Test methods for water vapor permeability transmission rate through films can be divided into gravimetric method and sensor method. At present, only gravimetric method is used for domestic film water vapor permeability testing, which is not favorable for product evaluation in international trade. In addition, although electrolytic sensor method has been adopted in some industrial standards (for example, medical packaging standards), there is no corresponding international support, which also impedes the popularization of industrial standard. Comparing with gravimetric method, electrolytic method has more advantages and can better satisfy present test requirements on high barrier packaging materials; the enactment of corresponding test standards is of urgent necessity. In early 2007, the enactment starts formally. Now it has entered approving stage of the standard.
1．The Enactment of Film Water Vapor Permeability Test Standard Using Electrolytic Sensor Method
According to the National Packaging Standardization Committee specified in its requirement ‘About Enactment and Revision of National Standard Project’, China Package Scientific and Research Testing Center will undertake the standard enactment task entitled Test Method for Water Vapor Transmission Rate through Plastic Films and Sheets-Electrolytic Sensor Method. National Package Quality Inspection Centre (Jinan) and Jinan Labthink Instruments Co. Ltd. are specially invited to draft this standard. The standard draftsman group, organized by technicians from these three institutes, drafted the recommendatory national standard proposal and submitted it to National Package Standardization Committee after investigation, analyses and discussion on the significance and feasibility, the involved international and foreign standards as well as the social and economical benefits. Based on ISO15106-3:2003 Plastics - Film and Sheeting - Determination of Water Vapor Transmission Rate - Part 3: Electrolytic Detection Sensor Method and combining the rich barrier property research experiences of Labthink, the standard draftsman group finished questionnaire draft of the standard that completely conforms to application characteristics of plastic packaging industry. After soliciting broad opinions from domestic enterprises, colleges and industry experts, the draft standard has been revised for many times. The approval for draft was finalized on 24th July 2007 and the draft standard has been submitted for inspection. This standard will be formally issued recently.
Test principles of electrolytic method are as follows: electrolytic cell is used as humidity sensor. Permeation cell of instrument is divided into a dry cell and a humidity-controlled wet cell by film or sheet. Water vapor transmits from wet cell through film into dry cell, and is further carried by carrier gas to electrolytic cell, where the electric signal is measured and the output can be used to calculate transmission rate of water vapor through specimen.
2． Calibration of Electrolytic Sensor Method
Electrolytic sensor method, together with dynamic relative humidity testing method and infrared testing method, belongs to sensor method water vapor permeability testing. It was gradually introduced since 1970. Requiring the same test environment as that of water method, electrolytic sensor method is distinguished for its rapid test speed, high resolution and less influencing factors. Therefore, this method is suitable for rapid determination of water vapor transmission rate through high barrier film, sheet and multi-layer structure materials with plastic ingredient.
However, due to sensor depletion or test accuracy, this instrument should be calibrated with gravimetric method test data periodically, which has been introduced in related test standards. Reference films are adopted for calibration and the data of reference films are obtained according to gravimetric method. For example, In ASTM E 398-03, there is such description: “Standard Films, which have been calibrated by gravimetric means……A standard, calibration film who’s WVTR has been gravimetrically determined…….”
Because electrolytic sensor is of consumptive type, electrolytic sensor itself cannot maintain stable test data, and therefore, needs calibration periodically. For this reason, calibration is an important item for such test method. During standard enactment this time, there is a detailed explanation of the calibration of electrolytic sensor method. The experts think that to make test data of electrolytic sensor method comparable with that obtained from other methods, this standard should specify requirement on traceability of test data. Therefore, in the chapter ‘Instrument’ of this standard, it is stated that, “Standard Films are used to calibrate instrument. Standard film is film with known WVT or the WVT of which is determined using gravimetric means’, which is consistent with the tractability requirements of ISO 15106-1:2003 and ISO 15106-2:2003on test data. Such calibration can ensure water vapor permeability data obtained with electrolytic sensor method is consistent with that of gravimetric method.
Presently, according to the requirements of international water vapor testing standards, test data sensor methods should be calibrated by test data obtained with gravimetric means so as to secure consistency. At the same time, since gravimetric method is the basic method for water vapor permeability testing, it can be guaranteed that tests performed under the same test environment can obtain the same test data. In this way, field test data of electrolytic sensor method can maintain consistency with that of international water vapor permeability test data.
3． Advantages of This Standard
Comparing with ISO 15106-3:2003, this national standard for electrolytic sensor method has the following advantages:
3. 1 The addition of methods for humidity realization: ISO 15106-3:2003 specified relative humidity realization by using sulfuric acid solution only. However, the safety of using sulfuric acid solution in actual operation requires attention. Therefore, this standard adds the method of using saturated saline solution to realize relative humidity based on the content of original standard while referring to ASTM E 104-02 Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions and DIN 53122.2 (1982) Testing of Plastics Films, Elastomer Films, Paper, Board and Other Sheet Materials Determination of Water Vapor Transmission Rate Electrolytic Method,.
3. 2 Reducing some restrictions. For example, this electrolytic sensor method standard does not regulate operating voltage of electrolytic sensor, and has looser restrictions on flow range and transmission area. At the same time, to make this standard more applicable, no particular specifications relating structure and components of test instruments is mentioned in this standard; and categories of materials are also increased. Such measures are beneficial for the applications of new types of materials and creative design of testing instruments, which paves the way for this standard to adapt to technological development.
3.3 Increasing judging conditions of transmission equilibrium. As we all know, transmission of water vapor goes through a non-equilibrium state before reaching equilibrium state. However, water vapor permeability parameters must be observed under equilibrium state. Otherwise, error test data will be obtained. For specific film, the time required to reach equilibrium state from non-equilibrium state varies. For electrolytic sensor method, as long as the quantity of water vapor transmitting into sensor per unit time does not exceed upper measurement limit of sensor, equilibrium process can be prolonged indefinitely until the sensor is depleted. Therefore, the end of the test cannot be determined by length of test duration. If transmission equilibrium is estimated only by human observation, accuracy of test data is lowered and repeatability of test data becomes poor. So, it is necessary to set terms of transmission equilibrium like that of gravimetric method, and there is such statement relating the determination of transmission equilibrium in this standard: “measure electrolytic current variation in fixed interval. When amplitude of three consecutive current fluctuations do not vary more than 5%, the current can be considered as stable and water vapor transmission enters into equilibrium state. Actual application of Labthink has proved that using this method to determine transmission equilibrium and provide test data can greatly improve repeatability of test data and stability of instrument, thus avoiding errors in judgment.
3.4 Introducing the definition of reference film: this standard adds requirements on traceability of test data. In addition to its reference to calibration methods of ISO and ASTM during standard enactment, the definition of reference film is introduced for the first time. It is required that ‘calibrate instrument using reference film’. The using of reference film makes its calibration method more clear and simple. At the same time, this can ensure that test data of electrolytic sensor method are comparable to test data obtained with other methods.
4． Conclusion
Electrolytic sensor method is applicable to rapid determination of water vapor transmission rate through plastic film, sheet and plastic-containing multi-layer materials. The enactment of electrolytic sensor method standard not only enriches test methods of barrier property testing for domestic packaging materials, it can also better satisfy test requirements of high barrier packaging materials. Electrolytic sensor method standard is formulated on the basis of ISO 15106-3:2003 with reference to the rich experience of barrier property testing. It is believed that this standard can better adapt to actual application requirements of our country.