Stability and performance of tannin-accelerated PF resins for plywood

Abstract

Materials and methods A phenol-formaldehyde (PF) resin was prepared at pH 11 at phenol:formaldehyde molar ratio = 1:2.2 as follows: 100 parts by weight of technical phenol (90%) are charged in a stainless steel jacketed reactor equipped with heating and cooling facilities, with continuous mechanical stirring and with condenser. To it are added 211 parts by weight of 30% formalin solution and 27 parts of 50% caustic soda solution. The reaction mixture is then brought to reflux (102 ~ over a period of 15 minutes and under continuous mechanical stirring. After 30 minutes at reflux a 35% solution of industrial quebracho tannin already adjusted to pH 9.5 has been added to the phenolic resin at a ratio of 6% tannin solids content on total liquid PF resin (=13.8% tannin solids content on PF resin solids content) at a temperature just under reflux. At the end of the addition the reaction mixture was cooled down and 3% urea on total liquid resin was added as a stabilizer. The final resin had a pH of 10.9, a total solids content of 42%, initial viscosity of 180 centipoises at 25 ~ Its viscosity increased to 230 centipoises over a period of 5 days from its preparation. Gel time at 100 ~ of the PF atone was of 75 minutes, and of the PF/tannin coreaction product was of 24 minutes. Gel time on addition of 1% glycerol triacetate accelerator at a level of 1% on total liquid PF/tannin resin decreased the gel time to 11 minutes with a pot-life of i00 minutes. Duplicate five plys laboratory plywood panels of beech veneers of 2 mm thickness, and of dimension 400 x 400 x 10 mm were prepared by spreading a glue mix based on the above PF/ tannin resin additioned of 20% coconutshell flour filler to reach a viscosity of 2000-2500 centipoises, with and without 2% glycerol triacetate accelerator, at a spread of 230 g/m 2 single glue line, with a 15 minutes cold prepressing of 6 kg/cm 2 and then hot pressed at 130 ~ at 15 kg/cm z for 6.5 and 8.5 minutes. A commercial, pure PF resin of 100 ~ gel time of 40 minutes was used as a control. The plywood specimens were then cut and tested according to European Norm EN 314 for exterior grade plywood (4 hours boiling + 16 hours drying at 60 ~ + 4 hours boiling, tested wet). The results are shown in the table. Press glue mix Strength results time pot life EN 314 (minutes) & shelf-life (Mpa)