The chemistry and development of branched PRF wood adhesives of low resorcinol content

Phenol–Resorcinol–Formaldehyde (PRF) resins of good adhesive performance, and of resorcinol content much lower than in traditional PRF cold‐setting adhesives, were prepared by introducing small amounts of urea as a branching unit during preparation of the resin. Resorcinol contents as low as 10.6% were obtained by this method while still maintaining an acceptable pot‐life of the resin. Resorcinol contents as low as 8%, still with a good adhesive performance, were obtained, although the pot‐life obtained was too short. GPC, FT–IR and C 13 NMR indicated that the small amount of urea branching unit used leads to the reduction of resorcinol content mostly by rapid doubling of the average linear length of the PF intermediates and of the final PF oligomers. Tridimensional branching in the liquid resin, contrary to what was previously thought, does not appear to be an important contributory factor and better adhesives are often prepared when branching is totally, or almost totally, absent. Reduction of resorcinol content is achieved simultaneously by two mechanisms: (i) the decrease of resorcinol percentage on resin solids due to the rapid lengthening (doubling) of average oligomers length and (ii) the need for these resins to operate at lower solids content, due to the increased viscosity obtained during their preparation.