Often, when gluing, it is not easy to achieve good adhesion and strong connections between two bonding partners. However, when a surface is functionalized with plasma before bonding, the resulting adhesions show a significant improvement in adhesion.
Atmospheric pressure plasmas possess a remarkable combination of properties that enable unique surface treatment. Large quantities of highly reactive but short-lived chemical species are produced, which functionalize and finely clean surfaces, thus preparing them for gluing, painting, and printing.
In contrast to wet-chemical primers, no outgassing of the surface is necessary after plasma treatment. This is due to the almost instantaneous decomposition and evaporation of surface organic contaminants as well as the generation of polar molecular end groups on plastic surfaces (see Figure 1).
The effect of the plasma on the surface can be determined by the improved wettability after treatment. To quantify this effect, the so-called contact angle measurement is used. Here, the angle formed by a droplet of a test liquid with the solid surface is measured. If the droplets (see Figure 2) of a polar and a non-polar liquid (e.g., water and diiodomethane) are measured on a surface in this way, the so-called surface energy of the solid can be determined. This is consequently composed of a polar and a dispersive part and is a measure of the wettability of the surface. In principle, the following applies: Better wetting leads to flatter droplets with a smaller contact angle and is measurable in an increased value of surface energy in the unit mN/m. Surface energies can also be tested with so-called test inks.
While conventional high-performance plasma jets are often used in line production for maximum process speeds, the Piezoelectric Direct Discharge (PDD®) technology is suitable for the manual use of atmospheric plasma. This operates with a few watts of power consumption and produces a highly efficient plasma that operates at less than 50 °C without significant heat losses. The use of this technology in handheld devices such as the piezobrush® PZ3 enables the activation of plastics and fine cleaning of almost all material groups at typical speeds of 20 mm/s with a treatment width of up to 29 mm.
Figure 4 shows the surface energies of selected materials before and after treatment with cold plasma. It becomes clear here that atmospheric plasma, thru active oxygen species, particularly increases the polar component of the surface energy of many materials.
This polar component of the surface energy corresponds to the polar components in liquids such as adhesives and sealants, printing inks, or paints. This allows for the formation of numerous stable chemical bonds, which in turn leads to improved adhesion. Figure 3 shows a comparison of the peel strength when bonding untreated plastics and the same plastics with prior plasma treatment using 2K epoxy adhesive. The comparison in Figure 3 shows a significant improvement in the peel strength.
Relyon Abb. 1
Many plastics do not have polar molecular groups available that could form a bond with adhesives and sealants. Thru plasma treatment, these anchor groups are quickly and easily generated.