Magnetic resonance spectroscopy and imaging as applied to the forestry sector : a thesis submitted to Massey University in partial fulfilment for the degree of Doctor of Philosophy

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The forestry sector in New Zealand ranks as the third largest export earner. The individual industries within the forestry sector have, in some cases, grown up on the basis of institutionalised knowledge, sometimes without a full understanding of the underlying fundamental physico-chemical relationships and the changes that occur during wood material processing. At the same time the commercial pressures of operating within the forestry sector have resulted in demand for more uniform, high quality, fit-for-purpose product, faster throughput and less downgrade from what is becoming a lower quality feedstock as harvest ages decline. In the 21st century, the forestry sector is being transformed into an "engineered ligno-cellulosic materials processor" and this in turn is requiring a more sophisticated knowledge of the material feedstock and the processes involved in wood products manufacture. The aim of this work was to use magnetic resonance techniques to explore aspects of ligno-cellulosic materials processing at points along the value-added process chain, namely drying, chemical modification (preservation) and re-engineering (gluing) of wood products. Magnetic resonance mini-imaging studies of the water transport during the drying of radiata pine boards rave shown differences in the directional movement of water within the wood structure. These effects show a dependence on the surface area to volume ratio of timber and the orientation of the annual rings with respect to the larger drying face. Narrow, flat-sawn boards exhibit anomalous drying behaviour in terms of water mass transport, whereas thicker boards display more conventional core-shell drying behaviour. These restrictions to flow have been further investigated using diffusion tensor imaging via a modified pulsed field gradient spin-echo sequence to elucidate the nature of anisotropic diffusion in wood. The direction of least restriction to self-diffusion is in the longitudinal direction, as would be expected with it being the direction of active transport within a tree stem, whereas the direction of greatest restriction to self-diffusion is in the radial direction, with the higher density latewood acting as a barrier. Preservation of radiata pine sapwood with novel boron-based preservatives has been investigated using magnetic resonance imaging to determine the penetration and retention of the incipient compounds. An apparent anomaly in retention for trimethylborate-treated Pinus radiata sapwood was investigated by 11B MAS NMR spectroscopy of excised sections of latewood and earlywood, which showed hydrolytic decomposition of trimethyl borate to form boric acid. The rate of hydrolysis of trimethylborate was monitored by 11B MAS NMR spectroscopy and was shown to occur very rapidly in the latewood (within 24 hours), and over a longer time scale of several days in the earlywood. The resulting publication has reported some of the first published 11B MRI images. Magnetic resonance spectroscopy has provided (in conjunction with separate mass spectrometry studies) mechanistic evidence for the accelerated curing of phenol-resorcinol formaldehyde resols, using ammonia in combination with the conventional paraformaldehyde hardener - a process known as GreenWeldTM. Carbon-13 and nitrogen-15 NMR spectroscopy has shown evidence of both benzylamine and aniline type bridging structures formed during cure, compared to only methylene structures being formed under conventional curing conditions. Poly(vinyl acetate) emulsion polymer adhesive has also been examined using NMR spectroscopic methods, with particular focus on the effect of addition of aluminium chloride, which is often commercially added to PVAc formulations as a crosslinking agent. Multinuclear magnetic resonance spectra obtained during the cure of AlCl3 modified PV Ac adhesive, shows a low frequency 27 Al shift of ca. 3 ppm suggesting a local change in environment as the aluminium changes from a solvated to a covalently bonded octahedral environment. Finally, as a preliminary study, a new device for measuring uniaxial extension of visco-elastic solids was trialled on poly(vinyl acetate) hydrogels to study the effect of addition of aluminium chloride on the visco-elastic properties of the adhesive. A similar, manually operated device was used to obtain NMR spectroscopic data during compression of the gel. These studies have shown that addition of aluminium chloride as a crosslinking agent, in fact produces a cured adhesive with fewer crosslinks than the corresponding unmodified adhesive, but with increased resistance to shear-induced creep.
Wood products testing, Wood preservatives, Wood preservatives, Rheology, Nuclear magnetic resonance spectroscopy, Magnetic resonance imaging, Adhesives, Moisture