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

The effect of two wet strength agents, butanetetracarboxylic acid (BTCA) an d polyamide-amine epichlorohydrin resin (PAE), on the pore size, pore size distribution and swelling of the fibre wall of unbleached kraft pulps has b een investigated using H-1 and H-2 NMR relaxation techniques and WRV measur ements. 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 po res. BTCA treatment resulted in a more significant pore size reduction, a w ider pore size distribution and considerably less fibre swelling than was o bserved in PAE-treated samples. The PAE resin was also fractionated into tw o molecular weight fractions. The unfractionated PAE and the two molecular weight fractions had approximately the same effect on the fibre swelling an d the pore size distribution. The tensile strength properties of sheets mad e 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 b een 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 th e low molecular weight PAE fraction. From our results it seems reasonable t o 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 d ifferent effects of BTCA and PAE treatment were interpreted as being due to the structural differences between BTCA and PAE, especially molecular weig ht, and the different mechanisms by which BTCA and PAE act.