CORONA TREATMENT IN GENERAL
Corona is a high frequency electric discharge directed at a surface. It results in an improvement in the chemical connection (dyne/cm) between the molecules in the surface and the applied media/liquid. This surface treatment neither reduces nor changes the strength or appearance of the material.
Plastic is a man-made synthetic material, which contains long homogeneous molecular chains that form a strong and uniform product. The chains of molecules are normally joined end to end forming even longer chains, leaving only a few open chain ends, and providing only a small amount of bonding points at the surface.
During Corona discharge treatment, electrons are accelerated into the surface of the plastic causing the long chains to rupture, producing a multiplicity of open ends, and the formation of free valences. The free valences are then able to form carbonyl groups with the atoms from the ozone created by the electric discharge, which gives improved adhesion.
The adhesion is further enhanced by the cleaning effect of the ozone, by oxidation on the surface of the material.
Whether a liquid wets a material well or poorly depends primarily on the chemical nature of both the liquid and the substrate. Wetting is defined as the ratio between the surface energies of the liquid and substrate.
In general, the following rule is true: ‘A material will be wetted, if its surface energy (dyne/cm) is higher than that of the liquid, and if it is not, there will be an adhesion problem.’
To obtain sufficient wetting and adhesion on plastic films or metallic foils inline, pre-treatment just before the printing, laminating, or coating unit is necessary. The way to optimise wetting and adhesion is to use a Corona discharge unit because this technique has proved to be highly efficient, cost effective, and can take place inline.
The increase in surface energy (dyne/cm) that is induced into a film begins to decay immediately following treatment, with the additional considerations of storage conditions and ambient temperature. Generally speaking, the more difficult a material is to treat, the quicker it is likely to decay. It has been established that film with very high slip additives (over 1200 ppm) can be totally resistant to printing just 24 hours after treatment, and it may be necessary to process the film immediately following treatment, or boost the treatment inline with the printer. Ageing of such films can render them impossible to treat if they are not treated during production.
Dyne is a unit of measurement used to determine the surface energy of a film or foil. Use the test method described by DIN 53364 and ASTM D2578-67, which is a liquid test, to carry out the measurement of the surface energy of a film. The liquid is applied as a broad line in a thin layer with either a brush or a pen.
The surface tension is then determined visually by estimating how the liquid reacts within the first two seconds following application. The test liquid can shrink and/or form itself into globules (individual droplets) or it can remain unchanged.
When a test liquid shrinks or forms into droplets it indicates that the film has a lower surface energy than the liquid applied. The test should be repeated as many times as necessary with a liquid of a lower surface tension until it remains unchanged for a period of two seconds or more. Once this has been achieved, the film can be said to have a level of surface energy at least equal to that of the liquid applied. Further applications should be made until shrinkage or droplets occur within two seconds. This last measurement should be taken as failure, and the surface energy of the liquid used for the previous measurement should be taken to be the surface energy of the film.
Ozone is a gas that is formed during the process of Corona discharge, and plays an important part in the chemical and molecular changes that take place in the surface of the plastic. Ozone is hazardous to health and has an unpleasant smell, even at relatively low levels. The average human can smell ozone at 0.01 ppm.
The safety threshold in an area occupied by personnel is 0.1 ppm. All Vetaphone equipment is designed to remove ozone effectively from the treater station by means of an ozone-protected blower/fan, and a specially designed exhaust system.
When treating paper with Corona you will get the benefits of cleaning the surface and removing any unwanted paper fibres that could create pin holes during the coating process.
The treatment level lasts for a minimum of six months. The only way to measure the dyne level is by using a water drop measurement.
Vetaphone is able to offer equipment for ATEX environments; however this must be designed and calculated according to customer requirements. Normally, Vetaphone offers pressurised units which also prevent solvents from reaching the Corona discharge.
The ceramic electrode is used for treatment of both electrically conductive (aluminium foil and metallised film) and non-conductive substrates (PE, PP, etc.) in widths of 0.1 m to 3 m. Each individual ceramic bar can take a load of approximately 1.25 kW/meter, and with a maximum of six tubes in one electrode cassette, the total output can be 7.5 kW/m. The electrodes consist of square section ceramic tubes, with metal inner conductors. They are mounted in special insulators, and each tube can be changed individually, with provision for expansion, ensuring uniform treatment of the film.
