Liquid Lubricants and Lubrication

Transcription

1 10 Liquid Lubricants and Lubrication Lois J. Gschwender Air Force Research Laboratory David C. Kramer Chevron Global Lubricants Brent K. Lok Chevron Global Lubricants Shashi K. Sharma Air Force Research Laboratory Carl E. Snyder, Jr. Air Force Research Laboratory Mark L. Sztenderowicz Chevron Global Lubricants 10.1 Introduction 10.2 Lubricant Selection Criteria Operating Environment Viscosity and Fluid-Film Lubrication Boundary Lubrication Performance Stability Fire Resistance Compatibility Biodegradability and Toxicity Additive Susceptibility 10.3 Conventional Lubricants The Evolution of Base Oil Technology Early Base Oil Processing Solvent Refining Additives Improve Performance Hydrotreating Hydrocracking Catalytic Dewaxing and Hydroisomerization Group II Modern Conventional Base Oils Group III Unconventional Base Oils Group IV Traditional Synthetic Hydrocarbon Base Oils (PAO) Group III vs. PAO Performance Future Evolution 10.4 Synthetic Lubricants Reasons For Use 10.1 Introduction Liquid lubricants have been around for a long time and have evolved from the conventional mineral-oil based to the more exotic synthetics. In this chapter, a systematic approach to the selection of a lubricant will be addressed. The evolution of the base oil technology for the conventional lubricants will be discussed in detail. Different classes of the synthetic-based specialty and high-temperature lubricants and their key features will also be addressed Lubricant Selection Criteria Although this chapter is primarily about liquid lubricants, it is worthwhile to spend a little time discussing some of the other popular lubricant types, i.e., greases and solid lubricants and hard coatings, to get a better idea of why liquid lubricants are selected most often. Also, the primary lubricant in grease is a liquid lubricant, so greases will be discussed to a minor extent throughout this section Operating Environment When selecting a lubricant for a specific application, a number of factors must be considered. First, and most important, is the environment in which the lubricant must function; generally, that is the temperature range of the application. However, there are sometimes other environmental considerations, e.g.,

2 outer space, liquid oxygen, or other reactive medium. After the above factors have been considered, one must decide which type of lubricant is required for the application. For some applications, greases can be selected which provide a simple design for the lubrication system, i.e., pack the bearing or mechanism with the appropriate grease. For other applications, solid lubricants or hard coatings are preferred. Again, they provide a simple lubrication system, although they frequently require some kind of relubrication process to provide long-time service. Finally we come to the most widely used form of lubricant, the liquids. Besides providing for lubrication of the mechanisms, they also provide damping and cooling. Cooling occurs by removing either the heat from the environment, e.g., a turbine engine bearing compartment, or the heat generated by the friction in the mechanism. By appropriate designing of the lubrication system, a liquid lubricant can control the temperature of a mechanical assembly within a very narrow temperature range, if required. For example, as a lubrication system using a pump to distribute the lubricant becomes hotter, the oil becomes less viscous, flow increases, and therefore the oil removes more heat. The liquid and grease lubricants also provide an efficient relubrication mechanism, i.e., if a grease or liquid lubricant film is broken, the grease or liquid flows back into that contact area, once again providing lubrication. For this reason, liquid and grease lubricants are more widely used than solid lubricants or hard coatings, but the solid lubricants and hard coatings are gaining wider acceptance as improvements in performance and lifetime are being achieved Viscosity and Fluid-Film Lubrication As previously stated, the operational environment for a liquid or grease lubricant usually imposes some specific property requirements on the lubricant and guides its selection. These property requirements are both physical and chemical. Perhaps the most used physical property is the viscosity temperature relationship. The low temperature operational capability of a liquid lubricant is defined by the maximum viscosity at which the lubricant can be pumped; for a grease it is defined by the maximum thickness at which the mechanical assembly or bearing can operate. The high-temperature operating capability of a liquid lubricant may be defined by the minimum viscosity that can provide fluid-film lubrication. The lubricant has to be effective in both the fluid-film lubrication and the boundary lubrication regimes. In fluid-film lubrication, a fluid film separates the interacting surfaces. In boundary lubrication the interacting surfaces react with the lubricant components to form protective physisorbed, chemisorbed, or reaction films. Fluid-film lubrication can be further divided into two broad categories hydrodynamic lubrication and elastohydrodynamic lubrication. In fluid-film lubrication, the physical properties of the lubricant, such as viscosity, pressure viscosity, and traction, determine the performance of the lubricated contact. While fluid-film lubrication is the desired mode of operation, the boundary lubrication regime cannot be avoided. Even in fluid-film lubrication, boundary lubrication occurs during start-up and stopping, and during occasional asperity interaction during operation. Therefore, the material (both the surfaces and the lubricant) properties that are important for the boundary lubrication regime are equally important for fluid-film lubrication. For thicker fluid films, a combination of higher viscosity and higher pressure viscosity coefficient are needed (Hamrock, 1994). Both these properties are affected by the molecular structure of the lubricant. Linear and flexible structures provide better viscosity temperature characteristics compared to the branched and rigid structures. On the other hand, the branched and rigid structures have higher pressure viscosity coefficients at a given temperature. The pressure viscosity coefficient for the branched and rigid structures decreases more sharply with temperature increase than that for the linear and flexible structures (Sharma et al., 1995). The lubricant selected should be able to maintain adequate film thickness throughout the operating temperature range. Lubricant traction coefficient (rolling/sliding friction coefficient) determines the power losses and stability of the rolling-element bearings (Gupta, 1984). For lubricants, a lower traction coefficient is desired for lower power loss and lower heat generation, whereas a higher traction coefficient is desired for the traction-fluids. The molecular structures that result in higher film thicknesses also provide higher

