Chemical treatment ofbacksawn Tasmanian Oakwith Polyethylene Glycol (PEG) prior to drying

A series of experiments was conducted with the view of obtaining baseline informationon the use of polyethylene glycol (PEG) on Tasmanian Oak for the purpose of improvingthe quality of the seasoned structural timber product. Tasmanian Oak is the marketingname 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 toseasoning follows on from investigations in northern hemispheric timber species such ashoop pine and spruce in the middle of the 20th Century.PEG penetrates freshly sawn Tasmanian Oak in a manner which is considerate ofincubation time, temperature, PEG molecular weight/size and timber density.Histological examination indicated that PEG penetrated completely throughout thestructure 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 isnegligible. The rate of moisture content loss in Tasmanian Oak was shown to be retardedby PEG pre-treatment although the ability to prevent moisture loss was not concomitantwith dimensional stability. An investigation to explain the change in rate of moisture lossexamined effects on the thermodynamic property, water activity. Results indicated that achange in solution water activity could partly expain changes in the rate of moisturecontent loss, but more research is required to better divine this relationship.Shrinkage in Tasmanian Oak was reduced after treatment with aqueous PEG 400solutions at or above 30% (v/v), with a greater percentage reduction in tangentialshrinkage 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. Thereduction in shrinkage was consistent with PEG concentration in the incubating medium.A decrease in the formation of drying defect, such as surface and internal checkingaccompanied the improvement in keeping sawn dimensions.Backsawn Tasmanian Oak obtained from young trees (less than 20 years) from plantationresource presents a challenging profile for commercial timber drying and will becomemore prevalent as the logging of old-growth forests is phased out. Timber seasoners maybe faced with options of longer drying times or lower yields due to drying defect unless amethod can be developed to provide added protection to the sawn timber product duringdrying. At this stage, pre-treatment of Tasmanian Oak with PEG shows the hallmarks ofproviding a solution to this emerging dilemma.