Hindered Amine Stabilizers HAS

The protection mechanisms of HAS are based on a complex series of chemical transformations, with many unresolved details. The primary step includes oxidation in situ of secondary or tertiary HAS into nitroxides NO by the various oxidants present during oxidative degradation [Eq. (86)]. The substituent can be H, CH3, OR, or COCH3, and the oxidative species 02, O2, O3, POO, POOH, PC(0)00, or PC(0)00H. 

Stable nitroxide radicals are traditionally considered as the principal species which account for the free-radical scavenging activity of HAS. Nitroxide radicals can trap alkyl, alkylperoxy and acylperoxy radicals. The probability of direct radical scaveng- 

As with the formation of peracids, it is suggested that scavenging occurs inhomo-geneously and is enhanced in regions of accumulated hydrogen-bonded complexes, owing to the formation in situ of R [Eq. (88)). 

The formation of hydrogen-bonded associates with hydroperoxides or phenolic additives is favored by the resonance-stabilized dipolar structure of nitroxides represented by Eq. (89). 

The transformed hydroxylamines and dialkylhydroxylamine ethers are assumed to play a key role in the mechanism of stabilization of HAS, through a regenerative inhibition cycle known as the Denisov cycle [Eqs. (90a)-(90c)] [50]. 

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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. 

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... 

Photostabilization processes have not received much attention in the last year other than a multitude of review and specialist industrial articles. These include stabilization of polypropylene fibres, new stabilizer developments, sorting of stabilizers for polyethylene, trends in UV absorbers, hindered piperidines (hindered amine stabilizers (HAS)) for polyacrylics and urethanes, new stabilizers for polyolefins, enhanced service life applications, stabilizer mas-terbatches for polyester, stabilizers for UV cured acrylic overcoats for PVC, ° stabilization of clearcoats and various commercial packages. ... 

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. 

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]. 

In this article the focus will be on the application of one-dimensional (1-D) and two-dimensional (2-D) spectral-spatial ESRI to the photo- and thermal degradation of poly(acrylonitrile-butadiene-styrene) (ABS) (19-55,58-63). The ESRI approach was applied to ABS because it represents a polymer that is exceptionally important in technological applications, yet is also vulnerable to photo- and thermal degradation, and can be used only in the presence of protective additives. Hindered amine stabilizers (HAS), for instance bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770, (8)), are added for stabilization of polymeric materials (101,102). 

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. 

Fig. 10. X-band ESR spectrum at 300 K of a nitroxide radical derived from Tinuvin 770, a hindered amine stabilizer (HAS), in heterophasic ABS. Fast and slow components are indicated. The extreme separation of the slow component is 63 G.

Fig. 4. The ID ESRI at 340 K of a cylindrical sample (height 3 mm, diameter == 4 mm) of heterophasic propylene-ethylene copolymer (HPEC) containing a nitroxide derived from a hindered amine stabilizer (HAS). X-band ESR spectrum recorded in the absence (a) and in the presence of a vertical magnetic field gradient of 206 G cm b).

Many polymer light stabilizers along with their relevant main functions have additional capacity to be heat stabilizers or antioxidants. These molecules often simultaneously bear fragments responsible for polymer protection against the destructive effects of both light and thermal oxidations. For instance, a number of hindered amine stabilizers (HAS) serve as both light and heat stabilizers [40-51]. 

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... 

Fig. 15.21. Structure of a typical commercial hindered amine stabilizer (HAS).

Figure 23.6 (a) Two-dimensional (2D) spectral-spatial perspective plot of nitroxide biradicals derived from hindered amine stabilizer (HAS) doped in HPEC (copolymer of ethylene and propylene [39c,b]) on one side of the... 

Nitroxide species are also formed in polymeric materials containing hindered amine stabilizers (HAS an example is TINUVIN 770 figure 23.9b) these are oxidized during the stabilization processes to form stable HAS-NO radicals (Figure 23.10) that have been used successfully as EPR spin probes of the host material [39,68]. 

Gijsman P. The mechanism of action of hindered amine stabilizers (HAS) as longterm heat stabilizers. Polym Degrad Stab 1994 43 171-6. 

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