Moisture in softwoods: fiber saturation point, hydroxyl site content, and the amount of micropores as determined from NMR relaxation time distributions

Abstract

Distributions of nuclear magnetic resonance (NMR) relaxation times provide detailed information about the moisture absorbed in wood. In this work, T2*, T2, and T1 distributions were recorded from fresh sapwood and heartwood samples of pine (Pinus sylvestris) and spruce (Picea abies) at various temperatures. Below the melting point of bulk water, free water is frozen and its signal disappears from the distributions. Then, the low-temperature distributions of the unfrozen bound water contain more information about its components, because the large free water peaks hiding some smaller bound water peaks are absent and the exchange between free and bound water is prevented. Comparison of the total moisture signal integrals above and below the bulk melting point enables the determination of fiber saturation point (FSP), which, in this context, denotes the total water capacity of cell wall. T2*, T2, and T1 distributions offer different kinds of information about moisture components. All the peaks in the distributions were assigned, and it was demonstrated that the accessible hydroxyl site content and the amount of micropores can be estimated based on the peak integrals.

The states of water in Norway spruce (Picea abies (L.) Karst.) studied by low-field nuclear magnetic resonance (LFNMR) relaxometry: assignment of free-water populations based on quantitative wood anatomy

“…The T 2 value of Peak 1 ( Table 5) was similar to the T 2 value that was ascribed to cell wall water in previous studies (Menon et al 1987;Araujo et al 1992Araujo et al , 1994Labbé et al 2002Labbé et al , 2006Thygesen and Elder 2008;Telkki et al 2013). A significant difference was seen between mature and juvenile wood as well as between EW and LW (Table 6).…”

“…The 1 st peak, generally at <3 ms (Menon et al 1987;Araujo et al 1992Araujo et al , 1994Labbé et al 2002Labbé et al , 2006Thygesen and Elder 2008;Telkki et al 2013) is assigned to the water bound in the cell wall, while peaks with a longer T 2 are assigned to free water. This differentiation between bound water and free water is made because the former has a T 2 which is constant and independent of MC changes above the fibre saturation point (FSP) (Araujo et al 1992).…”

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

“…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.…”

Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging

“…Another important advantage of NMR over other methods is its ability to distinguish between bound and free water by relaxation analysis (T 2 ), i.e. the decay of the NMR signal (Riggin et al 1979;Menon et al 1987;Araujo et al 1992;Telkki et al 2013). For detailed information, please see section ''Relaxation analysis of water in wood''.…”