Plasma cleaning / deep cleaning = plasma treatment
PWIS is a English acronym for substances that might impair surface wetting in the coating process. These substances prevent an even wetting of the surface to be coated, thus causing pit-like imperfections and pinholing in the coat of lacquer. Since the introduction of lacquering with solvent-free (or to be more precise, low-solvent) lacquers in the automotive industry, “pwis-free” production material, equipment and tools is a requirement. Since it is not known which substances cause these faults, materials, parts and assembly groups are tested to ensure they are “pwis-free”.
Whilst in the case of metals and many plastics intensive cleaning reliably removes any materials used in manufacturing (separating agents, coolants, etc.) stuck to the surface, surface cleaning is not sufficient in the case of elastomers. Depending on the compound, it is not only necessary to remove from the surface any residual materials used in manufacturing. All traces of substances used in manufacturing that are diffused in the material as well as several non-bonded compound ingredients have to be removed.
OVE has developed a process in which elastomers are cleaned “pwis-free” in as far as possible. In the case of compounds containing a high proportion of labs substances, renewed contamination may occur depending on storage and operational conditions. The best results are obtained with the OVE cleaning process. Following intensive wet-cleaning with a fat solvent, the parts undergo deep cleaning using low-pressure plasma cleaning with an oxygen flush.
The plasma principle
Plasma is a gaseous mixture of atoms, molecules, ions and free electrons. A low-pressure plasma emerges if a gas is at low pressure (0.1 - 100 Pa) in an electrical field (such as a 50 kHz alternating field, 1000 V) (see Fig. 1). The few free electrons and negatively charged ions existing in any gas are accelerated towards the cathode. All positively charged ions are accelerated towards the anode. On account of the low pressure, the particles have a long free path and are accelerated to around 100 eV. Should these highly energetic particles collide with the molecules of the gas, they likewise split them into ions, free electrons and free radicals. This way, plasma marked by a high proportion of reactive particles emerges.
The OVE method
The elastomer or plastic parts to be treated are placed in baskets and subsequently taken to the process chambers. These are evacuated. After that, some process gas is let in. At an internal pressure of 10 to 500 Pa (fine vacuum) the process gas is ionised by a high-frequency alternating field.
Oxygen is used as a process gas. As a result of the negative pressure, the ionised gas particles have a sufficiently long mean free path until they collide with other gas particles. The likelihood of a collision with the elastomer surface to be treated is therefore sufficiently high. Primarily oxidation processes and cracking take place on the elastomer surface. As a result, polar chemical groups form on the surface, in the form of carbonyl, carboxyl and hydroxide groups. The effect described also brings about, among other things, a measurable increase in the free surface energy. The depth of the effect amounts to only a few molecule layers.
Fig. 2 shows the fundamental structure of a plasma device, including gas supply, plasma processor and vacuum pump.
The reactive particles remove the dirt from the parts to be cleaned, either by reacting chemically with the dirt molecules or by "blasting" the dirt "away" by releasing their high kinetic energy upon impact. When removing the dirt by means of chemical reactions, the impurities are split up into water vapour, carbon dioxide and low-molecular volatile organic particles (see Fig. 3). The cleaned surfaces are PWIS-free.
The proof of being PWIS-free is provided through the VW 3.10.7 test specification.
Testing according to VW specifications
The VW PV 3.10.7 is a widely used standard test specification. The parts to be tested are wetted with a solvent compound and the solvent compound evaporated on a test plate; the test plate is then lacquered. There should be no scratches on the lacquered surface.
|In low-pressure plasma treatment, oxygen is activated in the vacuum through a supply of energy. Oxygen radicals (O) and ozone (O2 ) form. Reactive residues (oils, fats,…) are oxidised and eliminated as gas (CO, CO2 , H2O or dust).|
|„pwis-free“, surface activation|
|All types of elastomer|
|No coating application|
|No change in hardness|
|- Computer-controlled procedure|
- Finished part fulfils VW test specification no. 3.10.7
- Elastomer is permanently „labs-free“
- No change in the physical properties of the treated elastomer
- „labs-free“ available for all products
|2 – 3 weeks|