School of Agriculture, Food and Ecosystem Sciences - Theses

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    Drying and treatment of veneer for high-performance panel products
    Subarudi ( 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.