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 OPEN ACCESSPolymers 2014, 6 3020 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.

Recent Updates on the Barrier Properties of Ethylene Vinyl Alcohol Copolymer (EVOH): A Review

“…Additionally the cooling step to prevent shrinkage after production and the cooling rate affect the crystallization of the polymer and therefore also the permeability. Buntinx et al (2014) also found that trays with straight corners performed better than round corners possibly due to the stretching of the EVOH. [39] 2.5.4.…”

“…Buntinx et al (2014) also found that trays with straight corners performed better than round corners possibly due to the stretching of the EVOH. [39] 2.5.4. Retort processing and similar treatments Careful attention needs to be paid when multilayer structures containing EVOH are exposed to a retort process (e.g., sterilization of ready meal and pet food).…”

“…However, it should be noted that other parameters such as orientation and (re)crystallization can partly counteract the effect of thinning on the permeability of thermoformed packaging. [39] This will be addressed more in depth later in this paper.…”

Thermoformable high oxygen barrier multilayer EVOH/LDPE film/foam

“…Thermoforming imparts nonuniform deformation in the material which affects several properties such as mechanical strength, crystallinity of the constituent layers, thickness in corners, bottom and walls, and specifically gas transmission rates . There is a concern, particularly in the food packaging industry, whether the final formed product will maintain the barrier properties after the thermoforming process …”

“…Moreover, the samples need to be very carefully prepared and sealed to avoid leakage problems during testing. There is a strong interest to estimate oxygen transmission rate (OTR) in the package based on the OTR and thickness of the unconverted film or sheet …”