COMPARISON OF THE EFFICIENCY OF N-2 AND NH3 PLASMA TREATMENTS TO IMPROVE THE ADHESION OF PP FILMS TO IN-SITU DEPOSITED AL COATINGS - STUDY OF AGING PHENOMENA IN TERMS OF ACID-BASE PROPERTIES

This paper reports on the characteristics of polypropylene (PP) treated at two different frequencies, 70kHz and 13.56 MHz, in two plasma atmospheres, NH3 and N-2 plasmas. Chemical modifications created on the plasma-treated material were investigated by different complementary surface analytical techniques (X-ray photoelectron spectroscopy (XPS) and contact angle measurements), which revealed a higher reactivity of the NH3 plasma. The incorporation of nitrogen plays an important role as revealed by the adhesion study of thermally evaporated aluminium with treated PP: the measured peel strengths were two times higher for NH3-treated surfaces than for N-2-treated ones, with XPS measurements showing a sharper increase of N/C ratio as a function of treatment time for the former treatment. Ageing of the PP pretreated in NH3 was also studied in terms of surface acid-base properties. The work of adhesion, calculated from contact angle measurements, showed a distinct dependence on the pH value of the test liquids and good correlations were obtained with peel test results. The basic character of the NH3-treated PP disappears with ageing, leading to an amphoteric surface.