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Polyalphaolefin Base Oil

Polyalphaolefin Base Oil

Synthetic lubricants have been available for many years; in the early 1930s. Development of a catalytic polymerization process of olefins in the United States led to the formulation of automotive crankcase lubricants with improved low-temperature performance. These products were not commercialized due both to the inherent cost of these new synthetic base fluids and to performance improvements of mineral oil-based lubricants. In Germany, low-temperature performance drove the development of similar products, although the main objective was to overcome the general shortage of petroleum base stocks. Other than the special supply circumstances of the Second World War, synthetic lubricants were not commercially significant until after the war.

In general, the improved properties of lubricants achieved with early synthetic base stocks could be obtained more cost effectively by improved formulations based on mineral oils. But the requirement for lubricants to perform over increasing temperature ranges, led by military and aero-engine performance, stimulated 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.

Synthesis of Polyalphaolefin

The term polyalphaolefin, PAO, when used for lubricant base stocks refers to hydrogenated oligomers of an α-olefin, usually α-decene which shows the best performance. After oligomerization, the unsaturated products are separated from the reaction mixture, unwanted monomer removed and then the intermediate hydrogenated using supported nickel or palladium catalysts. Fractionation then gives the required viscosity grades, commonly 2, 4, 6 or 8 cSt at 100◦C.

Polyalphaolefin Base Oil

.Friedel–Crafts-Catalyzed Oligomerization

Reaction Initiation Step: The Lewis acid catalyst (BF3) undergoes reaction with the alkene, resulting in the formation of an electrophilic carbocation.

Reaction Propagation Step: insertion of a monomer between the olefinic cation and its associated gegenion.

Reaction Termination Step: occurring by proton transfer from the oligomeric cation.

The trimer of 1-decene, and star-branched oligomers containing a quaternary carbon center have been shown to have the best lubricating properties.

Trimer of 1-Decene
Star-branched Configuration

Properties of Polyalphaolefin


In addition to controlling temperature/viscosity properties to give base oils with low pour points and high viscosity indices, the oligomerization process also gives high-purity base fluids. These can be used to formulate lubricants with very good thermal and oxidative stability. Interestingly, in some oxidation tests on base oils without additives, mineral base oils appear to be superior to PAOs, e.g. the onset of oxidation measured by differential scanning calorimetry, DSC. This is ascribed to naturally occurring antioxidants present in mineral oils which survive the refining process. However, PAOs are more responsive to added antioxidants which leads to the superiority of fully formulated lubricants based on PAOs

The higher degree of branching in the final product gives very good low-temperature properties, especially as the products are wax-free. In addition, the molecular rearrangements are intramolecular and molecular weights are therefore predictable. Thus, PAO volatilities are lower than those of equiviscous mineral-derived products, whether solvent refined, hydrocracked or hydro-isomerized. Overall, the PAOs are able to operate effectively over a very wide temperature range.

Physical propertySolvent RefinedHydro-isomerizedPAO
Viscosity @100°C (cSt)5.435.645.98
Pour point (°C)-12-15-64
NOACK (% loss)157.86.1
Viscosity Index98125137


The composition purity of PAOs, and their low polarity, gives poor solvency for polar compounds such as the additives required for fully formulated engine lubricants. The very low polarity can also cause problems with seal performance. Therefore, PAO base fluids tend to be combined with other, more polar base fluids such as aliphatic esters (as synthetic lubricants) and solvent-refined mineral oils (as part synthetic lubricants).

Applications of Polyalphaolefin

The wide temperature performance range for PAO-based lubricants together with their excellent physical, chemical and thermo-oxidative stabilities has increased the use of PAOs in a wide variety of applications. Traditional application areas such as aerospace, transmissions and hydraulic systems continue to require the lubricant performance benefits of PAOs. In addition, the stresses of both increased performance and longer lifetimes placed on automotive and marine lubricants have increased growth in the use of PAO-based synthetic lubricants in these applications.


Abdelrhman Sabry

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