Thermooxidative breakdown of Lubricants

Under friction and heat free radicals are formed in lubricants which can be detected by ESR/EPR

Background

When used in mechanics, oils degrade over time. Viscosity decreases and solid particles are dispersed. Both factors are worsening the lubrication effect. Starting from this point, a degradation cycle begins, which constantly accelerates (see schematic below).

During the decomposition process, radicals are formed that indicate thermooxidative breakdown. ESR spectroscopy is a simple and fast method to determine the amount of radicals and monitore the lubricant’s quality.

Application example: Features of lubricant additives

Shimonaev et al. 1966

  • testing lube additives:

    • sulfonates, alkylphenolates or dialkyldithiophosphates of Me2

    • Me = Ca, Ba, Zn

  • expectation:

    • additives are going to dissociate into free radicals

    • Me-S and Me-O bonds are breaking easier than C-C bond

  • result:

    • most additives form solutions of free radicals

    • only two additives show no radicals

    • one additive forms radicals upon heating (160 °C)

 

 

References

  • G. S. Shimonaev, G. V. Zakharov (1966) Chemistry and Technology of Fuels and Oils 2, 510-513