Distribution of the equilibrium moisture content in four hardwoods below fiber saturation point with magnetic resonance microimaging

The distribution of liquid and bound water in wood samples under equilibrium moisture contents (EMC) below fiber saturation point (FSP) was assessed by magnetic resonance (MR) microimaging. Two Amazonian hardwoods, huayruro (Robinia coccinea) and cachimbo [Cariniana domesticata], a plantation grown eucalyptus (Eucalyptus saligna), and a temperate species red oak (Quercus rubra) were studied. Desorption tests were performed at 21 °C from full saturation state for huayruro, cachimbo, and red oak, and from green condition for eucalyptus. The EMC was reached under three desorption conditions [58, 76, and 90 % relative humidity (RH)]. MR microimages were obtained based on T ₂ times and on ¹H concentration. Scanning electron microscopy images helped us to interpret MR microimages. The results showed that wood structure plays a major role in liquid water drainage and in water diffusion. Eucalyptus saligna and red oak showed liquid water entrapped in parenchyma tissues, even below FSP (90 % RH). At this same RH level, all liquid water was, however, drained for cachimbo and huayruro. For these woods, bound water was not uniformly distributed in wood structure, concentrating it more in fibers for both species. Huayruro showed the highest heterogeneity in hygroscopicity, which is explained by its particular wood anatomy.