3 16 40 o C, cst MIL-PRF (No VI-improver) MIL-H-5606 (contains VI-improver) Test Hours FIGURE 10.1 Viscosity of fluid samples from pump tests. traction coefficients. Thus, the branched and rigid structures result in higher traction coefficients compared to the linear and flexible molecules (Hentschel, 1985). When selecting a lubricant, a balance must be achieved between the film-thickness and the traction properties. In the lubricant industry, the temperature response of viscosity is expressed as viscosity index (VI). The less the oil viscosity changes with temperature, the better its lubricating performance at high and low temperatures and the higher its VI. Generally, higher VI is preferred for lubricants. To increase the viscosity index of a lubricant, high-molecular-weight VI-improving additives are sometimes added to the lubricant. These additives increase the lubricant s viscosity at the higher temperatures as well as at the lower temperatures (to a lesser degree), thereby increasing the viscosity index of the lubricant. The VI-improvers have been extensively used in automotive lubricants to formulate the multigrade oils. The viscosity measurements under low shear rates do not directly translate into effective viscosity or film thickness in the lubricated contact for the VI-improved fluids. Under the high-stress and shear conditions, the VI-improved fluids do not contribute to the film thickness and traction to the predicted levels (Palacios and Bajon, 1983; Sharma et al., 1993a). In aircraft hydraulic pump tests, the fluid viscosity dropped sharply during the tests for the VI-improved fluid, whereas there was no viscosity loss for the non-viimproved fluids, as shown in Figure 10.1 (Sharma et al., 1998; 1999). It is interesting to note that the sheared, reduced viscosity fluid did not adversely affect the pump performance, and perhaps an even lower viscosity fluid would have worked equally well. Equivalent performance was shown in pump tests with a lower viscosity (8 40 C) non-vi-improved fluid (Gschwender et al., 1988), and no viscosity loss was observed. In selecting the lubricant viscosity for an application, one should not look just at the low-shear viscosity, but also the effective viscosity in the contact inlet Boundary Lubrication Performance When metal-to-metal contact occurs between two surfaces in a lubricated contact, the asperities on the surfaces shear, thereby exposing a fresh metal surface. The appropriate lubricant reacts with the exposed fresh metal to form protective surface films. The surface films thus formed are generally low-friction and wear-resistant, and protect the surfaces from early failure/wear. During operation, these surface films can wear and regenerate. Antiwear or lubricity additives are generally added to the base oil to enhance the formation of the protective surface films during boundary lubrication. Thus, in boundary lubrication, the chemistry of the lubricant along with the material properties of the interacting surfaces determines the performance of the lubricated contact. Some metals perform better in boundary lubrication than others. Surface coatings (Rai et al., 1999) can also enhance boundary lubrication performance. Therefore, when selecting a lubricant, the lubricated surfaces should be given proper consideration. While full-scale system/component testing is the best way to evaluate a lubricant s performance, more economical tests, such as the four-ball wear test (e.g., ASTM D4172) and Cameron Plint reciprocating

4 tribometer (e.g. Helmick et al., 1997), are available to measure the boundary lubrication performance of a lubricant/formulation. The full-scale component tests are preferred because sometimes the laboratory tests do not correlate with the lubricant s performance in the application Stability If the upper operating temperature limit for a liquid lubricant is not defined by the viscosity temperature properties, it is generally defined by the stability of the lubricant. For greases, the viscosity at elevated temperature is not generally the critical property. For greases, the upper operating temperature limit is generally defined either by stability or volatility. A lubricant must be stable in the environment in which it is being used so that it provides adequate lubrication for a finite lifetime. Some mechanisms are lubricated for life, whereas other mechanisms operate in such a severe environment that the lubricant must be changed on a regular basis. For example, most automobiles require that the engine lubricant be changed every 3000 or 5000 miles, whereas most small electric motor bearings do not require relubrication for their useful life. There are typically three different stabilities to consider that are inherent properties of the base fluid when a class of lubricants is selected for a specific application. They are thermal, thermal oxidative, and hydrolytic. (For greases, a fourth stability is the ability of the grease to remain a grease and not separate into the base fluid and the thickener.) We must carefully analyze the environment in which the lubricant must operate to determine what kind of stability is required. In an aircraft hydraulic system, for example, the lubricant, or in this case the hydraulic fluid, is not exposed to much air or moisture since the aircraft hydraulic systems are generally closed. Therefore, thermal stability is the most important stability for aircraft hydraulic fluids. On the other hand, if a shipboard hydraulic system is the intended application, hydrolytic stability also becomes very important since the lubricant will be operating in a wet environment. Thermal oxidative stability becomes important in air breathing systems, such as automotive engines, aircraft engines, etc., where the lubricant will be exposed to both high temperatures and oxygen. This puts another very severe limitation on the selection of the lubricant since many classes of lubricants have limited thermal oxidative stability. In applications where oxidative stability is of paramount importance, e.g., lubrication of valves coming in contact with liquid oxygen, liquid lubricants must not contain any hydrogen atoms and must be totally halogenated, e.g., chlorofluorocarbons or perfluorinated lubricants. The various classes of liquid lubricants and their thermal, hydrolytic, and oxidative stabilities will be discussed later in this chapter Fire Resistance Another important property to consider when a liquid lubricant is selected is fire resistance. Liquid lubricants can range from extremely flammable to nonflammable. Fire resistance is extremely important in hydraulic fluids. Three different aerospace hydraulic fluids have been developed and are in use today strictly to reduce the fire hazards in aircraft hydraulic systems. They are hydraulic fluids based on phosphate esters, specifically AS1241, and synthetic hydrocarbons, MIL-PRF and MIL-PRF Fire resistance is especially important for hydraulic fluids since they are used at high pressures (up to 5000 psi), which significantly enhances their potential ignition if the fluid is in the form of a spray. Hatton described the many hazards involved with hydraulic fluids and some of the more widely used methods to determine fire resistance, but even in his book, he failed to recognize one of the most important fireresistance characteristics, flame propagation (Hatton, 1966; Snyder et al., 1981). Most flammability test methods concentrate on a fluid s ignition characteristics, and, while that indeed is an important property, they fail to assess the potential damage from a fire once ignited. The AS1241 and both MIL-PRF fluids are ignitable, but they have extremely low flame propagation characteristics, making them significantly safer to use (Snyder et al., 1981). Although nonflammable hydraulic fluids have been developed, e.g., MIL-H-53119, a moderate-molecular-weight chlorofluorocarbon, they have not found their way into applications as yet. This has been

