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

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Ralph, J (2006) Chemical treatment of backsawn Tasmanian Oak with Polyethylene Glycol (PEG) prior to drying. PhD thesis, University of Tasmania.

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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.

Item Type: Thesis (PhD)
Keywords: PEG, polyethlene glycol, Tasmania, tasmanian oak, timber, treatment
Journal or Publication Title: bb
Publisher: University of Tasmania Library, Special & Rare Materials Collections
Date Deposited: 19 Jul 2007
Last Modified: 11 Mar 2016 05:54
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