Improvement of the adhesion of low-energy polymers by a short-time plasma treatment

Foils of polyethylene, polypropylene and polyethylene terephthalate are exposed to low-pressure plasmas. The plasma is produced by microwaves (f=2.45 GHz) at the electron cyclotron resonance, with electron densities of about 10(11) cm(-3) and electron temperatures between 2 and 4eV. Oxygen and hydrogen are used here for plasma treatment at a pressure of typically 3 Pa. Quartz-like and polyethylene-like thin films are produced on the polymers as adhesion agents. The adhesion power between the polymers and the adhesive is measured by pull-off tests. The adhesion strongly increases by a plasma treatment, for polyethylene by a factor of 20 after treatment of a few seconds. The surface tension of the polymer foils is measured using the Cahn surface tensiometer. It is shown that the polar part of the surface tension of the tested polymers increases with plasma treatment, whereas the dispersion part is only weakly influenced. The adhesion power and the polar part of the surface tension of the polymers are correlated. The polymer surfaces are investigated by scanning electron microscopy and by infrared spectroscopy.