Effect of Montmorillonite Content on the Thermal, Mechanical, and Barrier Properties of PVA-Coated BOPP Films

Polyvinyl alcohol (PVA) is a biodegradable, water-soluble polymer widely used in coatings, while montmorillonite (MMT) is a nanofiller capable of improving the mechanical and barrier properties of packaging materials. This study aimed to apply PVA/MMT coatings onto biaxially oriented polypropylene (BOPP) films and evaluate their thermal, mechanical, morphological, and barrier properties. Coatings containing 5% PVA and 1, 2, or 3% MMT were applied onto flame-treated BOPP films using an automatic bar coater, producing coatings approximately 30 µm thick. The samples were morphologically characterized by scanning electron microscopy (SEM) and thermally characterized by differential scanning calorimetry (DSC). Thickness, adhesion, mechanical properties [tensile strength (MPa), elongation at break (%), and elastic modulus (MPa)], water vapor transmission rate (WVTR), and fat permeability were also determined. SEM micrographs revealed good coating dispersion and homogeneity, although small bubbles were observed in localized regions. DSC analysis showed lower onset temperatures for coated films than for neat BOPP, while melting temperatures remained unchanged. A reduction in melting enthalpy, particularly for BOPP/PVA/MMT2%, suggested changes in the crystalline organization of the multilayer system. The coated films exhibited an average thickness of 20.56 ± 1.27 µm and low adhesion to BOPP (>60% coating removal). WVTR averaged 1.34 ± 0.04 g H₂O/m²·day, with no significant differences among formulations. No significant differences in tensile strength were observed in the machine direction (MD), whereas increasing MMT concentration significantly reduced tensile strength in the transverse direction (TD). Elongation at break increased in the MD (73–112%) and decreased in the TD (23.2–11.2%) with increasing MMT content. Elastic modulus was not significantly affected in the MD but increased significantly in the TD, from 1343 to 1904 MPa. Fat permeation was observed in all samples after 144 h at 60 °C, indicating limited lipid barrier performance. Overall, MMT incorporation modified the mechanical and thermal behavior of the coated films, increasing stiffness in the TD without affecting melting temperature. However, improvements in adhesion and barrier performance are still required for packaging applications.