Synthesis, characterization, and performance of polyacrylamide-based cationic copolymers as flocculants

This study describes the synthesis, characterization, and testing of cationic graft copolymers as flocculants. Graft-type copolymers are more effective as flocculants than their linear random counterparts because of the concentration of charges along the backbone of the macromolecular chain. The polymerization involves the grafting of cationic polydiallyldimethylammonium chloride (pDADMAC) onto the backbone of non-ionic, high-molecular weight polyacrylamide (pAM). The grafting of pDADMAC onto pAM was performed in dilute solution using potassium persulfate (K2S2O8) as a free-radical initiator. The synthesis parameters examined were reaction time and temperature, pAM:pDADMAC feed ratio, initiator concentration, and molecular weight of pDADMAC.; The graft copolymer was characterized in terms of grafting efficiency and degree of grafting (graft ratio) utilizing the compositional analysis capability of a gel permeation chromatograph (GPC). The GPC system is equipped with a Refractive Index (RI) detector and a Photodiode Array (PDA) detector. It is easier to determine grafting parameters from GPC measurements than with other methods because of the absence of time consuming preparative steps involved in purifying the reaction product. However, an adequate peak fitting model is essential in order to achieve this. The analysis is very sensitive to noise and impurities in the eluent and system as well as fouling of the chromatography columns. PeakFitRTM, a non-linear peak-fitting computer program, was used to determine the reaction parameters from the GPC chromatogram.; The effectiveness of the grafting reaction is very dependent on the combination of reaction time, reaction temperature, initiator concentration and the pAM:pDADMAC ratio in the blend. The maximum degree of grafting and grafting efficiency were obtained for samples reacted at temperatures of 75°C and 90°C for 60 minutes at 2 wt.% initiator concentration and 7:3 pAM to pDADMAC ratio using a high molecular weight pDADMAC.; The flocculation performance of selected graft copolymers was compared with homopolymers and their blends on a model dilute titanium dioxide suspension at doses of 2 and 5 mg/g of TiO2. The flocculation performance of graft copolymers was found to be superior to that of blends due to the concentration of ionic charges on the graft copolymer macromolecule. Flocculation performance was better with polymer concentrations of 2 mg/g TiO2 compared to polymer concentrations of 5 mg/g TiO2 because of overdosing and charge-reversal at higher polymer doses.