At low temperature, the activity of molecular chains in the polymer materials of flexible packaging bags decreases, and the ability to resist external deformation decreases. Macroscopically, the ability to resist impact decreases, which leads to the decrease of flexibility and increase of brittleness of flexible packaging. When subjected to external impact, the problems of bag breakage and air leakage are very easy to occur.

So how to improve the flexibility of packaging bags at low temperature and reduce the breakage rate?

Firstly, packaging materials should be selected reasonably according to content characteristics and cost control. For example, OPP/LDPE material has good moisture-proof, cold-resistant, low-temperature heat-sealing tension and cost-effective. PA/LDPE material has good cold-resistant, impact-resistant and puncture-resistant properties, which is suitable for packaging products with harder contents, but its cost is higher. In addition, PE, PET/PE, BOPP/PE, BOPP/CPP and other materials can also be selected according to actual needs.

Secondly, the cold resistance of the package was monitored. Cold resistance is an important performance index of flexible plastic packaging materials. It is also an important index for evaluating the physical properties of flexible packaging materials under low temperature. It is also a key concern index for product packaging circulating in low temperature environment. Packaging bag manufacturers such as material cold resistance can not meet the requirements, in the case of low temperature, the material can not maintain the original physical properties, resulting in packaging bag cracking and cracking, unable to meet the protection of content. The cold resistance of packaging bags can be effectively improved by choosing suitable resin raw materials, improving the processing technology of packaging materials, adding appropriate plasticizers and choosing packaging materials with good low temperature resistance, so as to prevent the increase of damage rate of packaging bags under low temperature environment.

Cold resistance can be tested by pendulum impact test. The test method is to compare the impact resistance of pendulum under standard environment with that under low temperature environment (mostly 18 below zero). The cold resistance should be no less than a fixed percentage ratio, such as QB/T1871-93 Bidirectional Tensile Nylon (BOPA) Low Density Polyethylene (LDPE). The cold resistance of the materials stipulated in "Composite Film and Bag" should be no less than 60%. The testing methods are as follows:

The anti-pendulum impact energy of the films was measured according to GB8809 after the samples were placed at - 18 C for 2 hours. The cold resistance was calculated according to the following formula, and two valid figures were taken.

In addition, we should not neglect the comprehensive physical performance testing of packaging bags. For example, tensile properties, packaging puncture resistance testing. Soft packaging materials used for frozen food should have certain flexibility. For example, at room temperature, the material has poor flexibility and low elongation. When air pressure is formed inside the finished product packaging, the packaging materials are prone to form resistance, leading to bag breakage. And the frozen food is hard. During transportation and storage, the products are extruded. Only with good puncture performance can the packaging manufacturers resist the puncture force from the contents. Therefore, the testing of comprehensive physical properties of materials such as tensile properties and puncture resistance should not be neglected in frozen food packaging materials.