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Measurement of Water Vapour Ingress in PET Bottles and Correlation with Oxygen and Carbon Dioxide Permeation
Schneider, J., Weber, I. and Pahl, R.

The beer and beverage industry is using ever more barrier enhanced plastic bottles for the filling of its products. The quality of the products can be considerably affected by the permeation of oxygen into the bottle and carbon dioxide out of it. The quality control of the bottles with particular emphasis on the gas barrier is thus of great importance. However, the conventional gas permeation measuring method needs too much time. In order to respond effectively and quickly to barrier defects, bottle production or incoming goods inspection measuring time must be shortened, for example by 2 hours. A physical problem of a quick measurement of oxygen is the comparably long unsteady state of permeation due to desorption of oxygen into the bottle after filling. In order to overbear this difficulty methods are tested which use other gases or as in this instance water vapour. Instead of a complete permeation only the migration of water from PET into the bottle inner is measured. The ruggedness of the method meets the requirements of the practical measurement conditions. The correlation of the water vapour migration rate with the permeation of carbon dioxide and oxygen measured with a real-time method is linear. Active barriers employing scavenger material can not be detected by the water vapour ingress measurement. This article is the second part of a study of which the first part has been published before [15]. The role of plastic bottles and the necessity of the quality control were discussed in the first paper already: an objective comparison of the barrier properties of various bottles and closure types is an important prerequisite in being able to forecast a product's shelf life, and hence in selecting the most appropriate container-cap combination, as several authors dealing with this topic agree [7, 14, 18]. Since in the food packaging industry plastic packaging materials have been used for a long time a carrier gas method for flexible food packaging has become a matter of a German standard (DIN 53380-3) [3, 19]. A modification of this method permits the measurement of packaging systems such as bottles [14]. Beside other difficulties as the creep effect (expanding of carbonised bottles) the problem is the minimum test time of this method. It is physically limited to about 2 days. Other so-called short-time tests are the manometric methods (absolute pressure method) which are not specific to a specific kind of gas, and a method that employs gas chromatography after taking samples [4, 5, 9, 10, 11, 12, 13, 16, 19].In order to investigate PET bottles with a high degree of accuracy, a so-called real time method can be used today [14, 19]. Real time means that the test time corresponds to the real time of the product shelf-life. In the case of PET bottle production or purchase thereof no useful tool is existent. Even a test time of 2 days as is possible with the DIN method is not quick enough. The aim is to provide a method that is fast, rugged and accurate enough for decisions in either bottle production control or incoming goods control. The measuring apparatus was developed as a quick tester by the company SIG Corpoplast (Hamburg, Germany). A humidity sensor inside of a bottle measures the humidity that migrates from the bottle wall through a perhaps existing inner coating layer into the bottle inner (Fig. 1). With the help of the tester the barrier layer is supposed to be evaluated within a few minutes. The increase of humidity is measured. Water is comparably well soluble in PET and through a defined exposition of the bottle in a climatic chamber the moisture content can be increased and adjusted with a high precision. Afterwards the bottle is flushed with compressed air wile one minute in order to obtain a dry atmosphere in the inner of the bottle. At this point a dew point humidity sensor (Fa?300-2, CS-Messtechnik) is introduced according to Figure 1. The working range is from -80??C to 20??C. Simultaneously the inner of the bottle is tightened to the environment. The measuring time including the flushing is 6 minutes. Every 20 seconds a measuring of the humidity takes place. As a reference method the real-time measurement oxygen ingress and carbon dioxide loss is used. This method described separately [19,?15] has gained acceptance in the brewing industry because of its high reliability and the accordance to the product shelf-life.A number of exemplary monolayer and barrier enhanced PET bottles were used in order to challenge the test method with a wide spread of measuring values. The characterisation of the bottles is described in Table 1. Multilayer bottles consist of more than one layer, typically 3 or 5 layers. The inner and outer layers provide the mechanical stability and the enclosed layer or layers represent the gas barrier. Active barriers (scavengers) are also implemented additionally in the passive barrier. Scavengers are able to chemically bind passing oxygen. However an active carbon dioxide barrier is not available. Bottles subsequently called "coating" consist of a monolayer PET bottle with the addition of an inorganic (used here) or organic coating such as a gas barrier.In Figure 2 two series of measurements demonstrate exemplary that the sensor can detect the changes in humidity properly. In the initial phase a steep raise of humidity is displayed through the measuring device. This is due to the technique of the measuring sensor and could not be avoided. The glass bottle was therefore exposed to a temperature of 23 ?C and 50 % relative humidity for 1 h (repeated determination). The experiment with the glass bottles show in difference to plastic bottles (Fig. 3) a steady drop of humidity within the measuring time of 320 s. These measurements were used for the setting and the calibration of the drying of the bottle material.During the exposition of the bottle in defined climatic conditions the PET takes up water according to the solubility. The curve progressing in Figure 3 represents the water that migrates out of the PET and that actually permeates through the internal coating layer. Apparently a time of 220 s is necessary before the linear increase of the humidity can be measured. It seems to make sense to use a fixed period between t1 = 220 s and t2 = 320 s as measuring time. The change of humidity in this period was set as the final measuring size "H" (water vapour migration rate) for the following experiments.

Descriptors: multilayer, oxygen, PET bottle, permeation, quick test, water vapour

BrewingScience - Monatsschrift fr Brauwissenschaft, 61 (May/June 2008), pp. 105-112