Present Situation and Prospects of Gravimetric Method Water Vapor Permeability Testing 3-1

March 23, 2009 - PRLog -- This is just an abridgement. For full and more thesis, please visit: http://www.labthink.cn/service/list1.aspx
Oxygen and water vapor have obvious influence on active substance of food, pharmaceuticals and some cosmetics, which make chemical property of these products unstable. Therefore, commercial packaging usually raises demand on barrier property of specimen materials. At present, the fundamental method of water vapor permeability testing-gravimetric method is provoking criticism in terms of its test efficiency and test repeatability, etc. The expansibility of gravimetric method and whether gravimetric method will be replaced have become the focus of concern. This article presents a systematic analysis of test principle, present situation and prospect of gravimetric method.
1. Gravimetric Method
1.1 Principle of Gravimetric Method
     In gravimetric method, the two sides of specimen maintain a certain pressure difference at a specified temperature. Then the weight variation of permeable cup is measured using gravimetric balance or analyzing balance. Based on the size, thickness, weighing interval and humidity difference on two sides of specimen, water vapor permeability parameters can be calculated. Since the characteristic testing component in gravimetric method is permeable cup, this method is also called cup method. Although certain relative humidity difference is specified in gravimetric method as its testing condition, it is not pointed out which side should be the high humility side. Therefore, whether the inner permeable cup to be the high humidity side and outer cup to be the dry side(water method)or vise versa (desiccant method) conforms to test principle of gravimetric method.
1.2 Data Comparison Between Desiccant Method and Water Method
      Whether test data obtained by two methods based on the same test principle, i.e., desiccant method and water method, are consistent has become the concern of the whole testing industry. According to test process and calculate formula provided in ASTM E96, test data of the two methods should be consistent under perfect condition. However, such deduction is less persuasive than actual comparison. In 2007, proficiency testing to 'Barrier Property Testing for Plastic Package Material-Determination of Oxygen and Water Vapor Transmission Rate' is held in our country nationwide, which is sponsored by CNCA and organized by National Packaging Product Inspection and Testing Center (Jinan). There are altogether 67 labs (all the domestic authoritative testing institutions are participated) participating in the testing of water vapor transmission rate (66 labs provided effective test data and 10 labs did not adopt gravimetric method). This proficiency testing is unprecedented in the world in terms of its scale, testing level of the members and authority of its organizers. Based on the results of data comparison, field test data of desiccant method and water method have no obvious difference.

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