The mechanism of action of hindered amine light stabilizers

Since the discovery of hindered amines as effective stabilizers against the photodegradation of polymers, the stabilization mechanism of Hindered Amine Light Stabilizers (HALS)has been a subject of discussion. Nevertheless, a complete mechanism has not yet been formulated. In this paper, the degradation of an unstabilized and two HALS-stabilized polyethylene (PE)films is described. The degradations are characterized by measuring: the oxygen uptake, the formation of CO and CO2, the FT-IR spectra, the stabilizer concentration and the oxygen content. The oxygen uptake of the unstabilized PE film led to the expected changes in the IR spectra and to the embrittlement of the film. The oxygen uptake by the HALS-stabilized films caused only minor changes. At an oxygen uptake of 900 mmol per kg of polymer, the HALS-stabilized PE films showed only minor differences in their IR spectra and in their mechanical properties, while the unstabilized material showed an enormous change in its IR spectrum and became totally brittle. Thus, for the HALS-stabilized materials and for the unstabilized materials, the mechanism of oxygen uptake must be different. The difference between the results for the unstabilized and the HALS-stabilized polymers are explained assuming that the initiation of the photodegradation of PE is due to charge transfer complexes. © 1992.