Enhancing exterior performance of clear coatings through photostabilization of wood. Part 2: coating and weathering performance

Abstract

Clear-coated boards have not been recommended for use in exterior conditions since irradiation with visible and UV radiation darkens them and photodegrades the lignin in the wooden surface beneath the coating, leading to delamination and subsequent catastrophic coating failure due to the continued action of sun, rain, and biological factors. Many approaches to rectify this problem have been explored. Chemical modification of the surface with hexavalent chromium, reaction with various anhydrides, grafting of UV absorbers, and esterification are among the methods attempted. A second approach has been via the clear coating itself where UV absorbers, antioxidants, and ultrafine titanium and iron oxides have been added. However, these have had limited or no success in stopping photodegradation processes. Since the main cause of photodegradation is photooxidation of lignin in the wooden surface as a consequence of free radical reactions initiated by UV irradiation, the approach taken in the present study, in an attempt to enhance the weathering performance of clear-coated boards outdoors, was to delignify the surfaces of wooden boards and then apply clear coatings to try and retard possible photodegradation. Two different pretreatments were used. Firstly, chemical surface delignification with a peracetic acid treatment created a partial delignification to a depth of 2-3 mm while still retaining the structural integrity of the surface. Secondly, a preweathering treatment, which resulted in a 100-lmdeep delignification zone, was compared. The coatings applied to the exposure surface of the pretreated boards were either polyurethane or an acrylic varnish. The clear-coated boards were exposed to exterior and accelerated weathering regimes for 3 years or 3000 h, respectively. Pretreated coated boards did not darken and yellow on exposure but untreated coated boards did. However, despite apparently arresting photodegradative processes on board surfaces, there were no significant gains in the performance ratings of coated pretreated boards over those of coated untreated control boards. Explanations for this involve the effectiveness of design factors incorporated into boards for exposure trials. These factors were the fungicidal dipping of boards before coating, precoating the exposure surface with a reactive primer, and applying a full polyurethane system to the back side and edges of boards. Both pretreatments resulted in clear-coated board surfaces that performed very similarly on exposure outperforming systems reported previously. It was surprising to observe that the preweathering treatment, which resulted in a 100-lm-deep delignification zone, performed as effectively as the chemically pretreated boards with 2-to 3-mm treatment zone. However, preweathered surfaces had lost all lignin in the middle lamella and there was cell separation, whereas in peracetic acid-treated boards, there was more or less complete lignin removal from the cell corner middle lamella only and partial lignin removal from other cell wall regions. ...

Weathering Properties of Wood Species Treated With Different Coating Applications

“…This yellowing or browning occurs after only several months of exposure in sunny, warm climates. In the absence of micro-organisms, wood can weather to a soft silver gray color as a result of the leaching of the decomposition products of lignin (Dawson et al 2008). The influence of the species on the member's characteristics are related to the content of extractives, and results of UV absorber performances on pine wood photostabilization was found to be different than that from beech samples (Aloui et al 2007).…”

“…It also has been reported that the acrylic resin provides a good performance, demonstrating that a resin coat that contains a UV absorber is an adequate method for the protection of wood for exterior applications (Decker et al 2004;Custódio and Eusébio 2006). However, waterborne coatings present the problem of the extractive bleeding in the case of pine because of the presence of a resin that has water-soluble phenolic or polyphenolic compounds (Dawson et al 2008). The typical anatomical structure of softwoods and hardwoods shows distinct differences.…”

“…The tracheids in the surface layers of the pretreated beech woods tended to separate in the region of the middle lamella, and the cells of this region would be one distinct point of weakness if the coating did not penetrate sufficiently and uniformly enough to stabilize this region (Dawson et al 2008).…”

Enhancing exterior performance of clear coatings through photostabilization of wooden surfaces. Part 1: Treatment and characterization

“…Such surfaces may be surface coated with clear coatings. In Part 2 22 of this series, the weathering performance of clear-coated delignified boards on exterior and accelerated exposure is assessed.…”

“…This article, Part 1 of a two-part series, reports the characterization of changes in P. radiata surfaces, after peracetic acid or accelerated preweathering treatments, using a range of chemical, microscopic, and spectroscopic techniques. Part 2 22 of the series describes the coating of these boards, their accelerated-and exterior weathering performance, and demonstrates their effectiveness in resisting exterior weathering forces.…”