Surface modification of polymeric materials by atmospheric plasma treatment

Abstract

We have been able to generate the wide and stable plasma in open air (discharge distance, 35 cm; discharge-electrode length, 16 m at maximum) using a pulse with a high voltage and narrow wave form. This was applied to treat the surface of rather non-polar plastics intended for the improvement of adhesion of over-coated layers such as coatings, adhesives and printing inks. The treating system (APPS) consists of the apparatus for generating the plasma and the treating process. Polypropylene (PP) and tetrafluoroethylene perfluorovinyl ether copolymer (PFA) have been examined as typical examples of the plastics. The adhesion strength of urethane paint on PP molding and of a PFA film on steel was significantly improved by the APPS treatment. The characteristics of the surface layer were evaluated by means of scanning electron microscopy, electron spectroscopy for chemical analysis, atomic force microscopy, and contact angle measurement, and it was found that hydrophilic functional groups were introduced into the surface layer of the plastics. The level of the improvement changed with time after treatment; this is discussed from the viewpoint of functional group movement from the surface to the interior. Application of paints on PP bumpers by the electrostatic spray method was also accomplished. The use of a small amount of nitrogen-containing compound following APPS treatment decreased the electrical resistance of the PP surface from 1016 to 1011Ω, and highly effective electrostatic coatings of PP bumpers could be realized.