1. Testing Characteristics of the Materials with High Transmission Rate
The materials with high transmission rate may show the following characteristics in barrier property test. First, as the increase of volume of the permeated gas, the gas supply is required to be sufficient. Second, as the transmission rate is much higher, the transducer is required to be more sensitive or have a wider test range. Third, time is usually not sufficient for the gas transmission to reach the equilibrium state required by some standards. At present, most of the testing instruments are designed for test of the materials with high barrier property with which the operator may encounter some difficulties in testing the materials with high transmission rate.
It should be especially noted that it is critical to determine the equilibrium state of the gas transmission in barrier property test. The conditions for determination of equilibrium should be strictly followed otherwise the test may be finished before the equilibrium state is reached. It usually takes several hours to reach the equilibrium state for the materials with high or medium barrier property. While for the materials with low barrier property i.e. the materials with high transmission rate, such as coating textiles, non-woven fabrics and battery diaphragm etc., gas transmission may reach the equilibrium state in a quite short period. That’s why the process for the gas transmission reaches equilibrium state has been omitted from the standards for water permeability test of textiles.
For the materials with high transmission rate, the increased volume of permeated gas and the high transmission rate make it difficult to test the barrier property. For example, the equilibrium for differential pressure method is determined by the pressure change of the lower test chamber. For the specimen with high transmission rate, the pressure of the lower test chamber would rise quickly as soon as the gas is charged into the upper test chamber. As for equal pressure method gas transmission rate tester, the equilibrium is determined by oxygen concentration in the test chamber, which would be directly measured by the transducer. However, when testing materials with extremely high transmission rate, the oxygen concentration in the testing chamber would exceed the upper limit of the test range of the transducer which may influence its lifespan. And for water vapor transmission rate testers, water vapor permeability would be increasing in process of the whole test, hence the solution consumption should be controlled and make sure the solution would not be exhausted. As well the ambient humidity should be controlled effectively.
Base on the experience of numeral tests, the following instructions are provided for reference which may help the operator to extend the test range and improve the repeatability of test results for barrier property test of the materials with high transmission rate.
2. Gas Transmission Rate Test of the Materials with High Transmission Rate
2.1 Differential Pressure Method
The test with differential pressure method mainly depends on the data obtained by the pressure sensor in the lower test chamber. As the pressure changes within a quite short period when testing the materials with high transmission rate, the pressure sensor is required to have excellent sensitivity and high precision.
The test range could be extended in the following ways for differential pressure method.
First, change the test mode. As the equilibrium is reached in a very short period in test of the materials with high transmission rate, the start pressure and the end pressure could be preset before testing. Then, gas transmission rate of the material could be calculated by measuring the duration for the pressure change. It has proved in actual tests that the test range is expanded and the repeatability of test results is greatly improved in this way. However, since the pressure changes too fast for test of the materials with high transmission rate, only automatic testing instruments can be used. Labthink i-7100, i-7200 and i-7210 Gas Permeability Testers are featured with standard mode which is specialized for permeability test of the materials with high transmission rate.
Second, change the test area. By changing the test area, the volume of test gas permeated through the specimen within unit time could be effectively controlled. Labthink provides the customers with a test accessory “MASK” which could change the area for gas transmission so that the test range could be extended as required.
2.2 Equal Pressure Method
For equal pressure method testing instruments, the gas volume in the test chamber is directly obtained by the sensor. If the gas volume exceeds the test range, the lifespan of the sensor would be influenced. Therefore, it is critical to reduce the gas volume in test chamber within unit time for test of the materials with high transmission rate. There are 3 ways as follows:
First, change the test gas. Usually, the test range could be expanded by reducing the oxygen concentration difference between the two sides of the specimen. For example, to replace pure oxygen with air or with other mixed gases and it should be noted that when calculating the test results, the concentration of test gas should be converted to that of oxygen.
Second, improve the flow rate of carrier gas so as to reduce the concentration of the test gas in test chamber. Since both the concentration of test gas and the flow rate of carrier gas are applied in calculation of test results, it would not influence the test data by improving the flow rate of carrier gas.
Third, change the test area. Labthink provides the test accessory “MASK” which could change the test area and thus reduce the volume of permeated gas within unit time.
3. Water Vapor Transmission Rate Test of the Materials with High Transmission Rate
3.1 Gravimetric Method
When testing the materials with high transmission rate with gravimetric method, the precision of the measuring parts and volume of the diffusion cup should be taken into consideration. Gravimetric method includes dry cup method and wet cup method. Based on the experience of numeral tests, dry cup method is limited by the moisture absorption of desiccant and adding desiccant may cause the weight of diffusion cup exceeds the test range of load cell. Therefore wet cup method is ideal for water vapor transmission rate test. The operator should observe the volume of the distilled water or saturated salt solution in the cup and make sure there is remaining distilled water or solution when the test is finished. Replace the cup with higher volume cup or customized cup when it could not meet the requirement. As well the humidity of the test chamber should be controlled effectively in testing process. At present, Labthink i-Hydro 7300, i-Hydro 7310 Water Vapor Transmission Rate Tester and i-Hydro 7320 Water Vapor Transmission Rate Testing System are featured with the function of automatic adjustment of the humidity of the test chamber which is of great importance to the water vapor permeability test of the materials with high transmission rate.
In order to extend the test range of the water vapor transmission rate tester, the customized cups could be used as well the test accessory “MASK” provided by Labthink could be applied to change the test area so as to extend the test range which would be suitable for test of materials with high transmission rate.
3.2 Sensor Method
The working principle and instrument structure of sensor method water vapor transmission rate tester are similar to those of equal pressure method gas transmission rate testers. However, the ways to extend test range for equal pressure method do not apply to sensor method. As for sensor method, the only way to extend test range is to change the test area with the test accessory “MASK”.
Since some materials are sensitive to water vapor, different test results of water vapor transmission rate may be obtained under distinct testing conditions. Therefore it is improper to extend the test range by changing type of test solution or reducing the concentration difference between the two sides of the specimen.
With the introduction of new materials and expanding of the applications, the materials with high transmission rate have found more applications in different fields. This article has put forward some effective, convenient, rapid and accurate methods to extend the test range for permeability test of materials with high transmission rate. Meanwhile, leakage, especially leakage at the edges of the packaging materials should be also paid attention to, which may influence the test results of the transmission rate of the test specimen.