Introduction to Plasma

Plasma technology – endless possibilities

1. Was ist Plasma?

Wird Materie kontinuierlich Energie zugeführt, erhöht sich ihre Temperatur und sie geht vom festen über den flüssigen in den gasförmigen Zustand über. Wird die Energiezufuhr weiter fortgesetzt, wird die bestehende Atomhülle aufgebrochen und es entstehen geladene Teilchen (negativ geladene Elektronen und positiv geladene Ionen). Dieses Gemisch wird als Plasma oder der "vierte Aggregatzustand" bezeichnet.

Kurz: Änderung des Aggregatzustands unter Energiezufuhr:

fest ⇒ flüssig ⇒ gasförmig ⇒ Plasma

In der Natur kommt Plasma z. B. in Blitzen, Polarlichtern, Flammen und der Sonne vor. Künstlich erzeugte Plasmen kennt man unter anderem durch die Neonröhre, vom Schweißen und von Blitzlichtern.

2. Wofür können Plasmaanlagen von Diener electronic eingesetzt werden?

Plasma wird in Bereichen eingesetzt, in denen es darauf ankommt, Materialien zu verbinden oder deren Oberflächeneigenschaften gezielt zu verändern. Mit dieser zukunftsweisenden Technik lassen sich die verschiedensten Oberflächen modifizieren. Es bieten sich daher vielseitige Anwendungsmöglichkeiten wie z. B. die

  • Feinstreinigung von Klein- und Mikrobauteilen
  • Aktivierung von Kunststoffbauteilen vor dem Kleben, Lackieren usw.
  • Anätzen und teilweise entfernen verschiedener Materialien wie PTFE, Fotolack usw.
  • Beschichtung von Bauteilen mit PTFE-ähnlichen Schichten, Barriereschichten, hydrophoben bzw. hydrophilen Schichten, reibungsreduzierenden Schichten usw.

Die Plasmatechnik hat sich mittlerweile in nahezu allen industriellen Bereichen etabliert. Es kommen laufend neue Anwendungen hinzu.

2.1 Plasma effects

3. Wie wirkt Plasma?

Bei der Niederdruck-Plasmatechnik wird Gas im Vakuum durch Energiezufuhr angeregt. Es entstehen energiereiche Ionen und Elektronen sowie andere reaktive Teilchen, die das Plasma bilden. Damit lassen sich Oberflächen wirkungsvoll verändern. Es werden drei Plasmaeffekte unterschieden:

Mikrosandstrahlen: Die Oberfläche wird durch den Ionenbeschuss abgetragen.

Chemische Reaktion: Das ionisierte Gas reagiert chemisch mit der Oberfläche.

UV-Strahlung: Die UV-Strahlung bricht langkettige Kohlenstoffverbindungen auf.

Durch die Variation der Prozessparameter wie Druck, Leistung, Prozesszeit, Gasfluss und -zusammensetzung ändert sich die Wirkungsweise des Plasmas. So lassen sich in einem einzigen Prozessschritt mehrere Effekte erzielen.

Plasma entfernt Trennmittel (auch Silikone und Öle) von der Oberfläche. Diese werden von z. B. Sauerstoff chemisch angegriffen und in flüchtige Verbindungen umgewandelt. Durch den Unterdruck und die oberflächliche Aufheizung verdampfen die Trennmittel bzw. deren Reste zum Teil. Durch die energiereichen Teilchen im Plasma werden die Trennmittelmoleküle in kleinere Molekülfragmente aufgebrochen und lassen sich dadurch absaugen. Außerdem entsteht ein "Mikrostrahleffekt" auf atomarere Ebene. UV-Strahlung kann Trennmittel aufbrechen.

Auf frisch produzierten als auch auf gelagerten Produkten befinden sich meist unsichtbare Ablagerungen wie Fette, Öle, Silicone, Feuchtigkeit, Oxidationsschichten. Um diese Oberflächen fehlerfrei beschichten zu können, müssen diese LABS-frei sein (LABS = LAckBeschichtungsStörende), was durch eine Plasmareinigung erzielt werden kann.

