Biaxially-stretched polyester film (BOPET) has excellent comprehensive properties, its high mechanical strength, good optical properties, wide use temperature, excellent barrier properties, oil resistance, corrosion resistance, etc., so its application is very extensive. BOPET film is most commonly used in printing, compounding, vacuum aluminum plating and other applications. PET resin is a polar polymer material. In general, the surface properties of BOPET, such as printing, lamination, and vacuum aluminum plating, have met the process requirements. However, for high speed, shallow screen printing, in order to improve the fastness of the printing ink and to increase the adhesion of vacuum aluminum plating, some requirements on the surface properties of the BOPET film still have to be raised, because these surface properties will affect the BOPET film. Printing and aluminizing effect. The requirements for surface properties of BOPET films for printing and aluminum plating are briefly described below and are for reference only.
First, the surface tension of BOPET film
When the BOPET film is not surface-treated, its surface tension is more than 40mN/m, but in order to further improve the bonding force between the printing ink or the vacuum aluminizing layer and the surface of the BOPET film and increase the adhesion fastness, it is often necessary to perform the BOPET film. Surface treatment. Surface treatment methods for plastic films include: corona treatment, chemical treatment, mechanical hair-dyeing, coating, flame, etc. The most commonly used method is corona treatment.
The basic principle of the corona treatment method is to apply a high-frequency, high-voltage power supply between a metal electrode and a corona treatment roll (usually a high-temperature, ozone-resistant, highly-insulating silicone rubber roll) so as to generate a discharge, so that The air ionizes and forms large amounts of ozone. At the same time, high-energy EDM strikes the film surface. Under their joint action, the surface of the plastic film is activated and the surface energy is increased. By corona treatment, the surface wet tension of BOPET film can reach 52~56 mN/m or even higher. The corona treatment of the surface of the plastic film wet tension and the level of the voltage applied to the electrode, the electrode and corona treatment roller and other factors. Of course, the corona treatment should be moderate, and the higher the corona treatment intensity, the better. It is worth mentioning here that the trapping of air between the PET film and the corona treatment roll should be avoided, and if air is trapped between them, it is possible to corona-treat the opposite side of the film. The consequences of the back side corona are: 1) Anti-sticking phenomenon may occur in ink printing; 2) Aluminum plating transfer occurs during aluminum plating, and transfer of the adhesive layer occurs during the coating process. These are what we least expect to happen. 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 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 requirement is less than 0.05 millimeters. The purpose is to ensure that the PET film enters the corona roller flatly to prevent air entrapment, thereby avoiding the phenomenon of reverse corona. .
The problem of corona treatment is time-efficient. This is known to everyone. Especially in the summer of high temperature and high humidity, the wet tension of the surface of the plastic film after corona treatment is attenuated more severely. Therefore, the plastic film is preferably printed or aluminized in time after the corona treatment. If the PET film is left for a long time after corona treatment, the wet tension of the surface will gradually decrease. Even if the surface wet tension of the printing surface and the non-printing surface tends to be the same, the printed film surface is prone to ink reversal after coiling. Stick to the non-printing surface, the so-called "anti-stick" phenomenon. In order to prevent "anti-adhesion", on the one hand, the ink must be completely dried during film printing; on the other hand, it is necessary to ensure that the printing surface has enough surface wet tension so that the difference between the wet tension of the surface of the printing surface and the non-printing surface is as large as possible.
The coating method is also another effective way to improve the surface properties of BOPET film. The so-called coating method is to apply a layer of a polymer solution such as a polyacrylate or a modified polyester on a surface of a thin film. For BOPET film, the coating process is: In-line coater between longitudinal draw machine (MDO) and horizontal draw machine (TDO), longitudinally stretched PET film immediately passed in-line coater The polymer solution coating is applied, and then TDO is transversely stretched. After the solvent of the polymer solution is dried and volatilized in the horizontal drawing machine, a layer of coated high molecular polymer is left on the surface of the PET film. Experiments show that after the surface of BOPET film is coated with modified polyester coating, the wet tension of the film surface can be increased to 58 mN/m or more. In addition, a major advantage of the coating method is that the wet tension of the film surface is not attenuated by the high-temperature, high-humidity climatic conditions, and the water resistance and the solvent resistance are excellent.
After the aluminum coating is coated on the coating film, the adhesion is strong, and the aluminum plating layer delamination phenomenon does not occur due to the contact with the aqueous solution at a high temperature.
