Antioxidants groups

Coefficients correlate with the quantity of functional antioxidant groups in the studied compounds (hydroxyl or thiol), which allows the measurement of total AOA in the solutions. The AOA value of the investigated object is expressed in mole-equivalent per liter of mediator system oxidized form, reacted with probe antioxidants. 

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. 

Antioxidant group Chemical designatiim Abbreviatimi Proptaties... 

Polymeric antioxidants have also been prepared by the chemical modification of preformed polymeric materials with antioxidant groups. For example, the polymeric antioxidant (Scheme 5.8) is prepared by the condensation of 2,4-dimethylphenol with polymethacrolein in the presence of a strong base or acid as a catalyst [38],... 

Other widely used type of food additives is the synthetic antioxidant group in which compounds such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiary butyl hydroquinone (TBHQ), 2,4,5-trihydroxybutyrophenone (THBP), di-tertbutyl-4-hydroxymethylphenol (IONOX-100), propyl gallate (PG), octyl gallate (OG), nordihydroguaiaretic acid (NDGA), 4-hexylresorcinol (4HR), 2-naphthol (2NL), 4-phenylphenol (OPP), and 2,4-dichlorophenoxyacetic acid (2,4-DA) are included. 

The same structure fBciency relationship is also observed in the phosphorus-containing antioxidant group. Thus it has been established that stabilization systems with tris(nonylphenyl) phosphite and distearyl pentaerythritol diphosphite are significantly inferior to systems containing tris(2,6-di-tert-butylphenyl) phosphite and tetra-kis(2,4-di-tert-butylphenyl)-4,4 -diphenylene-bisphosphite [10]. 

The effect of migration of stabilizers to the surface of the polymer, and their subsequent loss by volatilization, is well documented. To circumvent this problem antioxidant groups are incorporated in the polymer as a conventional vinyl or condensation polymer such that the antioxidant function. A, occurs repeatedly at short intervals along the polymer chain (see reaction 5). 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

Cost bilizers. In most cases the alkyl tin stabilizets ate particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyl tin mercaptides. Mercaptans are quite effective costabilizets for some of the alkyl tin mercaptides, particularly those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyl tin mercaptide ate currendy the most efficient stabilizers for PVC extmsion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roU mill performance of some methyl tin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

There is a health benefit associated with hindering hydrogen bonding. Alkylphenols as a class are generally regarded as corrosive health hazards, but this corrosivity is eliminated when the hydroxyl group is flanked by bulky substituents in the ortho positions. In fact, hindered phenols as a class of compounds are utilized as antioxidants in plastics with FDA approval for indirect food contact. 

This addition is general, extending to nitrogen, oxygen, carbon, and sulfur nucleophiles. This reactivity of the quinone methide (23) is appHed in the synthesis of a variety of stabili2ers for plastics. The presence of two tert-huty groups ortho to the hydroxyl group, is the stmctural feature responsible for the antioxidant activity that these molecules exhibit (see Antioxidants). 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

It is also possible to graft an aromatic amine antioxidant bearing a sulfhydryl group on to the backbone of an elastomer. 

At first, these highly reactive free radicals react with the antioxidant, but as the antioxidant is consumed, the free radicals react with other compounds. Hydrogens on methylene groups between double bonds are particularly susceptible to abstraction to yield the resonance stabilized free radical ( ) ... 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

---