Phenols, sterically hindered, antioxidant

Synergism based on the mixture of a chain-breaking antioxidant (sterically hindered phenol) and a hydroperoxide decomposer (organic sulfides or phosphites). 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Mesitylene. One of the principal derivatives of mesitylene is the sterically hindered phenol of the structure shown in Figure 4. Its trade name is Ethanox 330 and it is produced by Albemade Corporation (formedy Ethyl Corporation) (31). Ethanox 330 is an important noncoloring antioxidant and thermal stabilizer for plastics, adhesives, rubber, and waxes (qv) (32,33) (see Antioxidants). The oral toxicity of Antioxidant 330 is extremely low (oral 1D. in rats > 15 g/kg) since its large size, Cc4H7 0, effectively eliminates absorption from the gastrointestinal tract. 

High-performance LC is also a suitable method for separating BHA isomers. Commercially available BHA is a mixture of two positional isomers, an approximately 85 15 ratio of 3-BHA and 2-BHA. The former is approximately 2.4 times more effective as a food antioxidant than is 2-BHA, and half as effective as an inhibitor of benzo(a)pyrene-induced for stomach neoplasia in mice. For the separation, Ansari (116) used isocratic elution with 7% of 2-propanol in hexane on a Pirkle Type I-A column packed with 5- 01 y-aminopropyl packing, modified with lV-3,5-dinitrobenzoyl derivative of D-phenylglycine. Column eluent was monitored at 288 nm, with a detection limit between 1 and 2 ng. Under these conditions, isomers were separated without derivatization, where the phenolic group of 3-BHA was sterically hindered by an o-rm-butyl group and therefore could not interact with stationary phase that resulted in its rapid elution. 

Processing Additive Stabilizers Primary antioxidants (sterically hindered phenols, seoaryl amines) hydroperoxide decomposers (organophosphites, thioesters), acid absorbers (lead salts, Ca/Ba-Ba/Cd-Ba/Sn salts, organotins, epoxidized oils)... 

Flow and fusion promoters Thixotropic agents Antiaging Antioxidants PMMA and acrylate ester copolymers, MBS Fumed silica, clays Sterically hindered phenols, sec-aromatic amines, phosphates, thioethers... 

Unsaturated and Vulcanized Rubbers. Oxidation occurs most readily at polymers with structural double bonds, such as natural rubber, polybutadiene, or polyisoprene. Aromatic amines and sterically hindered phenols are effective antioxidants. From the rubber antioxidants, 96.8 million pounds were amines, and 20 million pounds were phenols. Amines act also as antiozonants whereas phenols are not effective. Furukawa shows that amines have a lower oxidation potential which is a prerequisite for antiozonant action. 

Aromatic amines, such as phenyl- -naphthylamine or condensation products of diphenylamine with acetone condensates, are excellent antioxidants and antiozonants but cause color development. From the sterically hindered phenols, monocyclic phenols, such as 2,6-di-teit-butyl-p-cresol, are less effective antioxidants but remain white and nontoxic during aging. They are, however, volatile and provide poor protection at elevated processing temperatures. Polycyclic phenols, such as 2,2 -methylene-bis (4-methyl-6-teit-butylphenol), are relatively nonvolatile, but become discolored by oxidation to a conjugated system. O Shea... 

Phenolic products, especially sterically hindered phenols, are widely used throughout the PVC industry - for instance, as chain-stoppers and antioxidants in PVC polymerisation, to terminate the reaction and prevent degradation of the virgin resin in the stripper and dryer. Methyl methacrylate butadiene-styrene (MBS), a frequently used impact modifier for rigid PVC, needs highly efficient protection against oxidative degradation... 

A sterically hindered, temperature-stable, phenolic antioxidant. It is a highly effective, noncoloring, odorless stabilizer for plastics, resins, rubber, and waxes. It... 

Controlled ortho alkylation of phenol by 1-hexene to produce the mono- or di-alkylated product has been achieved by using aluminum phenolate as the catsJyst under carefully optimiz reaction conditions. Sterically hindered phenols such as (15) are of particular interest as antioxidants (Scheme 4). 

