Packaging forms vary widely, but oxygen and water vapor are not the only factors causing product deterioration and failure. With the increasing popularity of MAP and CAP packaging, the permeability performance of packaging materials by gases (including some inert gases), which were previously less of a concern, is gradually gaining attention. Although oxygen and water vapor barrier testing is relatively common, how can we test the permeability of common gases such as nitrogen, carbon dioxide, and air to packaging materials? Are the data acquisition methods used in practice accurate? This article will explore these questions in depth. 1. Conventional Methods for Data Acquisition For the testing of non-oxygen common gas permeability, data acquisition methods have always been a key focus. One method is direct equipment testing. Currently, only differential pressure permeability testing equipment can test the barrier performance of materials to various gases (He, N2, Air, O2, CO2, etc.). If users can ensure proper control of the gas source and proper exhaust gas treatment (especially for flammable, explosive, and toxic gases), this testing principle can also be used to test the permeability of some special gases. Compared to oxygen testing, changing the test gas basically does not increase the testing cost, and the test process is the same as oxygen testing. The isobaric method cannot be a universal gas barrier property test method due to its detection principle. Another method is estimation. Previously, there were few devices capable of detecting non-oxygen conventional gases. To obtain the permeability of these gases, a specific ratio was sometimes used to estimate the permeability through oxygen. This estimation ratio often came from data in technical literature (since the referenced technical literature often differs, the estimation ratio itself is not a fixed value), and it often did not consider variations in sample material and testing environment. However, in reality, the data obtained by these two methods are not very consistent. Undoubtedly, directly measured data is accurate and valid. So, what causes significant deviations in the calculated data during estimation? Can these deviations be corrected? I will discuss this from both theoretical analysis and experimental verification perspectives below. 2. Theoretical Analysis The factors affecting the gas permeability of polymer films or sheets can be broadly categorized into three aspects: polymer structure, permeated gas characteristics, and environment. This study mainly investigates the influence of permeated gas characteristics, including the size, shape, polarity, and ease of condensation of gas molecules. Environmental factors and polymer structure are only considered appropriately. The size and shape of molecules affect the diffusivity of gases within materials. Molecular size can be represented by the kinetic diameter of the gas molecule; the smaller the kinetic diameter, the easier the diffusion in the polymer, and the larger the diffusion coefficient. However, for diffusing gases of different shapes with comparable molecular weights, elongated molecules have the strongest diffusivity and permeability. Molecular polarity and ease of aggregation primarily affect the solubility of gases on material surfaces. Since different polymers have varying polarities, changes in solubility coefficients become a major factor influencing the permeation of various gases between different materials. If the polymer lacks functional groups that can interact with the permeating gas, the critical temperature is the main factor controlling solubility; gases with higher critical temperatures tend to have greater solubility in the polymer. Of course, the solubility of gases in polymers generally follows the "like dissolves like" principle. If the polymer contains chemical structural elements that greatly increase the solubility of a specific gas, the selective permeability of the polymer to that gas can be significantly increased. Due to the influence of solubility, when comparing the permeation rates of several gases from the same polymer, it is possible to observe a phenomenon where larger molecular diameters result in larger gas permeability coefficients. The above analysis shows that different test gases will not exhibit completely consistent characteristics in the permeation process of the same material. Furthermore, different materials have different structures, so estimating data using ratios is inherently unscientific. 3. Experimental Verification To obtain an accurate comparison between estimated and measured data, the following experimental project was designed. The Labthink Barrier Laboratory used a Labthink VAC-V1 differential pressure gas permeation analyzer to detect the gas permeation of materials such as PC, PET, PVDC, and aluminum foil. Five gases were tested: He, N2, Air, O2, and CO2. Several test temperatures were set: room temperature, 35℃, 40℃, and 45℃. Some experimental data are listed in Table 1. The patterns in the data in the table are not immediately apparent, so they were converted into a proportional relationship table (Table 2) with the oxygen permeation at each sample and temperature point as the baseline value (aluminum foil data is not considered because, considering testing errors, its data variation is very small). Considering the influence of temperature on the barrier properties of materials, and based on the data for each gas and each sample at 35℃, the permeation rate of the same gas at other temperature points was calculated to