Thickness and Gas Permeability Properties of PLA and PLA/EVOH/PLA before and after Thermoforming into Variable Tray Types.

Abstract

Thermoformed trays are very popular in food packaging thanks to their ease of production, filling and sealing as well as to the relative low production costs. It is also known that thermoforming affects different properties of the sheet material. In collaboration with Belgian food and packaging companies, the oxygen, carbon dioxide and water vapor permeability and the thickness is studied before and after thermoforming 400-µm poly(lactic acid) (PLA) monolayer and PLA/EVOH/PLA multilayer sheets into trays with variable drawing depths of 25, 50 and 75 mm and two extra 50-mm trays with a variable radius of the corners and with ribs in the walls. Thinning increases with drawing depth, especially in the corners of the trays and in the walls of the 75-mm trays. The EVOH layer thins proportionally, representing ~3% of the total thickness before and after thermoforming. Co-extrusion of PLA and EVOH results in a multilayer sheet with an oxygen barrier that is ~35x better than monolayer PLA in dry conditions and ~22x better in humid conditions. The carbon dioxide barrier of PLA/EVOH/PLA sheets is ~77x better than PLA sheets. The oxygen transmission rate (OTR) of PLA/EVOH/PLA trays is 26x lower for the 25-mm tray, 27-30x lower for the 50-mm trays and 36x lower for the 75-mm tray as compared to the respective PLA trays (in humid conditions). The OTR increases with drawing depth as a consequence of thinning and surface increase, but no significant effect on PLA crystallinity is observed. Therefore the theoretical OTR is in good agreement with the measured OTR. In addition, PE-based adhesive layers in PLA/EVOH/PLA can improve the water vapor barrier properties of PLA 3-5x depending on the thickness of the adhesive layers. In conclusion, this study confirms the applicability of PLA in food packaging, especially after co-extrusion with EVOH, thereby creating a high barrier packaging material.