Studies on topochemical modification of cellulose fibres. Part 5. Comparison of the effects of surface and bulk chemical modification and beating of pulp on paper properties

The relative roles of surface and bulk charges of bleached softwood kraft pulp for subsequent paper properties have been investigated. The bulk charge was modified by carboxymethylation of the pulp in an alkaline isopropanollmethano1 reaction medium. The surface charge was modified by grafting carboxymethyl cellulose (CMC) onto the surfaces of fibres. Both methods increased the WRV of pulp, and the effects of these treatments were therefore compared with the effects of beating (PFI-mill). The chemically modified pulps were transferred to different counter-ion forms (Na and Ca) and papers were made from these pulps. When the pulps were compared in their Ca-form, both chemical treatments gave better paper properties than PFI-beating, in the sense of combinatory properties (e.g. tensile strengthllight scattering coefficient-relationship) and paper property1WRV-relationships. CMC-grafting was found to be superior to the other treatments, and this procedure thus has a potential to replace the beating operation in many papermaking applications. CMC-treatment leads to a higher paper strength without any significant sheet consolidation (increased sheet density) or change in the light scattering coefficient, but there was no difference with regard to paper properties between the two ionic forms (Na and Ca). It was concluded that CMC-treatment leads to an increase in relative bond strength, independent of the counter-ion. Papers made from bulk carboxymethylated pulps were, as expected, stronger if the pulp was in its Na- than its Ca-fonn. The difference in tensile strength was, however, much less than could be expected from the difference in fibre swelling. Evidently, the improvement in the strength properties by the bulk carboxymethylation treatment is also more related to the improved bond strength than to the enhanced cell wall flexibility. Altogether, the results shown in this paper suggest that the improved bond strength dominates the changed paper properties by beating rather than the changes in fibre swelling and hence, in cell wall flexibility. It is therefore concluded that the traditional interpretation of the role of ionic groups contributing to paper strength, by means of their effects on fibre swelling, needs to be revised.
By comparing the effects of these treatments with the effects of beating, it was also suggested that the development of strength during beating is primarily due to an increased relative bond strength rather than cell wall flexibilisation, as has been also suggested in recent investigations by other authors.