Temperature-dependent Oxygen Permeation through PET/MXD6-Barrier Blend Bottles with and without Oxygen Absorber Markus Schmid1, Kajetan Müller1, Sven Sängerlaub1, Christian Huber2, Karsten Fritsch3 Fraunhofer Institute for Process Engineering and Packaging (IVV), Giggenhauser Straße 35, 85354 Freising, Germany email to:
[email protected], phone: +49 8161 491 509 2 PreSens Precision Sensing GmbH, Josef-Engert-Str. 11, 93053 Regensburg, Germany 3 Invista Resins & Fibers GmbH, Ludwig-Hermann-Str. 100, 86368 Gersthofen, Germany
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Results and Discussion
Introduction The oxygen permeability of PET bottles has a significant influence on the shelflife of beverages that are sensitive to oxygen. This article places emphasis on the temperature dependence of permeation and on the activation energies of the oxygen permeation through PET bottles with passive and active MXD6 barrier grades for PET. The knowledge about the activation energy is of high interest for industrial applications: 1. According to the standards for permeation testing the oxygen permeation is measured at 23°C or 38 °C. However, the storage temperature will differ depending on the filled good and depending on the regional circumstances. 2. Based on knowledge about the activation energy of the oxygen permeation the permeation tests can be performed at higher temperatures resulting in shorter test periods. The oxygen permeability at lower temperatures can be calculated by applying Arrhenius’s law. The current study is part of a standardization campaign for oxygen absorbers: The results will be utilized to establish a new test standard describing the testing of the kinetics of oxygen absorbers for PET bottles as well as for other package components like closures, sachets and labels with oxygen absorbers.
Passive Barrier The results of all measurements with the passive barrier are summarized in Table 2. For a better comparability the oxygen permeation coefficients are calculated using the mean thickness of the PET bottle walls as indicated in Table 2. The permeation area of the PET bottles was approximately 400 cm² and the mean thickness of the PET bottles (without screw thread) was 387 μm. The indication of the oxygen permeation coefficient related to 100 μm thickness and 1 m² permeation area instead of the oxygen transmission rate has the advantage that the values can be used to predict the oxygen permeability of PET bottles with other bottle weight and other permeation area. Table 2: Activation energies and oxygen permeation coefficients of 0.5 L PET bottles with different concentrations of passive barrier measured at 5, 23, 38 and 55 C Bottle
Oxygen Permeability mg 100μm d m² bar
Experimental PET bottles The oxygen permeation measurements were performed with PET bottles made of MXD6/PET-blends with different MXD6-concentrations. The MXD6 acts as passive barrier and by adding a catalyst as oxygen absorber. The bottles were produced by INVISTA™ (Gersthofen, Germany). Table 1 summarizes the investigated materials with MXD6 acting either as passive barrier only or additionally as oxygen absorber. Table 1: Bottles tested in this study Barrier PET bottles MXD6 (passive barrier)
MXD6 (with catalyst)
PET (reference)
-
PET with 2 wt-% MXD6
PET with 2 wt-% MXD6
PET with 5 wt-% MXD6
PET with 5 wt-% MXD6
PET with 8 wt-% MXD6
PET with 8 wt-% MXD6
Figure 1: measurement set up
Measurement of the oxygen ingress into the PET bottles In this study, trace oxygen sensor spot type PSt6 from Presens (Regensburg, Germany) was placed onto the transparent closure which was adapted to the respective PET bottle. Three seals ensure the gas tightness of the closure (cf. Figure 1), in order to avoid side diffusion between closure and bottle finish. The reading was achieved by screwing the polymer optic fibre integrated in a metal holder on the outside of the package directly above the trace oxygen sensor. The following equation describes the increase in concentration versus time dc/dt of dissolved oxygen in a bottle filled with water:
ct S F pair
With: symbol QO2 c(t) SF pair c0 VG VF
QO2 V S p F F air c0 S F pair e
mg 1320 VG L bar 1 V S F F
Illustration Oxygen flux into the bottle
Unit mg day-1
Oxygen concentration in water, depending on time t Oxygen solubility in water (23°C) Atmospheric pressure in air (average absolute pressure 965 mbar) Oxygen concentration in water after filling (t=0) Volume of headspace Volume of water
mg L-1
mg L-1 =0.005L =0.52L
Poster presentation at the 5th international Symposium on Food Packaging, 14-16 November 2012, Berlin
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t
PET bottle (reference) 2 mass.-% MXD6 5 mass.-% MXD6 8 mass.-% MXD6
5 °C 23 °C
38 °C
55 °C
9.4 8.7 4.2 3.5
38.0 35.6 29.6 26.0
80.9 72.7 54.4 53.4
18.3 15.9 9.5 6.7
32.8 2.9 32.7 3.6 40.4 4.6 43.4 4.2
Active vs Passive Barrier The results of the optical oxygen measurement of PET bottles with and without oxygen absorber in Figure 2 show the oxygen content versus time. The oxygen content in the passive barrier bottles filled with water increases and consequently there is an oxygen uptake into the bottle. On the contrary, the oxygen content in the PET bottles with 2%, 5% or 8% wt. MXD6 with catalyst remains at a low level. Even after 180 days the oxygen partial pressure was below 0.2 mg l-1 for the absorber bottles with 2% MXD6, below 0.04 mg l-1 for the absorber bottles with 5% MXD6 and below 0.02 mg l-1 for that with 8% MXD6. These low oxygen levels indicate that the absorber with 2%, 5% and 8% wt. is able to protect the filled beverage for at least 180 days the better the higher the MXD6-content is. The measured values and the confidence interval for 2% wt. MXD6 with catalyst are significantly higher compared to that of 5% and 8% wt. MXD6. Since beer is especial sensitive to oxygen and has to be protected against loss of CO2, 5% or 8% wt. MXD6 with catalyst should be used for beer.
Figure 2: Oxygen uptake of water in PET bottles with 2%, 5% and 8% passive and passive and active barrier MXD6 stored at 23 C (values are mean value of five single measurements of different bottles).
Reference
40mg L bar 0.202 bar -1
Activation energy of oxygen permeation [kJ∙mol-1]
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K. Müller, S. Sängerlaub, A. Kramer, C. Huber, K. Fritsch, Temperaturedependent oxygen permeation through PET/MXD6-Barrier blend bottles with and without oxygen absorber. Brewing Science 2011, 64, 161-167.
