Fabrication of thermally and mechanically stable superhydrophobic coatings for cellulose-based substrates with natural and edible ingredients for food applications

This study successfully fabricated double-layer superhydrophobic coatings possessing good thermal and mechanical stability for cellulose-based substrates using natural and edible ingredients by a simple and low-cost casting and spray-coating technique. The first layer of the coatings was prepared by spray-coating the surfactant-free beeswax or camellia wax emulsion for providing high surface roughness and low surface energy to the coatings. Wax concentration (5, 10, and 20 mg/ml) and spray coating time (5, 10, 20, 30 s) were investigated on coating density and superhydrophobicity. The underneath second layer composed of zein particles (ZP)/cellulose nanofiber (CNF) or precipitated calcium carbonate (PCC)/CNF with different thickness (0.1, 0.3, 0.5, and 0.7 mm), was prepared by casting. The prepared coatings had micro/submicron-scale structure surface morphology. A variety of liquid foods could freely roll on the coated filter paper surfaces in spherical without leaving a trace. The introduction of PCC/CNF as the second layer significantly (P < 0.05) increased the thermal and cold stability of the superhydrophobic coatings. The apparent contact angle of PCC/CNF coatings remained to be above 150 degrees after 30 min heating under 80 degrees C or 24 h cold storage under 20 degrees C. The mechanical stability of the two-layer coatings with PCC/CNF was also improved significantly (P < 0.05) compared with wax only single-layer coating during abrasion test. Superhydrophobic coatings prepared with beeswax showed more homogenous morphology compared to candelilla wax due to lower melting point of beeswax. The developed thermo- and abrasion-resistant edible superhydrophobic coatings can potentially be employed for a broad range of applications in food packaging.