigment in the top-coat was significantly influenced by the number of coats, (the effect was evidently higher for two versus one coat), wood species (for black locust was higher than for Norway spruce), as well as by the positive supporting effect of the final water-repellent, AquaStop (Table 2). The influence of the initial roughness of the wood surface before painting (rough R compared with smooth S) on the color stability of painted wood during natural weathering was not statistically significantly in most cases, regardless of the wood species, pigmentation of coatings, and the number of coats (Table 2, see Duncan’s tests).
A positive effect of the light pine and light larch pigments was confirmed by Duncan’s tests (Table 3, see effect No. 1), showing that the effect of pigments was more apparent after 36 months of weathering compared to 6 months of weathering. This result agrees with the previous results by Evans and Chowdhury (2010) and Grüll et al. (2011). An affirmative effect of pigments on the color stability of painted wood samples after 36 months of weathering was not demonstrated when only one-layer of PerlColor was applied; a complete degradation of transparent and also pigmented coating systems occurred when the system included PerlColor-T(1x), PerlColor-P(1x), and PerlColor-L(1x) (Tables 2 and 3).
An improved long-term color stability was demonstrated for both transparent and pigmented top-coats if they were applied in two layers, i.e., PerlColor(2x). The best color stability was determined for coating systems in which two layers of pigmented PerlColor were combined with the final water-repellent, AquaStop (AS), i.e., systems with PerlColor-P(2x) + AS and PerlColor-L(2x) + AS (Tables 2 and 3). A positive effect of more layers of pigmented coatings on the color stability of painted wood was manifested mainly in the final phase of outdoor weathering, after 36 months (Table 2), and it was also confirmed by the Duncan’s test (Table 3, see effect No. 2).
The color stabilization effect of the final AquaStop layer was different for black locust compared with Norway spruce samples (Tables 2 and 3). This water-repellent layer had a statistically significant effect in stabilizing the colors of black locust wood that was previously painted with the pigmented, PerlColor coatings. However, a positive color stabilization effect of AquaStop on the Norway spruce samples was evident during the application of both the transparent and pigmented PerlColor coatings. These results were confirmed by Duncan’s tests (Table 3, see effect No. 3).
The painted surfaces of wood treated with AquaStop were more resistant to defects from weathering. This water-repellent layer apparently increased the stability of the coating systems against surface defects (Table 4). Generally, this hydrophobic layer extended the durability of painted wood during weathering. This result agrees with Ghosch et al. (2009) and Samyn et al.(2014), as hydrophobic layers reduce a synergistic degradation effect of UV-radiation and water (Owen et al. 1993; Sudiyani et al. 1999).
Table 2. The Total Color Differences ΔE* of Painted and Reference Black Locust and Norway Spruce Samples During Natural Weathering from 6 to 36 Months
S: smooth; R: rough; AS: AquaStop. Values represent a mean of 18 measurements with the associated standard deviation in parentheses.
Duncan’s tests (effect of initial roughness of wood) were evaluated at the (a) 99.9% significance level, (b) 99% significance level, and (c) 95% significance level; when no significant difference was detected at the P < 0.05 significance level, values were assigned (d)
Table 3. Duncan’s Test of the Total Color Differences E* of the Norway Spruce (Picea abies) and Black Locust (Robinia pseudoacacia)
a): smooth samples/ 6th month of weathering; b): smooth samples/ 36th month of weathering.
Evaluated Effects: (No. 1) Presence of Pigment in Paints, i.e., Pigmented versus Transparent; (No. 2) Number of PerlColor Layers, i.e., 2-layers versus 1-layer; (No. 3) Using of Final AquaStop, i.e., Use versus Non Use; and (No. 4) Wood Species, i.e., Black Locust versus Norway Spruce. Values represent a mean of 18 measurements with the associated standard deviation in parentheses. Duncan´s tests for effects No. 1 to No. 4: (a) 99.9% significance level, (b) 99% significance level, (c) 95% significance level; when no significant difference was detected at the P < 0.05 significance level, values were assigned (d)
Out of the two wood species, the black locust was a better substrate for maintaining the original color of painted samples during natural weathering; e.g., after 36 months, the total color difference (ΔE*) ranged from 2.2 to 38.0 and 14.1 to 45.2 for painted black locust and Norway spruce samples, respectively (Table 2). Results were confirmed using Duncan’s test (Table 3, see effect No. 4).
The influence of the duration of outdoor weathering on the color stability of painted woods was also shown in this work. Results from the Duncan’s tests showed a higher statistical influence of all coating system effects, No. 1 through No. 4, for the 36th month compared with the 6th month of weathering (Table 3).
Fig. 1. Changes in the color parameters, ΔL*, Δa*, Δb*, and ΔE* for smooth wood samples treated with two layers of PerlColor(2x) during outdoor weathering from 6 to 36 months.
Note: The other types of coating systems (see Table 2) painted on the smooth or rough wood surfaces obtained similar color changes in ΔL*, Δa*, Δb*, and their total color difference, ΔE*, is presented in Table 2
In summary, wooden structures, exposed to exterior weathering, required painting with a coating systems that had enough thickness, because during outdoor exposure the coatings are gradually degraded (Masaryková et al. 2010; Grüll et al. 2014). Clearly, the positive influence of more layers of coating on the color stability, i.e., two layers of PerlColor or two layers of PerlColorcombined with one final layer of AquaStop, was found to be beneficial in this work. The coating thickness was more important for the color stability at the end of age testing (after 36 months), independent of wood species (Table 3, see effects No. 2 and No. 3).
