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Surface checking of CCA-treated radiata pine decking timber exposed to natural weathering
2000 - IRG/WP 00-40165
A field trial was established at four locations in Australia to assess the effect of CCA and CCA-wax treatment on the checking (cracking) of radiata pine decking timber exposed to natural weathering. Untreated decking timber and timber that had been pressure treated with water acted as controls. After 1 year's exposure the number and sizes of checks occurring in treated timber and controls were assessed. Timber treated with CCA-wax had fewer checks, which were significantly smaller in size than those in CCA treated timber and the untreated controls (except for check length in untreated timber). Checking in CCA treated decking timber resembled that occurring in water treated controls in that checks tended to be larger (longer and wider) and fewer in number than those found in untreated (exposed) boards. This finding tends to suggest that the pressure treatment process may have influenced the susceptibility of wood to check during exterior exposure. The difference in check number and size between CCA treated and untreated controls was, however, statistically insignificant. The effect of the different treatments on checking was consistent across the four exposure sites. The implications of these findings for the development of preservative formulations that reduce the checking of wood during exterior exposure are discussed.
P D Evans, P J Beutel, C F Donnelly, R B Cunningham

Effect of Coatings on the Durability of Birch and Spruce Plywood. Part 1: Weathering Performance
2010 - IRG/WP 10-40526
Several different coated birch and spruce plywood types were investigated in Finland using outdoor weathering. The weather performance of the tested coated birch plywood was good compared with uncoated birch and spruce plywood. The paint base film coated and painted birch plywood performed well in outdoor testing at VTT's test site. Phenol film coated plywood was sensitive to UV light, but only a little cracking and mould growth was found after several years outdoor weathering. Edge sealing with acrylate paint protected well the plywood specimens during the exposure. Moisture content of coated birch plywood was less variable and it did not exceed the critical level needed for the early development of decay. The overall results clearly show that birch plywood is better substrate for film coatings.
A Nurmi, H Viitanen

Gloss and hardness variations as early indicators of wood coating failure during weathering tests
2019 - IRG/WP 19-40866
The objective was to track early changes which ultimately will lead to coating failure when exposed to artificial weathering. Two different coatings (one waterborne acrylic paint and one solventborne alkyd stain) were exposed to artificial weathering for 2016 hours according to EN 927-6. Every 168 hours, samples were removed from the artificial weathering device for assessment (general appearance, gloss, colour, Persoz hardness, cracking). The relation between the visual metrics and the Persoz hardness was studied. Results have shown that the increase in Persoz hardness due to weathering was paralleled by the decrease in gloss. For the first time, and for both coatings, a correlation was established between hardness and gloss often considered as pure aesthetical parameter. This demonstrates that the gloss loss usually observed during weathering tests of coatings is a sign of a change in mechanical properties of polymers. Therefore, a special attention should be given to gloss variations to anticipate cracking development and time for coating maintenance. Two models have been proposed to describe early changes for Persoz hardness and gloss variations. They should be useful to calculate acceleration factors produced by artificial weathering tests for each coating using data from natural weathering tests when available.
L Podgorski

The influence of chemical compounds on wood cell wall to surface cracks
2020 - IRG/WP 20-40908
Degradation due to cracking and dimensional changes caused by drying, have a significantly negative impact on the preservation and durability of wood. Therefore, the prevention of surface cracking, which tends to occur during the drying process, is vital. High temperature set drying is one of the most effective methods for preventing wood surface cracking. It begins with softening the wood at a high temperature (100°C–120°C), allowing the surface to dry all at once, which deliberately forms a drying set. However, the mechanism by which wood surface cracks are prevented using the high temperature set method is yet unclear. In attempting to identify the said mechanism, this study focused on changes in the chemical components of the wood cell wall, particularly changes in hemicellulose, and discusses the hygrothermal (moisture and heat) impact on the chemical constituents of wood cell walls. There was no quantitative change to the cell wall’s chemical components due to hygrothermal treatment (HTT) at approximately 120°C. However, analysis of arabinoglucronoxylan (AGX) showed that its molecular weight was decreased significantly by the said treatment, indicating that the main chain structure of hemicellulose in the cell wall is significantly affected.
R Suzuki, Y Mori, K Yoshihiro, K Yamashita, M Kiguchi

Influence of weathering on the mechanical properties and performance of exterior wood coatings
2022 - IRG/WP 22-40951
Three commercial coatings were exposed to artificial (EN 927-6) and natural weathering (EN 927-3) on wood samples. Cracking was visually assessed as well as Persoz hardness. Free films of the same coatings were also exposed to the same weathering tests before their tensile properties were measured. Results show that artificial and natural weathering modified the overall mechanical properties of wood coatings. The elastic modulus and the strength increased whereas the strain at break dramatically decreased from the first hours of exposure. For selecting good performing coatings, our results show that it is useful to consider the variations of the elastic modulus and to calculate the retention of the initial strain at break after weathering instead of considering the strain at break result. A significant increase in the elastic modulus lead to cracking. Coatings performed better when their modulus remained below 400 MPa and their retention in strain at break was higher than 20%. The study shows that short artificial ageing tests (< 500 h) on free films are relevant to highlight changes in strain at break observed in natural weathering. They are therefore a valuable tool in the formulation of high-performance products. The mechanical properties measured using the Persoz hardness test are also interesting to take into consideration in order to anticipate the risk of cracking. For the three coatings, the Persoz hardness increased more or less due to weathering. The results show that coatings with an initial Persoz hardness higher than 80 seconds should not be chosen for wood exposed outdoors because their risk of cracking is higher.
L Podgorski, J-D Lanvin