5 primarily due to the need to develop hydraulic systems around their unique properties, e.g., high density, specific seal compatibility, etc., and the fact that no one is willing to accept the high risk associated with being the first to use a new lubricant. Nonflammable lubricants are currently used only in small-volume applications Compatibility For a lubricant to be acceptable for a specific application, it must be compatible with all of the materials with which it will come into contact. This is especially true when trying to replace an existing lubricant with a new one. For this case, it is also important that the new lubricant be compatible and miscible with the former one to facilitate the changeover process from the old to the new lubricant. Significant costs could be experienced if parts of the system, e.g., O-rings, gaskets, and other seals, had to be replaced in order to realize the benefits the new lubricant would bring to that application. Since it is difficult to ensure that the new lubricant will not leak out of the system and get on other materials in the vicinity, e.g., paint, wiring insulation, composites, etc., the new lubricant will need to demonstrate compatibility similar to the old lubricant it is scheduled to replace. That has caused frustration over the years for operators of hydraulic equipment who would like to take advantage of the greater safety afforded by the fire-resistant hydraulic fluids based on phosphate esters. If this equipment was originally designed for use with mineral oil or other hydrocarbon-based hydraulic fluids, many of the materials contained therein will not be compatible with phosphate esters and would have to be changed if phosphate esters were to be used. This has caused many of these operators to stay with the rather flammable hydraulic fluid they have been using instead of exploring other options. In a number of cases, it has been possible to gain a significant increase in fire resistance by converting to a synthetic hydrocarbon- or ester-based hydraulic fluid that is compatible with all the materials of their systems. In addition, the compatibility of these fluids with the previous fluid has made the conversion to the new fluid simple and cost effective Biodegradability and Toxicity Biodegradability and toxicity have been combined because they both pertain to similar characteristics of lubricants. The degree of biodegradability of a lubricant relates to the environmental friendliness of a lubricant, whereas the toxicity of a lubricant relates to the friendliness of the lubricant to the user. Biodegradability is becoming a more important property as environmental regulations become more restrictive with regard to protecting the integrity of water supplies and the environment in general. Toxicity has been an important issue for quite some time, and safety to the user will continue to gain attention. So far, the most important aspect of a lubricant for a specific application is that it work well, and the environmental friendliness is secondary. In the future, it could be that the environmental aspects of the lubricant will take precedence over performance Additive Susceptibility In order for a lubricant to perform its required function, it is typically necessary to formulate the base oil with performance-improving additives. Some of these additives are typically oxidation inhibitors, lubricity additives (both antiwear and friction modifying), rust inhibitors, antifoam additives, and, in the case of greases, thickeners. It is critical that additives be effective in the base oils for a lubricant to provide satisfactory performance. In the case of liquid lubricants, the additives must also be soluble. For the majority of the hydrocarbon-based lubricants, this is not a problem. They have been in use long enough for effective, soluble additives to have been developed. However, for some of the newer fluids, e.g., the perfluoropolyalkylethers (PFPAE), the traditional additives are not soluble. Therefore, soluble, effective additives have had to be developed. The lack of effective, soluble additives for the PFPAEs has significantly delayed their introduction into the industry (Snyder and Gschwender, 1991). However, their limited introduction speaks well for their unique properties, which will be discussed later in this chapter. There can be cases where additive solubility is not an issue, but effectiveness is. This is the case for the

