EFFECTS OF THE INCORPORATION OF HIGH-MOLECULAR-WEIGHT NOVOLAC ON THE DURABILITY OF PHENOL-MELAMINE-FORMALDEHYDE ADHESIVE

Incorporation of powdered high-molecular-weight novolac greatly improves the durability of phenol-melamine-formaldehyde resin (PMF) glue joints in plywood. To clarify the fortifying action, the effects of the addition of novolac on the boiling-water-resistance of PMF-glue joints were compared with those of resin thickening and the addition of powdered resite (cured phenolic resin). Also the curing behavior of the PMF-novolac system was investigated. Resite powder showed no fortifying effect. Resin thickening brought about a considerable increase in boiling-water-resistance of glue joints. However, this effect of resin thickening was much less than that of the incorporation of powdered novolac. Plywood joints glued with the PMF-novolac system with stood 72 h of boiling, keeping high levels of shear strength and percentages of wood failure which were almost comparable to those of plywood joints glued with a phenolic resin adhesive. These observations indicate that the effects of the novolac can be ascribed to the fortification of the cured gluelines themselves rather than to the suppression of over-penetration of glue into the veneers. Analysis of the curing course of the PMF-novolac system revealed that the novolac was changed easily into gel by the reaction with the melamine part together with the phenol part of the PMF. Thus a large phenol/melamine composition ratio for the three-dimensionally cross-linked fraction was achieved in a short curing time.