Analysis of Mechanical Properties of Flexible Packaging and Plastic Films

[China Packaging Network News] has reliable mechanical properties, is the soft packaging to carry out the best protection of the basic indicators of the contents, if the flexible packaging mechanical properties do not meet the standard, in the course of use is very easy to damage, and then the possibility of leakage of the contents Case. Therefore, flexible packaging companies will conduct rigorous testing of their mechanical properties before they leave the factory, and will also evaluate various mechanical performance indicators for almost all materials used in flexible packaging production. However, many companies still have some doubts about the selection of projects and the application of standards when testing the mechanical properties of flexible packaging. In this paper, the author systematically analyzes the mechanical properties of soft packaging and plastic films and their reference standards, hoping to bring some help to people in the industry.

Peel strength

Peel strength, also known as composite strength or composite fastness, mainly examines the bond strength between the composite film layer and the layer. The main production methods of composite membranes are dry composite and solvent-free composites. The quality of the inter-membrane bonding directly affects the strength, barrier properties and service life of the composite flexible packaging. If the adhesive strength is too low, the soft packaging produced by it can easily cause separation between layers during use, resulting in leakage and other issues.

The test standard for peel strength shall refer to GB/T 8808-1988 "Test method for peeling of soft composite plastic materials". Peel strength test specimen clamping diagram shown in Figure 1, when the composite film layer can not be completely peeled or the composite layer breaks, the peel strength is judged acceptable, but the premise to ensure that the tensile strength of the composite film meets the relevant standards .

Heat seal strength is used to assess the quality of the film and film or film and other substrates (such as aluminum foil, etc.) when heat-sealed. Soft packaging is generally packaged using hot-pressing methods. The integrity of the package is largely determined by the quality of the heat seal. In the process of product preservation and transportation, if the heat seal strength of the flexible package is too low, it may cause cracking of the seal, resulting in leakage and other issues.

The heat seal strength test standard should refer to QB/T 2358-1998 "Test method for heat seal strength of plastic film packaging bags", which is applicable to the determination of the heat seal strength of various plastic film packaging bags.

Right angle tear strength

Right angle tear strength is generally used to assess the tear resistance of plastic film, refers to the tensile load applied to the standard sample, make the sample tear at the right angle mouth, measure the tear force of the sample.

The right-angled tear strength test is based on QB/T 1130-1991 "Test Method for Plastic Right-Angle Tearing Properties", which applies to films, sheets and other similar plastic materials. The right-angled tear test specimen is shown in Fig. 3. If the specimen is too thin, multiple specimens can be stacked and tested. However, the test results of single and superposed specimens cannot be compared.

Impact resistance

The impact resistance is used to characterize the impact strength of plastic films under impact loading. It is used to evaluate the plastic film's ability to withstand toughness or resist fracture under high-speed impact. In many occasions, flexible packaging or other plastic film products are inevitably subjected to sudden shocks and can withstand this sudden load to meet the use requirements. Therefore, the impact resistance of plastic film is an important performance indicator in engineering applications, which can reflect the ability of different materials to resist high-speed impact.

GB/T 8809-2015 "Plastic film anti-pendulum impact test method" provides a variety of plastic film anti-pendulum impact test. The pendulum type film impact tester (Fig. 4) was used for the test. During the experiment, the hemispherical punch impacted at a certain speed and passed through the film sample, and the energy consumed by the hemispherical punch was measured to evaluate the plastic film. The anti-pendulum impact ability.

Pressure resistance

Under normal circumstances, the flexible packaging is subjected to external pressure during storage and transportation. Inevitably, stack failure, extrusion damage, etc. will occur, which will affect the quality of the finished product. GB/T 10004-2008 “Plastic composite film for packaging, bag dry compounding, extrusion compounding” stipulates that when the contents are powdery, liquid, or need to be inflated or vacuum packed, the pressure resistance of the flexible packaging must be tested and Drop performance. The pressure resistance of flexible packaging is to test the pressure resistance of the sample by simulating the stacking, squeezing, and other actions of packaging, storage, and transportation. Its purpose is to measure the pressure bearing capacity of flexible packaging bags.

