Degradação termo-oxidativa de óleos lubrificantes. Caracterização termoanalítica e reológica.

Abstract

Lube oils are used to prevent premature wear of moving parts, caused by attition. These oils also prevent deposits and suspended contaminant formation, protect against corrosion, clean-up components and maintain operation temperatures. In order for these oils to properly play their role, their thermal and rheological properties must be maintained during use. Metal particles are liberated during automotive motor use contaminating the lubricating oil. These particles can promote and/or to catalyze chemical reactions leading to oil structural changes and poor performance. In the present work, a synthetic and a mineral based oil were thermal aged as received and in the presence of metal particles from two motor parts obtained at a rectifying business. Changes in rheological properties and chemical structure of these oils were monitored as a function of aging time (thermal exposure times). The data suggests that thermal exposure leads to lube oil oxidation, generating carboxyl compounds and conjugated aliphatic carbonyl containing dienes as seen by spectroscopic analysis (FYIR, UV-Vis and 1H NMR) and that metal particles accelerate degradation. Thermo-gravimetric analysis (TG) has shown the synthetic oil to display a more uniform degradation pattern which indicates that synthetic base oils display a more regular structure if compared with mineral base oils which are composed of a complex mixture of hydrocarbons. Rheological measurements indicate that oil viscosity did not significantly change over long thermal exposure times at 210ºC (1148 for synthetic and 1450 for mineral base lube oils). However, metal particle addition accelerated degradation of both oils, indicating that a new protocol for oil change might be useful.