Permeability test of flexible packaging materials

As we all know, in food, medicine, precision electronic components and other industries, the gas barrier performance of product packaging materials is a very important indicator, which can directly affect the storage life, storage environment, sales environment and so on. The influence of gas on food and drugs was first widely concerned by the flexible packaging industry. The penetration of oxygen may lead to food deterioration, drug failure, etc. now for food and drug packaging, industry standards have given preliminary instructions on the permeability and testing of packaging film. This paper introduces the permeability mechanism of the film, the influence of gas transmission on the contents, and the widely used gas barrier test method. This article only introduces the test of common gases, not organic gases

1. gas permeation process

in a broad sense, the permeability of packaging materials is through the material or the material transfer between the material and its contents. From the thermodynamic point of view, the permeation of small molecular substances into packaging materials is a single molecule diffusion process, and the speed of the whole process is determined by two factors: solubility parameter and diffusion coefficient. Without discussing the leakage of packaging, the barrier of the film to gas is the penetration of gas into the film: gas molecules enter the material surface from the high concentration area, diffuse to the material, and desorb from the other surface of the low concentration area

1.1 diffusion phenomenon

diffusion phenomenon is caused by mass transfer and mass flow caused by uneven distribution of component density. From the microscopic point of view, the diffusion phenomenon in gas is directly related to the thermal motion of gas molecules

1.2 the influence of penetrating gas on the contents

for food, medicine, precision electronic components and other industries, the barrier performance of product packaging materials is a very important indicator. For example, for fruit preservation, the oxygen resistance and moisture resistance of the packaging film (that is, the barrier of the film to oxygen and water vapor) are the determinants of the fruit preservation period. Another example is Shangshi Zhitong ointment, which is a drug that especially needs oxygen barrier packaging. If PVDC film packaging with good gas barrier and humidity barrier is selected, the smell will not be lost and the curative effect will not be affected

oxygen in the air has a certain destructive effect on the nutrients in food: oxygen oxidizes the oil in food, even at low temperature; Peroxide produced by oil oxidation can make food lose its edible value, and produce strange odor and toxic substances; Oxygen can make vitamins and various amino acids in food lose their nutritional value; Oxygen can also aggravate the reaction of oxidative browning of food; For food microorganisms, most bacteria multiply and grow due to the existence of oxygen, resulting in the corruption and deterioration of food. For the preservation of fruits and vegetables, fresh fruits and vegetables are still breathing to maintain their normal metabolism during storage, transportation and circulation, absorb a certain amount of oxygen and release a certain amount of CO2 and water, and consume some nutrients. The low oxygen permeability of the film can reduce the released water and consumed nutrients, prolong the preservation of fruits, and encourage large shipping enterprises to enter Hainan fresh period. In the packaging of carbonated soft drinks, pet plastic containers account for 40% of the whole carbonated soft drink packaging, replacing some glass bottles and metal cans, because pet has good transparency and strength, as well as the most important and longest impermeability to CO2

for the secondary packaging of drugs, packaging materials can play an important role in the quality, validity, packaging form, sales, cost, etc. of drugs. General drug production, whether tablets, granules, injections, liquids, powders, or ointments, is in a situation of small quantity and many varieties, and there are relatively complex ways from the delivery of drugs to the arrival of patients. Drug packaging materials should have stability, barrier performance, structural performance and good processability. Oxygen has the greatest impact on drugs in the air. Oxygen can oxidize and deteriorate many drugs, such as vitamins C, a, D, adrenaline, phenol and so on, so that the content of their main components decreases, and the curative effect is weakened or ineffective

although many drugs have protective films, the gas resistance of drug packaging is still crucial. At present, the world's six general plastics are still used for packaging: PE, PP, PS, PVC, PA and PC. PE, PVC, PP and polyester are mostly used in domestic drug packaging. In addition to meeting the general performance of packaging materials, pharmaceutical packaging materials should also meet special requirements such as safety and applicability. For example, pharmaceutical blister packaging materials include pharmaceutical aluminum foil, plastic hard sheet, heat sealing coating and other materials. Pharmaceutical aluminum foil takes hard industrial pure aluminum as the base material, and has the advantages of non-toxic, corrosion-resistant, impermeable, heat resistant, moisture-proof, light resistant, high-temperature sterilization and so on. Because drugs are very sensitive to moisture, humidity and light transmission, it is required that the blister materials used have high barrier to water, light, steam, etc. PVC, PVDC, or composite materials PVC/PVDC, PVC/PE, PV have force displacement, force time, displacement time curves C/PVDC/PE, PVDC/OPP/PE, etc

2. Permeability test methods

from the perspective of test principle, there are two kinds of permeability test methods: differential pressure method and sensor method through electrical analysis. At present, differential pressure method is widely used to test the permeability of thin films

2.1 measuring principle of differential pressure method

use the film to be tested to measure the

test chamber to separate into two independent spaces, fill one side (high-pressure chamber) with test gas, and form a certain differential pressure on both sides of the sample. The gas in the high-pressure chamber penetrates into the low-pressure chamber through the film, and the gas permeability can be obtained by measuring the pressure or volume change of the low-pressure chamber. GB/t1038-2000 adopts the differential pressure method. There are ASTM dl434-1998, iso15105-1, iso2556:2001 and other international standards for testing the permeability of thin films by differential pressure method

the pressure difference method permeability meter has a wide test range and is simple to use. It can test a variety of gases. The test cost is low, and the equipment price is generally lower than that of the electricity analysis sensor method

Jinan Languang is the first manufacturer in the world to launch the atmospheric pressure differential permeability tester. Its bty-b1 permeability tester uses the air with the test lower chamber (low-pressure chamber) of 1 atmospheric pressure, which effectively avoids the possible impact of the lower chamber leakage on the test

2.2 testing principle of electric quantity analysis sensor method

the principle of electric quantity analysis oxygen transmittance tester is to use the film to be tested to separate the test chamber into two independent air flow systems, one side is the flowing gas to be tested (but pure oxygen or mixed gas containing oxygen, the relative humidity of the gas can be set), and the other side is the flowing nitrogen. The pressure difference on both sides of the sample is equal, but the oxygen partial pressure is different. Under the effect of concentration difference, the oxygen through the film is sent to the electric quantity analysis sensor under the transportation of nitrogen gas flow, so as to calculate the oxygen transmittance of the material

electric quantity analysis equipment has high low-end accuracy. Due to the same pressure on both sides of the sample, it is conducive to reducing the possible leakage during the test. However, this kind of instrument is expensive, and the oxygen sensor is consumable, so the test cost is high. This kind of instrument has high detection accuracy for samples with low oxygen transmission, but it is not suitable to often detect samples with high oxygen transmission due to the limitation of the service life of the sensor. In addition, this kind of equipment has certain selectivity to the test gas

3. conclusion

the penetration of gas into packaging materials may cause the contents to deteriorate and fail, which has a great impact on the shelf life of products. In the world, there are two methods to detect gas permeability: differential pressure method and electric quantity analysis sensor method. China's flexible packaging testing industry has developed rapidly in the field of permeability testing

due to the certain differences between the common inorganic gas test methods and the water vapor and organic gas test methods, the author will introduce the water vapor permeability test of flexible packaging and the organic gas permeability test of flexible packaging in the future

Author: Zhang Weisheng