Effective modification using phenol formaldehyde resin (PF) requires it to be cured within the wood cell wall. Researchers have proposed that diffusion plays a role in transferring PF from the cell lumens into the wall. Rate of diffusion is influenced not only by PF concentration, but its molecular size, cell wall chemistry/architecture, interaction of resin with wood and temperature, only some of which can be controlled. Maintaining a diffusion gradient requires removal of water from the lumens by drying to concentrate PF. Conventional kiln drying schedules have been developed to remove sap from fresh-sawn stems by control of temperature and relative humidity. Schedules are often specific to species and piece thickness and intended to dry wood quickly while minimising drying defect formation.
This publication deals with the challenges associated with developing drying schedules for PF impregnated pine. Challenges include: the re-distribution of PF at the macroscopic level results in it concentrating at piece surfaces which may negatively influence drying; the need for drying to be conducted at low temperature to prevent PF polymerization and the influence PF on evaporation of water. Methods used to investigate drying are described and showed the benefits of using traditional procedures, such as piece sampling and prong tests, along with the monitoring of relative humidity (%RH) with instrumentation adjacent to pieces being dried, which showed differences of up to 30% RH throughout the drying cycle.
Rate of drying and conditioning of pieces was investigated at temperatures ranging from 35-55°C, at differing rates of %RH reduction, fan speed, resin solids contents and in different piece thicknesses. In all cases the rate of drying was greater over the moisture content (%MC) range 70-30% than the 30-10% range. Rate of drying increased with increasing temperature, being twice and three times as great at 55°C than 35°C over the %MC ranges 70-30% and 30-10% respectively. Increasing the rate of %RH reduction increased the rate of drying by 11% and 23% over the same %MC ranges respectively. Increasing airflow through increased fan speed increased drying rate by 39% and 233% over the same %MC ranges respectively. Reducing the resin solids content by 25% was found to increase drying rate, while increasing piece thickness from 25mm to 50mm was found to reduce drying rate by 50% over the %MC range 70-30% and 55% over 30-10% range. Defects observed during drying include honeycombing, collapse and washboarding. Work showed it was important to ensure the %MC gradient between outer ‘faces’ and core did not exceed 20% to prevent defect formation. Using the schedules developed, kiln drying required 30 days which compared to 124 days for air drying and 18 days when vacuum dried at 45°C and 125 mbar. The resin present in air dried wood was viable to cure after this period, allowing completion of the industrial wood modification process.