Synthetic Lubricant Base Stocks
There are six major base stock types used in the development of synthetic lubricants, with each offering its own set of unique properties and applications.
Silicones are valued for their low volatility, inertness to most chemical contaminants and thermal stability in severe applications, as well as their performance in low-temperature environments.
These qualities make them an excellent candidate for use as heat transfer fluids, specialty grease applications and DOT Type 5 automotive brake fluids. However, there are two limitations of silicones that must be considered for lubricating applications.
First, they cannot be used in the cylinder lubrication of internal combustion engines because the combustion by-product is silicon dioxide.
Second, extreme pressure performance is limited, and common extreme pressure additives are incompatible with them. In their proper applications, the fluid life and hydrolytic stability of silicones is unsurpassed.
Diesters, or dibasic acid esters, were developed during World War II and are the reaction product of long-chain alcohols and carboxylic acids. Historically, they have been effective as reciprocating compressor lubricants due to their low coking tendency at temperatures of 400°F or higher. They also provide excellent solvency and detergency. The aggressiveness of diesters toward elastomers, seals and hoses has limited the usefulness of these fluids. Newer fluids, such as polyol esters, meet the needs of many applications formerly filled by diesters.
Polyol esters, or Neopentyl poly esters, have largely replaced diesters in high-temperature applications where oxidative stability is critical. Common applications include their use as lubricants in aircraft engines, high-temperature gas turbines, hydraulic fluids, and as heat exchange fluids. They can also be used as a co-blended basestock with PAOs to enhance additive solubility and reduce the tendency of PAOs to shrink and harden elastomers.
PAOs are hydrocarbon polymers manufactured by the catalytic oligomerization of linear alpha olefins like alpha-decene. They are considered high-performance lubricants and provide a high viscosity index and hydrolytic stability. PAOs are the most commonly used, and are generally less expensive than other synthetic lubricants. They have been used in passenger car motor oils, as well as numerous industrial lubricant applications.
Phosphate Esters are valued in applications where safety and fire resistance are critical considerations, which include fire-resistant hydraulic fluids and aircraft fluids. High flash points and fire points enhance their resistance to ignition, and their low heat of combustion makes them excellent self-extinguishing fluids. However, they do have several weaknesses including poor hydrolytic stability, which can lead to the formation of aggressive acidic by-products. Care must be taken when used because they can also react and degrade a variety of commonly used sealants and paints.
PAG oils offer quality lubricity, high natural viscosity index and good temperature stability. PAG base fluids are available in both water soluble and insoluble forms, and in a wide range of viscosity grades. They offer low volatility in high-temperature applications and can be used in high- and low-temperature environments. They are commonly used as quenchants, metalworking fluids, food-grade lubricants and as lubricants in hydraulic and compressor equipment. However, the water soluble PAG oils are incompatible with petroleum oil, and care must be taken in transitioning equipment from hydrocarbon oils to PAG oils.