Corona treatment is widely used for the surface treatment of plastics, paper and metal foils to improve the adhesion of inks, paints, adhesives and paints. The reason why it is widely adopted is that it is effective, controllable and easy to master. This is especially true because of the continuous improvement in the efficiency and effectiveness of corona treatment equipment, and keeping pace with high-production processing equipment.
First, wettability and adhesion
Corona treatment is an effective means of changing the surface properties of many non-absorbent substrates, enabling the ink to have better wetting and adhesion properties, so-called wettability, which is what plastics, such as inks, primers, coatings, etc. Paper or foil attached to the foundation. Wetability determines the ductility of a liquid on a solid surface. Water droplets spread on the hydrophilic surface into a thin layer of water, while small droplets form on the lipophilic surface. The contact angle (angle) formed between the tangent of the outer surface of the water droplet and the solid surface indicates the wettability of the surface, and the greater the contact angle, the worse the wetting property. Wettability varies depending on chemical composition and surface structure. When the plastic is printed or hot-foiled, the wettability of the plastic surface must be higher than the wettability of the ink or foil, otherwise it will be difficult to extend, transfer and attach. The surface properties of several polymers are roughly as follows:
Polyethylene (PE) 31-33mN/m Polypropylene (PP) 29-30mN/m Polyester Polyester (PET) 4142mN/m
The surface energy usually used as an ink solvent is: Ethanol 22mN/m, Ethyl acetate 24mN/m, and water 72mN/m, it is difficult to wet plastics, so water-based inks are generally not used to print plastics.
Plastic is a composite material containing one or more polymers and various additives such as fillers, antioxidants, lubricants, antistatic agents, pigments, and the like. Although the chemical structure (group) of the host polymer of the plastic determines the wettability and adhesion of the ink or primer, its additives will migrate from the interior of the plastic to the surface, affecting the surface energy of the plastic. Therefore, the longer the plastic is stored, or the higher the content of certain additives (such as lubricants), the greater the change in surface energy. Therefore, the surface energy of the plastic must be tested before printing, and corona treatment, in order to avoid the transfer or adhesion problems. In general, 20-40 kilohertz IF processing is mostly sufficient. Special treatments (such as complex products) can be handled with specific electrodes.
UV inks require higher surface energy for plastic films than solvent inks. Water-based ink systems also require high surface area for high alcohol content, and require a narrow range of adhesion for good adhesion.
When printing polyethylene with different types of inks; the surface energy of polyethylene is required to be not less than the following:
Solvent-based ink 38-42mN/m UV ink 4246mN/m Water-soluble ink 4248mN/m Water-based ink 38-42mN/m
For elbows coated with polyethylene, the surface energy of polyethylene is required to be not less than the following:
Solvent-based adhesive 38-42mN/m uV, paint 42-46mN/m emulsion 42-48mN/m solvent-free adhesive 4448mN/m
Second, the wettability and adhesion test
In industrial practice, the determination of the surface energy of a polymer is performed by testing the ink in accordance with DINIS 08296 or ASTM D 2578-99a.
According to the DINIS08296 method, an ink about 100 mm long is brushed with an ink having a known surface energy and a film of about 100 mm in length is drawn on the film to be measured, and whether more than 90% of the edge of the ink strip is contracted and ink droplets are formed within 2 seconds is observed. If there is, then the ink with a lower level of surface energy is used to re-ink the ink and the same observation is performed until no shrinkage or ink drop occurs. The surface energy of the test ink corresponds to the surface energy of the film. This method is a method for testing the intermediate value of the surface energy, which is small and does not count.
