The evolution of synthetic lubricants
Josh Fernatt, Contributing Editor | TLT Webinar January 2013
While more expensive than mineral oils, these complex compounds can result in an overall lower cost of machine maintenance
KEY CONCEPTS
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Synthetics are part of the ongoing process of lubricants keeping up with evolving machinery.
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Synthetics have advantages and weaknesses, so you must understand their chemistry and characteristics when choosing one for an application.
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Synthetics can be designed with specific traits like higher flash points that often make them safer for specific machine applications.
WEBINARS: A NEW SERIES FROM TLT
This article kicks off a series of articles based on Webinars originally presented by STLE University. In some cases the Webinar presenter will author the article, and in others, like this one, the Webinar is adapted by a TLT writer.
Dr. Ken Hope, CLS, is a research fellow and team leader for NAO/PAO Technology for Chevron Phillips Chemical Co. LP. in Houston. He is a member of the STLE board of directors, serves on the Finance Committee and chairs the Editorial and Publications Committee. You can reach Ken at
hopekd@cpchem.com.
STLE University has sponsored dozens of Webinars and podcasts on a wide range of technical topics. To see Dr. Hope’s Webinar in its entirely, review all STLE University offerings and view the lineup of future events, log on to
www.stle.org. Webinars are $39 to STLE members and $59 for non-members.
Dr. Ken Hope
ADVANCES IN TECHNOLOGY have historically been achieved when innovators examine existing structures and identify opportunities for improvement. The rise of synthetic lubricants has resulted from isolating and combining the best properties of mineral oil-based lubricants to form compounds capable of outperforming what has come before.
MINERAL OIL VERSUS SYNTHETICS
While numerous differences exist between the two, mineral oils have played an important role in the evolution of synthetic lubricants. They are complex compounds that contain a myriad of 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 highly designed materials with strengths and weaknesses all their own. There are a wide variety of types of synthetics, and choosing the right one for a specific application is key.
“Since it’s a synthetic process, you’re really looking at getting good uniformity and consistency; that’s very typical of most synthetics,” says Dr. Ken Hope, CLS, research fellow and team leader for NAO/PAO Technology for Chevron Phillips Chemical Co. LP. in Houston, in a Webinar presentation by STLE University. The types of uniformity and fluid properties that can be attained with synthetics are often unreachable by mineral oil lubricants and can deliver better performance in certain applications.
While synthetics are typically more expensive than their mineral oil counterparts, it is important to consider the benefits when deciding which material is the best choice for the job. Synthetic lubricants are more uniform by design, which can mean lower product rejection and reduced analytical testing costs.
Another possible money saver is the longer oil life that stems from enhanced thermal and oxidation stability. This quality of synthetics can lead to less oil usage, less machine downtime and longer filter life. Machines running synthetics with lower volatility also require fewer top-offs between oil changes. In general, when compared with mineral oils, synthetic lubricants provide a wider range of safe operating temperature limits in both continuous and intermittent operating scenarios.
Hope cites how using synthetics designed for a specific role may translate into safer machine operation in very critical circumstances. In a dramatic test of a mineral oil-based hydraulic fluid versus a polyalphaolefin (PAO-based) synthetic, the U.S. Air Force filled two one-gallon metal cans with each 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.
Photos courtesy U.S. Air Force, Materials and Manufacturing Directorate
BUILDING FROM THE GROUND UP
Synthetic lubricants can be divided by class and type. Each class represents the chemical compound, such as synthesized hydrocarbons, organic esters and others, including polyglycol ethers, phosphate esters, silicones, silicate esters, halogenated hydrocarbons and polyphenyl ethers. It is also possible to blend almost any synthetic to combine sets of desired properties. Some materials are not compatible, though, and care must be taken to ensure proper formulation.
Though no overall synthetic lubricant is best for every situation, the varying properties of the synthetic classes make excellent problem solvers for specific lubrication needs.
COMPARING SYNTHETIC TYPES
As mentioned earlier, the synthesized hydrocarbon class contains PAOs, alkylated aromatics and polybutenes. PAOs are all typically made in the same fashion; that is, 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. Using different catalysts can result in higher viscosities, but the process remains essentially the same: reacting an alphaolefin, polymerizing it and then hydrogenating it.
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. The PAOs also have impressive flash and fire points and are known to perform well in extreme temperatures. PAOs are also generally compatible with mineral oils and additives.
