Ten commercial fire-retardant coatings (FRCs) designed for wood in outdoor applications, either film-forming or non-film forming (stains), and five top-coatings (used in combination with a FRC to increase its durability) were characterized by microscale combustion calorimetry (MCC) and cone calorimetry (50 kW/m2). All coatings were applied on western red cedar and according to the manufacturers specifications. A standard accelerated-weathering protocol alternating cycles of UV, spray and condensation was used to evaluate the outdoor durability of the coated wood samples. Prior to accelerated-weathering, four of the FRCs showed a significant reduction on the flammability of western red cedar wood with a decrease in peak of heat release rate (PHRR) and total heat release (THR). After accelerated-weathering equivalent to an estimated outdoor time of (14 to 28) days, the cone results revealed that all FRCs became ineffective (no significant reduction in PHRR and THR of the wood substrate). The use of five top-coatings was investigated to extend the durability of FRCs. Only two dual-coated systems based on FRCs and top-coatings maintained good compatibility (no suppression of the intumescent char expansion) and fire behavior. An outdoor durability of up to seven months was estimated based on accelerated-weathering for the best FRC/top-coating combination. After this period, this dual-coated system did not show any flammability reduction by cone calorimetry. The results of this study suggest that weathering is drastically reducing the durability and fire resistance of FRCs used on wood-based products.
Technical Note (NIST TN) - 2094
fire, flammability, wood, coatings, wildland urban interface communities