The exact mechanism of the surface treatment remains unknown. Despite continuous efforts, using complex instruments and sophisticated laboratory methods, we still do not have an accurate understanding of processing. Fortunately, we don't need this information to use this process.
Solid surfaces have surface energies specific to each material. In order for a droplet to spread across a given surface, the surface tension of the liquid needs to be lower than the critical surface tension of the solid. Metals and glasses have high surface energies, while plastics have low surface energies. Pretreatment increases the surface energy and thus its wettability. It also eliminates weak boundary layers, improving adhesion.
In flame processing, the high temperature of the combustion gases dissociates oxygen molecules to form free highly chemically reactive oxygen atoms. In addition, due to the energy in the high-temperature combustion process, oxygen atoms may also lose electrons and become positively charged oxygen ions. This electrically neutral gas, made up of equal amounts of positively and negatively charged particles, is called a plasma. Plasma may be hot or cold.
During flame treatment, these high-velocity, high-energy, highly reactive oxygen ions or free oxygen atoms bombard the surface of the plastic and react with the molecules. This process oxidizes the surface and requires an oxidation flame, which is an excess of oxygen.
In corona treatment, a high voltage field causes the oxygen molecules to break down into free atoms, which can react with the plastic. Those that do not react with the surface recombine into normal diatomic oxygen molecules or unstable ozone molecules - triatomic oxygen. In addition, surface treatment involves many other complex reactions.
In many cases, the goal is to treat the surface to a predetermined critical surface tension expressed in dynes/cm. ASTM specification D-278-84 describes a method for evaluating the level of surface preparation. An increase in surface energy is often associated with improved adhesion. However, sometimes the substrate may be wettable and still not provide the desired level of adhesion. Flame healing requires a consistent level. This requires keeping the operating parameters constant and having a uniform substrate surface.
