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Knowledge on moisture transport in wood is important for understanding its utilization, durability and product quality. Moisture transport processes in wood can be studied by Nuclear Magnetic Resonance (NMR) imaging. By combining NMR imaging with relaxometry, the state of water within wood can be identified, i.e. water bound to the cell wall, and free water in the cell lumen/vessel. This paper presents how the transport of water can be monitored and quantified in terms of bound and free water during water uptake and drying. Three types of wood from softwood to hardwood were selected covering a range of low to high density wood; pine sapwood and oak and teak. A calibration is performed to determine the different water states in each different wood type and to convert the NMR signal into moisture content. For all wood types, water transport appeared to be internally limited during both uptake and drying. In case of water uptake, free water was observed only after the cell walls were saturated with bound water. In case of drying, the loss of bound water starts only after vanishing of free water, irrespective of the position. Obviously, there is always a local thermodynamic equilibrium of bound and free water for both uptake and drying. Finally, we determined the effective diffusion coefficient (D eff ). Experimentally determined diffusion constants were compared with those derived by the diffusion models for conceptual understanding of transport mechanism. We found that diffusion in the cell wall fibers plays a critical role in the transport process.

The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound.To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.

The water phases in a MgSO4·7H2O crystal during heating were studied with the help of NMR. The thermogravimetric analysis (TGA) data showed that the heating rate has a strong effect on the dehydration process. NMR experiments showed that pore water, i.e., an aqueous solution of MgSO4, was produced during dehydration. Additional NMR measurements indicate a water diffusion coefficient of this pore water that is 10 times lower than that of bulk water. We showed on the basis of a model that pore water is produced as the vapor release is not sufficient at crossing a phase transition temperature.

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Nylon-6 is widely used as an engineering plastic. Compared to other synthetic polymers, nylon-6 absorps significant amounts of water. Although the typical sorbed amounts and diffusivity of water are well-known, less is known about the relation between the diffusivity and the water content. Attempts have been made in the past to obtain such relationship from moisture content profiles as measured with NMR imaging. However, these studies were mainly performed at high temperatures and without a proper calibration of the signal. In particular, at room temperature, far below the T g of dry nylon, plasticizing effects of water will result in a strong contribution of the polymer signal. Therefore, we have studied water uptake in 200 μm nylon-6 films in this temperature range near room temperature with NMR imaging. By calibrating the NMR signal with vapor sorption data, we were able to obtain moisture content profiles. A strongly nonlinear relation between the NMR signal and the moisture was observed at room temperature, which proves that contribution of the polymer to the NMR signal can neither be neglected nor assumed to be constant in time. Furthermore, glass transition temperature measurements combined with the water distribution provide plasticization profiles during water uptake. On the basis of the moisture content profiles, the moisture content dependency of the diffusion coefficient for water uptake is deduced through a Matano−Boltzmann analysis. This relation appeared to be highly nonlinear at room temperature. The self-diffusion coefficient was calculated through combination of the sorption-isotherm and the diffusion coefficient. Exposure of a nylon film to heavy water showed that water affects only a small fraction of the amorphous nylon phase. Water transport most likely occurs in this fraction of the amorphous phase. It is concluded that the heterogeneity of the amorphous phase is an important issue for a profound understanding of water transport in nylon-6 films. ■ INTRODUCTIONPolyamides, also known as nylons, are widely used as engineering plastic and textile fiber mainly due to their excellent properties. In particular, the mechanical properties are attractive for many applications and remain unaffected in a wide range of temperatures. Nylon is also easy to process, for example by extrusion molding, which is reflected in the large variety of geometries encountered in every day life.The chemical structure of nylons consists of amide groups separated by a number of methylene units. Therefore, a variety of polyamides exist, consisting of either one single α,ω aminoacid monomer like nylon-6 (PA6) and nylon-12 or two monomers, a dicarboxylic acid and a diamine like nylon 4.6 or 6.6. The number of successive carbon atoms in the polymer backbone between the amide groups is given by the index and influences material properties such a stiffness, melting point or water absorption.1 The latter feature is especially caused by the hydrophilic character of the amide functionality.Nylons absorb amounts of wat...

Water uptake by multilayer films plays an important role in their performance. Individual layers may consist of different polymeric phases. Understanding the water uptake in such systems requires knowledge of the water distribution, its state in the polymer, and influence on the polymeric phases. This study illustrates the application of high-resolution NMR and relaxometry for measuring water distributions and evaluating water–polymer interactions. We studied water uptake in a two-layered base coat/top coat system, where the base coat consisted of acrylic, polyurethane, pigment particles, and a polymeric dispersant. Water and the polymer phases in the base coat were identified with NMR relaxometry. The water diffusivity in the base coat was determined. At high water contents water is highly mobile and is loosely bonded to the polymer. Reversible plasticization of the dispersant was observed. The polymeric dispersant seems to play a key role in the sorption of water by the coating.