According to the ASTM D2578-99a method, a block of about 25 x 25 mm is coated with a cotton pad tester and the same method as described above is used to observe, and the lowest surface energy value of the film is measured. This kind of test method is difficult to grasp due to the thickness uniformity of the ink layer, its accuracy is inferior to DINISO 8296 method, the error of DINIS08296 is in the range of ±1mN/m, and the error of ASTMD2578-99a is about ±2mN/m. Therefore, in the industrial practice, the DIN S08296 method is used more often and is easier and faster. (In China, there is national standard GBl0003-88 for the test method of corona treatment effect.) However, no matter which method is used, the same Sofial test ink can be used. There are twenty to twenty-nine surface-level test inks of 30 to 72 mN/m. (each difference is 2mN/m). Dyne test pen (38mN/m) can be used as a rapid test tool for surface energy after corona treatment, but it is not suitable as a system test for printed or coated surfaces. When the test pen draws a line on the corona-treated surface, if it is continuously lined, it means that the surface energy of the material is not less than 38mN/m, such as intermittently not connected, indicating that the surface energy of the material is less than 38mN/ m. Insufficient or even untreated, not meeting the printing process requirements. If the plastic film contains a large amount of additives or is coated with a coating film, chemical reactions tend to occur when testing with the above test ink or test liquid, which affects accuracy. In this case, distilled water is recommended for contact angle testing. The adhesion of paints, coatings, inks, etc. can be quickly measured and can be used as a tape adhesion test according to ASTM D3359-97.
Third, corona treatment
The corona treater consists of an electrode, a high potentiometer, and a take-off guide roller. When the voltage exceeds the ionization resistance of the bull gap of 1-2 mm, continuous discharge occurs, and the discharge is uniformly distributed due to the dielectric on the guide roller. The electrodes are housed in a hood to prevent contact. In order to reduce the temperature and remove the generated ozone, an exhaust fan was used to blow out the air near the corona processor. However, in order to prevent the ozone from being emitted, the exhaust must first pass through an air purifier.
Corona treatment increases substrate adhesion by the following principles:
• Removal of absorbed atoms and molecules on the surface.
Promotes atomic contact and enhances wetting.
• Improve surface energy and adjust polarity.
· Create atoms or functional groups that can react chemically.
The physical and chemical effects of corona treatment on the plastic surface are complex and its effects are mainly controlled by: aspects, a specific electrode system, 2; dielectric on the guide roller, 3, specific electrode power.
As for the speed of take-up, the width of the roll, and the variety of plastics, only the motor power is adjusted, and some are controlled by the door, so the repeatability of the effect is guaranteed. The role of corona treatment is:
· Electrons released from the electrode are accelerated by the high pressure and rushed toward the roll.
· Electrons collide with air molecules to produce some ozone and nitrogen oxides.
• After the electrons collide with a plastic film (for example, polyethylene), the carbon chain or carbon chain is broken.
· The air affected by corona reacts with these radicals, mainly oxidation.
Because different chemical structures have different atomic bonds, the effect of plastic corona treatment also varies depending on the chemical structure of the plastic. Different plastics require corona treatment with different strengths. It has been confirmed that the structural state of BOPP film will change after production. In a few days, the polymer will change from amorphous to crystalline, which will affect the effect of corona treatment. After corona treatment, the functional group of the plastic surface layer has high mobility. In storage, many plastics have the negative effect of corona treatment on the adhesion. actual. Upper relative humidity also affects the effect of corona treatment, but in general it is not serious. If electromechanical halo treatment is used, it does not need to be considered. To achieve a certain surface energy of the plastic surface treatment, it is necessary to reach a certain amount of corona treatment (D), the formula is: D = P ÷ (CBXV)
D = corona throughput P = motor power (W) CB = corona treatment width (m) V = roll speed (m/min)
In general, the corona processor is set to the highest power required, and for lower-threshold films, the power is adjusted down. The effect of corona treatment has a great relationship with the design of the electrode. Multi-piece electrodes perform best (eg, Softal's patent). This system is characterized by the ability of the corona treatment to be spread out through parallel rows of electrode pads. In the case of thermal expansion, the electrode sheets can move without changing the gap of the pole pieces. Another advantage is that long continuous discharge channels can be avoided due to the uniformity of the discharge. According to comparison, the multi-sheet electrode may be 5 to 10 mN/m higher than that of a general metal electrode such as a monolithic electrode or a U-shaped electrode. Moreover, after the treated plastics have been stored for one month, the former has a weaker surface energy attenuation than the latter.