Triplet quenching and antioxidant effect of several carbon black grades in the photodegradation of LDPE doped with benzophenone as a photosensitiser

In a previous paper it was demonstrated for the first time, that carbon black pigments have the ability to operate as effective singlet and triplet quenchers. This work was undertaken directly in the polymer using the inherent long-lived luminescent species as triplet donor species. In order to ascertain the effectiveness of the quenching mechanism and its inter-relationship with carbon black parameters, a total of eight different carbon black pigments have been incorporated into low density polyethylene at different concentrations (0.1, 0.2, 0.4% w/w) and their effects determined an the quenching the triplet state of a known triplet photosensitiser, benzophenone (0.05% w/w). Using phosphorescence analysis at 77 K a large variability was observed in the benzophenone triplet lifetime quenching, the effect varying with the pigment type and the concentration of pigment. Photodegradation rates on the films were also performed in order to relate the quenching effect of the carbon black with films exposed to UVA light at 70 degreesC. An unusual synergistic effect between the carbon black and the benzophenone sensitiser was observed compared with films containing only carbon black. The implications of carbon black pigments to operate as "super quenchers", as well as antioxidants in this study and under practical concentration conditions is discussed. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. However, the presence of surface containing oxygen groups can be related to the synergistic effect reported, using experimental design statistical analysis. (C) 2000 Elsevier Science Ltd. All rights reserved.