Highly concentrated cationic flocculants based on 80/20 wt % [2-(acryloyloxy)ethyl]trimethylammonium chloride/acrylamide in an inverse microemulsion: Influence of the copolymerization variables on the flocculation performance

The influence of the total comonomer concentration (TCC; 30-42 wt %), emulsifier concentration (EC; 7.3-23 wt %), hydrophilic-lipophilic balance (HLB; 9-9.9), crosslinking agent (N,N'-methylene bisacrylamide) concentration (CAC; 0-122.8 ppm with respect to TCC), and isopropyl alcohol (chain-transfer agent) concentration (IPC; 0-1.5 wt % with respect to TCC) on the flocculation performance (FP) of 80/20 wt % acrylamide (AM)/[2-(acryloyloxy)ethyl] trimethylammonium chloride (ADQUAT) copolymers obtained by semicontinuous inverse microemulsion copolymerization was studied with capillary suction time testing for FP assessment on anaerobic digested sludges. FP increased as TCC decreased, was nearly unaffected by EC, was maximum with an HLB of 9.5, decreased strongly with CAC, and showed a peak value with an IPC of 1 wt %. At a very high TCC, copolymer growth in a highly collapsed state resulted in greatly structured, high weight-average molar mass flocculants with decreased swelling capacities (SCs), which did not favor bridging flocculation, and in increased shielding of their positive charges, which did not favor charge neutralization flocculation. However, industrially needed latices with both high TCC and good FP could be obtained by the addition of isopropyl alcohol, which, below a concentration of 1 wt %, improved FP by decreasing the weight-average molar mass and thereby enhancing SC while maintaining long enough chains to be effective for bridging flocculation. On the basis of the results, new star-shaped ADQUAT/AM copolymers are envisioned as flocculants with superior FP. A synthetic route is proposed.