6 polydimethylsiloxane fluids, a.k.a. silicones. Although typical antiwear additives like aromatic phosphates dissolve in silicone fluids, they do not provide improved lubricity for metal-to-metal contacts. This has prevented their widespread use as lubricants for wide-temperature-range applications for which they would be ideal candidates due to their excellent viscosity temperature properties. Additive susceptibility is an important property to consider when selecting a class of lubricants for specific applications Conventional Lubricants The Evolution of Base Oil Technology In this section, we cover some of the major milestones in the evolution of base oil technology used to make conventional lubricants. Conventional lubricants are defined here as those that have historically been formulated with mineral oils derived from crude oil. But we will see that as base oil technology continues to evolve, the performance gap continues to close between conventional mineral-based oils and the traditional synthetic hydrocarbon oils described later in the chapter Early Base Oil Processing From its humble beginnings over 3000 years ago, base oil technology has seen many phases of evolution. In the first phase, animal fats were used as lubricants. Ancient inscriptions dating back to 1400 BC show beef and mutton fat (tallow) being applied to chariot axles. Very little changed over the ages except that the oils sometimes came from more exotic animals such as whales. In 1852 petroleum-based oils first became available. They were not widely accepted at first because they did not perform as well as many of the animal-based products. Raw crude did not make very good lubricant. The base oil industry was on the very steep part of the learning curve. But demand grew with the appearance of the automobile. Soon lubricant manufacturers learned which crudes made the best lubricants. Further improvements were made by refining the crude into narrow distillation cuts with varying viscosity. By 1923 the Society of Automotive Engineers classified engine oils by viscosity; light, medium, and heavy. Engine oils contained no additives and had to be replaced every 800 to 1000 miles. In the 1920s more lubrication manufacturers started processing their base oils to improve their performance. There were three popular processing routes Clay Treating Clay was used to soak up and remove some of the worst components in the petroleum base oil. These compounds were usually aromatic compounds and highly polar compounds containing sulfur and nitrogen Acid Treating Concentrated sulfuric acid was used to react with the worst components in the base oil and convert them into a sludge that could be removed. This process effectively cleaned up the oil, but it was expensive. This technology has virtually disappeared from North America due to environmental concerns about the acid and sludge (National Petroleum Refiners Association, 1999) SO 2 Treating This was a primitive extraction process to remove the worst components in the lube oil by using a recyclable solvent. Unfortunately, the solvent was highly toxic. Although it also has been virtually phased out (National Petroleum Refiners Association, 1999), it was a useful stepping stone to conventional solvent extraction Solvent Refining By about 1930 solvent processing emerged as a viable technology for improving base oil performance using a fairly safe, recyclable solvent. Most base oils in the world today still use this process. Solvent refining for base oil manufacturing was pioneered mostly by Texaco and Exxon under trade names such

7 TABLE 10.1 API Base Stock Categories Group Sulfur, Wt % Saturates V.I. I >0.03 and/or < II <0.03 and > III <0.03 and > IV All Polyalphaolefins (PAOs) V All Stocks Not Included in Groups I-IV (Pale Oils and Non-PAO Synthetics) as Texaco MP, Duo Sol, DILCHILL, and EXOL. About two thirds of the base oils in North America are currently manufactured using this route. Solvent-refined base oils are commonly called Group I base oils. Group I base oils are those having less than 90% saturates (>10% aromatics) and more than 300 ppm sulfur. Table 10.1 shows all the base oil groups as defined by the American Petroleum Institute (API). Solvents and hardware have evolved over time, but the basic strategy has not changed since Solvent refining is still the most widely used route for making base oil. Aromatics are removed by solvent extraction to improve the lubricating quality of the oil. Aromatics make good solvent but they make poor-quality base oil. Aromatics are among the most reactive components in the natural lube boiling range. Oxidation of aromatics can start a chain reaction that can dramatically shorten the useful life of a base oil. The viscosity of the aromatic components also responds poorly to changes in temperature (low VI). Aromatics are removed by feeding the raw lube distillate (vacuum gas oil) into a solvent extractor where it is contacted counter-currently with a solvent. Popular choices of solvent are furfural, NMP (Nmethylpyrrolidone), and a mixture of toluene and MEK (methylethyl ketone). Phenol is very rarely used due to environmental concerns. Solvent extraction typically removes 50 to 80% of the impurities (aromatics, polars, sulfur, and nitrogen-containing species). The resulting product of solvent extraction is usually referred to as a raffinate. The second step is solvent dewaxing. Wax is removed from the oil to keep it from freezing in the sump or crankcase at low temperatures. Wax is removed by diluting the raffinate with a solvent. This is done to thin the oil and enhance the dewaxing rate. Popular dewaxing solvents are MEK/toluene, MEK/MIBK(methylisobutyl ketone), or (rarely) propane. The diluted oil is then chilled to 10 to 20 C. Wax crystals form, precipitate, and are removed by filtration Additives Improve Performance Over the next several decades, the solvent refining process did not change very much. Improvements in finished oil quality came mainly from the appearance of additives. Additives began to be widely used in 1947 when the API began to categorize engine oils by severity of service: regular, premium, and heavy duty. Additives were used to extend the life only in premium and heavy-duty oils. In 1950, multigrade oils were introduced which were formulated with polymers to enhance the hot and cold performance of the oil. The additives changed the VI of the oil. This trend continued for several more decades. Lubricant quality improved significantly only when the additive chemistry improved. This was the only viable strategy for progress until a significant improvement in base oil quality was available Hydrotreating Hydrotreating was developed in the 1950s and first used in base oil manufacturing in the 1960s by Amoco and others. It was used as an additional cleanup step added to the end of a conventional solvent refining process. Hydrotreating added hydrogen to the base oil at elevated temperatures in the presence of catalyst to stabilize the most reactive components in the base oil, improve color, and increase the useful life of the base oil. This process removed some of the nitrogen and sulfur-containing molecules but was not severe enough to remove a significant amount of aromatics. This was a small improvement in base oil technology that would become more important later.

