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Plastic films are widely used in the field of packaging. Plastic film can be used for food packaging, electrical product packaging, daily necessities packaging, clothing packaging and so on. They have a common point, that is, the plastic film are to color printing, and as food packaging but also multi-layer composite or vacuum aluminizing and other process operations. Therefore, the requirements of the plastic film surface free energy to be high, wet tension to be large, in order to facilitate the printing ink, adhesive or aluminum layer and plastic film adhesion; in the production of plastic film winding and high-speed packaging process, the film surface is required to have a certain friction properties to prevent film adhesion or slippage; in the electrical appliances, electronic products, packaging, etc., then the film is required to have a certain level of anti-static properties, and so on.
Surface Tension of Plastic Film
The surface tension of plastic film depends on the size of the surface free energy of the plastic film, and the surface energy of the film depends on the molecular structure of the film material itself. Most plastic films such as polyolefin films (LDPE, HDPE, LLDPE, PP) is a non-polar polymer, its surface free energy is small, the surface wet tension is low, generally about 30 dynes / cm. Theoretically, if the surface tension of the object is lower than 33 dynes/cm, ordinary ink or adhesive will not be able to adhere to the firm, so it is necessary to treat its surface. Polyesters (PET, PBT, PEN, PETG) are polar polymers with high surface free energy and surface wet tension above 40 dynes/cm. But for high-speed color printing or to increase the bonding force between the vacuum aluminized layer and the surface of the BOPET film, it is also necessary to carry out surface treatment of the BOPET film to further improve its surface wet tension.
Plastic film surface treatment methods are: corona treatment method, chemical treatment method, mechanical linting method, coating method, etc., which is often used is the corona treatment method.
Corona treatment method is the basic principle: through the metal electrode and corona treatment roller (generally high temperature, ozone resistance, high insulation silicone rubber roller) between the application of high-frequency, high-voltage power supply, resulting in a discharge, so that the air ionization and the formation of a large amount of ozone. At the same time, high-energy electric spark impact film surface. Under their joint action, the surface of the plastic film to produce activation, surface energy increases. Through the corona treatment can make the polyolefin film wet tension increased to 38 dyne / cm; can make the polyester film surface wet tension of 52-56 dyne / cm or more. Corona treatment of plastic film surface wet tension of the size of the voltage applied to the electrode, the electrode and the distance between the corona treatment rollers and other factors. Of course, corona treatment should be moderate, not the higher the intensity of corona treatment is better. Here it is worth noting that the plastic film and corona treatment rollers should be avoided between the air entrapment, otherwise it is possible to make the reverse side of the film is also corona treatment. Reverse corona caused by the consequences of: 1 may produce ink printing anti-sticking phenomenon; 2 in the aluminum plating will occur in the transfer of aluminum plating layer, in the adhesive coating will occur in the transfer of adhesive coating layer. The main measure to prevent the reverse side of the film corona is to adjust the pressure of the rubber pressure roller before the corona treatment rollers, pressure on both ends of the pressure roller should be consistent and the size of the pressure should be appropriate. In addition, the corona roller and the pressure roller shall carry out strict static and dynamic balance test, radial runout requirements of less than 0.05 mm, the purpose is to ensure that the plastic film flat into the corona roller, to prevent air entrapment, thus avoiding the phenomenon of reverse corona.