On almost all surfaces there are fine soiling invisible to the human eye, which severely impairs further treatment of surfaces such as gluing, printing, painting or coating. These fine impurities on metals, plastics or inorganic materials can be removed by plasma cleaning without the use of additional chemicals. In contrast to plasma coating or plasma etching, the surface of the workpiece is not removed or additionally coated, but merely modified.
Ultrafine cleaning of surfaces with cold atmospheric pressure plasmas is a process of removing organic, inorganic and microbial surface contaminants, as well as strongly adhering dust particles. It is highly efficient and at the same time very gentle to the treated surface. At higher strength, it can remove weak surface boundary layers, cross-link surface molecules and even reduce hard metal oxides. Plasma cleaning promotes wettability and adhesion enabling a wide spectrum of industrial processes preparing surfaces for bonding, gluing, coating and painting. While being performed using air or typical industrial gases including hydrogen, nitrogen and oxygen, it avoids wet chemistry and expensive vacuum equipment, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Typical surface contaminants
Multiple layers of contaminants usually cover surfaces, even if the latter visually appears to be clean. Contaminants form naturally due to exposure to air. They include layers of oxides, water, various organic substances and dust. Furthermore, technological processes leave surfaces covered with oils, release agents, compounding ingredients, monomers and exuded low molecular weight species. Contaminants can drastically reduce the quality of the adhesive bonding by introducing a weak intermediate layer. Moreover, their typically low wettability results in incomplete surface covering by the adhesive, further reducing the bonding strength.
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Cold atmospheric plasmas
Plasma is a partially ionized gas. Electric arcs, dielectric barrier, corona and piezoelectric direct discharges ionize gases at atmospheric pressures creating plasmas. The charged particles – electrons and ions – accelerate to very high energies. Only a small fraction of the gas molecules is turned into the energetic electrons and ions; the rest of the gas remains neutral and cold. Its temperature reaches only 50 C in the case of the piezoelectric direct discharge, and 250-450 C in the case of the arc discharge. At the same time, the very energetic electrons and ions collide with the gas molecules producing large quantities of short-lived chemical species, such as atomic H, N and O species, OH and ON radicals, ozone, nitrous and nitric acids, as well as various other molecules in metastable excited states. They make this plasma chemically very active.
Plasma cleaning mechanisms
Upon contact with the treated surface, the chemically active cold atmospheric plasma initiates a multitude of physical and chemical processes. The main reaction agents are highly reactive short-lived neutral chemical species. Additionally, when the electric discharge touches the treated surface, the latter is also irradiated by VUV light and bombarded by the energetic electrons and ions. Although the quantities of the charged particles are small, their highly reactive nature strongly enhances the cleaning effect. The following processes contribute to the ultra-clean surface:
- Plasma breaks organic bonds of heavy organic molecules producing lighter and more volatile molecules evaporating from the surface.
- Reactive chemical species oxidize organic contaminants forming carbon oxides and water vapor.
- These processes also destroy and remove microbial contaminations sterilizing the surface.
- With the increased treatment strength, plasma removes surface layers with the lowest molecular weight. It also oxidizes the uppermost atomic layer of the polymer.
- The bonds of the polymers, broken by the plasma, cross-link forming a stronger surface layer.
- Plasma discharges, ignited in the forming gas, typically containing 5 % of hydrogen and 95 % of nitrogen, produce large quantities of reactive hydrogen species. By contact with oxidized metal surfaces, they react with metal oxides reducing them to metal atoms and water.
- Electric discharges having direct contact with the substrate erode the substrate surface on the micrometer scale. This creates microstructures that are filled by the adhesives improving the mechanical binding of the adhesives.
- Plasma deposits polar OH and ON groups on the cleaned surface significantly increasing the energy of the surface and its wettability. As the results, the subsequently applied adhesives wet the surface efficiently and fill the microstructures due to the capillary action.
Advantages of plasma cleaning
As the plasma processes, initiated on the treated surface, break contaminants turning them into vapor, no residues are left on the surface, leaving the latter in the ultra-fine clean state. Most importantly, the plasma cleaning process works at atmospheric pressure. Its advantages comparing to standard chemical and vacuum plasma cleaning processes include:
- Ultra-fine cleaning, no residues
- Gentle surface treatment
- No wet chemistry
- Air or cheap non-toxic working gases
- Environmental friendliness
- No expensive vacuum equipment
- Fast processing speeds
- Easy integration into existing production lines
- Promotion of wetting and adhesion
Plasma cleaning products by Relyon Plasma GmbH
To cover a wide spectrum of industrial, medical and laboratory applications, Relyon Plasma GmbH developed a series of products well suited for plasma cleaning:
- Plasmabrush® PB3 is a universal highly reliable plasma generator based on our proprietary Pulsed Atmospheric Arc (PAA) Technology. With its power of 1 kW, very compact dimensions and exceptional long-time stability, this generator is well suited to integration into high-speed industrial production lines.
- Plasmacell P300 is a complete “turn-key” plasma cleaning solution. It includes all components required to establish an effective plasma cleaning process conforming to industry standards and regulations: plasmabrush PB3 mounted on a programmable high-speed X-Y-Z positioning system, compressed air supply and exhaust filtration systems. Enclosures with electronics and the treatment chamber, together with an electronic display, create a clean, safe and efficient working environment.
- Plasmatool is a handheld instrument based on plasmabrush PB3, ergonomically optimized for safe manual operation. Together with a portable module including a high voltage supply, an air compressor and controlling electronics, it enables highly efficient plasma cleaning of large structures, areas that are difficult to access, or where automation is not possible or practical
- Piezobrush® PZ2 is a low power handheld plasma generator, which is based on our proprietary Piezoelectric Direct Discharge (PDD) Technology. It enables manual plasma cleaning work in laboratories. It can create corona and dielectric barrier discharges and apply them for precision ultrafine plasma cleaning of small components.