PHOTODEGRADATION OF POLYPROPYLENE FILMS .2. PHOTOLYSIS OF KETONIC OXIDATION PRODUCTS

Oxidation products can be expected to be inadvertently produced during the processing of polypropylene. In order to study the nature and photolytic mechanisms of these products, polypropylene films were prepared from resin which had been extensively air oxidized at 225° for various times. These films were then degraded under vacuum by irradiating at various wavelengths in the region 2200-3800 Å. Photolytic changes in the oxidized film samples were followed by glpc and ir analysis, and by film weight loss. The ir spectra of the air-oxidized films were compared with the ir spectra of model compounds related to polypropylene. From this comparison, and a consideration of the changes detected in the oxidized films during irradiation, the main oxidation products were believed to be the polymeric ketones ~CH(CH3)CH2C(=O)CH2CH(CH3)~ (A) and CH(CH3)CH2C(=O)CH3 (B). A study of I he ir changes and the composition of the volatile products produced during irradiation indicated that ketone A is photolyzed primarily by a Norrish type I scission to give carbon monoxide and two macroradicals. whereas ketone B is photolyzed by a Norrish type II scission to give acetone and an unsaturated polymer chain end. The quantum yield for each photolytic reaction was determined from measurements of ir change and weight loss at various irradiation times. Both quantum yields were found to be approximately 0.08. The Norrish type I cleavage of ketones A and B is believed to be a possible source of initiation in the very early stages of the photooxidative deterioration of polypropylene. In later stages of deterioration, however, the photooxidation is probably dominated by other initiation steps, such as hydroperoxide photolysis. © 1969, American Chemical Society. All rights reserved.