Rapid Determination of Package Water Vapor Permeability

by Buddy Vapesit

Rapid Determination of Package Water Vapor Permeability

Abstract: Barrier property is one of the key parameters affecting shelf life of products, but traditional test methods for water vapor permeability still fall short not only in testing efficiency but also in terms of operation convenience. For that reason, a method that can rapidly determine water vapor permeability of package is introduced in this article.
Keywords: water vapor permeability, container, test, and electrolytic method

 

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Containers are directly used in liquid package. Since raw materials of containers are often processed with high temperature and cooling, some indexes of the raw material cannot indicate the final properties of container. The special shape of container adds difficulty to its barrier property testing. In the past, gas permeance and water vapor permeability are estimated by testing package sheet material. However, container packages are not uniform in thickness, and the property of materials will be changed in the process of production, which will cause certain disparity between estimated and tested results. Traditional water vapor permeability test methods still fall short not only in test efficiency but also in their operation conveniences. For that reason, a method that can rapidly determine water vapor permeability of package is introduced in this article.

1.  Traditional Methods to Determine Water Vapor Permeability of Containers

The commonly used methods of package water vapor permeability testing at present are developed on the basis of gravimetric method. Relevant test standards are GB/T 6981-1986, GB/T 6982-1986 and ASTM D 3079-94. According to the nature of materials, there are water vapor permeability testing of flexible package and rigid package. Although these two methods vary slightly in their detailed operation procedures, the basic test methods are similar.

Test procedures are as below: put desiccant (placement accessories can be used) into sample container, and then seal the container. Quickly place the preheated specimen into constant humidity and temperature cell for moist heat testing. After that, the specimen should be weighed periodically according to water vapor permeability of materials. When the transmission reaches equilibrium, calculate water vapor permeance of the specimen using the increment of specimen’s weight.

Although this method is developed on the basis of gravimetric method film testing, automatic detection is hard to realize. Therefore this method has the disadvantages of inconveniences in actual application, lower test efficiency and lower credibility of testing results as follows:

1. Longer test duration. According to specifications of standards, to determine transmission equilibrium, at least three weight data should be obtained. Although weighing interval of specimen is decided by operators in accordance with water vapor permeability of packaging materials, the interval time below is recommended in standards: for materials possessing high water vapor permeance property, the shortest weighing interval recommended is 2?3 days. While for lower water vapor barrier materials, an interval of 15?30 days is recommended. In this way, even packages made of very lower barrier materials need seven or eights days to complete one test. To test container package that is made of high barrier materials, several months are needed.

2. Test cannot be completed under a stable condition. During weighing process, the specimen (or desiccant and its placing accessories) is moved between testing environment and weighing environment. In principle, transmission of water vapor should be tested under an equilibrium state. But the movement during weighing will destroy transmission equilibrium established under testing state, which in turn affect the accuracy of testing results. Moreover, the shorter the weighing interval, the more obvious the affection will be. There are also such explanations in standards. For example, it is clearly pointed out in standards that frequent weighing affects accuracy of test.

3. A long time stable differential pressure of water vapor is hard to maintain on two sides of specimen. Because the weight of desiccant contained in specimen container are usually within 80-100g, moisture absorption ability of desiccant decreases after a period of test. Correspondingly, differential pressure on two sides of specimen will also change. In such a case, the desiccant should be replaced to continue testing. However, the desiccant can hardly be replaced completely consistent in quantity. Therefore, test will be influenced to some extent.

4. The sealing lacks reliability, especially for container packaging that cannot be sealed repeatedly. When desiccant and its

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