Absorption of Water in Thermally Modified Pine Wood As Studied by Nuclear Magnetic Resonance

Abstract

Thermal modification is an environmentally friendly method to increase the lifetime and improve the properties of timber. In this work, we investigate absorption of moisture in thermally modified pine wood (Pinus sylvestris) immersed in water using various nuclear magnetic resonance (NMR) methods. Magnetic resonance images (MRI) visualize the spatial distribution of absorbed free water. Spin−echo spectra measured both below and above 0 °C reveal that thermal modification partially blocks the access of water to cell walls; even modification at 180 °C slightly reduces the amount of bound water, and the amount decreases about 80% in the case of the sample modified at 240 °C. The spectra and MRI show that, above the modification temperature of 200 °C, the amount of free water decreases, indicating that high modification temperature tends to close the pits connecting the wood cells. T 2 relaxation time distributions measured using the Carr−PurcellMeiboom−Gill sequence show four components, two associated with bound water and two with free water. NMR cryoporometry measurements indicate that the bound water sites are mostly below 2.5 nm in size. A unique combined NMR cryoporometry and relaxometry analysis showed that the size of cell wall micropores is between 1.5 and 4.5 nm, and thermal modification significantly hinders the access of water to the pores.

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

“…It results in a relatively short relaxation time, called bound water. When the MC increases towards the FSP, as in case II, Table 2 The measurement settings of HSE and CPMG pulse sequences used during water uptake and drying of all selected wood types t e (ls) t ww (ls) (Menon et al 1987;Kekkonen et al 2014). The wood cellulose has very short T 2 around tens of microseconds, which is too short to be observed in the used NMR set-up.…”

“…These studies provide spatially resolved one dimensional moisture profiles, but no spatially resolved relaxation analysis. The role of the porosity on water transport was studied by Kekkonen et al (2014). They investigated absorption of water in thermally modified wood by applying various NMR methods.…”

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

“…Low-field nuclear magnetic resonance spectroscopy and magnetic resonance imaging Low-field (LF) nuclear magnetic resonance (NMR) spectroscopy, also known as time-domain NMR (Thygesen and Elder 2008) or NMR relaxometry (Kekkonen et al 2014), can distinguish between water-bound hydrogen located in different physical and chemical environments. Consequently, it is a good technique for determining the distribution of water within different parts of the wood structure (Araujo et al 1992).…”

“…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.…”

Magnetic resonance imaging study of water absorption in thermally modified pine wood

“…Although the NMR spectrum in this context typically shows only a single and broad peak, bound water and free water can be distinguished by means of T 2 or T 1 relaxation time distributions ( Riggin et al 1979 ;Araujo et al 1992 ;Hartley et al 1994 ;Xu et al 1996 ;Labb é et al 2002 ;Fantazzini et al 2006 ;Almeida et al 2007 ;Thygesen and Elder 2008 ;Cox et al 2010 ;Hoffmeyer et al 2011 ;Elder and Houtman 2013 ;Telkki et al 2013 ;Zhang et al 2013 ;Kekkonen et al 2014 ). NMR self-diffusion measurements enable the observation of moisture transport in wood ( Wycoff et al 2000 ;Hietala et al 2002 ;Topgaard and S ö derman 2002 ;Meder et al 2003 ;Johannessen et al 2006 ;Kekkonen et al 2009 ;Telkki and Jokisaari 2009 ;Kekkonen et al 2010 ).…”