Studies on topochemical modification of cellulose fibres. Part 4.Toposelectivity of carboxymethylation and its effects on the swelling of fibres

A toposelective carboxymethylation of laboratory cooked ECF-bleached softwood kraft pulp using monochloroacetic acid in an alkaline isopropanollmethanol medium is described. The toposelectivity was changed by subjecting the pulp to different drying conditions prior to carboxymethylation. It was found that the harsher the drying conditions (high temperature, low RH), the greater the extent of hornification of the pulp. An increase in hornification resulted in a more surfaceselective reaction, but made the fibres less reactive towards monochloroacetic acid. The surface - selectivity (ratio of surface charges to total charges introduced by the reaction) varied bctween 8 and 42 % depending on drying conditions and the amount of monochloroacetic acid added. A method was also developed to quantify the relative contributions of surface and bulk charges to the swelling properties of pulps. Assuming a two-phase model for fibre swelling, it was possible to quantify the effects of bulk and surface charges on the swelling of the pulps. The method is based on the blocking of surface charges using a high MW (> 1O6) poly-DADMAC. The surface blocking effect was verified with the help of carboxymethyl cellulose (CMC) that has been shown to attach to the surface of fibres with a very high surface-selectivity. Using CMC-attachment followed by neutralisation of the surface charges using poly- DADMAC, it was shown that the polyelectrolytic surface complex did not retain any water. It was therefore concluded that surface swelling had been achieved, by comparing the WRV's before and after blocking the surface charges with high MW poly-DADMAC. A similar approach was made to calculate the contribution of bulk charge to the swelling. The swelling model was validated with carboxymethylated pi~lps and it was found that, at a given charge level, the surface charge contribution to swelling is larger than that of the bulk charge. From the swelling results, it was possible to calculate the apparent bulk and surface moduli of the carboxymethylated fibres to 1.64 and 0.34 MPa, respectively. This is consistent with the assumption that the fibre structure is looser on the surface than in the cell wall.