Mechanical property testing methods for packaging bags and plastic films

Having reliable mechanical properties is a basic indicator for packaging bags to best protect their contents. If packaging bags their mechanical properties do not meet standards, they are prone to damage during use, which could lead to content leakage. Therefore, soft packaging enterprises strictly test various mechanical properties of their products before they leave the factory, and also assess the mechanical property indicators of almost all materials used in soft packaging production. However, many enterprises still have some doubts regarding the selection of projects and the application of standards when testing packaging bags mechanical properties. Professional soft packaging manufacturer Shunxingyuan Packaging has systematically analyzed the mechanical properties of soft packaging and plastic films and their reference standards, hoping to provide some assistance to you.

1. Peel Strength

Peel strength, also known as composite strength or composite fastness, primarily examines the adhesion strength between layers of composite film. The main production methods for composite films are dry lamination and solvent-free lamination. The quality of inter-film adhesion directly affects the strength, barrier properties, and service life of composite flexible packaging. If the adhesion strength is too low, the flexible packaging produced from it is highly prone to delamination during use, leading to issues such as leakage.

The testing standard for peel strength should refer to GB/T 8808-1988 "Peel Test Method for Flexible Composite Plastic Materials". The schematic diagram for clamping peel strength test specimens is shown in Figure 1. When the composite film layers cannot be completely peeled or the composite layer breaks, its peel strength is judged as qualified, but the premise is to ensure that the tensile strength of the composite film meets relevant standard requirements.

2. Heat Seal Strength

Heat seal strength is used to evaluate the quality when film is heat-sealed with film or with other substrates (such as aluminum foil). Soft packaging generally uses heat-pressing to seal, and the integrity of the packaging's seal largely depends on the heat seal quality. During product storage and transportation, if the heat seal strength of soft packaging is too low, it may lead to seal cracking and subsequent issues like leakage.

The testing standard for heat seal strength should refer to QB/T 2358-1998 "Test Method for Heat Seal Strength of Plastic Film Packaging Bags". This standard is applicable to the determination of heat seal strength for various plastic film packaging bags. The shape and dimensions of heat seal strength test specimens are shown in Figure 2.

3. Right-Angle Tear Strength

Right-angle tear strength is generally used to assess the tear resistance of plastic films. It refers to applying a tensile load to a standard specimen, causing the specimen to tear at the right-angle notch, and then measuring the tearing force of the specimen.

The basis for right-angle tear strength testing is QB/T 1130-1991 "Test Method for Right-Angle Tear Performance of Plastics". This standard is applicable to films, sheets, and other similar plastic materials. The schematic diagram for right-angle tear strength test specimens is shown in Figure 3. If the specimen is too thin, multiple specimens can be stacked for testing, but the test results of single-ply and stacked specimens are not comparable.

4. Impact Resistance

Impact resistance is used to characterize the impact strength of plastic films under impact load, and to evaluate the toughness or resistance to fracture of plastic films when subjected to high-speed impact. In many situations, soft packaging or other plastic film products inevitably encounter sudden impacts. The ability to withstand such sudden loads is necessary to meet usage requirements. Therefore, the impact resistance of plastic films is a very important performance indicator in engineering applications, reflecting the ability of different materials to resist high-speed impact.

GB/T 8809-2015 "Plastic Film Pendulum Impact Test Method" specifies the pendulum impact test for various plastic films. The test is conducted using a pendulum film impact tester (as shown in Figure 4). During the test, a hemispherical impactor strikes and penetrates the film specimen at a certain speed, and the energy consumed by the hemispherical impactor is measured to evaluate the pendulum impact resistance of the plastic film.

5. Pressure Resistance

Under normal circumstances, soft packaging is subjected to external pressure during storage and transportation, inevitably leading to stacking breakage, extrusion damage, and other issues, thereby affecting product quality. GB/T 10004-2008 "Plastic Composite Films and Bags for Packaging: Dry Lamination, Extrusion Lamination" stipulates that when the contents are powdery, liquid, or require inflated or vacuum packaging, the pressure resistance and drop performance of the soft packaging must be tested. The pressure resistance of soft packaging is tested by simulating stacking, extrusion, and other behaviors during packaging storage and transportation processes to determine the specimen's pressure bearing capacity. Its purpose is to check the pressure bearing capacity of soft packaging bags.

