Gravimetric Method Is the First Choice for Testing Moisture Permeability of Drug Packaging Materials

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Moisture permeability is as important as air permeability in evaluating materials barrier performance. The moisture permeability test method can be divided into gravimetric method and sensor method. Gravimetric method is the basic method (also known as cup method). The test data of sensor method needs to be calibrated by gravimetric method data, including infrared sensor method, humidity detection method and electrolytic detection method. As a result of the use of a unified gravimetric method data system, the data comparability between various methods is good. In 2003, the State Food and Drug Administration issued YBB00092003 “Test for Water Transmission”, including the first method cup method, the second method electrolytic detection method and the third method weighing method. The first method and the second method are suitable for medical film, thin film and medical aluminum foil, and the third method is suitable for medical container.

1. Cup method

The cup method (gravimetric method) can be divided into two test methods: the method of increasing weight when water vapor penetrating into the test cup and the method of reducing weight when water vapor penetrating away from the test cup. Desiccant is put in the test cup of the dry cup method (this method is used in ybb00092003). It can be considered that the inside of the test cup is 0% RH and the test environment is 38 ℃ and 90% RH; Distilled water or saturated salt solution is filled in the test cup of the wet cup method. If distilled water is filled, it can be considered that the inside of the test cup is 100% RH and the test environment is 38 ℃ and 10% RH. The essence of measurement of the two methods is the same. Both methods fix the sample on the test cup and calculate the water vapor transmission of the medical film, thin film and medical aluminum foil by measuring the weight change of the test cup. Therefore, the test data of the dry cup method and the wet cup method are the same, which is clearly described in ASTM E 96.

The rise of wet cup method is later than that of dry cup method, but it has prominent advantages. The internal environment of the test cup can be maintained at 100% RH for a long time (containing distilled water inside). The air flow above the sample takes away the permeated water vapor, which is convenient for automatic detection.

Because of the absorption limit of desiccant, the dry cup method cannot achieve full-automatic detection. The wet cup method on the other hand has a strong advantage in test environment humidity control, it therefore has achieved automatic testing with great development potential. The detection environment of the wet cup method is consistent with several sensor methods (electrolytic detection, infrared detection, etc.), the distilled water or saturated salt solution provides a stable humidity inside the sample, while the outside (test chamber environment) is kept dry by external control means, and the control of drying can be carried out stably without interrupting the test process, so the stability of test data is better. Wet cup method becomes the trend of gravimetric method development.

2.  Electrolytic detection

The electrolytic cell is used as the humidity sensor in the electrolytic analysis method. The permeation chamber is divided into a dry chamber and a humidity controlled wet chamber by film or sheet. After the water vapor permeates through the sample from the wet chamber, it will be carried to the electrolytic cell by the carrier gas flow. The humidity of the carrier gas is measured by the electrolytic cell and the electrical signal is output, and then the water vapor transmission of the sample can be calculated. Special attention should be paid to the fact that several sensor methods, including electrolytic analysis, need to be calibrated with the data of gravimetric method so that the measured data is in good agreement with the data of weighing method.

The international standards adopting electrolytic method include ISO 15106-3 and DIN 53122-2, of which ISO 15106-3 is more detailed and has a wider range of practical use. Therefore, if it is necessary to use electrolytic detection, ISO 15106-3 is recommended.

3.  Weighing method

The third method stipulated in ybb00092003 is weighing method, which are specially formulated for the particularity of drug packaging containers. The test principle is derived from cup method. The specific test process is relatively simple, but the test time is much longer than the film detection.

4.  Use of methods

Since weighing method is applied to all the container tests, only the two testing methods for film and sheet are analyzed here in combination with the specific implementation of the drug packaging container (material) standard.

Table1. Test method for moisture permeability of drug packaging materials

It can be seen from table 1 that most of the moisture permeability test methods of drug packaging materials listed in the table are gravimetric method, and only two materials need to be tested with electrolytic analysis method. It can be seen that the gravimetric method is the first choice for the moisture permeability test of flexible packaging materials for drug packaging.

5.  Conclusion

At present, only GB / T 1037-88 (gravimetric method) is one of the corresponding moisture permeability test methods in the national standard, and there is no sensor method standard. Therefore, the test standard of the moisture permeability of drug packaging materials puts forward that the electrolytic analysis method is a useful supplement in the test method based on the national standard. The moisture permeability test instrument based on the gravimetric method can be a choice that best meets the test standard of drug packaging materials.