Development of fast-curing, phenolic resin adhesives

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The effects of various factors on the curing-rate of phenolic resin adhesives were investigated quantitatively. The main factors affecting the rate of cure, other than temperature, are the molecular sizes of the resin, resin concentrations, formaldehyde to phenol molar ratios, alkali to phenol molar ratios, and additives. The rate of cure increases in a linear fashion with the increases in the molecular size and in the resin concentration. The rate of condensation reaction increases proportionately with the square of the formaldehyde/phenol molar ratio. The optimum alkali/phenol molar ratio changes with the formaldehyde/ phenol molar ratio. These results show that a fast-curing phenolic resin can be obtained by increasing the resin molecular size, the resin concentration, and the formaldehyde/phenol molar ratio and by choosing the optimum alkali/phenol molar ratio. However, upgrading along this line brings about problems: an enormous increase in viscosity and an extremely short pot-life.To overcome these problems a suspension-type adhesive system was designed. A suspension-type adhesive formulated from an ordinary phenolic resin and a powdered novolac with number-averaged molecular weight of 2,500 worked well in bonding high-moisture content veneers.