Evaluation of Oxygen Transmission Rate and Thickness before and after Thermoforming Mono- and Multilayer Sheets into Trays with Variable Depth

Abstract

During thermoforming, an (inline) extruded sheet is heated to its softening temperature and subsequently deformed through application of mechanical stretching and/or pressure into a tray. This process directly impacts several properties of the sheet. An important characteristic of food packaging, especially under modified atmosphere, is the oxygen permeability. The food and packaging industry have a strong interest in the specification of the barrier properties of the thermoformed tray, based on the oxygen transmission rate (OTR) and thickness of the sheet or film. As a result of the increased surface area and thinning of the sheet, the OTR of the tray will be increased. However, various studies have shown that OTR values of the virgin material cannot easily be extrapolated to thermoformed packaging. The MaProDe_Ox project was conducted in collaboration with companies from the Belgian food and packaging industry. The aim was to evaluate and quantify the impact of the thermoforming process on the OTR of selected commercial packaging materials. This paper reports on the results of a part of this study, where 5 commercial sheet materials (monolayer polypropylene (PP); PP/ethylene-vinyl alcohol co-polymer/PP (PP/EVOH/PP); polystyrene/EVOH/polyethylene (PS/EVOH/PE); amorphous polyethylene terephtalate/PE (APET/PE); and APET/PE/EVOH/PE) were thermoformed into 3 trays with the same top dimension of 190x132 mm and with variable depths of 25 and 50 mm, and a variable radius of the corners for the 50 mm tray. The thickness distribution after thermoforming into 3 different trays was visualised, showing the regions that were most affected by the deep drawn process for all test materials. The OTR of the trays increased with the drawing depth, but this effect could be improved by increasing the radius of corners. This study indicates that the predicted OTR based on a homogenous material distribution, can be used as a rough approximation of the real OTR. However, detailed analysis of unequal thinning and reorientation remains necessary to explain the deviation (upwards or downwards) of the measured OTR as compared to the predicted one. Certainly in the case of monolayer PP, there is potential for improvement of the OTR as compared to the predicted value, due to reorientation of PP polymers during deep drawing. This study quantifies the excellent effect of ~3% EVOH in differerent multilayer materials as compared to materials without this barrier layer. The significance of the OTR obtained in this study requires a translation in terms of oxygen level in the package, which will determine the shelf life of a food product in a modified atmosphere packaging (MAP).