Dialkyl benzene manufacturing is a common method for creating the second synthesized hydrocarbon, alkylated aromatics. This process involves adding an olefin to the aromatic, benzene in this example, and introducing the catalyst to produce a simple alkylation. The key features of alkylated aromatics include good low temperature behavior and compatibility with mineral oils, additives and elastomers. Alkylated aromatics also aid in solubility, and for this reason they are sometimes added to PAOs to increase a compound’s ability to solubilize additives.
Polybutenes are made by polymerizing isobutene. Their key features include the ability to volatize, leaving essentially no residue. This makes them very popular for use in two-cycle engines, and they are very common where exhaust is a factor in urban areas. Polybutenes are generally considered non-toxic.
The next class of synthetics, organic esters, 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.
Other synthetics of note include phosphate esters, polyalkylene glycols and silicones.
Phosphate esters 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, or PAGs, 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 but are limited by hydrocarbon solubility. Most PAGs are not compatible with other organics. An interesting feature of PAGs is that unlike most materials that dissolve more completely when heated, PAGs have an inverse solubility relationship, which means they have better solubility characteristics at low temperatures.
Silicones 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 and also show good resistance to water, solvents and chemicals. However, silicones can be costly and uncooperative with additives, especially those designed for mineral oils.
Comparisons of Synthetic Lubricants
SYNTHETICS AT WORK
Because each synthetic has an individual set of strengths and weaknesses, it is important to understand their chemistries and characteristics when deciding where and when to use them. PAOs have a wide range of acceptable temperature ranges but do not work well with some additives unless teamed up with an ester or alkylated aromatic. Polyol esters exhibit thermal oxidative stability but can be extremely costly. Some common areas of use for synthetics that most people are familiar with include: engine oils, gear oils, automatic transmission fluid, brake fluids and greases. Some other familiar places where you might find synthetic lubricants include refrigeration, food grade applications and compressors, where every class of synthetic may be found at work.
Other technological advances rely on the work of synthetic lubricants to succeed. 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. This application type presents a real opportunity for the use of customized synthetic lubricants. 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 8,000-10,000 hours. In this particular test, a comparison of the initial cost, total material use and labor revealed a savings of 67% by switching to the synthetic.
“When you’re looking at a synthetic, it’s useful to make sure that you’re going to use the right synthetic,” says Hope. “It’s good to look at everything that you’re concerned with, like the product value that’s lost due to downtime or the labor that you have to pay extra due to maintenance repairs, parts and the cost of the lube due to different oil change requirements.”
With the advent of 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.
ON THE HORIZON
As machine technologies evolve and extreme pressures are placed on lubricants to keep up, STLE members are busy developing synthetics to meet the challenge. New materials to keep an eye on include high viscosity metallocene PAOs and oil-soluble PAGs (OSPs).
The new high-viscosity metallocene PAOs (mPAO) are created using a metallocene catalyst, which differs from traditional PAO catalysts and is capable of yielding materials with a higher VI, lower pour point and better viscosity at lower temperature. Some also offer an improved friction coefficient. mPAO is commonly developed in two grades: 40 cSt and 100 cSt. A unique attribute of these synthetics can be seen in their ability to maintain very low pour points while achieving higher viscosity levels.
OSPs are capable of a greater functionality than traditional PAGs in that they can be used in combination with hydrocarbon oils to increase performance. Adding OSPs to hydrocarbon oils significantly improves deposit control and extends lubricant life. The Dow Chemical Co., one company working on OSP development, also 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. Based on these qualities, OSPs have extended the functionality of traditional PAGs.
“Traditional use of PAGs includes compressor and refrigeration oils, hydraulic fluids, textile lubricants and gear and bearing oils,” Hope says. “They’re also used as additives to build viscosity in water-glycol hydraulic fluids and a lubricity aid in water-miscible metalworking fluids.
“OSPs are primarily base oils in formulations— compressor/refrigeration oils, hydraulic fluids, gear/bearing oils and engine/transmission oils,” Hope adds. “They can be used as a co-base oil with Group I to Group III mineral oils and also PAOs, and also can be used with additives for deposit control, friction modifier and viscosity builder.”
SYNTHETIC ADVANTAGES
When comparing synthetic lubricants to mineral oil-based lubricants, the big improvements are seen in the crucial areas of energy savings, increased efficiency, reduced total operating costs, reduced maintenance and downtime, increased equipment life and extended lubricant life. And while synthetics might not always outperform mineral oils in every situation, their adaptability and combinability with other lubricants often make them the perfect problem solver in tasks where mineral oils fall short.
Josh Fernatt is a free-lance writer who can be reached at josh.fernatt@gmail.com.