The laboratories proficiency testing project “Barrier Property Test of Plastic Packaging Material——Determination of Oxygen and Water Vapor Transmission Rate” has accomplished. Through proficiency testing we have investigated the barrier property testing capabilities and gained tremendous achievements. However, a number of widespread problems are exposed. Some laboratories did not realize them fully, and testing data from them were considered to be “outlier” and “suspiciousness”. In this paper we present a summary of these problems.
1． Experimental conditions of barrier property testing
Some experimenters, having got used to the routine tests, consider that a 23°C temperature is suitable to barrier property testing. In fact, we must follow the standards of testing method and adopt experimental conditions when testing the oxygen transmission rate and water vapor transmission rate. But sometimes there is more than one standard experimental condition in a standard. Taking water vapor transmission rate testing for example, there are 5 standard experimental conditions in ISO 2528: 25±1°C, 90±2%RH; 38±1°C, 90±2%RH; 25±1°C, 75±2%RH; 23±1°C, 85±2%RH; 20±1°C, 85±2%RH. 5 experimental conditions in the series of ISO 15106: 25±0.5°C, 90±2%RH; 38±0.5°C, 90±2%RH; 40±0.5°C, 90±2%RH; 23±0.5°C, 85±2%RH; 25±0.5°C, 75±2%RH. Standard experimental conditions of GB/T 1037 are: 38±0.6°C, 90±2%RH; 23±0.6°C, 90±2%RH. Other standards such as ASTM E96 are not consistent with conditions above. After comparison, only experimental conditions of 38°C, 90%RH can satisfy the demands of water vapor transmission rate testing, excluding the differences in errors of temperature and humidity. It is the same condition to oxygen transmission rate testing. So, after comprehensive consideration of requirements of various testing standards, we adopt 23°C, 0%RH in oxygen transmission rate testing and 38°C, 90%RH in water vapor transmission rate testing; and they are not consistent with the experimental condition of 23°C, 50%RH in GB/T 2918.
The changes in temperature and humidity affect the material performances, and greatly influence barrier property testing data at the same time. Therefore, carrying out proficiency testing project according to national standard experimental conditions is premise. The oxygen transmission rate testing is under condition of 23°C, 0％RH and 38°C, 90% for water vapor transmission rate testing. Some laboratories did not follow the demands of experimental conditions, and the results from them are suspicious. This would be solved as soon as those laboratories have changed their experimental conditions under direction.
2． Error analysis of instrument in non-automatic gravimetric method
67 laboratories took part in the proficiency testing project, of which, 66 laboratories offered effective data; only 10 laboratories did not use gravimetric method instrument, 6 of them used infrared sensor instrument, and 4 of them used electrolytic detection sensor instrument. 2 laboratories employ gravimetric method are both considered to be outliers. Though judging from the statistical results of the first overall testing, we saw that most outlier and suspicious laboratories are using gravimetric method instruments; and through further investigation and classification, we have learned that there is a big difference between laboratories using instruments of non-automatic gravimetric method and automatic gravimetric method.
None of the laboratories using non-automatic gravimetric method instrument is interlaboratory outlier, but covers 50% of the suspicious laboratories. And laboratories employing non-automatic gravimetric method instrument make up 87.5% of within-laboratory outliers as well as 62.5% of within-laboratory suspicion. However, the laboratory using automatic gravimetric method instruments, none is outlier; only 3 laboratories are within-laboratory suspicion including the effects of experimental conditions. Because no laboratory using instrument based on gravimetric method is outlier, the systematic error of gravimetric method instrument is within the range of national recognition. But the testing precision of instruments with non-automatic gravimetric method are greatly influenced by testing errors, laboratories using those instruments cover 87.5% of within-laboratory outliers.
Gravimetric method is the basic method of water vapor transmission rate testing; its testing data is the foundation of other test methods. Testing error of desiccant method is inevitable, so the development of water method and using automatic gravimetric method instruments are the only solution at present. The testing error of instrument with non-automatic gravimetric method were not so obvious in the past because the lack of effective testing methods. However with the popularization of other automatic testing instruments, the shortage of non-automatic gravimetric method instrument has become more obvious. Data from laboratories with non-automatic gravimetric method instrument are not satisfying, 52.2% of which are considered to be suspicious and even outlying, popular attention are paid to the conditions of instrument usage due to the surprising data. And this also helps institutes and labs select barrier property testing instrument.
3． Stability estimation and experimental condition of oxygen transmission rate testing
The oxygen transmission rate testing method includes differential-pressure method and equal-pressure method; of which, the differential-pressure method is more widely used. In this proficiency testing, 69 laboratories participate in oxygen transmission rate testing; only 13 of them using equal pressure method instruments. Many factors, depending on testing method, would affect the test data of oxygen transmission rate testing.
The problem that has gained special attentions is the vacuum limitation of system and the time spend in vacuum pumping. Experiment demonstrates that short time of vacuum pumping could affect experimental results. On one hand, it is because the gas from vacuum pipes is mixed into the permeation gas, which leads to slightly large and instable of testing data. On the other hand, gas and impurity would infiltrate from the surface of material through the specimen; time spent on vacuum pumping would affect the degree of gas and impurity infiltration. The more thoroughly the vacuum is, the better the effect of exclusion and more stable the testing data. The volume of testing cavity is relevant to the time spent on vacuum pumping. Less time is spent on smaller volume, but it is hard for vacuum method instrument to keep pressures if time spent on vacuum pumping is too short. The degree of vacuum and time spent on vacuum pumping is definitely prescribed in gas transmission rate testing, such as GB/T 1038 standard requiring 3 hours of non interrupted pumping after having arrived at the specified degree of vacuum (27Pa). Laboratories is suspicious and outlying caused by insufficient time on vacuum pumping would get satisfying testing data by prolonging time spent on vacuum pumping.
Special attentions need to be paid on purging and permeation equilibrium of testing system when equal-pressure method is used. System purging is to put carrier gas into oxygen sensor to reach the very low state of oxygen content. Purging affects the precision of testing, and is a key step in equal-pressure method. The time spend on purging which commonly considered being several hours, is specifically prescribed. Different from differential-pressure method, the estimate of permeation equilibrium in equal-pressure method depends on the observation of experimenter at present. According to experiences, the longer the time spent on testing after entering testing status, the more stable the testing data is. Experimental results received at the beginning of experiment are often different from the real testing data after arriving at permeation equilibrium. If it only depends on the observation of operators to determine the arrival of permeation equilibrium, the accuracy of testing data would be affected seriously and led to the status that data obtain from permeation equilibrium differ from person to person. So we suggest prolong the time spent on testing especially for high barrier specimen.
4． Summary
There are many obvious problems in this proficiency testing as illustrated above. Since barrier property testing is a microscopic testing, we must pay attention to many other problems in test operation. For example, specimen clamping, the time spent on purging in equal-pressure method, cup operation, specimen preparation and the effective protection of sensors etc. It is optimistic that many problems have been solved in the proficiency testing; other problems have also aroused attentions from operators. It is a good beginning of regulating operation procedure, reducing artificial errors and unifying testing data.