A comparison of polypropylene-surface treatment by filamentary, homogeneous and glow discharges in helium at atmospheric pressure

Three different dielectric barrier-controlled discharge regimes in helium a t atmospheric pressure under sinusoidal excitation have been obtained by va rying the excitation frequency or the gas chemical composition: the filamen tary discharge, which is the discharge that is usually obtained; the glow d ischarge, which is controlled by cathode secondary emission; and the homoge neous discharge, which is of a nature in between those of the filamentary a nd the glow discharges. All the characteristics that have been studied, suc h as the discharge current, the emission spectrum, the wettability and the chemical transformations of a polypropylene film, are related to the discha rge-regime variation. The glow discharge is clearly more efficient than the others as a means of increasing the polypropylene-surface energy. Values a s high as 62 mJ m(-2) are obtained with this discharge whereas the maximum value after interaction with the filamentary one is 45 mJ m(-2). This impro vement in wettability is due to there being more O atoms implanted at the s urface as well as to the addition of N atoms. The differences among in surf ace transformations have been correlated to the characteristics of these di fferent discharges and more specifically to the localization of the electri cal energy transfer into the gas and to the nature of the ions created duri ng the discharge.