BORATE TREATMENT OF DOUGLAS FIR AND GRAND FIR LAMINATES FOR A MORE DURABLE CROSS-LAMINATED TIMBER

Abiola Teslim Adeniran

doi:10.7273/000008367

This research investigated the possibility of adding borate preservative treatment to the production of cross-laminated timber (CLT) using Douglas-fir and grand fir laminates. The goal was to improve biological durability without harming adhesive bond strength or structural integrity. Experimental approaches based on pressurized pre-treatments and non-pressure methods were tested. Borate-treated boards were evaluated for borate retention and bonded with one-component polyurethane resins. The adhesive bond strength of bonded boards was thereafter evaluated per AWPA standards. The study also investigated the influence of post-treatment planning on borate retention and bond performance. Results revealed the influence of species and treatment method on borate retention and adhesive bond performance. The study observed that grand fir consistently absorbed more borate preservative and showed more widespread borate distribution than Douglas-fir. While pressure-based treatments met the required retention levels, the non-pressure methods used prior to optimization did not, indicating the importance of developing optimized non-pressure treatment conditions. Non-pressure dip treatment was optimized using an exponential-square-root model, and the predicted optimal parameter was used to achieve adequate borate retention levels. Experimental validation of the model showed that borate uptake followed diffusion-controlled kinetics, with temperature and moisture content strongly influencing the kinetics. Higher temperatures (40-60 °C) and lower moisture content (6-12%) raised diffusion rates and equilibrium retention. Planning was observed to reduce borate levels while improving adhesive bond performance. The study also developed a spray-dry cycle regime that can achieve targeted retention levels and satisfactory bond performance with post-treatment light-sanding. Adhesive bond performance of treated laminates with a properly prepared surface is satisfactory, with wood failure over 80%, and delamination under 5%, meeting ANSI A190.1 and PRG-320 standards. The study concludes that borate preservative treatment, whether via pressure or optimized non-pressure methods, can be integrated into CLT production under specific process conditions without compromising the mechanical or adhesive properties. These findings provide a feasible process for producing preservative-treated CLT suitable for indoor and protected structural applications, offering a practical way to enhance the durability and lifespan of mass-timber systems.

BORATE TREATMENT OF DOUGLAS FIR AND GRAND FIR LAMINATES FOR A MORE DURABLE CROSS-LAMINATED TIMBER

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