Hindered Amine Light Stabilizers
The ability of hindered amine light stabilizers (HALS or HAS) to scavenge radicals which are produced by weathering, may be explained by the formation of nitroxyl radicals through a process known as the Denisov Cycle. The nitroxyl radical(R-O•) combines with free radicals in polymers:
R-O• + R'• → R-O-R'
Although they are traditionally considered as light stabilizers, they can also stabilize thermal degradation.
Even though HALS are extremely effective in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride (PVC). It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized by hydrochloric acid (HCl) that is released by photooxidation of PVC. The exception is the recently developed NOR HALS which is not a strong base and is not deactivated by HCl.
Read more about this topic: Stabilizers For Polymers
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