Abstract Determining the Oxygen Transmission Rate of Carton Packages Ellinor Folkeson Verification & Validation, Package & Distribution Solutions, Carton Bottle, Tetra Pak Department of Chemical Engineering, Lund University, Sweden 2012-06-08 How to predict shelf life for different products in different packages is currently under investigation at Carton Bottle, Tetra Pak. The shelf life of the packed product, i.e. how long the package and product can be stored before consumption, is determined by several different factors. This report focuses on one of the factors: oxygen. The main aim of this study was to serve as a basis for deciding on suitable methods/measuring techniques for determining the oxygen transmission rate (OTR) of the three packages (entire packages as well as the package subsystems) Tetra Top, Tetra Rex and Tetra Evero Aseptic at the Carton Bottle platform. For future work with the methods investigated in this report it is suggested to empirically test the OxySense/PreSense method, to continue working with the oxygen indicator solution method to improve the filling and possibly to make a root case analysis of the hydrogen leakage device. Keywords: Oxygen Transmission Rate, OTR, Carton Packages. Introduction When producing packages designed for keeping a low internal oxygen level, it is crucial to be able to measure the amount of oxygen entering the package, i.e. the oxygen transmission rate (OTR) of the package. Without a proper measuring method it is difficult to find out whether changes to the package design are improving the oxygen barrier of the package or not. The currently used method for measuring the oxygen transmission rate uses an instrument from Mocon Inc. Packages are mounted on the equipment, with pure nitrogen flowing through the package. Oxygen molecules entering the package are transported by the nitrogen flow to a sensor detecting the number of molecules. When a steady flow of oxygen molecules entering the package have been achieved the OTR value is determined. The method has been questioned for several different reasons and other methods have been requested for quite some time. The method cannot measure packages with active barrier systems (scavengers), the method does not reflect real life conditions with product in the package, the method is time consuming and it has a low capacity. These drawbacks make it hard to focus development of the most important parts of the package. New methods would ideally include both a quick method to easily determine the impact of a change as well as a method that reflects real life and thus can be used for verification and validation of new concepts. Preferably, one should also be able to establish a correlation between these methods. 1
Theory The oxygen transmission rate is the amount of oxygen passing a material during a specified time at a given temperature and partial pressure of oxygen. The common unit of OTR is cm 3 O2/package/day (at 0.21 atm, 23 C, 50% RH). Strategy Since the main aim is to find new methods, the strategy has been to first create a broad overview of the present possibilities. Creating such an overview requires searching different instances and fields. Articles and patents as well as the web pages of companies and contacts at Tetra Pak have been investigated. Rating, evaluating and comparing methods that are very different is difficult and in order to evaluate the methods, a list of general requirements of the methods has been set. The complexity of measuring on produced packages with product has been clarified and the theoretical ways of solving the problems have been considered together with measuring techniques/instruments. Possible methods have been identified by using homepages on the internet as well as discussing with companies providing the instruments/methods. Investigating methods more closely have been made through trials in the lab as well as through personal contact with previous developers and users of the methods. In order to create an understanding of the methods and further investigate the possibilities, the methods have, when possible, been tried in practice. Since it has not been possible to evaluate all methods in practice in this study, as much relevant information as possible about the investigated methods have been provided for future work within the area. Several calculations have been made from a theoretical model based on Henry s Law, see equation 1.,, /, 769,23 /, Results and Discussion There are many possible ways of measuring the OTR or the oxygen concentration. However, the number of possible ways to determine the OTR for a carton package with headspace is limited. There is not always a clear way of how to apply the measurement system to the actual carton package. Instead of measuring the actual OTR, the change in oxygen content over time can be measured and the OTR calculated. This requires, however, an initial low concentration of oxygen in the package content. If equilibrium is established between the two phases (gas and liquid) the total oxygen concentration can be determined by measuring only one of the phases. Otherwise the oxygen concentration of both phases has to be measured. Literature study Several different techniques have been found in the literature and are suggested as good ways to measure OTR or measure the oxygen concentration in a sample. Reducing agents can, with different techniques be used to indicate the incoming oxygen. Visual indicators can for example be luminescence based or colorimetric. Several different examples of gas phase measuring techniques have been found. It might also be possible to measure the transmission by using and detecting another gas. Almost all the techniques found in the literature have been found on the market as well, often in the form of ready to use systems for measuring OTR/oxygen concentration. (1) 2
