Accelerated Weathering of Coated and Uncoated Beech Wood Modified with Citric Acid

Chemical modifi cation of wood can minimize wood drawbacks in exterior application, such as moisture absorption, swelling and shrinkage, susceptibility to photodegradation and microbial attack. We modifi ed a beech wood with 7.0 % water solution of citric acid and 6.5 % sodium-hypophosphite monohydrate (SHP) as a catalyst and investigated the color stability and weathering performance of coated and uncoated wood under accelerated weathering conditions. The modifi ed and unmodifi ed beech wood samples were coated with transparent water-borne stain and transparent solvent-borne stain, and with reference stain. The accelerated outdoor exposure was conducted in a QUV weathering tester (Q-Panel Company) equipped with UVA-340 fl orescent lamps for 56 days (1344 hours). The surface of samples was examined for color and gloss changes, adhesion and appearance of fl aking, cracking, blistering and chalking. The overall color change (ΔE*) of coated beech wood samples at the end of exposure was smaller in unmodifi ed wood samples. The trend of gloss changes of modifi ed and unmodifi ed wood samples was very similar during the exposure. At the end of exposure unmodifi ed beech wood samples exhibited better weathering performance than modifi ed beech wood samples. In order to achieve the optimal protection of wood modifi ed with citric acid the new type of fi nishes should be developed.


INTRODUCTION 1. UVOD
In exterior application wood is susceptible to weathering and attack by microorganisms.Weathering is the general term used to defi ne the slow degradation of materials exposed to the weather (Williams, 2005).The weathering process often results in discoloration, a physical deterioration of the wood surface, and loss of paint-retaining properties.Sunlight (especially UV and visible light) and water play a major role in weathering of wood.
The UV light causes photochemical degradation mainly in lignin polymer in the cell wall.As the lignin is degraded, water leaches out degradation products and washes away loosened surface cellulose fi bers, causing a rough surface.Water also causes the wood to swell, and upon drying, checks and cracks develop that expose new material to UV degradation (Feist et al. 1991).Wood can be chemically modifi ed to minimize specifi c problems such as moisture absorption, swelling and shrinkage, as well as susceptibility to photodegradation and microbial attack.It has been established that chemical modifi cation of wood can also infl uence the behavior of wood under weathering and improve the performance of coatings (Plackett et al, 1992; Beckers et al, 1998; Evans et al, 2000;Xie et al, 2005Xie et al, , 2006Xie et al, , 2008;;Tomažič et al, 2004;Temiz et al, 2007).Modifi cation of fi r and beech wood with citric acid has been shown to improve the dimensional stability and biological durability (Bischof Vukušić et al, 2006;Despot et al, 2008;Šefc et al, 2009).The citric acid crosslink with wood and enhance dimensional stability of the modifi ed wood, which might help to improve the coating performance because the coating should be less stressed by movements of the substrate.However, the modifi cation of wood may have an impact on wood wettability and adhesion of coatings (Podgorski et al, 2000;Hakkou et al, 2004;Petrič et al, 2007).The aim of this preliminary research was to investigate the color stability and weathering resistance of coated and uncoated wood modifi ed with citric acid under accelerated weathering conditions.
The samples were coated with two commercial wood stains: transparent water-borne stain (mark: W) and transparent solvent-borne stain (mark: S), and with reference stain (mark: R) according to HRN EN 927-3 provided by Belinka Belles d.o.o.All coatings were applied on wood manually by brush in three layers with a 24 hour drying time between layers.The amount of applied water-borne stain was 90 g/m 2 per layer and of solvent-borne and referent stains 50 g/m 2 per layer.The average dry fi lm thickness was 50 μm for the waterborne and referent system, and 60 μm for the solventborne system.The dry fi lm thickness was measured at fi ve positions on two samples for each coating.
The accelerated outdoor exposure was conducted in a QUV weathering tester (Q-Panel Company) equipped with UVA-340 fl orescent lamps.Seven panels of each type of coated samples and two panels of uncoated chemically modifi ed and unmodifi ed samples were exposed to UV light directly at the distance of 5 cm for 56 days (1344 hours).The exposure cycle consisted of 24 h condensation period at (45±3) °C followed by 2.5 h UV irradiation period of 0.77 W/m 2 /nm at (60±3)°C and by 0.5 h water spray period of 6-7 l/min without irradiation.The periods of UV irradiation and water spray were alternately repeated 48 times.It took a whole week to complete the cycle (168 h).
The surface of samples was examined for color and gloss changes, adhesion and appearance of fl aking, cracking, blistering and chalking before and after 2, 4, 7, 14, 28, 42 and 56 days of exposure.
Color changes were measured with a portable spectrophotometer Microfl ash 100d produced by Datacolor (d/8° measuring geometry, 10° standard observer, D65 standard illuminate, xenon fl ash lamp source) always on the same eight locations.The overall color change ∆E* was measured using the CIE L*a*b* color measuring system by the following equation: where ∆L*, ∆a* and ∆b* are the differences between the initial and fi nal values (before and after UV irradiation) of L*, a* and b*, respectively.Gloss changes were measured with a portable glossmeter produced by Erichsen, model 507.The measurements were made at the angle of 60° on the three locations parallel to the wood grain.
The examination of wood samples for appearance of cracking, blistering, fl aking and chalking was performed according to HRN EN ISO 4628-2, 4, 5 and 6.
The adhesion of coatings was determined by cross-cut test according to HRN EN ISO 2409 at two positions on the samples.

