6 Discussion and Conclusion
6.1 Influence of high pressure processing of packages with modified atmospheres on packaging integrity
6.1.1 Reversible structural changes
In order to understand the effects on the polymeric packaging, the mechanisms during high pressure treatment must first be understood. Up until now, no in-situ measurements of gas permeation under hydrostatic pressures of up to 2,000 bars have been carried out.
The development of a new measurement method based on fluorescence quenching of oxygen has enabled the quantification of oxygen permeation processes in polymers (Chapter 4).
The results of the permeation measurements indicated a decrease in permeability by up to a factor of 70. This was comparable to results concerning the transport processes of aromatic compounds under similar pressure conditions (RICHTER ET AL. 2010). The reduction in permeability was put down to the fact that the free volume of the polymer is markedly reduced under hydrostatic pressure. However, simulation studies of Sarrasin predicted a decrease in the amorphous volume of polyethylene by only a factor of 1.06 at pressures up to 2,000 bars (SARRASIN ET AL.2015). Hence the detected strong reduction of the oxygen permeability cannot be explained by merely this factor. An explanation might by the marked decrease in chain motion at high pressures which is necessary for the fluctuation of voids in the matrix and therefore responsible for the movement of diffusing molecules. The chains are arranged in a quasi-crystalline state during pressure
DISCUSSION AND CONCLUSION
174
treatment which diminishes the permeation of gases in polymers. Related comments were made by Gorbachev who blamed “increasing intermolecular interaction, decreasing chain flexibility and increasing packing density of the macromolecules” for the reduced permeability of gases in polymers (GORBACHEV ET AL.1977).
The results provide an important contribution for understanding the processes taking place under pressure, even though the determination of the solubility and diffusion coefficients is not yet possible. As the breakthrough time of oxygen in a polyethylene film was faster than the pressure build-up time (about 2 minutes), those coefficients could not be determined using the ‘lag time’ method. The breakthrough time of oxygen in a polyethylene film of 100 µm thickness is about 55 seconds under ambient pressure and temperature conditions, as calculated using an averaged diffusion coefficient in PE (MOISAN 1985). As a decrease both in diffusion and solubility coefficient was observed for other substances (RICHTER,STERR ET AL.2010), this is likely to have happened also in the case of oxygen.
Future work should use polymers with lower diffusion coefficients to determine the gas solubility under high hydrostatic pressures. Also, the measurement of the permeability of other gases such as CO2 with optical sensors is desirable.
6.1.2 Irreversible changes
The review in Chapter 3 summarises the irreversible structural changes in polymers due to pressure application. Knowledge of these changes is of great importance for predicting the suitability of a polymer for HPP as the structure, namely the crystallinity and density, influence the barrier and mechanical properties. In the present work, structural changes were determined via Raman spectroscopy (Chapter 5.3.4). The influence of headspace volume and composition, film thickness, initial polymer density, maximum applied pressure and treatment time was investigated. The results clearly show that the changes in crystallinity are not uniform or homogeneous throughout the polymeric matrix but seem to be dependent on thermal gradients. The pressure range, pressure holding time and temperature (under moderate temperature conditions) did not significantly influence the crystalline and amorphous phases of the polymer. In contrast, an increase in headspace volume increased the amorphous content of polyethylene. A possible reason for this is the increased amount of dissolved gas in the polymer and the concomitant plasticising of the intermediate phase. The crystalline phase did not undergo any significant
DISCUSSION AND CONCLUSION
175 transformation, possibly indicating that the structure of the polymer crystals is in general insensitive to hydrostatic pressures of up to 6,000 bars.
In summary, none of the directly and indirectly induced changes in polymer morphology which were detected will play a crucial role for industrial application of HPP as the changes were small in absolute terms.
6.1.3 Changes in oxygen permeability
The oxygen permeation measurements after high pressure treatment on pouches and tray packaging with headspace (Chapter 5.4.3) showed good agreement with results from literature for experiments on vacuum packaging (LARGETEAU 2010; LÓPEZ-RUBIO ET AL. 2005; MASUDA ET AL.1992). In these studies ethylene - vinyl alcohol copolymer (EVOH) was used as a barrier layer. Even in the presence of supercritical gases, the barrier properties of EVOH did not show any significant changes after HPP. This barrier material can therefore be particularly recommended for HPP. Only if there is delamination or blister formation, the oxygen transmission will increase significantly. Also, inorganic barrier layers were found to be sensitive to blistering as well as to mechanical stress under HPP conditions. The result is a total loss of barrier properties in most cases. These materials should therefore be avoided for the use in modified atmosphere packaging with HPP.
6.1.4 Migration
No significant additional migration (global or specific) due to a possible extraction of substances with supercritical carbon dioxide (scrCO2) was detected (Chapter 5.4.4). This was expected due to the small amount of CO2 present in the packaging. For common extraction processes such as used for decaffeinating coffee, higher amounts of gases are necessary. The findings agree with results in the literature on the global migration of packaging constituents into food simulating liquids after high pressure treatment of vacuum packaging (AYVAZ ET AL.2016; JULIANO ET AL.2010). The findings also agree with experiments on the extraction of plasticisers out of polyamide (PA11) using CO2 at pressures of up to 40 bars and temperatures of up to 120°C (FLACONNÈCHE ET AL.2001).
It can be concluded that if a polymer is in conformity with the migration limits under atmospheric conditions as specified by regulations and if the polymer remains undamaged under HPP treatment, then the requirements will also be met under HPP conditions.
DISCUSSION AND CONCLUSION
176
An overview of the results of this work is given in Figure 6-1.
Figure 6-1 Overview of the results of the present study