Heat and Moisture Resistance of a Urea-Resin in Plywood Joints

Describes the results of testing urea-resin bonded plywood under varying conditions of heat and moisture. Yellow birch plywood was made with urea-resin adhesives according to a previously described technique (see Gillespie, R. H. 1965. Accelerated aging of adhesives in plywood-type joints. Forest Prod. J. 15(9):369-378). Specimens were exposed to dry heat conditions at 100?, 120?, and 140?C and water soaking conditions at 400, 55?, and 70?C using precision temperature controlled apparatus. Other specimens were exposed to controlled moisture conditions over distilled water or saturated salt solutions. Results are shown as the time in days required for the specimens to lose one-half their original shear strength (called shear strength half-life). For specimens exposed to dry heat, the shear strength half-life was found to be inversely temperature dependent. As the exposure temperature decreased, the shear strength half-life increased. An activation energy of 23.9 kilo calories per mole was calculated. For the specimens exposed to water soaking, the shear strength half-life was also found to he inversely temperature dependent. For water soaking, an activation energy of 34.4 kilo calories per mole was calculated. Controlled variable moisture content effects were also shown to effect the shear strength half-life in a manner that was inversely temperature dependent. The activation energies for the controlled moisture content specimens were consistent with those calculated for the dry heat and the water soaking specimens. In addition, the variable moisture content test conditions generated free formaldehyde from the glue lines, indicating clearly that hydrolytic degradation of the adhesive was taking place.

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