Impact of Material, Process & Design on the gas permeability of thermoformed packaging

Abstract

Thermoforming is widely used by the food packaging industry to produce trays, that can be used to package prepared meals, fresh products, frozen or other foods. Depending on the preparation of the food product, preformed trays are filled and sealed, or packaging machines that form, fill and seal a package on the same machine are used. The thermoforming process, using heat and pressure (and vacuum), to form trays with a specific design out of flat sheets, has a direct impact on several physical-chemical properties of the origin material. Cooling e.g., is essential to avoid shrinkage after production. Thinning of the material, in the bottom, walls and/or corners can lead to unwanted and often unpredictable properties of the final tray. An important characteristic of food packaging, especially under modified atmosphere (MAP), is the oxygen permeability. The food and packaging industry often uses the oxygen transmission rate (OTR) of the sheet or film to specify the barrier properties of the thermoformed packaging. However, as a result of the increased surface area and thinning of the sheet, an increased OTR of the thermoformed package is expected. Various studies have shown that OTR values of sheets cannot easily be extrapolated to thermoformed packaging. Namely, the increase in OTR is not only related to thinning of the material and hence, cannot be modeled simply on the basis of the final surface and thickness of the packaging. Furthermore, corners, walls or bottoms are in some cases more susceptible to the physical stretching, which may lead to unequal thinning. In addition, the effect of physical thinning on the OTR can be thwarted by a possible reorientation (crystallization) of the polymer molecules during thermoforming. In this study, monolayer polypropylene (PP) and a selection of conventional multilayer packaging materials such as 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); PA/EVOH/PA/PE and PE/PA/EVOH/PA/PE were thermoformed from sheets of 2 different thicknesses into trays with 2 different depths; 25 and 50mm and 50 and 75mm respectively. An additional variant of the design was manufactured by thermoforming trays of 50mm depth with round corners in the bottom (r=5mm). All trays were thermoformed with plug-assist, except the flexible materials and PP and PP/EVOH/PP thermoformed in depth 25mm. Process parameters were optimized according to extensive industrial experience. The physical thinning in the bottom, walls and corners of all trays was measured precisely as compared to the thickness of the sheets. Thickness of individual layers were measured microscopically in selected positions. The OTR of sheets and thermoformed trays was measured using a Mocon Ox-Tran (ST or SH) in accordance with ASTM F-1927, at 23°C and 50% relative humidity (RH) and 90% RH for sheets (cc/[m².day.atm]) and ASTM F-1307, at 23°C with 50% RH outside and 90% RH inside the package for trays (cc/[package.day.0,209 atm]). In this project, a conditioning system was developed to enhance the capacity of the Mocon OTR measurements. Using this system, films and trays were flushed with formier gas (5% hydrogen and 95% nitrogen) at 23°C and the preferred % RH for 5 to 21 days, in order to reach a steady state of the flow rate apart from the Mocon module. Then, the film or tray was transferred quickly from the conditioning system to the Mocon and the OTR was measured until steady state with respect to the oxygen pressure gradient was observed. For all 7 materials, OTR measurements from the sheets in 2 thickness variants and 5 different tray designs were analysed by comparing the obtained data after recalculating the OTR into units of cc/[m².day.atm], [cc.20μm]/[m².day.atm] and cc/[package.day.atm]. In addition, the measured OTR values were also compared to predicted OTR values that were calculated according to the equation (1, not shown). Using a simplified formula, one can calculate the theoretical thickness of the tray according to the assumption of constant volume of the sheet used in thermoforming (2, not shown). The theoretical OTR value for the trays was calculated using equations (1) and (2). Plots of OTR, expressed as (cc/[m2.day.atm]) versus the drawing depth deviated from the predicted OTR values for most tested materials. This indicates that the OTR might not only be affected by thinning due to drawing, but in some cases also by orientation. Crystallinity is an important factor, since the permeant must seek out amorphous zones in order to penetrate a material. A lower degree of crystallinity yields greater permeability. Increased molecular orientation reduces permeability, in effect making the path to permeate more difficult. Chemical analyses using differential scanning calorimetry (DSC) were further performed to interpret the OTR results. The OTR of a selected tray (APET/PEdu50) was also measured at a lower (10°C) and higher (38°C) temperature, to predict the OTR e.g. at refrigerator temperature using the Arrhenius equation. Finally, the CO2TR of selected sheets and trays (APET/PE and APET/PE/EVOH/PE) was also measured according to ASTM F2476 (23°C and 0% RH outside the tray; 23°C and 0% RH inside the tray). In conclusion, in this study the effect of material characteristics, processing parameters and various designs on the thickness and final OTR of the thermoformed trays of 7 conventional rigid and flexible packaging materials was measured and analysed. An overview of all results can be consulted via the MaProDe_Ox tool.