Sustainable paper coating for food packaging: Calcium-modification for enhanced water barrier and durability

As a potential alternative to petroleum-based plastics in food packaging, paper suffers from poor water and oil resistance, which limits its applications. In our previous work, we developed a paper-based barrier coating based on sodium alginate (SA) and alkyl ketene dimer (AKD). While this coating enhanced the water and oil barrier properties of paper, it exhibited swelling and detachment when exposed to large amounts of water. To overcome this limitation, we introduced a 1.5% calcium lactate solution as a crosslinking agent, which effectively stabilized the coating. The resultant coating maintained good oil resistance- meeting general food packaging standards-while significantly enhancing water resistance. Specifically, the water vapor transmission rate (WVTR) decreased from 296.05 g/(m²·d) to 106.30 g/(m²·d) at 23 °C/50% RH, the water vapor permeability (WVP) reduced from 28.24 × 10^-13 g/(cm∙s∙Pa) to 10.37 × 10^-13 g/(cm∙s∙Pa), and the Cobb30 value dropped from 25.67 g/m² to 5.43 g/m². These improvements were attributed to the formation of a dense three-dimensional network structure on the paper surface. Furthermore, the coating demonstrated remarkable durability, retaining its barrier properties even after repeated folding, friction, and finger-touch tests. Since calcium lactate is an additive approved by the U.S. Food and Drug Administration (FDA), its use as a crosslinking agent helps ensure food-grade regulatory compliance, facilitating potential applications in food-contact materials and reducing barriers to commercialization. This work presented a sustainable and facile approach to preparing paper with excellent barrier properties, offering a promising avenue for the development of environmentally friendly food paper packaging based on bio-based materials.