Due to its obvious effects on preservation of products, barrier-packaging material has become one of the fastest growing functional materials in application. However, the development of its key technical parameter testing-barrier property testing is out of balance in our country. Actual testing instruments vary from each other to different degrees, resulting in the inconsistent barrier property testing level and data of various institutes. Therefore, unified test data is expected greatly in the market. A large number of laboratories including domestic authoritative testing institutions participated in this proficiency test of barrier property testing for plastic packaging materials, which is undoubtedly the most objective, accurate and efficient way to make a census of the testing ability in the whole barrier property testing field.
1. Proficiency Test Requirements of the Certification and Accreditation Administration of China
To strengthen the capacity construction of laboratories, and to verify and enhance laboratory testing level in focused fields that merits special attention, in 2007, The Certification and Accreditation Administration of China (CNCA for short) decided to continue the proficiency testing among laboratories.
In April 24th, 2007, CNCA issued Document Guo Ren Fa (2007) No. 60 Notice Regarding 2007 Proficiency Test Plan. According to opinions from relevant departments of quality inspection institutes and society, CNCA selects 22 among the 138 registered projects as the proficiency test plan of 2007, which are divided into 11 categories with A and B of each category respectively. The project ‘Barrier Property Testing of Plastic Packaging Materials- Determination of Oxygen and Water Vapor Transmission Rate ’ submitted by National Package Product Quality Supervision & Testing Center (Jinan) is listed as the third one in category A. As to A category proficiency testing projects this notice, it is stipulated as follows: various quality supervision and testing centers having obtained metrology accreditation while possessing A category test proficiency product quality supervision, inspection institutes at provincial levels (including deputy provincial level city and city directly under state planning), government-operated technical centers (labs) of Administrations for Entry-Exit Inspection and Quarantine, various relevant departments and ministries or industrial product quality inspection centers must participate relevant A category proficiency test. Written report must be submitted to CNCA for absence reason. As to B category projects, it is stipulated in the notice that laboratories can enter their name for B category proficiency test voluntarily according to their own needs. It is thus clear that the proficiency test this time is record-breaking in terms of test scale, testing level of participating labs and authority of organizers.
2. Introduction to Proficiency Test
2.1 Organizational Institute and Institutions Cooperating
National Packaging Product Quality Supervision and Testing Center is responsible for the daily operation and coordination of Proficiency test program ‘Barrier Property Testing of Plastic Package Materials- Determination of Oxygen and Water Vapor Transmission Rate ’. As a technical supporting institute, Labthink undertakes the work of selecting specimen, sampling, packaging, seal and pack, specimen uniformity and stabilization of testing.
2.2 Participating Laboratories
‘Barrier Property Testing of Plastic Package Materials- Determination of Oxygen Water and Vapor Transmission Rate’ belongs to A category project of 2007 proficiency test. Therefore, all eligible laboratories must participate in the proficiency test. At the same time, local technical supervision system and laboratories of other relevant industries are encouraged for participation. To improve mutual technical understanding, laboratories of overseas testing institutions stationed in the mainland are also invited. There are altogether 69 laboratories participating oxygen transmission rate testing and 67 laboratories participating water vapor transmission rate testing.
2.3 Specimen Preparation
To investigate the difference between and within participating laboratories, that is, investigating the systematic error and random error of the participating laboratories respectively. And to secure the reliability of proficiency test data, ‘split level’ sample design is adopted, That is to say, samples are divided into A category and B category. There is subtle difference between the two categories of specimen and testing results. However, samples of the two categories are of the same material.
Specimens for this proficiency test are PE films. During the selection of specimen, uniformity and stability of specimens are tested using single factor method analysis of variance as well as the test method stipulated in CNAS-GL03 Evaluation Guide for Specimen Uniformity and Stability in this Proficiency Test. 108 uniformity test data and 44 stability test data received general consent from experts, indicating that the selected materials meeting requirements specified and can be used for the proficiency test. Specimen processing condition of stability evaluation is 38℃/50％RH and 48℃/90％RH, which can better simulate the representative summer temperature and humidity of in most areas of our country. Test data proves that specimens can maintain stable barrier property during transportation.
2.4 Test Methods
As to test methods of the proficiency test, GB/T1037-1988 Standard Test Method for Water Vapor Transmission Rate through Plastic Film and Sheeting Using Cup Method and GB/T1038-2000 Stand Test Method for Gas Transmission Rate Through Plastic Film and Sheeting Using Differential-pressure Method are recommended by National Packaging Product Quality Supervision and Testing Center (Jinan). However, since each project has more than one test methods and other barrier property test methods are actually used sometimes in our country (although some methods have not obtained international support), gravimetric method or differential pressure method is not regulated as the only method for this proficiency test. Therefore, laboratories can use other test methods. But they must note the method used in their test reports.
According to the summary of adopted test methods by laboratories for proficiency test, 10 laboratories did not use gravimetric method in water vapor permeability testing (6 used infrared sensor method and 4r used electrolytic sensor method). 16 laboratories did not use differential pressure method in oxygen transmission rate testing (13 used equal pressure method-oxygen sensor method)
2.5 Statistical Results of Proficiency Test
Data statistics are carried out in accordance with CNAS-GL02 Evaluation Guide for Specimen Uniformity and Stability in Proficiency Testing using Robust method to evaluate testing ability of participators. Proficiency evaluation of labs mainly includes inter-lab ZB value and internal lab ZW value. The evaluation results also include seven comprehensive statistics: result data, median, IQR, CV, maximum value, minimum value and Range. In the proficiency test, there is no laboratory outlier using test instruments based on gravimetric method or differential pressure method, that is, system error of instruments is within allowable range of our country.
3. Conclusion
Data analysis of the whole project shows that proficiency-test project-‘Barrier Property Testing of Plastic Package Materials – Determination of Oxygen and Water Vapor Transmission Rate’ undertaken by National Packaging Product Quality Supervision and Testing Center achieves complete success. The important role Labthink plays in this project is highly appraised by relevant governmental bodies. This proficiency test completes not only the actual status assessment of domestic barrier property testing level, but also a large-scale system and objective data comparison between various barrier property test methods for the first time in the world. It points out the way and provides sound data evidence for the unification of barrier property data system.