Plastic film is widely used in the packaging field. 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 is required for plastic films, and multi-layer composite or vacuum aluminum plating is required for food packaging. Therefore, it is required that the surface free energy of plastic film should be high and the wet tension should be large, in order to facilitate the strong adhesion of printing ink, adhesive or aluminum coating to plastic film; In the process of plastic film production and high-speed packaging, it is required that the film surface has certain frictional properties to prevent film adhesion or slipping; When used for packaging of electrical and electronic products, it is required that the film has certain anti-static properties, etc.

Surface tension of plastic film

The surface tension of the plastic film depends on 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), are non-polar polymers with low surface free energy and low surface wet tension, typically around 30 dynes per centimeter. In theory, if the surface tension of an object is below 33 dynes per centimeter, ordinary ink or adhesive cannot adhere firmly, so its surface must be treated. Polyester (PET, PBT, PEN, PETG) are polar polymers with high surface free energy and a surface wet tension of over 40 dynes per centimeter. However, for high-speed color printing or to increase the binding force between the vacuum aluminum coating and the surface of BOPET film, BOPET film also needs surface treatment to further improve its surface wet tension.

The methods for surface treatment of plastic films include corona treatment, chemical treatment, mechanical roughening, coating, etc. Among them, corona treatment is commonly used.

The basic principle of corona treatment method is to apply high-frequency and high-voltage power supply between the metal electrode and the corona treatment roller (usually high-temperature, ozone resistant, and highly insulated silicone rubber roller), causing discharge, thereby ionizing the air and forming a large amount of ozone. At the same time, high-energy electric sparks impact the surface of the thin film. Under their combined action, the surface of plastic films is activated and the surface energy increases. The wet tension of polyolefin films can be increased to 38 dynes/cm by corona treatment; The surface wet tension of polyester film can reach over 52-56 dynes/cm. The magnitude of the surface wet tension of the corona treated plastic film is related to factors such as the voltage applied to the electrode and the distance between the electrode and the corona treated roller. Of course, corona treatment should be moderate, not 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 negative corona are: 1. There is a possibility of ink printing sticking back; During aluminum plating, there will be transfer of the aluminum coating layer, and during adhesive coating, there will be transfer of the adhesive layer. The main measure to prevent corona on the reverse side of the film is to adjust the pressure of the rubber compression roller in front of the corona treatment roller. The pressure at both ends of the compression roller should be consistent and appropriate. In addition, the corona roller and compression roller must undergo strict dynamic and static balance tests, with a radial runout requirement of less than 0.05 millimeters, in order to ensure that the plastic film enters the corona roller smoothly, prevent air from being trapped, and thus avoid the occurrence of reverse corona.