New synergists for hindered amine light stabilizers

The development of hindered amine light stabilizers (HALS) in the seventies led to a tremendous increase in the outdoor use of polyolefins. Although since that time a great deal of insight has been gained into the UV-degradation mechanism of polyolefins and the mechanism of action of HALS, no new UV-stabilization chemistry has been discovered. In this paper, the most recent thoughts about the UV-degradation mechanism of polyolefins (initiation of photo-oxidation by polymer oxygen charge transfer complexes (CTCs)) and new insights into the mechanism of action of HALS stabilizers (quenching of these CTCs) are explored as a basis for designing new UV-stabilizers. A mechanism is proposed that explains the action of HALS as a quencher. Based on this mechanism several other possible quenchers (bridged amines) have been suggested. It is shown that these bridged amines are active as UV-stabilizers in films and plaques and that they show a synergism with HALS stabilizers. These new stabilizers are the most effective when they are present at concentrations higher than 0.1% and at higher concentrations of HALS. In such cases the addition of these synergists to a HALS stabilized system can lead to an improvement of a factor of 2–3.