School of Ecosystem and Forest Sciences - Theses
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ItemMicrowave conditioning and drying of wood veneer( 2000)Several aspects of microwave veneer drying are described. The final moisture content profiles and quality of two dried veneer thicknesses A and B (3 and 1.5 mm in thickness respectively) are determined. Veneer gluability of the two veneer thicknesses, dried by two different methods, are examined. The dimensional stability of veneer and plywood samples made from these dried veneers are also examined. Green weight per unit area was used as an indicator of moisture content during drying. A wide variation in final moisture content of dried veneer was found to be due to the presence of earlywood and latewood in veneer sample A and juvenile wood in veneer sample B. Green veneer sorting, based on water content per area or volume, green weight per area and percent saturation, can be used to improve drying performance. Statistical analysis indicates that the final veneer moisture content profiles estimated from green weight per unit area for two dried veneer thicknesses A and B dried by microwave, are not significantly different from those under conventional drying. However, the result indicates that microwave drying resulted in a higher degree of moisture content uniformity for both veneer thicknesses, whereas excessive drying occurred on the outer sections for both veneer thicknesses under conventional drying. A dark color was found in the dried veneer of both thicknesses under microwave drying compared to those of conventionally dried veneers. Statistically, both drying methods resulted in similar veneer quality, except microwave drying resulted in better veneer quality with respect to flatness, and has a low flatness ratio. Microwave dried veneer also had low percent shrinkage for both veneer thicknesses compared to conventionally dried veneer samples. Veneer gluability for veneer A is better than for veneer B, higher in the percentage of wood failures and greater glueline shear strength. Microwave drying resulted in better veneer gluability. Although microwave-dried plywood samples had a higher percentage of water absorption for both veneer thicknesses compared to conventional samples, they had lower percentage thickness swelling. In addition, the percent failure did not represent the glue bond quality of plywood samples.
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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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ItemGreen sorting and alternative timber pre-treatments for drying and preservative treatment of radiata pine (Pinus radiata D. Don.)( 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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ItemDrying and treatment of veneer for high-performance panel products( 1995)At the present time, plywood has lost market share to competitors such as waferboard and oriented strand board (OSB). Improvement in plywood production and plywood performance is required to make plywood competitive with other wood based panel products. The objectives of this study are: (1) to examine the possibility of using microwave for drying rotary cut veneer, (2) to investigate treatment methods for veneer both before and after fabrication of panels using diphenyl-methane diisocyanate (MDI), (3) to test the characteristics of veneer and panels visually and mechanically. The experimental (preliminary and main research topics) approach to addressing this question involves investigating the characteristics of high performance panel products. All experimental designs used in this study are factorial ANOVA models for different factorial levels. A one factorial design was used to analyse the effect of moisture content (0% and 10 %) on the % solid MDI uptake of veneer samples, and the effect of % MDI concentrations (100%, 80%, 60%, 40%, and 20%) on the % solid MDI uptake of small cube wood samples. A two factorial design was used to investigate uptake of MDI in veneer and plywood treatments. The variables tested included three concentrations of MDI (50%, 60% and 70%) and three levels of soaking times (5. 10, and 15 minutes). Another two factorial design was used to analyse the effects of five levels of % MDI concentrations (100%, 80%, 60%, 40%, and 20%) and three levels of temperature setting (60 , 100, and 140 C) on the curing time of MDI solution for small cube wood samples. Analysis of the effects of edge sealing (sealed and unsealed) and pressure type (low and high pressure) on the uptake MDI and Copper Naphthenate was analysed using a two factorial design. The study shows that microwave drying of veneer is technically feasible to cope with veneer drying problems, which at the present time poses a "bottleneck" and is the major consumer of thermal energy used in plywood production. Microwave drying is characterised by having low within charge variability in moisture content after drying; no drying defects and has no effect on the glueability of plywood. However, some importance aspects of the use of microwaves should be taken into account- in designing a commercial drying process. These include: the position, condition, type of wave guide covers and the power level of microwave. Microwave power had a significant effect on drying time. The higher the microwave power used the shorter the drying time. The highest uptake of solid MDI in plywood samples was obtained with 70 MDI following 10 minutes soaking (51 kg/m3). This was followed by 60 MDI (45 kg/m3) after 15 minutes soaking and 50 % MDI after 15 minutes soaking (41 kg/m3) respectively. The highest solid MDI uptake in radiata pine veneer was obtained with 70 % MDI following 10 minutes soaking (115 kg/m3). This was followed by 60 % MDI (94 kg/m3) after 15 