The Weighing Method – the First Choice for Water Vapour Permeability Testing of Drug Packaging
10 April 2007
Water vapour permeability is an important index in evaluating barrier property of materials and is no less important than auto gas permeability. Water vapour permeability can be tested with the weighing method or sensor method. The weighing method, or cup method, is the basic method, while test data of the sensor method should be calibrated by the data of the cup method (including infrared method, dynamic relative humidity method and electrolytic method).
As the uniform data system of the weighing method is adopted, data comparability of various methods becomes rather good. In 2003, the state Drug Administration issued YBB00092003 Water Vapour Permeability Measurement, which includes the first method (the cup method), the second method (the electrolytic method), and the third method (the weight method). Among that, the first and the second methods are applicable to film, sheets and aluminium foil of drug packaging. The third method applies to drug container packaging.
1. CUP METHOD
The cup method (weighing method) can be further divided into the desiccant method and the water method. In the desiccant method (the method used in YBB0009200), there is desiccant in the permeable cup. The humidity of the inner cup is considered as 0% RH and the test condition is 38°, 90%RH.
In the water method, the disk contains distilled water or saturated saline solution. If the cup is filled with distilled water, the humidity of the inner permeable cup can be considered as 100% RH and the test condition is 38°, 10% RH. The principles of these two methods are basically the same, in either of which the specimen is sealed to the permeable cup and then the rate of water vapour movement through drug film, sheets or aluminium foil can be calculated with the tested weight variation. Therefore, the test data of these two methods is the same, which is clearly specified in ASTM E 96.
The water method arose a little later than the desiccant method. However, it has obvious advantages in some aspects. For example, the inner permeable cup (when containing distilled water) of this method can maintain 100% RH for a long time. Since gas flow above the specimen carries away the water vapour, the automatic test is easily realised.
Due to the upper adsorption limit of desiccant, the desiccant method is not able to realize the full automatic test. In contrast, the water method has a great advantage in humidity controlling and has realized automatic test at present, thus has great development potential.
The test environment of the water method conforms to that of several other sensor methods (electrolytic method, infrared method, etc.); that is to say, on one side of the specimen, stable humidity is provided with distilled water or saturated saline solution while the other side is the controlled dry environment.
Moreover, such dryness can be stably controlled without interfering to the process of test. Therefore, the test data of the water method is more stable. It is for these reasons that the water method becomes the development trend of the weighing method.
2. ELECTROLYTIC METHOD
In the electrolytic method, an electrolytic cell is used as the humidity sensor. The gas transmission cell is divided into a dry chamber and a controlled-humidity chamber by film or sheet. Water vapour transmits through the specimen from the controlled-humidity chamber and then is carried away by the carrier gas into an electrolytic cell, where the humidity of carrier gas is measured and the electric signal is outputted. Users now can calculate the permeance of water vapour. Note that sensor methods including electrolytic method should be calibrated with the data of the cup method. Therefore the measured data is uniform with that of the cup method.
There are two international standards for the electrolytic method: ISO 15106-3 and DIN 53122-2. ISO 15106-3 is more comprehensive in content and wider in application. For this reason, ISO 15106-3 is recommended for electrolytic method testing.
3. WEIGHT METHOD
Based on the specialty of drug packaging, the weight method, or the third method of YBB00092003, can be further divided into three detailed test methods. The test principle of the weight method originates from the first method (the cup method). Though the weight method is simpler in terms of test process, the time of the test is much longer than that of the film test.
4. THE APPLICATION
Since all the tests of the container adopt the weight method, this author only introduces the application of film and sheets test based on the executive standards of drug container packaging (material).
Almost all the water vapour permeability tests of drug packaging are based on the weighing method. Only two kinds of materials require the electrolytic method. Therefore, the weighing method is the first choice for water vapour permeability testing of flexible drug packaging.
5. CONCLUSION
At present, the GB/T 1037-88 (weighing method) is the only Chinese national standard for water vapour permeability tests, and there is no sensor method standard. Therefore, the introduction of the electrolytic method for water-vapour permeability test standards of drug packaging is a helpful supplement. Therefore, the selection of water vapour permeability testers based on the weighing method can better satisfy the requirements of drug packaging test standards.
