Analysis on the Permeability of Non-Oxygen Conventional Gases


Oxygen and water vapor are not the only factors that cause the products become deteriorated which packed in various types of packages. With the popularization of MAP package and CAP package, the permeability of those gases, which were not usually neglected (including some inert gases), should be taken into consideration in package testing. Oxygen and water vapor permeability test have been applied widely. This article is to discuss the test method of permeability of nitrogen, carbon dioxide, air and other conventional gases to various packaging materials.



For non- oxygen conventional gas permeability test, data collection method has been always a focus of concern. The data could be obtained directly by the testing instruments. At present, only the differential pressure method instrument could test the permeability of various gases including He, N2, Air, O2 and CO2 etc. Some special gases especially combustible, explosive and toxic gases could be also tested with differential pressure method on the condition that the operator control the gas supply and handle the exhaust gas properly. The operation procedures for permeability test of those gases are totally the same as the oxygen permeability test. However, due to the limitation of its testing principle, equal pressure method testing instrument could not be used for universal gases barrier property test. The other method is to obtain result by estimation. There are very few testing instruments for permeability test of the non-oxygen gases in the past. Therefore it is a common way to obtain the permeability of non-oxygen gases by estimation with the obtained data of oxygen permeability and a certain proportion specified in technical document. Actually the proportion specified in technical documents is different from each other. As well, the material of the specimen and environmental factors are not taken into consideration when calculate the permeability of non-oxygen gases. As a result, the results obtained with the 2 methods mentioned above are not consistent with each other. There is no doubt that the data obtained by testing instrument is true and valid result. 



There are 3 factors which may influence the permeability of polymer films and sheets. They are polymer structure, characteristics of the gas and environmental factors. In this article, the focus is the influence of the characteristics of the gases, including the size, shape, polarity and the cohesion strength etc. of the gas molecule. The molecule shape and size will influence the gas diffusion inside the materials. The molecule size can be expressed by the dynamic diameter of the molecule. The smaller the diameter is, the easier it is to spread in the polymer and the greater the diffusion coefficient is. But for the gases with comparable molecule weight and different shape, long strip molecules own the strongest diffusion capability and permeability. The molecule polarity and cohesion ability are the main factors that influence the solubility of the gas on the material surface. As polymer materials have different polarity, the change of solubility coefficient has become the main factor that influences the permeability of gases in various materials. If the polymer does not have functional group which could interact with gases, the critical temperature is the key to control the solubility. Usually, the higher the critical temperature is, the greater the solubility is. Of course, the solubility of gases in the polymer usually follows "like dissolves like" theory. If certain chemical structure which could greatly dissolve specific gas exists in the polymer, the selective permeability of this polymer against the specific gas could be greatly improved. Therefore it is possible that the permeability coefficient of gas in some polymer with greater molecule diameter is also high. It could be obtained from the analysis above that various gases would show different characteristics in permeation of the same material. As well the structure difference would also influence the gas permeation. To be concluded, the estimation method is not scientific.



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