REZULTATI I RASPRAVA
The results of color and gloss changes are presented in Figure 1 and 2. As could be expected, the most prominent color changes were in uncoated wood sam-ples.Modifi ed uncoated wood changed the color more than unmodifi ed uncoated wood during accelerated exposure.This could be expected because modifi cation of wood with citric acid modifi es cellulose not lignin, which is susceptible to photodegradation.
At the end of exposure the overall color change (∆E*) of coated wood samples was smaller in unmodifi ed wood samples.After 56 days of exposure the samples coated with referent stain exhibited the smallest color change, followed by samples coated with solvent-borne stain, and samples coated with water-born stain.Referent stain shows the best color stability because it contains pigments that protect wood from UV light.Table 1 shows the initial values of color parameter for all the samples.They were apparently different for modifi ed and unmodifi ed wood, and for coated and uncoated wood.After modifi cation with citric acid, wood changed color due to the high temperature during thermocondensation and the decomposition of citric acid at high temperatures (Katović et al, 2005).
The trend of gloss changes of modifi ed and unmodifi ed wood samples was very similar during the exposure as can be seen in Figure 2. The gloss of uncoated modifi ed and unmodifi ed wood samples was basically unchanged during the exposure (Figure 2d).The referent stain exhibited the most prominent gloss change during the exposure (Figure 2c), followed by solvent-borne stain (Fig 2a) and water-borne stain (Figure 2b).The solvent-borne stain exhibited higher gloss changes between unmodifi ed and modifi ed wood samples than other stains.
The results of adhesion and visual assessment of surface properties after accelerated exposure are presented in Table 2.
According to the results presented in Table 2, it can be concluded that unmodifi ed coated beech wood samples exhibited better weathering resistance than ........Miklečić, Jirouš-Rajković: Accelerated Weathering of Coated and Uncoated Beech... modifi ed coated beech wood samples.During accelerated weathering, the cracking behavior of uncoated modifi ed and unmodifi ed wood samples was very similar.These results suggested that cross-linking reaction between cellulose and citric acid was not sufficient to stop the effects of weathering.At the end of the accelerated exposure the most prominent changes were the changes in adhesion and cracking.Of all tested stains, at the end of accelerated weathering the referent stain showed the best performance, followed by the solvent-borne stain and the water-borne stain.These results are opposite to majority of literature data concerning the weathering of chemically modifi ed wood but there are also results that indicate that some methods of modifi cation do not improve the weathering performance of wood.For example Evans (1998) established that esterifi cation of wood with dicarboxylic acid anhydrides did not increase the resistance of wood to weathering.Podgorski and Roux (1998) showed that acetylation treatment did not improve the coating behavior during artifi cial weathering.Feist and Rowell (1982) also found that butylenes oxide-or butyl isocyanate-modifi ed southern pine wood performed no better than untreated controls during accelerated outdoor weathering.Unfortunately there is no literature data regarding the coatability of wood modifi ed with citric acid.Modifi cation is generally expected to diminish adhesion by making the wood surface less polar and less porous resulting in worse coating wetting of wood and fewer chemical bonds between the two surfaces (Hunt et al, 2007).Trajković et al. (2007) established the decrease of pH value, increase of wettability with water and decrease of total surface free energy after modifi cation of beech wood with citric acid, but which of these properties contributed mostly to change adhesion should be further investigated.

ZAKLJUČCI
The modifi cation of beech wood with citric acid did not improve the color stability of neither uncoated nor coated samples during accelerated weathering.
These preliminary results show that wood modifi cation with citric acid and sodium-hypophosphite monohydrate (SHP) as catalyst negatively affects the performances of tested commercial water-borne and solvent-borne wood stains.This indicates that in future research the wood modifi ed with citric acid should require the novel type of fi nishes developed especially for this type of substrate.a Class 0 -no changes, class 5 -the greatest changes./ Ocjena 0 -bez promjene, 5 -najveća promjena.