Formation of Resins in Used Oils as a Result of Conformation-Structure Change of Their Hydrocarbons

The mechanism of formation of resinous compounds during use of industrial and motor oils was studied by IR and NMR spectroscopy. Based on a comparative analysis of the spectra of samples of the used oil and the resins separated from it and the spectrum of fresh oil it was concluded that, besides oxidative mechanism, resinous compounds are formed in the oils as a result of oligomerization of the hydrocarbons. The molecules of the latter suffer, in the thermomechanical deformation process, profound stereostructural change at the level of distortion of the torsion angles and bond lengths. This leads to increase of the potential energy, corresponding dissociation of the R–C⋯H bonds in the hydrocarbons, and increase of their acidity, which stimulates oligomerization at these protonated bonds, i.e., initiates the oligomerization process and formation of aromatic hydrocarbons, or aging of the oil. The distortion of the stereostructure of the hydrocarbons in the oil during use remains in the mechanochemical memory of the molecules. This is why the purified used oil, even without oxidation of the hydrocarbons, becomes dark during storage because of the chromophoric –C–H acids that are formed. The latter are neutralized in the presence of alkaline reagents.