The surface tension of a plastic film depends on the free energy of the surface of the plastic film, which in turn 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, typically around 30 dynes/cm.
In theory, if the surface tension of the object is less than 33 dynes/cm, ordinary inks or adhesives cannot adhere firmly, so the surface must be treated. Polyester (PET, PBT, PEN, PETG) is a polar polymer with a high surface free energy and a surface wet tension of 40 dynes/cm or more. However, for high-speed color printing or to increase the bonding between the vacuum-aluminized layer and the surface of the BOPET film, it is also necessary to surface-treat the BOPET film to further increase the surface wet tension.
The methods for surface treatment of plastic film include corona treatment, chemical treatment, mechanical roughing, coating, etc., and the most commonly used is corona treatment.
The basic principle of the corona treatment method is: by applying a high-frequency, high-voltage power source between the metal electrode and the corona treatment roller (generally a high-temperature resistant, ozone-resistant, high-insulation silicone rubber roller), so that discharge is generated, thus The air ionizes and forms a large amount of ozone. At the same time, high-energy electric sparks impact the surface of the film. Under their joint action, the surface of the plastic film is activated and the surface energy is increased.
The wet tension of the polyolefin film can be increased to 38 dynes/cm by corona treatment; the surface wet tension of the polyester film can be made 52-56 dynes/cm or more. The surface tension of the corona-treated plastic film is related to the level of the voltage applied to the electrode, the distance between the electrode and the corona treatment roller, and the like. Of course, the corona treatment should be moderate, not the higher the corona treatment strength, the better.
It is worth noting here that air should be trapped between the plastic film and the corona treatment roller, otherwise the reverse side of the film may be corona treated. The consequences of the reverse corona are: 1 it is possible to produce the anti-stick phenomenon of ink printing; 2 when the aluminum plating occurs, the aluminum layer transfer occurs, and the rubber layer transfer occurs when the glue is applied.
The main measure to prevent the corona on the reverse side of the film is to adjust the pressure of the rubber pinch roller before the corona treatment roller. The pressure at both ends of the pinch roller should be consistent and the pressure should be appropriate. In addition, the corona roller and the pinch roller must undergo strict dynamic and static balance test, and the radial runout requirement is less than 0.05 mm. The purpose is to ensure that the plastic film enters the corona roller smoothly and prevents the air from being trapped, thereby avoiding the phenomenon of reverse corona.