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Funding Information: The financial support of the Austrian Federal Ministry of Digital and Economic Affairs and the National Foundation for Research, Technology and Development , Austria, is gratefully acknowledged. We also thank the industrial partners Mondi, Canon Production Printing, Kelheim Fibres, and SIG Combibloc for their support. Publisher Copyright: © 2021 Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Hydrophobization of cellulosic materials and particularly paper products is a commonly used procedure to render papers more resistant to water and moisture. Here, we explore the hydrophobization of unsized paper sheets via the gas phase. We employed three different compounds, namely palmitoyl chloride (PCl), trifluoroacetic anhydride/acetic anhydride (TFAA/Ac2O)) and hexamethyldisilazane (HMDS) which were vaporized and allowed to react with the paper sheets via the gas phase. All routes yielded hydrophobic papers with static water contact angles far above 90° and indicated the formation of covalent bonds. The PCl and TFAA approach negatively impacted the mechanical and optical properties of the paper leading to a decrease in tensile strength and yellowing of the sheets. The HMDS modified papers did not exhibit any differences regarding relevant paper technological parameters (mechanical properties, optical properties, porosity) compared to the non-modified sheets. XPS studies revealed that the HMDS modified samples have a rather low silicon content, pointing at the formation of submonolayers of trimethylsilyl groups on the fiber surfaces in the paper network. This was further investigated by penetration dynamic analysis using ultrasonication, which revealed that the whole fiber network has been homogeneously modified with the silyl groups and not only the very outer surface as for the PCl and the TFAA modified papers. This procedure yields a possibility to study the influence of hydrophobicity on paper sheets and their network properties without changing structural and mechanical paper parameters.

Ultrasonic liquid penetration (ULP) measurements of porous sheets have been applied for a variety of purposes ranging from determining liquid absorption dynamics to surface characterization of substrates. Interpretation of ULP results, however, is complex as the ultrasound signal can be affected by several mechanisms: (1) air being replaced by the liquid in the substrate pores, (2) air bubbles forming during penetration, and (3) structural changes of the substrate due to swelling of the substrate material. Analyzing tailored liquids and substrates in combination with contact angle measurements we are demonstrating that the characteristic shape of the ULP measurement curves can be interpreted in terms of the regime of liquid uptake. A fast and direct decline of the curve corresponds to capillary penetration, the slope of the curve indicates the penetration speed. A slow decline after a previous maximum in the signal can be related to diffusive liquid transport and swelling of the substrate material.