Permeability testing and control of flexible packaging materials

First, the study of the permeability of flexible packaging materials
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Traditional metal and glass packaging are completely impervious to oxygen and water vapor, and are the preferred materials for food and pharmaceutical packaging. However, with the continuous development of packaging technology, high barrier plastic packaging materials capable of providing shelf life, freshness preservation, flavor protection, and extended shelf life are available because of their low cost, light weight, impact resistance, transparency, microwave heating, convenience, and packaging design. The large selection of properties has become a strong competitor for metal and glass packaging materials, and it has been rapidly developed and widely used in the food, pharmaceutical and other packaging industries.
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The main structure of plastic flexible packaging material is a plastic polymer, which has a certain permeability to gas, and therefore cannot completely replace metal and glass packaging. However, the high barrier material can effectively prevent oxygen from invading to make the commodity oxidatively degenerate, prevent the penetration of water or water vapor to make the product wet and mold, and prevent the permeation of aroma, carbon dioxide and odor from deteriorating the product.

With the development of the food flexible packaging industry, gas packaging and modified atmosphere packaging have also been widely used. The most commonly used gases are O2, CO2, and N2. According to the rational change of gas, N2 and CO2 can inhibit the growth and reproduction of bacteria, microorganisms and fungi, and slow down the oxidation and respiration of foods as a medium for preservation and preservation of food packaging. N2 and CO2 can block the oxidation of foods and prevent them from deteriorating due to changes in the fat content of foods. However, the gas inside the package will penetrate more or less.

The gas and atmosphere in the flexible packaging product undergo gas exchange through the packaging film, and the gas concentration constantly changes. The gas permeation rate is affected by the pressure of the mixed gas in the package, and is affected by the ambient temperature. Therefore, the gas permeation is in an unstable state.

The solution to these problems requires quantitative analysis of the air permeability of flexible packaging materials. Therefore, testing of the permeability (or barrier property) of flexible packaging materials is increasingly important in packaging testing.

Second, the soft packaging permeability test status

1. Several methods of permeability testing for flexible packaging materials are based on the pressure change, volume change, and concentration change of the test gas through the membrane. The test methods for the permeability of plastic films are the pressure method, volume method, and concentration method, and the gas phase using gas chromatography. Chromatography and heat conduction. Most gas permeability testers now use differential pressure.
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2. The standard condition of air permeability test in China
China has actively introduced advanced foreign standards. As early as 1970, the National Standard for Testing the Air Permeability of Plastic Films (GB/T1038-70) was proposed. The standard is based on the differential pressure principle. The GB/T1038-2000 "Test Method for Gas Permeability of Plastic Films and Sheets" proposed in 2000 is also a differential pressure method. The main difference between this standard and the original GB/T1038-1970 "Test method for permeability of plastic films" is that :
1 The scope of application of this standard extends from the original standard plastic film to plastic film and sheet;
2 This standard only stipulates the working principles and elements of the instrument. It does not make specific requirements for the instrument. At the same time, the instrument can carry the computer computing device. 
3 Because the state prohibits the use of "atm (atmospheric pressure)" as the pressure unit, it uses the pressure unit "Pa (Pa)" that the country permits; the time unit is consistent with ISO2556, and the unit of measurement is "d (day)" after revision. .

Third, the principle and technology of permeability testing of flexible packaging material pressure difference method

The test principle of differential pressure method is to maintain a certain gas pressure difference on both sides of the sample at a certain temperature, measure the gas pressure change on the low pressure side of the sample, and calculate the air permeability coefficient and air permeability. 
The object of gas permeability detection is microscopic gas, and the permeation amount of the membrane is also extremely small. Therefore, how to ensure the accuracy of measurement is the key to measuring the measuring instrument.

