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Antioxidant is a substance added in small quantities to hydrocarbons which are
susceptible to oxidation, such as rubbers, plastics, foods, and oils to inhibit
or slow oxidative processes, while being itself oxidized. Antioxidants work in
two different ways. In primary antioxidants (also called free-radical
scavengers), antioxidative activity is implemented by the donation of an
electron or hydrogen atom to a radical derivative. These antioxidants are
usually hindered amines (p-Phenylene diamine, trimethyl dihydroquinolines,
alkylated diphenyl amines) or substituted phenolic compounds with one or more
bulky functional groups such as a tertiary butyl at 2,6 position commonly.
Butylated hydroxytoluene (BHT) is a common example of hindered phenolic
antioxidant. The reaction rate, or carbocation stability, in SN1 mechanism is 3° > 2° > 1° > CH3
(no SN1) so, tertiary alkyl moiety exists in lots of phenolic antioxidant
compounds. Primary antioxidants are free radical scavengers which combine with
peroxy radicals and break autocatalytic cycle. In secondary antioxidants ( also
called peroxide decomposers), activity is implemented by the removal of an
oxidative catalyst and the consequent prevention of the initiation of oxidation.
Examples of peroxide decomposer type of antioxidant are trivalent phosphorous
and divalent sulfurcontaining compound such as sulfides, thiodipropionates and
organophosphites. Synergistic effect is expected when primary antioxidants are
used together with secondary antioxidants as primary antioxidants are not very
effective against the degradation by UV oxidation. Sometimes, chelating agents
are added to scavenge metal impurities which can initiate decomposition. All four major antioxidants
classified by functional moiety include
phenols amines, phosphites and thioesters.
http://ksundaram.tripod.com/
...The
Primary antioxidants consist mainly of hindered phenols
and hindered aromatic amines. They scavenge and destroy
the chain propagating peroxy and alkoxy radicals before
they can react with the polymer. These materials contain
hydrogen atoms that can be easily donated to the chain-propagating
peroxy, alkoxy,and hydroxy radicals. The alkoxy and
hydroxy radicals are converted to alcohols and water,
respectively, which are inert and do not hamper polymer
stability. The antioxidant radicals formed are stable
and do not abstract more hydrogens from polymer. A typical
primary antioxidant of the hindered aromatic amine variety
is AO-TMQ (Polymerized 1,2-dihydro-2,2,4-trimethylquinoline)....
The
amine antioxidants are generally more powerful than
the hindered phenols. This is due to a cyclic process
which the amine antioxidant undergoes in which a nitroxyl
radical is regenerated and consumes more radicals. A
drawback of the aminic AO¡¯s is that they are oxidized
to products which are more discoloring and staining
than their hindered phenol counterparts. AO-1010 is
exceptional among hindered phenolic AO¡¯s. It has one
of the highest molecular weights in its class and is
non-staining, nondiscoloring, and effective in non-black,
colorable rubber compounds.......
http://www.sciencedirect.com/
Antioxidants can be divided into two broad classes, primary and secondary,
depending upon their mode of action. The most broadly used primary antioxidants
are hindered phenolics. Phenolic antioxidants have traditionally been based on
2,6-di-t-butyl-4-methylphenol (BHT). This functional moiety has been
incorporated into larger molecules affording products with lower volatility or
greater polymer compatibility. More recently, molecules have been introduced
which vary in steric hindrance about the phenol and also the way in which
multiple phenol functional groups are linked to form larger molecules. This has
led to structures which have lower colour contribution. Surprisingly, in some
cases these molecules have shown higher levels of efficiency relative to other
antioxidants with similar phenol/molecular weight ratios. This has included
enhanced levels of synergism with secondary antioxidants. An attempt is made to
correlate structural features of these molecules to the enhanced performance
and/or lower colour.
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