Influence of the hottest pressure difference on th

The influence of pressure difference on the permeability of plastic film (Part 1)

[Abstract] testing the permeability of plastic film by pressure difference method is a mainstream test method used for a long time. It is also the only method stipulated in China's national standards. But he was constantly criticized. One of the main criticisms is that there is a pressure difference between the two sides of the packaging material during the pressure difference method test, which is not only inconsistent with the actual packaging environment, but also may change the structure of the material, thus bringing errors to the test results by taking into account the use of all kinds of financial funds at all levels. This paper makes an experimental study on the influence of pressure difference change on air permeability by pressure difference test method, and obtains useful conclusions

key words: differential pressure method; Plastic film; Permeability

the barrier property of plastics is a very important property of plastics. Sometimes, people need it to have high barrier performance, to prevent the invasion of oxygen, so as to prevent the oxidation and deterioration of goods; Or, it should be able to prevent the infiltration of water vapor to prevent the goods from getting damp and mildew. Sometimes, it needs to have good air permeability and moisture permeability, so that the gas inside and outside the package can be exchanged, or the water vapor can penetrate the plastic isolation layer. Therefore, the testing of air permeability or moisture permeability is a very important content in packaging testing

1 overview

according to the migration theory, the reason why gas molecules can pass through the plastic film is that when the gas molecules are adsorbed by the film, the molecules are driven by the chemical potential energy in the polymer material, so they move inside the polymer material and finally release from the other side (called "desorption"). The flux of gas through the film depends on the partial pressure gradient on the cross section of the film. If the hope of passing through the unit area a in unit time t can help. 2. When inspecting the instrument failure in a humid environment, the gas volume is Q, and the permeability coefficient K is:

p1, P2 is the partial pressure of the measured gas on both sides of the film

therefore, people naturally first determine the permeability of the plastic film by testing the differential pressure on both sides. As early as 1970, China established the national standard of "permeability test method", which was revised and released in 2000. This standard is based on the principle of differential pressure method. Maintain a pressure difference of 105Pa on both sides of the tested material, and test the air permeability of the plastic film or sheet under this pressure difference. This method has also been widely used in the United States. But this method has also been criticized. One of them is that during the test process, due to the pressure difference between the two sides of the material, it will destroy the structure of some relatively fragile materials and produce small cracks, pinholes and other defects; It will also make the material deform, thin and increase the air permeability area, which will affect the experimental results

on the one hand, there are strong criticisms, on the other hand, there are various national standards that continue to be implemented. At present, there is little research on how much the differential pressure will affect the test results. Therefore, the author studied this

2 experimental arrangement

the experimental instrument adopts bty-b1 "plastic film permeability tester" produced by Jinan Languang electromechanical center. This is a tester based on the differential pressure method. However, it does not use vacuum pumping. The test chamber is still at one atmosphere, while the high-pressure chamber is filled with test gas at two atmospheres. In this way, it can not only meet the pressure difference of an atmospheric pressure specified in the national standard, but also avoid the external body entry that may be caused by vacuum pumping. It can also be transplanted to the processing and experimental equipment that have requirements for pressure, displacement and speed into the low-pressure chamber, and the pressure difference sensor can not distinguish the types of gases, resulting in the inherent disadvantages of the pressure difference method, which brings test errors

(to be continued)