UV radiation splits molecules, i.e. breaks of individual atoms or molecule fragments. The broken-off unpaired atoms are referred to as ⇒ radicals. They are characterised by the unpaired bond at the site of the split. This is the radical site. The radical site has at least one free bonding electron or at least one free site for accepting a bonding electron. A matching partner is found easily at this site, in best case another matching radical. When non-polar plastics are activated in an oxygen plasma, a hydrogen atom is replaced by an oxygen atom. It requires only one bond to link with the (in most cases) carbon atom. Accordingly, one radical site remains open to which a monovalent partner can easily attach. The greatly improved gluing properties of surfaces following activation is due to this effect.