Influence of Drying on the Pore Structures of the Cell Wall

Abstract

Using a recently proposed concept of cell wall structure based on a multiplicity of lamellae coaxial with the cell, the effect of drying on the structure of the cell wall of bleached spruce sulphite tracheids has been examined by means of nitrogen adsorption. The data suggest that in the fully water-swollen wall there are up to several hundred lamellae, each of the order of 100 Å thick, with a median separation of about 35 Å. During drying, the lamellae draw together progressively into thicker and thicker aggregates, decreasing the total pore volume, but leaving approximately the same median separation in the spaces that remain. At dryness, the pore volume remaining in the wall is negligible.

During drying,the first pores to close  do not reopen when the fibres are treated with water,whereas the pores that close during the later stages of drying do so. The lamellae separation after drying at 25°C and reswelling remains at a median value of about 35 Å, but it drops after drying at 105°C and reswelling to 25 Å, owing to a greater permanency of pore closure in the pores of larger size. It is tentatively suggested that a fibre dries radially inwards towards the lumen and that the pores that tend to remain closed after drying are located towards the outside of the fibre, whereas the pores that reopen easily upon rewetting after drying are located towards the lumen.

Fibres of very different composition, chemical treatment and morphology, as well as native and regenerated cellulose fibres, possess a wide range of pore volumes, yet are shown to have very similar pore size distributions and it is suggested that this distribution is not therefore of biological origin, but is based on a property of the cellulose molecule. No obvious correlation was found between the fractional extent of irreversible pore closure upon drying and the composition of the fibres.

In the swollen cell wall of spruce sulphite pulp fibres, which contain almost 1 cm³ water per gram of dry material, about 20 percent of the water is present in macro reticular pores (spaces between lamellae) and 80 per cent in micro reticular pores(spaces within lamellae). This ratio persists throughout the drying cycle.

The relationship between cell wall porosity and the papermaking properties of fibres is discussed briefly in terms of the loss of porosity during drying and its regain during beating.