Recently, the consumer’s demand to use fire-retardant treated wood as an exterior use is elevatingin Japan. However, the fire-retardant chemicals are basically water soluble and they are easily leaching outby rainwater. There are few reports on weatherability and comubustibity of fire-retardant wood for outdooruse in Japan. There are also few data indicating the effects on surface coatings to prevent leaching of fireretardantchemicals. This study evaluates weatherability and combustibility of fire-retardant woods withcoatings during 2000 hours of accelerated and 18 months of natural weathering tests.As a fire-retardant, polyphosphatic carbamate was impregnated into sugi (Cryptomeria japonica)sapwood. Four coating systems (water-borne or solvent-borne, pigmented or clear, film-forming orpenetrating) were used to study their effects for preventing leaching of the chemicals.In accelerated weathering, all of the coated specimens maintained the Quasi-noncombustibleMaterial fire performance until 1000 hours, although uncoated specimen could only show the Fire-retardantMaterial performance at 250 hours. In natural weathering, coated and uncoated specimens maintainedQuasi-noncombustibility at 6 months, however the uncoated specimen’s chemical retention was thesmallest and decreased to less than 70kg/m3 at 9 months.The fire-retardants distribution was examined by SEM and x-ray microanalysis for acceleratedweathered specimens. The fire-retardant chemicals were accumulated in both cell walls and luminathroughout the wood tissues. Without coatings, the fire-retardants were leached from the surfaces. Leachingwas observed down to a depth of 150μm from the surface after 500 hours, and extended throughout thespecimen after 1000 hours of accelerated weathering. However, the fire-retardants were remained in thesamples finished with coatings even after 1000-hour weathering test.
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