Studies of the mechanism of polyolefin photodegradation
Through studies of the photodegradation of commercial polymers and their model compounds, the roles of ketone carbonyl and hydroperoxide functional groups in the overall photodegradation mechanism have been elucidated. Besides undergoing photochemical reaction themselves, polymeric ketones are capable of sensitizing the decomposition of the more reactive hydroperoxide functionality by energy transfer involving an exciplex intermediate. It Is now well established that the primary mechanism of photodegradation of hydrocarbon polymers when exposed to terrestrial UV involves the process of photooxidation. Although there has been much controversy in recent years about the relative roles of different absorbing groups in polyolefins, it seems clear that in most cases initiation of the photooxidation process is caused primarily by absorption of light by only two groups, namely the hydroperoxide group and the ketone carbonyl. The relative importance of these two groups in the photooxidation process may vary significantly depending on the structure of the particular polyolefin. In polyethylene, for example, it is well known that the steady state concentration of hydroperoxides during a photooxidation in the solid phase remains very low, while in polypropylene, substantial concentrations of hydroperoxide are produced and maintained in the photostationary state primarily because they exist in a sequential hydrogen bonded form. These hydrogen-bonded peroxides may be significantly more stable to UV photodecomposition than an isolated hydroperoxide group. In polyethylene, careful studies of photooxidation rates under constant UV irradiation in the solid phase indicate that the process follows an exponential relationship, as shown in Fig. 1. Since under Fig. 1. Build-up in earbonyl absorbance during continuous irradiation of low density polyethylene in air. weathering conditions the concentration of polymer and oxygen would be expected to remain relatively constant through the process, It is tempting to ascribe the increase in rate of photooxidation observed experimentally to an increase in the number of initiating sites. The exponential relationship of this 285 Accelerometer. h 286 JAMES E. GUILLET particular curve is shown more clearly when the logarithm of the absorbance due to the carbonyl group in the polymer is plotted versus time of irradiation, as shown in Fig. 2.