Polymer stabilization nitroxyl radical

Apparently, pre-irradiating polyethylene containing a HALS with a low pressure Hg lamp imparts some surface stabilization toward sunlight exposure. Anthracene-HAS stabilizer molecules are found to be more effective stabilizers than the separate molecular moieties, while the nature of the polymer composition affects the distribution of nitroxyl radicals in stabilized materials. " Polyethylene with terminal HAS groups has been found to be very light stable, as were reactive acrylic coatings with acrylated HAS systems. A new epoxide amine based HAS has been found to be quite effective in... 

The early recognition of the role of stable nitroxyl free radicals, e.g., 2,2,6,6-tetramethyl-4-oxopiperidine, and their hindered amine precursors, in polymer stabilization soon led to the development of the hindered amine light stabilizer (HALS) class of photoantioxidants. The first HALS, Tinuvin 770, AO-33, (commercialized in 1974) proved to offer much higher UV-stabil-ity to polymers than any conventional UV-stabilizer available at the time such as UV-absorbers, nickel compounds and benzoates. Table 3). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

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],... 

In the present work, we performed the hydroperoxidation of unsaturated polymers as EPDM or polyoctenamer by using singlet oxygen photo-generated at 365 nm by anthracene / O2. The resulting hydroperoxidized polymer (POOH) was then decomposed by UV irradiation (X > 300 nm) in the presence of unsaturated monomers or nitroxyl radicals. The efficiency of these new grafted polymers as new photo- and thermo-stabilizers will be finally demonstrated. 

There is some evidence to suggest that in the case of bifunctional compounds a proportion of the nitroxyl radicals becomes chemically bonded to the polymer backbone (80MIU507). Such attachments further increase the effectiveness of this class of stabilizer by improving their long term retention by polymeric materials. 

Some workers suggested that nitroxyl radical stabilizes the polymer by forming a hydrogen-bonded complex with a hydroperoxide group of the type VUI... 

The mobility of low molecular weight additives in the presence of fillers is important for the same reasons. Recent studies show that UV stabilizers are immobilized on the surfaces of filler particles. Nitroxyl radicals were used as spin probes in silica filled polymers. Experimental work confirms that absorption occurs on the OH groups of silica but it was shown that a certain minimum concentration of filler is required to trigger this absorption effect. 

Great attention in literature is paid to stabilizers - spatially difficult amines and then-derivatives [10, 70] which act as inhibitors of radical processes of polymer destruction. Difficult piperidines and their nitroxyl radicals are among them ... 

This chapter is a comprehensive overview of the progress in the field of generation, chemistry, and application of nitroxyl radicals and their precursors, for example, hindered amines of the 2,2,6,6-tetramethy1-piperidine series. Because of the importance of nitroxyl radicals to polymer stabilization, this application is discussed at length, while the others are touched upon briefly. 

Photoexcitation increases nitroxyl reactivity appreciably. The photochemical reaction of the radical (6) with toluene yields quantitatively the corresponding hydroxylamine and its benzyl ether (41). This reaction simulates to a certain extent the process of polymer stabilization by nitroxyl radicals. 

The possibility of stabilizing polymers with hindered nitroxyl radicals is based on their reaction with polymer radicals which are engaged in propagating polymer oxidation. It was established that hindered amines, the precursors of nitroxyl radicals, are also effective photostabilizers and, as such, they are more effective than most of the common photostabilizers for polymers. 

Several reactions may be involved in providing protection to polymers against photodecomposition destruction. The contribution of each of the processes depends on the nature of the polymer and the oxidation conditions. The time during which a hindered amine is transformed into nitroxyl in polypropylene is short compared to the induction period. Therefore, piperidines and corresponding nitroxyl radicals are almost equally effective for polymer stabilization. In rubber, which is more rapidly oxidized than polypropylene, nitroxyl radicals are fairly more effective than hindered amines. Subsequent transformations of nitroxyl radicals, formed through Reactions 6-8 are shown in Scheme 6 (53). 

Other fields of nitroxyl radical application are not as far advanced as the spin-label method and polymer stabilization technique. Therefore, they will be discussed only briefly or just mentioned in passing. 

Elimination of the yellowing introduced by the stable nitroxyl radicals was essential for commercial development of these excellent stabilizers. Since phenolic antioxidants are necessary for the thermal stabilization of polymers during processing, we turned our attention to ways in which unfavorable interactions between the hindered phenols and the stabilizing nitroxyl radicals could be avoided. In this section we describe the discovery of the light-stabilizing activity of hindered amine compounds, an improved synthetic method for these compounds, the synthesis of a number of derivatives, and the evaluation of their stabilizing activity. 

These results suggest that hindered amine compounds could be converted to stable nitroxyl radicals through the corresponding amino radicals in polymers that is, they are the precursors of the stable radicals. In fact a key compound, 2,2,6,6-tetramethyl-4-oxopiperi-dine (42), showed high light-stabilizing activity in polypropylene, comparable to that of the nitroxyl radicals, lending support to this interpretation. 

A mechanism for a reaction between HALS and hydrogen peroxide has been proposed by Scott and co-workers [8], These hydroperoxides are formed in the polymer and react with the HALS to form stable nitroxyl radicals that are believed to be responsible for the stabilization [3], Figure 4.5 shows this reaction with the nitroxyl radical, which is a very effective scavenger of alkyl or macroalkyl radical as well as the substituted hydroxylamine formed by this reaction. Substituted hydroxylamines can react with peroxy radicals to regenerate nitroxyl radicals [3],... 

D.K. Hodgeman, Formation of polymer-bonded nitroxyl radicals in the UV stabilization of polypropylene by a bifunctional hindered amine light stabilizer, J. Polym. Sci. Polym. Chem. Ed. 1981, 19, 807. 

G. Geuskens and G. Nedelkos, The oxidation of hindered amine light stabilizers to nitroxyl radicals in solution and in polymer, Polym. Degrad. Stab. 1987, 19, 365-378. 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

If the reaction of the hindered piperidine were to stop at this point, it would be difficult to explain why these materials are so effective. However, tile polymeric hydroxyl amine is itself an effective stabilizer, and it probably reacts [134], as in the reaction given by Equation 3.9, to form the hydroxyl amine equivalent of the hindered amine (IV), which can then react with a peroxyl radical to reform the hindered nitroxyl radical. The regeneration of the nitroxyl radical permits hindered piperidine compounds to be effective in preventing a polymer radical from decomposing in a manner that degrades the polypropylene fiber ... 

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