Questions and answers on physical pre-treatment

The purpose of surface pre-treatment is to improve adhesion to surfaces. This is necessary because the surfaces of polymers, metals, ceramics, glass and many types of paper are inherently non-polar or poorly wettable. 

This can lead to poor adhesion results when coating with paints, varnishes, adhesives, etc. The consequences can be a poor print image, a splintering of the paint or a loosening of the bonding. To avoid this, the surfaces must be prepared before coating. There are several ways to do this:

• Mechanical processing, such as roughening or sandblasting
• Chemical pre-treatment, such as washing or the use of adhesion promoters (primers)
• Physical surface activation with corona, plasma or a lean flame (with or without silica coating)

Chemical pre-treatment processes produce hazardous waste products (sometimes even hazardous goods) that have to be disposed of in a complex and cost-intensive manner. In addition, the products cannot be processed until the chemicals have completely dried. Mechanical pre-treatment is time-consuming and not usually compatible with in-line processes. 

For these reasons, the physical processes of corona, plasma or flame pre-treatment under atmospheric pressure are becoming increasingly important. They are cost-effective, compatible with in-line processes, have low operating costs and do not generate waste.

The mode of action of the physical pre-treatment is always based on the production of a plasma. While this is not thermally generated by an electrical discharge during corona or plasma pre-treatment, a plasma is formed in the hot flame during the flame treatment. 

Activated molecules and atoms of the plasma are integrated into the plastic surface and generate polar groups there. The paint, varnish or adhesive molecules can bind to these. The effect is an increase in surface tension. In almost all cases, this also involves an improvement in adhesion.

By adding gases or precursors, the surfaces can be activated even more specifically. Using a silane compound (arcosil® process), very thin silicate layers, only a few nanometres thick, can be produced, which have very good adhesive properties.

To check whether the pre-treatment has achieved its goal – increasing the surface tension – there are two methods: contact angle measurement and test inks.

Test inks have a defined surface tension. For testing, simply apply a line of ink to the pre-treated substrate. If the surface tension of the test ink matches that of the substrate, the line will neither contract or spread. This test can be carried out at specific points or over a large area, with the test surface stationary or in motion. The accuracy of this method is +/-1.0 mN/m.

Even more accurate results can be achieved with a contact angle measurement in the laboratory. These provide only results for specific points and require much more time and material effort than the use of test inks.

This cannot be answered in a general way. All four physical processes (corona, plasma, flame and silica coating) are equally suitable for the pre-treatment of common plastics (PP, PE, PA, PMMA, ABS, PC, etc.). In practice, the choice of the best pre-treatment method is usually determined by external circumstances. These include, but are not limited to, the following factors.

Application:

• Coating small areas
• Coating large areas

Shape of the part to be pre-treated:

• Type of part (e.g. film, moulded part)
• Surface structure
• Height differences on the surface
• Thickness
• Desired treatment width

Production conditions:

• Machine speed
• Cycle time
• Handling of the part
• Integrability into existing production lines
• Heat sensitivity of the pre-treated material

Yes, in addition to the activation of plastic surfaces, metal and glass surfaces as well as ceramics and paper can also be pre-treated.

Yes, especially for metal and glass surfaces, the cleaning effect of the plasma plays an important role. In the manufacturing process, contaminated surfaces can be subjected to fine cleaning after washing, which in turn shows up in an increase in surface tension. 

Fats or oils that come to the surface during production can be combusted, vaporised or burnt in. This improves adhesion to the surfaces. However, the effect of this measure should always be tested on a case-by-case basis.

Arcotec GmbH has a team of physicists, engineers and technicians who are happy to test your samples in our in-house laboratory. We also help you find the best pre-treatment method for your material.

If you want to carry out simple tests yourself, we offer you a cost-effective and compact laboratory system for this purpose. It is also ideal for mobile use in your production or at your customers’ premises.

We design a tailor-made solution for your application task. We can use all four methods of physical pre-treatment under atmospheric pressure. In this way, you always receive a system that is optimally adapted to your needs.

As an Arcotec customer, you also benefit from our comprehensive after-sales services. Our support team and technicians are at your side 365 days a year.