On a paper machine the retention of fillers and fines during the first pass can be very different from the retention during a subsequent pass, because fillers and fines have much more time to interact in the short circulation loop than during the period between the injection of a retention aid and the forming section of the machine. Also in the short circulation loop reconformation
of adsorbed polymer can occur, often reducing the flocculation efficiency of the polymer and interfering with subsequent adsorption. Thus for a meaningful modeling of retention on a paper machine first pass retention and second pass retention should be distinguished. Retention of fillers and fines can result from deposition on fibers, either freely suspended in the papermaking
suspension or immobilized in the forming sheet, or by capturing fines and filler aggregates in the sheet by mechanical entrapment. Aggregates are more likely to be formed in the short circulation loop than on the paper machine. Effects of retention aids, detachment from fibers and the importance of polymer transfer are discussed.