Wettability of Southern Pine Veneer By Phenol Formaldehyde Wood Adhesives

Wettability of southern pine veneers was judged by measuring the contact angles made by 36 phenol formaldehyde resins. Formulation of the resins was by factorial design, the molar ratios of sodium hydroxide to phenol being 0.4, 0.7, and 1.0; the levels of resin solids content in the reaction mixture 37, 40, and 43 percent; and the molar ratios of formaldehyde to phenol 1.6, 1.9, 2.2, and 2.5. The molar ratio of sodium hydroxide to phenol was the dominant factor affecting contact angle; molar ratio of formaldehyde to phenol was second in importance. As molar ratios of sodium hydroxide to phenol increased and ratios of formaldehyde to phenol decreased, the contact angle decreased. Contact angle was not correlated with solids content. Contact angle on earlywood was less than that on latewood, apparently because earlywood surfaces were rougher. Contact angle was positively correlated with glue bond quality as tested by wet shear strength, percent, of wood failure, and percent of delamination. High contact angle of the resin–and low wetting of the veneer– may prevent excess glue penetration, which often causes poor bonds in southern pine plywood.

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