Thin Layers for Optimized Characteristics Optimized pre-treatment and activation is essential for reliable, sustainably stable paint adhesion. Processes involving solvents are frequently used to this end. Plasma technology represents an environmentally sound, efficient alternative, and offers numerous additional effects as well.
In the case of metals, it’s usually oil and grease that gets in the way of continuously reliable paint adhesion – especially where ecological water-based paints are used. In order to remove quality impairing contamination which usually originates from production, the coated parts are run through an environmentally harmful, energy and time-intensive pre-treatment process. The fact that this can be accomplished in an environmentally sound, economic fashion is substantiated by plasma technology. Not only all technical metals, but rather practically all substrate materials as well can be treated by means of this process. Processing is dry, contactless and non-abrasive. Correct selection of the various process parameters such as type of utilized gas, gas throughput, pressure, electrical power and treatment time allows for ideal matching to the respective application.
Plasma – the Fourth State of Aggregation Plasma is formed by adding energy: The smallest building blocks of solid material (atoms and molecules) initially start to move, and then pass over into the liquid state. Adding more energy causes these building blocks to abandon their respective compounds and move about freely – a gas is created. If even more energy is added, the gas is ionized: The particles move so quickly that the charge carriers (electrons) detach themselves from the atoms and molecules. Surfaces can be precision cleaned, activated/modified and coated with these active gas particles, without causing any thermal stress to the surface.
Different Process Variants Two different technologies are available to this end: In the case of low-pressure plasma, treatment is carried out in closed chambers in a partial vacuum. This makes it possible to clean work pieces with complicated shapes as bulk goods or individual parts. The use of a great variety of process gases is also possible, because treatment takes place in an evacuated, sealed space. With rates ranging from 0.1 to 0.5 liters per minute, gas consumption is significantly lower than for atmospheric plasma systems.
Direct and indirect corona discharge (dielectric barrier discharge) takes place under ambient pressure. With the first variant, the discharge (plasma) strikes the work piece directly. In the case of indirect atmospheric-pressure plasma, which makes use of so-called plasma heads (nozzles), discharge takes place at the plasma head and is directed to the surface to be processed by means of compressed air. The range of applications includes above all the treatment of substrates with large surface areas, for example coils, aluminum and steel strip, and foils. Systems with widths of greater than ten metres can be implemented with direct corona discharge. Thanks to simpler technology without vacuum components, the investment costs are lower and atmospheric-pressure plasma systems can also be more easily incorporated into automated production lines.
The plasma-spot has been developed as a device for inline capable processing, and lies between atmospheric and low-pressure plasma. It consists of a plasma head which is individually matched to the surface to be treated, and a supply unit with integrated, fully automated controller. The plasma head is set onto the surface during treatment, and the low-pressure plasma process is started. Processing times range from one second to ten minutes, depending upon the material. This process is used in particular for localised, selective treatment before painting, bonding and printing, as well as for repairing paint damage.
Pre-Treatment for Painting with Plasma Above all thin organic contamination such as auxiliary processing materials can be effectively removed with low-pressure and atmospheric plasmas. For example, fast and effective cleaning results can be obtained for oil films with thicknesses of up to 100 nm. In most cases air is used as the process gas for cleaning before painting processes, and the material removal rate increases along with oxygen concentration. The removal of inorganic contamination with plasma is only possible to a limited extent, and in some cases not at all.
The surface is simultaneously cleaned and activated during plasma treatment. This dual function is based on the physical and chemical characteristics of the process: The atoms released in the low-pressure plasma “bombard” the surface of the component to be cleaned. This functions like a miniature sand-jet in the nano-range, thus removing organic contamination which adheres to the surface such as oil and grease, and to some extent inorganic contamination as well.
With both low-pressure and atmospheric-pressure plasma, organic contamination is broken down into short, volatile chains, and is oxidized into water and carbon dioxide by means of chemical reaction with the oxygen. At the same time, free ions and electrons react with the surface, thus forming polar groups. Consequently, surface tension is adjusted to an ideal value for the subsequent painting process. Surface tensions of greater than 72 mN/m can be achieved in this way by means of plasma treatment. As a result, the surface becomes highly wettable which assures ideal painting conditions when water-based paints are used on metals, as well as when coating difficult to paint plastics, thus contributing to reduced scrap rates.
Coatings for Good Adhesion and Corrosion Protection Plasmas can also be used to apply thin layers to metal surfaces, for example the siliceous precursor hexamethyldisilazane (HMDS), which promotes good adhesion to metals, as well as good chemical bonding of the paint. For example, the adhesion of epoxy paints to metals can be improved with a coating of this sort.
The deposition of temporary corrosion protection layers is also possible as an alternative to the use of oil. This coating has to be removed or activated before painting, but this can be accomplished simply and efficiently with a second run through the plasma system. This eliminates the necessity for the separate, wet-chemical cleaning step which is required for the application of oil which is currently common. Research institutes and industry are now working on the development of coatings which provide good temporary corrosion protection and promote good adhesion without further treatment before painting.
Optimizing Painted Surfaces In particular, low-pressure plasmas can also be used to improve resistance to scratching, and to increase the hardness of UV resistant paints. In this application, the UV component of a plasma generated from, for example, argon gas is used for better cross-linking of the UV resistant paint. With low-pressure plasmas, the benefits of UV resistant paints can also be taken advantage of for geometrically complex parts, which normally only coat the readily accessible areas under normal cross-linking conditions.
The possibility of depositing layers onto surfaces in a targeted fashion during the plasma process makes it possible, for example, to furnish surfaces which have already been liquid painted with special characteristics. This includes the targeted deposition of glass-like layers in order to increase hardness and resistance to wear.
Priming with Plasma Plasma technology offers problem solving advantages when joining and painting components made of different materials such as metal and rubber. In this case, a matching primer coat can be deposited onto the rubber part in combination with cleaning and activation. On the one hand, this allows for permanent bonding to the metal part, and on the other hand assures adequate, homogeneous paint adhesion to both components for a good external appearance.
A plasma coating used for grates and other production equipment which are contaminated by overspray provides for intentionally poor paint adhesion. The coating, which adheres well to galvanized and hot-dip galvanized steel, stainless steel, aluminum, plastics and painted surfaces, demonstrates a distinct anti-adhesion tendency where water and solvent-based liquid paints are concerned. As a result, paint can be easily removed from affected production equipment with the help of a high-pressure water jet without any residues in an environmentally sound fashion.
Thanks to diverse possibilities offered by targeted surface modification such as cleaning, activation, coating and metallization, numerous tasks throughout the painting process can be technically and economically optimized by means of plasma technology.
Submitted by: Doris Schulz
Leading International Trade Fair for
Industrial Coating Technology
13 - 16 April 2010
Exhibition Centre Karlsruhe, Germany