Having reliable mechanical properties is the basic indicator for the best protection of the contents of flexible packaging. If the mechanical properties of the flexible packaging are not up to standard, it will be easily damaged during use, and the contents may leak. Therefore, flexible packaging companies will rigorously test their mechanical properties before leaving the factory, and will also evaluate the mechanical properties of almost all materials used in flexible packaging production. However, many companies still have some doubts about the choice of the project and the application of the standard when testing the mechanical properties of flexible packaging. In this paper, the author systematically analyzes the mechanical properties of soft packaging and plastic film and its reference standards, hoping to bring some help to the industry.

Peel strength

The peel strength is also called composite strength or composite fastness. The adhesive strength between the composite film layer and the layer is mainly investigated. The main production methods of composite membranes are dry composite and solvent-free composite. The quality of adhesion between membranes directly affects the strength, barrier properties and service life of composite flexible packaging. If the bonding strength is too low, the flexible packaging produced by it is extremely easy to cause interlayer separation during use, thereby causing problems such as leakage.

The test standard for peel strength shall refer to GB/T8808-1988 "Test method for peeling of soft composite plastic materials". The peeling strength test sample clamping diagram is shown in Figure 1. When the composite film layer can not be completely peeled off or the composite layer is broken, the peel strength is judged as qualified, but the premise needs to ensure that the tensile strength of the composite film meets the relevant standards. 

Heat seal strength is used to assess the quality of a film and film or film and other substrates (such as aluminum foil, etc.) when heat sealed. Flexible packaging is generally packaged by hot-press sealing. The tightness of the packaging is determined by the heat seal quality. During the storage and transportation of the product, if the heat-sealing strength of the flexible package is too low, the seal may be cracked and leakage may occur.

The heat sealing strength test standard should refer to QB/T2358-1998 "Test method for heat sealing strength of plastic film packaging bags", which is applicable to the determination of heat sealing strength of various plastic film packaging bags.

Right angle tear strength

The right angle tear strength test is based on QB/T1130-1991 "Test method for plastic right angle tear performance", which is applicable to films, sheets and other similar plastic materials. The schematic diagram of the right angle tear strength test sample is shown in Figure 3. If the sample is too thin, multiple pieces of sample can be used for the test, but the test results of the single piece and the laminated sample are not comparable.

Impact resistance

Impact resistance is used to characterize the impact strength of plastic films under impact loading and is used to evaluate the toughness or resistance to fracture of plastic films when subjected to high speed impact. In many cases, flexible packaging or other plastic film products are inevitably subject to sudden impact and can withstand such sudden loads to meet the requirements of use. Therefore, the impact resistance of plastic film is an important performance index in engineering applications, which can reflect the ability of different materials to resist high-speed impact.

GB/T8809-2015 "Plastic film anti-pendulum impact test method" specifies the anti-pendulum impact test of various plastic films. The pendulum type film impact tester (Fig. 4) was used to test the plastic film by impacting the hemispherical punch at a certain speed and passing through the film sample to measure the energy consumed by the hemispherical punch. Anti-pendulum impact capability.

Pressure resistance

Under normal circumstances, soft packaging is subject to external pressure during storage and transportation, and inevitably, stacking cracks, crushing damage, etc. may occur, thereby affecting the quality of the finished product. GB/T10004-2008 "Packaging plastic composite film, bag dry composite, extrusion compound" stipulates that when the contents are powder, liquid or need to be inflated, vacuum packaging, the pressure resistance and drop of the flexible packaging must be tested. performance. The pressure resistance of flexible packaging is to test the pressure resistance of the sample by simulating the stacking and squeezing behaviors of packaging, storage, transportation, etc., and the purpose is to test the pressure bearing capacity of the flexible packaging bag.

The pressure test diagram is shown in Figure 5. In the pressure test, the upper and lower pressure plates should be kept horizontal, and the pressure plate is not deformed. The contact surface with the test bag should be smooth, and the area of the upper and lower pressure plates should be smooth. Should be larger than the test bag. Add the weight according to the regulations and keep it for 1 min. When the weight of the weight is added to the rupture of the test bag, the test is finished. At this time, the sum of the mass of the weight and the mass of the pressure plate is the maximum tolerance of the test bag.

Tensile properties

Tensile properties are important physical and mechanical properties of plastic films. Whether the tensile properties of plastic films are qualified or not determines the quality of flexible packaging.

The tensile properties of the plastic film can be obtained by the tensile strength (longitudinal/transverse) test. The plastic film sample is stretched in a specific direction (longitudinal or transverse direction) at a certain experimental speed under the action of the stretching jig until the test The fracture strength is expressed by dividing the maximum force of the plastic film sample during the test by the cross-sectional area of the plastic film.

Elongation at break refers to the ratio of the increase in the distance between the lines of the plastic film to the initial gauge length during the tensile test. This value is used to measure the elongation of the plastic film when it is not broken.

Tensile strength and elongation at break shall be tested in accordance with the provisions of GB/T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for thin plastics and sheets". According to the standard, the plastic film samples are cut at equal intervals in the longitudinal and transverse directions, taking long strip samples with a sample width of 15mm, the total length of the sample is not less than 150mm, and the test speed is 200mm/min. No less than 5, the gauge length (ie the distance between the test fixtures) is 100mm. It is worth noting that if the deformation rate of the sample is large, the gauge length can be reduced, but the minimum is not less than 50 mm. The preparation of the sample requires smooth edges without damage or chipping, because the small defects tend to cause stress concentration. As a result, the internal deformation of the sample is concentrated and the test is invalidated. It should be emphasized that if the broken portion of the sample appears in the experimental fixture, the test is invalidated.

Because the plastic film is a high molecular polymer, it has viscoelasticity, and its stress relaxation process is closely related to the deformation rate. When stretching at low speed, the polymer segment has time to shift and rearrange, showing ductile behavior; when stretching at high speed, the movement of the polymer segment can not keep up with the application speed of external force, showing brittle behavior. Therefore, in the tensile strength test of plastic film, the tensile force should be uniform and constant, and should not suddenly increase or decrease.

Puncture resistance

Puncture resistance of soft-packed meat products is one of the important performance indicators, such as poultry meat products, in which the bone has a strong puncture ability to the outer packaging, so the puncture strength of the composite film should be tested to reduce the internal and external factors on the packaging. The effect of the bag reduces the chance of deterioration or leakage of the contents.

Puncture resistance of soft-packed meat products is one of the important performance indicators, such as poultry meat products, in which the bone has a strong puncture ability to the outer packaging, so the puncture strength of the composite film should be tested to reduce the internal and external factors on the packaging. The effect of the bag reduces the chance of deterioration or leakage of the contents.

Coefficient of friction

In the process of production and processing, material filling, and packaging bag stacking, the friction coefficient of plastic film is an important performance index. The friction coefficient is divided into static friction coefficient and dynamic friction coefficient, which can be used to evaluate the smooth performance of the inner and outer layers of the flexible packaging. It should be tested to ensure that the flexible packaging has good opening performance and smooth conveying on the high-speed packaging production line.