Proton magnetic resonance techniques for characterization of water in wood: application to white spruce

Summary

Two new proton magnetic resonance techniques, relaxation spectra and relaxation selective imaging, have been used to investigate the distribution of water in samples of normal white spruce sapwood, heartwood, and juvenile wood as well as two rehydrated heartwood samples containing incipient decay and compression wood respectively. It is demonstrated that the spin-spin (T₂) relaxation behavior in wood is best presented as a continuous spectrum of relaxation times. Spectra of T₂ for white spruce show separate peaks corresponding to the different water environments. Bound water gives a peak with an T₂ time of about 1 ms and lumen water gives a distribution of T₂ times in the range of 10 to 100 ms. The lumen water T₂ time is a function of the wood cell radius. Consequently, the different cell lumen radii distributions for spruce sapwood, juvenile wood, and compression wood are readily distinguishable by the shape of their T₂ spectra. Water environments which are separable on a T₂ spectrum may be imaged separately. Imaging has been carried out in one dimension for bound water and lumen water of a spruce sapwood sample at four different moisture contents ranging from 100% to 17%. For the first time, we demonstrate that above the fibre saturation point the moisture density profile of the bound water is largely independent of moisture content. The feasibility and utility of using these techniques for internal scanning of logs and lumber is discussed. These techniques should provide new insights into the wood drying process.