School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemAn evaluation of timber drying problems in terms of permeability and fine structure( 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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ItemMethods of improving the preservative treatment of radiata pine and eucalyptus with emphasis on the use of novel boron compounds( 1998)This thesis identifies the effect of raw materials and processing parameters on the application of TMB on wood and wood products. It also describes the effect of TMB on the properties of wood and wood products. This work proposes the application of TMB by momentary immersion following by storage in a close container. The effect of wood moisture content, temperature and time were studied. Low wood moisture content (3%) is required to produce high penetration of the TMB in radiata pine (Pinus radiata D. Don) sapwood. High temperatures (40°C) during storing stage can result in adequate penetration. The storage stage is described in two phases, the emission of chemicals from the wood and the diffusion of the chemical from the head space of the container into the wood. Parameters affecting the process include temperature, wood moisture content and time. Wood moisture contents as low as 3% were found necessary to achieve high penetration of chemicals into wood. During the storage of treated timber, high temperatures reduced the emission and vapour diffusion phases. The levels of TMB and methanol in the headspace of the container were lower at high temperatures. The concentration of TMB and methanol in the head space of the desiccator achieved an equilibrium concentration at 100 hrs for samples stored at 40°C and 200 hrs for samples stored at 20°C. Methanol emission produced by the interaction between water in the wood and TMB was found in low levels. Samples of radiata pine were preconditioned to 3% wood moisture content, treated by momentary immersion in TMB and then stored at 20°C. After vapour diffusion the samples were submitted to an airflow of 0.25 L/min for 24 hrs. Emissions were collected in a water trap. For samples treated at 3% moisture content and stored at 20°C, the analysis showed that only 9.6% of the total production of methanol was found in the emissions. For radiata pine this equals to 28 g/m3/hr. Methanol in the wood is removed easily by water. However a strong bond between wood and methanol is formed. Samples of different species were saturated with methanol and then dried at 105°C for 24 hrs. Twenty nine grams of methanol was found in radiata pine after this process. Pre-conditioning of wood prior to momentary immersion improves the penetration and retention of TMB. Several pre-conditioning processes were studied. Steaming reported better absorption and penetration than microwaving, high temperature and air drying. TMB penetration and a retention of 2.48L/m3 was achieved when steamed radiata pine samples were immersed in TMB for 30 secs. These values were statistically different from values obtained with other processes. Momentary immersion of particleboards in TMB following by a storing stage in a close container was studied. Wood moisture content required to achieve total penetration was 8% for particleboards of thickness 12 and 18 mm. Thicker panels required lower moisture contents. Stiffness (MOE) and strength (MOR) was reduced as maximum as 13 and 17% respectively. Momentary immersion in TMB following by vapour diffusion in a close container was successfully trialed in sapwood of radiata pine, messmate (Eucalyptus obliqua L'Herit), jarrah (Eucalyptus marginata Donn. Ex Sm) and blue gum (Eucalyptus globulus Labill). The effect of non-polar solvents as carriers upon vacuum-pressure impregnation of timber with TMB was studied. Total penetration in sapwood of radiata pine was achieved at wood moisture contents below 14%. The rate of hydrolysis of TMB in non- polar systems was measured and compared with other systems. Non-polar systems have a rate of hydrolysis 26 times slower that polar systems. This allows TMB to penetrate deeper into the timber before hydrolysis begins. Different polymers of trymethylborates were also tested in these systems. Methylpolyborates, a polymer of TMB, were used in the manufacture of pastes and rods for remedial treatments of wood. Methylpolyborates in combination with copper and fluoride compounds were used as active in the manufacture of pastes and rods. Diffusion in stubs of red gum (Eucalyptus camaldulensis Dehnh.) after 60 days demonstrate the feasibility of these TMB polymers as diffusible wood preservatives.