Liquid penetration in hydrophobised cellulose based sheets

Abstract Controlling the liquid transport within cellulose-based materials is crucial for numerous applications, including printing, bio-assays, packaging, and cleaning. To control liquid transport and quality, post-processes such as calendering, a way of compressing and smoothen the paper using hard pressure rollers, and hydrophobisation, are commonly employed. To understand how these processes influence liquid uptake, this study uses an Ultra-Fast Imaging (UFI) NMR method to analyse moisture profiles during liquid uptake in various cellulose-based paper sheets with diverse levels of hydrophobisation and calendering. It is demonstrated that calendering decreases penetration speed and increases swelling. The reduction in penetration speed could be linked to a decrease in permeability upon calendering, as measured by the Gurley air permeance. Additionally, it is observed that hydrophobisation delayed and slowed down liquid uptake in the paper samples, and, in extreme cases, completely altered the liquid uptake phenomena. With substantial hydrophobisation, liquid penetration no longer proceeded with a well-defined liquid front but exhibited huge levels of fingering. Furthermore, is was observed that within highly hydrophobised paper, fibres were first prewetted, initiating a first swelling, before the pores between fibres could be filled. Subsequently, water could enter the pores between, allowing fibre bonds to be broken, leading to a second swelling of the paper sheet. The improved understanding will contribute to better control of the flow within cellulose-based materials, benefiting applications such as printing, packaging and microfluidics.