Scavengers Hindered Amine Light Stabilizers

Much attention has centred on hindered amine light stabilizers (HALSs), which are efficient scavengers and function by inhibiting degradation of polymers that have already formed free radicals (see also Chapter 7). Commercially, they are now the single most important light stabilizers, followed by benzophenones and benzotriazoles. 

Tetramethyl piperidines were first identified as potent traps for free radicals in the 1960s and the first generation of HAS was commercialized in the 1970s as effective light stabilizers for polyethylene, polyamides, and acrylics. But the mechanism by which HAS act to stabilize polymer radicals is still not entirely clear. 

The mechanism is not fully understood. Hydroperoxide decomposition and free radical scavenging certainly play a part, as also does the regeneration of HALSs, where UV absorbers are frequently consumed as a result of their operation. There are several theories for how this works - possibly by energy transfer, free radical termination, or peroxide decomposition. Significant stabilization is achieved at relatively low concentrations and it appears that the HALS is actually regenerated by the stabilization process, rather than consumed by it. Theory suggests that the hindered amine oxidizes to form amine-ether, which is a non-radical species. 

Claimed to be the first tertiary HALS of its type to be sanctioned by the US Food and Drug Administration as an indirect food additive, a new high-performance additive has been introduced by Cytec Industries, under the name Cyasorb UV 3 529. The company adds that it has applied for the European equivalent to FDA approval. 

It has low reactivity towards co-additives and environmental agents, making it a good choice for pesticide resistance and lower pigment interaction. Lower pigment interaction leads to dramatic improvements in colour yield and UV stabilization in coloured plastics applications. High molecular weight provides the polymer permanence required for products today. 

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Light Stabilizers Pigments (carbon black, iron oxides), UV absorbers (hydroxyphenones, benzotriazoles), excited-state quenchers (organic Ni complexes), free-radical scavengers Hindered amine light stabilizers [piperidines, hindered amine light stabilizers (HALS)]... 

Trapping of the radicals formed during polymer degradation by radical scavengers (hindered amine light stabilizers, HALS)... 

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Primary phenohcs (free-radical scavengers) or secondary phosphite (hydroperoxide decomposer) may be used as heat stabilizers for WPC, while light stabilizers commonly used in WPC include UV absorbers (e.g., benzotriazole or benzophenone), radical scavengers or hydroperoxide decomposers, and hindered amine light stabilizers (HALS). 

Radical Scavengers (Sterically Hindered Amines). Two typical sterically hindered amines (HALS = Hindered Amine Light Stabilizer) follow ... 

Ultraviolet-light stabilizers are of two types those that preferentially absorb UV, thus shielding the polymer structure from UV light and those that protect by quickly scavenging the free radicals as they form. The latter belong to a group of compounds known as hindered amine light stabilizers (HALS). 

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. 

Hindered amine Light stabilizers Hindered amine scavenges radictils which are produced by light. This effect may be expltiined by the formation of nitroxyl radictils... 

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... 

Improvement in the performance of hindered amine light stabilizers [2] (see the two articles on these stabilizers later in this book) as well as their resistance to pesticides (important for greenhouse films) can be achieved by acid scavengers as well. 

The proper choice of the acid scavenger system is particularly important for the additive packages containing hindered amine light stabilizers (HALS). The amine functionality of HALS is easily protonated in the presence of acidic species, yielding an inactive ammonium salt which... 

Hindered amine light stabilizers have attracted enormous scientific and commercial interest due to their efficiency as photo-antioxidants. The stabilization process in the presence of HALS involves the interference of secondary >NH or tertiary (>NCH3) groups with hydroperoxides POOH generated by oxidation chain mechanism of polymer alkyl radicals P. HALS-derived nitroxides >NO, formed through scavenging peroxyls POO by hydroxylamine species >NOH, are the key intermediates in the HALS stabilization mechanism because they are able to quench alkyl radicals arising in the initiation step of oxidation [100, 101]. The involvement of HALS in the stabilization process can be attributed to their ability to remain an... 

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