Abstract Printing on the surface of plastics, paper and metal foil often causes quality problems due to insufficient adhesion. This article briefly introduces the corona treatment principle and method for improving the adhesion of ink to plastics.
Keywords: ink plastic printing surface treatment
I. Introduction
Printing on the surface of plastics, paper and metal foil often causes quality problems because of insufficient adhesion. The most common method of treating surfaces such as plastics to improve the adhesion of inks, paints, adhesives, and coatings is corona treatment.
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, corona treatment of the surface according to the test results, in order to avoid the problem of insufficient transfer or adhesion.
Second, wettability and adhesion
Corona treatment is an effective means of changing the surface properties of many non-absorbent substrates, allowing 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 commonly used in the industry are 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 plastics, so water-based inks are generally not used to print plastics.
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 type adhesive 38-42 mN/m
UV coating 42-46 mN/m
Emulsion 42-48 mN/m
Solventless adhesive 44-48 mN/m
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. In China, there is a national standard GB10003-88 for the corona treatment effect test method for reference.
Regardless of the method used, the same Softal test ink can be used, with test inks of 30 to 72 mN/m of twenty-one surface levels (each differing by 2 mN/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.
Third, the corona treatment method and mechanism
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 plastic surfaces are complex, and their effectiveness is mainly controlled in three ways:
1. Specific electrode system
2, the dielectric on the guide roller
3, the specific electrode power.
Fourth, the role of corona treatment
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 some ozone and nitrogen oxides.
3. After the electrons collide with plastic film (such as polyethylene), they break the hydrocarbon chain or carbon-carbon chain.
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 (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 experience of corona treatment, it is known that the corona treatment amount of the above-mentioned films should be approximately:
PET 5.0 (up to 42 mN/m from 42)
LDPE 7.5 (up to 38 mN/m from 38)
PP copolymer 12.5 (from 40 to 45 mN/m)
PP homopolymer 25.0 (from 39 to 45 mN/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 up to 25×1.6 m×350 m/min = 14,000 watts and the PP interpolymer It is 7000W, LDPE is 4200W, and 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 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: Ink Information