Catalytic activation of wood adhesive polycondensation by the adhesion forces to a cellulosic substrate

Abstract

The reactions of polycondensation of phenol–formaldehyde resins, melamine–formaldehyde resins, and diisocyanates with wood residual water to polyureas, and of autocondensation of aldehyde-free tannin adhesives all have lower energies of activation on a cellulosic substrate than when such resins harden alone. This lower energy of activation has been identified as being due to (1) the catalytic self-condensation induced by the adherend carbohydrates such as crystalline and amorphous cellulose, (2) the diffusion hindrance exercised by the substrate on the hardening reaction of the resin, and (3) the formation of resin–substrate covalent bonds. The first of these is shown to be by far the major cause of the lowering of the activation energy of the adhesive polycondensation and hardening, with the second exercising some contribution and the third most often a negligible contribution under the conditions pertaining to thermosetting wood adhesive applications.