Technical test of air permeability tester:
1. With reference to the national standard GB/T1038-2000, differential pressure method can be used to measure various gases such as O2, CO2, and N2, which can be widely used in flexible packaging enterprises. 
2. Use the most advanced micro differential pressure sensor and high-end SMC pneumatic components to ensure stable and reliable test accuracy and safe operation. 
3. Throughout the testing process, the computer is stepwise and bidirectionally controlled to ensure accurate test results. 
1 reference valve closes waiting
The setting of the reference valve closing wait time (xxxS) is an operation for sampling the system. Theoretically, during the period when the reference valve is closed, the pressures at the reference and test ends are unbalanced, and the pressure in the two chambers is damped and eventually reaches a new equilibrium point. The setting of the reference valve closing wait time is to take out a new balance point and filter out the interference caused by the operation of the electric control valve and the air control valve. 
2 threshold test
By setting the threshold value (xxxPa), the diaphragm is brought to an approximately linear initial value, which has the effect of eliminating the initial disturbance of the system. 
3 wait for you
The setting of this value is used to predict the approximate air permeability of the diaphragm. Within the waiting value range, the system will automatically track the change of the pressure value in the test chamber to estimate the air permeability and air permeability of the test sample. Then the system calculates the approximate time required for the test process and the pressure value that can be reached in the test chamber during the period according to the upper limit set by the user, that is, the length of the vertical and horizontal coordinates of the test process. In the waiting interval, interference in various aspects of the system can also be filtered out. 
4 official test
The time and pressure curves are displayed in real time, and the air permeability and the air permeability coefficient are calculated. 
4. Calibration uses a standard high-precision (0.5‰) pressure calibrator that is intuitive, accurate, and convenient to avoid the use of reference materials because the storage requirements of the reference materials are extremely stringent and difficult to trace.

Breathability tester test process 1. Test conditions sample and pretreatment
1 Specimen 120mm x 120mm square; sample without wrinkle, surface clean; 3 samples per group; sample dried in anhydrous calcium chloride desiccator for 24h, or according to product standard regulations. 
2 Temperature: 25 ± 2 °C or according to product standards. 
3 Select the type of gas according to the application requirements, using 99.99% high-purity O2, N2, CO2 or other gases. 
2. Test procedure
The sample was placed on a coarse filter paper (diameter 66 mm), sealed in an air permeable chamber, the air source was opened, and the main interface of the test and test software was started.
After the end, the breathability tester report pops up.

Fourth, breathable test should pay attention to matters

1. The physical properties of the tested diaphragm cannot be destroyed. (eg flat, no scratches, no perforations, no other attachments on the surface, no elastic or inelastic stretching)
2. The permeability of the membrane is related to the test temperature and humidity. “Test temperature” is one of the parameters in the calculation formula. It is necessary to ensure the laboratory temperature stability in order to reduce the test error. The permeability of the membrane is directly related to the humidity of the membrane. The membrane must be subjected to stringent drying prior to testing. 
3. The single-layer film has the same front and back surface test results, and one side can be measured. The front and back surfaces of the coating film, composite film, and multilayer co-extruded film should be measured separately.
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Fifth, the application of breathability test data
Can be used to guide the production of packaging film. The components of the packaging film can be reasonably selected, including basic resins such as LDPE and LLDPE; the laminated structure of the composite film can be rationally selected; barrier coating materials such as PA and PVDC can be selected; PVDC is currently the only one approved by the US FDA to be in contact with food. High-barrier transparent materials, for example, BOPP, the oxygen permeability after coating is 1/1000 times that before coating; the process of barrier coating research can be single-sided or double-sided or multi-layered Barrier layer. Or the choice of coating methods such as vacuum aluminum plating, physical or chemical deposition coating silicon oxide and so on. 
As a company that uses packaging films, batch-by-batch incoming inspections can be conducted, and each batch of products meets the required standards. This ensures that each batch of packaged goods can meet the required shelf life, thus preventing the reputation of the company from being damaged, eliminating customer complaints, and improving Corporate image. It helps to choose the thickness of packaging film reasonably and saves cost; it provides an effective basis for determining the shelf life of packaged products.