UV‐stabilization of ABS with sterically hindered amines. Results of an ESR‐study

Abstract

AbstractABS polymers have been limited in outdoor applications due to their poor weathering characteristics. On exposure to light the polybutadiene phase on the exposed side of the sample is photooxidized resulting in loss of impact strength and discoloration. We have found that the combination of a hindered amine light stabilizer (HALS) and a benzotriazole UV absorber dramatically outperforms either component alone. This L‐S blend provides a level of impact strength retention many times that of unstabilized ABS and also protects the polymer from discoloration. The present study describes the results of an E.S.R. investigation intented to explain the synergistic effect between HALS and benzotriazole UV absorber. Since the photostabilizing effect of HALS is associated with the reaction of the nitroxide radicals formed upon irradiation, the concentration of these radicals, as a function of light exposure time, was determined. Two types of . radicals were found. First, a broad . signal with an anisotropic hyperfine nitrogen interaction, was detected. This signal is characteristic of the . radicals localized in the SAN matrix. Second, a narrow . signal with an isotropic hyperfine nitrogen interaction, corresponding to nitroxide radicals localized in the polybutadiene phase, was also found. The . concentration in the SAN matrix and in the polybutadiene phase could, therefore, be determined separetely, as a function of light exposure time.In both phases, a maximum of . is observed after approximately 50 hours exposures. However, the maximum . concentration is ten times higher in the polybutadiene phase of ABS stabilized with a blend 0,5% HALS/0,5% UV Absorber than in the BR phase of an ABS containing only 0,5% HALS. On the other hand, there is only a slight increase of . radicals in the SAN phase when 0,5% UV absorber is added on top of 0,5% HALS. A comparable effect is observed in the polybutadiene phase when 8% titanium dioxide is added to an ABS stabilized with 0,5% HALS.Since the polybutadiene phase is the most sensitive toward photooxidative degradation, these results rationalize the observed synergistic effect of the combinations of HALS/Benzotriazole UV absorber.