Durability assessment of one-component polyurethane adhesives for bonding of preservative treated wood subject to artificial ageing

Advancements in the use and applications of engineered wood products is receiving much interest in the construction industry. This is because wood has highly credible characteristics in relation to sustainability. It acts as a carbon store and is a natural renewable material. However, achieving adequate durability requirements and ensuring satisfactory adhesive bondline integrity is fundamental for such engineered products particularly when subjected to an exposed exterior service class environment. Little information exists on the durability performance of preservative treated wood bonded using one-component polyurethanes (1C-PURs). This study therefore involved the accelerated ageing of two 1C-PUR adhesives for the bonding of untreated, copper chromate arsenic and micronised copper azole preservative treated wood of fast growing Radiata pine. The ageing involved both vacuum-pressure soak drying cycling and exposure to a constant long-term high temperature and humidity environment. Resorcinol formaldehyde bonded specimens and solid wood specimens were used as controls. The testing showed that the standard requirements for delamination were satisfied when bonding preservative treated wood with 1C-PUR adhesive. The shear strength of specimens bonded with 1C-PUR adhesive subjected to vacuum-pressure soak drying compared well to the performance of the controls. The predominant failure behaviour was wood failure. The shear strength of all bonded preservative treated wood specimens with both 1C-PUR and resorcinol formaldehyde which were exposed to a constant long-term high temperature and humidity environment demonstrated a general and comparable reduction in strength over time which was statistically significant. High wood failure percentages were recorded at the bond interface which indicated weakening of the wood adherend when subject to the ageing rather than weakening of the adhesive bond. Chemical analysis of bonded specimens after ageing demonstrated that no significant variations were experienced.