Properties of Phenolic Adhesives as Related to Bond Quality in Southern Pine Plywood

Experimental panels of southern pine plywood were glued up with 72 phenolic resins (36 resins with replication) representing a wide range of properties. In the range tested, contact angle (57-105?), heat of curing reaction (95 to 235 cal/gm. and glueline thickness (8 to 21 millimicrons) were linearly and positively correlated with wet shear strength and percentage of wood failure. Resin shrinkage (11 to 21 percent) during cure also was linearly correlated with shear strength and percentage of wood failure, but the relationship was negative, i.e., resins with the most shrinkage yielded the poorest bonds. Surface tension, time to cure, and pH were in general negatively correlated with wet shear strength and wood failure; the regression relationships were parabolic with maxima. The most effective bonding occurred when the resin wetted–but did not over-penetrate –the veneer surfaces. For the formulations tested, this condition resulted when the resin had a high contact angle and a surface tension of approximately 68 .4 dynes/cm. The optimum glue bonds were from resins with high chemical reactivities. Such resins appeared to produce a high degree of cross linking when cured and required short cure times.

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