Several factors are typically considered when selecting an antioxidant, including required concentration, compatibility with the substrate, stability, toxicity, ease of use and cost. Organophosphites are the most commonly used antioxidants. They are used to protect the color and molecular weight of the polymer during processing and have been shown to decompose peroxides, as well as chelate and react with metals. Other antioxidant groups include sterically-hindered phenols (BHT), thioesters and amines. 

The use of antioxidants in human-contact applications, e.g., food-contact, medical, and pharmaceutical, present a challenge in terms of their safety and level of migration into the contact media, e.g., food and body fluids. The biological antioxidant vitamin E, which is a suitable candidate for such areas of application, is a fat-soluble, and sterically hindered phenol antioxidant with the most bioactive form of the vitamin being the a-tocopherol (Table 1, AO 10). 

Yellowing after heat treatment of elastane can also be done to antioxidants in the fibre, especially of the sterically hindered phenol type. 

Not only can molecular design improve the miscibility with other oils, it can also confer ILs with special functions, such as antioxidation and the ability to adapt to a broader temperature range. We synthesized three imidazolium-based ILs containing sterically hindered phenol groups with antioxidant functions and evaluated the tribological behaviors of these ILs as additives for PEG appfication in steeFsteel contacts on an Optimol SRV-IV oscillating reciprocating friction and wear tester, as well as on MRS-IJ four-ball testers [100]. The rotary bomb oxidation test (RBOT) test, thermal analysis, and Cu strip test results revealed that synthesized ILs possessed excellent antioxidation capability. 

Sterically hindered phenols (Scheme 3) are the most widely used antioxidants in polymers. They are characterized by having a labile hydrogen atom with low dissociation energy and by not reacting further by hydrogen abstraction from the polymer backbone. They react primarily with alkoxy radicals (Scheme 4) and their oxidation products, quinones, react with peroxy radicals. 

An alternative reaction mechanism to Reactions (4.31) and (4.32) has been suggested concerning how sterically hindered phenols act as antioxidants for lubricants [27]. Based on the fate of 2,6-di-ferft fl/7-butyl-p-cresol in a turbine oil used in a conventional steam turbine, it was concluded that there is a strong possibility that this phenol is consumed not through the inhibition of oxidation, Reactions (4.31) and (4.32), but via direct oxidation with oxygen. Reaction sequence (4.33) ... 

It has been demonstrated that the nitroxyl radical, X, reacts with a secondary alkyl radical to form XI which, under high-temperature conditions (>120°C), regenerates the original diphenylamine molecule, Reaction (4.36). In essence, this group of stabilisers acts catalytically by scavenging alternately peroxy (ROO-) and alkyl radicals (R-). As stated earlier, sterically hindered phenols deactivate only two peroxy radicals per phenol molecule. Hence, under high-temperature conditions, aromatic amines are far superior to their phenolic counterparts. As shown in Table 4.3, the stoichiometric factor of the diphenylamines depends on the substituents in the para position [33]. The efficacy of the diphenylamine antioxidant is improved by alkylating the para positions. The stabilisation mechanism for phenyl-a-naphthylamines. Reaction sequence (4.37) [34], is described as follows ... 

The patent describing this development discloses that a mixture of a thermally stable phenolic antioxidant and a sterically hindered phosphite was used in a hydrotreated base stock. However, the heterosynergism between the radical scavenger (phenolic antioxidant) and the hydroperoxide decomposer (phosphite) was not shown in solvent-refined base stocks. An additional benefit of this technology is energy saving, due to extremely good control of viscosity. 

ZnDTPs, sterically hindered phenols, alkylated diphenylamines, organomolybde-num compounds and dithiocarbamates are the product groups meeting the present needs of the lubricant industry for the protection of lubricants against oxidation. Will they still be the products in the year 2020 There are important driving forces leading to future antioxidants ... 

It is an important fact, that there is an explicit synergistic effect observed by combining primary and secondary antioxidants. A well known example of such synergistic mixtures is the use of thiodipropionates together with sterically hindered phenols for long-term stabilization of polyolefins. Of course, the different polymers and different applications require laborious optimization of each case. 

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