12th International Symposium on Nondestructive Testing of Wood

Identification of free and bound water content in wood by means of NMR relaxometry GUZENDA, Ryszard, OLEK,Wieslaw Department of Mechanical Engineering and Thermal Techniques BARANOWSKA, Hanna.M Department of Physics Agricultural University of poznan ul. Wojska Polskiego 28 60-637 poznan, poland Corresponding Author Contact: Email: RGuzenda@woodcock.au.poznan.pl

ABSTRACT

The advanced techniques of Nuclear Magnetic Resonance (NMR) relaxometry enable to determine not only moisture content in wood but also contents of free and bound water. Simultaneous identification of both subsystems of water in wood is especially important in investigations of moisture transfer in wood. This paper presents the analysis of the relationship between the spin-lattice relaxation time and moisture content of the most common Polish wood species i.e. Scots pine. Samples for the relaxation time determination were obtained from both heartwood and sapwood of pine. The CracSpin computer program using the spin-grouping method was applied to perform the separation of the spin lattice relaxation times. For moisture contents above the Fiber Saturation Point (FSP) there were obtained two components of the relaxation time related to two free and bound water. While below the FSP only one component was identified. The obtained results were generalized by empirical models. Differences between relaxation times for heartwood and sapwood were discussed.

INTRODUCTION

The progress in research of mass transfer in wood requires effective, non-destructive methods determining water content in wood as well as separating bound (hygroscopic) and free (capillary) water subsystems. The standard methods, which are presently used to measure the moisture content in wood, allow determining only the total amount of bound and free water (Skaar 1988). The most promising non-destructive method already applied to investigate water in wood is computer tomography based on X-ray absorption (Antti 1992, Davis and Wells 1992, Habermehl and Ridder 1993a, 1993b). The method allows obtaining images of internal futures of wood. However, intensities or colours of singles pixels of images depend not only on water content but also on properties of wooden tissue. Moreover, the method is unable to separate the both subsystems of water in wood i.e. to determine contents of free and bound water.

The possible identification of water subsystems is especially important for validation of heat and mass transfer models, which can be successful tools for predicting wood drying or moisture changes in wood products during their use. The experimental methods based on the Proton Nuclear Magnetic Resonance (H¹ NMR) seems to be the only techniques, which allow identifying free and bound water in wood (Araujo et al. 1992, 1994, Hartley et al. 1994). The application of the method for moisture content determination requires however the very basic research on NMR relaxometry parameters, which values are responsible for the construction of NMR images.

The objective of the paper is to present relationships between the very basic parameter of NMR relaxometry i.e. the spin-lattice relaxation time (T₁) and moisture content of Scots pine wood (Pinus sylvestris L.). The analysis of the results will be performed separately for sapwood and heartwood. Additionally, the T₁ relaxation time will be separated to the components, which are attributed to free and bound water.

MATERIALS AND METHODS

Material characteristics and samples preparation
Scots pine wood of freshly cut trees is characterised by remarkably different moisture contents of heartwood and sapwood of the transverse cross-section of a trunk. It is reported for softwoods that moisture content of sapwood, which performs functions of water transport in a living tree, is several times higher than moisture content of heartwood (e.g. Wood handbook... 1999). Moreover, heartwood of Scots pine is characterised by the higher content of extractives (i.e. resins) in comparison to sapwood (Kollmann and Côté 1968). It is also reported that sapwood of conifers is characterised by very limited aspiration of pits, which are responsible for free water transport in wood (Siau 1984, 1995). The presented above differences between structure and properties of sapwood and heartwood inclined us to perform measurements separately for both zones of Scots pine wood.

Experimental material was obtained from wood, which was firstly air dried to moisture content of ca. 10%. Samples had rectangular geometry and dimensions of 11·12·18 mm. The longitudinal anatomic direction of wood was parallel to cyclic magnetic field in the spectrometer. In order to obtain wood moisture contents higher than the Fiber Saturation Point (FSP), samples were immersed in distilled water and subjected to controlled soaking. Moisture levels under the FSP were obtained by samples equilibration in air of controlled relative humidity. After obtaining the assumed moisture contents each sample was covered with aluminium foil and stored in a desiccator at temperature of 4°C in order to equilibrate moisture content in samples. The real values of moisture contents of samples were always determined with the gravimetric method after relaxometry experiments.

Relaxometry experiments
Measurements of water relaxation times in wood were performed with the use of a pulsed NMR spectrometer operating at 30 MHz. The spin-lattice relaxation times (T₁)were measured using the p-t-p/2 pulse sequence (inversion recovery). The distances between pulses were changing from 0.5 to 1000 ms depending on moisture content of samples. During each experiment 30 Free Induction Decay (FID) signals were recorded. The repetition time was always TR = 10 s. For samples of law moisture content there were used up to 10 accumulation of a FID signal. All measurements were performed in temperature of 20±1° C

The values of the T₁ ofrelaxation time were obtained using the CracSpin program ( Haraczyk et al. 1999). The spin-grouping method applied in the program allowed performing the two-dimension analysis of FID signals in the time domain, which enabled to increase accuracy of separation of the T₁ relaxation time components.

