20 The Light Stabilization of Polystyrene CHRIS SAVIDES, JOSEPH A. STRETANSKI, and LEO R. COSTELLO
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American Cyanamid Co., Organic Chemicals Division, Bound Brook, N. J. It is well recognized that polystyrene discolors appreciably on exposure to both outdoor and artificial light sources. Certain phenolic antioxidants, in combination with ultra violet absorbers, afforded greater light stability to the poly mer than the ultraviolet absorbers alone. Correlation studies between accelerated devices and actual fluorescent lamp exposure indicated that high output, cool white, fluorescent lamps are the best sources for predicting performance char acteristics of light-stabilized polystyrene. Phenolic antioxi dants inhibit both the rate of discoloration and the rate of photooxidation under irradiation with ultraviolet sources rich in energy above 3000 A. '"phe discoloration of polystyrene on exposure to light has long been recognized as an undesirable property. This deficiency is particularly important to the lighting industry since polystyrene has gained wide acceptance for use influorescentlightingfixtures.Because of the many advantages of polystyrene for this application—ease of fabrication, good dimensional stability, clarity, and especially low cost—considerable efforts have been made to improve its light stability. While these efforts have been met with some success and polystyrene lightfixturesare now com mercially produced in large quantities, there is continued demand for improved stabilization. The most common approach for photostabilizing polymers is by incorporating ultraviolet absorbers. These materials are light-stable organic compounds which absorb strongly in the region 3000-4000 A. Their mode of action is mainly through competitive ab sorption of the ultraviolet energy responsible for the polymer degradation. Figure 1 shows the activation spectrum of polystyrene measured by Hirt and co-workers (8). It represents the yellowing, as measured by the increase in absorbance at 4000 Α., caused by the various wavelengths of ultraviolet radiation between 2800 and 4000 A. The ultraviolet source used for these measurements was a xenon arc equipped with a borosilicate 287
Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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glass filter. Maximum degradation occurs with wavelengths of 30003200 A . The activation spectrum is of practical importance because it can be used to select ultraviolet absorbers for stabilization. Among the types of ultraviolet absorbers that may be used as stabi lizers are salicylates, o-hydroxybenzophenones, o-hydroxyarylbenzotriazoles, and certain acrylonitriles. The stabilization of polystyrene by other additives has also been reported. Matheson and Boyer (10) found that certain aliphatic amines and amino alcohols improved the light stability of the polymer.
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Figure 1. Activation spectrum of polystyrene measured by Hirt et al. (8) This paper deals with a study of phenolic antioxidants and their usefulness i n the light stabilization of polystyrene ( 3 ) , especially as applied to indoor lighting applications, and their mode of action. It is generally agreed that light degradation of polystyrene is a photooxidation reaction caused by ultraviolet radiation. Evidence for such reaction has been derived through spectrophotometric measurements, both infrared (2) and ultraviolet (12), mass spectrometry ( I ) , and oxygen absorption studies ( 5 ) . Achhammer (2) and co-workers, who originally studied the effect of ultraviolet radiation on this polymer, reported that on exposure to a sunlamp the infrared absorption bands increase in the regions of 2.8μ. and
Platzer; Stabilization of Polymers and Stabilizer Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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SA VIDES E T A L .
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Light Stabilization
5.8/x. This increase was attributed to the formation of hydroxyl and carbonyl groups, respectively. Figure 2 shows the typical increase in the carbonyl absorption band of a commercial polystyrene film irradiated with a carbon arc.
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