The treatment is effected by direct exposure of the substrate to a gas flame for a short period of time. The flame carries some excess oxygen, which takes part in the combustion, building intermediate products in form of active ions such as hydroxyl and carboxyl groups. These ions are bound into the surface, thus building polar groups, which enable a reaction with the coating to be applied.
The surface ,which is nonpolar by nature, becomes polar and can be bonded with ink, varnish, adhesion etcetera.Therefore, flame treatment is not a heat application, but the controlled use of a chemical property of the flame.
The precise setup of the air/gas mixture is very important to create the excess of oxygen. The presence of polar groups increases the surface energy.