8 Hydrocracking Hydrocracking is a more severe form of hydroprocessing. It is done by adding hydrogen at even higher temperatures and pressures than simple hydrotreating. Molecules are reshaped and often cracked into smaller molecules. A great majority of the sulfur, nitrogen, and aromatics are removed. Molecular reshaping of remaining saturated species occurs as rings are opened and paraffin isomers are redistributed, driven by thermodynamics with reaction rates facilitated by catalysts. Clean fuels are by-products of this process. A primitive version of the hydrocracking process was attempted for lube oil manufacturing in the 1930s but was soon abandoned for economic reasons when solvent refining appeared (Sequeira, 1994). After WWII, more modern hydrocracking catalyst technology was imported from Germany. This technology was commercialized for fuel production in the late 1950s by Chevron (Stormont,1959). In 1969 the first hydrocracker for base oil manufacturing was commercialized in Idemitsu Kosan Company s Chiba refinery using technology licensed by Gulf (Anonymous, 1972). This was followed by Sun Oil Company s Yabucoa refinery in Puerto Rico in 1971, also using Gulf technology (Sequeira, 1994) Catalytic Dewaxing and Hydroisomerization Then came first-generation catalytic dewaxing and hydroisomerization in the 1970s. BP, Shell, and others used hydroisomerization technology coupled with solvent extraction to manufacture very high-vi base oils in Europe. In the U.S., Mobil used catalytic dewaxing in place of solvent dewaxing, but still coupled it with solvent extraction to manufacture conventional neutral oils. Catalytic dewaxing was a desirable alternative to solvent dewaxing, especially for waxy high-vi oils. This process removed n-paraffins and waxy side chains from other molecules by catalytically cracking them into smaller molecules. This lowered the pour point of the base oil so that it flowed at low temperatures like a solvent dewaxed oil. In the case of hydroisomerization, the majority of remaining aromatics were saturated, and the majority of remaining sulfur and nitrogen species were also removed. Chevron was the first to combine catalytic dewaxing with hydrocracking and hydrofinishing in its Richmond, California, base oil plant in 1984 (Zakarian et al., 1987). This was the first commercial demonstration of an all-hydroprocessing route for lube manufacturing. In 1994, the first modern wax hydroisomerization process was commercialized by Chevron in the same plant. This was an improvement over earlier catalytic dewaxing because it lowered the pour point of the base oil by isomerizing (reshaping) the n-paraffins and other molecules with waxy side chains into very desirable branched compounds with superior lubricating qualities rather than cracking away the n-paraffins. By combining three catalytic hydroprocessing steps (hydrocracking, hydroisomerization, hydrotreating), molecules with poor lubricating qualities are transformed and reshaped into higher-quality base oil molecules. Pour point, VI, and oxidation stability could be independently controlled. This was fundamentally different than the previous approaches that relied solely on subtraction. Among the many benefits of this combination of processes is greater crude oil flexibility; that is, less reliance on a narrow range of crude oils from which to make high-quality base oils. In addition, the base oil performance is substantially independent of crude source, unlike solvent-refined base oil Group II Modern Conventional Base Oils The modern hydroisomerization process licensed by Chevron under the name ISODEWAXING gained acceptance rapidly in the 1990s. In fact, about one third of all base oils manufactured in North America are now manufactured using this process (Figure 10.2). ISODEWAXING technology is also moving through Asia as well. Rapid growth of this technology in the U.S. prompted the API in 1991 to categorize base oils by composition (API publication 1509), as shown in Table Most of the base oils made using modern hydroisomerization are categorized as Group II. Table 10.1 shows that they are differentiated from Group I base oils because they contain significantly lower levels of impurities (<10% aromatics, <300 ppm sulfur). They also look different because Group II oils are normally very clear, almost water-white. From a performance standpoint, improved purity means that the base oil and the additives in the finished product can last much longer. This technology, along with

9 Percent % Base Oil in 10 North America FIGURE 10.2 ISODEWAXING trend in North America. specially designed additives, has already had a tremendous impact on finished oil performance. In some applications, lubricating oils formulated with Group II base oils outperform expensive traditional synthetic hydrocarbon oils made from PAO Group III Unconventional Base Oils The difference between Group II and III base oils is simply viscosity index (VI). Base oils with a conventional VI (80 to 119) are Group II. Base oils with an unconventional VI (120+) are Group III. They are also sometimes called unconventional base oils or UCBOs. Still another commonly used name for these oils is VHVI, or very high VI base oils. Group III base oils were not widely available in North America until a few years ago. Earlier generations of Group III base oils (Min, 1998) were produced in Europe, primarily by Shell and BP, but were not produced using the now commonly accepted all-hydroprocessing route. Consequently, they do not have the exceptional stability and low temperature performance of these modern Group III oils. Fortunately for North American consumers, those older-generation stocks are made and sold in different markets. Many of these plants are being upgraded to modern technologies that will enable them to make the modern Group III oils (Howell, 2000). Modern Group III base oils perform at a level that is significantly higher than conventional base oils, both Group I and Group II, and nearly match existing levels of performance in finished lube applications already established by their traditional synthetic hydrocarbon counterparts. The most notable exception, arctic oils, have very small market presence. From a processing standpoint, the higher VI in modern Group III base oils is achieved by increasing the temperature or time in the hydrocracker. This is sometimes collectively referred to as the severity. Alternatively, the product VI could be increased simply by increasing the feed VI, which is typically done by selecting the appropriate crude. Group III base oils are now widely available in North America because they can be manufactured by most of the companies that currently make Group II oils. Many companies have started adding them to their synthetic hydrocarbon product lines Group IV Traditional Synthetic Hydrocarbon Base Oils (PAO) Synthetic lubricants will be discussed in detail later in the chapter. However, we thought it would be appropriate to say a few words about synthetic hydrocarbon base oils here because they are directly influencing the future direction of mineral-based base oil technology. The hydrogenated polyalphaolefin (PAO) commercial market can be traced as far back as the early 1970s when specialized products were formulated from PAOs (Bui, 1999). PAOs became a major consumer-sought lubricant product when Mobil Oil commercially marketed its Mobil 1 product 25 years ago. However, Mobil s involvement with synthetic hydrocarbons can be traced back to the 1960s when the oil company resolved a problem plaguing military planes based on aircraft carriers. Mobilgrease 28 was developed to protect against wheel bearing failure caused by atmospheric cold temperatures. Mobil s SAE 5W-20 engine oil is derived from this base fluid technology. Since then, the PAO market has traveled

