Gaseous plasma technology for tailoring surface properties of polymers
The surface properties of polymer materials are rarely adequate; therefore, they should be modified prior to painting, printing, gluing, or deposition of various coatings. A popular technique for tailoring surface properties of polymer materials is a brief treatment with gaseous plasma. Depending on the required surface finish, a variety of gases are used, and plasma is created using different discharges. The most common surface finish is activation; it stands for removal of any surface impurities, functionalization of polymer surface with polar functional groups, and often also roughening of the polymer material on the sub-micrometer scale. Such a surface finish is usually achieved by treatment of polymer products in weakly ionized oxygen plasma. Oxygen plasma is usually rich in charged particles as well as neutral reactive gaseous particles, especially neutral O-atoms in the ground state. When exposing a polymer to oxygen plasma, both functionalization, etching, and sometimes even modification of the subsurface film occur simultaneously. In many cases, such a combination of different effects is beneficial since they may result in a super-hydrophilic surface finish.
Oxygen plasma treatment is obsolete for hydrophilization of fluorinated polymers because etching is predominant, so functionalization with polar oxygen functional groups is marginal. Such polymer should be activated by a two-step method: in the first step, the surface film of thickness a few nm is depleted from fluorine by irradiating with vacuum ultraviolet (VUV) radiation. Such radiation will break C-F bonds in the surface film of thickness that corresponds to the absorption depth of VUV radiation. The best source of such radiation is hydrogen plasma. The H-atoms will interact with the free F-atoms to form HF molecules, and the C- dangling bond is occupied with an H-atom. The surface layer of a fluorinated polymer thus resembles a polyolefin after treatment with hydrogen plasma. Polyolefins are easily activated by using oxygen plasma, but there is a limited range of fluences that assures for super-hydrophilic surface finish.
