| Abstract: | The pigment retention efficiency, in a model papermaking system, of anionically and cationically modified polyacrylamides was compared to that of the unmodified homopolymer. The anionic polyacrylamide is a much less efficient retention aid, especially at high pH's; the nonionic and cationic polymers perform similarly and without any marked pH dependence. However, it is the nonionic and anionic polyacrylamides which are similarly poor at fiber flocculation, at pH's between 4.5 and 7.5, whereas the cationic polymer is a good fiber flocculant. All three polymers stabilize pigment suspensions. The anionic polymer is not effective in heteroflocculation of mixed dispersions of fibers and pigment, whereas both the nonionic and cationic are good flocculants, the latter being less susceptible to overdosing. Equilibrium adsorption isotherms were determined; the cationic polymers is, unlike the other modified polyacrylamide, very well adsorbed by cellulose fibers. Onto titanium dioxide, polymers adsorbed in the order anionic < nonionic < cationic. A further differentiation of the cationic polymer is that it gives high pigment retention in sheet formation without markedly increasing the resistance to fluid flow through the forming sheet. Although a process of heteroflocculation is postulated for both nonionic and cationic polymers, in the former case it is thought that the primary adsorption is onto titanium dioxide, whereas with the latter it is onto cellulose. |
