WOOD-INDUCED CATALYTIC ACTIVATION OF PF ADHESIVES AUTOPOLYMERIZATION VS PF WOOD COVALENT BONDING

The reaction of polycondensation of phenol-formaldehyde (PF) resins in the presence of wood was confirmed to have a lower energy of activation than of the PF resin alone. Under the low temperature and short curing times characteristic of the application of PF resins as thermosetting wood adhesives DSC, TGA, chemical kinetics, and IR of PF resins and relevant model compounds were carried out. These indicated that two effects appear to be present when a PF resin cures on a wood surface, both induced by the polymeric constituents of the substrate, namely carbohydrates and lignin. These appear to be (1) the catalytic activation of the resin self-condensation induced particularly by carbohydrates such as crystalline and amorphous cellulose and hemicelluloses and (2) the formation of resin/substrate covalent bonding, particularly in the case of lignin. The first appears to be, by far, the major cause of the lowering of the activation energy of PF resins curing, The contribution of the second has been found to be very small and often negligible under the conditions pertaining to thermosetting wood adhesives applications. Molecular mechanics results appear to indicate that the marked catalytic activation of PF resins autocondensation and curing appears to be induced by the strong set of PF adhesive/substrate secondary forces interactions which appear to weaken bonds which, by cleavage, lead to PF resins autocondensation. (C) 1994 John Wiley & Sons, Inc.