Compressive deformation of wood impregnated with low molecular weight phenol formaldehyde (PF) resin I: effects of pressing pressure and pressure holding

Abstract

The deformation behavior of low molecular weight phenol formaldehyde (PF) resin-impregnated wood under compression in the radial direction was investigated for obtaining high-strength wood at low pressing pressures. Flat-sawn grain Japanese cedar (Cryptomeria japonica) blocks with a density of 0.34 g/cm³ were treated with aqueous solution of 20% low molecular weight PF resin resulting in weight gain of 60.8%. Oven-dried specimens were compressed using hot plates fixed to a testing machine. The temperature was 150°C and the pressing speed was 5 mm/min. The impregnation of PF resin caused significant softening of the cell walls resulting in collapse at low pressures. The cell wall collapse was strain-dependent and occurred at a strain of 0.05–0.06 mm/mm regardless of whether the wood was treated with PF resin. Thus, pressure holding causing creep deformation of the cell walls was also effective in initiating cell wall collapse at low pressure. Utilizing a combination of low molecular weight PF resin impregnation and pressure holding at 2 MPa resulted in a density increase of PF resin-treated wood from 0.45 to 1.1 g/cm³. At the same time, the Young’s modulus and bending strength increased from 10 GPa to 22 GPa and 80 MPa to 250 MPa, respectively. It can be concluded that effective utilization of the collapse region of the cell wall is a desirable method for obtaining high-strength PF resin-impregnated wood at low pressing pressures.