10 a long and winding road battling a slow, steady growth and criticisms of the higher cost compared to conventional oils. In the last 10 years, the PAO market significantly took off, first in Europe and then in North America, experiencing as much as a double-digit growth. In part, the growth might be attributed to the stricter lubricant specifications in Europe that created a market niche for synthetic hydrocarbons and semisynthetic hydrocarbon products. The demand has since extended to North America and other continents. The use of the word synthetic in the lubricant industry has historically been synonymous with polymerized base oils, such as PAOs, and other synthetics, such as esters, polyphenylethers, etc., which are made from smaller molecules. Primarily this use evolved because these types of base oil were the only components available for high-performance lubricants. That changed a number of years ago when some lubricant manufacturers, primarily in Europe, began replacing PAOs with newly available Group III base oils, which are made from feed molecules of substantially the same size as the final product. With the recent availability of modern Group III base oils in North America, this practice is now spreading to North America. This is currently causing a controversy in the lubricant industry, as some synthetic hydrocarbon base oil producers and lubricant manufacturers see only polymerized base oils or other oils made by combining smaller molecules as the true and only synthetic hydrocarbons. PAOs have historically had superior lubricating performance characteristics, such as viscosity index, pour point, volatility, and oxidation stability, that could not be achieved with conventional mineral oils. Now, in modern base oil manufacturing, viscosity index, pour point, volatility, and oxidation stability can be independently controlled. Modern Group III oils today can be designed and manufactured so that their performance closely matches that of PAOs in most commercially significant finished lube applications Group III vs. PAO Performance As well-designed Group III base oils become abundant in the market place, the performance gap between Group III and PAO (Group IV) is closing. Here are some key examples: Pour Point Pour point is the one place where Group III oils allegedly fall short of PAOs. While it is certainly true that the pour point of the neat VHVI base oil is substantially higher than that of a PAO of comparable viscosity, it is important to understand that what matters is the pour point of formulated lubricants, which are comprised of both base oils and additives. Fully formulated Group III-based lubricants are very responsive to pour point depressant additives, and where pour point depressants may be used, these lubricants can demonstrate pour points of 50 C or below when they are manufactured with modern isomerization catalysts. Products such as motor oils made with the lighter-grade PAOs, on the other hand, typically have higher pour points than the base fluid, so the gap in final product pour point between PAO-based and UCBO-based lubricants is smaller than in the base fluids themselves. Moreover, it is entirely possible with modern Group III manufacturing technology to produce base oils of even lower pour point, although this is not currently common practice in the industry precisely because there is very little customer demand or specifications for pour performance below 50 C. Cold Crank Simulator Viscosity in engine journal bearings during cold temperature startup is a key factor in determining the lowest temperature at which an engine will start. Cold Cranking Simulator (CCS) viscosity, as measured by ASTM D5293, is determined under conditions similar to those experienced in engine bearings during starting. For base oils, this viscosity is determined almost entirely by viscosity and viscosity index. Since VHVI stocks have a VI comparable to that of 4 cst PAO, one would expect comparable CCS performance. This is demonstrated in Figure 10.3, where it can be seen that a 4 cst Group III base oil with a kinematic viscosity of C and a VI of 129, and PAO 4, with a viscosity of 3.9 cst and VI of 123, have similar CCS values, both about half that of a 4 cst Group II base stock of about 100 VI. This performance makes the UCBO very effective for formulating fuel-efficient multiviscosity engine oils in the 0W-20 to 0W-50 range, one that has historically been achieved only with PAO-based product.

