Chemical treatment of backsawn Tasmanian Oak with Polyethylene Glycol (PEG) prior to drying

Abstract

A series of experiments was conducted with the view of obtaining baseline information on the use of polyethylene glycol (PEG) on Tasmanian Oak for the purpose of improving the quality of the seasoned structural timber product. Tasmanian Oak is the marketing name for a triad of Tasmanian-grown eucalypt species (E. delegatensis, E. obliqua, and E. regnans). Incubation of freshly-milled timber in aqueous PEG solutions prior to seasoning follows on from investigations in northern hemispheric timber species such as hoop pine and spruce in the middle of the 20th Century. PEG penetrates freshly sawn Tasmanian Oak in a manner which is considerate of incubation time, temperature, PEG molecular weight/size and timber density. Histological examination indicated that PEG penetrated completely throughout the structure of the wood substance in three orientations (transverse, radial and tangential). During air-drying of PEG soaked timber, further migration of PEG into Tasmanian Oak is negligible. The rate of moisture content loss in Tasmanian Oak was shown to be retarded by PEG pre-treatment although the ability to prevent moisture loss was not concomitant with dimensional stability. An investigation to explain the change in rate of moisture loss examined effects on the thermodynamic property, water activity. Results indicated that a change in solution water activity could partly expain changes in the rate of moisture content loss, but more research is required to better divine this relationship. Shrinkage in Tasmanian Oak was reduced after treatment with aqueous PEG 400 solutions at or above 30% (v/v), with a greater percentage reduction in tangential shrinkage compared to reduction in radial shrinkage. This is significant as backsawn (a.k.a. flatsawn) timber, with its broader tangential face, was in particular focus. The reduction in shrinkage was consistent with PEG concentration in the incubating medium. A decrease in the formation of drying defect, such as surface and internal checking accompanied the improvement in keeping sawn dimensions. Backsawn Tasmanian Oak obtained from young trees (less than 20 years) from plantation resource presents a challenging profile for commercial timber drying and will become more prevalent as the logging of old-growth forests is phased out. Timber seasoners may be faced with options of longer drying times or lower yields due to drying defect unless a method can be developed to provide added protection to the sawn timber product during drying. At this stage, pre-treatment of Tasmanian Oak with PEG shows the hallmarks of providing a solution to this emerging dilemma.