Effect of kaolin on the barrier properties of cross-linked PVA composite coatings for food packaging

Polyvinyl alcohol (PVA), as an eco-friendly coating, holds potential for replacing polyvinylidene chloride (PVDC) and ethylene-vinyl alcohol copolymer (EVOH). However, its application is limited by challenges such as poor adhesion, lack of vapor barrier properties, high humidity sensitivity, and insufficient thermal stability. To overcome these limitations, this study integrated chemical cross-linking modification of PVA with blending modification using layered nanoscale silicate to develop a melamine-formaldehyde (MF) cross-linked PVA/kaolin (KL) (MF-PVA/KL) composite coating. Composite membranes with varying KL content were prepared via solution casting, and their chemical structure, morphology, and properties were characterized. The coating was subsequently dip-coated onto polyethylene terephthalate (PET) substrate films, and the barrier properties and peel strength of the resulting composite coating membranes were evaluated. Results indicated that the MF-PVA/KL composite coating exhibited excellent leveling and wettability on PET substrates. Hydrogen bonding formed between components within the composite membranes. While KL incorporation influenced the crystallization behavior of PVA, the PVA molecules primarily adsorbed onto the KL surface without intercalating into its layered structure. The barrier effect of KL's layered structure, synergistically combined with hydrogen bonding interactions, enhanced the thermal stability of the composite membranes. Tensile strength and Young’s modulus of the composite membranes initially increased and subsequently decreased with increasing KL content. Conversely, the elongation at break decreased continuously from 14.7% to 0.9%, indicating a significant reduction in material toughness. Furthermore, limiting the KL content to ≤5% ensured that both the haze and peel strength of the MF-PVA/KL//PET composite coating membranes met the application requirements for food packaging. Barrier performance testing revealed that at 0% relative humidity (RH), the oxygen transmission rate (OTR) initially increased and then decreased with rising KL content. Notably, under high-humidity conditions (85% RH), with the increase of KL content, the OTR of MF-PVA//PET composite coating membranes decreased significantly from 65.50 mL/m²·day to 3.1 mL/m²·day, demonstrating excellent barrier performance. This result is attributed to the MF cross-linking enhancing structural stability and the KL lamination suppressing the penetration of water vapor in a high-humidity environment. The water vapor transmission rate (WVTR) decreased monotonically with increasing KL content. By regulating the KL content, this study elucidates its influence on the thermal properties, barrier performance, and peel strength of the cross-linked PVA coating. These findings provide theoretical support for enhancing the applicability of PVA in high-barrier food packaging.