The principle of the weighing method is to first put a certain amount of desiccant (usually anhydrous calcium chloride) into a moisture-permeable cup, put the detected film in 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. After the water vapor passes through the test material, it is absorbed by the desiccant, and the weight of the permeated cup is pumped at an appropriate time to calculate the water vapor transmission rate. As a development variant of the moisture-permeable cup, the container can be a bag, a bottle or some other container. The weighing method has the advantages of simplicity, convenience, and low cost of equipment and equipment. China's GB/T1037-1998 "Plastic Film and Sheet Water Vapor Test Method Cup Test", GB/T6982-1997 "Packaging Materials Test Method Moisture Permeability Rate". GB/T 6981-1986 “Hardness test methods for hard packaging containers†and GB/T 6982-1986 “Test methods for moisture permeability of flexible packaging containers†all use the weighing method. However, we can see from its experimental design and practice that the weighing method has the following obvious disadvantages:
1 The weighing method cannot be tested in a stable state. Originally the water vapor transmission was measured in a penetrating equilibrium state. It takes a certain time for the diffusion and infiltration to go from a non-equilibrium state to an equilibrium state. This is what we call the equilibrium time. In some methods (such as GB/T 1037-1988), the samples are weighed after being equilibrated for 30 minutes under absolute dry conditions at 23°C. This will inevitably destroy the diffusion and osmotic equilibrium under the original test conditions, which will affect the experimental results. accuracy. Although GB/T has noticed this problem, it states that "Weighing should preferably be carried out in a test environment, otherwise the weighing time cannot exceed 30 seconds." However, in practice, very few high-precision balances are placed on the balance. The use of 38°C, 90% relative humidity, weighing time is less than 30 seconds, it is very difficult to do.
2 poor repeatability. There are many links in the testing process (especially the cup method). The sample preparation habits and weighing habits of the operators have a great influence on the experimental results. Therefore, the repeatability of the experiments is poor. According to the US ASTM report, the water vapor transmission rate of materials measured by different laboratories using the same material, the error can be as high as 20%, and the error of infrared detection method is only 3%.
3 poor reliability. The cup method in the weighing method is sealed with a sealing wax. The composition and quality of the sealing wax have a great influence on the experimental results. On the one hand, if the sealing wax is of poor quality, it will easily produce micro leaks during sealing, which will cause errors. On the one hand, sealing waxes stored for a long time at 38°C cause weight changes. Although GB/T 1037-1987 also takes into account this problem, it stipulates that the sealing wax should be at 38°C and 90% relative humidity. Exposure does not soften the deformation. If the exposed area is 50cm2, the mass change within 24h cannot exceed 1mg." From this sentence we can clearly see that the test accuracy of this method cannot be higher than 0.2g/m2.24h.
4 long measurement time. Due to the poor accuracy and precision of the weighing method, the weight method generally requires a long detection time, and the detection time is 20 times that of the infrared detection method. Generally, 3 g/m2.24h of the packaging material requires about 10 days of detection time. .
5 Low accuracy and narrow application range. Because of the large measurement error in the weighing method (especially the cup method), some standards (such as ISO2528-1995, GB/T16928-1997) clearly stipulate that the moisture permeability is less than 1g/m2.24h of the packaging material. The test is not applicable to the dry cup 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 the other is an absolutely dry oxygen flow. The water vapor passes through the film from the side of the moist nitrogen flow. A dry nitrogen flow is reached, and as the dry nitrogen flow flows to the infrared detector, the content of water vapor in the nitrogen is measured, and the water vapor transmission rate is obtained. Infrared detection method is fully automatic in the whole experimental process, without destroying the balance of diffusion and infiltration, so the result is accurate and reliable. At the same time, due to the high sensitivity of the detection sensor by the infrared detection method, it can measure high barrier material in a short time. . The testing accuracy of the infrared test method can reach 0.005m2.24h for the material and 0.000052g/24h for the package. The accuracy of the infrared test method is 100 times that of the weighing method.
China's existing water vapor transmission rate detection instruments include the weighing method and the infrared detection method. Currently, the national standard only has the weighing method. For cups with a higher water vapor transmission rate, cup weighting can be used. That is, GB/T 1037-1988, for water vapor transmission rate is small, and double heat-sealable materials, can be used bag weighing method, namely GB/T method B; for water vapor transmission is small, In the case of non-heat-sealable materials or structures containing materials with high hygroscopicity (such as paper, cellophane, nylon, etc.), it is generally advisable to use an infrared detection method. The infrared test method has good repeatability, high precision, and wide adaptability to materials. When there is a high requirement for the moisture permeability of the packaging material, or when precise measurement of the moisture permeability of the material is required, infrared detection method is recommended for measurement. Unfortunately, there is currently no national standard for measuring moisture permeability of packaging materials by infrared testing.
3. Precautions The permeability of packaging materials, whether water vapour transmission rate or oxygen transmission rate, should pay attention to the following issues in the application process of detection and detection results:
1 The concept of permeability is that the film is based on Fic Rian's law. For oxygen, in addition to individual oxygen absorbing materials, it is generally consistent with Hooke's law. However, due to the interaction of water vapor and organic matter with many polymers, the non-Hooke's law generally spreads.
2 For composite materials, the structure is not necessarily symmetrical, so there are problems with the positive and negative sides of the sample. Some materials, such as PVDC coated BOPP or PVDC and PVC composite hard plates, have significant differences in the oxygen transmission rate measurement on the front and back sides, and sometimes even one time difference. 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. PVDC and PVC composite hard films used in pharmaceutical blister packaging, some manufacturers use PVDC surface and PTP aluminum foil heat seal, some manufacturers use PVC surface and PTP aluminum foil heat seal, some manufacturers use PVC surface and PTP aluminum foil heat seal, From the principle, it should be a high-resistance cadmium layer PVDC and PTP aluminum foil heat sealing is reasonable, because it can most effectively seal the drug.
3 For the absorbent hygroscopicity packaging materials, the influence of adsorption and desorption on the experimental results should be considered in the test process, and it should be clear that the balance time is generally longer, and even in the same environment, through different processes The state of equilibrium is not necessarily the same, which means that the equilibrium state of the material is not only related to the balanced environment but also to the process.
4 Leakage problems in the testing process should be highly regarded. The water vapor 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.) all have a significant impact on the leakage during the test.
5 Packaging materials and packaging are two different concepts. With high-resistance cadmium-based packaging materials, it is not always possible to produce high-barrier packaging. From the packaging material to the package, from the package to the consumer, many factors in this process will affect the final barrier properties of the product.
Third, the leakage of the packaging There are generally two types of leakage in the packaging: one is the unavoidable leakage (such as the leakage of the bottle and the cap) that is caused by the design and the structure itself. The leakage is to change the design of the packaging. Structure can solve the problem (such as the bottle and bottle cover leakage can be solved by adding a bottle of heat sealing gasket to solve the leak); the other is due to the defects caused by the packaging in the manufacturing, transportation and other defects caused by the leakage. Without changing the design and structure, adjustments can be resolved. Determining whether there is a leak in a package and determining the location of the leak is very important for the packaging project. The current domestic and international commonly used packaging leakage testing methods are shown in Table 3.
Table 3 Test Method for Packing Airtightness