The influence of two wet strength agents on pore size and swelling of pulp fibres and on tensile strength properties

The effect of two wet strength agents, butanetetracarboxylic acid (BTCA) and polyamide-amine epichlorohydrin resin (PAE), on the pore size, pore size distribution and swelling of the fibre wall of unbleached kraft pulps has been investigated using H-1 and H-2 NMR relaxation techniques and WRV measurements. The results showed that both BTCA and PAE resulted in less swelling of the fibres and a shift of the pore size distribution towards smaller pores. BTCA treatment resulted in a more significant pore size reduction, a wider pore size distribution and considerably less fibre swelling than was observed in PAE-treated samples. The PAE resin was also fractionated into two molecular weight fractions. The unfractionated PAE and the two molecular weight fractions had approximately the same effect on the fibre swelling and the pore size distribution. The tensile strength properties of sheets made from the pulps were also studied. The wet tensile strength increased for both BTCA- and PAE-treated samples but was lower for the samples that had been treated with PAE. The samples treated with unfractionated PAE and with the high molecular weight PAE fraction developed both a higher dry tensile strength and a higher wet tensile strength than the samples treated with the low molecular weight PAE fraction. From our results it seems reasonable to conclude that crosslinking of the fibre wall, the formation of a network surrounding the fibres and the formation of bonds at fibre-fibre crossings are important mechanisms for the development of wet tensile strength. The different effects of BTCA and PAE treatment were interpreted as being due to the structural differences between BTCA and PAE, especially molecular weight, and the different mechanisms by which BTCA and PAE act.