Source：Link Testing Instruments Co.,Ltd.
Food flexible packaging products are not prone to deterioration in a cold environment, and the shelf life can be relatively extended. Because low temperatures can inhibit the reproduction and growth of microorganisms and food nutrients are not easily oxidized, the probability of food mildew, rancidity and other problems is generally reduced. However, the low temperature environment also brings side effects. While reducing the probability of product deterioration, it will adversely affect the packaging and integrity of the product during transportation, storage and sales. For example, the bag body of quick-frozen dumplings did not appear to be broken during the production line, but the bag body or the seal appeared to be broken after being frozen and stored. How to make full use of the low temperature environment, select suitable packaging materials or improve the performance of packaging materials, and reduce the rate of product damage in the cold environment is a quality problem that plagues food companies.
Under low temperature conditions, polymer flexible packaging materials have a tendency to increase brittleness, and brittleness is easy to occur. Because under low temperature conditions, the mobility of the molecular segments inside the corresponding polymer material of the flexible plastic packaging is reduced, the deformability under the action of external force is reduced, and the impact resistance is weakened, which causes the flexibility of the flexible plastic packaging to decrease and the brittleness to increase. The flexibility and impact resistance of the material deteriorate at low temperatures, resulting in inability to withstand external impacts, causing damage such as bag breakage and air leakage. In order to improve the resistance to brittleness of flexible plastic packaging materials at low temperatures, it is necessary to add additives such as toughening agents to the raw materials. However, its cold resistance and whether it can withstand low temperature environments should be verified by cold resistance tests. It is the pendulum impact resistance measured by comparing standard environmental conditions and low temperature environments (mostly minus 18 ℃). For example, the cold resistance is not less than a fixed percentage ratio (for example, the cold resistance of BOPA/LDPE composite film requires Not less than 60%), it means that the physical and mechanical strength of the packaging material at low temperatures has not significantly deteriorated. For example, the lower the ratio, the worse the cold resistance of the material.
Test methods and instruments
1. Introduction to test equipment
LTJM-06 film impact tester: developed according to the principle of conservation of energy. The semi-circular punch of the instrument impacts and penetrates the film sample at a certain speed. According to the energy consumed by the punch, the impact resistance of the film is evaluated.
The impact energy of the equipment has three ranges of 1 J, 2 J, and 3 J, and can be extended to 5 J with a resolution of 0.01 J; the punch diameter is divided into three types: Φ25.4 mm, Φ19 mm, and Φ12.7 mm. There are two types of specimen clamping diameters: Φ89 mm and Φ60 mm; the specimen is clamped pneumatically. The equipment can be used to accurately measure the pendulum impact energy of plastic films, sheets, composite films, metal flakes and other materials, and can also be used to test the pendulum impact energy of paper and cardboard, such as aluminized cigarette paper, Tetra Pak Packaging aluminum-plastic-paper composite film, etc.
2. Test procedure
The test method of pendulum impact energy that needs to be tested in cold resistance refers to GB 8809 "Plastic Film Anti-Pendulum Impact Test Method".
(1) Cut samples: Cut 20 pieces of 10 cm × 10 cm samples from the surface of No. 1 and No. 2 samples, and place them at 23±2℃ and 45% ~ 55%RH for at least 4 hours.
(2) Thickness measurement: test the thickness of No. 1 and No. 2 samples respectively. One point is tested at the center of each specimen, and the thickness of the two specimens is the arithmetic average of the test results of 20 specimens.
(3) Low-temperature treatment: randomly select 10 cut samples from each of the two samples, and put them in an environment of -18℃ for low-temperature treatment for 2 hours.
(4) Equipment calibration: select a punch with a suitable diameter and impact range so that the test result of the sample is between 10% and 90% of the selected range. In this test, a punch with a diameter of Φ12.7 mm was selected, the jaw diameter of the sample was Φ89 mm, and the impact range was 3 J. Calibrate in accordance with the requirements of the equipment.
(5) Normal temperature sample test: clamp the sample piece in the equipment, step on the pneumatic pedal to clamp the sample. Click the test option, the punch will drop and impact the sample, and the device will automatically display the test result of the sample. Repeat this operation until the 10 specimens of No. 1 and No. 2 samples without low temperature treatment are all tested.
（6）Low-temperature treatment sample test: After the sample has been treated at low temperature, take it out and perform the test according to the steps in (5).
The pendulum impact resistance of the film measured after being placed in an environment of -18°C is divided by the pendulum impact resistance of the film measured under standard environmental conditions, which is the cold resistance, and the unit is %.
After testing, the cold resistance of No. 1 sample is 85.4%, and the cold resistance of No. 2 sample is 95.2%. The cold resistance of No. 1 sample is significantly lower than that of No. 2, that is, the flexibility of No. 1 sample is greatly dropped in a low temperature environment, the brittleness is obviously increased, the impact resistance of the packaging is significantly reduced, and the impact resistance of the package is significantly reduced. Deterioration of destructive power such as pressure, drop, etc. is greater, while the flexibility of No. 2 sample under low temperature environment is small, and it still maintains high impact strength. This is caused by the damage rate of No. 1 instant noodle sample package in the northern cold area. It is an important reason why the damage rate of No. 2 instant noodle sample packaging has not changed significantly.
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