Natural ageing of one-component polyurethane bonded preservative treated wood evaluated using fracture energy tests

Abstract

At present, increasing emphasis is being placed on the use of sustainable materials such as wood for construction. In exterior environments, preservative treatments are regularly required to achieve adequate durability. In laminated timber elements that use such preservative treatments, limited technical information exists on the ageing resistance of one-component polyurethane (1C-PUR) adhesives. A natural ageing study which involved full exposure for twelve months was undertaken to assess the durability of two different 1C-PUR adhesives for the bonding of untreated, chromated copper arsenate (CCA) and micronized copper azole (MCA) preservative treated wood. Resorcinol formaldehyde (RF) bonded specimens and solid wood specimens were used as a control. Reduced delamination resistance was experienced by both 1C-PUR adhesives in comparison to RF bonded specimens. Readings were higher for all adhesives when bonding CCA preservative treated wood. Latewood delamination was noticeability highest because of its higher density and the increased swelling that it experiences. The MCA bonded specimens experienced minor delamination. Mode I fracture testing demonstrated that because of the ductile nature of the 1C-PUR adhesives, higher energy release rates could be achieved with aged bonded specimens in comparison to unaged solid wood specimens. Mean wood failure percentage readings exceeded 90% for all intervals of ageing. Scanning electron microscopy revealed greater effective penetrations in the earlywood in comparison to the latewood. The 1C-PUR with lower viscosity achieved higher effective penetration and improved delamination resistance. Because of differences between the two 1C-PUR adhesives, no clear evidence existed to state the preservatives reduced the effective penetration.