Corona treatment
The corona treatment is an electrochemical process for surface modification of plastics .
Reasons for Corona Treatment
Most plastics have a non-polar, electrically well insulating and water-repellent surface. It is difficult to wet with printing inks , solvents , aqueous plastic dispersions , adhesives or adhesion promoters . This is especially true for polyethylene , polypropylene and polyester films . The printing of such plastics (foils or three-dimensional bodies) or their further processing by lamination or coating is therefore not possible at all. The printing inks would not adhere and delamination would occur during the production of composite films .
Corona treatment, the most frequently used form of surface treatment, helps here . Alternative methods would be flame treatment , fluorination and plasma treatment . The goal of all of these methods is to increase the polarity of the surface, which significantly improves wettability and chemical affinity .
Practical implementation
In film production, the corona treatment is usually carried out " in line " at the end of the production process. The film web is exposed to a high-voltage electrical discharge. This occurs between a grounded, polished roller made of steel or aluminum and a tightly fitting insulated electrode. Depending on the area of application, it is also possible to use an insulated roller against uninsulated electrodes. The film rests on the polished roller so that only the side facing the electrode is treated. If there is an air gap between the roller and the web, the reverse side is also treated. The electrode is supplied with an alternating voltage of 10 to 20 kV and a frequency between 10 and 60 kHz by a high-frequency generator .
Spatial bodies (such as yogurt pots) are treated by rotating around the electrode.
A corona work is arranged in special offset printing machines in front of the actual printing units.
Mode of action
The mechanism of corona treatment is not yet fully understood, despite its widespread use and numerous published works. The main role, however , is played by oxidation processes , through which, depending on the film, various polar functional groups (e.g. alcohols , aldehydes , carboxylic acids , esters , ethers , peroxides ) are formed. A degradation (radical cleavage) of the polymers has also been proven. On the other hand, crosslinking of the macromolecules can be assumed, since the corona treatment worsens the sealability (increase in glass transition temperature T g and melting temperature ). The electrical discharge leads to an ionization of the air, which leads to the local formation of ozone . The ozone can also have an oxidative effect on the film surface.
effectiveness
The effect achieved by the corona treatment is an increase in surface tension (Dynung) to 38 to 44 mN / m. Since the surface tension arises from dispersive and polar interaction components, the introduction of polar functional groups in particular increases the polar portion of the surface tension.
To avoid mistakes, it is important that the surface tension of the film is higher than that of the coating material. In addition, the ratio between the polar and dispersive interaction component of the surface tension should be as similar as possible for the film and the coating material. This is the only way to achieve good flow and optimal wetting.
The impact of the treatment will decrease over time. The shelf life of the treated films is therefore limited. In the case of a polyethylene film, the surface tension is 30 to 32 mN / m without treatment. Directly after the treatment, depending on the setting, values between 38 and 44 mN / m are achieved. A minimum of surface tension occurs after 4 weeks of storage. The values are then around 10% lower than immediately after the treatment.
Influencing factors
The type of film production ( blowing , calendering or extrusion ), the temperature of the polymer melt, the stretching conditions and the type of cooling all influence the subsequent effectiveness of the corona treatment. Additions to the processed polymers usually have a negative effect. The higher z. B. the proportion of lubricants in a film, the more difficult the corona treatment becomes.
Possible problems
Despite its widespread use, corona treatment is not easy to master. An uneven distribution of the electrical discharge (asymmetrical structure, fluctuations in film thickness and conductivity) can lead to "breakdown". The discharge penetrates through the film and leaves burn holes.
Test methods
The ink test is a quick test. It is carried out with so-called "Dyn test inks", each of which has different surface tensions. You start with low surface tensions and work your way up until the test ink no longer wets the surface. Polar and dispersive interaction components of the surface tension cannot be differentiated with this method.
Another test method is the contact angle measurement or contact angle measurement . Here, a drop of liquid is placed on the film surface and the contact angle (contact angle) of the drop compared to the surface is determined with a large increase. The smaller the angle, the better the wetting.
By measuring the contact angle with several chemically pure test liquids with known surface tension and known dispersive and polar fractions, the polar and dispersive fractions of the surface tension of the film can be determined.
Occupational safety
Both the discharge currents and the ozone generated represent a potential hazard. Danger to persons can be avoided through preventive protective measures such as isolation and a sufficiently dimensioned suction system. Since the high voltage is discharged against an earthed roller, the use of residual current circuit breakers is excluded.