Oxygen permeation testing on PET bottle caps
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The need for measuring the oxygen transmission rate on PET bottle caps

Oxygen, the life giving gas in the atmosphere, has potential to destroy aroma, vitamin C and the taste of many different beverages. 1,2,4

In recent years packaging materials have been improved tremendously with respect to their oxygen barrier properties. The barrier of a PET bottle can be improved down to 0,015 cm³/bottle day for a 1 liter size.

While the packaging material involved is down gauged year by year, the barrier properties are being improved in order to extend the product’s shelf life, despite the fact that the overall material content is being significantly reduced. As the packages become more sophisticated it is recommended to analyse areas of a package separately in order to understand where oxygen permeation occurs.

Although caps and closures equate to only 0.5% to 6% of the total package, it is worthwhile to check their contribution to any oxygen ingress. This paper provides some ideas and guidelines regarding the testing of caps and closures with respect to their oxygen transmission rate contribution of the package.

There are several challenging tasks that bottle caps need to perform:

  • It should be easy to open
  • It must be tamper proof
  • It must be tight under all conditions for transportation and storage
  • Its integration into the complete package

Oxygen permeation testing is a well-established method to test the barrier properties of films and packages. Systech Illinois have performed permeation tests in order to separate the contribution of the closure from the total oxygen transmission of the complete package.


When testing the cap it is important to understand that the closure must be tested under the same or very similar conditions to where it fulfils its task. That means a cap should be tested on the thread of the bottle and the closure for a carton package should be tested when it is connected to the package material. Therefore a bottle closure is tested with a bottle neck cut off from a preform.

 

It is also important to measure a preform neck together with a aluminium top. The results will be used to determine the permeation rate of the cap independently from the permeation rate of the threaded region of the preform. In most cases a preform with a suitable thread from any supplier can be used as a standard for testing cap modifications. However, for a final permeation result it is better to test with a preform that is later blown into the bottle together with the correct cap, for the finished product. This eliminates variations in the selection of the PET master batch. The interface between the closure and a carton package however, is highly sophisticated and it is best to include this area in the test.

The closure of a carton package is prepared for testing by cutting out the closure from the package, without opening or breaking the seal. This is important because most closures have a double seal and/or a tamper proof feature that enhances the barrier. Once opened the barrier performance will be altered from the sealed condition. The cut-out will include the cardboard as the integration of the closure into the carton material is part of the sophisticated technology. Therefore it makes sense to test it in this condition. The cut out is glued (using a 2 part epoxy resin) into a mask made from an aluminium foil, a perfect oxygen barrier.


Systech Illinois' state of the art oxygen permeation analysers are capable of testing the oxygen barrier properties of packages with a detection threshold of 4E-05 cm³/pack a day. Several different packages from various supermarket shelves have been used for testing. The oxygen transmission rates for the closures have been measured in the range between 0,25 and 0,01 cm³/cap day. It does not cover all available systems; this is simply a demonstration of how the test principle works.

In the case of bottle caps, the bottle necks with the caps and the preform threads with the aluminium shims are glued directly on the platen of the permeation analyser. Using a rapid setting epoxy resin, sealing the caps and preforms onto the platen is a quick and simple procedure.

All the cap samples and preforms tested have been collected from recent packaging industry shows and exhibitions. The pairing of the caps to the performs have been done arbitrarily. In this case study, it is the test principle and the description of the procedure which is of interest. The variety of available cap systems in the market cannot be measured in the frame of an application report. Oxygen transmission rates for this random selection of caps have been measured between 8,3 E-02 and 1,47E-02 cm³/cap and day including a contribution from the perform neck in the range of 2,6 and 2,16 E-03 cm³/neck day.

When focusing on beer packaging, the PET/glass discussion is always accompanied by the crown cork as the preferred closure for glass bottles. Oxygen permeation rates are detectable, although the metal of the crown cork and the glass bottle are perfect barriers. A detailed measurement showed values of typical 1 x10E-03 cm³/cap/day. 5

Footnotes
1. Modelling of Oxygen Diffusion through a Model Permeable Package and Simultaneous Degradation of Vitamin C in Apple Juice. F. H. Barront, 6. Hart#*, J. GiacinS and R. Hernandez* 
Food Science and Packaging Department. Clemson University, Clemson, SC 29634, USA; SSch
PACKAGING TECHNOLOGY AND SCIENCE VOL 6 301-309 (1993)
2. The role of oxygen in brewing. The BREWER International • www.igb.org.uk • March 2002, p45
3. Courtesy of KHS Plasmax Hamburg
4. Vitamin C contents of citrus fruit and their products: a review. Nagy, Steven, Florida Dep. of Citrus, Agric. Res. & Education Cent., Lake Alfred, Florida 33850, USA. 1980 Journal-of-Agricultural-and-Food-Chemistry; 28 (1) 8-18, many ref.
5. Untersuchung der Permeation von Sauerstoff durch EPT Flaschen Bachelorarbeit. Thomas Meier, Verpackungstechnick/Packaging Technology der Technischen Fachhockschule Berlin, University of Applied Sciences.

February 2014
By Peter Heumeller, Business Friends Market Entry Consultants, Wachtberg, Germany and Graham Jennings, Systech Illinois, Thame, UK.

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