Wetting tension is generally referred to as surface tension, which is a manifestation of molecular force, and is usually referred to as wetting tension for thin films. Wetting tension is the tension acting on any boundary along the surface of the liquid surface layer due to the unbalanced molecular gravity. The international unit is N/m, but usually we use dyn/cm (dyne) to express, 1dyn/cm= 1mN/m.

BOPET (biaxially oriented polyester film) has excellent comprehensive properties, so its application fields are very wide, and the most common are in printing, lamination, vacuum aluminum plating, etc. When the BOPET film is not surface-treated, its surface tension is above 40mN/m, which can already meet the general printing requirements. , It is often necessary to perform surface treatment on the BOPET film to increase the wetting tension of the BOPET film, thereby enhancing the printing and aluminizing effect of the BOPET film. The methods of surface treatment of plastic films include: corona treatment, chemical treatment, mechanical roughening, coating, flame, etc., among which corona treatment is the most commonly used.

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, high insulation silicone rubber roller) to generate a discharge, so that the The air is ionized and large amounts of ozone are formed. At the same time, high-energy sparks impinge on the surface of the film. Under their combined action, the surface of the plastic film is activated and the surface energy is increased. Through corona treatment, the wetting tension of BOPET film can reach above 52-56mN/m, or even higher.

The wetting tension of corona-treated plastic film is related to the level of voltage applied to the electrode, the distance between the electrode and the corona-treated roller and other factors. Of course, the corona treatment should be moderate, not the higher the corona treatment intensity, the better. It is worth mentioning here that air should be avoided between BOPET and the corona treatment roller. If air is caught between them, it is possible that the reverse side of the film will also be corona treated.

The consequences of the corona on the reverse side are: first, there may be a reverse sticking phenomenon in ink printing; second, the transfer of the aluminized layer will occur when aluminized, and the transfer of the adhesive layer will occur during gluing.

The main measure to prevent corona on the reverse side of the film is to adjust the pressure of the rubber pressure roller in front of the corona treatment roller. The pressure at both ends of the pressure roller should be consistent and the pressure should be appropriate. In addition, the corona roller and the pinch 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 BOPET enters the corona roller smoothly and prevents air from being trapped, so as to avoid the phenomenon of reverse corona.

Corona treatment has an aging problem, especially in summer with high temperature and high humidity, the wetting tension of plastic film after corona treatment is relatively attenuated. Therefore, it is best to print or aluminize the plastic film in time after corona treatment. If BOPET is left for too long after corona treatment, the wetting tension will gradually decrease, and even because the wetting tension of the printing surface and the non-printing surface tends to be the same, the printed film surface is prone to reverse sticking of the ink after winding. to the non-printing side, the so-called "anti-stick" phenomenon. In order to prevent "anti-sticking", on the one hand, the ink should be completely dried during film printing; on the other hand, it is necessary to ensure that the printing surface has sufficient wetting tension, so that the difference between the wetting tension between the printing surface and the non-printing surface is as large as possible.

Coating method is also another effective way to improve the surface properties of BOPET. The so-called coating method is to coat a layer of a certain polymer solution on the surface of the film, such as a coating of a polymer solution such as polyacrylate and modified polyester. For BOPET, the coating process is: between the longitudinal drawing machine (MDO) and the transverse drawing machine (TDO), there is an online coating machine, and the longitudinally stretched PET film is immediately passed through the online coating machine. The polymer solution is coated, and then enters the TDO for lateral stretching. After the solvent of the polymer solution is dried and volatilized in the cross-drawing machine, a layer of the coated polymer is left on the surface of the BOPET.

Experiments show that the wetting tension of the film can be increased to more than 58mN/m after the modified polyester coating is applied on the surface of BOPET. Moreover, a major advantage of the coating method is that the film wetting tension will not be attenuated due to high temperature and high humidity climatic conditions, and it has excellent water and solvent resistance. After aluminized on the coating of the coated film, the adhesion is strong, and the delamination of the aluminized layer will not occur due to contact with the aqueous solution at high temperature. Good adaptability when printing on top of a coated film. However, attention should be paid to the compatibility of the coating layer with the printing ink and the solvent resistance of the coating layer.