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

• Published in: JCT research 2008-06, Vol.5 (2), p.207-219
• Main Authors: Dawson, Bernard S. W., Singh, Adya P., Kroese, Hank W., Schwitzer, Michael A., Gallagher, Suzanne, Riddiough, Stephen J., Wu, Shuhong
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemistry and Materials Science; Chromium compounds; Coatings; Coatings industry; Coatings. Paints, varnishes and inks; Corrosion and Coatings; Degradation and preservation; Exact sciences and technology; Industrial Chemistry/Chemical Engineering; Lignin; Materials Science; Polymer industry, paints, wood; Polymer Sciences; Radiation; Surfaces and Interfaces; Thin Films; Tribology; Uses; Wood; Wood. Paper. Non wovens
• ISSN: 1547-0091; 1945-9645; 1935-3804; 2168-8028
• DOI: 10.1007/s11998-008-9090-z

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-μm-deep 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 v