Hindered Amine Light Stabilizers
One main category of light stabilisers consists of what are known as hindered amine light stabilizers (abbreviated as HALS). They are derivatives of 2,2,6,6-tetramethyl piperidine and are extremely efficient stabilizers against light-induced degradation of most polymers.
HALS do not absorb UV radiation, but act to inhibit degradation of the polymer. They slow down the photochemically initiated degradation reactions, to some extent in a similar way to antioxidants (visit the Antioxidants Center).
One advantage of the hindered amine light stabilizers is that no specific layer thickness or concentration limit needs to be reached to guarantee good results. Significant levels of stabilization are achieved at relatively low concentrations. HALS' high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process.
The mechanism of hindered amines stabilizers against thermooxidation appears to be complex. Because of the regenerative nature of this process, as well as the typically high molecular weights of the stabilizers, hindered amine stabilizers are capable of providing extreme long-term thermal and light stability.
What tends to be important for the choice of the light stabilization system is the chemical nature of the polymers used and tackifier resins, as described in the next pages. Interaction has also been observed with fillers and pigments.
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