its ratio, resulting in Table 3 (data for aluminum foil is not considered because temperature changes have almost no effect on the barrier properties of metallic materials). Table 2. Proportion Table of Barrier Properties of Materials for Multiple Gases. From the data in Tables 1, 2, and 3, it can be seen that the characteristics exhibited by different gases when permeating different materials are mainly as follows: First, the patterns exhibited by the same gas when permeating different samples are not the same, with the influence of temperature changes being the most significant. For example, comparing the permeation rate of a gas at 40℃ with that at room temperature, when the permeating gas is He, for PC film, GTRHe40/GTRHe25 = 1.25, while for PET film, GTRHe40/GTRHe25 = 1.31, but for PVDC, GTRHe40/GTRHe25 = 1.48. However, analysis of the data in Table 3 shows that the helium permeation of several films is less affected by temperature, while the nitrogen permeation is significantly affected. For example, for PC film, GTRN240/GTRN225 = 1.42, for PET film, 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. It is particularly noteworthy that although the permeation of Air and CO2 in PC and PET films is less affected by temperature, the effect of temperature becomes prominent when these two gases permeate through PVDC materials, at which point GTRAir40/GTRAir25 = 2.56 and GTRCO240/GTRCO225 = 3.05. Moreover, overall, the permeation of various gases with temperature increases faster for PVDC films than for PET and PC films. Figure 1. Schematic diagram of nitrogen permeation growth under different materials and temperatures. Secondly, the permeation ratios of different gases through the same sample do not differ. For example, at room temperature, the ratio of GTRHe∶GTRN2∶GTRAir∶GTRO2∶GTRCO2 for PC film is 9.17∶0.21∶0.40∶1∶4.54, but for PET film it is 48.20∶0.18∶0.39∶1∶6.23, and for PVDC material it is 30.89∶0.12∶0.23∶1∶3.47. Since the materials selected in this experiment are relatively representative and have low correlation with each other, it is indeed difficult to obtain a stable gas permeation ratio for polymer films (the influence of temperature is not considered here). Even excluding He, which exhibits the most significant variation, and using the approximate ratio of GTRN2∶GTRAir∶GTRO2∶GTRCO2≈0.17∶0.34∶1∶4.75 (average value) for data estimation, the known arithmetic error already exceeds 20%. Furthermore, the estimated ratios used are not necessarily derived from the same literature, potentially leading to even greater errors. Third, considering both temperature and gas type, the data regularity is even worse (although there are patterns for each gas at each temperature). For example, for PET film, at room temperature, GTRHe∶GTRN2∶GTRAir∶GTRO2∶GTRCO2=48.20∶0.18∶0.39∶1∶6.23, while at 40℃, GTRHe∶GTRN2∶GTRAir∶GTRO2∶GTRCO2=36.00∶0.17∶0.33∶1∶4.94. O2 is more significantly affected by temperature than the other gases. However, for PVDC materials, at room temperature, GTRHe∶GTRN2∶GTRAir∶GTRO2∶GTRCO2=30.89∶0.12∶0.23∶1∶3.47, and at 40℃, GTRHe∶GTRN2∶GTRAir∶GTRO2∶GTRCO2=22.64∶0.13∶0.29∶1∶5.24. The effect of temperature on O2 is less significant than on Air and CO2. It can be determined that as temperature changes, the difference between the actual ratio of the permeation amounts of these gases and the average ratio calculated at room temperature becomes increasingly significant. However, when testing aluminum foil, the results obtained for each film using different test gases at different temperature points are basically consistent. This well demonstrates that temperature changes and differences in test gases primarily affect polymer materials. Fourth, as can be seen from the data in Table 1, the characteristics of the permeating gas significantly affect the gas permeation amount, which effectively proves the correctness of the previous theoretical analysis. Let's first compare the molecular weights of various gases and their kinetic diameters (see Table 4). As can be seen from the data in Table 1, since N2 molecules have the largest diameter and He molecules have the smallest diameter, under the premise of similar molecular solubility, materials with smaller molecular diameters will have a greater gas permeation. Therefore, for each sample, He has the highest permeation, while N2 has the lowest. However, you may notice that the kinetic diameter of CO2 is similar to that of O2, meaning their diffusion coefficients are relatively similar. Yet, the carbon dioxide permeation of several samples in Table 1 is several times that of oxygen permeation in the same material. Why is this? This is due to the influence of the solubility coefficient. For inorganic gases, there are no functional groups in polymers that interact with them specifically. Therefore, the critical temperature becomes the main factor controlling solubility. The critical temperature of CO2 is 31℃, much higher than other common inorganic gases, so its solubility on the material surface is greater. Consequently, the CO2 permeation of the material is significantly higher than that of O2. 4. Conclusion In summary, it is impossible to use a single estimation ratio to apply to all materials. Material differences and environmental factors should be considered. Therefore, it is not recommended to use a ratio to estimate the permeation of other gases based on oxygen permeation. This article only discusses single-component materials; it's easy to imagine that the situation is much more complex with modified materials and composite materials. Editor: He Shiping