Gloss and hardness variations as early indicators of wood coating failure during weathering tests

IRG/WP 19-40866

L Podgorski

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.

Keywords: coating, wood, weathering, hardness, gloss, cracking

Conference: 19-05-12/16 Quebec City, Canada

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