Synthetic base fluids

Abstract

Synthetic lubricants have been used for many years. In the early 1930s, synthetic hydrocarbon and ester technologies were being simultaneously developed in Germany and the US. In the US, the development of a process for the catalytic polymerisation of olefins led to the formulation of automotive crankcase lubricants with improved low temperature performance (Sullivan et aI., 1931;Sullivan and Vorhees, 1934). These products were not commercialised due to the inherent cost of the new synthetic base fluids and to improvements in the performance of lubricants based on mineral oils. In Germany, low temperature performance was one of the driving forces behind the development of similar products by Zorn (Gunderson and Hart, 1962) although the main objective was to overcome the general shortage of petroleum base stocks.With the exception of the special circumstances of World War II, synthetic lubricants did not become commercially significant until after the war. In general, the improved properties of lubricants achieved with the early synthetic base stocks could be obtained more cost effectively by improved formulations based on mineral oils. However, the requirements for lubricants, particularly military and aero-engine lubricants, to perform over increasing temperature ranges, has stimulated the continuing development of synthetic lubricant technology. Synthetic lubricants are now found in all areas of lubrication such as automobiles, trucks, marine diesels, transmissions and industrial lubricants, as well as aviation and aerospace lubricants.Many compounds have been investigated as possible base stocks for synthetic lubricants. Gunderson and Hart (1962) identified over 25, of which seven types are of major importance: • polyalphaolefins • alkylated aromatics • polybutenes • aliphatic diesters R. M. Mortier et al. (ed.), Chemistry and Technology Lubricants

The effect of aviation anti-wear additives on tribofilm formation and micropitting propensity

“…These species were initially not separated out due to cost but happened to contribute to enhancing performance without the need for further purification. 1,2 As gas turbine technology progressed and turbine temperatures increased to improve overall efficiency, there was a need for superior high temperature stability, which subsequently led to the use of synthetic group V polyol ester base oils. 3 Consequently, the natural tribological properties imparted from the crude oil are now absent in these synthetic formulations, and as a result, extra additives are added to provide the required wear protection.…”

“…The mechanism of boundary lubrication involves a chemical reaction between the anti-wear additives and the metal surface to create a chemisorbed tribofilm that serves as a constantly replenished sacrificial film that is preferentially sheared off instead of the substrate metal. 1,4…”

“…The most common aviation anti-wear additive is tricresyl phosphate (TCP), Figure 1, and is usually added in concentrations of ∼1–3% in the lubricant formulation. 1 This is an effective anti-wear additive, but it forms much thinner tribofilms than ZDDP and therefore generally provides less wear protection. 23 It has been found in the literature that ZDDP tribofilms stabilise at a thickness of approximately 120 nm and TCP films tend to form additive films around 30 nm thick.…”

“…23,24 However, it is more stable at the higher temperatures experienced within a gas turbine oil system. 1 It has been frequently observed that TCP forms a coloured film on the surface of the metal, and this film has been attributed to the reduction of wear and friction in the boundary lubrication regime. 25–28…”

On the effect of temperature in friction force measurements for a simplified piston cylinder set-up: Squeeze film combined with minimum oil film thickness measurements and viscosity index effect

“…Shear stability is a common lubricant requirement that can be achieved with specially formulated VI improvers. 6…”