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 to change the surface characteristics of many non-absorbent substrates, can make the ink have better wetting and adhesion properties, the so-called wettability, which is such as ink, bottom The basis for the attachment of plastic, paper, or foil to plastics, coatings, etc. 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 (PE31-33mN/m) Polypropylene (PP29-30mN/m) Polyester Polyester (PET) 41-42mN/m
Typically the surface energy of the solvent as the ink: Ethanol 22 mN / m, ethyl acetate 24 mN / m, and the water is 72 mN / m, it is difficult to wet plastic, the water-based ink for printing plastic generally do not. 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 treated with specific electrodes.
UV ink than solvent inks requires a plastic film has a higher surface energy and high alcohol content of water based ink systems also require high surface capacity, but also requires a narrow range in order to have 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: 38-42 mN/m UV ink: 42-46 mN/m Water soluble ink: 42-48 mN/m
Water based ink: 38-42 mN/m
For polyethylene coating, the surface energy of polyethylene is required to be not lower than the following:
Solvent-based adhesive: 38-42 mN/m UV coating: 42-46 mN/m Emulsion: 42-48 mN/m
Solvent-free adhesive: 44-48 mN/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 D 2578-99a.
According to the DIN ISO 8296 method, an ink about 100 mm long is brushed on the film to be measured with an ink having a different surface energy, and whether more than 90% of the ink strips contract and form within 2 seconds is observed. The ink droplets, if any, are replaced by ink of a lower surface energy level and the ink strips are brushed again for the same observation until no shrinkage or ink droplets 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 D 2578-99a, a block of approximately 25 x 25 mm is applied using a cotton padding test ink, and the minimum surface energy value of the film is measured by the same method as described above. This test method is difficult to grasp due to ink layer thickness uniformity, its accuracy is less than DIN ISO 8296 method, DIN ISO 8296 error is about ± 1 mN / m range, while the ASTM D 2578-99a error is about ± 2 mN /m. Therefore, in commercial practice, the use of DIN ISO 8296 method, a more simple, rapid No matter which method is used, the same are available Softal test ink, the ink has a test 30 to 72 mN / m twenty-one kinds of surface level (each difference is 2mN/m). Dyne test pen (38mN/m) can be used as a quick test tool for surface energy after corona treatment, but it is not suitable as a system test for printed or coated surface. 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. Rapid determination of the adhesion of paints, coatings, inks, etc., can be used for tape adhesion testing according to ASTM D 3359-97.
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:
Removes absorbed atoms and molecules from 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 toward the roll.
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. Within 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, and the migration of additives from the interior to the surface is also a factor that causes the surface energy to drop and affect the adhesion. This negative effect cannot 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 due to the fact that 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 (watts) CB = corona treatment width (m) V = take-off 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 experience of corona treatment, it is known that the corona treatment amount of the above-mentioned films should be approximately:
PET 5.0 (from 45 to 45 mN/m) LDPE 7.5 (from 38 to 45 mN/m) PP interpolymer 12.5 (from 40 to 45 mN/m) PP homopolymer 25.0 (from 39 to 45 mN/m)
Then the power of its corona-treated (P) also can be calculated from P = D × CB × V out, wherein the power requirements of PP homopolymer is at most 25 × 1.6m × 350 m / min = 14,000 watts, while PP copolymer 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 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.
Source: Weinan Printing Network
First, wettability and adhesion Corona treatment is an effective means to change the surface characteristics of many non-absorbent substrates, can make the ink have better wetting and adhesion properties, the so-called wettability, which is such as ink, bottom The basis for the attachment of plastic, paper, or foil to plastics, coatings, etc. 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 (PE31-33mN/m) Polypropylene (PP29-30mN/m) Polyester Polyester (PET) 41-42mN/m
Typically the surface energy of the solvent as the ink: Ethanol 22 mN / m, ethyl acetate 24 mN / m, and the water is 72 mN / m, it is difficult to wet plastic, the water-based ink for printing plastic generally do not. 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 treated with specific electrodes.
UV ink than solvent inks requires a plastic film has a higher surface energy and high alcohol content of water based ink systems also require high surface capacity, but also requires a narrow range in order to have 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: 38-42 mN/m UV ink: 42-46 mN/m Water soluble ink: 42-48 mN/m
Water based ink: 38-42 mN/m
For polyethylene coating, the surface energy of polyethylene is required to be not lower than the following:
Solvent-based adhesive: 38-42 mN/m UV coating: 42-46 mN/m Emulsion: 42-48 mN/m
Solvent-free adhesive: 44-48 mN/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 D 2578-99a.
According to the DIN ISO 8296 method, an ink about 100 mm long is brushed on the film to be measured with an ink having a different surface energy, and whether more than 90% of the ink strips contract and form within 2 seconds is observed. The ink droplets, if any, are replaced by ink of a lower surface energy level and the ink strips are brushed again for the same observation until no shrinkage or ink droplets 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 D 2578-99a, a block of approximately 25 x 25 mm is applied using a cotton padding test ink, and the minimum surface energy value of the film is measured by the same method as described above. This test method is difficult to grasp due to ink layer thickness uniformity, its accuracy is less than DIN ISO 8296 method, DIN ISO 8296 error is about ± 1 mN / m range, while the ASTM D 2578-99a error is about ± 2 mN /m. Therefore, in commercial practice, the use of DIN ISO 8296 method, a more simple, rapid No matter which method is used, the same are available Softal test ink, the ink has a test 30 to 72 mN / m twenty-one kinds of surface level (each difference is 2mN/m). Dyne test pen (38mN/m) can be used as a quick test tool for surface energy after corona treatment, but it is not suitable as a system test for printed or coated surface. 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. Rapid determination of the adhesion of paints, coatings, inks, etc., can be used for tape adhesion testing according to ASTM D 3359-97.
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:
Removes absorbed atoms and molecules from 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 toward the roll.
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. Within 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, and the migration of additives from the interior to the surface is also a factor that causes the surface energy to drop and affect the adhesion. This negative effect cannot 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 due to the fact that 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 (watts) CB = corona treatment width (m) V = take-off 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 experience of corona treatment, it is known that the corona treatment amount of the above-mentioned films should be approximately:
PET 5.0 (from 45 to 45 mN/m) LDPE 7.5 (from 38 to 45 mN/m) PP interpolymer 12.5 (from 40 to 45 mN/m) PP homopolymer 25.0 (from 39 to 45 mN/m)
Then the power of its corona-treated (P) also can be calculated from P = D × CB × V out, wherein the power requirements of PP homopolymer is at most 25 × 1.6m × 350 m / min = 14,000 watts, while PP copolymer 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 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.
Source: Weinan Printing Network