Antiozonants
Para-Phenylenediamines

Ozone and sunlight rapidly attack unprotected polymers which can significantly reduce their service life. Particularly polymers with high unsaturation, i.e. rubbers, are prone to suffer from ozone degradation because the double bonds in unsaturated polymers readily react with ozone. To prevent or to slow down ozone initiated oxidative degradation, antiozonants are frequently added. The most powerful and most common class of antiozonants are para-phenylene diamines (PPDs) which have the general structure

PPD

Paraphenylenediamines are not only efficient antiozonants but are also very effective primary antioxidants. Their reactivity and efficiency depends on the substituents on the nitrogen. In general, dialkyl substituted amines are the most reactive PPDs followed by arylalkyl-substituted and bisaryl-substituted amines; probably because the N-H bond of alkyl substituted PPDs has a lower bond dissociation enthalpy than those of aryl substituted amines. However, the activity of the different types of PPDs also depends on their solubility in the rubber, the temperature, and aging conditions. For this reason, blends of PPDs are often used. In some cases, the efficiency of antiozonants can be further increased by incorporation of waxes and certain synergistic antioxidants.1-3 All three types of PPDs are effective antiozonant in natural rubber and polyisoprene because they are only moderately soluble in these rubbers and, thus, can migrate to the rubber surface to provide good ozone protection. Diaryl PPDs, on the other hand, are usually more effective in polychloroprene than the others.

There are at least three competing mechanism of ozone protection:

  1. The antiozonants react faster with ozone than the rubber and, therefore, acts as ozone scavengers.

  2. The ozone-antiozonant reaction products form a protective film on the rubber surface preventing ozone from reacting with the rubber.

  3. The antiozonants react with the radical sites of the rubber fragments, forming new cross-links and, thus, restore the rubber network.

The reaction between N-alky-substituted para-phenylenediamines and ozone is shown below:

PPD

Ozone oxidizes the amino group of the phenylenediamine yielding a hydroxylamine. Subsequent elimination of water yields quinonediimine which can be further oxidizes by peroxyl radicals to produce nitrones and dinitrones (not shown).

References and Notes
  1. Waxes provide effective protection under static load. However, protection under dynamic loads
    is possible only with antiozonants.2
  2. P.B. Sulekha, R. Joseph and S. Prathapan, J. Appl. Poly. Sci., Vol 81, 2183-2189 (2001)
  3. D. A. Lederer and M. A. Fath1, Rubber Chem. Technol., Vol. 54, No. 2, pp. 415-426 (1981)
  4. R.P. Latimer, E.R. Hooser, R.W. Layer and C.K Rhee, Rubber Chem. Technol. , Vol. 56, No. 2, pp. 431-439 (1993)

  • Summary

    Antiozonants

    The most powerful and most common class of chemical antiozonants are alkyl / aryl substituted paraphenylene diamines (PPDs).

  • PPDs not only provide protection against atmospheric ozone but also act as powerful antioxidants and antifatigue agents.

  • Effective antiozonants must migrate to the vulcanizate surface and must react with ozone faster than the rubber.

  • The reaction products with ozone (the so called ozonides) form a continuous protective film against further ozone attack.

  • Often microcrystalline waxes are added to the rubber which migrate to the surface and form a protective film. However, waxes are usually not effective during dynamic flexing.