4. What types of plasma system are there?

A distinction is made between low-pressure and atmospheric plasma systems

Low-pressure plasma technology

Atmospheric-pressure plasma technology

5. How are low-pressure plasma systems designed and how do they work?

In the low-pressure plasma technology, the gas is excited by supplying it with energy in a vacuum. This results in energetic ions and electrons, as well as other reactive particles, which constitute the plasma. Surfaces can then be effectively altered. There are three plasma effects:

  • Micro-sandblasting: The surface is removed by ion bombardment.
  • Chemical reaction: Chemical reaction of the ionized gas with the surface.
  • UV radiation: UV radiation breaks down long-chain carbon compounds.

The effect of the plasma changes by varying the process parameters such as pressure, power, process time, gas flow and composition. Several effects can therefore be achieved in a single process step.

Example of typical process parameters:

Performance: 500 watts
Process chamber volume: 100 litres
Process gas: Air or oxygen
Pressure: 0,2 - 0,6 mbar
Duration: 1 - 5 minutes

A variety of process gases are available (e.g. air, oxygen, argon, argon-hydrogen, oxygen-tetrafluoromethane) and chemicals (e.g. hexamethyldisiloxane, vinyl acetate, acetone, fluorine-containing chemicals).

Basically the following applies: Process knowledge is crucial. The plasma must match the material so it can be specifically set to achieve all the desired effects.

Low-pressure plasma systems: Generation with an LF or RF generator
Low-pressure plasma systems: Generation with a microwave generator

6. How are atmospheric pressure plasma systems designed and how do they work?

With atmospheric plasma technology, gas is excited by means of a high voltage under atmospheric pressure, such that a plasma is ignited. The plasma is expelled by compressed air from the nozzle. There are two plasma effects:

  • Activation and precision cleaning is carried out by the reactive particles contained in the plasma jet.
  • In addition, loose, adherent particles are removed from the surface by the compressed air accelerated active gas jet.

The treatment performance can be affected in different ways by varying the process parameters such as treatment speed and distance from the substrate surface.

6.1 Corona facilities ("Gliding Arc" principle)

According to the "Gliding Arc" functional principle, an electric arc discharge is generated at low voltage in a "hot" plasma zone and then protruded through the air in the flow direction (voltage increases to about 10 kV). In this way, a "cold" plasma zone is formed, which can be used as a treatment tool.

The treatment width is approximately 50 - 60 mm. Treatment distance can be up to about 20 mm.

The devices are equipped with a micro-controller for the plasma generation. That means the following parameters are set at the factory:

  • Pulse width of the discharge
  • Pause time in the discharge
  • Air quantity

The mentioned parameters affect the temperature and the efficiency of the plasma.

The following applications are possible with the devices:

  • Activation by the reaction of reactive particles (radicals) contained in the plasma jet.
  • In addition, loose adherent particles are removed from the surface by the compressed air accelerated plasma jet.

The devices are designed for the pretreatment of surfaces of plastic moulded parts for the improved adhesion of

  • Print colors
  • Paint
  • Adhesives
  • Foam etc.

The following is important for good surface treatment:

  • Only non-conductive materials may be treated.
  • The treatment speed and the distance between corona head and the surface to be worked are the most important parameters for achieving the desired surface properties. Small changes in these parameters can drastically change the pretreatment effect.
  • Lower speeds and/or multiple treatment leads to more uniform surface activation.

6.2 PlasmaBeam

With atmospheric plasma technology, gas is excited by means of a high voltage under atmospheric pressure, such that a plasma is ignited. The plasma is expelled by compressed air from the nozzle. There are two plasma effects:

Activation and precision cleaning is carried out by the reactive particles contained in the plasma jet.

In addition, loose, adherent particles are removed from the surface by the compressed air accelerated active gas jet.
The treatment performance can be affected in different ways by varying the process parameters such as treatment speed and distance from the substrate surface.

The atmospheric plasma processor PlasmaBeam is mainly used for local pretreatment of different surfaces (cleaning, activation):

  • Polymers
  • Metal
  • Ceramic
  • Glass
  • Hybrid materials

The PlasmaBeam is suitable for robots and can be installed in existing, automated production lines without great effort.

6.3 Plasma systems - PlasmaBeam

7. How long can treated parts (activation) be stored before further processing?

The shelf life of the components is dependent on the activation time and the material and varies between a few minutes and several months. Therefore, it is often necessary to carry out tests on site.

