Hindered amine
Hindered amines are chemical compounds containing an amine functional group surrounded by a crowded steric environment. They have uses such as gas scrubbing, as stabilizers against light-induced polymer degradation, and as reagents for organic synthesis.
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. These advantages result in an expected considerable increase of demand for HALS, especially in products made from polypropylene and polyethylene. [1]
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.
Mechanism of action of Hindered amine
Current work regarding how the hindered amine light stabilizers work suggests that a number of different reactions are responsible for the stabilizing effect they have on photooxidation. Proof has been found of the radical entrapment qualities of carbon radicals, the reaction of HALS metabolites with peroxy radicals and the decomposition of hydroperoxides.
An acidified hindered amine cannot easily enter into the free radical scavenging cycle.
N-H and N-R hindered Amine Stabilizers fit most needs regarding light stability but can be alkaline.
N-OR type enter the UV stabilization cycle quickly and are much less alkaline than N-H or N-R type of HALS.
References
- U.S. Patent 4,405,579 Sterically hindered amino acids and tertiary amino acids as promoters in acid gas scrubbing processes. S. Guido and W. A. Thaler
- ↑ Market Study Stabilizers, published by Ceresana, May 2011