The application of plastic film in the packaging industry* is extensive. Plastic film can be used for food packaging, electrical product packaging, daily necessities packaging, clothing packaging, and so on. They have one thing in common, that is, color printing on plastic films, and multi-layer composite or vacuum aluminum plating as food packaging. Therefore, the surface free energy and wet tension of the plastic film are required to be high in order to facilitate the firm adhesion of printing ink, adhesive or aluminized layer to the plastic film; in the process of plastic film production and high-speed packaging, The surface of the film is required to have a certain friction performance to prevent the film from sticking or slipping; when used in the packaging of electrical appliances and electronic products, the film is required to have a certain anti-static performance and so on.
Surface tension of plastic film
The surface tension of the plastic film depends on the free energy of the surface 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) are non-polar polymers with low surface free energy and low surface wet tension, generally about 30 dyne/cm. Theoretically, if the surface tension of the object is lower than 33 dyne/cm, ordinary inks or adhesives cannot adhere firmly, so the surface must be treated. Polyesters (PET, PBT, PEN, PETG) are polar polymers with high surface free energy and surface wet tension above 40 dyne/cm. However, 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 perform surface treatment on the BOPET film to further increase its surface wet tension.
The surface treatment methods of plastic film include: corona treatment, chemical treatment, mechanical roughening, coating, etc. Among them, the most common method is corona treatment.
The basic principle of the corona treatment method is: by applying a high-frequency and high-voltage power supply between the metal electrode and the corona treatment roller (usually a high-temperature, ozone-resistant, and high-insulation silicone rubber roller) to generate electrical discharge, so The air ionizes and forms a large amount of ozone. At the same time, high-energy electric sparks hit the surface of the film. Under their joint action, the surface of the plastic film is activated and the surface energy increases. The corona treatment can increase the wet tension of the polyolefin film to 38 dyne/cm; the surface wet tension of the polyester film can reach 52-56 dyne/cm or more. The size of the surface wet tension of the corona treatment plastic film is related to factors such as the voltage applied to the electrode and the distance between the electrode and the corona treatment roller. Of course, corona treatment should be moderate, not that the higher the intensity of corona treatment, the better. It is worth noting here that air should be avoided between the plastic film and the corona treatment roller, otherwise the reverse side of the film may also be corona treated. The consequences of the reverse corona are: 1 may produce the anti-stick phenomenon of ink printing; 2 the transfer of the aluminized layer will occur when the aluminum is plated, and the transfer of the glue layer will occur when the glue is applied. The main measure to prevent corona on the reverse side of the film is to adjust the pressure of the rubber pressing roller in front of the corona treatment roller. The pressure at both ends of the pressing roller must be consistent and the pressure must be appropriate. In addition, the corona roller and the pressure roller must undergo a strict dynamic and static balance test, and the radial runout is required to be less than 0.05 mm. The purpose is to ensure that the plastic film enters the corona roller smoothly and prevents air from being trapped, thereby avoiding the phenomenon of reverse corona. .