Packaging material barrier test

1, breathability test

Breathability is one of the most important physical properties of polymers. In particular, high polymer products such as plastic sheets, films, and coatings have special requirements for air permeability. Ventilation performance and aging resistance are closely related, but also with the structure of the polymer, so the determination of permeability has important theoretical and practical value.

There are many methods for measuring the permeability of polymers. There are many pressure and volumetric methods used, and the most widely used method is the pressure method. Because the pressure method is highly accurate and repeatable, it is easy to record automatically and is easy to implement.

Divided from the test principle, the permeability test of packaging materials includes two methods: differential pressure method and composition analysis method through a coulometric sensor.

The measurement principle of the differential pressure method is to divide the test film into two separate spaces, one of which (high pressure chamber) is filled with the measuring gas, and the other (low pressure chamber) is evacuated, so that both sides of the sample are There is a certain pressure difference. The gas in the high pressure chamber will permeate through the membrane to the low pressure chamber. By measuring the pressure or volume change in the low pressure chamber, the gas permeability can be obtained. The differential pressure method has the advantages of simplicity, convenience, ability to measure a variety of gases, and low equipment and equipment costs. China's only national standard for gas transmission rate GB/T1038-2000 adopts the differential pressure method. The current gas transmission rate test instruments used by enterprises and institutions in China are basically instruments for differential pressure.

The principle of the coulometric oxygen permeability tester is to divide the test membrane into two independent gas flow systems. One side is a flowing gas (which can be pure oxygen or a mixed gas containing oxygen, and the relative humidity can be set). On the other side is a flowing oxygen with stable relative humidity. The total pressure on both sides of the sample is equal, but the partial pressure of oxygen is different. Under the effect of oxygen concentration difference, oxygen passes through the membrane. Oxygen through the membrane is sent to the coulometric sensor under nitrogen flow. The coulometric sensor can measure the amount of oxygen contained in the gas stream and calculate the oxygen transmission rate of the material.

The coulometric oxygen permeability tester can control different test conditions such as humidity, temperature and gas with different oxygen content, and can more effectively simulate the conditions of action in packaging. The pressure on both sides of the sample during the test is the same. It is beneficial to reduce the leakage during the test and damage to the sample, and its detection service life is not long. For materials with high oxygen permeability, the test probe has little effect on the life of the test probe, and the test cost is high.

At present, China's oxygen permeability testing instruments use mostly differential pressure products. The domestic oxygen transmission rate is only GB 1038 "Plastic Film and Diaphragm Gas Permeability Test Method Differential Pressure Method", which is also a reason why testers choose to purchase a differential pressure oxygen permeability tester. It is necessary to establish a national standard for measuring the oxygen transmission rate using a coulometric method similar to ASTM D3985.

2, moisture permeability test
Divided from the detection principle, the moisture permeability test methods mainly include weighing method and infrared detection method.

Weighing method is divided into weight gain method and weight loss method. The principle of weight gain method is to put a certain amount of desiccant (usually anhydrous calcium chloride) into a moisture-permeable cup, place the detected film on the moisture-permeable cup, and seal it with wax to form a moisture-permeable cup. In a closed space, the moisture-permeable cup is placed in a constant-temperature and wet environment. The water vapor passes through the test material and is absorbed by the desiccant. The weight of the moisture-permeable cup is weighed in an appropriate time to calculate the water vapor transmission rate. . The weight loss test method is similar to the weight gain method, except that the moisture permeable cup contains distilled water or salt solution. The sample is placed on a moisture permeable cup and sealed with wax to form a closed space in the moisture permeable cup. The moisture-permeable cup is placed in a constant-temperature and wet environment, the water vapor in the moisture-permeable cup is absorbed by the dry substance in the constant-temperature and humidity box after passing through the test material, and the weight of the moisture-permeable cup is weighed in an appropriate time to calculate the water. Vapor permeability. As a development variant of the moisture permeable cup, the container can be a bag, a bottle, or other type. The weighing method has the advantages of simplicity, convenience, and low cost of equipment and equipment. China's GB/T1037-1998 "Cup test method for water vapor permeability test of plastic films and sheets", GB/T6985-1997 "Testing methods for packaging materials moisture permeability", GB/T6981-1986 "Hardness test for hard packaging containers" "Method", GB/T6982-1986 "Hardness testing method for hard packaging containers" are used weighing method.

The principle of the infrared test method is to divide the test film into two independent air flow systems, one side is a nitrogen flow with a stable relative humidity, and with the dry nitrogen flow to the infrared test sensor, measure the water vapor transmission rate in nitrogen gas. . Infrared detection method is fully automatic in the entire experimental process, without destroying the balance of diffusion and infiltration. The result is accurate and reliable. At the same time, due to the high sensitivity of the infrared detection method for detecting the sensor, it is possible to measure high barrier materials in a short time.

At present, China's national standard is only the weighing method standard, namely GB/T1037-1998. For the water vapor transmission rate is relatively small, and the material can be heat sealed, can be used bag weighing method, namely GB/T 16928-1997 B method. When the amount of water vapor transmission is small and non-heat-sealable materials or structures contain materials with high hygroscopicity (such as paper, cellophane, nylon, etc.), infrared detection methods are generally appropriate. At present, there is no relevant standard for measuring the moisture permeability of packaging materials by infrared testing.

3, matters needing attention

The barrier properties of packaging materials, whether water vapour transmission rate or oxygen transmission rate, should be noted in the following aspects in the application process of detection and detection results;

1 The concept of permeability is obtained when the film meets Fick's law. For oxygen, Fick's law is generally followed except for individual oxygen absorbing materials. However, due to the interaction of water vapor and organic matter with many polymers, it generally belongs to non-Fick law diffusion.

2 For composite materials, the structure is not necessarily symmetrical, so there are problems with the front and back of the sample. Some materials, such as PVCD coated BOPP, or PVDC and PVC composite hard plates, have very different oxygen transmission rates on the front and back, sometimes up to 1 times. This is because during the actual test, the measured result is the sum of the penetration through the sample and the penetration of the seal.

3 For absorbent and hygroscopic packaging materials, the effects of adsorption and desorption on the experimental results should be considered during the test, and it should be clear that the equilibrium time is generally longer, and even in the same environment, through different processes. The equilibrium may not be the same, which means that the equilibrium state of the material is not only related to the balanced environment, but also related to the process.

4 The leakage problem during the testing process should be highly valued. The water vapour transmission rate and oxygen transmission rate obtained from any experiment are the sum of penetration and leakage. Only when the leakage is negligible, the measured penetration rate is high. Is accurate. The details of the operation and some auxiliary materials (such as sealing wax, vacuum grease, etc.) have a significant impact on the leakage during the test. Packaging materials and packaging are two different concepts. Using high-barrier packaging materials does not necessarily produce high-barrier packaging. From the packaging material to the package, from the package to the consumer, many factors affect the final barrier properties of the product during this process.