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Characterization of water in wood below the fibre saturation point

University of British Columbia

The purpose of this study was to explore the nature of water in the wood below the fibre saturation point. The approach was two-fold: (1) to experimentally determine the sorption isotherms for sapwood and heartwood of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and sitka spruce (Picea sitchensis (Bong.) Carr.), and use this data to evaluate theoretical concepts; and, (2) to use pulse proton nuclear magnetic resonance (NMR) with matched specimens at moisture contents below the fibre saturation point to determine the interaction of water to the wood structure. Also, water clustering analysis of the sorption data was done using the Zimm-Lundberg water cluster theory. The inital desorption, adsorption and secondary desorption isotherms were determined and non-linear regression techniques were used to fit four sorption models to the data. The BET model did not fit very well to the data. The Dent and the H-H models were mathematically identical. Since the Le-Ly model had more parameters, it fit the data well. The equilibrium moisture contents were generally lower for the heartwood region compared to the sapwood region for the same species. The adsorption to secondary desorption ratios were found to be: hemlock sapwood: 0.830; hemlock heartwood: 0.823; spruce sapwood: 0.898; and, spruce heartwood: 0.869. Tangential and radial dimensional changes were calculated based on oven dry measurements during the sorption experiments, and it was found that they were independent of the sorption process. From the NMR experiments, moisture contents and second moments were determined from the free induction decay curve. Average relative spin densities, which were needed to calculate the moisture contents from NMR experiments, were calculated from known moisture contents and were: hemlock sapwood: 0.616; hemlock heartwood: 0.537; spruce sapwood: 0.679; and, spruce heartwood: 0.421. The second moments were higher at low moisture contents as compared to those near the fibre saturation point. The condition as to how the equilibrium moisture content was attained did not influence the second moments for hemlock; however for spruce the second moments were sorption dependent. The decay curve from the CPMG experiments was decomposed into two components for each species; a long component was determined with a T₂ > 5 ms and a short component with T₂ < 2 ms. Unfortunately, the heterogeneous and complex chemical composition of wood-water systems produced a mixture of NMR signals that were difficult to interpret relative to verifying specific concepts of cluster theory. An attempt was made to extract as much information as possible and speculate about the nature of the bound water in wood the results.

Thesis/Dissertation

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2009-04-14

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http://hdl.handle.net/2429/7083

Forestry

University of British Columbia

UBCV

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