EFFECT OF POLYBORATE PRETREATMENT ON THE SHEAR-STRENGTH DEVELOPMENT OF PHENOLIC RESIN TO SITKA SPRUCE BONDS

Pre-treating the constituents of wood-based composite panels with fung itoxic chemicals prior to lay-up and pressing offers some advantage ov er post-pressing treatment. Boron is a strong candidate pre-treatment chemical for this application but it influences the bonding mechanisms that must occur during hot pressing. In the present study, the effect s of boron loading level (0 to 15% boric acid equivalent, BAE) on the strength development rates of small phenol formaldehyde-to-wood (sitka spruce) test bonds was investigated. Each bond (15 mm x 15 mm overlap ) was formed under pre-selected time (20 to 240s) and temperature (70, 80 or 95-degrees-C) conditions and immediately thereafter tested in s hear mode. This enabled three approximately isothermal bond strength d evelopment curves to be constructed for each boron loading level inves tigated. All levels of boron pretreatment hastened initial strength de velopment rates, although levels above 7.5% BAE had diminishing effect s on isothermal rates - with possible decreases at very high (15% BAE) levels. Though not exactly reproducing conditions that occur around b onding sites in panels during pressing, the techniques described may a id in our basic understanding of the mechanisms affected by chemical p re-treatment.