Hindered phenohc antioxidants

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

The principal use for 2,6-di-/ f2 -butylphenol is in the production of hindered phenoHc antioxidants and this appHcation accounts for 80—90% of all of this compound produced. Reaction of 2,6-DTBP with formaldehyde under base catalysis forms the methylene bisphenoHc,... 

A large number of hindered phenoHc antioxidants are based on the Michael addition of 2,6-di-/ f2 -butylphenol and methyl acrylate under basic catalysis to yield the hydrocinnamate which is a basic building block used in the production of octadecyl 3-(3,5-di-/ f2 butyl-4-hydroxyphenyl)propionate, [2082-79-3], tetrakis(methylene-3(3,5-di-/ f2 butyl-4-hydroxylphenyl)propionate)methane [6683-19-8], and many others (63,64). These hindered phenolic antioxidants are the most widely used primary stabilizers in the world and are used in polyolefins, synthetic and natural mbber, styrenics, vinyl polymers, and engineering resins. 2,6-Di-/ f2 -butylphenol is converted to a methylene isocyanate which is trimerized to a triazine derivative... 

The tendency of aliphatic ethers toward oxidation requires the use of antioxidants such as hindered phenoHcs (eg, BHT), secondary aromatic amines, and phosphites. This is especially tme in polyether polyols used in making polyurethanes (PUR) because they may become discolored and the increase in acid number affects PUR production. The antioxidants also reduce oxidation during PUR production where the temperature could reach 230°C. A number of new antioxidant products and combinations have become available (115,120,124—139) (see Antioxidants). 

These compounds are used most frequentiy in combination with hindered phenols for a broad range of apphcations in mbber and plastics. They are also able to suppress color development caused by oxidation of the substrate and the phenoHc antioxidant. Unlike phenols and secondary aromatic amines, phosphoms-based stabilizers generally do not develop colored oxidation products. 

The molecular weight of the polymers is controlled by temperature (for the homopolymer), or by the addition of organic acid anhydrides and acid hahdes (37). Although most of the product is made in the first reactor, the background monomer continues to react in a second reactor which is placed in series with the first. When the reaction is complete, a hindered phenoHc or metal antioxidant is added to improve shelf life and processibiUty. The catalyst is deactivated during steam coagulation, which also removes solvent and unreacted monomer. The cmmbs of water-swoUen product are dried and pressed into bale form. This is the only form in which the mbber is commercially available. The mbber may be converted into a latex form, but this has not found commercial appHcation (38). 

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

As early as 1990, vitamin E has been considered an attractive substitute for other hindered phenolic antioxidants used in polyolefin food packaging, such as BHT [12]. The drawback of BHT and other hindered phenols was that oxidation products could leach into packaged food, causing an undesirable aftertaste. Vitamin E was considered a natural alternative to synthetic phenohc antioxidants used in food packaging [14]. Many smdies of vitamin E... 

Phosphite stabilizers behave synergisticaUy with hindered phenolics providing good processing protechon and in some cases, they enhance stability during ultraviolet exposure. They nonnaUy are the recommended secondary antioxidants for use in combination with primary phenohc antioxidants. 

High inherent chemical efficiency of phenolics is a prerequisite for the successful protection of plastics against degradation. It is controlled by the architecture of the stabilizer molecule. Steric hindrance of the phenohc moiety is one of the factors governing antioxidant efficiency. Most hindered phenols contain one tertiary butyl group combined with a methyl group or two tertiary butyls in positions 2 and 6 [3]. Compounds I and II are typical representatives of broad-spectrum antioxidants for plastics. Some phenolic antioxidants have one of the ortho positions (relative to the HO group) unsubstituted. Compounds III-V are examples. 

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