Biocomposites of Whey Protein isolate/Polyvinyl alcohol/Nano-silica for food flexible packaging - Conventional and bilayer corona discharge-treated films

Abstract

The present work proposes the unprecedented addition of colloidal nano-silica (NS) to the matrix of whey protein isolate (WPI) and poly (vinyl alcohol) (PVOH), forming a new biocomposite with high tensile strength and flexibility for application as flexible packaging for food. Furthermore, it proposes the use of the lamination method with corona discharge in WPI and PVOH films, to modify tensile mechanical properties and moisture barrier. For this purpose, blends with 70% WPI and 30% PVOH plus 0%, 1%, 2%, 3%, and 4% NS were developed using the casting method. The biocomposites produced were characterized according to the properties: mechanical (tensile test), morphological (Scanning Electron Microscopy - SEM), interaction (Infrared Region Spectroscopy - FTIR), glass transition temperature (Differential Scanning Calorimetry), permeability, diffusivity, and solubility to water vapor, and interaction with moisture under different water activities and temperatures (sorption isotherms). The addition of 4% NS led to a biocomposite with a tensile strength of 10.2 MPa (43% higher than that of the film without NS), and with a tensile modulus of 78.2 MPa, which are excellent results for application as flexible packaging. At 4% of NS there was also a reduction of 17% in sorption, 58% in diffusivity, and 40% in water vapor permeability, and a lower spontaneity of the sorption process compared to film with 0% of NS, showing a greater ability to maintain the integrity of the food and the packaging itself during application. In a second phase of the work, new bi-layered biocomposites were produced via casting, with a first layer of WPI/NS and a second layer of PVOH/NS, fixing the amount of NS (4% m/m polymer) and applying different corona discharge periods (0s, 30s, 60s, 90s) to improve adhesion between layers. The analysis of FTIR, water contact angle, and surface energy of the first layer showed that the longer the corona discharge time, the greater the wettability and surface energy obtained, leading to greater adhesion between the layers. The final laminated biocomposites were evaluated under the properties: morphological of the joining region between the layers (MEV), mechanical (tensile test), and permeability, diffusivity, and solubility to water vapor. Laminated biocomposites treated with 90s corona discharge showed a more homogeneous joining region between the layers (compared to the other treatment periods), and a tensile strength of 12.6 MPa, being 72% and 23% more resistant, respectively, than the laminated film not treated with corona and that the non-laminated film (single layer). Furthermore, lamination with 90s of corona provided a 7.4 times greater flexibility compared to non-laminated film. Despite the increase in tensile strength and flexibility, lamination together with corona discharge increased the permeability and affinity of the film with water.