RESULTS AND THEIR ANALYSIS

For all samples of moisture contents lower than the FSP there was obtained only one component of the spin-lattice relaxation time (T₁₁). Figure 1 presents an example of results obtained for that kind of samples after performing the spin-grouping analysis. The analysed wood was heartwood of moisture content equal to 12.5%. The CracSpin program identified the component of the value T₁₁= 19.3 ms with the error of ±1.4 ms.

For samples of moisture content above the FSP there were identified two components of the T₁ relaxation time. They were called the shorter component (T₁₁) and the longer component (T₁₂). Figure 2 presents an example of the T₁ relaxation time decomposition for a sample of heartwood of moisture content equal to 80%. The value of the shorter component (T₁₁= 10.0 ms) has significantly shorter value than the longer component (T₁₂ = 95.8 ms). The obtained results for different level of moisture content are discussed below.

Fig 1: The spin-lattice relaxation time component (T11) for Scots pine wood of moisture content of 12.5%

Fig 2: The spin-lattice relaxation time components (T11 and T12) for Scots pine wood of moisture content of 80%

The empirical models were used to describe the results obtained from the spin-lattice relaxation time measurements. It was performed separately for each component i.e. T₁₁ and T₁₂ and both zones of wood i.e. sapwood and heartwood. The shorter component (T₁₁) was described by the model of the form:

(1)

where: MC; % is wood moisture content, a, b, c are coefficients of the T₁₁model. Table 1 contains values of coefficients of the model.


The longer component (T₁₂) of the spin-lattice relaxation time was described by the model of the following form:

(2)

where:f and g are coefficients of the T₁₂model.

Zone of the cross-section	Coefficients of the T11 model			Coefficient of determination R2
	a	b	c
Sapwood	3.0912	159.98	7.3911	0.878
Heartwood	9.9916	502.31	4.7297	0.862
Table 1: Coefficients of the empirical model of the T11 component for sapwood and heartwood

Zone of the cross-section	Coefficients of the T12 model		Coefficient of determination R2
	f	g
Sapwood	110.10	-16785	0.7868
Heartwood	103.51	-14082	0.844
Table 2: Coefficients of the empirical model of the T12 component for sapwood and heartwood

The results of the separation of the spin-lattice relaxation time components as a function of moisture content are presented in Figure 3. The plot shows the T₁₁ and T₁₂ values for sapwood and heartwood obtained from the CracSpin program as well as the representation of the empirical models. For moisture contents lower than the FSP the T₁₁ relaxation time was the only one identified component. Therefore, the component was describing bound water in this area of moisture contents. The values of the T₁₁ were decreasing with moisture content increase. The reported values for the lowest moisture contents were ca.120 ms,while close to the FSP the T₁₁ was 3-10 ms.

Fig 3: The spin-lattice relaxation as a function of moisture content(HW -heartwood,SW-sapwood

Above the FSP there were identified two components of the spin-lattice relaxation time. The values of the T₁₁ component were practically constant and equal to ca. 3 ms for sapwood and ca. 10 ms for heartwood. The very low values of the component above the FSP suggest very little mobility of water molecules of the subsystem.Therefore,it was decided to identify the subsystem as bound water (Baranowska et al. 1997).

The values of the T₁₂ component increase with moisture content above the FSP. For moisture contents close to the FSP the T₁₂ is equal to ca. 20 ms for sapwood and ca. 30 ms for heartwood. For the highest reported moisture contents in this study i.e. for ca. 100% the component obtains the value of ca. 90 ms for both zones of the cross-section of wood i.e. for sapwood and heartwood. The T₁₂ relaxation times are lower than the spin-lattice relaxation time of bulk water, but higher than reported for the T₁₁ component and simultaneously increase with moisture content (Baranowska et al. 1998). Therefore, the T₁₂ component can be identified as the relaxation time describing the water subsystem consisting of molecules of free water, which are kept in the lumens or voids of wood. The limited mobility of the molecules of free water is mainly caused by capillary forces(Siau1984,1995)

FINAL REMARKS

1. The observed changes of the both components of the spin-lattice relaxation time with moisture content had the same character for sapwood and heartwood. The components differed only in their values.
2. The values of the spin-lattice relaxation times in sapwood were always lower than in heartwood at the same moisture contents. The lower values of the T₁₁ and T₁₂ relaxation times obtained for sapwood suggest that water molecules have easier access to the active sorption sites in sapwood than in heartwood.
3. The differences in the values of the T₁₂ component for sapwood and heartwood were significantly decreasing with moisture content and practically disappeared at 100% when mobility of free water is the same for both zones of the wood cross-section.
4. The possible application of Magnetic Resonance Imaging, as a standard procedure for water investigations in wood, should be preceded not only by the detailed analysis of the spin-lattice relaxation time (T₁) of both water subsystems, but also by the same analysis of the spin-spin relaxation time (T₂)

ACKNOWLEDGEMENTS

The authors wish to thank the State Committee for Scientific Research for financial support of the work as the PB 567/PO6/95/09 research grant

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