11 FIGURE 10.3 Cold cranking performance of mineral oils and PAO. FIGURE 10.4 Noack volatility of mineral oils and PAO. Noack Volatility Noack volatility of an engine oil, as measured by ASTM D 5800 and similar methods, has been found to correlate with oil consumption in passenger car engines. Strict requirements for low volatility are important aspects of several recent and upcoming engine oil specifications, such as ACEA A-3 and B-3 in Europe and ILSAC GF-3 in North America. Figure 10.4 shows that from a blender s perspective, Group III base oils are similarly effective as PAOs for achieving these low volatility requirements in engine oil applications. The viscosity index of modern Group III oils typically matches or exceeds that of PAO so they can match the volatility of PAOs at a reasonable distillation cut width. Oxidation Stability Oxidation and thermal stability are among the most important advantages that synthetic hydrocarbons bring to the table. Better base oil stability means better additive stability and longer life. High stability is the key to making the premium-quality finished oils of the future with longer drain intervals. Here Group III oils routinely challenge PAO performance. The stability of modern Group III stocks depends mostly on their viscosity index, because VI is an indication of the fraction of highly stable isoparaffinic structures in the base oil (Kramer et al., 1999). However, because modern Group III stocks also undergo additional severe hydrofinishing after hydrocracking and hydroisomerization, they achieve an additional boost in stability because only trace amounts of aromatics and other impurities remain in the finished stocks. The benefit of all-hydroprocessed Group III base oils in oxidation stability is illustrated in Figure 10.5, for hydraulic oils formulated by using the same additive system in four different base oils. Here, the time required to reach an acid number of 2.0 (defined by neutralization of 2.0 mg of KOH/g of oil) in the Universal Oxidation Test (ASTM D4871), a common measure of oil oxidation, was substantially longer for the Group III formulation than for either the Group I or II products. Moreover, the performance of the Group III product was essentially the same as that for the oil formulated with PAO. Table 10.2 lists a variety of North American lubricants of which the authors are aware which are based upon allhydroprocessed Group III base stocks. These products include engine oils, industrial oils, and driveline fluids like automatic transmission fluid, and are targeted at the same performance levels achieved by traditional synthetic hydrocarbon formulations.

12 FIGURE 10.5 Oxidation stability, acid number of hydraulic oils. TABLE 10.2 Synthetic Quality Products Utilizing All-Hydroprocessed Group III Base Stocks Available Now Semi and full synthetic PCMO Semisynthetic HDMO DaimlerChrysler ATF+4 Ford Mercon V ATF Compressor oil Upcoming GF-3 PCMO (semi and full synthetic) Extended drain gear oil High performance automotive (racing) oils Motor oils Gear and transmission oils Future Evolution Looking to the future, the trend is toward base oils with even higher purity, higher VI, lower volatility, and longer life. The base oils will probably look even more like PAO as they become more concentrated in isoparaffins, the most stable component in the base oil. It is likely that as base oil technology continues to evolve, the base oils of the future will surpass current benchmarks such as PAO in the critical performance areas such as oxidation stability and VI. There are many possible routes for improving base oil quality. Continued evolution of the all-hydroprocessing route is one likely possibility. Selectivity toward desired molecular compositions could be improved by improving the catalysts and the processing technology. Improving the feedstock can also improve the product. Very paraffinic (waxy) feedstocks such as Fischer Tropsch wax from natural gasto-liquids plants can potentially be further processed into high-quality base oils. Volumes and applications are expected to grow, as more ultra-waxy feedstocks become more widely available. Other competing technologies are likely to emerge. New routes for manufacturing PAOs have been proposed that use cheaper feedstocks such as ethylene and propylene rather than 1-decene (Heilman et al., 1999). In summary, the conventional base oil technology evolved slowly from ancient times until the middle of the 20th century. Then solvent refining brought base oil performance to a new level. Starting in the 1960s, hydroprocessing technologies were applied and combined to improve base oil purity and performance. Group II base oils were born. An all-hydroprocessing route for Group II base oil manufacturing was commercialized. Modern hydroisomerization technologies, such as ISODEWAXING became widely accepted and have grown exponentially since They are now used to make one third of all base oils in North America. The next wave appears to be Group III base oils. They offer most of the performance advantages of traditional PAO-based synthetic hydrocarbon oils at a fraction of the price. Most of the manufacturers that make Group II base oils can make Group III base oils. As base oil technology and additive technology continue to improve, mineral-based oils will continue to close the performance gap with traditional synthetic hydrocarbons. Traditional synthetic hydrocarbon oils such as PAO should continue to coexist with Group III oils as they have for years in Europe, but the widespread availability of highquality Group II and III mineral oils is accelerating the rate of change in the finished oil markets. New improved base oils are helping the engine and equipment manufacturers meet increasing demands for better, cleaner lubricants.

