Effectively control the humidity of breathability test

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 practical application environment, and it can provide a detection method for avoiding design errors caused by the environmental change. At present, the humidity control of the test gas in the equal pressure method can be achieved by the dry-moisture mixing method and the double-pressure method. The SHG-01 Humidity Generator developed by Labthink Languang has for the first time achieved humidity control of differential gas test gas.

With the gradual deepening of the research on the relationship between the permeability of packaging materials and the preservation quality of packaging contents, air permeability testing and equipment have become an important means and tool for the scientific and rational selection of packaging materials. As the temperature and humidity of the test environment will change the permeability of the material, the gas permeability test data obtained under different conditions of temperature and humidity are not comparable. In order to solve this problem, the equipment must strictly control the temperature and humidity of the test environment, which also contributed to the realization of temperature and humidity control as one of the development directions of the air permeability testing equipment. Since the effect of temperature on the permeability of the material is very obvious, it has received extensive attention, and the temperature control of the air permeability testing equipment has also been developed very well. However, the control of humidity still needs improvement. Because there are technical difficulties in the control of humidity, especially the control of the gas pressure in the differential pressure test. The difficulty is that the humidity is not only affected by factors such as temperature, but also its control can not rely on other media for conduction, but can only directly control the test gas.

The effect of humidity on the permeability of the material

Flexible packaging materials are mainly made of a variety of high molecular polymers. Some high-molecular polymers (such as cellulosic materials, nylon 6, PVA, and EVOH) contain hydroxyl-OH, amido-CNH-, and the like, and these polymers are sensitive to water. If the ambient humidity increases, the moisture in the environment will diffuse into the polymer, which is equivalent to adding a certain amount of plasticizer, not only will increase the free volume in the material, but also make some of the polymer's movement units. The rearrangement motion is easy to perform, providing more instantaneous gaps for the diffusion of gas molecules, thereby increasing the permeability coefficient of the gas. However, for some other high-molecular polymers, such as those containing ester group-CO-, cyano-C=N, the increase in humidity does not affect the air permeability of the material. Obviously, the change of humidity will only affect the permeability of a part of the material, and the size of the influence 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 in accordance with the same conditioning environment before the test is conducted. Such test data is comparable. As specified in GB/T 1038-2000, the sample shall be placed in a desiccator at a temperature of 23±2°C as stipulated in GB/T 2918 for 48 hours or more, or it shall be treated according to product standards. Other breathability test standards also require that the test specimen be similarly dried prior to testing.

Isobaric gas humidity control

The isobaric testing principle is to use a test film or sheet to separate the permeation chamber into two independent gas flow systems. One side is a flowing test gas (which can be pure O2 or a mixed gas containing O2), and the other side is Flowing dry N2. The pressure on both sides of the sample is equal, but the partial pressure of O2 is different. Under the influence of the concentration difference of O2, O2 passes through the membrane 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 to calculate the O2 transmission rate of the material.

During the entire isobaric testing process, continuous airflow was passed through both the upper chamber and the lower chamber. The formation of a partial pressure difference between the two sides of the specimen was a dynamic equilibrium. Although this dynamic balance is not conducive to the accurate measurement of parameters and the accurate control of the balance, it is beneficial to the humidity control of the test gas, which is mainly reflected in the convenience of humidity measurement and the uniformity of humidity. At present, humidification methods with good practical effects include dry-moisture mixing method and double-pressure method. The accuracy of the humidity control of these two methods is generally ±2%RH.

Differential pressure gas humidity control

The differential pressure test principle uses a sample to separate the permeation chamber into two separate spaces, first evacuating both sides of the sample into a vacuum, and then filling one side with 0.1 MPa (absolute pressure) test gas. One side remains vacuumed.

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 permeates through the membrane to the low pressure side and causes a change in the pressure on the low pressure side. By using a high-precision pressure gauge to measure the amount of pressure change on the low-pressure side, the amount of gas permeation can be calculated. The differential pressure method can be used for the detection of many common inorganic gases such as O2, N2, CO2, and air.

Different from the equal pressure method, the total amount of test gas in the upper chamber and the lower chamber under test is constant during the entire test process. The formation of a pressure difference between the test gas on both sides of the test is a stable static equilibrium. The static balance is very favorable to the measurement of various test parameters, but it brings certain difficulties to the test gas humidification. In the past, the differential pressure test under a certain humidity has not been realized, so that the humidity control of the method has been regarded as unachievable. Recently, Labthink has relied on its rich experience in the development of testing equipment to summarize the advantages and disadvantages of various humidification methods. It combines the saturated salt solution humidity generation method with the dry-moisture mixing method to develop a unique gas dedicated to vacuum differential pressure method. Permeameter humidity generation system SHG-01. The system can realize the occurrence of various humidity in 0%~100% RH, thus achieving the breakthrough of the differential pressure humidity control technology.

to sum up

Realizing the temperature and humidity control of the test environment is one of the prerequisites for testing the permeability of packaging materials in practical applications. At present, the humidity control of the test gas in the equal pressure method can be achieved by the dry-moisture mixing method and the double-pressure method. The SHG-01 Humidity Generator developed by Labthink Languang has for the first time achieved humidity control of differential gas test gas.