Biaxially oriented polyester film (BOPET) has excellent comprehensive properties, such as high mechanical strength, good optical properties, wide application temperature, barrier property, oil resistance and corrosion resistance, so it is widely used. BOPET film is widely used in printing, laminating and vacuum aluminizing. PET resin is a polar polymer material. Generally speaking, the surface properties of BOPET, such as printing, laminating and vacuum aluminizing, can meet the technological requirements. However, for high-speed shallow screen printing, in order to improve the fastness of ink and increase the adhesion of vacuum aluminizing, there are still some requirements for the surface properties of BOPET film, because these surface properties will affect the printing and aluminizing effect of BOPET film. The requirements of printing and aluminizing on the surface properties of BOPET film are briefly described as follows for reference only.
Surface tension of biaxially stretched polyester film
Without surface treatment, the surface tension of BOPET film is above 40mn/m, but in order to further improve the adhesion between ink or vacuum aluminized layer and BOPET film surface and increase the adhesion fastness, BOPET film often needs surface treatment. The surface treatment methods of plastic film include corona treatment, chemical treatment, mechanical roughening, coating and flame treatment, among which corona treatment is the most commonly used.
The basic principle of corona treatment is to apply high-frequency and high-voltage power supply between metal electrode and corona treatment roller (generally silicone rubber roller with high temperature resistance, ozone resistance and high insulation) to generate discharge, which ionizes air and forms a large amount of ozone. At the same time, high-energy EDM impacts the film surface. Under their interaction, the surface of plastic film is activated and the surface energy is increased. Through corona treatment, the surface wet tension of BOPET film can reach above 52~56 mN/m, or even higher. The wet tension on the surface of corona treated plastic film is related to 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 mentioning that air should be avoided from entering between PET film and corona treatment roller. If there is air between them, the reverse side of the film can also be corona treated. The consequences of back corona are: ① it is possible to produce anti-sticking phenomenon of ink printing; ② Transfer of aluminized layer will occur when aluminized, and transfer of gelatinized layer will occur when gelatinized. These are the last things we want to happen. The main measure to prevent corona on the reverse side of the film is to adjust the pressure of the rubber press roller in front of the corona treatment roller, and the pressure at both ends of the press roller should be consistent and appropriate. In addition, the corona roller and the pressure roller must undergo a rigorous dynamic-static balance test, and the radial runout is required to be less than 0.05 mm, so as to ensure that the PET film can smoothly enter the corona roller, prevent air from being trapped, and thus avoid the reverse corona phenomenon.
It is well known that corona treatment has the problem of aging. Especially in summer with high temperature and humidity, the surface wet tension of plastic film after corona treatment is seriously attenuated. Therefore, it is best to print or aluminize the plastic film in time after corona treatment. If the PET film is left for too long after corona treatment, the surface wet tension will gradually decrease, even because the surface wet tension of the printed surface and the non-printed surface tends to be the same, the surface of the printed film after winding is prone to ink sticking to the non-printed surface, which is the so-called "anti-sticking" phenomenon. In order to prevent "anti-sticking", on the one hand, the ink should be completely dried when printing the film; On the other hand, it is necessary to ensure that the printed surface has enough surface wet tension, so that the greater the difference between the surface wet tension of the printed surface and the non-printed surface, the better.
Coating is another effective method to improve the surface properties of BOPET films. The so-called coating method is to coat a layer of polymer solution on the surface of the film, such as polyacrylate, modified polyester and other polymer solutions. For BOPET film, the coating process is as follows: an online coating machine is set between the longitudinal drawing machine (MDO) and the transverse drawing machine (TDO). The longitudinally stretched PET film is immediately coated with polymer solution by an online coating machine, and then enters TDO for transverse stretching. After the solvent of polymer solution is dried and volatilized in the transverse stretching machine, a layer of coated polymer is left on the surface of PET film. The experiment shows that the wet tension of BOPET film surface can be increased to more than 58 mN/m after the modified polyester coating is coated on the film surface. Moreover, one of the advantages of coating method is that the wet tension on the surface of the film will not be attenuated due to high temperature and high humidity weather conditions, and the water resistance and solvent resistance are excellent.
