A lab approach to Simulate the Effects of Thermoforming on the Gas Permeability of Commercial Multilayers

Abstract

In the search for sustainable food packaging, critical reflection on the choice, combination and quantity of (bio)materials for specific applications is becoming increasingly important to prevent food waste. Our previous research has shown the combined effects of material thinning and polymer reorientation on the gas permeability of thermoformed multilayer trays. This study aims to investigate the feasibility of simulating the thermoforming process by heating and deforming (bio)plastic multilayers in a lab scale set-up to determine the maximum thinning and the associated gas permeability properties. First, thermal and tensile properties of commercial PE/EVOH/PE and ecovio®/G-Polymer/ecovio multilayer films (~85 µm) are characterized. Next, tensile testing at selected temperatures is applied to determine the maximum stretch, based on the elongation at break. Then, hot stretching of 70x60 mm2-films in machine and/or cross direction (MD, CD, MD+CD) or 45°, is done in an oven at optimal temperature by applying maximum gravity without breaking the multilayer. The microscopical thickness resulting from these situations is compared with the thinning in the bottom, walls and corners of thermoformed trays. The results show that thinning of both films is proportional to an increase in the water vapor transmission rate (WVTR), with the PE-layers providing a better water vapor barrier than the ecovio-layers. In contrast, the oxygen transmission rate (OTR) is not proportional to thinning of the total film, nor the thickness of the barrier layer. Here, polymer reorientation comes into play, even resulting in improved oxygen permeability coefficients as compared to the base films. We conclude that this approach can support the optimization of thermoforming processes by determining the maximal stretch of the individual layers while safeguarding the gas barrier properties of the final packaging, e.g. in thermoformed fiber-based trays.