A Combination Of Pigment Filling/Coating And Surface Hydrophobication For Fabrication Of "Sticky" Superhydrophobic Paper

In the pulp and paper industry, mineral fillers and coating pigments, e.g., calcium carbonate, talc, and clay, are widely used in the manufacture of paper-based products. The use of these mineral additives is an indispensable element associated with traditional products including printing/writing papers and some packaging paper grades, delivering significant enhancement in paper properties and reduction in product cost. On the other hand, hydrophobication (or sizing) of cellulosic paper is highly desirable for meeting end-use requirements pertaining to printing/writing, packaging, etc.On the basis of the commercially-available processes associated with paper production, an integrated process concept involving wet-end filler addition, pigment coating, and surface hydrophobication for converting cellulosic fibers into superhydrophobic paper was proposed. Given the widely-accepted theory that surface roughness and surface free energy are essential for the fabrication of superhydrophobic materials, the key elements involved in the proposed process concept are:(1) wet-end filler addition and pigment coating can be effectively used for surface roughness control; (2) surface hydrophobication (or surface sizing) of cellulosic paper can create a low-free-energy surface, and it may also contribute to the construction of hierarchical surface roughness in certain cases, e.g., when a hydrophobic material dissolved in an ionic solvent is used as a sizing agent. It is possible that the delicate combination of these two elements would allow for tunable surface hydrophobicity.In accordance with the proposed process concept, the use of micro-sized precipitated calcium carbonate particles as the filling material and the coating material was integrated with surface post-hydrophobication of paper with a bio-derived material (i.e., beeswax or rosin), and the impact of key process parameters such as the dosage of precipitated calcium carbonate particles on paper hydrophobicity was investigated. Interestingly, the combination of internal and surface applications of precipitated calcium carbonate particles with surface hydrophobication using beeswax dissolved in ethanol resulted in the fabrication of superhydrophobic paper with a water contact angle of about 153°. Under the conditions studied, this superhydrophobic paper exhibited a very pronounced "sticky" character, and water droplet on the paper surface could not move at any tilt angle even when the paper surface was turned upside down. Similarly, rosin dissolved in ethanol was also used for the surface hydrophobication of precipitated-calcium-carbonate-containing paper, which merely resulted in a paper-based material having a very low water contact angle (i.e., much less than 150°); however, further treatment of the paper with alum solution delivered significant increment in hydrophobicity, and the treated paper had a water contact angle of about 151°, which also exhibited a pronounced "sticky" character. The interesting role of alum was due to its delicate interaction with rosin (anchored to paper surface), and the hydrophilic carboxyl group of rosin can be blocked upon paper surface treatment with alum solution.Based on the above-mentioned research findings from this preliminary work, it can be concluded that the integrated process involving wet-end filler addition, pigment coating, and surface hydrophobication can be effectively used for the fabrication of "sticky" superhydrophobic paper. The "sticky" superhydrophobic paper-based products can potentially find use in various applications, including high-quality packaging, non-loss liquid transportation, microfluidic monitoring/detection, and chemical/biological sensing.