Durability of Adhesives in Plywood: Dry-Heat Effects By Rate-Process Analysis

Douglas-fir and yellow birch plywood specimens bonded with phenol-resorcinol-, melamine-, melamine-urea-, urea-, thermosetting polyvinyl acetate-, and acid-catalyzed phenol- resin adhesives were subjected to dry heat for varying periods of time. The rate at which wet shear strength changed with exposure time was measured. Strength loss was primarily due to wood degradation with the highest rates of loss occurring with adhesive-species combinations that were highest in acidity, i.e., acid catalyzed phenol with Douglas-fir. The differences in strength loss due to dry heat paralleled those found after long-term storage of duplicate specimens at ambient room conditions. The rates of loss in wet strength were correlated with temperature by the Arrhenius equation. Using this relation ship, estimates of the life expectancy of the joints were in the range normally expected of wood in dry service environments. It was concluded that known differences among the adhesives when in service environments was due to their relative resistance to moisture rather than to any thermal instability. Aging with dry heat and analysis by the rate process procedure can be a useful method for detecting adverse effects of adhesive systems on bond quality. The method can be beneficial in selecting adhesive-substrate combinations suitable for long-term structural performance.

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