School of Ecosystem and Forest Sciences - Theses

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    An evaluation of timber drying problems in terms of permeability and fine structure
    Kininmonth, John Alexander (1931-) ( 1970)
    The relationships of difference in rate of drying to permeability and wood structure were determined for two angiosperms and one gymnosperm. These investigations took two particular drying problems as a basis for study and attempted to explain why: - heartwood of Nothofagus fusca (red beech) takes many times longer to dry than sapwood. - green sapwood of Pinus radiata (radiata pine) dries readily but, if dried and pressure-treated with water-borne preservatives, its subsequent drying is greatly retarded. Test material was used from 14 trees of N.fusca from New Zealand, four trees of Eucalyptus regnans (mountain ash) and seven trees of P.radiata from Victoria, Australia and the experimental work was carried out under three headings: (a) Unidirectional drying. Small specimens, sealed on all except one pair of grain faces, were dried in a laboratory kiln at temperatures up to 60C. Comparisons were made between radial and tangential drying in sapwood and heartwood or in green and resaturated specimens; effects of treatments such as steaming were also assessed. Moisture gradients were determined to show the contribution of free water movement to overall drying. (b) Permeability studies. A method was developed to measure the transverse permeability of green wood to the flow of micro-filtered water; established methods were used for longitudinal permeability. Data for P.radiata met the requirements allowing application of Darcy's Law for flow of fluids through inert porous media and N.fusca approximated them. Pathways of flow were determined with chemical stains. (c) Wood structure. The transmission electron microscope was used to compare the appearance of pit membranes and the cell walls in sapwood and heartwood of N.fusca. In P.radiata, emphasis was on determining the percentage of bordered pits that were aspirated in sapwood - green, after drying and resaturation and after various treatments - and relating this to differences in drying and permeability. The main conclusions drawn from this study are: (a) The green sapwood of N.fusca and E.regnans is permeable to micro-filtered water in the radial and tangential directions. After drying and resaturation, the permeability of N.fusca is unchanged but that of E.regnans is drastically reduced, particularly in the tangential direction. The heartwood of both species is impermeable when tested at a pressure differential of 40 cm.Hg. (b) Differences in the permeability of N.fusca can be explained by differences in the appearance of pit membranes in sapwood and heartwood: in heartwood, the membrane surfaces are usually completely occluded when viewed as replicas in a transmission electron microscope; in sapwood, the surfaces are always less occluded often exhibiting a clean primary well texture. It is inferred from studying the effects of various extraction treatments that the pit membrane surfaces in sapwood are less occluded than indicated by the appearance of replicas. (c) Plasmodesmata may provide pathways for mass movement of liquids in the radial direction in the wood, but, in other pits, without obvious pores, permeability probably results from movement through the general structure of the pit membrane. (d) Heartwood of N.fusca takes several times longer to dry than sapwood because of its reduced permeability coupled with lower rates of moisture diffusion. (e) Contrary to previous reports, at least 80 percent of the bordered pits in green sapwood of P.radiata are open, irrespective of distance from the outside of the tree. After drying and resaturation most pits are aspirated and the wood is much less permeable than in the green state. (f) The condition of the bordered pits has an effect on the rate of drying in the tangential direction - causing a marked reduction in resaturated material - but has no appreciable effect on radial drying which is little different in green or resaturated wood.
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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.
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