A Study on the Cause for Variation in Plywood Bond Quality with Open Assembly Time

No change in number average molecular weight occurred in phenol-formaldehyde (PF) resin after an open assembly time as long as 4 hours. Rates of quinone methide formation in PF resin solution were measured at 30, 45, and 72?C by measuring the absorbance at 570 mp with time. The activation energy for the quinone methide formation was calculated to be 4 Kcal/mole. Increasing the oxygen or sodium hydroxide concentration in the PF resin solution had no effect on the reaction rate. The addition of sodium borohydride to the PF resin solution reduced the quinone methide structure and the reduction reaction followed first-order kinetics. Kinetic rate measurements showed that the quinone methide formation in PF resin was too slow to affect bond quality with open assembly time, and a parameter such as wetting capacity of the resin is probably the controlling factor in determining bond quality with open assembly time. To improve the wetting capacity of the resin, polyethylene glycol 200 (PEG) was added in concentrations of 10, 20, and 30 wt. percent PEG. A significant improvement in bond quality after long open assembly time was found and this improvement increased with increasing wt. percent PEG. Wet and dry shear strength and wood failure values were used as criteria to evaluate bond quality.

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