PHENOL-FORMALDEHYDE RESIN CURING AND BONDING UNDER DYNAMIC CONDITIONS

To better understand the curing and bonding behavior ai phenol-formald ehyde (PF) resin under dynamic conditions, flakeboards were manufactur ed either by conventional pressing at 7% or 12% mat moisture content o r by steam injection pressing with 10 or 20 seconds steaming duration. Resin-impregnated glass-cloth samples and lap-shear tension specimens were embedded in the core of each flakeboard. After the flakeboards w ere pressed for various periods of times, the samples and specimens we re quickly retrieved. The degree of resin cure was determined on the r esin-impregnated glass-cloth samples by dynamic mechanical analysis. T he bond strengths were measured from the lap-shear tension specimens o n a mechanical testing machine. The results of resin curing and bondin g were then correlated to the performance of the resin-bonded boards, which were evaluated by internal bond, modulus of rupture, modulus of elasticity, and thickness swelling. Resin curing and lap-shear bonding did not proceed simultaneously. In conventional pressing, the mat moi sture content (MC) at 12% favored resin curing, but slightly retarded lap-shear bonding, as compared to 7%MC. In steam injection pressing, t he rates of resin curing and lap-shear bond strength development were much faster for 20 seconds than for 10 seconds of steaming duration. L onger press times were needed to obtain boards with maximum strength i n the 12%MC conventional pressing and the 20-s steam duration steam in jection pressing than in those conditions where moisture content was l ower or steam time was less. The need for longer press times must be a ttributed to higher internal vapor pressures and/or lower wood and res in component strengths, rather than to incomplete cure or bonding.