Adhesion and performance of exterior wood coatings on chemically and thermally modified wood – Results from 5.5 years outdoor exposure

IRG/WP 23-40964

L Emmerich, H Militz

Non-durable softwood and hardwood species were treated with the water-soluble cyclic N-methylol compounds 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and methylated DMDHEU. One half of the modified specimens were coated with a waterborne acrylic coating system prior a natural weathering for 5.5 years according to EN 927-3 (2020). By frequent evaluations, the impact of chemical modifications and combined thermo-chemical modifications on the moisture dynamics and crack sensitivity of solid wood boards was assessed, further the aspect how such modifications impact on the paint adhesion and performance of a waterborne coating system. In general, natural weathering processes put forth changes in solid wood boards in the form of cracking and discolorations due to UV degradation. Principally, the crack performance was almost unaffected by DMDHEU and mDMDHEU modifications for the pine species of choice (P. radiata, P. sylvestris), whereas poplar and TMT poplar occurred more crack sensitive after the chemical modifications. Similar observations were made related to the paint adhesion, which was almost unaffected by a modification with DMDHEU but led to a slight decrease by treatments with mDMDHEU. Finally, thermally modified and thermo-chemically modified wood blocks turned out as the most complex and challenging material related to the paint adhesion and the durability of this coating system in service. Specifically, for TMT wood and partly treatments with a methylated DMDHEU, further optimization work related to the compatibility of such β€˜new’ surfaces with existing coating systems is needed.


Keywords: acrylic wood coatings, EN 927-3, field tests, hardwood, modified wood, natural weathering, softwood

Conference: 23-05-28/06-01 Cairns, Australia


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