minutes soaking and 50 % MDI after 15 minutes soaking (71 kg/m3) respectively. Investigation into the curing processes indicated that the best temperature for curing MDI solution was 60 C. Microwave energy could not be applied for curing MDI treated samples. Microwaving resulted in substantial losses of MDI solution from the wood blocks. Pressure impregnation of MDI, increased the uptake of resin compared to soaking techniques. The highest uptake of solid MDI for plywood samples was obtained with 70 % MDI in unsealed samples using high pressure treatment (359 kg/m3). This was followed by the treatment of plywood which had been edge sealed and high pressure treated (208 kg/m3), sealed and low pressure treatment (71 kg/m3) and unsealed and low pressure treatment (51 kg/m3), respectively. The highest uptake of Copper Naphthenate in plywood samples was obtained for edge-sealed and high pressure treatment (0.92 % wt/wt Cu) followed by unsealed and high pressure treatment (0.88 % wt/wt Cu), unsealed and low pressure treatment (0.45 % wt/wt Cu) and unsealed and low pressure treatment (0.51 % wt/wt Cu), respectively. MDI solution applied to plywood and veneers by immersion improved the surface qualities of these products. The surfaces were smoother, more rigid; there was water resistance compared to untreated samples. Water absorption was limited to 8 kg/m3 after 5 minutes exposure compared to 84 kg/m3 for untreated plywood. The average surface hardness of MDI treated veneers (82) was 4 points (in Shore C scale) higher than untreated veneers (78). The greatest hardness was obtained for samples treated with 50 % MDI solution (85), followed by 60 % (83) and 70 % (78) MDI solutions. The average surface hardness of MDI treated plywood using soaking treatment (85) was 6 points higher than untreated plywood (79). When MDI was applied by pressure impregnation surface hardness scores of 90 were obtained. The average shear strength of MDI treated veneer (0.183 kN) is 35 % higher than untreated veneers (0.135 kN). The highest values were obtained for samples treated with 50 % MDI solution (0.202 kN), followed by 70 % (0.178 kN) and 60 % (0.169 kN) MDI solutions. The average shear strength of MDI treated plywood using soaking treatment was 0.125 kN. This was 24 % higher than untreated plywood (0.101 kN). This shear strength value is 19 % higher when compared to that for Cu Naphthenate treated plywood using pressure treatment (0.105 kN). It can be concluded that MDI resin can be used to improved the water resistance, surface hardness and strength properties of veneer and plywood. Cu naphthenate (in kerosene) preservative can also be used for treating finished plywood without affecting the strength properties of treated plywood.
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ItemThe effect of desiccation on the germination of some tropical tree seeds( 1991)The aim of this study was to investigate the influence of desiccation on the germination behaviour of some tropical tree seeds when tested under controlled conditions following exposure to different moisture regimes. The results were used to classify the seed of the observed species into the physiological categories of recalcitrant or non-recalcitrant and to provide practical recommendations for effective storage. Six species of tropical tree seeds, Cryptocarya triplinervis, Neolitsea caalbata, Acmena hemilampra, Podocarpus elatus, Azaarachta indica and Pittosporum rubiginosum were either forced dried at 300 C (22-28% RH), or dried slowly at 200 C (52-64% RH). The water content of the seeds was measured during drying and the percentage loss in water content and the rates of drying varied for different species. All seeds, except for A. indica were stored for two months at room temperature (230 q and at 40 C. A. indica seeds were stored in either (i) a desiccator with silica gel as a desiccant, (ii) a jar where the seeds were sterilized on the surface or (iii) laminated - polyethylene bags placed in a freezer. The period of storage was 6 weeks and the seeds were removed periodically to assess the viability. The embryo and the female gametophyte of P. elatus seeds were measured after 7 and 14 days of incubation to determine structural changes in the seeds due to the effect of desiccation. An investigation of the effect of the pattern of drying on subsequent seed quality of some of the species (C. triplinervis , A. indica and P. rubiginosum ) showed that the rapid drying gave better viability. On the other hand, the rapid drying method resulted in a gradual loss of viability of N. dealbata and P. elatus seeds. However, both of drying patterns did not so damage A. hemilampra seeds. The critical seed moisture content below which seeds could not recover from desiccation were 30% for C. triplinervis, 7% for N. dealbata, 35%- 37% for P. elatus and 13 5%f or P. rubiginosum. A. hemilampra seeds remained viable at a moisture content of 29% The lowest moisture content for A. indica was not determined because the viability of seeds was reduced during the experiment. The viability of seeds of C. triplinervis, A. hemilampra and P. elatus can be maintained after 10 weeks storage at 40 C given the maintenance of high moisture content. However, N. dealbata and P. rubiginosum seeds should be kept at 40 C because of the relatively low moisture content. Placing the seed in a desiccator gave the best result for A. indica. C. triplinervis, P. elatus and A. indica seeds may be categorized as recalcitrant. N. dealbata and P. rubiginosum seeds should be classified as non-recalcitrant species. A. hemilampra seed cannot yet be classified because the critical moisture content is not yet known. There were structural differences in the seed components of P. elatus during the course of germination due to desiccated either by rapid or slow drying.