Studies on topochemical modification of cellulosic fibres Part 1. Chemicalconditions 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 t he irreversible attachment of a bifunctional molecule such as carboxymethyl cellulose (CMC) onto cellulosic surfaces have been determined. The influen ces of electrolyte concentration, pH, temperature, pulp consistency, pulp t ype and properties of CMC (D.S. and M-W) on the attachment of CMC were stud ied. The topochemistry of attachment was investigated using the colloid tit ration technique for the determination of surface charge. It was found that by choosing the optimal conditions (e.g. high temperature, high electrolyt e concentration, high cellulose content of pulp and low D.S. of CMC), CMC c an be attached quantitatively. Furthermore, the functionalisation was surfa ce selective, provided that the CMC has a sufficiently high M-W not to pene trate the cell wall of the fibres. The surface charge density of bleached k raft pulp was found to increase by an order of magnitude (from 3 to 29 mu m ol/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 respons ible for the strong sorption onto cellulosic surfaces (co-crystallisation). and the other modifies the charge density of fibres. Thus. the effect of e lectrolytes is simply to screen the charge interaction between the negative ly charged CMC and the anionic cellulose surface so that the polymer can co me into close proximity to the cellulose surface. The effect of D.S. may be st be understood in terms of electrostatic repulsive forces between cellulo se and CMC, the incipient insolubility of low D.S. CMC, and the fact that c o-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 metho d may be practical for commercial use. If so, this surface modification met hod will have a number of applications in the chemical technology of paper- and boardmaking.