4.3.6.2 Corona Discharge

In a corona discharge process, a hard-to-bond plastic is exposed to an electrical discharge, usually in the presence of air and at atmospheric pressure, which creates a plasma ‘field’. This roughens the surface, which provides sites for mechanical interlocking and introduces reactive sites on the plastic’s surface, consequently increasing the wettability and reactivity of the surface. The reactive functionalities, which are theorised to be introduced to the surface, may include carbonyl, hydroxyl, hydroperoxide, aldehyde, ether, ester and carboxylic acid groups, as well as unsaturated bonds.

4.3.6.3 Flame Treatment

Flame treatment increases the bondability of plastic by oxidising the surface through brief exposure to flame. The oxidation proceeds as a free radical mechanism, accompanied by chain scissions and some crosslinking. The functionalities introduced by oxidation are hydroxyl, carbonyl, carboxyl and amide groups, with a typical oxidation depth of approximately four to nine nanometers. Wettability is improved by increased surface energy. The chain scissions on the surface contribute to higher interfacial diffusivity.