Plasma pretreatment: Varnishing
Plasma surface pretreatment improves the adhesion of paints and varnishes to the surfaces and thus improves the quality of the coating and varnishing. Many components are made of materials such as metals, glass, ceramics and even natural materials such as wood and textiles, whose surfaces are very difficult to varnish. Plasma pretreatment: Varnishing with atmospheric plasma cleans the components on the one hand and functionalizes them on the other. As a result, many polymers with typical non-stick surfaces can be successfully coated. This process also enables solvent-free coating. The result is a uniform and faultless varnish that has been produced using an environmentally friendly process.
Plasma processing achieves the surface functionalization effect through a combination of ultra-fine surface cleaning from organic contaminants, modification of the surface topography and deposition of functional chemical groups. In the case of metals, plasma can also reduce hard metal oxides exposing the bare metal surface.
Plasma functionalization of surfaces can be performed at the atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen. It avoids expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Advantages of the plasma processing
Plasma cleans, strengthens and chemically functionalizes the surface. All these effects, which are required for improved adhesive bonding, are achieved simultaneously in a single step. Most importantly, the plasma processing works at atmospheric pressure. Its advantages comparing to standard chemical and vacuum plasma cleaning processes include:
- Ultra-fine cleaning, no residues
- Improved and uniform varnish appearance
- Degreases the surfaces
- Produces an antistatic effect
- No washing and priming processes necessary
- Gentle, non-destructive 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
Improvement of adhesion of varnishes and paints by plasma treatment
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, which are produced by the plasma in large quantities. 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 effects of the plasma. The following processes contribute to the promotion of the adhesion by the plasma treatment:
- Plasma cleans the surface. Plasma breaks organic bonds of heavy organic molecules producing lighter and more volatile molecules evaporating from the surface. Further, reactive chemical species oxidize organic contaminants forming carbon oxides and water vapor. As the plasma breaks contaminants turning them into vapor, no residues are left on the surface, leaving the latter in the ultra-fine clean state.
- Plasma reduces metal oxides. 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 vapor.
- Plasma strengthens the surface. With the increased treatment strength, plasma removes nm-scale weak surface layers having the lowest molecular weight. The bonds of the polymers, broken by the plasma, cross-link forming a stronger surface layer.
- Plasma deposits chemically functional groups and increases the surface wettability. By reacting with the polymer molecules, plasma species deposit 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. Moreover, by adding specific chemical substances, plasma can deposit specialized functional groups or even polymerize the surface to enable the strongest chemical adhesion.
- Plasma microscopically roughens surface. Electric discharges having direct contact with the substrate, especially the electric arcs burning on the metal substrate, when the latter is used as a cathode, erode the substrate surface on the micrometer scale. This creates microstructures that are filled by the adhesives improving their mechanical binding to the substrate.
Requirements for strong, high quality adhesive bonding
Strong high quality adhesive bonding between dissimilar materials, such as varnishes and plastic or metal substrates, can be formed by fulfilling the following requirements:
- Ultra-clean surface. The surface has to be free from contaminants. Even visually clean surfaces can contain contaminants such as adsorbed organics, water, monomers, release agents, oils. Ultra-fine clean state is difficult to achieve with conventional cleaning techniques which often leave residues.
- Oxide-free metal surface. The best adhesion to metal surfaces is achieved when they are free from metal oxides. However, in the air metals oxidize quickly. Thus, the time between the removal of the oxide layer and the application of the adhesive usually needs to me minimized down to milliseconds.
- Strong surface. In the case of plastic polymers, formed by molding and extrusion processes, the uppermost nm-width surface layer consists of low molecular weight and not cross-linked polymer molecules. This surface layer is mechanically weak. Removing this weak layer and cross-linking the polymer molecules in the remaining layers improves the adhesion strength.
- Wettable surface. For the adhesive to cover (wet) the surface efficiently, the surface energy of the adhesive should be lower than that of the surface they should be bonded to. However, mechanically strong adhesives and paints usually have high surface energy. This poses a serious problem for their use with most plastic polymers, which typically have very low surface energy.
- Chemically functional surface. On the molecular level, the adhesion between two materials is mediated by either electric attraction between the surface molecules and the molecules of the adhesive, or by their chemical bonds. The former type, called dispersive adhesion, is strong when polar molecules cover the surface. While surfaces of plastic polymers are typically non-polar, formation of the layer of polar molecules functionalizes the surface. The latter type, called chemical adhesion, forms the strongest joints. However, chemical bonding between dissimilar materials in not possible. An intermediate layer of molecules having chemical affinity to both, the surface and the adhesive, functionalizes the surface enabling very strong chemical bonding.
- Microscopically rough surface. When the surface is well wettable by the adhesive, the latter can efficiently fill the surface pores and irregularities due to the capillary action. This increases the mechanical strength of the adhesive bonding.
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. In the case of the piezoelectric direct discharge, its temperature reaches only 50 C. In the case of the arc discharge, the arc volume reaches temperatures of 6.000 – 12.000 C. However, after leaving the discharge volume, the gas cools quickly to 250-450 C. These temperatures do not damage the surfaces by fast processing speeds. While the plasma remains cold, 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 treatment products by Relyon Plasma GmbH
To cover a wide spectrum of industrial, medical and laboratory applications, Relyon Plasma GmbH developed a series of plasma processing products designed to improve adhesive bonding, such as printing, coating, painting and gluing:
- 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 for integration into high-speed industrial production lines.
- Plasmacell P300 is a complete “turn-key” plasma treatment solution. It includes all components required to establish an effective plasma processing 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, ready for operation.
- 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 processing 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 processing work in laboratories. It can create corona and dielectric barrier discharges and apply them for precision ultrafine plasma cleaning and chemical functionalization of small components.