Amines as antioxidants

In this work, the activities of amines as antioxidants and antiozonants are discussed in relation to their chemical structures. As a possible mecha-... 

Amines as Antioxidants or Antiozonants for Rubber (13). Oxidative degradation of vulcanized rubber is evaluated from the depression in the tensile properties during aging in the Geer oven. 

Both trialkyl- and triarylstibine sulfides and selenides are known. Trimethylstibiae sulfide [15082-97-6], C3H9SSb, has been prepared from trimethylstibine oxide and hydrogen sulfide (164). It is monomeric in benzene and chloroform. Trialkylstibine sulfides and selenides have been prepared from tri-alkylstibines and sulfur or selenium, respectively (165). Unlike triphenylstibine oxide, the structure of triphenylstibine sulfide is tetrahedral, as shown by both Mu ssbauer and x-ray diffraction studies (153). A patent covering the synergistic use of triphenylstibine sulfide with aromatic amines as antioxidants in lubricating oils has been issued (166). 

From this follows a rational method of increasing the effectiveness of amines as antioxidants their preliminary oxidation to azotoxide stable radicals and the use of the latter for stabilization purposes. 

Environmental Impact of Ambient Ozone. Ozone can be toxic to plants, animals, and fish. The lethal dose, LD q, for albino mice is 3.8 ppmv for a 4-h exposure (156) the 96-h LC q for striped bass, channel catfish, and rainbow trout is 80, 30, and 9.3 ppb, respectively. Small, natural, and anthropogenic atmospheric ozone concentrations can increase the weathering and aging of materials such as plastics, paint, textiles, and mbber. For example, mbber is degraded by reaction of ozone with carbon—carbon double bonds of the mbber polymer, requiring the addition of aromatic amines as ozone scavengers (see Antioxidants Antiozonants). An ozone decomposing polymer (noXon) has been developed that destroys ozone in air or water (157). 

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

Radical Trapping. Figure 2 shows some of the reactions of aromatic amines that contribute to their activity as antioxidants and to their tendency to form highly colored polyconjugated systems. 

Aromatic amines possess weak N—H bonds and the latter are attacked by peroxyl radicals when amines are used as antioxidants [1-9]. 

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],... 

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. 

The antioxidant and acid neutralization properties of most fuel stabilizer formulations help to prevent the initial formation of most acids. Compounds such as tertiary-alkyl primary amines, as shown by the example in FIGURE 6-5, are quite effective distillate fuel additives. These oil-soluble amines can be formulated into stabilizer formulations at relatively low concentrations and can provide a significant boost in overall performance of a fuel stabilizer. 

Secondary aromatic amines, such as phenyl beta-naphthylamine. have been used as antioxidants in elastomers, but the preferred antioxidants for plastics have been 2,6-disubstituted and 2,4,6-trisubstituted phenols. These hindered phenols serve as chain transfer agents with the macroradicals which are produced by the degradation of polymers. 

A second mechanism in the. aging of CTPB propellants also exists and proceeds concurrently with the reactions proposed above. It consists of an attack at the reactive points of unsaturation in the backbone polymer, which causes additional crosslinking and hence an increase in propellant modulus, particularly at the surface. The exposed surface of CTPB propellants changes, as indicated by an increase in hardness. Heavy metal ions are particularly harmful, and it was found that an increase from 10 to 80 p.p.m. of iron caused a significant increase in surface hardening by catalytic attack on the double bonds. Antioxidants in general provide sufficient protection for polymer storage. In CTPB propellants the antioxidant selected to protect the double bond is very important. Amine-type antioxidants have provided better surface stability than phenolic compounds. 

Like some phenolic groups, amino groups bound to an aromatic system (e.g., benzene) may be oxidized in the environment. In some cases such transformations may lead to the formation of products that are of considerable concern (Chapter 14). Finally, some aromatic amines that form stable radicals are actually also used as antioxidants (see example given in Fig. 2.18 and Kirk-Othmer, 1992). 

This paper shows that Fukui s theory is plausible for interpreting the presence of the optimum oxidation potentials for amines to be effective as antioxidants or antiozonants. 

Antistatic additives are designed to be present on the surface of the molded part to achieve the full antistatic benefit. The types of additives used to enhance antistatic properties include quaternary ammonium salts, alkyl sulfonates or phosphate plus alkali metals, ethoxylated amines, or gylcerol esters. Antistatics are typically used at higher levels than other additives such as antioxidants. Therefore, antistatics are likely to increase light scattering, making it more difficult to achieve the higher chroma colors. 

A-Nitrosamines in rubber products are by-products of the reactions taking place during the vulcanization of rubber mixtures [80]. They are formed from some chemical compounds such as secondary amines (accelerators, antioxidants) via nitrosation by nitrogen oxides present in the surrounding air. These reactions occur inside the rubber product and on its surface. The resulting A-nitrosamines from the interior of the rubber can diffuse to the surface of the article, and then to the environment or media in which the product is used. A-Nitrosamines can also be incorporated in the rubber mixture by contamination of raw materials during preparation. 

The diesel needs to be stabilized with diesel fuel additives that will inhibit diesel polymerization and inhibit oxidation, darkening and agglomeration of certain components of the diesel. One such effective stabilizing additive is Octel FOA-6. Octel FOA-6 and FOA-3 are amine-based antioxidants that are recommended for antioxidant protection of distillate fuels such as diesel. FOA-3 and FOA-6 together with AO-22, generally give then-best performance when added hot and early to the fuel, usually to a cracked component in the run-down from the cracker. 

The discovery that exposure to exogenous chemicals could lead to cancer in humans was first made in the late 18th century, when Percival Pott demonstrated the relationship between cancer of the scrotum and the occupation of chimney sweepers exposed to coal tar/soot. Other examples noted later were scrotal cancers in cotton spinners exposed to unrefined mineral oils, and cancers of the urinary bladder in men who worked in textile dye and rubber industries due to their exposure to certain aromatic amines used as antioxidants. Experimental induction of cancer by chemicals was first reported in detail by Yamagiwa and Ichikawa in 1918, when repeated application of coal tar to the ear of rabbits resulted in skin carcinomas. Over the next few years, Kennaway and Leitch confirmed this finding and demonstrated similar effects in mice and rabbits from the application of soot extracts, other types of tar (e.g., acetylene or isoprene), and some heated mineral oils. These researchers also observed skin irritation sometimes accompanied by ulcers at the site of application of the test material. Irritation was thought to be an important factor in skin tumor development. However, not all irritants (e.g., acridine) induced skin cancer in mice and conversely, some purified chemicals isolated from these crude materials... 

Compounds that are classified as antioxidants (such as alkylphenols, aromatic amines, thiophenols, and imines) generally exhibit moderately high reactivity toward organic peroxy radicals (ROO ). A plausible reaction sequence for the formation of ROO is given in Figure 12.3b. 

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