School of Agriculture, Food and Ecosystem Sciences - Theses

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    Green sorting and alternative timber pre-treatments for drying and preservative treatment of radiata pine (Pinus radiata D. Don.)
    Semple, Kate ( 1999)
    This project contains research in the disciplines of wood drying and preservation. It first explored two different avenues for possible improvement in the drying stage of sawn timber production and then studied a novel pre-conditioning method for preservative treatment of radiata pine. The first avenue of investigation for improving drying was the sorting the green timber into more uniform charges for drying usina the large and consistent difference between heartwood and sapwood moisture content. The impetus for this arose from the results of a preliminary mill-based study of moisture content variation within a typical kiln charge of mixed radiata pine heartwood and sapwood. Green density can be quickly and easily determined from scanned board volume and loadcell-derived board weight and was found to be closely correlated to average board MC and heartwood/sapwood content. Green density was a very good predictor of wood % saturation also. Although little used here in Australia, such a sorting system has potential to improve efficiency in the drying and conditioning stages of radiata pine sawn timber production. Pre-drying treatments (i.e. pressure-steaming and pre-steaming) to improve permeability and drying speed in radiata pine were examined as the second avenue for possible improvement in drying efficiency. The effects of these pre-treatments on drying parameters including measured radial permeability were assessed. Pressure-steaming (a preconditioning. method to improve permeability of radiata pine heartwood for preservation) rendered the wood highly susceptible to checking during drying, resulting in severe degrade. Pressure-steaming also appeared to retard drying rates, especially in the sapwood -- a phenomenon contrary to the significant improvements to permeability found in the wood samples subjected to this pre-treatment. Pre-steaming at 100 C for 4 hours prior to drying was found to be only of marginal benefit to permeability and drying rates, although its use is necessary for drying heart-in studs and reducing early surface checking. The second part of the project tested the application microwave energy as a novel alternative pre-conditioning method to pressure-steaming for preservation treatment of radiata pine. The microwave pre-conditioning caused very rapid drying of the wood (especially the heartwood) and significantly increased the uptake of preservative solution. However, the distribution of preservative was variable compared with that in wood which had been pressure-steam conditioned. Complete and even preservative distribution was achievable only in wood which had been microwave pre-conditioned for longer time periods at high intensity irradiation. These conditions also resulted in severe checking of the wood and excessive use of preservative solution. The effects on the structure of the wood were somewhat different to those of pressure-steaming in that increased permeability and ease of preservative penetration were facilitated by visible checks of various sizes whereas with pressure-steaming, the wood was altered at the microstructural level, by destroying the soft contents of the rays and thereby significantly increasing permeability in the radial direction. The important conclusions from the study were: (a) There is considerable scope for developing and applying simple but effective pre-drying sorting of green sawn radiata pine to generate greater- uniformity in drying charges and better tailor drying schedules to their specific requirements. This would lead to improved drying efficiency and reduced degrade. (b) Contrary to initial expectations, pressure-steaming is a pre-conditioning method suited to preservation but not to drying, as demonstrated by the retarded drying and severe degrade in pressure-steamed wood samples. (c) Microwave pre-conditioning for preservation treatment of radiata pine has considerable advantages over pressure-steaming in terms of rapidity of drying and deserves further consideration and development. The microwave processing conditions used in this study yielded variable results, demonstrating that much further research is required to optimise the microwave processing conditions to better suit radiata pine.