Adhesion improvement by plasma treatment

Adhesion between the substrate and the material applied onto it plays a critical role for adhesive bonding, printing and for coatings such as varnish, metallization, etc. In this case, a number of mechanisms and factors may be responsible for the adhesion, such as the wetting of the substrate, its morphology, polarity, formation of chemical links between the substrate and coating, etc. Minimal contamination on the surface as well as the formation of a so-called weak boundary layer can drastically reduce the adhesion levels.

Numerous methods are used, depending on the "contact partners", to achieve good adhesion, including treatments with solvent based and aqueous solutions, flame treatment and corona treatment. However, these are also subject to numerous restrictions and disadvantages, such as material consumption and environmental contamination in the case of wet treatments, or reproducibility problems in the case of flame treatment or corona. Pretreatment of products before adhesive bonding or coating may take several hours.

Plasmas influence a number of surface characteristics which are critical for good adhesion:

• the surface can be cleaned and
• roughened
• it is made polar and thus wettable
• the sub-surface material is cross-linked, increasing its mechanical and chemical stability, and
• the surface can be provided with chemical functional groups resulting in to a chemical interaction (e. g. by reactions with the functions of the adhesive) between the substrate and the coating or the adhesive.

In this case, even a treatment duration of only a few minutes or seconds is sufficient to drastically improve adhesion.

• Metallization of plastics

A considerable proportion of all plastic products are metallized. The fields of application are, in particular:

• electrically conductive layers in electrical engineering:

• printed circuit boards
• metallization of housing for electromagnetic screening
• 3-dimensional electronic assemblies (3D molded interconnect devices; 3D MIDs)

• decorative and protective layers (in particular in automotive applications).

The metallization can be achieved by various processes, for example by adhesive bonding of a metal foil, by direct metallization using PVD (physical vapor deposition) or by wet chemical metal deposition.

• Adhesive bonding, varnishing, printing of polymers
• Composite materials (e. g. tires)

Plasma changes the surface in many respects, although not all of these changes have a positive effect on adhesion: in fact, a plasma treatment can also contribute to the creation of weak boundary layers. The correct choice of the parameters (type of gas, power, treatment duration, etc.) is thus critical. It is also very important to be able to characterize the surface changes resulting from the treatment.

The best known for adhesion improvement on polymer surfaces is the use of argon, oxygen and air plasma. With the parameters chosen correctly, this approach is often successful. However, these apparently simple plasmas lead to very complicated chemical actions on the surface (particularly if subsequent contact with air is involved). These relatively aggressive plasmas frequently destroy or weaken the sensitive substrate surface. The reactions initiated by the plasma may continue during storage, and may adversely affect the reproducibility.

Therefore, we mainly use custom tailored plasma systems, which lead to a defined surface and to substantially reduced extent of secondary reactions. These belong to one of the following two classes: functionalizing or depositing plasmas.

The type of plasma treatment must be selected and developed for each specific case under consideration of the goods to be treated, the treatment location withing the technological chain, and financial viability. Speak to us, we will be happy to help you.