Filtration mechanics of sheet forming. Part II: Influence of fine materialand compression

Recent results with a high-speed drainage apparatus demonstrated an increas e in viscous resistance coefficient with basis weight. Two mechanisms are r easonable to explain this increase: (1) compression of the fibre mat caused by fluid drag, and (2) variable retention of fine material throughout the sheet forming process. Rapid dewatering results of pulp suspensions with di fferent levels of fines indicate that the fines level affects the viscous r esistance coefficient development. Fine material retained irt the mat seem to decrease local void volume of the mar and increase the resistance coeffi cient. The fine particle retention coefficient increases with basis weight and displays an increase similar to the increase in viscous resistance coef ficient with basis weight. The influence of compression was isolated by con ducting experiments at different pressures. The resistance coefficient decr eased with increasing pressure gradient This decrease is the opposite behav iour expected based on compression mechanisms and appears to be linked to t he decrease in retention of fines. Both compressibility and fines level in the,nar are thought to contribute to the increases in viscous resistance wi th basis weight, but the fine material retention seems to be the dominant m echanism.