Synthetic organic ester basestocks for lubricants
Trudy E. Bell, Contributing Editor | TLT Webinars June 2014
Esters can be highly tailored to the needs of your specific application.
KEY CONCEPTS
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Synthetic lubricants have a higher initial cost but often a lower overall lifetime cost when factoring in reduced replacement, maintenance, energy use, downtime and disposal.
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Esters are synthesized by reacting organic acids with organic alcohols—the choice of both determines the ester's properties.
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Compared to mineral oils, esters typically have superior high temperature performance, cleanliness, low temperature fluidity and biodegradability.
MEET THE PRESENTER
This article is based on a Webinar originally presented by STLE University. “Organic Ester Lubricant Basestocks” is available at
www.stle.org: $39 to STLE members, $59 for all others.
Gene Zehler is a technical support manager with the Fuel & Lubricant Solutions Division of BASF Corp. in Cincinnati. He has been developing and supporting synthetic lubricants since 1981 and received his master’s degree in chemistry from the University of Cincinnati. You can reach Gene at
gene.zehler@basf.com.
Gene Zehler
ORGANIC ESTERS are the oily basestock for many synthetic lubricants. Until the late 1800s, natural esters were the predominant basestock in lubricants from animal or vegetable sources. But in the late 1800s, with the advent of the petroleum industry, mineral oils eliminated most uses of esters as lubricants until the 1940s. During World War II and soon after, in both Germany and the United States, much research was conducted on synthetic esters for use as lubricants in low- and high-temperature applications. During this same period, the development of high-performance applications such as aircraft turbine engines demanded lubricants that would be more stable than petroleum-based lubricants at high temperatures. Consequently, by the late 1940s, synthetic esters were being used as synthetic basestocks in some high-performance applications—specifically diesters because of their good stability and cleanliness at high temperatures, along with their excellent fluidity at low temperatures.
As gas turbine engines continued to get more powerful, however, they ran hotter and, thus, needed a lubricant that was even more stable. Polyol esters were extensively studied in the 1950s and ’60s for this application and eventually became critical to the advancement of gas turbine engines and their use in all of our industries today. In the 1970s, transportation and heavy industry took a keen interest in developing esters for normal applications—ones neither extremely hot nor extremely cold, such as crankcases, compressors or gearboxes—that still needed high performance. Although synthetic lubricants were significantly costlier than petroleum, by the 1980s it became clear that when you factor in all the dynamics of actual operation, including all the reductions in replaced parts, maintenance, energy use, downtime and disposal, syn lubes often turned out to have a lower final lifetime cost.
Thus, understanding different types of synthetic esters (diesters, polyol esters, dimer esters, aromatic esters and monoesters) and how they can be used to tailor the characteristics of synthetic lubricants was the topic of a December 2013 STLE University Webinar by Gene R. Zehler, an STLE member and technical support manager with the Fuel & Lubricant Solutions Division of BASF Corp. in Cincinnati.
SYNTHESIZING ESTERS
Simply put, synthetic organic esters are created by reacting organic acids with organic alcohols. The product is an ester, and the byproduct is water. Although the chemical equation is simple (acid + alcohol = ester + water), the sheer diversity of acids and alcohols gives lubricant engineers very flexible design capabilities. Indeed, esters can be custom-tailored to an application. So as applications and equipment needs grow and diversify, so does the technology for synthesizing new esters.
Sometimes a single synthetic ester may be used as a basestock for a lubricant. Other times, the basestock is a blend of synthetic esters. In other cases, esters are used as a co-base stock—they are blended with other types of basestocks such as polyalphaolefins (PAOs), mineral oils, or other synthetics.
Many important properties of a lubricant are influenced by the choice of the acids and alcohols (
see Table 1). Thus, “the science and art of synthesizing esters is matching up the needs of an application with the desired characteristics of the lubricant basestock,” Zehler noted. “Although it is not possible to be perfect in all the characteristics all of the time with one molecule, the goal is to find a good fit.”
Table 1.
One last point: the terms
biodegradability and
bio-based refer to two entirely different concepts. Terminology can sometimes be confusing, as the two may be discussed in the same context. Biodegradable means that the natural microbial life on Earth will be able to degrade and break down the lubricant into smaller components not harmful to the environment. In contrast, bio-based or renewable means that a molecule, such as an ester, is composed of all or varying amounts of renewable carbon. Today carbon can be generated from vegetable or animal sources—“Young carbon, if you will, as opposed to fossil carbon as in the case of petroleum,” Zehler observed.
DIESTERS
Diesters were among the first commonly used synthetic esters. Diesters are produced from di-functional acids and mono-functional alcohols. In organic chemistry, functional groups are the atoms that give a molecule its characteristics. In an alcohol, the functional group is the hydroxyl, that is, the oxygen-hydrogen group (-OH).
