Microwave Surface Modification of Radiata Pine Peeler Cores for Preservative Treatment
K Sugiyanto, G Torgovnikov, P Vinden
Radiata pine peeler cores are classified as a by-product derived from the cores of logs left over after the veneer peeling process. The core can be potentially developed for value added solid wood products as it is abundantly available and has a constant diameter. However, the core needs to be preservative treated because of low durability in ground contact. An additional problem is that the wood is primarily heartwood and difficult to impregnate with wood preservatives. Recently, microwave (MW) technology has been developed for improving the permeability of wood. Microwave energy absorbed by timber generates steam pressure within the wood cells and creates narrow micro voids in the radial-longitudinal plane. A several thousand-fold increase in wood permeability can be achieved in wood species that previously were considered impermeable. In Australia peeler cores measuring 130 mm in diameters need to be preservative treated to at least 8 -20 mm depth depending on the hazard class (above ground or ground contact) before being utilized. This means that the preservative is required to penetrate the surface only. A special microwave applicator (PC-1) that operates on a frequency of 2.45 GHz has been developed for timber surface treatment. The applicator has been designed and built based on computer modelling of the interaction between timber and microwave energy.
A key aim of this research is to examine the effect of surface treatment using applicator PC-1 that meet peeler core preservative treatment standards. The methodology includes temperature distribution measurements in the peeler core after microwave heating and preservative distribution and uptake measurement after heating. The results show that after microwave treatment, the highest temperatures are recorded on the surfaces of the core. There is then a gradual decrease in temperature with depth in the timber. Experiments show that applicator PC-1 can modify radiata pine peeler cores to meet Australian Standards requirements. CCA uptake after pressure impregnation could be controlled to within the range between 94 to 314 L/m3 depending on the applied microwave energy (in the range of 55 – 90 kWh/m3) and processs parameters. Preservatives penetrate through the induced microvoids in the radial longitudinal direction of the wood to a limited depth of penetration depending on the intensity of power application. Control samples had an average uptake of 35 L/m3, which means microwave modified logs had 3-9 times higher uptake than the controls.