Commercialisation of a low maintenance transparent coating is expected to assist wood products to maintain residential market share in the face of competing materials and potentially expand markets in recreational property and non-residential applications. Four areas for improvement were identified: optimizing the UV blocking capability of the coating, improving black stain resistance of the coating, improving UV resistance of the underlying wood and improving black stain resistance of the underlying wood. This study focussed on improving UV/visible light resistance of the underlying wood and the effect of the UV protectants on resistance to black stain. Samples of ponderosa pine sapwood were pre-treated with a range of individual compounds with potential as UV protectants and a range of combinations of these compounds. Half of each sample was finished with two coats of the first step and one coat of the second step of a two-step water based transparent coating. The other half was finished with three coats of transparent water based urethane. The first set of samples was exposed for 2000 hours in an Atlas Weather-Ometer®. The weathering cycle was full time UV and full time misting using diffusers over the spray nozzles, except the mist was turned off for one hour each workday. The samples were evaluated using a set of criteria developed by the USDA Forest Products Laboratory at Madison. Comparable samples were pre-treated with the same series of UV protectant systems then one half was finished with two coats of the first step and one coat of the second step of a two-step water based transparent coating. The other half was left unfinished. Four combinations of biocides were added to this part of the experiment. The second set of samples was inoculated with a spore suspension of black stain fungi and exposed for 2000 hours in damp chambers at 20° C. These samples were evaluated for stain intensity on a 0 to 5 scale.
Uncoated samples exposed in the Weather-Ometer showed severe weathering leaving the wood completely white with loose surface cells. Controls with no pre-treatment showed virtually the same level of damage under the water based urethane though the coating itself remained largely in place albeit with considerable cracking. The only individual UV protectant that showed substantial protection against UV under the water based transparent urethane was a UV absorber (UVA), Tinuvin 1130. The two-step transparent water based coating proved so effective at stopping UV that there was no damage to the underlying wood, even for the control with no pre-treatment. It did suffer from discolouration with whitening and blackening in places, types of colour change not seen under normal service conditions. The suggested the constant UV and water may have created conditions conducive to chemical reactions not seen in service such as conversion of transparent iron oxides from iron III to iron II.
None of the other UV protectants combined with the UVA provided any substantial improvement to its performance. Several of the UV protectants had adverse effects on performance. The colloidal zinc oxide caused blistering of the water based transparent urethane. The trans iron oxides turned black possibly due to reduction from iron III to iron II. Lignostab caused a yellow discolouration of the water-based transparent urethane in reference samples not exposed to light.
Several of the UV protectants appeared to increase the growth of mold and stain fungi on the samples. All four biocide combinations were very effective at controlling mold and stain fungi. The results of these tests were used to design a field test of UVA and biocide combinations.