Effect of fire-retardant treatment on plywood pH and the relationship of pH to strength properties

Summary

This paper investigates the relationship between wood pH and the strength properties of fire-retardant-treated (FRT) plywood, as it is affected by fire-retardant (FR) formulations, processing variables, and extended high temperature exposure conditions. The objectives of this study were to (1) identify the effect of post-treatment kiln-drying temperature, followed by high temperature exposure, on wood pH; (2) identify the effect of various mixtures of FR components, followed by high temperature exposure, on wood pH; (3) determine if treatment effects on strength and pH are affected by plywood thickness; and (4) quantify the relationship between changes in wood pH and strength loss and whether pH can be used as a predictor of strength loss. Results indicate that the differences in pH resulting from the initial redry temperature became insignificant after extended periods of high temperature exposure. All FR treatments studied caused large, rapid decreases in pH, with the most rapid decreases occurring with formulations containing phosphoric acid. Additions of borate compounds, especially disodium octaborate tetrahydrate (Timbor), produced a measurable buffering effect that slowed or lessened the decreases in pH. No differences in the effect of FRT on the wood pH-strength relationship were noted between the two plywood thicknesses evaluated. A strong relationship was noted between changes in pH of the plywood and reductions in strength and energy-related properties. These findings suggest that the pH of FRT plywood is a good indicator of its current condition and may have potential as a predictor of future strength loss as the plywood is subjected to elevated in-service temperatures.