A requirement to achieve adhesion between an adhesive and a substrate is that the former wets the latter. If an adhesive does not wet the surface that you want to bond, there are two options: 1) Change adhesive, or 2) Pre-treat the substrate surface.
There are many pre-treatment methods for plastics, including roughening, primer, etching, Corona and plasma treatment. In the video below, you will see how a difficult-to-bond plastic (polypropylene, PP) is affected by plasma treatment.
Surface tension & wetting
Plasma treatment (in the featured video using a low-pressure plasma equipment from Diener) has several functions that you can read more about in our previous blog post about plasma. One of them is to increase the surface tension of plastics. The surface tension of an adhesive and the surface that is to be bonded determines if the adhesive will wet that surface. The general rule is that an adhesive will wet a surface that has a higher surface tension than itself. In other words, plasma treatment makes a plastic surface easier to wet and therefore easier to bond!
To evaluate wetting, special testing fluids, or inks, can be used. These have a known surface tension and can thereby indicate the surface tension of the substrate. In the video, a fluid with a surface tension approximately equal to that of an epoxy adhesive is used.
Results from plasnma treatment of polypropylene
Before pre-treatment (only cleaning), this ink does not wet the polypropylene plastic at all, i.e. the plastic has a lower surface tension than the ink. After plasma treatment for 5 minutes, however, the result is the opposite. The ink now spontaneously wets the plastic which means that the surface tension of the polypropylene now exceeds that of the ink.
According to the results of this simple test, the polypropylene should now be significantly easier to bond.
How does a low pressure plasma work?
- Details are placed in the plasma chamber.
- Using a vacuum pump, the pressure in the chamber drops to approximately 0.2-0.4 mbar.
- A gas (in this case water vapor) enters the chamber and is transformed into plasma. The plasma, in turn, changes the properties of the surface exposed to it. The bright purple color is the plasma.
- After a few minutes, the pressure is released and the details can be taken out of the chamber and be bonded.