Phenolic Resin Adhesives

Abstract

Phenolic resins are the polycondensation products of the reaction of phenol with formaldehyde. Phenolic resins were the first true synthetic polymers to be developed commercially. Notwithstanding this, even now their structure is far from completely clear, because the polymers derived from the reaction of phenol with formaldehyde differ in one important aspect from other polycondensation products. Polyfunctional phenols may react with formaldehyde in both the ortho and para positions to the hydroxyl group. This means that the condensation products exist as numerous positional isomerides for any chain length. This makes the organic chemistry of the reaction particularly complex and tedious to unravel. The result has been that although phenolic resins were developed commercially as early as 1908, were the first completely synthetic resins ever to be developed, and have vast and differentiated industrial uses today, and great strides have been made in both the understanding of their structure and their technology and application, several aspects of their chemistry are still only partially understood. It may be argued with some justification that such a state of affairs is immaterial, because satisfactory resins for many uses have been developed on purely empirical grounds during the past 90 years. However, it cannot be denied that the gradual understanding of the chemical structure and mechanism of reaction of these resins has helped considerably in introducing commercial phenolic resins designed for certain applications and capable of performances undreamed of in formulations developed earlier by the empirical rather than the scientific approach. Knowledge of phenolic resin chemistry, structure, characteristic reactions, and kinetic behavior remains an invaluable asset to the adhesive formulator in designing resins with specific physical properties. The characteristic that renders these resins invaluable as adhesives is their capability to deliver water, weather, and high-temperature resistance to the cured glue line of the joint bonded with phenolic adhesives, at relatively low cost.