| Abstract: test accuracy and test repeatability of | | | | not exceed 10%(there is similar specification for |
| water vapor permeability testing using gravimetric | | | | hygroscopic capacity of other desiccant), there is |
| method are controversial at present. The | | | | no data proof whether hygroscopic capacity of |
| expansibility of gravimetric method and whether | | | | desiccant changes obviously when its hygroscopic |
| gravimetric method will be replaced by new | | | | moisture content approaches the upper limit. |
| methods have become the focus of concern. This | | | | However, water method of gravimetric method |
| article presents a systematic analysis of test | | | | does not have the above defects and has realized |
| principle, present situation and prospects of | | | | automatic testing at present. Then why the |
| gravimetric method. | | | | improvement of water method could not attract |
| Keyword: water vapor permeability?gravimetric | | | | enough attention? The author thinks that this is |
| method?cup method | | | | related with the misunderstanding of GB/T 1037 |
| Oxygen and water vapor have obvious influence | | | | among some insiders. ASTM E96 is the most |
| on active substance of food, pharmaceuticals and | | | | widespread and historic water vapor |
| some cosmetics, which make chemical property | | | | permeability-testing standard. It is also the most |
| of these products unstable. Therefore, | | | | excellent standard in terms of its completeness of |
| commercial packaging usually raises demand on | | | | describing test methods. This standard |
| barrier property of specimen materials. At | | | | simultaneously introduces these two test |
| present, the fundamental method of water vapor | | | | methods: desiccant method and water method, |
| permeability testing-gravimetric method is | | | | both of which are considered as fundamental test |
| provoking criticism in terms of its test efficiency | | | | methods, and should be selected according to |
| and test repeatability, etc. The expansibility of | | | | application environment of materials in actual test. |
| gravimetric method and whether gravimetric | | | | Other water vapor permeability testing standards, |
| method will be replaced have become the focus | | | | such as GB/T 1037, do not have such complete |
| of concern. This article presents a systematic | | | | description of test methods. The first edition GB |
| analysis of test principle, present situation and | | | | 1037-70 ‘Determination of Water Vapor |
| prospect of gravimetric method. | | | | Transmission Rate for Plastics' simultaneously |
| 1. Gravimetric Method | | | | includes gravimetric method and water |
| 1.1 Principle of Gravimetric Method | | | | method. However, the revised edition GB |
| In gravimetric method, the two sides of specimen | | | | 1037-88 ‘Determination of Water Vapor |
| maintain a certain pressure difference at a | | | | Transmission Rate for Plastic Films and |
| specified temperature. Then the weight variation | | | | Sheets-Cup Method' simplifies its description of |
| of permeable cup is measured using gravimetric | | | | water method and only lists the method of |
| balance or analyzing balance. Based on the size, | | | | putting desiccant into permeable cup when |
| thickness, weighing interval and humidity | | | | explaining how to realize relative humidity, which |
| difference on two sides of specimen, water | | | | makes some users think that it is not allowable to |
| vapor permeability parameters can be calculated. | | | | use distilled water or other reagents. However, |
| Since the characteristic testing component in | | | | the emphasis upon GB 1037-88 on ‘cup |
| gravimetric method is permeable cup, this method | | | | method' in the standard title indicates that its |
| is also called cup method. Although certain relative | | | | content does not confines to ‘desiccant |
| humidity difference is specified in gravimetric | | | | method' only. It is because the above-mentioned |
| method as its testing condition, it is not pointed | | | | misunderstandings that results in the cold |
| out which side should be the high humility side. | | | | reception of water method, which actually |
| Therefore, whether the inner permeable cup to | | | | conforms to GB/T 1037. At the same time, |
| be the high humidity side and outer cup to be the | | | | various investigation rules including present market |
| dry side?water method?or vise versa (desiccant | | | | access QS certification requires implementation of |
| method) conforms to test principle of gravimetric | | | | GB/T 1037 gravimetric method, and not confining |
| method. | | | | to desiccant method. |
| 1.2 Data Comparison Between Desiccant Method | | | | In addition, the impact of sensor method makes |
| and Water Method | | | | some people violate justice in evaluating desiccant |
| Whether test data obtained by two methods | | | | method. Although some people speak highly of |
| based on the same test principle, i.e., desiccant | | | | sensor method, they do not have any actual |
| method and water method, are consistent has | | | | application experience. Test parameters and the |
| become the concern of the whole testing | | | | reasonability of precision will not be discussed in |
| industry. According to test process and calculate | | | | this article. The limitation of desiccant method in |
| formula provided in ASTM E96, test data of the | | | | terms of specimen thickness and specimen sealing |
| two methods should be consistent under perfect | | | | also exists in those traditional sensor methods. |
| condition. However, such deduction is less | | | | Moreover, the sealing problem is still a key factor |
| persuasive than actual comparison. In 2007, | | | | restraining lower test limit of testing instrument. |
| proficiency testing to ‘Barrier Property | | | | Therefore, in terms of test principle, the lower |
| Testing for Plastic Package Material-Determination | | | | test limit of several water vapor testing methods |
