Effect of elevated temperature on the bond strength of wood block shear specimens

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University of New Brunswick


This report examined the effect of elevated temperature on the shear strength of wood block shear specimens bonded using two types of adhesives. The adhesives used were phenol formaldehyde (PF) and polyurethane (PUR). Each type had two recipes, SPF and CPF for surface and core layers of oriented strand boards, and 1-component (1-PUR) and 2- component (2-PUR) for PUR. The species of wood used was sugar maple (Acer saccharinum) of an average air-dry density of 0.739g/cm3. Four groups of block shear specimens were made, and four temperature levels were employed, 20°C, 80°C, 150°C, and 200°C. Each group had 11 replicates, generating a total of 176 specimens tested. It was found that (1) The shear strength of all PF specimens linearly decreased as the temperature increased from 20°C to 200°C, with CPF decreasing by 51.4%, and SPF decreasing by 65.9%. There was also a decline in the shear strength of PURs specimens in a non-linear way as the temperature increased from 20°C to 200°C. The most significant decrease in shear strength was observed to be 89.6% and 53.7% for 1-PUR and for 2-PUR, respectively. (2) In the block shear tests, the highest peak loads differed among groups with 13,879 N for 1-PUR, 29,588 N for 2-PUR, 30,011 N for CPF, and 27,103 N for SPF. (3) In 200°C, 1-PUR and 2-PUR had higher coefficients of variation of more than 50%, 58% and 53% for 1-PUR and 2-PUR, respectively. At 200°C, the stability of PF adhesive was higher than that of PUR. 2-PUR performed much better than 1-PUR at all levels of temperature used. (4) CPF and SPF did not show remarkable difference in bond strength, which was because the Tg of PF was about 200°C, being higher than that used in this study. (5) The shear strength of groups 1-PUR and 2-PUR did not have a significant difference when the temperature being 150°C and over since PUR had a Tg of around 80°C. (6) In the block shear test, the percentages of wood failure at maximum load were 58.72%, 35.70%, and 47.92% for groups 2-PUR, CPF and SPF, respectively. The group SPF recorded the highest wood failure percentage of 61.40% at 80°C. (7) The percentages of wood failure of group 2-PUR were 58.72%, 53.26%, 37.54%, and 25.58% at the temperatures of 20°C, 80°C, 150°C, and to 200°C, respectively. The percentage of wood failure decreased as the temperature increased. It could be recommended to conduct the tests at a temperature of higher than 200°C for PF-bonded specimens and a temperature of 100°C, 120°C, and 180°C for PUR-bonded specimens with an aim to gather more first-hand information about how the elevated temperature impacts the block shear strength. Key words: adhesive, elevated temperature, block shear test, shear strength, percentage wood failure