Preparation, application, and mechanism of starch modified dicyandiamide formaldehyde polymer–bentonite microparticle retention and drainage aid system

Modified dicyandiamide formaldehyde polymer (MDDF) and calcined bentonite were used to produce a MDDF-bentonite microparticle retention system. The authors investigated its retention, drainage-aid effect, and its mechanism through measuring the pulp charge density, particle size of the fine particles, and other factors. The results revealed that under neutral conditions and the addition of 0.08% MDDF and 0.3% bentonite, the initial floc was broken via high-speed shearing. Meanwhile, the inorganic filler was added, and the retention effect of the MDDF-bentonite system was close to that of the cationic polyacrylamide (CPAM)-bentonite system. The MDDF-bentonite system exhibited a patch-bridging mechanism. First, the authors added cationic polymer to form a sizeable initial floc, which was then dispersed at high shear force. Then, the authors added inorganic fillers with opposite charges. A smaller floc with more compact structure reformed and was distributed on the wet paper sheet so that more fillers and fine fibers remained on the paper, thus improving the retention efficiency. Because the charge density of MDDF is higher than that of CPAM, the addition of MDDF enabled the pulp system to become closer to the isoelectric point, and the drainage effect was better than that of CPAM.