| of Oxygen and Water Vapor Transmission Rate' | | | | should be consistent. |
| is held in our country nationwide, which is | | | | 3. Prospects of Gravimetric Method |
| sponsored by CNCA and organized by National | | | | In 2007 barrier property proficiency testing, 85? |
| Packaging Product Inspection and Testing Center | | | | of the participators use gravimetric method |
| (Jinan). There are altogether 67 labs (all the | | | | instruments. And system error of all these |
| domestic authoritative testing institutions are | | | | laboratories is within the allowable range of our |
| participated) participating in the testing of water | | | | country. Statistical data from this proficiency |
| vapor transmission rate (66 labs provided | | | | testing have confirmed the fundamental position |
| effective test data and 10 labs did not adopt | | | | of gravimetric method, which simultaneously |
| gravimetric method). This proficiency testing is | | | | refutes those reports about lower accuracy of |
| unprecedented in the world in terms of its scale, | | | | gravimetric method. |
| testing level of the members and authority of its | | | | However, we must admit that the improvement |
| organizers. Based on the results of data | | | | space of desiccant method is becoming more and |
| comparison, field test data of desiccant method | | | | more restricted. With the accelerating of |
| and water method have no obvious difference. | | | | automatic testing process, its development is |
| 2?Present Situation of Gravimetric Method | | | | obviously lagging behind other test methods. The |
| 2.1 Present Situation | | | | pleasant thing is that problems encountered by |
| It is undeniable that the formulation and the | | | | desiccant method during the course of its |
| implementation of GB/T 1037 play a rather | | | | development can be well solved by water |
| important role in the wide application of | | | | method. Liquids such as distilled water contained in |
| gravimetric method. At the same time, the salient | | | | permeable cups of water method can maintain |
| features of gravimetric method in terms of its | | | | long-term water vapor concentration stably |
| easy operation and lower price of test | | | | without frequent human interference. Now the |
| instruments also accelerate the popularization of | | | | highly efficient and convenient automatic testing |
| instruments using this method. During its | | | | instrument using water method has been |
| application in the past twenty years, the negative | | | | introduced to the market. Actual application has |
| evaluation of gravimetric method is occurred now | | | | proved that desiccant method testing needs three |
| and then. Moreover, with the introducing of new | | | | days while full automatic water method |
| testing methods and the intensifying need to | | | | instruments only needs several hours to complete |
| replace traditional instruments; such negative | | | | the test. Test efficiency of water method |
| evaluation becomes more prominent. The | | | | instruments is no less inferior to that of sensor |
| negative side mainly exists in aspects of lower | | | | method instrument. Moreover, full automatic |
| test efficiency, accuracy and repeatability. Thus | | | | instruments possess advantages in terms of test |
| the newly rising sensor method instruments | | | | accuracy and repeatability. |
| become more favorable, which is thought to be | | | | To develop water method, the content of GB/T |
| more suitable to the booming development of | | | | 1037 should be further improved to eliminate |
| present flexible packaging industry. | | | | misunderstandings of GB/T 1037 in the society, |
| 2.2 Analysis of Present Application Status | | | | avoiding the ideas that ‘water method does |
| Is gravimetric method is really impractical as is | | | | not belong to gravimetric method'. At present, to |
| reported? When analyzing the reasons, it is not | | | | update their testing instruments, several |
| difficult to discover that those defects of | | | | authoritative testing institutions in our country |
| gravimetric method mainly result from the using | | | | have already began to purchase full automatic |
| of desiccant method instruments. In fact, there | | | | instruments using water method and stopped |
| are indeed weak points in desiccant method. | | | | purchasing desiccant method instruments, which |
| However, these problems are mainly result from | | | | indicates that they believe water method |
| the difference between testing environment and | | | | conforms to GB/T 1037. Although electrolytic |
| weighing environment and the difficulty to maintain | | | | sensor method water vapor permeability testing |
| a stable pressure difference of water vapor on | | | | standard was approved in 2007, the applications |
| two sides of specimen in a long term (test | | | | of this new method would not shake the position |
| environment is not stable). This is especially | | | | of gravimetric method, just as the issuing of GB |
| obvious for specimens with better hygroscopic | | | | T 19789 did not shake the authoritative |
| capacity. Although consolidation of testing | | | | position of differential-pressure method (in the |
| environment can solve errors caused by | | | | 2007 proficiency testing of barrier property, 74? |
| environmental changes, concentration difference | | | | participators use differential pressure method |
| of water vapor on two sides of specimen is | | | | instruments) in gas permeability testing during the |
| maintained totally on manual operation, which | | | | past two years. Therefore, those who want to |
| makes automatic testing and the consolidation of | | | | completely develop water vapor permeability |
| test environment difficult to realize. In addition, | | | | testing must start with gravimetric method, in |
| although it is specified in GB/T 1037 that | | | | which popularizing and improving water method is |
| hygroscopic moisture content of desiccant should | | | | the best way. |