Metals, ceramics, glass and elastomers: about 1 hour

Plastics (excluding elastomers): days, weeks, months

8. How should treated parts be stored?

After plasma treatment, it is recommended not to store the parts in the open, as they attract dust, organic contamination and humidity.

Shrink-wrapped parts have a substantially higher shelf life than those left in the open.

Parts treated by us for surface treatment are packaged in close consultation with the customer e.g. certified silicone-free PE bag, ESD packaging, or customized packaging material which is made available to us.

9. Why should the treated parts not be touched?

Plasma removes organic but not inorganic impurities. For example, salts (inorganic contaminants) are contained in the sweat of fingerprints, so components must only be handled with gloves.

10. How can a plasma activation be measured?

10.1 Contact angle/wetting angle

The contact angle is the angle formed by an observation of the projection of the resting drop onto the solid by the tangent to the drop shape at the surface of the solid body, at the triple point. According to the physical definition, a surface having a contact angle of less than 90° is hydrophilic (wettable), whereas if it is greater than 90° it is hydrophobic (non-wettable). Plasma treatment can change the contact angle (increase, decrease). An appropriate plasma process or application of a suitable coating in a plasma process can cause hydrophilic surfaces to become hydrophobic (by hydrophilic layers and vice versa).

Wetting angle measurement device
Contact angle measurement

10.2 Test inks

Measuring means for estimating the surface energy: If the test ink beads after application on the surface, the surface energy of the solid is less than that of the ink; however, if wetting is obtained, then the surface energy of the solid is equal to, or greater than that of the liquid. The total surface tension of a solid can be determined through the use of series of test inks with graduated surface energy. The polar and non-polar component of the surface energy can not be determined with this method however.

10.3 Cross Hatch Test

To test the adhesion of paints, a Cross Hatch Test (standards: EN ISO 2409 and ASTM D3369-02) is performed. After painting, the paint layer of the plastic part is cut in a lattice form. Then a standard adhesive tape is stuck to the cut grid, pressed and abruptly withdrawn again. If paint sticks on the tape, the adhesion of the coating is defective. The Cross Hatch thus shows the adhesion of paint coatings on plastic parts.

11. How can a plasma treatment be detected?

The indicator tags as well as the plasma indicator metal compound provide users of plasma systems with the ability to see at a glance whether a plasma treatment has taken place. The tests can be carried out in virtually no time. They can be used in any plasma system for any treatment whether cleaning, activating, etching or coating. The indicators identify plasma treatment previously carried out on your products and semi-finished products, even after weeks or months.

11.1 Indicator tags

The adhesive label is made of specially coated films, which can be used as a reference and are placed directly in the chamber, or glued to the components. As soon as the dark indicator point has disappeared, the plasma treatment has been successfully completed. The indicator labels can also be used for a system test. Here, a label is laid in the empty vacuum chamber and the plasma ignited.

Plasma indicator label

11.2 Plasma indicator metal compound

The plasma indicator is a liquid metal compound, which decomposes in the plasma so that the plasma-treated surface of the object has a shiny metallic surface. A drop applied to the component itself, or a reference sample is transformed in the plasma treatment into a shiny metallic coating that forms on most surfaces and forms a clear contrast to the originally colourless drops. The golden shiny metal film resulting from the plasma is distinguished from all the other colours of the object by its optical reflectivity.

On the right is the indicator before plasma treatment. On the left is the indicator after plasma treatment.
On the right is the indicator before plasma treatment and on the left after treatment

12. What are the advantages of plasma technology?

Plasma technology exhibits decisive advantages compared to other methods such as flame treatment or wet-chemical treatment:

  • Many surface properties can only be obtained using this method
  • Universally applicable method: on-line production capable and can be fully automated
  • Extremely environmentally friendly process
  • Almost independent of geometry, powders, small parts, plate materials, non-wovens, textiles, tubes, hollow bodies, circuit boards, etc. can be treated
  • Components are not mechanically altered
  • Heating of the parts is minimal
  • Very low running costs
  • High process and work safety

Particularly rational process.

13. Which applications are possible?

Explanations and further information can be found in Comparison chart LP/AP plasma and under Applications.