Evolution of Synthetics
By Suzi Wirtz
Editor’s Note: Some of the materials in this article is based on content originally published in Tribology & Lubrication Technology (TLT), STLE’s official monthly magazine.
Synthetics are part of the ongoing process of lubricants keeping up with evolving machinery. Like anything else, they have advantages and weaknesses and it’s important to understand those when choosing the right one for a specific application.
First, you know that mineral oil is a complex compound that contains myriad materials, some of which are great for lubrication and some that are detrimental to the cause. To take mineral oils to a more efficient and effective level, chemists and lubrication engineers zero in on their positive attributes and combine those building blocks into synthetics. The types of uniformity and fluid properties that can be attained with synthetics are often unreachable by mineral oil lubricants.
By way of example, consider this: The U.S. Air Force filled two one-gallon metal cans with fluid. One held a mineral oil-based hydraulic fluid; one held a polyalphaolefin (PAO-based) synthetic fluid. They then fired at each can with a 50-caliber armor-piercing incendiary round. Upon impact, the can with the mineral oil-based hydraulic fluid erupted in flames, while the can of synthetic hydraulic fluid merely smoked (see photo). The synthetic hydraulic fluid tested was designed with different fire and flash points, making it the clear choice for use in aircraft in combat situations.
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Synthetics by Class and Type
Each class represents the chemical compound, such as:
• Synthesized hydrocarbons – This class contains PAOs, alkylated aromatics and polybutenes
• Organic esters
• Polyglycol ethers
• Phosphate esters
• Silicones (silicate esters, halogenated hydrocarbons, polyphenyl ethers)
Synthesized Hydrocarbon Class
1. PAOs -- Either a normal or linear alphaolefin is reacted with a catalyst to oglimerize it, thus producing dimers, trimers and higher oligomers. The material at this stage is still unsaturated, so by using another catalyst the compound is hydrogenated and then distilled into the nominal viscosities of 2, 4, 6 and 8 centistokes (cSt) at 100 C. Different catalysts can result in higher viscosities, but the process remains essentially the same. With the principal PAO grades (4, 6, 8 and 100 [cSt]), the viscosity index (VI) and pour point stack up favorably compared to mineral oils. PAOs have impressive flash and fire points and are known to perform well in extreme temperatures. PAOs are generally compatible with mineral oils and additives.
2. Alkylated aromatics – These are created by dialkyl benzene manufacturing, which involves adding an olefin to the aromatic (benzene in this example) and introducing the catalyst to produce a simple alkylation. Key features of alkylated aromatics include good low temperature behavior and compatibility with mineral oils, additives and elastomers. They aid in solubility, and for this reason they are sometimes added to PAOs to increase a compound’s ability to solubilize additives.
3. Polybutenes -- Made by polymerizing isobutene, their key features include the ability to volatize, leaving essentially no residue. Popular for use in two-cycle engines, they are also common where exhaust is a factor in urban areas. Polybutenes are generally considered non-toxic.
Organic Ester Class
These are formed by taking a monobasic acid and an alcohol and adding a catalyst to form an ester and water. However, the reaction that formed the ester also can be reversed if the ester is introduced to water, causing it to hydrolyze and revert back to its alcohol and acid, a definite negative in some applications. Key features of esters include their easy customization, additive solubility and seal swell properties. Additional examples of esters include dibasic acid esters, or diesters, which have seen significant growth in the realm of industry, and polyol esters, which are well suited for high temperature environments such as jet engines. Because polyol esters are typically higher cost than diesters, their use in ground transportation applications has so far been cost prohibitive.
Phosphate Esters
These exude the positive traits of low volatility and chemical stability, but the hydrolytic instability can be troublesome in some applications. The decomposition products left behind following hydrolyzation of phosphate esters can be corrosive and may damage seal materials and elastomers, so care must be taken to choose compatible elastomers. The toxicity of some phosphate esters is also of concern.
Polyalkylene Glycols (PAGs)
These have a very high VI, modest pour points and reasonably good flash points. PAGs tend to have slightly better load carrying ability than most other materials. They are limited by hydrocarbon solubility and many are not compatible with other organics. Unlike most materials that dissolve more completely when heated, PAGs have an inverse solubility relationship. This results in better solubility characteristics at low temperatures.
Silicones
These exhibit a very high VI, superior thermal stability and oxidation resistance, a very wide operating temperature range and low volatility. Silicones are well-suited for nonmetallic lubrication and seal compatibility. They show good resistance to water, solvents and chemicals. Silicones can be costly and uncooperative with additives, especially those designed for mineral oils.
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The Future
Great opportunities exist for customized synthetic lubricants. In the example of compressors, new technologies are pushing operating temperatures higher and creating higher air throughput, while striving for greater energy efficiency and reduced machine downtime. In laboratory compressor oil tests, findings have shown that switching from a mineral oil to a synthetic increased the lubrication change interval from every 1,000 hours to every 8,000 to 10,000 hours and potentially a savings of up to 67% by switching to the synthetic.
With sealed-for-life units, a larger focus on biodegradable materials and the ever important economic impacts of operating machinery in the most efficient means possible, the capabilities and advantages of synthetic lubricants are taking center stage.
New materials to keep an eye on include:
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The Dow Chemical Co. is working on OSP development and has seen improved friction control, improved miscibility over conventional additives and higher viscosity grades as an alternative to bright stock and other materials. Dow also has determined that OSPs are easier on paints and elastomers and less hygroscopic than traditional PAGs. (OSPs are primarily base oils in formulations—compressor/refrigeration oils, hydraulic fluids, gear/bearing oils and engine/transmission oils. They can be used as a co-base oil with Group I to Group III mineral oils and PAOs; they can be used with additives for deposit control, friction modifier and viscosity builder.)
NOTE: It is possible to blend almost any synthetic to combine sets of desired properties, however, some materials are not compatible. Therefore, care must be taken to ensure proper formulation; the varying properties of the synthetic classes make excellent problem solvers for specific lubrication needs.
