| Microparticulate retention aids, consisting of a high molecular weight polymer used in conjunction with inorganic microparticles, are widely used to improve the process colloid retention in papermaking. Such additives are generally introduced to the paper stock prior to the web formation, so that fines and fillers are flocculated and retained into the forming web. This study investigated the mechanism of fines and filler retention under the high shear typical of papermaking. Retention and drainage were optimized using the newly built Laboratory Twin Former. The variables of interest include the concentrations of a cationic polyacrylamide polymer (CPAM) and bentonite, and their residence times with the furnish. The former variables provide information for optimal bridging efficiency between the furnish components, while the latter variables indicate the appropriate location for adding the retention aid to reach maximum retention. The experimental results indicated that the bridging theory well predicts a maximum retention when only CPAM is used. For the microparticulate system, a longer residence time of bentonite in the furnish is preferable so that the fines retention improves by allowing sufficient time for colloids coagulation and by promoting bentonite deposition on the polymer coated fines. We also studied the mechanism of fines retention and quantified the drainage rate at the jet impingement region, with and without retention aids. By individually varying the jet velocity and the jet angle, the two velocity components of the jet, i.e. VjetX and VjetY were controlled. The slice opening and the wire velocity were kept constant. For the range investigated, the drainage force proportional to V jetY had little effect on fines retention or drainage. A high shear impact on the wire by increasing the jet velocity, and consequently by augmenting VjetX relative to the wire speed, reduced the fines retention and restricted the drainage rate. |
