High-Intensity Flash Irradiation of Polymers

Abstract

IN an attempt to shorten exposures in the ultraviolet irradiation of polymers, we have used as light sources high-intensity flash discharge lamps, described by Murphy and Edgerton¹, which have been used extensively in flash spectroscopy^2–4 and flash photolysis^5,6. The lamps used were similar to Christie and Porter's fused silica spiral lamp No. III⁷ and usually were flashed with 648 µf. at 4,000 V. (5,184 j.). Each flash lasted about three milliseconds, and by uranyl oxalate actinometry⁸ was shown to yield 9.6 × 10¹⁹ quanta in the 2000–4900 A. region. The polymers were placed approximately along the axis of the lamp coil, either in fused silica or ‘Pyrex’ glass test-tubes, and were irradiated in air, streaming nitrogen, or in vacuo. In several polyethylenes, polystyrene, polyethylene terephthalate, polyiso-butylene, 6-10 nylon, polyvinylchloride, polytetra-fluoroethylene, regenerated cellulose and cellulose acetate, we found that, in addition to some minor homogeneous photolysis, a single flash caused violent heterogeneous decompositions within the polymer matrices. These decompositions usually were accompanied by a sharp report, the formation of gas bubbles and carbon spots within the polymer, and the deposition of fluorescent (under mild excitation by 3000–4000 A. radiation) substances near centres of decomposition. These heterogeneous decompositions do not seem to depend on the chemical properties of the polymers but rather on the presence of particulate impurities within the polymer matrices. This is shown by: (1) the wide spatial separations among decomposed areas (0.5–5 mm.); (2) the transparency of many of the polymers in the wave-length region used; (3) the reproducibility of the decompositions when certain particulate contaminants are added to the polymers before irradiating; and (4) the fact that clean polyethylene taken directly from a polymerization reactor (this polyethylene was kindly supplied by Dr. S. W. Hawkins of Imperial Chemical Industries, Ltd.) undergoes very much less decomposition than polyethylenes that presumably have become contaminated during normal processing.