After aluminizing on the coating of the coating film, the adhesion is strong, and the phenomenon of delamination of the aluminized layer will not occur due to contact with aqueous solution at high temperature.
It has good adaptability when printing on the coating of coated film. However, attention should be paid to the compatibility of the coating with printing ink and the solvent resistance of the coating.
Surface roughness of biaxially stretched polyester film
The surface of pure BOPET film is very smooth, and the smooth surface will stick when winding, so it can not be wound normally and is not easy to unwind. At the same time, smooth film surface is also very unfavorable to ink printing and vacuum aluminizing, because smooth surface will greatly reduce the adhesion between ink or aluminized layer and BOPET film, including the adhesion between adhesive and aluminum foil and BOPET film. In order to make the surface of PET film have a certain roughness and increase its adhesion with other substances, an anti-blocking agent is usually added to PET resin to make the surface of PET film have a certain roughness. The surface roughness of the film is related to the type and amount of additives (anti-blocking agents), the particle size and shape of additives, the dispersibility of additives, the surface treatment of additives and other factors. Commonly used additives are: SiO2 _ 2 _ 2, TiO2 _ 2, CaCO3 _ 3, A 12o _ 3, MgO, BaSO4 _ 4 _ 4, kaolin, etc. Choose different additives according to different uses 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.
Surface roughness refers to the comprehensive evaluation of micro-geometric size characteristics of micro-peak and valley unevenness at small intervals on the surface of thin films. Surface roughness (formerly called surface smoothness) is chaRacterized by Ra, Rz and Rt, where ra (average arithmetic deviation of contour); Within the measured length, the average value of the sum of the distances from each point on the measured contour to the centerline of the contour: Rz (average roughness height): the average value between the five highest peaks and the five lowest peaks on the contour in the measured length; Rt (peak-valley height): the maximum peak-valley value (peak-valley height) on the contour line within the measured length.
Proper surface roughness is beneficial to ink printing and vacuum aluminizing, which is certain. Of course, if the phase roughness is too large, ink or aluminum molecules may not be able to fill the surface depression of the film, forming gaps, affecting the adhesion between them, and may lead to the separation of ink or aluminum plating layer from the film in serious cases. General control Ra=0.08~0. 16.
Surface friction coefficient of biaxially stretched polyester film
In the production of plastic film and plastic packaging bag, the friction coefficient of plastic film is an important technical index. On the one hand, it becomes a quantitative evaluation index of plastic film opening together with film anti-blocking performance, on the other hand, it can be used as one of the reference data of automatic packaging machine running speed, tension adjustment and film wear in operation.
In the process of printing aluminum plating, there is also a certain requirement for the friction coefficient of plastic film, and the friction coefficient of film surface has a linear relationship with its surface roughness. Under certain conditions, the greater the surface roughness, the smaller the friction coefficient. In other words, reducing the friction coefficient of the film surface is beneficial to printing and aluminizing, increasing the bonding surface between them and plastic films and improving the adhesion between them. Generally, the friction coefficient is required to be around 0.4. The friction coefficient is also adjusted by additives.
Cleanliness of BOPET film surface
In the process of film production and use, it is necessary to prevent its surface from being polluted by the outside world, otherwise it will also affect the adhesion between ink or aluminized layer and the film surface.
1) oil stain-mainly refers to the oil stain caused by oil mist dripping on the film surface at high temperature if the chain clamp/track seal is not good or there is too much lubricating oil when PET enters TDO transverse stretching. Oil pollution is a taboo for printing and aluminizing.
2) During the processing of low molecular weight material PET, some low molecular weight volatiles will be produced, and this oligomer will condense into powder when it is cold. In the cooling section of TDO cross wire drawing machine, this oligomer powder is easy to deposit on the surface of the film, or after long-term accumulation in the cooling section, the oligomer powder falls off and pollutes the film.
3) Other pollution-for example, when PET film is corona treated, electrodes and ozone exhaust hoods are generally placed on it. In rainy season or when the air relative humidity is very high, condensed water droplets are easy to form on the inner and outer surfaces of the stainless steel exhaust hood above the corona treatment roller, and may also be mixed with low molecular weight and oil stains, and occasionally drop on the surface of the PET film to form white glue, causing pollution. This kind of pollution will bring great trouble to printing and aluminum plating and must be overcome.