The need for surface treatment was recognized shortly after the development of polyolefin materials, leading to the evolution of various treatments. WH Kreidl pioneered the use of oxidizing flames on polyolefins to produce surfaces that are easy to print and coat. Meanwhile, Kreidl's assistant, Kritchever, was developing a corona to produce the same result. The pair went their separate ways and both methods have been used extensively for the past 30 years for a variety of surface preparation applications.
Solid surfaces have surface energies specific to each material. In order for a droplet to spread to a given surface, the surface tension of the liquid needs to be lower than the critical surface tension of the solid. Metallic glasses have high surface energies while plastics have low surface energies. Pretreatment increases the surface energy and hence its wettability. It also eliminates weak boundary layers, thus improving adhesion.
In flame processing, the high temperature of the combustion gases causes the oxygen molecules to become dissociated, forming free, highly chemically reactive oxygen atoms. In addition, oxygen atoms may also lose electrons to become positively charged oxygen ions due to energy during high-temperature combustion . This electrically neutral gas- charged particle consisting of equal amounts of positive and negative electrodes is called a plasma. Plasma may be hot or cold.
During flame treatment, these high-velocity, high-energy, very reactive oxygen ions, or free oxygen atoms, bombard the plastic surface and react with the molecules. This process oxidizes the surface and requires an oxidizing flame, a flame with an excess of oxygen.
In corona treatment, a high voltage field breaks down oxygen molecules into free atoms, which can react with the plastic. Those molecules 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 units of dynes per centimeter. ASTM specification D-278-84 describes an evaluation of surface treatments. An increase in surface energy is often associated with improved adhesion. However, sometimes a substrate may be wettable and still not provide the desired level of adhesion. Flame treatment needs to be done to a consistent level. This requires keeping operating parameters constant and having a uniform substrate surface.
