School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemAcceleration of the chemical modification of wood using microwave heating and catalyst( 2011)Wood is a versatile material with many superior properties. However, dimensional instability and biodegradability are the most undesirable properties of wood. There are different methods of improving wood properties and chemical modification is one potential approach. Interest in the chemical modification of wood arises from the need to substitute conventional biocides currently used to protect timber from decay and insect attack while maintaining a substrate that is dimensionally stable. Lower moisture content, preferably 5-6%, is a prerequisite for most of the chemical modification reactions. Economically it is not feasibly to dry the timber to such low moisture content by conventional drying. However, microwave heating was evaluated and found to be effective in reducing the moisture content of radiata pine from 13% to 6% in a microwave cycle of 2 minutes without any drying degrade. The energy consumed to achieve this was 55 kWh/m3. Acetylation of wood is considered to be one of the most promising ways of enhancing wood properties. However, conventional heating leads to a long reaction time. Reduction in reaction time is an advantage both in terms of economy of the process and properties of the end product. The use of potassium acetate as a catalyst, methanol and water as solvent carriers for this catalyst and microwave heating were investigated to accelerate the acetylation reaction. The properties of the microwave assisted acetylated wood were compared with those of the conventional heated acetylated wood. Solvents imparted no significant effect on the rate and degree of reaction. Potassium acetate significantly accelerated the reaction. The rate of acetylation was 7.5 times faster in catalysed condition after 30 minutes of reaction. In the catalyzed reaction, an average weight gain of 20.6% was achieved after 30 minutes reaction time, while for the uncatalyzed reaction it was less than 3%. Microwave heating proved to be effective in the acetylation of wood in the presence of potassium acetate. However, a lower catalyst loading is desirable since higher loadings led to charring of samples, particularly where acetylation uses a limited supply of reagent. A weight gain of 21.6% was achieved in 10 minutes of microwave heating using a catalyst compared to 13.4% where no catalyst was employed. The presence of a catalyst provided more uniform acetylation across the sample thickness compared to the uncatalyzed reaction. The catalyst and mode of heating did not influence the adsorption behaviour of the acetylated sample once the catalyst was leached. Un-leached acetylated samples showed significantly higher EMC values, particularly at higher relative humidities, due to the hygroscopic nature of the catalyst. The mode of heating and catalyst also had no influence on the swelling coefficient, anti-shrink efficiency, modulus of elasticity, modulus of rupture, resilience and decay resistance of the samples. Improvement in the mechanical properties was anticipated with the reduction in reaction time. Although the use of a catalyst and microwave heating reduced the reaction time significantly, no improvement in the strength properties was achieved. Wood modification with furfuryl alcohol results in a wood polymer composite. Curing by conventional heating is a time consuming process. The dielectric properties of furfuryl alcohol indicate that it can be heated under a microwave field. Microwave curing of furfuryl alcohol caused complete polymerization but the resin yield was 7% lower compared to conventional curing. Oven curing of impregnated samples caused significantly higher weight percent gains compared to microwave curing. A microwaving time of 10 minutes was optimal in terms of weight percent gain and uniformity of colour. Microwave cured samples showed similar anti-shrink efficiency and decay resistance to oven cured samples even at lower weight gains.
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ItemPhysical and chemical modification of the bamboo species Dendrocalamus asper( 2011)In spite of its numerous beneficial properties, bamboo is dimensionally unstable and susceptible to attack by fungi and insects. Preservative treatment is therefore regarded as a necessity. Preservative treatment must provide sufficient uptake and even distribution in the culms for adequate protection. The treatability of bamboo, however, is generally poor. Microwave technology was evaluated for improving wood permeability. Microwave surface modification of radiata pine peeler cores using the PC-1 applicator was studied in this thesis. The temperature distribution, preservative uptake and distribution, and macro-structure changes were evaluated. Results show that the PC-1 applicator allowed microwave energy to modify predominantly the surfaces of peeler cores and significantly improve permeability as a result of the modification. The PC-1 applicator was suitable for bamboo culm because bamboo is hollow in the centre. The applicator allows microwave energy to heat the culm walls and not specifically heat the centre. The study of energy distribution in this thesis shows that the PC-1 applicator effectively distributed microwave energy onto the surface of the bamboo culm. Temperature distribution on bamboo culm surfaces was similar to those of the peeler cores. Macro-structure changes to the bamboo culm after microwaving were different from those in radiata pine as the structure of bamboo is different from wood. There is no specific fracturing pattern in the bamboo culm as they contain no ray tissue. Surface treatment of bamboo nodes resulted in checking only occurring in the culm walls. These checks created pathways for liquid to penetrate into the culm walls without necessarily filling up the central hollow section. The use of the PC-1 applicator with microwave energy to modify bamboo nodes was found effective in improving liquid uptake. Based upon successful chemical modification of wood, bamboo strips were subjected to two chemical modification processes (acetylation and furfurylation). Boiling bamboo strips with acetic anhydride achieved the highest weight percent gain compared to heating in a conventional oven. Bamboo strips boiled in acetic anhydride for an hour gained 16% weight compared to the conventional method (heating in an oven) that gave rise to a 3.5% weight gain after 24 hours of reaction time. Fourier transform spectra analysis confirmed an increase of the number of acetyl groups and a reduction in the number of hydroxyl groups in the acetylated bamboo strips. For furfurylation of bamboo, soaking, vacuuming and combinations thereof were evaluated. Results demonstrated that soaking bamboo strips for two days after vacuum treatment achieved optimum uptake of furfuryl alcohol solutions and gave rise to an 80% weight gain. Catalyst addition during furfurylation did not increase the weight percent gain. Water was an effective solvent carrier for achieving higher weight gains. The aim of chemical modification of bamboo strips was to reduce water sorption, improve dimensional stability and weathering performance and enhance natural durability. Results of the study suggest that water sorption of chemically modified bamboo strips was lower than non-modified strips, leading to dimensional stability enhancement. After undergoing accelerated weathering, acetylated and furfurylated specimens were still effectively protected from absorbing water. However, chemical modification did not reduce the greying process during weathering. Chemical modification was also effective in protecting bamboo stakes from decay in a soil bed test. Side effects of chemical modification included alteration of the mechanical properties and colour changes. These were examined as a measure of the quality of the end product. The results of the analysis suggested that bending stiffness and bending strength of acetylated bamboo strips were slightly decreased. Furfurylated bamboo strips showed a slight decrease in bending stiffness and a slight improvement in bending strength. The colour of chemically modified bamboo was darker and redder than the natural colour of bamboo.