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

Abstract

During thermoforming, plastic sheets are heated and subsequently deformed through the application of mechanical stretching and/or pressure. This process directly impacts sheet properties such as material thickness in walls, corners, and bottom, crystallinity in the constituent layers, and particularly the oxygen gas permeability. The aim of this study was to quantify the impact of thermoforming on thickness and oxygen transmission rate (OTR) of selected packaging materials (polypropylene (PP); PP/ethylene-vinyl alcohol co-polymer/PP (PP/EVOH/PP); polystyrene/EVOH/polyethylene (PS/EVOH/PE); amorphous polyethylene terephtalate/PE (APET/PE); APET/PE/EVOH/PE; polyamide/PE (PA/PE); and (PE/)PA/EVOH/PA/PE). These materials were extruded in two different thicknesses and thermoformed into trays with the same top dimensions and variable depths of 25, 50, and/or 75 mm and a 50 mm tray with a variable radius of the corners. The distribution of the material thickness in the trays was visualized, showing the locations that were most affected by the deep drawn process. The OTR results indicate that the calculated OTR, based on a homogeneous material distribution, can be used as a rough approximation of the measured OTR. However, detailed analysis of crystallization and unequal thinning, which is also related to the tray design, remains necessary to explain the deviation of the measured OTR as compared to the predicted one.