The color stability results of painted wood are shown according to changes in the individual color components, ΔL*, Δa*, and Δb*, and their influence on the total color difference of painted and weathered woods. Changes in ΔL*, Δa*, and Δb*, together with ΔE* for painted black locust and Norway spruce samples, are shown for one selected type of coating system, i.e., the PerlColor(2x) (Fig. 1).
Surfaces of both wood species, painted with the other eleven types of coating systems, exhibited similar trends in color change during weathering. In all cases, the influence of ΔL* and Δb* on the total color difference ΔE*of wood painted with the transparent coatings was observed. Because of a slight UV-inhibition effect of transparent coatings for the lignin degradation in wood surfaces (Hon and Chang 1984; Teacà et al. 2013), and the penetration of dust particles into surface layers of damaged coatings (Evans 2008), there in this experiment was a darkening (decrease of L* after ageing), bluing (decrease of b*), and transition from reddening to greening (firstly increase and then decrease of a*) occurrence to the wood (Fig. 1). When using transparent coatings, the darkening was manifested more significantly for the Norway spruce, which in a native state is lighter than the black locust; while changes of the chromaticity coordinates (a*; b*) were quite similar for both painted wood species (Fig. 1).
Surface Defects in Painted Woods during Weathering
Similar to the color stabilization effects, more layers of coating resulted in the best suppression of surface defects in painted wood during natural weathering (Table 4). A positive influence of the coating thickness was very important, especially after 12 months when compared with 2 to 3 years (Table 4).
A visual assessment of the defects created on painted surfaces showed a higher resistance when the coating systems were applied to black locust wood, especially for the pigmented coatings. The best coating systems were PerlColor-P(2x) and PerlColor-L(2x), in combination with the final, water-repellent layer, AquaStop (Table 4). A possible explanation that there was a more significant worsening of adhesion from the coatings on Norway spruce wood, which has a different anatomical and chemical structure than black locust wood. Another explanation is in the kinetics of water sorption, because spruce wood has a faster sorption and desorption of water, and therefore also a worse dimensional stability in comparison to black locust wood in wet conditions as was shown by Van Acker et al. (2014). However, these explanations are only hypotheses because tests of adhesion before and during weathering were not conducted in this experiment. Generally, a longer weathering duration caused more intensive destruction of the coatings (Table 4).
For both of the wood species, a positive effect of a finer grinding method (smooth initial surface), to measure of visual defects formed during outdoor exposure, was confirmed. Unlike results of color fastness evaluations usually carried out independently on different initial wood roughness, this result confirmed the effect of superior surface quality of wood for a longer period of coating quality, in accordance with Scrinzi et al. (2011) and Slabejová et al. (2014).
Table 4. Visual Rating of Defects on Painted Smooth or Rough Black Locust and Norway Spruce Woods after Natural Weathering
Values represent a mean of 6 replicates. The evaluation was based on the level of degradation: i.e., 0=none; 2=small aesthetical changes; 4=mild (easy to retreat); 6=moderate (maintainable); 8=striking (maintenance is difficult); 10=advanced (maintenance coat cannot restore the defects). De Windt et al. (2014)
Generally, the pigmented coating systems were, to a statistically significant degree, more resistant against surface defects than transparent ones, regardless of the wood species. This result, together with a better color stability of wood painted with pigmented coatings, meant that the use of light pigments in coatings is ideal for their application in harsh outdoor conditions that require the longest possible lifespan (Tables 2 and 4).
CONCLUSIONS
- Smaller color changes and less important surface defects in painted wood surfaces were observed when using more layers of coating. This also included coating systems containing light pine or light larch pigments.
- Color stability and resistance to surface defects was higher when the coating systems were applied on the black locust wood compared with Norway spruce wood.
- The color stability of the coatings was not usually influenced by a different initial roughness of the wood; however, on rougher woods the creation of more important surface defects on paintings was visually observed during weathering.
- A positive effect of the final water-repellent, AquaStop, layer in the coating systems improved the color stability and durability of painted woods, primarily in combination with pigmented top-coats.
- A sufficient time for exterior testing of coating systems is necessary, for example 1 or 3 years. Six months of outdoor ageing was generally not enough time for an accurate comparison of different wood factors, e.g., wood species and wood roughness, and different coating factors, e.g., pigment in coatings, number of coats, and the use of water-repellent layer, on the color stability and long-term quality of coatings on painted woods.
ACKNOWLEDGMENTS
This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0200-12, and by the Grant Agency of the Faculty of Forestry and Wood Sciences in Prague, Projects No. A07/15 and A31/16. The authors would also like to thank Ms. S. Bagócsiová for her technical assistance.
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Article submitted: February 3, 2016; Peer review completed: March 4, 2016; Revised version received: March 23, 2016; Accepted: April 1, 2016; Published: April 12, 2016.
DOI: 10.15376/biores.11.2.4663-4676