Substitution of phenol in phenol-formaldehyde (PF) resins by wood tar for plywood adhesives

Wood tars from Acacia confusa Merr. and Cryptomeria japonica D. Don. were produced by reduced pressure distillation at 76 mm Hg and 50 degrees C from the bottom layer of crude wood vinegars that had set for over 6 months. The feasibility of using water-soluble, resole-type, wood tar phenol-formaldehyde resins (WT-PF) was investigated. The resins were prepared from phenol (P) and wood tars (WTs) at weight ratios of 100:0, 80:20, 60:40 and 40:60 and F/P molar ratios of 1.5, 1.8 and 2.0 as plywood adhesives. The results demonstrated that WT-PF resins had a higher viscosity, shorter gel times, a higher polydispersity, a lower curing temperature and less heat of curing than conventional PF resins under the same conditions. The shear strength of plywood bonded with WT-PF resins was slightly lower than that of conventional PF resin. However, the WT-PF resin still met the CNS 1349 requirement for the dry test, and phenol substitution levels could be up to 60 %, but only 20 % for type I and type II plywood. The addition of A. confusa bark powder fillers could significantly improve the bonding strength of WT-PF resins, especially for the warm water soaking and soaking in boiling water tests. The required amounts of fillers added were up to 5 % for A. confusa WT-PF resins and up to 3 % for C. japonica resins.