The coulometric oxygen permeability tester can control different test conditions such as different humidity, temperature and gas with different oxygen content. It can simulate the actual conditions of use of the packaging more effectively. During the test, the two test pressures of the sample are the same. Reducing leakage and damage to the test specimen during the test. Since the coulometric oxygen permeability tester can accurately determine the oxygen component in the gas, the test result is more accurate and reliable. The Coulometric Oxygen Transmittance Tester has a high detection accuracy because of its reasonable design and high sensitivity of the detection probe. The minimum detection amount can generally reach the film: 0.01ml/m2.24h.1atm, and the package is: 0.001 Ml/24h.latm. For high-precision detectors, the minimum detectable film thickness is: 0.001 ml/m2.24 h.1 atm, and the package is 0.00005 ml/24 h.1 atm, which is 500 times more accurate than the differential pressure method. Of course, this instrument also has disadvantages, that is, the price is expensive, the manufacturer is not many, and its detection service life is not long. For materials with high oxygen permeability, the life of the test probe during the test process has little effect, and the test cost is high.
For high-resistance cadmium-containing packaging materials containing aluminum foil, the oxygen transmission rate is poorly measured by the differential pressure method, because the oxygen transmission rate is low, and is generally 0.5 ml/m2.24 h.1 atm or less. It is understood that high-resistance cadmium-containing packaging materials containing aluminum foil are often less than 0.2 ml/m2.24 h, 1 atm, measured using a coulometric oxygen permeability tester from Morgan. According to the theoretical calculation of the pinhole of the aluminum foil, the oxygen transmission rate of the generally good aluminum foil composite packaging material is less than 0.1 ml/m2.24 h.1 atm, and the measurement accuracy thereof is 0.3 ml/m2.24 h.1 atm. Obviously, it cannot meet the requirement of testing high barrier packaging materials containing aluminum foil. A drug packaging material inspection station in a sampling inspection, oxygen permeability using Japan's Toyo Seiki's oxygen permeability tester (differential pressure method), the results of all aluminum foil composites failed (0.5ml/m2.24h. 1atm), but sent to the Shanghai Pharmaceutical Supervision and Inspection Center of packaging materials using the American Mocon's oxygen transmission rate tester (electrical analysis type) testing all qualified. For packaging materials such as PA and EVOH, which have a great influence on the oxygen permeability and the ambient humidity, the differential pressure method can only be tested under the condition of relative humidity RH=0%, because the coulometric oxygen pass rate tester should generally be used. Testing.
At present, oxygen permeability testing instruments used in China are basically differential pressure methods, and most of them are Japanese Toyo Seiki. The domestic oxygen transmission rate is only GB1038 "Plastic film and sheet gas permeability test method differential pressure method" this standard, the test unit also because of the reasons for the standard, and buy the differential pressure oxygen permeability tester. At present, the development of a national standard for measuring oxygen permeability using a coulometric method similar to ASTM D3985 is very necessary to determine the oxygen permeability of high-barrier and wet-inspired composite packaging materials.
2. Detection of water vapor transmission rate
At present, the commonly used water vapour transmission rate test methods at home and abroad are shown in Table 2. From the detection principle, there are mainly two types of weighing methods and infrared detection methods.
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 single inside of the 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 moisture-permeable cup is weighed in 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 â€œHardness test methods for hard-packed containersâ€ and GB/T 6982-1986 â€œTest methods for moisture permeability of flexible packaging containersâ€ 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 23Â°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 16928-1997 has already taken note of this problem, it states that â€œWeighing should preferably be carried out in a test environment, otherwise the weighing time cannot exceed 30 secondsâ€. However, in practice, very few high-precision balances are placed on the balance. The use of 38Â°C, 90% relative humidity, weighing time is less than 30 seconds, it is very difficult to do.
2 poor repeatability. The weighing method (especially the cup method) has a lot of links in the testing process. The operator's sample preparation habits and weighing habits have a great influence on the experimental results, and 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, resulting in errors. On the one hand, sealing waxes stored for a long time at 38Â°C cause weight changes. Although GB/T 1037-1987 also takes into account this problem, it stipulates that the sealing wax should be at 38Â°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, the 3g/m2.24h 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 materials. 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 side is an absolutely dry oxygen flow. The water vapor passes through the side of the moist nitrogen flow. The film reaches a stream of dry nitrogen and flows with a stream of dry nitrogen to an infrared detector. The amount 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 detection accuracy of the infrared test method test instrument can generally reach 0.005g/m2.24h 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 large water vapor transmission rate, cup weighting can be used. That is, GB/T 1037-1988, for the water vapor transmission rate is relatively small, and the double heat sealable material, can be used bag weighing method, namely GB/T 16928-1997 method B; for water vapor transmission capacity When small, non-heat-sealable materials or structures containing materials with high hygroscopicity (such as paper, cellophane, nylon, etc.) should generally be inspected by infrared. 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.
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