Permeability analysis of the hottest conventional

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The permeability analysis of conventional gas

there are various packaging forms, but it is not only oxygen and water vapor that cause deterioration and failure of products. With the popularization of map packaging and cap packaging, the permeability of gases (including some inert gases) that were not paid much attention in the past to packaging materials has been paid more and more attention. Although the detection of oxygen and water vapor barrier is popular, how to detect the permeability of packaging materials by common gases such as nitrogen, carbon dioxide and air? Is the actual data acquisition method accurate? This paper will discuss these problems in depth

1. Conventional methods of data acquisition

for the detection of non oxygen conventional gas transmission, data acquisition methods have always been the focus of attention. One method is obtained directly through equipment detection. At present, only the permeability test equipment of differential pressure method can detect the barrier performance of materials to a variety of gases (he, N2, air, O2, CO2, etc.). If the user can ensure that the gas source is well controlled and the tail gas is well treated (especially for flammable, explosive and toxic gases), the equipment of this test principle can also be used to detect the permeability of some special gases. Compared with the oxygen test, changing the test gas basically does not increase the test cost, and the test process is consistent with the oxygen test. However, isobaric method equipment can not become a general gas barrier test method because of its detection principle. The other method is to obtain by estimation. In the past, there were few equipment that can detect non oxygen conventional gases. In order to obtain the transmission of these gases, sometimes a specific proportion is used to estimate the oxygen transmission. The estimated proportion is mostly from the data in the technical literature (because the technical literature referred to is often different, the estimated proportion itself is not a certain value). The change of sample material and test environmental factors are often not considered. However, in fact, the data consistency obtained by the above two methods is not good. There is no doubt that the directly measured data is true and effective. What is the reason for the significant deviation of the calculated data in the estimation? Can it be corrected? Next, I will discuss it from two aspects: theoretical analysis and experimental verification

2. Theoretical analysis

the factors affecting the gas permeability of polymer films or sheets can be divided into three aspects: polymer structure, gas permeability characteristics and environment. In this study, we mainly study the influence caused by the characteristics of permeable gas, including the size, shape, polarity of gas molecules and the difficulty of condensation, and only give appropriate consideration to environmental factors and polymer structure. The size and shape of molecules will affect the gas diffusivity in materials. The size of the molecule can be determined by the kinetic diameter of the gas molecule. 10. the number of tests can be programmatically controlled. The smaller the kinetic diameter of the molecule, the easier the diffusion in the polymer and the larger the diffusion coefficient. However, for different shapes of diffusing gases with comparable molecular weight, the long strip molecules have the strongest diffusing and permeability. The polarity of molecules and the difficulty of condensation mainly affect the solubility of gases on the surface of materials. Because the polarity of different polymer materials is not completely consistent, the change of solubility coefficient becomes the main reason affecting the permeability of various gases between different materials. If there is no functional group in the polymer that can interact with the permeable gas, the critical temperature is the main factor controlling the solubility. The higher the critical temperature, the greater the solubility in the polymer. Of course, the solubility of gas in polymer usually follows the law of similar miscibility. If there is a chemical structure factor with high solubility for a specific gas in the polymer, the selective permeability of the polymer to this gas can be greatly increased. Due to the influence of solubility factor, the phenomenon of large molecular diameter and large gas permeability coefficient may occur when comparing several gas permeability of the same polymer. From the above analysis, it can be seen that different test gases will not show completely consistent characteristics for the penetration process of the same material, and there are also differences in the structure of different materials, so it is not scientific to use the proportion estimation data

3. Test verification

in order to obtain the accurate difference between the estimated data and the measured data, the following test topic is specially designed. Labthink vac-v1 differential pressure gas permeameter is selected by Labthink barrier laboratory to detect the gas permeability of PC, pet, PVDC, aluminum foil and other materials. There are five kinds of test gases: he, N2, air, O2 and CO2. At the same time, several test temperature points of room temperature, 35 ℃, 40 ℃ and 45 ℃ are set. Some test data are listed in Table 1. Table 1 It is difficult to see the rule of the data in the measured data table

of various gas barriers at a glance, so it is converted into table 2 of the proportional relationship based on the oxygen transmission of each sample at each temperature point (the aluminum foil data is not considered, because the test data change is very small considering the test error). Considering the influence of temperature on the barrier property of materials, and based on the data of each gas and each sample at 35 ℃, calculate the transmittance of the same gas and its ratio at other temperature points, and then get Table 3 (aluminum foil data is not considered, because temperature change has little effect on the barrier property of metal materials). Table 2 Proportion Table of various gas barrier data of materials

