Polyalphaolefin is by far the most common major synthetic base oil used in industrial and automotive lubricants. It is a synthetic hydrocarbon (SHC) that mimics the best hydrocarbon (branched) structure found in mineral oils.
Polyalphaolefin Characteristics
Polyalphaolefin does not contain ring structures, double bonds, sulphur, nitrogen components or waxy hydrocarbons. The absence of these structures and materials results in a very non-polar base oil with a high viscosity index (of approximately 130), excellent low-temperature flow and pour-point characteristics, good oxidation stability and compatibility with mineral oils, paints and seals commonly found in lube oil systems. Because of their controlled structure, PAOs do not contain lighter, more volatile (small) hydrocarbons. This lowers their volatility, creates less hydrocarbon tailpipe emissions and raises the flash point.
PAOs are used extensively in automotive fluids as well as hydraulic, gear and bearing oils, working in extremely cold climates or hot applications. They are also employed as base fluids in some wide temperature range greases. One application in which they have not worked well is in high-temperature (high-pressure) reciprocating air compressors where valve deposits have been an issue.
However, nothing is perfect, and polyalphaolefin base oils do have a few negative characteristics. These include the tendency to shrink seals and trouble dissolving common oil additives. Therefore, they are commonly blended or combined with organic ester synthetic base oils to provide a blended base oil that does not have these negative characteristics. Polyalphaolefins also have poor fire resistance and biodegradability.
Synthetic Comparison Grid
The term synthetic hydrocarbon (SHC) is a general term. Several synthetic base oil types fall into the SHC category. These include PAOs and also relatively common polyisobutenes (PIB), which are sometimes used as oil additives or as base oils in two-stroke engines.
Strengths
- High viscosity index (VI)
- High thermal oxidative stability
- Low volatility
- Good flow properties at low temperatures
- Non-toxic
- Compatible with mineral oils
Weaknesses
- Limited biodegradability
- Limited additive solubility
- Seal shrinkage risk
Applications
- Engine oils
- Gear oils
- Bearing oils
- Compressor oils
- High-temperature grease
- Lube-for-life applications
Polyalphaolefin History and Facts
- Developed in the 1930s; first commercial engine oils in 1970s
- Made from ethylene gas; still crude oil/natural gas dependent
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Higher temperature resistance than mineral oil
- PAO up to approximately 160°C (320°F) continuous service; 270°C (520°F) intermittent
- No wax content, therefore best low temperature of all synthetics (approximately minus 50°C to minus 60°C or minus 70°F)
- Higher VI (approximately 130 to 140) than mineral oil (nearly 100)
- Less volatile than mineral oil (higher flash point, less flammable, lower hydrocarbon emissions)
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Lower natural lubricity than mineral oil
- Polyalphaolefin base oil not necessarily less wear than mineral base oil (additive-dependent property)
- Compatible with mineral oil
- Problems – very non-polar (low natural solvency, additive solubility, lower lubricity and film strength)
- Forms hard deposits in reciprocating compressors
- Must blend in 5 to 20 percent ester base oil for seal swell, additive solubility and lubricity
- Not biodegradable
- Costs four times more than mineral oil, less than other synthetics
