Gas and Water Vapor Barrier Performance of Cellulose Nanocrystal–Citric Acid-Coated Polypropylene for Flexible Packaging

Cellulose nanocrystals (CNCs) are promising materials for the packaging industry due to their nontoxicity, abundance in nature, biodegradability, and high gas barrier properties. Unfortunately, CNC barrier films and coatings suffer from brittleness, water sensitivity, and, similar to polar polymers, loss of barrier performance at elevated humidity. Citric acid is an inexpensive, nontoxic additive, and a potential cross-linking agent, which can be utilized for decreasing the moisture sensitivity of cellulose-based coating materials for packaging applications. In contrast, polypropylene (PP) films are flexible and water resistant but have high permeability of oxygen and carbon dioxide. In this study, CNC and CNC–citric acid films were coated on polypropylene films by shear coating to investigate the barrier performance of the coated films. The study shows that addition of citric acid (CA) to the resultant CNC film can increase hydrophobicity and maintain transparency of the CNC-coated PP films. Importantly, CA addition can increase the barrier performance of a CNC coating (i.e., lower transmission rate) for O2, CO2, and water, which makes the CNC–CA coatings a possible choice for packaging applications.