Hygroscopicity of resorcinol-formaldehyde resin and aqueous vinyl polymer-isocyanate resin in high humidity

The hygroscopicity and hygro-mechanical properties of the adhesives, resorcinol-formaldehyde (RF) resin and aqueous vinyl polymer-isocyanate (API) resin have been examined. Adsorption isotherms for cured RF and API resins with various degrees of crosslinking showed that the hygroscopicity of both resins decreased as the curing reactions proceeded, although the mechanisms for these decreases differed. In RF resins cured with a large amount of hardener, a highly crosslinked structure is formed, so swelling is limited by the network. Thus, the hygroscopicity of cured RF resins decreases due to the restricted swelling, while the number of hydroxyl groups, another source of hygroscopicity, are not changed by the curing. On the other hand, swelling in API resins cured with a large amount of hardener is not completely inhibited because of long segments between the crosslinks. The hygroscopicity of cured API resins is lowered as the curing extent increased, since this reaction decreases the number of hydroxyl groups.