Significance
With the increase of product types and the development of packaging technology and processes, there are more and more types of shrink films, such as PE, POF, PVC, BOPP, PETG, PVDC, etc., which are widely used in the packaging field, such as shrink sleeves, bundled packaging, casing film and other forms used to package food, medicine, tobacco, electronic products, automobiles, daily chemical products and other products. Different products have different shapes and sizes, which requires the excellent shrink performance of shrink film, to meet different product packaging requirements.
The factors that affect the shrink performance of the film include the shrinkage rate, shrinking force and contracting force generated during the shrinking process. If the three indexes mentioned above are poor or not applicable to the product, the film may not be tight and cannot fit the product properly, which may even cause the product to be crushed. At present, the test methods for film shrinkage are mainly oil bath method, drying oven method and air heating method.
Test Method
Oil bath method:
Specific sized specimen is placed into a heat transfer oil (e.g. silicone oil) at a certain temperature to be heated for shrinkage testing. After a certain period of time, the specimen is taken out, and the size of the specimen after heating process is measured. The ratio of the dimension before and after heating process is the shrinkage ratio. The ratio is the shrinkage of the sample. Related standards e.g. GB/T 13519-2016: Polyethylene heat-shrinkable film for packaging applications, requires an oil bath temperature of 140 °C and shrinking time of 20s. The testing equipment using oil bath method is shown in Figure 2.
Drying oven method
A sample of a certain size is placed in an oven at a certain temperature. After reaching a specified time, the sample is taken out and the size of the sample is measured. The ratio of the dimensional change value to the size before shrinkage is calculated as the shrinkage rate of the sample. Relevant standards are GB/T 12027-2004：Plastics-Film and Sheeting-Determination of Dimensional Change on Heating and GB/T 10003-2008: Biaxially oriented polypropylene (BOPP) film for general use. The former places samples in kaolin. On the bed, the temperature and time of the test are determined according to the specific sample, which requires the sample to be placed on a stain-less steel plate at a test temperature of 120° C. and a time of 120 s.
Air Heating Method
After the sample is heated by the hot air inside the closed chamber, the shrinkage rate is obtained according to the heat shrinkage time and size change of the sample, and the shrinkage force and the shrinkage force are calculated according to the variation of the force value of the sample during heating and cooling. At present, the domestic reference test method standard GB/T 34848-2017: Determination of Shrinkage Character of Heat-shrinkable Film will be formally implemented on May 1, 2018, before the implementation of this standard can refer to ISO14616-2004: Plastics – Heat-shrinkable Films of Polyethylene, Ethylene Copolymers and Their Mixtures - Determination of Shrinkage Stress and Contraction Stress. The test of the shrinkage performance of copolymers and their mixtures and heat shrinkable films is tested. The standard requires that the sample enters the chamber and shrinks when it reaches the set temperature in the test chamber. When the measured shrinkage force reaches the chamber conditions, the sample exits from the chamber and the equipment continues to monitor the contraction and force values of the sample until the end of the test. The shrinkage rate is calculated according to the change of the sample size automatically monitored by the equipment during the entire test process. The maximum force value measured during heating in the chamber is the shrinkage force, and the maximum force value measured during the cooling process after the sample exits from the chamber is the contracting force. The test equipment is shown in Figure 3.
Comparative Analysis
It can be seen from the test range that, all three test methods can be used in shrinkage ratio test of the films, but the air heating method can be also used in testing of shrinking force and contracting force. It can be seen from the test process that the oil bath method and drying oven method require manual measurement of the specimen size after shrinkage test, which can be done automatically in the air heating method meanwhile the real-time force can be displayed in air heating method instrument. In addition, the specimen is placed in the oil bath in heating process in oil bath method. The specimen is heated on kaolin or stain-less steel plate in drying oven method. In the air heating method, the specimen is exposed to the air and does not come in contact with any object. This difference of the heating environment will affect the shrinkage of the specimen, so the shrinkage test results obtained with those three methods are not comparable.
Conclusion
When testing the shrinkage properties of the film, a proper test method should be selected according to the material quality, product standard, test purpose and other factors. If there is a clear specification in the product standard, the test should be conducted according to the requirements in the product standard. if not, the test method can be selected according to the actual conditions. If it is required to monitor the shrinkage value of the film or complete the test automatically in high efficiency, or the shrinkage process with the high temperature steam or hot air duct is needed, it is recommended to use FST-02 Film Thermal Shrinkage Tester, using air heating method.
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