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

In a previous paper it was demonstrated for the first time, that carbon bla ck pigments have the ability to operate as effective singlet and triplet qu enchers. This work was undertaken directly in the polymer using the inheren t long-lived luminescent species as triplet donor species. In order to asce rtain the effectiveness of the quenching mechanism and its inter-relationsh ip with carbon black parameters, a total of eight different carbon black pi gments have been incorporated into low density polyethylene at different co ncentrations (0.1, 0.2, 0.4% w/w) and their effects determined an the quenc hing the triplet state of a known triplet photosensitiser, benzophenone (0. 05% w/w). Using phosphorescence analysis at 77 K a large variability was ob served in the benzophenone triplet lifetime quenching, the effect varying w ith the pigment type and the concentration of pigment. Photodegradation rat es 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 unu sual synergistic effect between the carbon black and the benzophenone sensi tiser was observed compared with films containing only carbon black. The im plications of carbon black pigments to operate as "super quenchers", as wel l as antioxidants in this study and under practical concentration condition s is discussed. Although not consistent throughout, there is some indicatio n that low surface area carbon black pigments can be the most effective que nchers of active excited states. However, the presence of surface containin g oxygen groups can be related to the synergistic effect reported, using ex perimental design statistical analysis. (C) 2000 Elsevier Science Ltd. All rights reserved.