Antioxidant chain-breaking donor

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

Chain-breaking donor antioxidants, often referred to as CB-D antioxidants, work by donating an H to the free radical, as in the following example, where (A)-H is the CB-D antioxidant ... 

Chain breaking - donor antioxidants such as hindered phenols are ineffective under conditions of photo-oxidation due to their rapid consumption in the higher rates of initiation present compared to thermal oxidation, and to photodegredant activity of their derived molecules such as quinones and quinone methides. Although they have limited photo-antioxidant ability when used alone, they can be protected from photoljnic destruction by, for example, UV absorbers, and for this reason they can produce good synergistic results with a variety of co-stabilisers. 

By themselves, chain-breaking donor antioxidants such as the hindered phenols, rV, are ineffective processing stabilisers for PVC since alone they are not able to inhibit the HCl unzipping reactions discussed above. However, they do inhibit hydroperoxide formation and are widely used as synergists with the basic stabilisation systems discussed above and are always included in commercial PVC stabiliser packages for plasticised formulations (see above, Section 4.1). 

The basis to the chain breaking donor (CB—D) mechanism, which was the first antioxidant mechanism to be investigated, was laid down by the late 1940s [10-12]. Many reducing agents, e.g., hindered phenols and aromatic amines, which reduce the ROO to hydroperoxide in a CB—D step have already been empirically selected and used for rubbers and by this time also for the newer plastics industry (e.g., Table la, AO 1-8 and 9-12). The major mechanistic landmarks of the antioxi-... 

Scheme 2 Schematic presentation of the cyclical oxidation process and some of the main reactions/products formed from the propagating radicals. The antioxidant mechanisms interrupting the oxidative cycles are also shown. AO antioxidant, CB-A chain breaking acceptor, CB-D chain breaking donor, PD peroxide decomposer, UVA UV-absorber, MD metal deactivator...

The primary antioxidants are normally broken down further into the classes of chain-breaking donor (CB-D) and chain-breaking acceptor (CB-A). CB-D additives interact with peroxy radicals, and are by far the commonest class of antioxidant in general use. They are represented by such additives types as hindered phenols and secondary aromatic amines. CB-A additives interact with alkyl radicals but, due to the rapid oxidation of such radicals, these additives are really useful only under low oxygen availability. CB-A types are represented by aromatic nitro and nitroso compounds, and a few speciality stable free radicals. Some transformation products of CB-D antioxidants can also act as CB-A species. 

Antioxidants interfere with the free-radical oxidative cycle to inhibit or retard the oxidation mechanism. On the basis of the mechanisms by which antioxidants function, they can be classified into two categories primary or chain-breaking antioxidants, and secondary or preventive antioxidants. Chain-breaking antioxidants are of two types chain-breaking donor (CB-D) antioxidants and chain breaking acceptor (CB-A)... 

To inhibit the oxidation process, certain chemical compounds (antioxidants or stabilisers) are added to the polyethylene, see Sect. 5.2.2. The so-called chain-breaking-donors or primary antioxidants, also called inhibitors, react with the chemical radicals. They thus intermpt the reaction chain. The so-called hydroperoxide decomposer or secondary antioxidants react with the hydroperoxide before it can disintegrate into radicals. Thus they prevent the start of new reaction chains. 

This class of antioxidants acts by removing either alkyl (P ) or alkylperoxyl (POO ) the first by oxidation to an olefin (chain-breaking acceptor, CB-A) and the second by reduction to a hydroperoxide, (chain-breaking donor, CB-D) [20] (see Scheme 1.4). Aromatic amines and hindered phenols fall into the... 

Hydrogen donors are referred to as chain breaking donors. In particular, hydrogen donors decompose peroxides into inert products. Hydrogen donors are classified as primary antioxidants, because they access secondary products in the chain of the autoxidation reaction. The mechanism that interferes the autoxidation reaction is shown in Eq. 19.5. 

Primary antioxidants, also termed as chain-breaking antioxidants, interfere with the chain reaction in Scheme 2.1, by trapping radicals or labile hydrogen atom donors. These are exemplified by hindered phenols and alkylarylamines. Scheme 2.8 schematically demonstrates the scavenging activity of a typical hindered phenol. 

Chain-breaking antioxidants (a) free-radical traps, (b) electron donors, (c) hydrogen donors. 

Vitamin E, a lipophilic vitamin, is a major free radical chain-breaking antioxidant located in cellular membranes. The presence of vitamin E in subcellular membranes is vital in protecting the membrane phospholipids against peroxidation (Chen et aL, 1980), presumably by functioning as an electron donor to free radicals (Tappel, 1968). A deficiency of vitamin E increases the susceptibility of microsomal lipid to peroxidation (Leung et aL, 1981). 

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