Figure 1 shows generic diester synthesis. The mono-functional alcohol is on the left. The chemist’s abbreviation for an alcohol is ROH, where R is the chemist’s abbreviation for other organic material such as a chain of a number of carbon molecules that is not important to specify at this point. The organic di-acid is on the right. In this generic case, it is a dicarboxylic acid, meaning an acid with two acid or carboxyl groups (the chemist’s abbreviation for an acid or carboxyl group is the carbon double-bond oxygen, single-bond hydroxyl group), one at each end of the molecule. In the diester resulting from the synthesis, shown below the arrows, each acid group from the di-acid is now converted to an ester group using the R portions from the two alcohol molecules. The R in the ester group is the same R group as in the alcohol. For every ester group that is produced, a molecule of water is also produced; so in the synthesis of a diester, two molecules of water are produced. The arrows pointing in both directions mean that this reaction can be reversible under severe circumstances.
Figure 1.
Zehler showed the actual synthesis of a very common diester used in industry called di-2-ethylhexyl azelate (
see Figure 2). It is synthesized from two molecules of the alcohol 2-ethylhexanol reacted with one molecule of azelaic acid, a di-acid with 9 carbons. Each alcohol molecule reacts with one end of the azelaic acid to produce the long diester along with water. Terminology tip: if you see -ol in the name, that’s usually a giveaway that it’s an alcohol. Furthermore, the names of esters usually begin with the name of the alcohol in some form and end in -ate attached to the acid name. So two 2-ethylhexanol + one azelaic acid = di-2-ethylhexyl azelate + water.
Figure 2.
The molecular structures of alcohols typically used tend to be branched rather than linear, because that branching gives the diesters better low-temperature performance. But linear alcohols also can be used for special applications. The most common alcohols used for diesters range from 8 to 13 carbons long.
The di-acids commonly used range from 6 to 10 carbons long. The most common type is adipic acid, which is 6 carbons long. Where bio-based content or biodegradability is more important, azelates and sebacates are used because they are bio-based and renewable acids, whereas adipic acid is typically not.
Why chose one alcohol or acid over another? Every alcohol and acid offers advantages and disadvantages. “That’s part of the fun of developing a lubricant that’s custom-designed for an application,” Zehler pointed out. “You work with the advantages, you stay away from disadvantages or you find applications where the disadvantages are not meaningful.” For example, advantages that diesters have over petroleum-based mineral oil are a low pour point, reduced volatility, better viscosity index and improved thermal and oxidation stability. All those characteristics spell longer service life. “Remember, that was the primary reason diesters were turned to in the young gas turbine engine industry,” Zehler noted. “It was a vital aspect of the lubricant to make those engines work well and have improved maintenance.” Also diesters have better solvency and cleanliness than mineral oil, as well as better lubricity and biodegradability.
Diesters also have disadvantages. They are not universally compatible with all elastomers, plastics and paints, whereas mineral oil has a broad compatibility profile. Specifically, diesters tend to swell up or soften some elastomers, plastics and paints. That being said, a wide variety of elastomers, plastics and paints are compatible with diesters and with synthetics in general. Moreover, that tendency to swell and soften can be used as an advantage where an ester will be blended with another synthetic basestock. For instance, PAO also has compatibility considerations but in the opposite direction, tending to shrink some elastomers that esters tend to swell. So it’s very common to blend lower levels of ester with higher levels of PAO to yield a blend with neutral (that is, good) elastomer compatibility.
Another possible disadvantage of diesters is their hydrolytic tendency. Recall that the synthesis of esters from an acid and alcohol with the byproduct of water is actually a reversible reaction. Hydrolytic tendency means that under extreme conditions of moisture and temperature, adding water to an ester can drive the reaction in reverse: the ester can hydrolyze with water and produce acid. Although such reversal is not common or seen much in industry, the possibility exists. In contrast, mineral oils or other synthetic lubricants such as PAOs and PAGs have no hydrolytic tendency.
Table 2 shows some typical properties of diesters (along with other esters this article covers) to give a rough idea of low temperature performance, viscosity index, pour point and flash point. It shows some useful patterns. Esters with higher molecular weight are built using longer- chain alcohols and longer-chain di-acids. As molecular weight increases, viscosity tends to increase as well. These common diesters have excellent fluidity at low temperatures, high viscosity indices (higher than is typical with routine mineral oils) and very low pour points, plus flash points that are a bit higher than mineral oils of equivalent viscosity.
Table 2.
POLYOL ESTERS
Polyol esters, a very large and diverse group, are produced from multifunctional alcohols—which are also called polyols—plus mono-functional acids.
