Hindered amine light stabilizer reactions

There are several claims in literature for the reaction of acetone and ammonia, at temperatures like 50°C, to give triacetoneamine (2,2,6,6-tetramethyl 4-piperidone), using Nafion and other CIERs (Eqn. (10)). This product is the starting material for the renowned hindered amine light stabilizers. 

Kimura, K., et al. Direct determination of a polymeric hindered amine light stabilizer in polyproplylene by thermal desorption-gas chromatography assisted by in-line chemical reaction, Analyst, 2000, 125, 465. 

Obviously, in a PA melt, phenols undergo reactions and the reaction products are unable to react with the amide peroxy radical. On the other hand, antioxidants of the hindered amine light stabilizer type interrupt the oxidation reaction of PA6 after an induction period. During this induction period, they are oxidized to the N-oxyl form, which is essentially reactive... 

Hindered amine light stabilizers (H ALSs) have been described in detail (25). They are derivatives of 2,2,6,6-tetramethyl piperidine and are extremely efficient stabilizers against the light-induced degradation of polymers. HALS do not absorb any UV radiation, but they inhibit the degradation of a pol5mer. In particular, they slow down the photochemically initiated degradation reactions in a similar way as antioxidants (26). 

Stabilizers inhibit the chemical reaction between two or more other chemicals, and inhibit the separation of suspensions, emulsions, or foams. Stabilizers include (1) antioxidants that prevent unwanted oxidation of food materials. (2) UV stabilizers that protect food materials from harmful effects of UV radiation, being (a) UV absorbers which absorb UV radiation and prevent it from penetrating the materials, as sunscreens, (b) Quenchers which dissipate the radiation energy as heat instead of letting it break chemical bonds, (c) Scavengers that eliminate the free radicals formed by UV radiation, as hindered-amine light stabilizers. (3) Sequestrants that inactivate traces of metal ions that would otherwise act as catalysts by forming chelate complexes. (4) Emulsifiers and surfactants that stabilize emulsions. 

Keywords photooxidation, UV absorbers (UVA), outdoor performance, weathering, accelerated weathering, kinetic chain length, photoinitiator, Norrish reactions, peroxide decomposer, hindered amine light stabilizers (HALS), photosensitizer, transition metal complex, UV stabilizer, time-controlled stabilization, reactive antioxidants, polymeric antioxidants. 

Some stabilizers act via cyclic reaction paths and are regenerated, remaining effective at lower concentrations or for longer periods of time than would otherwise occur. This is the case with hindered amine light stabilizers (HALS), an example of which is bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (often known by a commercial name Tinuvin 770), which are especially effective in improving weatherability in polypropylene and other polyolefins. 

The main difference in the various HALS is the substituent on the hindered nitrogen of the piperidine ring. Hindered amine light stabilizers reduce the photooxidation rate by scavenging free radicals and thus interfering with the oxidation cycle of R2-R3. The basic reactions that are important are summarized in reactions R6-R10.15... 

Sterically hindered amine light stabilizers (HALS) constitute another class of light stabilizers that efficiently reduce UV-induced degradation. They are compounds that intercept chemically highly reactive radicals formed hy photochemical reactions and transform them into harmless reaction products. Fig. 3.8. Their effectiveness is not based on a competing physical absorption effect, therefore no minimum layer thickness is required for HALS effectiveness, unlike for the UV absorbers. That makes them suitable for applications such as films and fibers (527). In particular, they provide surface protection, which plays a special role in visible automotive parts. Another essential difference to UV absorbers is the fact that photooxidation is not prevented, but after initial damage its propagation is either reduced or suppressed [510],... 

Hindered amine light stabilizers are reacting via the nitroxyl fimc-tion.The mechanism of action is shown in Figure 20.3. Examples of reactions of nitroxyl radicals are shown in Eq. 20.14. 

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