Hindered-amine stabilizers mechanism

A mechanism involving charge-transfer complexes formed by HAS, polymer, O2, and ROO, i.e. [HAS - -02], [polymer - - -O2], [HAS - -ROO ], has been proposed [111]. It is considered to contribute in the early stages of the hindered amine stabilization mechanism [87]. The oxidation of TMP derivatives as illustrated in Scheme 9.15 commences when the polymer is processed. It continues later, when the polymer is exposed to light. 

Schwetlick, K. Habicher, W. D. Antioxidant action mechanisms of hindered amine stabilizers. Polym. Degrad. Stab. 2002, 78, 35-40. 

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. 

Here are presented only modern trends in the chemistry of photo- and thermostabilizers based on hindered amines, the mechanisms of their action and the methods of selecting stabilizing compositions, including mixtures of hindered amines with other classes of stabilizers. 

The nitroxide concentration rises rapidly to a maximum then slowly decays. The slow decay of the nitroxide concentration In the hindered amine doped coatings together with the high rate of Initiation demonstrates the Importance of nitroxide regeneration as a stabilization mechanism. The effectiveness of the hindered amine stabilizers Is a function of the kinetic chain length and of the lifetime of the stabilizer In the coating. The results found here are contrasted with degradation and stabilization studies In other polymers such as polypropylene and polyethylene. 

The hindered-amine stabilizers readily form nitroxyl compounds. Denisov (10,11) has found facile formation in polypropylene. Rozantsev (12,13) indicates that the mechanisms by which these sterically hindered amines act is via the generation of stable piperidinoxyl radicals, and he proposes their formation. 

Hindered amine stabilizers are transparent to visible and terrestrial UV light (300-400 nm). In polymeric matrices, they are oxidized in a sacrificial reaction by way of a not yet fully understood mechanism to stable nitroxyl (aminoxyl) radicals >N-0. A mechanism based on the reaction of HASs with alkyl hydroperoxides and alkyl peroxyl radicals is presented in Scheme 9.14 [87]. 

Hindered amine stabilizers (HAS) are the most common class of the curative additives and their application is the state-of-the-art in photoprotection of carbon-chain polymers, polyolefins in particular. HAS shape future polymer development, promote their consumption in new areas and expand material performance by increasing its lifetime. Application of HAS is based on a long-term effective development and is connected with commercial benefits for polymers. An optimized technical application of HAS required explanation of their chemistry and activity mechanisms in different phases of the oxidative degradation of polyolefins [14-17]. 

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. 

Scheme 11. Regenerative chain-breaking mechanism of hindered amine stabilizers.

Besides providing effective protection of polymers against photo-degradation, some high molecular weight and oligomeric HALS (e.g. HALS-3) have been shown to act as effective long term heat stabilizers. For this reason, in some relevant literature, it is possible to find the abbreviation HATS (Hindered Amine Thermal Stabilizers) or HAS (Hindered Amine Stabilizers). A detailed overview of the various mechanisms proposed for HALS action can be found in [1]. 

Another group of chain breaking antioxidants of rapidly increasing importance is the so-called hindered-amine stabilizers (HAS). These were originally developed as photostabilizers. The mechanism of their action is discussed below and involves the initial oxidation of the amine to a nitroxide... 

Figure 23.10 Stabilization mechanism of hindered amine stabilizer.

Gijsman, P. The mechanism of action of hindered amine stabilizers (HAS) as long-term heat stabilizers. Polymer Degradation and Stability, 43 (1994), p. 171 176... 

Hindered amine stabilizers act as scavengers for alkyl radicals. They act as they are forming stable nitroxyl radicals. The mechanisms of action is shown in Figure 19.5. 

Cyclic mechanisms have been described with stable radicals in hindered amine stabilizers. 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

Mechanisms for the action of hindered amine light stabilizers are reviewed briefly. On the basis of these considerations, two aspects of the mode of action of these substances were examined more closely. 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

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