Superhydrophobicity of paperboard packaging by hierarchical surface roughness modification of polylactic acid electrospraying

Abstract

Water resistance of paperboard packaging was improved by developing a hierarchical surface roughness structure through double-layer coating. Combination coating methods for the first layer were performed by rod coating containing kaolin or calcium carbonate (CaCO3) particle mixture in native starch solution as a coating formulation. The second layer coating was conducted by polylactic acid (PLA) electrospraying. Kaolin or CaCO3 average particle sizes of 0.5, 1.0 and 10 μm were prepared at 5%(w/v) and mixed into a 1.0% native starch solution for the first layer coating, while PLA electrospraying was set at 1%, 0.25% and 0.125% (w/v). Coating weight significantly increased after first layer coating and after PLA electrospraying, proportional to increase of particle size and PLA concentration. Surface roughness significantly decreased after first layer coating and further decreased after second layer electrospraying due to deposition of PLA particles in the valley areas of the pre-coated layer. PLA electrospraying at all concentrations significantly promoted increase of water contact angle regardless of pre-coating formulations. Water contact angle reached superhydrophobic level at 151.29 degrees for 0.05 μm of CaCO3 pre-coating with additional 0.125% (w/v) PLA electrospraying. The PLA nanoparticles (Ra = 0.036 μm) reduced the contact area of water molecules, enhancing water repellency. For water vapor transmission rate (WVTR), 0.05 μm kaolin pre-coating with 0.125% (w/v) PLA showed optimal improvement as a plate-like shape obstructed the water vapor. Double-layer coating improved mechanical properties when small particles and lower PLA concentrations were applied.