New information on the role of cofactor in PEO-type retention aid systems

The flocculation of three colloidal dispersions (precipitated calcium carbonate (PCC), TiO2 and a calcined clay) using a combination of polymeric flocculants and cofactors was investigated. The flocculants used were PEO (MW = 9×10⁶). CPAM (a cationic copolymer of acrylamide) and POLYPAM-CO-PEG (a non-ionic comb copolymer (MW = 5×10⁶) consisting of a polyacrylamide backbone with ∼ 1mole % pendent PEG chains). The cofactors were based on poly(vinyl phenol-co-sodium acrylate) and poly(vinyl phenol-co-sodium styrene sulfonic acid). The amount of flocculation induced was dependent on the components present in each system. Cofactors containing sulfonic acid were more calcium ion tolerant than the cofactors containing carboxyl groups. These latter cofactors formed a precipitate when exposed to ≥ 0.6 mM Ca⁺⁺. Maximum flocculation of PCC was obtained by using POLYPAM-CO-PEG with a cofactor containing 23 mole % of sulfonated groups. TiO2 was not flocculated when PEO was employed due to the adsorbed layer thickness of this flocculant being approximately equal to half the Debye screening length in 0.001 M NaCl. However, TiO2 was flocculated by POLYPAM-CO-PEG and CPAM, the best flocculation being obtained when POLYPAM-CO-PEG was used with either a cofactor containing 19 mole % sulfonated groups, or a cofactor containing 14 mole % acrylic acid groups. Overall, the easiest colloid to flocculate was the calcined clay; maximum flocculation being obtained when PEO was combined with a cofactor containing 19 mole % sulfonated groups. In general, the flocculation was most sensitive to the charge of the cofactors. It is proposed that the hydrophobic character of the vinyl phenol based cofactor is important. Hydrophobic interaction may be a part of the cofactor interaction with PEO and subsequent complex adsorption on surfaces.