Pressure difference Imposes No Influence on Gas Permeability

Author: Xingdong Wang -National Packaging Products Quality Supervision & Testing Centre (Jinan)

Differential-pressure method and equal-pressure method are two commonly used methods in film gas permeability testing. Some holds that, in differential- pressure method, polymer materials adopted by flexible package will be affected in terms of structure and state due to the existing of pressure difference, which in turn affects results of gas permeability test. At the same time, this does not conform to actual application environment of flexible packing materials. The analysis of empirical test data and test methods has proved that the above view is very superficial and the conclusion is not correct.

1． Pressure difference Exists in Various Gas Permeability Test Methods

Based on the principle of differential-pressure method, a preset pressure difference with definite gradient direction will form on two sides of specimen. Therefore, users take measures to eliminate direct influence of pressure difference on materials. Empirical tests have proved that using porous paper to support low-pressure side can better eliminate the influences and prevent specimens from distortion. The state of specimen inside test chambers during tests of differential-pressure method is shown below (under the specimen is filtration paper):                     

Figure1. State of Specimen in Differential-pressure Method

Although some methods such as oxygen sensor method are considered as equal-pressure methods, it does not mean that there is no pressure difference existing on two sides of specimen. In fact, in equal-pressure method, gas flow needs to be regulated in order to control the concentration difference of gases on two sides of specimen. However, to make static gas flow, dynamic gas source is needed. Therefore, the pressure must be changed to regulate gas flow. Since the requirements on flow rate of upper and lower chambers vary in different standard test methods in factors such as manual regulating of flow, ambient temperature, and gas pressure, only very careful and strict performance during actual application can ensure identical pressure on two sides of specimen without any specimen distortion (specimen state is shown in figure 2). Otherwise, there is pressure difference on two sides of specimen. However, different from common views, in differential-pressure method, the factor of pressure difference has been taken into consideration and filtration paper is adopted to support specimen on low-pressure side. While in equal-pressure method, there is no support on either side of specimen. Therefore, although pressure difference on two sides of specimen is usually very small, the distortion rate of more flexible and thinner specimen during test is very high (as is shown in figure 3. The protrusion or notching of specimen depends on the gradient direction of pressure difference. As to whether there is distortion, users can open test chamber for examination after test).

Figure2.  State of Specimen in Equal-pressure Method (without pressure difference)

Figure3. State of Specimen in Equal-pressure Method (with pressure difference)

2． Pressure difference in Differential-pressure Method Imposes No Influence on Gas Permeability

The testing object of barrier property is the micro-property of materials. Therefore, to examine whether the existing of pressure difference causes structure and state variation of material in differential-pressure method, and to analyze and examine the relationship between the existing of pressure difference and test data of gas permeability, physical indexes of specimen before and after test are needed. In this article, two most relevant indexes are analyzed: repeatability of the test result for one same specimen in repeated tests and the variation of specimen thickness.

According to some people, the pressure difference on two sides of specimen will change loose material into compact structure. That is to say, it is a compacting process of loose materials. Moreover, they hold that the effect of such structure variation becomes obvious with the compacting time. If the above views hold ground, materials will become thinner under long-term pressure difference. At the same time, since barrier property of materials relates with their structures, permeability data should pose a reducing tendency with the increasing of test times.

During the research this time, Aluminum foil laminated film, PET film and PC film are selected for the verification test. They belong to higher barrier property, medium property and lower barrier property materials respectively. Therefore, the conclusion obtained from this group of data is applicable to any kind of barrier materials. As to testing instruments, Labthink VAC-V1differential-pressure gas permeability tester and CHY-C2 thickness tester are selected for testing. Measured data are listed in table 1 and table 2 respectively.

Table1. Measured Data of Oxygen Permeance

Note: 1. The unit of oxygen permeance is ml/m2·24h·0.1MPa.

Table 2 Measured Data of Specimen Thickness

Note: the thickness is tested within effective area of specimen. Other area is polluted by vacuum grease and there is no significance measuring it.

Oxygen permeability of each specimen is tested three times. For each specimen, filtration paper is not changed and test chamber is not opened between two tests. From table 1, it can be easily seen that repeatability of the three specimens is very good. Oxygen permeability data of the three specimens do not pose reducing tendency. However, during the testing of high barrier materials, data repeatability is worse than low and medium barrier materials, either in equal-pressure method or differential-pressure method. Therefore, sensor resolution becomes the main influential factor of data repeatability.

Specimen thickness in table 2 does not change before and after test. Therefore, it is not correct to say that pressure difference will affect material state and influence test results of gas permeability.

3． Conclusion

The pressure difference existing on two sides of specimen does not have any influence on permeability test data and material structure. It tells us that the conclusion can hold ground only when it is obtained with empirical tests. In addition, some holds that test environment in differential-pressure method differs very much with actual application environment of packing materials since there is no pressure difference on two sides of materials in actual application. This is fault too. Commonly used vacuum and gas packaging for food and drug also has pressure difference inside and outside package. That is why gas permeability test of differential-pressure method, already very mature in technology and theory, has the broadest application in the world.