Permeability of gas to materials (Permeability) was one of the important items in the physical performance test of materials. Materials with low Permeability were called as having certain Permeability to gases. Among these materials, those with very low gas Permeability were also called barrier materials, and were widely used as functional materials (such as commodity packaging).The material with high permeability is relative to the material with a certain barrier, their gas transmittance is very large, a variety of, the use is also very wide.

1. Air permeability test

We often say that the permeability test refers to the material with a certain gas barrier for the detection of specific gas permeability.These materials are mostly polymer or multilayer composite materials made from high polymers, which are widely used in the packaging of products in the fields of food, medicine, chemical industry, electronics, military industry and so on.Among them, the materials with excellent barrier property (very low gas permeability) can be used in the packaging of goods sensitive to oxygen and water vapor, which is the focus of the development of plastic packaging industry in recent years, and also the foundation of new packaging development such as aerated packaging, vacuum packaging and aseptic packaging.The permeability testing methods of materials mainly include pressure difference method and isobaric method, among which the pressure difference method is widely used in Lang ranges.Differential pressure method is a pure physical testing method, the principle of the test is clear, consistent with the permeability testing equipment mentioned later, is the fundamental method in permeability testing.Pressure difference method is divided into vacuum pressure difference method and positive pressure difference method. According to the detection standard, vacuum gauge or gauge pressure sensor with very high resolution should be adopted, and any small pressure change in the detection process should be collected.

Air permeability test has the following characteristics:

First of all, the vacuum degree of the test chamber is the most important index for the vacuum differential permeability test equipment.

ASTM D 1434-82(2003) requires that the low pressure side of the vacuum differential method be below 26Pa, IS0 2556:2001 and GB/T 1038-2000 require that the low pressure side be no more than 27Pa, and the air permeability tester should be equipped with a vacuum pump with high vacuum capacity.Since the vacuum degree is directly related to the state of the sample, whether the vacuum degree can meet the standard requirements will have a certain influence on the test data.Secondly, Vacuum Gauge accuracy has a direct impact on the accuracy of test results, and the selection of accuracy has certain limits on the range of Vacuum Gauge.Due to the low permeability of gas to this kind of material, the pressure change rate of the lower chamber in the test process is also very small, so the range of the vacuum gauge need not be too large.Table 1 lists the measured data of oxygen permeability detection for five high-barrier materials. In this group of tests, the maximum pressure change by the end of the test is no more than 20Pa.

Thirdly, the change of temperature can directly affect the barrier property of the sample.The permeability coefficient, solubility coefficient and diffusion coefficient of the material are affected by the temperature change according to the Arrhenius equation.

2. Breathability test

For plastic foam, cardboard, leather, textiles, paper, high gas permeability of materials, such as porous ceramics used in some specific areas need to quantify the permeability of gas for these materials, such as the permeability of cigarette paper choose whether or not can directly affect the appearance of YanZhi right, cigarette taste and the content of flue gas composition, and control of the textile air permeability is a direct effect on clothing wearing comfort of the key factors.For this kind of material, the test of gas permeability is called breathability test, and the test needs to use a professional breathability tester.The test methods of this kind of material can be divided into two types: constant pressure differential measurement and constant pressure differential measurement.The constant pressure differential measurement method is mainly used in the test of polyurethane foam plastics and soft or bovine hard porous elastic materials: the constant pressure differential measurement method is mainly used in textiles, non-woven fabrics, leather, geotextile, etc.The permeability test of paper, leather and other materials needs to test the time of passing through a certain volume of air under a certain pressure difference, which can be classified as the flow rate of constant pressure difference measurement, and then calculated by the flow rate measured.Compared with the permeability test introduced earlier, the permeability test has the following characteristics:

First, the test objects are different.The permeability test is mainly used to detect thin films, while the permeability test is mainly used to detect porous materials, non-woven fabrics, etc.

Secondly, the permeability test can detect the permeability of oxygen, helium, carbon dioxide, air and other gases to materials, while the permeability test mainly detects the permeability of air to materials.

Third, there is a big difference in the length of the test, the permeability test time is long, the permeability test time is much shorter, this is mainly because the two test materials have a very big difference in the permeability.

Fourthly, because the test of breathability mainly detects the materials with high breathability, the equipment does not need to use the vacuum gauge or pressure sensor with high sensitivity, and the vacuum pumping capacity of the vacuum pump is also relatively low, but the pressure sensor is required to have a wide range to meet the detection needs of different breathability.

Fifthly, the key components of breathability testing equipment include not only pressure sensors and vacuum pumps, but also the accuracy and range of flow meters.

Sixth, for different materials, due to the differences in testing standards and methods, the unit of test data varies greatly: for example, the final test result of textiles is permeability (mm/s), while the final test result of leather is permeability (ml/ CM2-H).With special operating software, the same transparency tester can complete the conversion of all kinds of test data above.At present, the development of breathability testing equipment is very uneven. For example, there are many types of breathability testing equipment used for textile, paper and other materials in the market, but few testing equipment used for polyurethane foam and porous elastic materials.

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