Urea Resins for Hot- and Cold-Press Hardwood Plywood

Bonding conditions for hardwood plywood must be closely controlled to produce quality plywood for today’s competitive market. Eleven common gluing defects are described, and their causes and cures are outlined. Bleed-through may be caused by too thin an adhesive mix, too short an assembly time, too high a moisture content in the veneer, or an improper proportion of extender (an inert filler such as nut shell flour should make up 10 to 25 percent of the extender). Blows or blisters may also be caused by too thin a mixture, too short an assembly time, or too high a moisture content (above 5 to 10 percent) in the stock to be bonded; other causes may include too high a press temperature, too short a press time, or defects, such as knots, worm holes, or checks in the stock. Dimpling is caused primarily by defects in the stock, too thin crossbands, defects in the lumber core, nonuniform moisture content in particleboard cores, or board surfaces which are pitted and full of voids. Face-checking can be caused by improper conditioning of the assembly, too long a press period, or too high a press temperature. Indentations indicate that cauls or platens are pitted or damaged. Low water resistance may be caused by too high an extension in the mixture or by incomplete cure. Causes for pre-cure include too long an assembly time, using stock directly from the kiln or dryer before it has cooled, too high a press temperature, too light a glue spread, or variability in the thickness of the stock. Spotty bonding can be caused by too long an assembly time, worn or damaged spreader rolls, careless veneer handling, face veneers which are sized or glazed, insufficient or non-uniform pressure, improper mixing, rough stock, or nonuniform glue-spread. Starved joints may occur with too short an assembly period, too thick a mixture, faulty extension, or a foamy mix. Warpage may occur with defects in core stock, improper conditioning of panels, nonuniform spread, over-curing in the hot press, too high a press temperature, or unbalanced construction. Weak bonds may be caused by an adhesive mix which is not reactive enough, too heavy a spread, too low a moisture content in the stock, too short a press cycle, or too low a press temperature.

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