This paper covers the use of thermoporosimetry to measure the pore size distribution (PSD) of pulp fibers. Thermoporosimetry is based on the melting temperature depression of an absorbate in a porous structure. A discreet or “step” melting procedure, rather than the usual continuous method, is used to melt the absorbate. This method eliminates thermal lag and gives the high temperature accuracy required for measuring large pores.
Measurement of water-saturated chemical pulp fibers using this technique, combined with solute exclusion, indicates a bimodal distribution of cell wall pores. The interpretation of data from water-saturated fibers is complicated by several factors: 1) distortion of the cell wall by ice crystal growth; 2) the depression of water’s melting temperature by osmotic pressure; and 3) inadequate range to cover the larges pores. One way to correct these problems is by replacing the water with cyclohexane. The major disadvantage of this approach is that the cell wall contracts in cyclohexane and its pore structure may change in other ways which are not understood.
Like water, the cyclohexane analysis shows a bimodal distribution of pores. The smaller pores, “micropores”, are less than about 5 nm in diameter, the “macropores” are about 15–700 nm. It was found that there is a quantity of cyclohexane in the cell wall which does not freeze. Analysis of nonfreezing cyclohexane indicates a surface area of about 400 m2/g for kraft pulp. The cyclohexane method is very suitable for studying beating, which primarily involves the opening of larger pores.