Thiobisphenolic antioxidant

The other class of primary antioxidants are the phenols (hindered phenols, hindered bisphenols, hindered thiobisphenols, polyhydroxy phenols) (Fig. 34). Phenolic antioxidants are generally used when the discolouration of the amine antioxidants cannot be tolerated. Phenols may produce coloured reaction products (yellow, tan or pink) but the discolouration is significantly less than produced with amines. 

Thanks to proper stabilization, polypropylene has found outdoor applications as a low cost fiber and engineering material. Phenolic antioxidants, such as methylenebisphenol and thiobisphenol, are highly potent, nonvolatile, and solvent resistant but yellow on exposure to nitrogen oxides, always present in urban atmospheres. 2,6-Didodecyl-p-cresol and 2,6-dioctadecyl-p-cresol have been found as highly effective, nondiscoloring, and nonvolatile antioxidants. Lappin describes an economic process for the second product. 

Some of these groups are common units in high molecular weight phenolic antioxidants, such as the methylenebisphenol types and the thiobisphenol types, all of which are gas yellowing. 

Various polyesters derived from phosphorous or phosphoric acids were prepared. Efiicient polyphosphites were synthesised in the early 196(. Polyphosphite prepared from 152 and 4,4 -isopropylidenebis(cyclohexanol) was tested as a thermal stabilizer for PC [199] or as secondary AO for radiation sterilized EPM [200]. Built-in phosphites obtained by transesterification of triallcylphosphite with 4,4 -isopro-pylidenebisphenol or 4,4 -thiobisphenol possess antioxidant properties in polyolefins. Stabilizer containing phosphite moiety 153 was prepared from tris(2-hydroxy-ethyl)isocyanate, decyl alcohol and triphenylphosphite [201]. Various phosphites were derived firom polynuclear phenols or dihydric phenols. For example, a polycondensate prepared by reaction of phosphorus trichloride with 2,5-di-rert-butylhydroquinone was tested as heat and light stabilizer for PP [202], A linear polyester with a built-in phenolic moiety was synthesised from (2,6-di-tm-butyl-4-methylphenyl)bis(6-hydroxyhexyl)phosphite and dimethyl terephthalate [203]. 

Phenolic sulphoxides and sulphones are formed in the transformation process of thiobisphenolic antioxidants (Chap. IV B). Sulphoxide restores the antioxidant capacity though to a lesser extent than that of sulphide266,267) while sulphone has no antioxidant properties. However, corresponding phenoxyls are formed from both these compounds22,260) they are less stable than phenoxyls from the starting sul-... 

Important information about the transformation of thiobisphenols under the conditions of inhibited oxidation were obtained on the basis of product analysis of model reactions. The detailed investigation was carried out with 4,4 -thiobis(2-methyl-6-tert-butylphenol) (CLXVIIIa), which is among the most effective thiobis-phenolic antioxidants and tert-BuOOH was chosen as a simple model of polypropylene hydroperoxide. The reaction was followed at 20 °C in benzene solution and in the presence of the catalytic amount of Co(II)acetylacetonate272). Under these conditions, the phenolic antioxidant was attacked by both alkylperoxyls and hydro-... 

The product study272) and the kinetic study261) form the experimental basis for the discussion of complex transformations of thiobisphenolic antioxidants. Their retardation effect in polypropylene after the induction period and the mechanism of formation of intermediates or products catalytically active in ROOH decomposition indicate the reactivity of both different centres in molecule and provide partial data for the discussion of intramolecular synergism of thiobisphenols5 57,258,26 ). 

It holds specifically for antioxidants with the structure of thiobisphenols and hydroxybenzyl sulphides that their active role in stabilization of polyolefins is not only accompanied by chemical transformations, but it is even determined by these transformations. Thus, phenolic sulphides, in addition to their chain-breaking function, are also precursors of active species which catalytieally decompose hydroperoxides. They can serve as an example of compounds in which the transformation product of the original additive is an active antioxidant. 

Thiobisphenols are less effective than hindered phenols in terminating peroxy radicals. They also function as peroxide decomposers (secondary antioxidants) at temperatures above lOO C. Typically, thiobisphenols are chosen for use in high-temperature resin applications. Users generally prefer hindered phenolics over thiobisphenols where high-temperature service is not involved. 

The two major groups among the primary antioxidants are hindered phenolics and aromatic amines. The most widely used antioxidants in plastics are phenolics. The products generally resist staining or discoloration. However, they may form quinoid (colored) structures on oxidation. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenolics. Hindered phenolics, such as butylated hydroxytoluene (BHT), high-molecular-weight phenolics, and thiobisphenolics, are the most popular of the primary antioxidants. 

Hindered phenolics, because of their nonstaining qualities, are the most preferred type of primary antioxidant for light colored applications such as footwear. This group can be further categorized into simple phenolics (1), bisphenolics (2), polyphenolics (3), and thiobisphenolics (4), as shown on the following page [17]. 

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