Plasma activation

Many industries employ surface preparation methods including wet chemistry, exposure to UV light, flame treatment and various types of plasma activation. Advantage of the plasma activation lies in its ability to achieve all necessary activation objectives in one-step without the use of chemicals. Thus, plasma activation is simple, versatile and environmentally friendly.

Many types of plasmas can be used for surface activation. However, due to economic reasons, atmospheric pressure plasmas found most applications. They include arc discharge, corona discharge, dielectric barrier discharge and its variation piezoelectric direct discharge.

Pulsed atmospheric arc technology improves the arc stability at low electric currents, maximizes the discharge volume, and together with it the production of reactive species for plasma activation, while at the same time reducing the size of the driving high voltage electronics. These factors make it economically very attractive for industrial applications.

Typical generator of an atmospheric pressure plasma based on a high voltage electric arc discharge. The arc is burning between the inner anode, biased with high voltage, and the grounded outer cathode. The vortex airflow stabilizes the arc and expels the plasma through a hole in the cathode.

There are two ways of using electric arcs for surface activation: non-transferred and transferred electric arcs. In the non-transferred technique, both electrodes are part of the plasma source. One of them also acts as a gas nozzle producing a stream of plasma. After the plasma stream leaves the arc region, the ions recombine quickly, leaving the hot gas having high concentrations of chemically active hydrogen, nitrogen and oxygen atoms and compounds, which is also called remote plasma. The temperature of this gas stream is of the order of 200 – 500 °C. The gas is very reactive allowing high surface treatment speeds when only a short-time contact with the substrate is sufficient to achieve the activation effect. This gas can activate all materials, including temperature-sensitive plastics. Moreover, it is electrically neutral and free from electric potentials, which is important for activation of sensitive electronics.

In the transferred technique of using the electric arcs, the substrate itself plays the role of the cathode. In this case, the substrate is subject not only to the reactive chemical species, but also to their ions with energies of up to 10 – 20 eV, to high temperatures reaching within the cathode spots 3000 °C, and to UV light. These additional factors lead to even greater activation speeds. This treatment method is suitable for conductive substrates such as metals. It reduces metal oxides by their reactions with hydrogen species and leaves the surface free from organic contaminants. Moreover, the fast moving multiple cathode spots create a microstructure on the substrate improving mechanical binding of the adhesive.

Corona discharges appear at atmospheric pressures in strongly non-uniform electric fields. Sharp edges of high voltage electrodes produce such fields in their vicinity. When the field in the rest space is negligible – this happens at large distances to the electric grounds – the corona discharge can be ignited. Otherwise, the high voltage electrodes may spark to the ground.

A dielectric barrier discharge at 30 kHz in air between metal electrodes separated by two dielectric mica sheets with a gap of 4 mm. The "foot" of the discharge is the charge accumulation on the barrier surface.

Due to the unique construction principles, the piezoelectric barrier discharge is the economic and compact source of the dielectric barrier and corona plasmas. Although its power is limited to about 10 W per unit, the low costs and small sizes of the units allow construction of large arrays optimized for particular applications.

The goal of the plasma generators is to convert the electric energy into the energy of charged and neutral particles – electrons, ions, atoms and molecules – which then would produce large quantities of chemical compounds of hydrogen, nitrogen and oxygen, in particular short-lived highly reactive species. Bombardment of the substrate with all constituent plasma species cleans and chemically activates the surface. In addition, at the contact points of discharge filaments the surface can locally reach high temperatures. This modifies the topography of the surface improving mechanical binding of the adhesive.

At the atmospheric pressure, the high collision frequency between the electrons and the gas molecules precludes the electrons from reaching high energies. Typical electron energies are of the order of 1 eV except for the electrode layers of 10 – 30 μm thickness where they can reach 10 – 20 eV. Due to the low electric currents of individual filaments in corona and dielectric barrier discharges, the gas present within the discharge volume does not reach thermal equilibrium with the electrons and remains cold. Its temperature rises typically only by up to a few 10 °C above the room temperature. On the other hand, due to the high electric currents of the arc discharge, the whole arc volume thermally equilibrates with the electrons reaching temperatures of 6,000 – 12,000 °C. However, after leaving the arc volume, this gas quickly cools down to a few 100 °C before it contacts the substrate.

Although it is not correct to speak of temperatures of non-equilibrium electron and ion gases, the temperature concept is illustrative of the physical conditions of the discharges, as the temperature defines the average energy of the particles. The average electron energy of 1 eV, realized typically within the plasma volume, is equal to average electron energy at temperatures of 10,000 °C. In the thin cathode and anode layers, the ions and the electrons reach average energies up to 10 times higher, corresponding to temperatures of 100,000 °C. At the same time, the molecular gas can remain cold.

Plasma of the atmospheric discharges or its product gas, rich with highly reactive chemical species, initiates a multitude of physical and chemical processes upon contact with the surface. It efficiently removes organic surface contaminants, reduces metal oxides, creates a mechanical microstructure on the surface and deposits functional chemical groups. All of these effects can be adjusted by selecting discharge types, their parameters and the working gas. Following processes result in surface activation:

Balance of the chemical reactions on the substrate surface depends on the plasma gas composition, velocity of the gas flow, as well as the temperature. The effect of the latter two factors depends on the probability of the reaction. Here one distinguishes two regimes. In a diffusion regime, with a high reaction probability, the speed of the reaction depends on the velocity of the gas flow, but does not depend on the gas temperature. In the other, kinetic regime, with a low reaction probability, the speed of the reaction depends strongly on the gas temperature according to the Arrhenius equation.