The action of oxygen corona and of ozone on polyethylene

Abstract

AbstractFilms of low‐density polyethylene which were exposed to a corona discharge in oxygen at a pressure of 1 atm. lost weight and decreased in thickness due to oxidation of the surface to volatile products (principally carbon dioxide and water). The weight loss was directly proportional to the time of exposure and was independent of film thickness. The infrared spectra of the films after exposure to the discharge showed a strong band at 1717 cm.–1 due to carbonyl groups. After a short time in the discharge, the formation of new carbonyl groups in the polymer was balanced by their destruction by oxidation to volatile products at the surface. The intensity of the carbonyl band initially increased rapidly with increasing exposure, but reached a maximum value after 4 hr. and then showed no further change for 90 hr., during which time the thickness of a film originally 4 mils thick was reduced to approximately 1 mil. The data indicate that the carbonyl groups are present in a relatively thin oxidized layer, estimated to be between 5 X 102 and 7 X 104 A. in depth. Carbonyl groups were also formed in films which were not in the discharge area but were exposed to the ozone produced by the discharge. The position of the carbonyl band in both types of oxidized film is consistent with either carboxylic acids or ketone. The relative amounts of the two were determined by soaking the films in ethanolic sodium hydroxide solution to convert the acid to carboxylate ion and measuring the decrease in intensity of the 1717‐cm.–1 band. In the sample which was exposed to the discharge, 60% of the carbonyl was carboxylic acid and 40% was ketone. In the sample exposed only to ozone, the proportion was 30% acid and 70% ketone. It is believed that the carbonyl groups in both cases are the result of attack of ozone on the carbon‐carbon double bonds of the polymer. Small amounts of oxalic acid and other low molecular weight acids were also produced in films exposed to the discharge, presumably due to oxidation of the surface by one or more of the unstable species of the corona.