Page 264 - Handbook of Plastics Technologies

ELASTOMERS

4.56 CHAPTER 4

fault points on the rubber surface. Instead of total surface involvement, the ozone attack
occurs at these relatively few fault areas, causing rather large cracks to develop. Thus,
other types of additives are needed for protection against the effects of ozone under dy-
namic conditions.
Chemical Antiozonants. The first effective chemical antiozonant was a dihydroquin-
oline type, 1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline (DETQ). However, polymer-
ised quinoline derivatives provide only slight ozone protection, although they are good
antioxidants. DETQ provides protection against the action of ozone, but it is highly stain-
ing and discoloring and is lost from rubber compounds because of its volatility.
p-Phenylenediamine antiozonants such as N,N´-di-sec-alkyl-p-phenylenediamines
were then introduced. They surpassed the dihydroquinolines in their ability to protect rub-
ber from ozone attack.
The success of the dialkyl-p-phenylenediamines led to the development of related anti-
ozonants, i.e., alkyl/aryl-analogs, N-isopropyl-N´-pheny-p-phenylenediamine (IPPD) and
N-cyclohexyl-N´-phenyl-p-phenylenediamine, and mixed N,N´-diaryl-p-phenylenedi-
amine mixtures.
The longer-chain alkyl substituents served to reduce the volatility. However, the fugi-
tive nature of the protection offered by the di-octyl-p-phenylenediamine was attributed to
the fact that it reacts directly with oxygen (O ) as well as ozone. Alkyl/aryl- or diaryl-p-
2
phenylenediamines are less subject to depletion by reaction with oxygen and are longer-
lasting in rubber compounds.
The antiozonants, N-(1,3-di-methylbutyl)-N´-phenyl-p-phenylenediamine (6PPD) and
the C and C alkyl-analogs were later introduced. In addition to their use as antiozonants,
5
7
p-phenylenediamines, primarily the more persistent mixed diaryl-derivatives, have re-
placed N-phenyl-β-naphthylamine as antioxidants flex crack inhibitors and synthetic poly-
mer antioxidant stabilizers.
All of these antiozonants are staining and discoloring. This has limited their use prima-
rily to carbon black-loaded compounds. We also note that the antiozonants and amine-
based antioxidants cause a reduction in scorch resistance.
Multiple Functions. The N,N´-disubstituted-p-phenylenediamines (PPDs) are unique
stabilizers. Many of them simultaneously are potent antioxidants as well as antiozonants
(especially the sec-alkyl-p-phenylenediamines). They are also good flex-crack inhibitors.
In general, the best antioxidant protection is afforded at levels slightly less than 1 phr.
Higher levels of use actually become detrimental, as the system can become pro-oxidative.
Nevertheless, to provide antiozonant protection, antiozonant levels in excess of 2 phr or
more may be necessary. The use of di-aryl-p-phenylenediamines with di-alkyl or alkyl/
aryl derivatives is beneficial, since it reduces the pro-oxidative effect. Their low volatility
and low extractability provide for long-term protection.
Differences between Polymers. The degree of required ozone protection varies with
the type of rubber. Saturated elastomers need no antiozonant protection, because they have
no sites for reaction with ozone. Rubbers such as EPDM, which have a low olefin concen-
tration, need essentially no protection against the effects of ozone. Styrene-butadiene rub-
ber (SBR) requires antiozonant, while NR and synthetic polyisoprene (IR) may require
somewhat increased dosages of antiozonant. Nitrile rubber (NBR) is very difficult to pro-
tect against ozone attack.
Antiozonant activity changes with time. For short periods of aging time, the dialkyl-p-
phenylenediamines are the most effective antiozonants, very closely followed by the alkyl/
aryl analogs, with the diaryl being less effective. However, with increased aging time, the
order of effectiveness is completely reversed, as oxidation and reaction with ozone oc-
cur—another reason for using mixtures of antiozonants, i.e, to provide protection for an
extended period of aging.

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