Hindered amine stabilizers stabilization

Proper selection of testing conditions revealed an outstanding efficiency of hindered amine stabilizers (HAS) [3,4] in the temperature range below 130°C as long-term heat stabilizers. 

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

Gijsman, P. Gitton, M. Hindered amine stabilizers as long-term heat stabilizers for polypropylene. Polym. Degrad. Stab. 1999, 66, 365— 371. 

There is only perhaps one significant case where low concentrations of an antioxidant shows flame-retardant behavior, in the case of certain hindered amine stabilizers (HAS) that at the normally used concentrations (<1 wt%), offer low levels of flame retardancy in polypropylene and show synergy with bromine-containing flame retardants.78 80... 

Table 2 Major commercial hindered amine stabilizers 566...

At concentrations of 0.25 and 0.50%, the zinc acetate behaved like the hindered-amine stabilizer—it afforded little or no photochemical protection to the wool fabric AE values of 2.47 and 2.21 were observed at the above concentrations after 550 kj/m2 exposure. Thus, one could assume it functions photochemically as an oxygen/peroxide scavenger. At concentrations of 1.0%, zinc acetate did afford moderate protection to the wool fabrics (AE value of 1.55 after 550 kj/m2 exposure), but its absorption/transmission curve exhibited a maximum absorption only at 250-300 nm. Thus, the moderate protection afforded to the wool fabric... 

Table III shows results obtained when three UV stabilizers, alone or in combination, were used in 5- and 12-mil films for protecting the blue wool fabrics. The hindered-amine stabilizer (UV-3) was not tested because it was ineffective in 8-mil films (Table II). In 5-mil-thick films, the trends in AE values for the hydroxybenzotriazole (UV-1B), hydroxy-benzophenone (UV-2), and the zinc acetate (UV- 1) stabilizers were the same as they were when these stabilizers were incorporated into 8-mil-thick films. However, after maximum exposure time, the AE values were somewhat higher than those observed with 8-mil films. Similar trends were observed with films of 12-mil thickness. Only the hydroxy-benzophenone (UV-2) had AE values less than 1.0 after 550 kj/m2 exposure. The relative effectiveness of a hydroxybenzotriazole (UV-1B), a hydroxybenzophenone (UV-2), and combinations of each of these stabilizers as a function of film thickness are shown in Figure 3. All films, even those containing no stabilizers, were most effective in protecting the wool fabrics from color change when they were 8-mil thick. Again, no advantage was observed in using combinations of stabilizers instead of a single stabilizer in the film.

The redox reaction also extends to the participation of hydroperoxides, but their efficient decomposition depends on the formation of a non-radical product such as an alcohol. Another example of a redox couple is found in the behaviour of the nitroxyl radical (R NO )- Depending on the structure of R, these are efficient radical scavengers and a redox couple between the mdical and the hydroxyl amine (R NO /R NOH) is formed (which is analogous to the galvinoxyl radical G-/GH). It is noted that the hindered amine stabilizers (e.g. Tinuvin 770 and the monomeric and polymeric analogues) are ineffective as melt antioxidants, possibly because of reaction with hydroperoxides or their sensitivity to acid. 

There is much to learn and admire in the hindered amine story. Chemists can take pride in how effectively they have worked together across national boundaries to make hindered amine stabilizers an important product group for the stabilization of polymers. This introduction is a modest effort to review some of the early history of stable-free radicals including triacetoneamine-N-oxyl. This chapter was intended to serve primarily as an introduction to the hindered amine review which took place at the symposium and intentionally avoids covering material which other participants were expected to present. It is a "light-touch" overview. 

The first synthesis of 2,2,6,6-tetramethyl-4-oxypiperidine-N-oxyl was described in a paper by M. B. Neiman, E. G. Rozantsev, and Y. G. Mamedova (2). The synthesis was achieved by the oxidation of triacetoneamine with hydrogen peroxide in the presence of sodium tungstate. The key properties of the tetramethyloxypiperidine-N-oxyl, which subsequently led to the whole family of hindered amine stabilizers, were outstanding thermal and chemical stability. 

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. 

Figure 7. Commercially available hindered amine stabilizers.

Hindered amine stabilizers have been the single most important development of the last decade in polymer stabilization" -Peter P. Klemchuk. 

Scheme 5. Structures of hindered amine stabilizers for polymers. Continued on next page.

Methods of hindered amine stabilizer synthesis are being developed with the purpose of reducing cost and using available raw materials. 

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