Corona treatment advantages and disadvantages

Corona treatment is widely used for the surface treatment of plastics, paper and metal foils to improve the adhesion of inks, paints, adhesives and coatings. 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) 41-42mN/m

The surface energy usually used as an ink solvent is: ethanol 22 mN/m, ethyl acetate 24 mN/m, and water 72 mN/m, it is difficult to wet the plastic, so the water-based ink is generally not used for printing 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 ink, the surface energy of polyethylene is required to be not lower than the following:

Solvent ink 38-42mN/m

UV ink 42-46mN/m

Water-soluble ink 42-48mN/m

Water base ink 38-42mN/m

For polyethylene coating, the surface energy of polyethylene is required to be not lower than the following:

Solvent type adhesive 38-42mN/m

UV coating 42-46mN/m

Emulsion 42-48mN/m

Solventless adhesive 44-48mN/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 DIN ISO 8296 or ASTM D2578-99a.

According to the DIN ISO 8296 method, an ink about 100 mm long is brushed with an ink of known different surface energy on a film to be measured, and whether more than 90% of the ink stripe sides shrink and form ink droplets 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.

In accordance with ASTM D2578-99a, a block of about 25 x 25 mm was coated with a cotton padding test ink. The same method as described above was used to observe and the lowest surface energy value of the film was measured. This kind of test method is difficult to grasp due to the thickness uniformity of the ink layer, and its accuracy is inferior to the DIN ISO 8296 method. The error of DIN ISO 8296 is in the range of ±1 mN/m, and that of ASTM D2578-99a is about ±2 mN/m. Therefore, in the industrial practice, the DINISO 8296 method is often adopted, and it is simpler and faster. (In China, there is a national standard GB10003-88 for the corona treatment effect test method provisions, for reference.

Regardless of the method used, the same Softal test ink can be used, with a test ink of 30 to 72 mN/m for twenty-one surface levels (each differing by 2 mN/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 air 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 mechanisms:

• 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 three aspects: 1. specific electrode system, 2. dielectric on the guide roller, 3. specific electrode power.

As for the speed of winding, the width of the roll, and the variety of plastics, only the motor power is adjusted, and some are automatically controlled, and the repeatability of the effect is guaranteed.

The role of corona treatment is:

1. The electrons released from the electrode are accelerated by the high pressure and rushed to the scroll.

2. The collision of electrons with air molecules produces part of ozone and nitrogen oxides.

3. After the electrons collide with the plastic film (for example, polyethylene), the carbon chain or carbon chain is broken.

4. The air affected by corona reacts with these radicals, mainly oxidation.

5. Hydroxy groups, ketone groups, ether groups, carbonic acid groups, and esters are all polar groups and are the basis for ink adhesion.

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 cross-linked structure of the plastic surface layer is less than the cross-linked structure of the inner layer, so the functional groups of the surface layer have a higher mobility. Therefore, in the storage, many plastics appear corrosive effect of the corona treatment, additives from the internal to the surface of the migration, but also the surface energy to reduce the impact of adhesion factors, this negative effect can not be completely suppressed.

In fact, relative humidity also affects the effect of corona treatment. Humidity is a depolarizing agent, but in general it is neglected because the effect is not serious and is often within the test error range. If you use electromechanical corona treatment, you don't have to think about it.

To treat the plastic surface to a certain surface energy, the amount of corona treatment (D) needs to reach a certain value, the formula is: D = P ÷ (CB × V)

D = corona throughput P = motor power (W) CB = corona treatment width (m)

V=Rolling speed (m/min)

Example: There is a plastic film printer that prints 1600mm wide film at a speed of 350m/min. There are different plastic films such as PET, LDPE, PP interpolymer, and PP homopolymer. Before the printing process, the surface energy of these films must be processed to 45mN/m or more. According to the corona treatment experience, it is known that the amount of corona treatment of the above films should be approximately:

PET 5.0 (up to 42mN/m from 42)

LDPE 7.5 (up to 38mN/m from 38)

PP copolymer 12.5 (from 40 to 45mN/m)

PP homopolymer 25.0 (from 39 to 45mN/m)

The power (P) of the corona treatment can then also be calculated from P=D×CB×V, wherein the power requirement of the PP homopolymer is a maximum of 25×1.6 m×350 m/min = 14,000 watts, while the PP copolymer is The body is 7000W, LDPE is 4200W, PET is 2800W.

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 be moved 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.