Realizing the temperature and humidity control of the test environment is one of the conditions for detecting the permeability of the packaging material in the actual application environment, and can provide a detection method to avoid design errors caused by environmental changes. At present, the humidity control of the test gas in the isobaric method can be achieved through the dry and wet gas mixing method and the double pressure method. The SHG-01 humidity generating device newly developed by Labthink is the first time to realize the humidity control of the test gas by differential pressure method. With the gradual deepening of the research on the correlation between the air permeability of packaging materials and the preservation quality of packaging contents, air permeability testing and equipment have become an important means and tool for scientific and reasonable selection of packaging materials. Because changes in the temperature and humidity of the test environment will cause changes in the material's breathability, the breathability test data obtained under different temperature and humidity conditions are not comparable. In order to solve this problem, the equipment must strictly control the temperature and humidity of the test environment, which has also contributed to the realization of temperature and humidity control as one of the development directions of air permeability testing equipment. Since the temperature has a very obvious influence on the gas permeability of the material, it has been widely concerned, and the temperature control of the gas permeability testing equipment has also developed very maturely. However, the control of humidity needs to be improved. Because there are technical difficulties in the control of humidity, especially the control of the humidity of the gas measured by the pressure difference method. The difficulty is that humidity is not only affected by temperature and other factors, but its control cannot rely on other media for conduction, but can only directly control the test gas. Effect of humidity on the permeability of materials Flexible packaging materials are mainly made of various high-molecular polymers. Some high molecular polymers (such as cellulose materials, nylon 6, PVA and EVOH, etc.) contain hydroxyl-OH, amido-CNH-, etc. These polymers are sensitive to water. If the environmental humidity increases, the moisture in the environment will diffuse into the polymer, which is equivalent to adding a certain amount of plasticizer, which will not only increase the free volume in the material, but also make some of the polymer's moving units The rearrangement movement is easy to perform, providing more instantaneous gaps for gas molecules to diffuse, thereby increasing the gas permeability coefficient. However, for some other high molecular polymers, such as those containing ester-CO- and cyano-C = N, the increase in humidity does not affect the permeability of the material. It can be seen that changes in humidity will only affect the permeability of some materials, and the size of the impact is related to the polarity and hydrophilicity of the material. In order to eliminate the influence of humidity, the standard requires that the state of the sample be adjusted according to the same adjustment environment before the test, so that the test data is comparable. As specified in GB / T 1038-2000, the sample should be placed in a desiccator under the environment of 23 ± 2 ℃ specified in GB / T 2918 for 48h or more, or processed according to the product standard. Other air permeability test standards also require that the samples should be dried similarly before testing. Isobaric gas humidity control The test principle of the isobaric method (shown in Figure 1) is to use a test film or sheet to divide the permeation chamber into two independent gas flow systems, with a flowing test gas on one side (which can be pure O2 or a mixture containing O2) Gas), on the other side is flowing dry N2. The pressure on both sides of the sample is equal, but the O2 partial pressure is different. Under the effect of the concentration difference of O2, O2 passes through the film and is sent to the oxygen sensor by the N2 flow. The oxygen sensor accurately measures the amount of O2 carried in the N2 flow, thereby calculating the O2 transmission rate of the material. In the entire test process of the isostatic method, there is continuous air flow in the upper and lower test chambers, and the formation of the partial pressure difference of O2 on both sides of the sample is a dynamic balance. Although this kind of dynamic balance is not conducive to accurate measurement of parameters and accurate control of balance, it is conducive to humidity control of the test gas, which is mainly reflected in the convenience of airflow humidity measurement and the uniformity of humidity. At present, the humidification methods with good practical effect are dry and wet gas mixing method and double pressure method. The humidity control accuracy of these two methods is generally ± 2% RH. Gas Humidity Control by Differential Pressure Method The test principle of the differential pressure method (shown in Figure 2) is to use the sample to divide the permeation chamber into two independent spaces, first evacuate both sides of the sample, and then fill one side with 0.1MPa (absolute pressure) ) Of the test gas, while the other side is kept in a vacuum state. In this way, a test gas pressure difference of 0.1 MPa is formed on both sides of the sample, so that the test gas penetrates and enters the low-pressure side through the membrane, and causes a change in pressure on the low-pressure side. By using a high-precision manometer to measure the pressure change on the low-pressure side, the gas permeation can be calculated. Differential pressure method can be used to detect many common inorganic gases such as O2, N2, CO2 and air. Different from the equal pressure method, the total pressure of the test gas in the upper and lower test chambers is constant during the entire test process. The pressure difference between the two sides of the sample is a stable static balance. Static balance is very conducive to the measurement of various test parameters, but it brings certain difficulties to the humidification of the test gas by this method. In the past, the differential pressure test under a certain humidity has not been implemented, so that the humidity control of this method has been regarded as unachievable. Recently, Labthink relied on its rich experience in testing equipment research and development, summing up the advantages and disadvantages of various humidification methods, combining the saturated salt solution humidity generation method with the dry and wet gas mixing method, and developing a unique special gas for vacuum pressure difference method. Humidity generation system of osmometer SHG-01. The system can realize the occurrence of various humidity in 0% to 100% RH, thus achieving a breakthrough in the humidity control technology of the differential pressure method. to sum up Realizing the temperature and humidity control of the test environment is one of the necessary conditions for testing the ventilation performance of the packaging materials in the actual application environment. At present, through the dry and wet gas mixing method and the double pressure method, the humidity control of the test gas in the equal pressure method can be achieved. The SHG-01 humidity generating device newly developed by Labthink is the first time to realize the humidity control of the test gas by differential pressure method. Ningbo Smile tools Co.,ltd , https://www.nbsmiletools.com
Figure 1 Schematic diagram of the isostatic test Click here to view all news pictures
Figure 2 Schematic diagram of differential pressure test Click here to view all news pictures