Factors affecting the free shrinkage of handsheets: apparent density, fines content, water retention value, and grammage, TAPPI JOURNAL June 2018
ABSTRACT: Industrial papermaking is a high-speed process during which a suspension of cellulosic fibers is formed into a continuous web via dewatering followed by drying. Dewatering in the paper machine occurs mechanically in the forming and pressing sections; however, most of the remaining water, whose removal requires applying high temperatures, evaporates in the dryer section. As a result, the paper web shrinks, due to the shrinkage of individual fibers in the paper web. On the paper machine, the paper web is under restraint in the machine direction (MD), whereas it can shrink in the cross-machine direction (CD). The edges of the web shrink more than the center of the web. A shrinkage profile is therefore created in the CD of the web. All machine-made papers exhibit a CD shrinkage profile. The CD shrinkage profile is significant because it affects the final product quality and manufacturing efficiency. The prime cause of the CD shrinkage profile during drying is free shrinkage. The effects of several wood pulp fibers on the free shrinkage of handsheets were investigated to obtain deeper understanding of the mechanism of paper shrinkage during drying processes.
Application: Mills can better understand the role of different fiber types, density, fines content, water retention value, and grammage on the free shrinkage of handsheets formed from a selection of softwood and hardwood pulps.
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