The three polyols most common in synthetic lubrication are shown in Figure 3: neopentyl glycol (NPG), which has two hydroxyl groups, trimethylolpropane (TMP) with three hydroxyl groups, and pentaerythritol (PE), with four hydroxyl groups. Higher functionality polyols are also sometimes used (for example, di-PE has six hydroxyl groups and tri-PE has eight hydroxyl groups).
Figure 3.
As shown with diesters, the typical synthesis of a polyol ester is shown in Figure 4. The polyol TMP (
shown on the left) is reacted with pelargonic acid, a linear organic acid with nine carbons (
shown on the right). Three molecules of the mono-functional acid react with the TMP’s three hydroxyl groups, producing a large synthesized ester called trimethylolpropane tripelargonate (“there’s your
-ate,” quipped Zehler). Because the alcohol was a triol, three molecules of water are produced.
Figure 4.
Polyol esters are dramatically influenced by the different alcohols utilized, as well as the length, branching and saturation of the acids, giving broad latitude in tailoring synthetic lubricants.
Turning first to the acids: branching is the existence organic side groups along an acid’s primary molecular chain. Branching can have a large influence on viscosity, viscosity index, low temperature fluidity and other properties of the esters.
Saturation is another useful aspect. All carbon atoms are most stable if they have four single bonds attached to other atoms. When carbon cannot have four single bonds attached to it, it will form double bonds between its neighboring carbon atoms. Double bonds are unsaturated, as they tend to be less stable and more prone towards oxidation and degradation. So, for maximum stability of an ester, go with a fully saturated ester. But there are times when you want or need to use unsaturation in your ester molecule such as to lower costs or improve low temperature fluidity.
The acids most commonly used in polyol esters range from five carbons to 18 carbons, either linear or branched chain. The prefix iso- in a chemist’s nomenclature typically indicates a branched chain. The colon is a chemist’s nomenclature indicating some unsaturation in the molecule. Specifically, :1 means there is one double bond in the molecule; :2 would mean there were two double bonds in the molecule and so on.
Turning now to the alcohols: hindrance in a polyol gives an ester an especially good stability at high temperature. Hindered polyols have no hydrogen atoms attached to what is known as the beta (β) carbon, that is, the second carbon in from the hydroxyl group. In Figure 3, the central carbon (in yellow) is the β carbon. Instead, all four bonds of the carbon are attached to other carbon atoms. The absence of β hydrogen eliminates an ionic degradation mechanism that otherwise is present at moderately high temperatures, allowing the hindered polyol ester molecule to be thermally stable to much higher temperatures.
Polyol esters are generally more stable than diesters at high temperature because of hindrance. As with diesters, however, polyol esters can swell and soften some elastomers, plastics and paints or can hydrolyze under severe conditions. It should be noted that these tendencies can be virtually eliminated, either by choosing different acids or by blending the esters with other syn lubes.
As with diesters, as the molecular weight increases, so does viscosity. Even higher molecular-weight esters are possible by using combinations of polyols, mono-ols, acids and di-acids, which can yield special characteristics such as solubility, lubricity, etc. These types of esters are called complex esters.
DIMER ESTERS
Dimer esters are produced from mono-functional alcohols (typically the alcohols discussed in the diester section) and dimer acids. Dimer acids are a class of higher molecular weight di- and tri-functional organic acid, which is derived from oleic acid. Consequently, these dimer acids are predominantly mixtures of 36 carbon-long diacids and 54 carbon-long triacids.
Compared to the esters described so far, dimer esters are higher in molecular weight and viscosity because they start with a larger acid molecule. The longer chains also provide higher viscosity index and excellent lubricity. Dimer esters are known for having very good frictional characteristics, as they tend to provide lower friction in loaded contacts. Moreover, due to their long chains, dimer esters have broad compatibility with elastomers, which is rather similar to mineral oil. On the other hand, because of their higher molecular weight, dimer esters generally are not as biodegradable as the other esters discussed so far, but they are still better than most non-ester basestocks of similar viscosity. Also, dimer esters usually have some residual unsaturation left in the molecule, making them a little more prone to oxidation and thus a little less stable than the other esters at high temperatures. As with polyol esters, it is very common to make complex dimer esters, if even higher viscosity is desired.
AROMATIC ESTERS
Aromatic esters are produced from mono-functional alcohols (again, the same as discussed in the diester section) plus aromatic acids. Aromatic acids (so called because early organic chemists noticed they smelled different from more familiar acids) have a benzene ring in their structure: six carbons in a stabilized ring instead of along a chain. The two most common types are phthalic acid (with two acid groups) and trimellitic acid (with three acid groups). Less commonly, a type called pyromellitic acid (with four acid groups) is also used for esters.
Compared to the other esters, aromatic esters are higher in viscosity and their oxidation stability is similar to diesters. Their viscosity indices and biodegradability tend to be much lower than the other esters. The benzene ring makes the molecule less flexible and thus unable to flex as much with temperature. The benzene ring is also stable and in a form that is difficult for most microbial life to biodegrade. The benzene ring, thus, has a very large influence on the properties of aromatic esters.
