The surface tension of plastic films depends on the size of the free energy of the plastic film surface, 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 a small surface free energy and low surface wet tension, generally around 30 dynes/cm. Theoretically, if the surface tension of the object is below 33 dynes/cm, ordinary inks or adhesives will not adhere firmly to it, 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 colour printing or to increase the bond between the vacuum aluminised layer and the BOPET film surface, surface treatment of BOPET film is also required to further improve its surface wet tension.

Corona treatment, chemical treatment, mechanical linting, coating, etc., of which the most commonly used is the corona treatment method.

By applying a high-frequency, high-voltage power supply between the metal electrode and the corona treatment roller (generally a high-temperature, ozone-resistant, highly insulating silicone rubber roller), an electrical discharge is generated, which then ionises the air and forms a large amount of ozone. At the same time, high-energy electric sparks impact the film surface. Together, they produce activation and an increase in surface energy on the surface of the plastic film. The wet tension of polyolefin films can be increased to 38 dynes/cm by corona treatment; the surface wet tension of polyester films can be increased to more than 52-56 dynes/cm. The surface wet tension of corona-treated plastic films depends on the voltage applied to the electrodes, the distance between the electrodes and the corona treatment rollers and other factors. Of course, the corona treatment should be moderate and not the higher the intensity of the corona treatment the better. It is worth noting here that air should be avoided between the plastic film and the corona treatment rollers, otherwise the opposite side of the film may also be corona treated. The consequences of corona on the reverse side are: 1) the possibility of back-sticky ink printing; 2) the transfer of the aluminium layer in the case of aluminium plating and the transfer of the adhesive layer in the case of adhesive coating. 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, which should be consistent and of the right size at both ends. In addition, the corona roller and the compression roller must be subjected to a strict static and dynamic balance test, with a radial runout of less than 0.05 mm, in order to ensure that the plastic film enters the corona roller smoothly and prevents the entrapment of air, thus avoiding the phenomenon of reverse corona.