Plstic Films Adhesion and Corona Treatment
The role of corona treatment is1. 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 (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 corona treatment experience, it is known that the amount of corona treatment of the above 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 a maximum of 25×1.6 m×350 m/min = 14,000 watts and PP interpolymers 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 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.
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