Reversibility of polymer-induced fiber flocculation by shear. 1. Experimental methods

Papermakers desire two seemingly incompatible outcomes. On the one hand, strong agglomeration of fibers and fines can help one to achieve rapid drainage and satisfactory fine-particle retention. On the other hand, papermakers also want uniform distribution of fibers in the sheet. Procedures have been developed in our lab to evaluate effect of different retention and drainage chemical programs under stressed conditions of salt content, fines content, or high levels of charged colloidal matter. In the work described here the same tests were used to compare the reversibility of agglomerative effects of some common classes of retention and drainage programs. Optical and viscometric tests showed increased flocculation following treatment with increased amounts of cationic polyacrylamide. Application of intense hydrodynamic shear caused essentially complete reversal of flocculation. By contrast, treatment with a highly charge density cationic polymer yielded a maximum in flocculation, according to the optical test, at a treatment level corresponding to the point of charge neutralization. Divergent results were obtained when comparing fine-particle retention tests to drainage tests. In general, retention results were consistent with a model in which polymer bridges, i.e. “hard flocs,” between fibers may be irreversibly broken by shear. Meanwhile, bonds formed between fibers and fine particles appeared to remain intact. In contrast, drainage results appeared to be more highly dependent on the electrokinetic properties of the furnish, i.e. factors related to “soft floc” formation.