The schematic diagram of the withstanding pressure test is shown in Fig. 5. In the pressure test, the upper and lower pressure plates should be kept horizontal and the pressure plate should not be deformed. The contact surface with the test bag should be smooth and the area of ​​the upper and lower pressure plates should be the same. Should be larger than the test bag. According to the regulations, add and release weights for 1min. When the weight is added to the rupture of the test bag, the test ends. At this time, the sum of the mass of the weight plus the mass of the compression plate is the maximum resistance of the test bag.

Tensile properties

Tensile property is an important physical and mechanical performance index of plastic film. Whether the tensile properties of plastic film is qualified or not will largely determine the quality of the use of flexible packaging.

The tensile properties of plastic films can be determined by the tensile strength (longitudinal/transverse) test. The plastic film samples are stretched in a specific direction (longitudinal or transverse) at a certain test speed under the action of a tensile clamp until the test. In the case of fractures, the tensile strength is expressed as the maximum force at which the plastic film specimen breaks during the test divided by the cross-sectional area of ​​the plastic film.

Elongation at break refers to the ratio of the increase in the distance between the reticle when the plastic film specimen is broken to the initial gauge length in 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 GB/T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for thin plastics and sheets". According to the standard, plastic film samples are cut at equal intervals along the longitudinal and transverse directions. Take long strips, the sample width is 15mm, the total sample length is not less than 150mm, and the test speed is 200mm/min. Not less than 5, gauge length (that is, distance between fixtures of the tester) is 100mm. It is worth noting that if the deformation rate of the sample is large, it can be considered to reduce the gauge length, but the minimum should not be less than 50mm; the preparation of the sample requires a smooth edge, no damage or notch phenomenon, because the tiny flaws tend to produce stress concentration, As a result, the internal deformation of the sample concentrates and the test fails. It should be emphasized that if the broken part of the sample appears in the test fixture, the test is invalidated.

Because the plastic film is a high molecular polymer and has viscoelasticity, its stress relaxation process is closely related to the deformation rate. When stretched at low speed, the polymer chain segments have time-rearranged rearrangements and exhibit ductile behavior. When stretched at high speeds, the movement of the polymer segments does not keep up with the application of external forces and exhibit brittle behavior. Therefore, in the tensile strength test of plastic films, the pulling force should be uniform and should not suddenly increase or decrease.

Puncture resistance

The puncture resistance performance of soft packaging of cooked meat products is one of the important performance indicators. For example, poultry meat products, in which the bones have strong puncture ability to the external packaging, the puncture strength of the composite film should be tested to reduce the internal and external factors on the packaging. The influence of the bag reduces the chance of deterioration or leakage of the contents.

The puncture resistance performance of soft packaging of cooked meat products is one of the important performance indicators. For example, poultry meat products, in which the bones have strong puncture ability to the external packaging, the puncture strength of the composite film should be tested to reduce the internal and external factors on the packaging. The influence of the bag reduces the chance of deterioration or leakage of the contents.

Coefficient of friction

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

The coefficient of static friction refers to the ratio of the maximum resistance of two contact surfaces at the start of relative movement to the normal force applied perpendicularly to the two contact surfaces; the coefficient of dynamic friction refers to the resistance and vertical application of two contact surfaces moving at a certain speed relative to each other. The ratio of the normal forces at the two contact surfaces. At present, the static (dynamic) coefficient of friction is based on the standard test GB/T 10006-1988 "plastic film and sheet friction coefficient determination method", the specific test is to put the surface of the two specimens together, under a certain contact pressure, The relative movement of the two surfaces of the specimen occurs, and the force at the time of the relative movement of the specimen and the force at the uniform movement are measured, and the static (dynamic) friction coefficient is calculated.