THERMAL AND OXIDATIVE STABILITIES OF HYDROCARBON VACUUM PUMP OILS
The predominant mechanism by which oils degrade or wear out in use is oxidation. Most lubricants have a higher level of thermal stability than oxidative stability. Thermal stability is dependent primarily on the chemical structure of the base stock. Oxidative stability is generally obtained by the use of an additive.
Studies have shown that oils oxidize through a carbon-carbon bond to form a peroxide which in turn forms a carbonyl as a primary oxidant product. The subsequent reactions of the primary oxidation products form condensation polymers which are precursors to sludge and deposits. The primary difference among these oils are the rates of these reactions. Vacuum pump oils are being subjected to ever-increasing thermal stresses.
The role of the lubricants thermal and oxidative stabilities is becoming an important factor in the ability of vacuum pump oil to survive at higher temperatures in oxidizing environments.
THERMAL STABILITY
It is unfortunate that literature is often not explicit concerning the term thermal stability. It is sometimes used interchangeably with thermal-oxidative stability. However, the proper definition is reserved for processes occurring in the absence of oxygen.
MECHANISM
In the case of hydrocarbons and most other fluid classes, thermal decomposition or pyrolysis proceeds through a free-radical chain reaction process yielding many products. Free radicals are organic fragments containing an unpaired electron, and are produced by homolysis (breaking) of the carbon-carbon bonds. These radicals can be generated by mechanical processes and thermal energy.
These radicals are highly reactive and start chain reactions by abstracting hydrogen atoms from the parent hydrocarbon.
This chain sequence will continue until the radicals are destroyed or all the reactants are consumed. Hence, a single reaction can bring about many changes in thousands of molecules. Radicals may be destroyed by recombination or through disproportionation reactions. Radicals themselves may also fragment producing new radicals and unsaturated species. These reactions are analogous to industrial cracking.
The most general change in properties of the oil is an increase in vapor pressure of the system. These reactions also produce higher molecular weight products which have higher boiling points. The formation of the higher molecular weight products is important as these products are generally believed to cause viscosity increase, acidity increase, varnish and sludge formations.
REFERENCES
William R. Jones : NASA Lewis Research Center
E.E. Klaus : Pennsylvania State University
Stephen M. Hsu : National Bureau of Standards
This information is derived from papers presented at an international conference sponsored by NASA Lewis Research Center.