The schematic diagram of the pressure resistance test is shown in Figure 5. In the pressure resistance test, the upper and lower pressure plates should remain horizontal, the pressure plates should not deform, the contact surface with the test bag should be smooth, and the area of the upper and lower pressure plates should be larger than that of the test bag. Weights are added as specified and held for 1 minute. When the weight of the weights added causes the test bag to rupture, the test ends. At this point, the sum of the mass of the weights added and the mass of the pressure plate is the maximum bearing capacity of the test bag.

6. Tensile Properties

Tensile properties are important physical and mechanical performance indicators for plastic films. Whether the tensile properties of plastic films meet standards largely determines the service quality of soft packaging.

The tensile properties of plastic films can be obtained through tensile strength (longitudinal/transverse) tests. Under the action of a tensile fixture, a plastic film specimen is stretched in a specific direction (longitudinal or transverse) at a certain test speed until the specimen breaks. Tensile strength is expressed as the maximum force applied when 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 gauge marks to the initial gauge length when a plastic film specimen breaks during a tensile test. This value is used to measure the stretchability of the plastic film before breaking.

Tensile strength and elongation at break should be tested according to GB/T 1040.3-2006 "Plastics - Determination of tensile properties - Part 3: Test conditions for thin plastics and films". According to the standard, plastic film specimens are cut at approximately equal intervals in the longitudinal and transverse directions. Long strip specimens with a width of 15mm and a total length of not less than 150mm are taken. The test speed is 200mm/min, with no less than 5 specimens per group, and the gauge length (distance between the grips of the testing machine) is 100mm. It is worth noting that if the specimen deformation rate is large, the gauge length can be reduced, but it should not be less than 50mm. When preparing the specimens, the edges should be smooth and free from damage or notches, as minor flaws easily cause stress concentration, leading to concentrated deformation inside the specimen and test failure. It should be emphasized that if the fracture of the specimen occurs within the test grips, the test is invalid.

Because plastic films are polymers with viscoelasticity, their stress relaxation process is closely related to the deformation rate. When stretched at low speed, the polymer segments have time to rearrange their positions, exhibiting ductile behavior; when stretched at high speed, the movement of the polymer segments cannot keep up with the application speed of the external force, exhibiting brittle behavior. Therefore, in the tensile strength test of plastic films, the tensile force should be uniform and constant, and sudden increases or decreases should be avoided.

7. Puncture Resistance

Puncture resistance is one of the important performance indicators of cooked meat product soft packaging. For example, in poultry meat products, bones have a strong puncturing ability on the outer packaging. Therefore, the puncture strength of the composite film needs to be tested to reduce the influence of internal and external factors on the packaging bag and reduce the probability of deterioration or leakage of the contents.

Puncture resistance is one of the important performance indicators of cooked meat product soft packaging. For example, in poultry meat products, bones have a strong puncturing ability on the outer packaging. Therefore, the puncture strength of the composite film needs to be tested to reduce the influence of internal and external factors on the packaging bag and reduce the probability of deterioration or leakage of the contents.

8. Coefficient of Friction

The coefficient of friction of plastic films is an important performance indicator during production, material filling, and packaging bag stacking. The coefficient of friction is divided into static coefficient of friction and dynamic coefficient of friction, which can be used to evaluate the smoothness of the inner and outer layers of soft packaging. It should be tested to ensure that the soft packaging has good opening performance and can be smoothly conveyed on high-speed packaging production lines.

The static coefficient of friction is the ratio of the maximum resistance at the beginning of relative movement of two contacting surfaces to the normal force applied perpendicularly to the two contacting surfaces; the dynamic coefficient of friction is the ratio of the resistance when two contacting surfaces move relative to each other at a certain speed to the normal force applied perpendicularly to the two contacting surfaces. Currently, the standard for the detection of static (dynamic) coefficient of friction is GB/T 10006-1988 "Determination of coefficient of friction of plastic films and sheets". The specific test is to place two test samples flat together, under a certain contact pressure, make the two surfaces of the samples move relative to each other, measure the force when the samples begin to move relative to each other and the force when they move at a uniform speed, and calculate the static (dynamic) coefficient of friction.