it can be seen from the data in Table 1, table 2 and table 3 that the characteristics of different gases passing through different materials mainly include the following points. Firstly, the same kind of gas shows different rules when passing through different samples, and the effect of temperature change is the most significant. For example, compare the permeability of gas at 40 ℃ with that at normal temperature. When the permeation gas is he, gtrhe40/gtrhe25=1.25 for PC film, and gtrhe40/gtrhe25=1.31 for PET film, but gtrhe40/gtrhe25=1.48 for PVDC. However, it can be seen from the data in Table 3 that the helium permeability of several films is less affected by the email:ang@ temperature, but the nitrogen permeability is more affected. For example, for PC films, gtrn240/gtrn225=1.42, while for PET films, gtrn240/gtrn225= 1.72, but for PVDC, gtrn240/gtrn225=2.20. Figure 1 is a schematic diagram of the growth of nitrogen permeation at different temperatures based on the proportional data in Table 3. However, it should be noted that although the penetration of air and CO2 into PC and PET films is less affected by temperature, the impact of temperature is highlighted when these two gases penetrate through PVDC materials. At this time, gtrair40/gtrair25=2.56, gtrco240/gtrco225=3.05. On the whole, for PVDC films, the transmission rate of various gases with the increase of temperature is faster than that of PET films and PC films. Figure 1 Schematic diagram of nitrogen permeation growth at different temperatures for different materials secondly, the proportional relationship between the permeation of different gases through the same sample is not the same. For example, PC film GT at room temperature "In recent years, rhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=9.17 ∶ 0.21 ∶ 0.40 ∶ 1 ∶ 4.54, but for PET films, gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=48.20 ∶ 0.18 ∶ 0.39 ∶ 1 ∶ 6.23, and for PVDC materials, the proportional relationship is gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=30.89 ∶ 0.12 ∶ 0.23 ∶ 1 ∶ 3.47. Because the materials selected in this test are relatively representative, they are related to each other.Low visibility It is really difficult to obtain a stable gas transmission ratio for polymer films (the influence of temperature is not considered here). Even if he with the most significant change is excluded, the ratio relationship of other gases is reluctantly used for data estimation according to gtrn2:gtrair:gtro2:gtrco2 0.17:0.34:1:4.75 (taking the average value), but it is known that the arithmetic error has exceeded 20%, and the commonly used estimation ratio may not necessarily come from the same literature, so the error may be greater. Third, if the temperature factor and the gas type are considered together, the data regularity is even worse (although there is a regularity for the data of each gas at each temperature point). For example, for PET film, at room temperature, gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=48.20 ∶ 0.18 ∶ 0.39 ∶ 1 ∶ 6.23, and at 40 ℃, gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=36.00 ∶ 0.17 ∶ 0.33 ∶ 1 ∶ 4.94, O2 is slightly more affected by temperature than other gases. However, for PVDC materials, gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco2=30.89 ∶ 0.12 ∶ 0.23 ∶ 1 ∶ 3.47 at room temperature, and gtrhe ∶ gtrn2 ∶ gtrair ∶ gtro2 ∶ gtrco at 40 ℃, the best spring testing machine on the market now is 2=22.64 ∶ 0.13 ∶ 0.29 ∶ 1 ∶ 5.24 imported from Japan, and O2 is not significantly affected by temperature as air and CO2. What is certain is that with the change of temperature, the difference between the ratio between the actual transmission of several gases and the average ratio calculated at room temperature will become more and more significant. However, when testing the aluminum foil, the results of each film using different test gases at different temperature points are basically consistent. It is well proved that the change of temperature and the difference of test gas mainly affect the polymer materials. Fourth, from the data in Table 1, it can be seen that the characteristics of permeated gas obviously affect the gas permeability, which effectively proves the correctness of the previous theoretical analysis. First, compare the molecular weights of various gases and their kinetic diameters (see Table 4). Table 4 The molecular weight and kinetic diameter table of various gases

compared with the data in Table 1, it can be seen that since N2 has the largest molecular diameter and he has the smallest molecular diameter, the smaller the molecular diameter is, the greater the gas transmission of the material will be under the premise of similar molecular solubility. Therefore, for each sample, the transmission of he is the largest, while the transmission of N2 is always the smallest. However, you may find that the kinetic diameter of CO2 is close to that of O2. It should be said that the diffusion coefficients of CO2 and O2 are close. However, the carbon dioxide permeability of several samples in Table 1 is several times that of the same material. What's the matter? This is the effect of solubility coefficient. For inorganic gases, there is no functional group that has special effect with them in the polymer, so the critical temperature becomes the main factor controlling the solubility. The critical temperature of CO2 is 31 ℃, which is much higher than other common inorganic gases, so its solubility on the material surface is greater, so the CO2 permeability of the material is significantly higher than that of O2

4. Summary

to sum up, it is completely impossible to use an estimated proportional relationship and make it applicable to all materials. Materials should be distinguished and the impact of environmental factors should also be considered. Therefore, it is not recommended to use the proportional relationship to estimate the permeability of other gases through oxygen permeability. What this paper says is only for single component materials. It is conceivable that the situation of modified materials and composites will be more complex. For details, please call Jinan Languang or login Officer

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