Effect of Fines Level and Compressibility on Filtration Resistance of Fibrous Suspensions, 1999 Papermakers Conference Proceedings

Joseph M. Genco, Douglas W. Bousfield, Vaughn J. Wildfong, Jay A. Shands
This paper presents a discussion of the filtration resistance which is a fundamental parameter used to describe the sheet forming process. A high speed drainage apparatus was developed to measure the filtration resistance of fibrous suspensions under the high drainage velocity, low basis weight conditions present during industrial gap forming [1]. Insight into filtration resistance and the sheet forming process can lead to improvements in former design, machine efficiency, sheet structure and furnish management.

Previous work evaluating the filtration resistance of fibrous suspensions had shown that the resistance changes during the mat forming process. Experiments performed using headbox suspensions obtained from a twin wire paper machine demonstrated an increase in viscous resistance coefficient with basis weight. The Darcy’s Law model used to evaluate the viscous resistance coefficient was derived for incompressible media[2]. The compression of the fiber mat was considered one possible mechanism causing the increase in resistance with basis weight. Another possible mechanism involved variable retention of fine material throughout the sheet forming process. Both mechanisms were investigated. The fines level of the fibrous suspension significantly affected the viscous resistance coefficient development presumably by plugging pores in the pre-formed mat causing an increase in specific surface area and decrease in local void volume of the mat. The fine particle retention coefficient increased with basis weight and displayed an increase similar to the increase in viscous resistance coefficient with basis weight. The contribution to resistance of compressibility of the fiber mat was isolated by removing the fines from a suspension. The viscous resistance coefficient still demonstrated an increase with basis weight which was small compared to the change with fines level. Both compressibility and fines level in the mat are thought to contribute to the increases in viscous resistance with basis weight, with fines level being the dominant mechanism.