Hydrophobization, smoothing, and barrier improvements of cellulose nanofibril films by sol-gel coatings
Single-layer films from cellulose nanofibrils on a plastic support were coated with sol-gel coated with inorganic-organic copolymers (ORMOCER®s), consisting of inorganic Si-O-Si-based networks combined with ceramic (Al-O- and Zr-O-) groups and special organic fluoroalkyl chain containing functional groups. Sol-gel coatings decreased the surface hydrophilicity and water vapor transmission rate. The water contact angle of uncoated films was 24°, indicating high affinity between water and the cellulose nanofibrils. All sol-gel coatings tested increased the surface hydrophobicity with the contact angles ranging between 54° and 102°. The water vapor transmission rates varied between 230 and 410 g/m2/day. With UV curable highly organically crosslinked coating, the water vapor transmission rate was decreased by 77% as compared to uncoated film. The uncoated film had oxygen transmission rates of 0.7 and 107 cc/m2/day at 50% and 80% RH, respectively. At high humidity conditions, the films tended to swell, thus allowing permeation to increase. Sol-gel coatings significantly improved the oxygen barrier properties especially at 80% RH. The transmission rates varied between 0.4 and 0.5 cc/m2/day (50% RH) and between 51 and 86 cc/m2/day (80% RH).