1. Tensile properties of high molecular polymers: When used as materials, high molecular polymers are required to have necessary mechanical properties. It can be said that for most applications of high molecular polymers, mechanical properties are more important than other physical properties. High molecular polymers have the widest range of mechanical properties among all known materials. This is due to the fact that polymers are composed of long-chain molecules, and the molecular motion has obvious relaxation properties. If the polymer material has a relatively high elongation, the break elongation of PE is generally 90% to 950% (including the linear elongation of low-density polyethylene LLDPE is higher), through special manufacturing process, some materials The elongation rate can be above 1000%, and the elongation at break of ordinary polymer materials is mostly between 50% and 100%. Generally, the heat-shrinkable film and stretched film are required to have higher tensile properties.
2. Tensile test: Tensile test (stress-strain test) generally clamps the two ends of the material sample on two fixtures at a certain distance. The two fixtures separate and stretch the sample at a certain speed. The stress on the specimen changes until the specimen fails. Tensile test is one of the most widely used methods to study the mechanical strength of materials. It requires the use of a constant-speed tensile testing machine. According to the different load measurement methods, the tensile testing machine can be roughly divided into two types of pendulum tensile testing machine and electronic tensile testing machine. At present, the electronic tensile testing machine is mostly used.
3. Selection index of electronic tensile testing machine: Since flexible packaging materials are mainly high-molecular polymers or related materials, as mentioned above, the elongation rate of high-polymer materials is far better than that of metals, fibers, wood, plates, etc. Therefore, there is a certain difference between the tensile machine for testing polymer and the normal material tensile property testing machine. In particular, it should be noted that the effective stroke of the electronic tensile machine and the sample fixture are two aspects.
3.1 Effective stroke: Although the size of the sample used in the tensile test is small, the elongation of the material is generally relatively high, so the tensile performance of the flexible packaging material needs to be equipped with a tensile machine with large stroke, otherwise the fixture Operation may exceed the service limit of the stroke and cause damage to the equipment. The calculation formula for the elongation at break or yield elongation (εt, unit is%) given in GB13022-91 "Test Methods for Tensile Properties of Plastic Films" is as follows: where: εt is the elongation at break or yield Elongation; L is the distance between the marking lines when the specimen breaks or yields; L0 is the distance between the marking lines. It should be noted that in the calculation of elongation, we only collect the elongation between the two markings on the sample. The marking lines are drawn on the finished sample by printing or manually (the addition of the marking line should have no effect on the sample), and what is the distance between the marking lines? Most of the distances given by different standards are different, and in the same standard, different sample sizes are often given for different materials, so the distance between the marking lines is also different, but this is conducive to the detection of extension Very long or very small materials and get accurate test results. For plastic films, the distance between the marking lines is usually between 25-50mm. Because the deformation and elongation of the sample in the tensile test are not only within the marking line, all samples between the two fixtures will get different degrees of tensile deformation. In the standard, the initial distance between the clamps corresponding to the distance of the marking line is within 80 to 115 mm. If the sample between the two clamps can maintain the same elongation and assume 500%, the effective stroke of the tensile machine needs to be 480 ~ 690mm, if it is a sample with an elongation of 1000%, the effective stroke of the tension machine should be at least 880mm to ensure the normal test.
3.2 Sample fixture: The description of the fixture in GB13022-91 "Test Method for Tensile Properties of Plastic Films" is: "The test machine should be equipped with an appropriate fixture, which should not cause the sample to break at the fixture, when any load is applied , The clamps on the testing machine should be able to be immediately aligned into a line, so that the long axis of the sample coincides with the direction of stretching through the center line of the clamp .... Place the sample in the two clamps of the testing machine to make the sample longitudinal The shaft coincides with the center line of the upper and lower fixtures, and should be tight and suitable to prevent the sample from slipping and breaking in the fixture. The fixture should be lined with an elastic material such as rubber. "Due to the special nature of the polymer mechanical properties and The special use of flexible packaging materials makes the thickness of the samples tested by the flexible packaging materials very thin, and the general fixtures cannot meet the requirements. Improper use will cause the specimens to break at the fixture and cause the test to fail.
3.3 TF-213 intelligent electronic tensile testing machine: Shanghai Tuofeng Instrument is an enterprise specializing in the production of flexible packaging material testing equipment. Especially for the characteristics of large elongation of flexible packaging materials, TF-213 intelligent electronic tensile testing machine has been made with a stroke of up to 1000mm, can carry out seven kinds of tests such as stretching, peeling, tearing, heat sealing, etc., and professionally designed various types of test fixtures dedicated to flexible packaging materials. It is a mechanical property tensile testing machine tailored for flexible packaging materials.
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