NMR cryoporometry to study the fiber wall structure and the effect of drying

NMR cryoporometry has been used for investigating the porosity changes of bleached wood pulp upon drying. This NMR method follows the same principles as thermoporosimetry, which has been used for the same purpose during the last decade and makes it possible to investigate porous material in the water-swollen state. In this study bleached softwood kraft pulp was exposed to a series of drying procedures where the decrease in porosity within the fiber cell wall could be characterized for pore radii below 100 nm. This decrease in porosity is called hornification, which is an irreversible collapse of the fiber wall structure during drying and results in decreased uptake of water and reduced swelling of the fiber upon rewetting. Our results have been compared to the traditionally used water retention value (WRV) and correlates well with these. Furthermore, this NMR method could show the reduction of hornification when adsorbing the hemicellulose glucuronoxylan to the fiber.