Plasma

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Plasma

In just two decades, plasma processes have become a versatile tool for industrial applications across numerous industries and technological fields, serving a wide range of tasks in surface treatment, cleaning, and coating, as well as the activation of plastics, the etching of semiconductor structures, and much more.

By definition, a plasma is a partially ionized gas, often referred to as the fourth state of matter. Phenomena such as lightning or the Northern Lights (aurora borealis) are naturally occurring plasmas. Technically, they can be generated by applying electric fields.

Plasmas are particularly interesting due to their physical and chemical properties. Highly excited particles and radicals are generated within them, which can trigger chemical reactions that are not possible under normal conditions. At the same time, the temperature of the workpieces being treated can be kept very low.

Highly accelerated ions in a plasma enable applications such as sputtering.

It is thanks to all these physical and chemical properties that plasma plays such an extraordinary role in science and industrial technology.

Plasma Activation

The activation of plastic surfaces is a specialized application of plasma technology. In this context, activation refers to an increase in surface energy, which results in improved adhesion properties and, consequently, wettability. The contact angle of a droplet on a plastic surface is a common measure of adhesion.

In many cases, polymers have low surface energies or are hydrophobic, which can cause problems, for example, when using water-based coatings. Therefore, they must be modified before coating or printing. Plasma has proven to be a very efficient method for this purpose. Thus, plasma activation serves as a substitute for conventional adhesion promoters.

Furthermore, there are virtually no limitations regarding the geometry of the workpieces or the type of polymer, since the temperature does not rise significantly during the process. In addition, the topography of the plastic remains virtually unchanged; only the outermost monolayers are modified.

For activation processes, the plasma typically consists of oxygen, but sometimes also of nitrogen or ammonia. When oxygen is used, it is possible to modify polymers in such a way that hydroxyl, carboxyl, or peroxide groups form on the surface. If, on the other hand, nitrogen or ammonia is used, this results in amine or imine groups, which may also be of interest for certain applications. These polar, hydrophilic groups improve the surface wettability. Activation processes generally take only a few seconds. After activation, the workpieces are ready for immediate further processing, which may even be possible within the same plasma system.

A typical application for this established process is the pretreatment of plastic parts in automotive manufacturing, such as painted bumpers.

An overview of some advantages

Process

Excellent adhesion
Improved homogeneity
High process stability
Simple process control
Suitable for temperature-sensitive materials

Cost-effectiveness

No disposal costs
No drying process required
Low plasma gas consumption
No primer required

Environmental Friendliness

Solvent-free
No hazardous waste
Integrated environmental compatibility

Plasma cleaning

Surface cleaning is one of the applications offered by plasma technology. Various methods of plasma generation are used for this purpose. The most common methods involve applying high-frequency alternating voltages (40 kHz or 13.56 MHz) or using microwave excitation. Typical gas mixtures contain oxygen, argon, and sometimes hydrogen and carbon tetrafluoride (_CF₄). The mixture depends on the specific requirements of the cleaning process, which are determined by the type of contamination and the material being treated.

There are virtually no restrictions regarding the material to be cleaned. Thus, temperature-sensitive plastics can be treated just as easily as metal, glass, and ceramics. However, the plasma system must always be adapted to the specific task at hand.

Depending on the gas mixture, the plasma acts as an oxidizing or reducing agent. Oxidizing plasmas are advantageous for removing organic contaminants (oils, fats), while reducing plasmas are more suitable for inorganic deposits (such as metal oxides).

Almost every surface becomes contaminated during various production steps. Workpieces must be cleaned before further processing steps, such as painting, curing, or sealing.
Plasma cleaning is the best way to achieve absolutely clean, grease-free surfaces.

Conventional cleaning methods often involve wet chemical processes and contain hydrocarbons (HC), chlorinated hydrocarbons (CHC), or harsh cleaners dissolved in water. All of these agents pose environmental problems both during use and upon disposal. Waste reduction is achieved through so-called end-of-pipe solutions, which incur additional costs.

In contrast to wet chemical cleaning, plasma cleaning meets the requirements for integrated environmental protection.

To understand how plasma cleaning works, one must examine a microscopically small area. What happens immediately on and at the surface?

In the case of organic contamination, the plasma gas contains oxygen and argon. The cleaning process is the sum of two processes: one physical and one chemical.

Physical:
When the workpiece is placed on the cathode or connected to it as a cathode, positive ions from the plasma are accelerated toward it due to the applied electric field. Upon impact, the ions detach atoms and molecules from the surface through direct momentum transfer. One could describe this process as “billiards with atoms.”

Chemical:
Excited oxygen and oxygen ions react with hydrocarbons to form carbon dioxide and water. The exact chemical equation is as follows:

Hydrocarbons are thus converted into relatively harmless chemical compounds. The products of incomplete combustion, if any, can be easily filtered out of the exhaust gases using activated carbon filters.

Plasma cleaning offers several notable advantages in terms of process efficiency, cost-effectiveness, environmental impact, and workplace safety:

Process

High degreasing efficiency
High crackability
Good preparation for further plasma processes, such as coating
High stability
Simple process control
Suitable for temperature-sensitive materials

Cost-effectiveness

No disposal costs
No drying process required
Low consumption of plasma gases

Environmental friendliness

Solvent-free
No hazardous waste
Minimal disposal effort
Integrated environmental protection

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Country
Germany