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

Abstract

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 T 2 for white spruce show separate peaks corresponding to the different water environments. Bound water gives a peak with an T 2 time of about 1 ms and lumen water gives a distribution of T 2 times in the range of 10 to 100 ms. The lumen water T 2 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 T 2 spectra. Water environments which are separable on a T 2 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.

A critical discussion of the physics of wood–water interactions

“…Firstly, the three phenomena do normally not occur at the same moisture content as shown by Stamm (1971). Secondly, during drying, liquid water may still be present in lumens as water desorbs from cell walls of other parts of the same sample (Araujo et al 1992;Hernández and Bizoň 1994;Almeida and Hernández 2006a, b;Hernández and Pontin 2006). Another definition of FSP was proposed by Stone and Scallan (1967) as the moisture content corresponding to ''the amount of water contained within the saturated cell wall''.…”

Experimental techniques for characterising water in wood covering the range from dry to fully water-saturated

“…Consequently, it is a good technique for determining the distribution of water within different parts of the wood structure (Araujo et al 1992). The technique uses the very, very slight magnetic polarisation in a material caused by a permanent magnet within the spectrometer (typically up to a few tesla in strength).…”

“…In particular, CPMG is a commonly used sequence for determining T 2 relaxation as it reduces effects of magnet inhomogeneity by multiple refocusing of the spins in the transverse plane. Several studies have employed LFNMR for characterising water in both untreated Araujo et al 1992Araujo et al , 1994Cox et al 2010;Flibotte et al 1990;Fredriksson and Thygesen 2017;Labbé et al 2002Labbé et al , 2006Menon et al 1987;Passarini et al 2015;Telkki et al 2013;Elder 2008, 2009) and modified wood (Elder et al 2006;Hietala et al 2002;Javed et al 2015;Kekkonen et al 2014;Elder 2008, 2009). Based on a deconvolution of the decay curves, signals from water in different compartments within the wood structure have been analysed Araujo et al 1992Araujo et al , 1993Fredriksson and Thygesen 2017;Passarini et al 2015) (see Fig.…”

An Investigation of the Drying Rate of Water in Wood at Different Relative Humidities Studied by Time Domain Nuclear Magnetic Resonance

“…The most popular method for investigating moisture content in wood is the nuclear magnetic resonance (NMR) technique; as a nondestructive and effective determination, it has been used for the determination of moisture content for some time (Nanassy 1976;Sharp et al 1978). Moreover, the free water and bound water NMR relaxation properties can be identified simultaneously using NMR (Schmidt 1991;Araujo et al 1992). Generally, free water connects with wood loosely and has spin-spin relaxation time (T2) values from tens to more than 100 ms.…”

“…Generally, free water connects with wood loosely and has spin-spin relaxation time (T2) values from tens to more than 100 ms. The bound water connects with wood tightly and has short T2 values with a few milliseconds (Menon et al 1987;Araujo et al 1992).…”