Requirements The determined requirements that possible methods should be evaluated according to have been: if there is a correlation to the oxygen molecule, if there is a correlation to real-life conditions (e.g. liquid product in the package) (mostly required for a verification/validation method), if it is possible to measure subsystems of a package, if it requires few man hours (during measuring moment and preparation work), if the lead time is short, if the capacity can be high, if it is a non-destructive method, if it is a non-destructive measuring moment and if the output is continuous data. Required resolution If the oxygen concentration in the package over time is described with equation 2 the oxygen concentration in a package with OTR 0.02 cm 3 O2/package/day (at 0.21 atm, 23 C, 50% RH) can be described with figure 1., 1 (2) Figure 1 warrants the conclusion that if OTR is determined by measuring the oxygen concentration over time and the lead time should be only ~4 days (for a package with OTR=0,02 cm 3 /pkg/day) the device has to be able to detect a change in 100 ppb dissolved oxygen. Dissolved Oxygen [mg/l] 10 8 6 4 2 0 Figur 1. The theoretical oxygen concentration in a 1000 ml package with OTR 0.02 cm 3 O2/package/day as a function of time. Ox-Tran, Mocon Inc. Dissolved oxygen, OTR=0,02 0 50 100 The Ox-Tran device used to measure OTR has a few major drawbacks. Since the samples are locked to the sensor during the measurement period only a very limited number of samples can be tested. There are moreover no possibilities to measure under real life conditions with liquid in contact with the tested material. Oxygen Indicator Gel The package is filled with a gel that changes color due to oxygen ingression. The method provides a good way to determine the difference in barrier between different parts of the package. The outcome of the method is however based on subjective judgments. Today there are no possibilities to quantify the color change due to oxidation of indigo carmine by incoming oxygen. PermMate and GasPorOx Time [days] 1000 ml package Solubility O2 The PermMate and the GasPorOx methods both measure the oxygen concentration in the gaseous part of the package. The big advantage with the GasPorOx device compared to the PermMate is that the GasPorOx measurement method can be completely non-destructive, 3
since it uses a laser. The advantage with the PermMate is that it can possibly measure completely non-transparent packages and through thick plastic packages, which might be difficult using GasPorOx. The Tetra Rex package can most likely not be measured with the GasPorOx device. It might, however, be possible with the PermMate, through piercing the cap. It is not known if the PermMate can measure the oxygen content in a gas saturated with water. OxySense/PreSens/OpTech The OxySense/PreSens/OpTech methods all measure the oxygen concentration in the packages through measuring a fluorescent light from a dot placed inside the package. The package has to be transparent on a small area or the dot has to be placed on a fixture with a glass window. The possibilities with the above mentioned methods are promising. The detection limit for the OpTech device seems higher than for the other devices. The fixtures used in the OIS method could possibly be used for attaching the dots and integrating them in to the package. The package then has to be refilled with degassed water and a headspace with a low concentration of oxygen. The increase of oxygen in the package can then be measured over time. Several measurements are possible with the method and the OTR can be calculated from the difference in oxygen concentration over time. The rental cost for testing the OxySense device is not high and it is recommended to do further evaluations of the device and the method. The Oxygen Indicator Solution The oxygen ingress in the package can be determined by filling the package with a colorless liquid which turns blue upon oxidation. The method is called the oxygen indicator solution and it has been investigated empirically. The color changing molecules have to be reduced by a reducing agent prior to filling the package. Adding too much reducing agent introduces a lag phase before oxidation of the color changing molecules starts. The variation in the length of the lag phase was very big, between packages filled equally. Due to the big variation in lag phase it takes at present about 30 days to achieve a result from one test. The lag phase most likely origins from deviations during the filling. The possible and probable reasons for the variation in lag phase were listed and investigated through both calculations and practical tests. The outcome of the investigations was: Always dry packages before filling. Filling without flushing first and using 0,6g reducing agent/l is not possible. Required amount of reducing agent in worst case is 25 times the initial recipe (1,5g/l). Use the new pressure regulator to the reactor 5s delay before the cap is attached does not change the oxygen content in the package Be careful with the packages during flushing and filling. Use the manufactured nitrogen ring. No definite solution, remedying the variation in lag phase, has been found so far. Decreasing the variation in lag phase, and receiving results in a few days, is necessary in order to motivate continuing the development of this method. The Hydrogen Leakage Method The hydrogen leakage method, with a device available at Tetra Pak, was evaluated through different tests. Nitrogen is flowing on one side of the sample and a carrying gas on the other side of the sample. Hydrogen molecules passing the sample are carried to a sensor by the carrying gas. Using the hydrogen leakage method as an OTR measurement device has not been successful. The device and the method is perhaps useful, but not with its current design. Encountered problems are: non-repeatable 4
tests; a startup-phase with constantly rising values; and changing the carrier flow (containing the permeating hydrogen) does not give the expected response (e.g. doubling the carrier flow does not result in half the concentration of hydrogen). The device is not built or specifically adapted to measure permeation. The device is built for measuring leakages, that usually give a bigger response (lets more hydrogen through the cap). When measuring permeation, as in this test, much smaller volumes of hydrogen should be measured. To be able to measure concentrations in the same interval, for which the sensor is initially calibrated (10 ppm), the carrying gas flow has to be decreased to a low level, not to dilute the permeated hydrogen. At this low level there might be a risk of fluctuations in the flow, influencing the measured value. Moreover, the flow of the carrying gas is much lower than the flow to the sensor during measurements. A larger flow to the sensor risks diluting the flow containing hydrogen, passing the cap, resulting in false values. Conclusion The most promising method is the OxySense/PreSens method. It can be useful for both subsystem measurements and for measuring entire packages. It measures both in the gaseous phase and in the liquid phase. The ability to do so will enable determining the difference in oxygen transport in a package filled with either gas or liquid or both. The accuracy of the equipment seems to be sufficient for oxygen concentration measurements over time. When using the OIS-method there is a big variation in lag phase that has to be decreased in order to further develop the method. The Hydrogen leakage method is not working in its present state, as it has been used in this study. 5