Hindered amine stabilizers free-radical reactions

And so, the work on mechanisms of autooxidation at the British Rubber Producers Association, the early work on the synthesis and reaction of stable free radicals, the recognition of the rale of stable free radicals in polymer stabilization, the discovery of stable triacetonamine-N-oxyl, and the search for practical candidates for commercialization, have led to the development of hindered amine stabilizers, a new class of polymer stabilizers. They are effective in many polymers against photodegradation and also are effective against thermooxidation in some polymers. The structures of the current commercially available products for polymer stabilization may be seen in Figure 7. These compounds are effective in meeting the stabilizer requirements in many commercial polymers however, others are under development to satisfy requirements not being met by them. 

As with radical-scavenging AOs, HALS (or sometimes more concisely called hindered amine stabilizers, HAS) deactivate free radicals created by reactions with UV light. HALS has been a successful additive technology by any measure. 

Chain-breaking acceptor (CB-A) antioxidants, on the other hand, act by oxidizing alkyl radicals in a stoichiometric reaction and hence are only effective under oxygen-deficient conditions (reaction lOd). Antioxidants with structures based on benzofuranone derivatives (lactones) and hydroxylamines, as well as on quinones and stable free radicals, are good examples of CB-A antioxidants (91-96). Hindered amine derivatives [often referred to as hindered amine stabilizers (HAS) eg, AOs 25-27, Table 3 also function by a chain-breaking mechanism and, through their transformation products, are able to trap both R. and ROO in a cyclical regenerative mechanism (50,55,62,94,97-100) for simplified reaction mechanism, see Scheme 11. 

Quenching agents do not absorb ultraviolet radiation but stabilize polymers by reacting with the free radicals generated by degrading polymers to stop the chain reaction. Hindered amine hght stabilizers (HALS) were introduced in 1975 and are the newest of the hght stabihzers. They have replaced benzo-phenones and benzotriazoles in polyolefins and are more cost-effective. 

The adherence to the polymer and elimination of diffusion to the environment are both essential however, they should be mobile enough to move to the surface layer. Novel stabilizers, based on steric hindered amines (HALS) are considered to be very efficient, albeit there is lack of agreement regarding their stabilization mechanism. It is assumed that there exists a mutual reaction of antioxidation and hydroperoxide decomposition combined with the destruction of free radicals. These stabilizers are essential for protecting polyolefins and other polymers. 

While amines and some annular hydrocarbons are suitable chain terminators, hindered phenols such as di-l-butyl-p-cresol (alias butylated hydroxytoluene or BHT) are most popular because they avoid discolorization and they eliminate two free radicals per BHT molecule (Fig. 54.3). The resonance-stabilized aryloxy radical is protected by the bulky electron-releasing l-butyl groups in the 2 and 6 positions, so the hindered phenol can combine with a second peroxy radical but cannot combine readily with molecular oxygen or with another aryloxy radical nor abstract H atoms from the polymer to initiate a new firee-radical chain reaction. 

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. 

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... 

Free radical scavengers act by free radical scavenging reactions. By this mechanism is explained the stabilization due to the addition of -butylamine-nickel-2.2 -thiobis(4-ier -phenolate), nickel-bis-3.5-di-ieri-butyl-4-hydrophosphonic acid mono butyl ester, 2-hydroxy-4-dodecyl oxybenzophenone. The hindered amine type light stabilizers (HALS) have the highest effectiveness. 

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