The Effect of Urea-Formaldehyde Resin Viscosity on Plywood Bond Durability

This study explores possible reasons for durability variation in the performance of urea-formaldehyde resin samples which were aged to various viscosity levels. The experiment had two phases. In the first phase hot-pressed, three-ply panels (1/16-inch face and back with 1/8-inch core) were made from hard maple veneer. Urea-formaldehyde resins varying in viscosity from 314 to 11,700 centipoises were used in panels cured for three minutes at 250?F and 150 psi. Glue spread was 20#/MSGL, of a commercial 65 percent solids resin which had been aged at 50?C to achieve desired viscosity levels. Dry bond quality was measured in a plywood strip shear test. Durability was measured by subjecting 1-inch by 10-inch specimens to repeated cycles of cooking in water at 135?F. The number of cycles necessary to produce visible delamination on 50 percent of the exposed glueline edges was taken as a measure of durability. In the second phase of the experiment, a similar set of panels was produced, but the viscosity range was extended to 41,600 cp. In addition, fillers, thickeners, and extenders were utilized. The spread rate was varied. Bond quality was measured as before. Viscometric molecular weight, hot water extractive content (solubilization), film shrinkage, and penetration of the resinous material into the wood cell wall (by use of a fluorescent dye) were measured. Results strongly indicated a relationship between resin viscosity and bond durability with the more viscous resins producing the more durable bonds. Results of the second phase of tests indicated that the improvement in durability with increasing viscosity is observable up to viscosity levels beyond which the resin becomes unworkable. There were no marked differences in the physico-chemical characteristics of hardened resin, regardless of its viscosity. However, thicker gluelines were the ones derived from the more viscous resins and had greater durability; adhesive penetration into the gross capillary structure of the wood, rather than into the wood cell walls, was concluded to be the major factor.

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