Benefits of antioxidants in lubricants and greases assessed by pressurised differential scanning calorimetry

Abstract

The long-term thermo-oxidative stability of lubricants and greases is a prerequisite to meet today's 'stay in grade' and extended drain interval requirements. To achiezie this goal, there is a need for a better understanding of thermo-oxidative lubricant degradation and of how antioxidants can affect this process. In this paper a tuio-stage degradation model is discussed. Primary degradation leads to the formation of very reactive species such as radicals and peroxides. These are the precursors for generating secondary degradation products, such as oligomers, sludge, or deposits. By applying the theory of reaction kinetics and the Arrhenius law, quantitative correlations may be derived between oxidation induction time (OIT) and temperature, and between OIT and antioxidant treat level. Pressurised differential scanning calorimetry (PDSC) data confirm these theoreticalfindings and demonstrate the benefit of the addition of aininic antioxidants i n comparison with high-molecular-weight hindered phenolic antioxidants.

On Monitoring Physical and Chemical Degradation and Life Estimation Models for Lubricating Greases

“…This decreases the test time at the same temperatures and allows the use of lower test temperatures [31]. It also avoids the interaction of the oxygen with the lubricant being controlled by diffusion [23].…”

“…Antioxidants protect the grease base oil and thickener against radicals until they are depleted. These two initial steps (free radicals formation and antioxidant scarification/depletion) are normally referred to as the grease primary oxidation phase [23] or induction period [24]. Ito [25] showed that the base oil oxidation, which is accompanied by acidic product formation, begins after antioxidant depletion, and that the acid number increases after antioxidant depletion.…”