A study of the surface modification of polyethylene by corona and plasma discharge and of some stable free radicals, using esca and other techniques

Abstract

The surface modification of polyethylene by Corona and Plasma discharges is studied by X-Ray photoelectron spectroscopy and multiple internal reflection spectroscopy. Wettabilities are used to observe changes in surface energy and the strengths of bonds formed using adhesives are measured. A short account is given of the importance to manufacturers of being able to increase the surface energies of polymers. The conditions needed to establish the corona discharge and the nature of the corona discharge are discussed. The reactions occurring within the plasma formed by the discharge and the various reactive species formed are considered and the effects of these species are discussed. Plasma discharges at low pressures and their effects on polymer surfaces are also considered. The surface modification of polyethylene by both corona and plasma discharges is shown to be associated with an increase in the oxygen functionality of the surface. The relationships between the increase in oxygen and carbon functionalities are discussed and the possible groups formed are suggested. Many of these are polar in character and include hydroxyl, carbonyl, carboxylic, carbonate and hydroperoxide groups together with ether and possibly peroxide (non polar) groups. These groups are found to be produced even when only traces of oxygen are present. The use of nitrogen gas is shown to produce high levels of amine and amide functions. The polar groups are believed to be largely responsible for the increased wettabilities of the surfaces and partly responsible for the increased adhesive strength. In this latter case cross linking at the surface is also believed to be important. Some evidence is put forward, from the heat treatment of plasma treated polyethylene, for free radicals being involved in some of the reactions at the surface- The fact that free radicals are not detected after the surface modification in many instances is thought tube due to the fact that they often react completely with oxygen as fast as they are formed. The surface charging of polyethylene during corona discharge is examined and shown to be largely due to deeply trapped electrons rather than merely superficial charges. A comparison is made between corona and plasma discharges and the suitability of each method in different situations is considered. The structure and bonding in the perchlorodiphenyl aminyl radical is investigated to see if this could helping the investigation of the effects of corona and plasma treatment of polyethylene. The results, though interesting in themselves, reveal that the multiplet splitting effect is too small for this effect to be used in investigating surface changes in polyethylene.