Curing Process of Phenol Formaldehyde Resin for Plywood under Vacuum Conditions

Abstract

The study characterized the curing behaviors of phenol formaldehyde (PF) resin under different vacuum degrees and explored the properties of 9-ply plywood panels hot-pressed under both vacuum and atmospheric conditions. The changes in core temperature and moisture content of the plywood mats during hot pressing were investigated as well. It was found that the gel times and gel temperatures of PF resin decreased with the increase of vacuum degree using a self-made device. FTIR spectra indicated the degree of polycondensation of hydroxymethyl gradually increased with the increase in temperature. It was also observed that a higher degree of vacuum led to a slower polycondensation reaction rate of PF resin. During different hot-pressing processes, the bonding strengths in the innermost and uppermost gluelines of the vacuum hot-pressed plywood panels were up to 30%-50% higher than their counterparts of conventional hot-pressed products. A less difference in the bonding strengths between these two gluelines was also observed for vacuum hot-pressed products. In addition, the core of vacuum hot-pressed plywood was found to have a greater heating rate and higher temperature at the final stage of hot pressing, which was beneficial to cure the PF resin. The results from this study indicate a promising potential of introducing a vacuum during hot pressing to improve the quality and productivity of plywood products and provide a basis for adopting vacuum to hot press wood composites.