Studies on topochemical modification of cellulosic fibres Part 1. Chemical conditions for the attachment of carboxymethyl cellulose onto fibres

A method has been developed by which it is possible to increase the surface charge density of cellulosic fibres. The optimal chemical conditions for the irreversible attachment of a bifunctional molecule such as carboxymethyl cellulose (CMC) onto cellulosic surfaces have been determined. The influences of electrolyte concentration, pH, temperature, pulp consistency, pulp type and properties of CMC (D.S. and M-W) on the attachment of CMC were studied. The topochemistry of attachment was investigated using the colloid titration technique for the determination of surface charge. It was found that by choosing the optimal conditions (e.g. high temperature, high electrolyte concentration, high cellulose content of pulp and low D.S. of CMC), CMC can be attached quantitatively. Furthermore, the functionalisation was surface selective, provided that the CMC has a sufficiently high M-W not to penetrate the cell wall of the fibres. The surface charge density of bleached kraft pulp was found to increase by an order of magnitude (from 3 to 29 mu mol/g) as a result of the CMC treatment. It is suggested that these results can be interpreted as if the CMC had two functions, one of which is responsible for the strong sorption onto cellulosic surfaces (co-crystallisation). and the other modifies the charge density of fibres. Thus. the effect of electrolytes is simply to screen the charge interaction between the negatively charged CMC and the anionic cellulose surface so that the polymer can come into close proximity to the cellulose surface. The effect of D.S. may best be understood in terms of electrostatic repulsive forces between cellulose and CMC, the incipient insolubility of low D.S. CMC, and the fact that co-crystallisation of CMC onto cellulose is more likely to occur for low D.S. than for high D.S. CMC. It is believed that the outlined attachment method may be practical for commercial use. If so, this surface modification method will have a number of applications in the chemical technology of paper- and boardmaking.