It is very adaptable when printed on a coating film coating. However, attention should be paid to the compatibility of the coating layer with the printing ink and the solvent resistance of the coating layer.
Second, the surface roughness of BOPET film
The surface of the pure BOPET film is very smooth, smooth surface will produce adhesion when the film is rewinded, it can not be normal rewinding, and it is not easy to unwind. At the same time, the smooth film surface is also very unfavorable for ink printing and vacuum aluminum plating, because the smooth surface will greatly reduce the adhesion between the ink or the aluminized layer and the BOPET film, including the adhesion between the adhesive and the aluminum foil and the BOPET film. . In order to increase the surface roughness of the PET film to increase its bonding strength with other substances, an anti-blocking agent is generally added to the PET resin to form a rough surface on the surface of the PET film. degree. The surface roughness of the film is related to the type of additive (anti-blocking agent), the amount of additive added, the particle size and shape of the additive, the dispersibility of the additive, and the surface treatment of the additive. Common additives are: SiO2, TiO2, CaCO3, A12O3, MgO, BaSO4, kaolin and so on. Different additives are used depending on the use of BOPET film. With the increase of additive content in BOPET film, the friction coefficient μs of the film decreases, and the surface roughness increases.
The surface roughness refers to the comprehensive evaluation of the micro-geometric features of the micro-peak-to-valley roughness on the smaller surface of the film. Surface roughness (usually often referred to as surface finish) is characterized by Ra, Rz, Rt, Ra (contour mean arithmetic deviation); the average of the sum of the distances from the points on the contour being measured to the midline of the contour over the measured length: Rz (average height of roughness): the average of the five highest peaks and the five lowest peaks on the contour line within the measured length; Rt (height from the peak to the valley): the largest on the contour line within the measured length Peak value (from peak to valley height).
Appropriate surface roughness facilitates ink printing and vacuum aluminum plating, which is positive. Of course, if the roughness is too large, the ink or aluminum molecules may not fill up the surface of the film and cause the gaps to form voids to affect the adhesion between the two. In severe cases, the ink or the aluminum layer may be separated from the film. General control Ra=0.08~0.16.
Third, the coefficient of friction of BOPET film surface
In the production of plastic films and plastic packaging bags, the friction coefficient of plastic films is an important technical indicator. On the one hand, it becomes the quantitative evaluation index of the opening of plastic film together with the anti-blocking property of the film. On the other hand, it can be used as one of the reference data for the running speed, tension adjustment and abrasion of the film during the operation of the automatic packaging machine.
In the process of printing and aluminum plating, the friction coefficient of the plastic film also has certain requirements. The coefficient of friction on the surface of the film is linear with the roughness of the surface. Under certain conditions, the greater the surface roughness, the smaller the coefficient of friction. In other words, reducing the coefficient of friction on the surface of the film is advantageous for printing and aluminum plating, and it is advantageous to increase the bonding surface between them and the plastic film, which is advantageous for improving the adhesion between them. General requirements of the friction coefficient is about 0.4. The coefficient of friction is also regulated by additives.
Fourth, the cleanliness of BOPET film surface
During the production and use of the film, the surface of the film is prevented from being contaminated by the outside world. Otherwise, the adhesion of the ink or the aluminized layer to the surface of the film is also affected.
1) Oil contamination ———When PET enters TDO horizontally, if the chain clamp/rail seal is not good or the lubricant is excessive, oil mist will fall on the surface of the film and cause oil pollution at high temperature. Oil pollution is a taboo for printing and aluminizing.
2) Low-molecular-weight material --- PET will produce some low-molecular volatiles during processing, and this oligomer will condense into powder when it is cold. In the cooling section of the TDO horizontal puller, the oligomer powder is easily deposited on the surface of the film, or the oligomer powder is dropped after the long-term accumulation of the cooling section to cause contamination of the film.
3) Other pollution—for example, in the corona treatment of PET film, electrodes and ozone exhaust hoods are generally set on the top, when the mildew season or the relative humidity of air is high, above the corona treatment roll Condensate beads are easily formed on the inner and outer surfaces of stainless steel exhaust hoods. These may also contain low-molecular-weight and oily condensed water beads. They may occasionally drip on the surface of the PET film to form white gels and cause contamination. This kind of pollution causes great troubles for printing and aluminum plating and must be managed to overcome it.
Author: Feng Shuming Source: Jiangyin payment of New Materials Co., Ltd.
Printing, Aluminizing the Surface Properties of BOPET Films