Unlike metallic electrodes, a ceramic electrode does not need a dielectric covering on the backing roller. This eliminates downtime associated with changing defective sleeves or coverings.
For correct maintenance of the station it is important that it is cleaned thoroughly once a month. When an insulator or other part applied to high voltage is covered with dirt, there is a big risk of an electrical flash-over in the station, which results in a breakdown. It is necessary to change the insulator after a flash-over has occurred.
Before start-up, it is necessary to check that the ceramic electrode bars and electrode holders (insulators) are clean and free from grease and dirt. Cleaning of the electrodes should be given special attention, especially cleaning between the ceramic bars. An accumulation of dirt will prevent an effective cooling of the bars, resulting in overheating, which again causes the ceramic surface to crack. This leads to machine
stop, and the defective ceramic bars will have to be replaced before production can start again.
Electrodes covered with dirt will result in an uneven Corona treatment of the film.
It is very important to keep bars and insulators clean. Cleaning can be done with spirits/alcohol.
Check out this quick video guide on how to clean the cartridge and electrodes:
The NIP is a pressure roller that is intended for use at speeds above 350m/min or with low web tension. The NIP roller will add pressure to the material passing though the treater station preventing pin holes or backside treatment. Remember that you cannot add a NIP roller on a silicone sleeve backing roller as this will be torn off.
The purpose of Tendency Drive is to help the film turn the backing roller and to avoid stretching or wrinkling a thin film when passing through the station. An AC-motor is mounted on the endplate of the station and a toothed timing belt drives the backing roller. The supplied control box is equipped with a main switch and a potentiometer for adjusting the torque for the motor. The film controls the rotation of the backing roller while the Tendency Drive motor helps the film to overcome the inertia of the roller and ball bearings.
The purpose of Synchronised Drive is to match the backing roller’s speed exactly to the speed of the rest of the production line to avoid stretching or wrinkling a thin film when passing through the Corona treater station.
A DC-motor is mounted on the endplate of the station and a toothed timing belt, between the pulley on the motor shaft and the roller shaft, drives the backing roller.
A tacho-generator is integrated on the motor which supplies the electronic controller with information about actual speed. From an external tacho-generator, which is driven by another roll in the production line, the signals are fed to the controller and compared with signals from the motor-tacho, which allows the motor speed and the backing roller to be synchronised with the main machine.
In order to leave untreated areas at welding points, it is essential that the Corona system is able to make intermittent treatment on printing lines. Vetaphone Skip Control is designed for this purpose.
The Vetaphone Skip Control operates with input from a roto pulse sensor connected to the printing cylinder. Using impulses from the sensor as a reference, the position and the length of the treated area can easily be adjusted even during production.
Adjustment can be made for one or two generators (A+B) and is done by setting four parameters in steps from 1 to 360 degrees of the circumference of the print cylinder, or alternatively in millimetre steps.
2.5mm, 5mm and 10mm width in aluminium.
Between 1.5–1.8mm, depending on the application and material.
Backside treatment occurs when both sides of the material are Corona treated instead of just the one side intended.
Unwanted treatment on the ‘wrong’ side can result in blocking problems, making it impossible to unwind the material, or creating pin holes in the material. In addition, unintentional treatment of the ‘wrong side’ can cause poorer efficiency on the correct side as a result of lower dyne levels.
The most common reason for backside treatment is build-up of dirt on the backing roller. To avoid this, simply clean the roller according the maintenance plan.
Another cause could be an increase in line speed. As the line speed increases (above 350m/min) there will be more air travelling with the material which can result in small air bubbles being created between the material and the roller. The air in this gap ionises and produces backside treatment. The use of a NIP roller can prevent this problem occurring because it ensures the film is wrapped perfectly around the roller and does not leave any spots with air bubbles.
The last major problem of backside treatment is the creation of wrinkles.
This depends very much on your raw material supplier. If they change the recipe then they also change the composition of the material. Each time you change your raw material supplier, remember to check the dyne level and adjust the material factor accordingly.
Keeping the correct air gap is a very important factor in obtaining the best performance. With the wrong air gap you risk having lower dyne levels or even different dyne levels across the web.
In general, Vetaphone has three types of treater station designs, each with its own way of adjusting the air gap settings. Please refer to the manual supplied with your equipment for further explanation.