SynopsisPrevious work has shown that a polymeric phenyl salicylate arid a polymeric dihydroxybenpophenone are formed during the irradiation of polycarbonate with nonspectrally dispersed ultraviolet light. In the present studies the photodegradation of polycarbonate film has been investigated as a function of irradiating wavelength using spectrally dispersed light from a xenon arc between 2300 and 6300 b. Maximum changes in absorbance at 3200, 3600, and 4000 b were induced in the sample by irradiating wavelengths between 2800 and 2900 A. The wavelength sensitivity of an extruded sample of polycarbonate is compared with results obtained for a solution-cast thin film of the material. The results of the present study support the stepwise photodegradation mechanism of polycarbonate previously reported and suggest the possibility of an additional photodegradation process.

The degradation of plastics by ultraviolet energy and the use of protective ultraviolet absorbers to improve stability necessitate studies along several lines. These include (a) the measurement of the ultraviolet spectral energy distribution of sunlight and of the indoor accelerated test sources, (b) the measurement of the extent of degradation as a function of the wavelength of the incident radiation, (c) an estimate of the absorbing effectiveness of the added protective agents, and (d) the stability of the protective ultraviolet absorbers themselves.

The activation spectrum of a material represents the relative amount of damage caused by individual spectral regions of the source to which the material is exposed. Experimental techniques used to determine activation spectra include (1) narrow-band radiation utilizing interference filters or a grating or prism spectrograph and (2) polychromatic radiation with sharp cut-on UV/visible transmitting glass filters to define the relative effects of radiation by individual spectral regions during exposure to all wavelengths longer than the cut-on of the filter. The spectral effects of radiation on a material are determined by measurement of spectral changes, i.e., by UV, visible or infrared spectroscopy, and, if sample size permits, by changes in physical properties. Applications of activation spectra include the development of light stable materials, the design of meaningful light stability tests, timing of exposures based on actinic radiation and prediction of service life by providing a means of determining effective dosage. Examples are given to demonstrate many of the applications of activation spectra and comparison is made with action spectra which represent the wavelength sensitivity of a material independent of the spectral emission properties of the radiation source.

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