Introduction
The sun emits UV light. The ozone in our atmosphere is produced by a chemical reaction triggered by UV light. The same chemical reaction can be used in UV light systems to treat surfaces.
Actual surface cleaning occurs through oxidation and cleavage of organic surface contaminants. Mainly carbon dioxide and water are formed. The oxygen radicals used for this are produced by splitting ozone molecules, which themselves are formed from oxygen molecules by irradiation with UV light. Both the formation of ozone and its decomposition are triggered by the high-energy UV irradiation of low-pressure mercury vapor lamps. These lamps are therefore ideally suited for such surface-cleaning applications.
- The 185 nm UV line splits oxygen molecules and leads to the production of ozone O3. The 245 nm UV line decomposes ozone, producing highly reactive free oxygen radicals O• (activated oxygen).
- Radicals such as •OH, COO•, CO• and •COOH are formed and increase the hydrophilic properties of the treated surface.
- Organic molecules are broken down by UV light and oxidized by the oxygen radicals. CO2 and H2O are formed and desorb from the surface.
- The surface is freed from organic contamination and becomes more hydrophilic.
Cleaning effects
Surface modification
Adhesion effects
After treatment with UV light, the surface is cleaned or modified and adhesion forces are increased. Wettability of the surface changes and this property is used to measure the cleaning effect. Several methods are applied:
Contact angle of a water droplet.
A good method to determine the hydrophilic nature of a surface is to measure the contact angle of a deposited water droplet. As wettability increases, the angle between the water droplet and the surface becomes smaller. The method is very accurate and can also be automated.
Wettability reagent.
Another method uses a special wetting agent whose spreading on the surface is observed. In this case it is necessary to wet reference surfaces and compare the spreading of the agent on the reference surface with its spreading on the surface to be tested. The accuracy of the test method depends on the experience of the tester.
Use of wetting grids.
In this method, an acrylic lacquer is applied to the surface and scored into a grid. A cellophane film is applied to the lacquer and then peeled off. Depending on the adhesion, more or fewer lacquer squares come off the surface. The number of detached squares is counted.
