Effect of wood moisture content on diffusion of boron-based biocides through Douglas-fir and western hemlock lumber

The effect of wood moisture content (MC) on the ability of borate to diffuse through Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and western hemlock (Tsuga heterophylla [Raf.] Sarg.) lumber at moisture levels representative of those found in many buildings (between 20% and 50% MC) was investigated by using three borate-based commercial formulations over a 3-month period. The formulations were: Timbor® (disodium octaborate tetrahydrate); Boracare® (disodium octaborate tetrahydrate plus glycols); and Diffusol® (boron salts plus thickener). As measured by an indicator, borate penetration into wood at 20 or 35 percent MC was generally shallow 1 month after treatment, but penetration increased with time. Diffusion appeared to be slightly better in Douglas-fir than in western hemlock, although the differences were sometimes slight. Boracare appeared to produce the most consistent results over the test period, although even this treatment produced variable penetration under certain conditions. Chemical analysis showed that most of the borate remained within 5 mm of the wood surface, although concentrations were variable with all three formulations. The results suggest that borate-based formulations can diffuse for short distances at levels that confer wood protection, but their ability to move deeper into wood at levels capable of controlling actively growing decay fungi is somewhat limited