Performance of clear coatings on modified wood exposed to the weather for 2 years in Australia

Abstract

We examine the performance of clear coatings on wood treated to improve its photostability. The treatments tested were: benzoyl chloride; chromic acid; modified PF resins; vinyl benzoate; and primers containing different photostabilisers. Treated panels were finished with clear coatings containing different types/levels of photostabilisers and exposed outdoors in Australia for 2 years. The coatings performed better on treated than untreated panels, except for coatings on panels treated with a primer containing cerium oxide nanoparticles. Clear coatings performed best on wood treated with chromic acid, vinyl benzoate or PF resin containing a hindered amine light stabiliser. However, the performance of the coatings was also strongly influenced by coating type. We conclude that the performance of clear coatings can be increased if they are applied to photostable wood substrates. However, coating performance is also dependent on coating flexibility, its propensity to form surface defects, and the level/types of photostabiliser in the coatings.

The Search for Durable Exterior Clear Coatings for Wood

“…This form of failure is quite common although other forms of failure occur. For example, very hard inflexible coatings fail even more quickly by cracking and large-scale delamination of the coating [34]. Clear coated wood products in tropical climates can fail very quickly due to decay of the wood before there is complete failure of the clear coating [10].…”

“…The cracking of the coating containing the binder with the high Tg began after 1 year of exposure while no cracking was observed in the finish containing the binder with a Tg of 10 °C [40]. Vollmer and Evans also observed that a hard inflexible acrylic coating formulated for metals failed far more quickly on wood than more flexible coatings [34]. The relationship between the flexibility of phenolic varnishes and alkyd clear coatings and their durability on wood exposed to the weather was examined by Ashton [39].…”

“…The ability of the brush-on PF resin pre-treatment to improve the performance of the clear coatings exposed to natural weathering was only slightly better than that of the water repellent pre-treatment mentioned above, but it was far less effective than an acid copper-chromate pre-treatment [86]. More recent research on the performance of clear coatings on PF resin treated wood has confirmed that PF resin pre-treatment is effective at improving the performance of water-borne acrylic and alkyd clear coatings, especially if the photostability of the coating is increased by doubling the level of UVA/HALS in the coating [34].…”

“…Other wood modification systems that have been tested to see if they improve the performance of clear coatings on wood include esterification with the aromatic compounds benzoyl chloride or vinyl benzoate and the etherification modifications, cyanoethylation, benzylation and allylation. The latter two modifications improved the performance of clear coatings [92] as did modification of wood with the aromatic compounds benzoyl chloride and vinyl benzoate [19,34]. The effects of wood modification with these two aromatic compounds on the performance of clear coatings are examined in more detail below.…”

“…This approach involves esterifying wood with benzoyl chloride or vinyl benzoate [104][105][106]. Both approaches photostabilize lignin, although they are not as effective as chromium VI, and they also increase the longevity of clear coatings [19,34]. However, wood esterified by benzoyl chloride or vinyl benzoate and then clear-coated becomes white with prolonged exposure to the weather, which is undesirable in a pre-treatment designed to improve the performance of clear finishes [34].…”