Phthalates (esters made from phthalic acids) tend to be clean-burning. They also have a tendency to swell many elastomers, so are commonly used as plasticizers to intentionally soften resins and form soft, flexible plastics. Trimellitates and pyromellitates, on the other hand, have very low volatility, so are commonly used in high-temperature applications.
Technically, “Phthalates can be called diesters because they are made from a di-acid,” Zehler noted. “But because the benzene ring has such a large influence on the performance of the basestock, I like to put them in the class of aromatic esters.”
MONOESTERS
Monoesters are produced from mono-functional alcohols and mono-functional acids. Since both the acid and the alcohol are small molecules, the synthesis produces lower molecular-weight esters.
Because lower molecular-weight esters typically have lower viscosity, compared to the other esters, monoesters are thin—the lowest viscosity of all the esters discussed so far. Thus, they tend to perform well at low temperatures, except for the long-chain oleate type of monoesters. Compared to mineral oil fractions or other natural hydrocarbons with similar viscosity, monoesters have lower volatility and higher flash point, although those characteristics are not as good as in the other synthetic esters. Biodegradability is generally very good because of the smaller molecule. Also because the molecule is small, elastomer compatibility is poor—they can migrate into plastics or elastomers and soften them.
OVERVIEW
In comparing the performance of synthetic lubricant basestocks to one another and to mineral oil, Zehler cautioned that all syn lubes (and even mineral oils) can be tailored to have a specific property. Additives to the lubricant also can have a large influence on properties. “There is no one best synthetic lubricant basestock,” Zehler reminded. “They are all good in different ways, and they all have some weaknesses. The interesting part of working with syn lubes is finding a good matchup between a syn lube basestock and an application.”
People often turn to syn lubes because of their stability at high temperature. Many esters have very good biodegradability and can be tailored to be extremely biodegradable if needed, much more so than other synthetic basestocks.
Unique to esters is their oxygen content. Oxygen is not present in typical mineral oil or PAOs. In an ester, the oxygen content influences how the electrons orbit or reside around an ester molecule. Specifically, the electrons tend to congregate in certain sections of the molecule, giving parts of the molecule a net negative charge and other parts a net positive charge. The molecule is thus polarized somewhat like a magnet or a battery, resulting in positive and negative sections in the same molecule. This polarity causes many of the differences between esters and other syn lubes. The polarity gives esters stronger intermolecular attraction than syn lubes without polarity—ester molecules tend to cling to each other more tightly and, thus, form stronger and more stable films. That intermolecular attraction yields reduced vapor pressure, reduced volatility and higher flash points. The polarity also allows the ester molecules to hold on to metal surfaces more tightly, giving higher film strength and lubricity. Finally, that polarity gives the molecule a natural cleanliness or subtle detergency, which helps to reduce insoluble residues and also helps to soften and remove pre-existing residues.
Zehler briefly addressed esters in greases. Organic esters can make very fine synthetic greases using all current conventional procedures. The ester basestocks can be tailored to characteristics important for the grease industry such as low-temperature pumpability, low-temperature torque, oil separation, seal compatibility and biodegradability. In short, everything discussed for liquid lubricants also holds true for synthetic greases. “When you make a synthetic grease, however,” Zehler cautioned, “you not only have to think about the high temperature stability of the basestock, but also of the thickener.”
SUMMARY
Organic esters are a class of lubricant basestock that can offer improved high temperature stability, biodegradability and a variety of other performance advantages over mineral oil or some other synthetic fluids. Ester properties can be precisely tailored to applications. Esters can also be used to produce partial synthetic lubricants or synergistically blended full synthetics, bringing unique improvements in lubricity, biodegradability, renewability (ie., bio-based content), cleanliness, low temperature fluidity or materials compatibility.
There is no universal basestock. Each has its own set of advantages and disadvantages. Each application should be studied and compared to the basestocks and lubricants in your arsenal to determine the performance requirements of the lubricant to find a good match between the lubricant, lubricant basestock and the application.
All synthetics commonly have a higher initial cost than mineral oil. So why and when should we use synthetics? “I believe the answer is only when the performance improvement of the synthetic justifies the higher initial cost,” Zehler concluded.
Often that justification can be shown by a detailed and dynamic analysis of various issues such as reduced replacement, maintenance, energy usage, downtime and disposal. Alternatively, some applications are so unique and so severe that an optimized synthetic lubricant is the only viable approach to keep the process effective and trouble-free.
Trudy E. Bell, M.A. (www.trudyebell.com) is a free-lance science and technology writer and editor who can be reached at t.e.bell@ieee.org.