The vacuumizing time and the vacuum degree reached by the system should be paid much attention when performing gas transmission tests with vacuum method. The vacuum degree would be reduced if the vacuumizing time is not sufficient, which would cause inaccuracy of the test results.
Gas permeability of the materials is mainly tested with differential pressure method or equal pressure method, of which the former one is the most widely used and the most influential method. Being the earliest test method of gas permeability, vacuum method is based on the time-lagged method of membrane test technology. It is applicable in permeability test of multiple kinds of gases. Moreover, test parameters including diffusion coefficient and solubility coefficient could be obtained with this method. With those advantages mentioned above, vacuum method has been well recognized worldwide. It is also a practical method in the research of material modification.
1.Vacuum Testing Environment
Test procedures of vacuum method are mainly as follows: Mount specimen, vacuumizing process, charge test gas, record pressure variation of lower test chamber when test gas permeates through specimen, finish test and calculate test results.
Vacuumizing process is an important step in the whole process of permeability test with vacuum method. It is the only way to form the test environment (one side of specimen is test gas with pressure 0.1Mpa and the other side is vacuum). Additionally the seal quality of the specimen could be checked in this step. If there is leakage in the specimen, it would take a long time to evacuate the system and the vacuum degree could not reach the expected level. As well by vacuumizing process, the impurity or other gases other than test gas could be expelled which may interrupt the repeatability, reliability of the test data greatly. Therefore, the effect of vacuumizing process is of great significance. Proper vacuumizing process can improve both data validity and data repeatability.
The permeability test should not started immediately after the vacuum degree reaches the required level. Otherwise the accuracy of the test results may be influenced. The vacuumizing time depends on the vacuum degree required to reach. If the vacuum degree required by the system is low, the vacuumizing time would be decreased. Usually when the vacuumizing time is reduced, the vacuum degree would be low and as a result the test precision would be decreased.
The required vacuum degree of test chamber specified in ASTM D 1434-82(2003) is less than 26Pa. The lower test chamber must reach required vacuum degree before the test gas is charged into upper test chamber. Otherwise, test results would be invalid and meaningless. When the required vacuum degree is reached, vacuumizing process should be continued for more than three hours.
3 The Influence of Vacuummizing Time on Test Data
The longer the vacuumizing time is, the higher the vacuum degree of lower test chamber is and the more accurate and stable the test data is. In order to study the influence of vacuumizing time on the test data, permeability test of several specimens with different vacuumizing time are performed.
Apparatus: Labthink i-GASTRA 7100 Gas Permeability Tester(http://www.labthinkinternational.com/product/i-gastra-7100-gas-permeability-tester.html)
Test Specimen: PET Film (25um)
Tests are performed with vacuumizing time of 24h, 18h, 2h, 1h and 0.5h respectively. For each vacuumizing time, at least 3 specimen tests are performed.
As the vacuumizing time is decreased, test time is shortened while the oxygen permeance result is rising, which is in accordance with the theoretical analysis. When vacuumizing time is less than four hours, oxygen permeance is increased obviously. And test data is not stable.
Although vacuumizing time is only one of the parameters of vacuum method gas permeability testers, it should be paid much attention to by the users. Otherwise, the test accuracy could not be guaranteed and test may be incorrect if the vacuum degree does not meet the requirements.
Labthink provides one data list of the vacuumizing time (i-GASTRA 7100 http://www.labthinkinternational.com/product/i-gastra-7100-gas-permeability-tester.html) for user's reference. In practice, it is reasonable to determine the vacuumizing time according to testing experience.
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