Reasonable selection of the measuring range of barrier equipment

The barrier properties of packaging materials are related to the shelf life of their contents, so the testing of barrier parameters of packaging materials has received widespread attention. However, some users do not fully understand the range of barrier testing equipment, so when selecting equipment, they pursue high indicators in terms of test range and accuracy, which ultimately leads to poor practicality of the equipment, insufficient use of the equipment, failure to give full play to the advantages of testing, and waste of equipment investment. This article will introduce how to reasonably select the range of barrier equipment from the aspects of the relationship between the range and accuracy of the testing equipment, the relationship between improving the detection accuracy and broadening the lower limit of the test and the equipment cost, and the barrier range of commonly used packaging materials.

1. Analysis of the measurement range and accuracy of barrier equipment

The test range and accuracy of barrier equipment are closely related, and the main determining factors are the range and accuracy of the key indicator test sensor. If the sample test exceeds the range of the sensor, it will not only have an adverse effect on the sensor, but also the test data will have no accuracy even if it is issued. For differential pressure equipment, excessive air permeability will cause the sensor to exceed the range during the permeation balance judgment process, resulting in test failure. For isobaric oxygen permeation equipment or electrolytic moisture permeation equipment, over-range not only makes the equipment unable to issue correct test results, but also is not conducive to the service life of the sensor. The range of the sensor and its accuracy affect each other. For any sensor, high test accuracy and wide measurement range are very important.

The test range is difficult to obtain simultaneously.

The lower test limit of the equipment represents the highest detection capability of the equipment. However, to obtain the ideal lower test limit, it is necessary to equip a sensor with extremely high precision. Correspondingly, the sensor range is relatively small. Therefore, there is no testing equipment that can achieve an excellent lower test limit and have a very high upper test limit at the same time, that is, the range of the equipment is regular. Although the range of the barrier test equipment can be widened by other methods, such as reducing the sample area, changing the test chamber volume or carrier gas flow rate, while increasing the upper test limit of the range by these methods, the lower test limit will also be increased in the same proportion. Assuming that the original permeability test range is 0.1 cm3/m2·24h·0.1MPa~5000cm3/m2·24h·0.1MPa, the test range after processing may become 1 cm3/m2·24h·0.1MPa~50000cm3/m2·24h·0.1MPa. At present, there is no method to widen the upper and lower test limits at the same time.

2. Expansion of test limits and equipment costs

As mentioned before, the test range and accuracy are mainly determined by the sensor and processing technology, and the ideal upper and lower test limits cannot be obtained at the same time. The lower test limit represents the highest detection capability of the equipment and therefore attracts much attention. However, the lower the lower test limit of the equipment, the higher the sensor test accuracy must be while ensuring that there are no leaks in the test system. Moreover, the lower the lower test limit to be achieved, the higher the sensor accuracy that needs to be selected. However, in the field of trace gas detection and vacuum technology related to barrier testing, the improvement in sensor accuracy is not proportional to the increase in cost. Within the conventional measurement range, the price may not differ much, but as the accuracy approaches the limit of current detection technology, the cost increase will be very high.

At present, the cost of differential pressure equipment with a test lower limit of 0.01cm3/m2·24h·0.1MPa and the cost of isobaric equipment with a test lower limit of 0.005ml/m2·day are also very high, so the selection of the range of the testing equipment should be carefully considered, and it is particularly important to purchase according to needs. Excessive pursuit of high indicators of the test lower limit will not only greatly increase the purchase cost of the equipment, but also improper range selection will increase the probability of equipment being idle.

3. Analyze the selection of equipment range based on the barrier property range of actual packaging materials

Flexible packaging materials can quickly replace metal packaging and glass packaging with excellent barrier properties in many fields mainly because they have reasonable packaging costs, appropriate barrier properties, good packaging effects, and varied packaging forms. Among them, low cost is the basis for the widespread application of flexible packaging materials.

At present, there is a great demand for high gas barrier materials, because the gas barrier properties of commonly used packaging materials (such as PP, PE) are poor, but the moisture barrier properties are good. The gas permeability range of flexible packaging materials is very wide, and materials with gas permeability ranging from 1cm3/m2·24h·0.1MPa to 10000cm3/m2·24h·0.1MPa are often used. It is generally believed that polymer materials with oxygen permeability less than or equal to 5cm3/m2·24h·0.1MPa are called high barrier materials. According to this definition, only ethylene-vinyl alcohol copolymer (EVOH) and polyvinylidene chloride (PVDC) can be called high barrier materials among polymer materials. However, even for these two materials, it is necessary to pay attention to the use of materials to achieve the expected gas barrier properties. For example, EVOH is very susceptible to moisture, but the gas barrier properties of this material will be greatly reduced after being damp, so it must be formed into a multilayer composite material with other materials for practical application. At present, only aluminum foil materials, vacuum aluminum-plated materials and some materials with special coatings can achieve air permeability below 1cm3/m2·24h·0.1MPa. Unfortunately, the cost of these materials has increased significantly, and there is almost no cost advantage compared to other packaging forms. The production of high-barrier materials is mainly concentrated in some material manufacturers in my country, so these manufacturers are also the ones who pay attention to the lower limit of the test of air permeability testing equipment. As for manufacturers of medium-barrier and low-barrier flexible packaging materials, they should pay attention to the upper and lower limits of the test of air permeability testing equipment, and should choose the range of the equipment according to their actual testing needs. According to this analysis, what the market needs most at present is air permeability testing equipment with a certain detection accuracy but a wider detection range.

4. Conclusion

In summary, it is impossible to obtain both wide range and high precision at the same time for any device. Therefore, for barrier devices, the selection of test range and accuracy should be based on the test needs of the device itself, and excessive comparison of test accuracy and lower limit should be avoided, resulting in the device range being unable to meet the test needs.