EFFECTS OF FIRE-RETARDANT RETENTION, BORATE BUFFERS, AND REDRYING TEMPERATURE AFTER TREATMENT ON THERMAL-INDUCED DEGRADATION

All wood products are prone to strength loss from extended exposure to temperatures above 65 degrees C (150 degrees F). The higher the tempe rature, the less time required to produce a given reduction in strengt h. In the United States, problems with fire-retardant-treated plywood roof sheathing and roof-truss lumber have occurred in the field; therm al degradation has occurred in as few as 2 to 5 years after installati on. Before the thermal-induced degradation of treated wood can be full y modeled, we must more completely determine the relationships of trea tment processing factors, mixtures of chemical components, post-treatm ent (redrying) temperature, and in-service moisture to performance, an d relate the results to in-service temperature-induced strength degrad ation. In the study reported here, Variation in redrying temperature f rom 49 degrees C (120 degrees F) to 88 degrees C (190 degrees F) had l ittle effect on the magnitude or rate of subsequent thermal degradatio n when the treated plywood was exposed at 65 degrees C (150 degrees F) for up to 290 days. The addition of borate-based buffers to fire-reta rdant-treatment chemicals was found to significantly mitigate thermal degradation. Finally, the results imply that the combined effects of p hosphate retention and accumulated thermal exposure (from both redryin g and in-service high temperatures) are additive and cumulative.