13 10.4 Synthetic Lubricants Reasons For Use In this section we will present an overview of synthetic fluids with unique properties. Synthetic fluids are selected because a mineral oil is deficient in some respect for a particular application. The U.S. military has often led research and development into new synthetics because of military requirements for high performance. Because of this and the authors backgrounds, many examples will be based on military lubricants. Driving the use of these synthetics are cost, availability, performance, and safety Cost Synthetic fluids are, as a rule, more expensive than mineral oils. An exception was a super-refined, deep dewaxed paraffinic mineral oil, used as the base stock for the SR-71 aircraft hydraulic fluid, MIL-H A. MIL-H-27601A cost over $100 per gallon, when a less expensive hydrogenated polyalphaolefinbased replacement was found. In cost analysis, an engineer must not only consider the initial acquisition cost of the lubricant, but also the life cycle cost. In life cycle cost analyses, the synthetic lubricants are often superior because equipment needs fewer repairs with synthetics, which are better inherent lubricants. Often equipment can be lubricated for life, never needing further lubrication, or at least extending maintenance intervals and thereby reducing down time Availability Synthetic fluids are generally more available in good quality than even the more stable of the mineral oils because the mineral oils come from various wells that may not be the same. Gradually as the better crude oils are depleted, less stable crude oils are used. In several cases in the military, mineral oil products no longer met the specifications that had been written around the original products. Synthetics, on the other hand, can be made consistently, as long as the starting materials are the same Performance Synthetic lubricants can be adjusted in the synthetic process to optimize property performance to a particular application. With mineral oils, the refiners have some, but much less latitude to adjust properties. High-temperature thermal and oxidative stability of synthetics exceeds those of mineral oils. Also viscosity temperature range and low-temperature flow can be improved and controlled in synthetics by controlling branching and chain length, for example. The upper limit may be from a thermal stability limit or from a viscosity limit. (A fluid s lowest useful viscosity depends on the application, but is generally considered to be 0.5 to 1.0 cst for adequate lubrication film formation in a bearing or other wear surface.) The lower temperature limit may be either from limiting viscosity or from pour point, two distinct properties of a fluid. The low-temperature viscosity depends on the application, e.g., size of fluid pumps, length of tubing lines, and other impediments to flow. For gas turbine engine oils, for example, 19,000 cp is considered the highest usable viscosity, while for hydraulic fluids in aircraft, 2100 cp is considered the highest usable viscosity. Most often, any fluid class is available in a number of viscosity grades. The commercial literature is often the best place to find current information to aid in selecting the proper grade of a fluid. Selecting the proper fluid class, however, is not so simple and requires research to weigh the pros and cons of the available fluid classes Safety Flammability of lubricants is often a major consideration in the design of equipment, and even in determining if a particular fluid should be used. Mineral oils are a distillation cut of a crude oil and so the final oil s flash point will be that of the most volatile part of the mixture. Synthetics, on the other hand, can be more closely controlled to be essentially the same molecular weight throughout. This means the flash point of synthetic oil will be higher for a certain viscosity of oil than it would be for a mineral oil of the same viscosity. For a phosphate ester fluid, fire resistance is even greater because the phosphorous atom acts as a fire suppressant.

14 FIGURE 10.6 Polyalphaolefin general formula. Flammability testing is a science unto itself (Snyder et al., 1981). The potential threat of an application must be considered, including ignition source, amount of oxygen present, air flow, temperature of the surroundings, and materials present, to name a few Fluid Classes Synthetic Hydrocarbons Synthetic hydrocarbons have gone from the laboratory to a major industrial product. Hydrogenated polyalphaolefin (PAO), Figure 10.6, is the most widely used synthetic hydrocarbon. It is most often synthesized from C 10 alpha unsaturated olefins using BF 3 catalyst to make C 20, C 30, C 40, etc., finished base oils. The synthesis conditions are adjusted to favor a certain molecular weight product, optimum viscosity temperature properties and low-temperature flow characteristics. The fluid is hydrogenated to remove residual olefins and fully saturate the molecules. PAOs have many of the advantages of highly refined paraffinic mineral oils such as those from now mostly depleted Pennsylvania crude oils. In fact, PAOs resemble paraffinic oils chemically, as they are largely straight chains, giving them high thermal stability and good viscosity temperature index. Some branching exists in PAOs, which prevents them from freezing (waxing) at cold temperatures. Mineral oils must be dewaxed to remove the totally linear molecules to prevent low-temperature freezing (waxing). PAOs are synthesized to avoid waxes. Ideal properties for a PAO are a careful balance of branch vs. linear structure. PAOs in general have better lubricating ability than comparable mineral oils because there are no other atoms besides carbon and hydrogen. The thermal stability of PAOs, when oxygen is excluded, is quite high, as shown by the MIL-PRF-27601C fluid upper bulk oil temperature, 288 C. (The MIL-H A specification changed from a mineral oil to a PAO-based fluid when it became MIL-H-27601B, as it is in the later revision MIL-PRF-27601C.) Also, PAOs respond to additives very similarly to mineral oils except that additives tend to be less soluble in PAOs (especially the higher-molecular-weight oils) than in mineral oils. PAOs cost less compared to other synthetic fluids. The first major use of PAOs was in military hydraulic fluid, MIL-H (now MIL-PRF-83282) ( 40 C to 200 C), based on decene trimer PAO (Gschwender and Snyder, 1999). This was developed as a fire-resistant fluid to counter the loss of military aircraft and lives from fires resulting from the use of a highly flammable mineral oil-based hydraulic fluid, MIL-H Aircraft damage directly attributable to hydraulic fluid fires went from over $10 million a year to fewer than $1 million a year with the conversion of the majority of Air Force aircraft to MIL-PRF MIL-PRF is now the primary hydraulic fluid for military aircraft in the U.S., Australia, and Israel, and is gradually going into European military aircraft. Other PAO uses in the military are MIL-PRF ( 54 C to 135 C), a fire-resistant hydraulic fluid for cold climate operation; MIL-H-46170, similar to MIL-PRF but with a rust inhibitor; MIL-PRF ( 40 C to 288 C), the high-temperature hydraulic fluid mentioned earlier. The C 20, PAO decene dimer is widely used as a dielectric and cold plate coolant for military radar systems (Flanagan et al., 1991), commercially as a solar heat transfer fluid and as a large computer coolant, and as a shock absorber for higher priced automobiles. PAOs are also used in moisture-resistant, widetemperature greases, e.g., MIL-PRF-32014, and instrument oils. A small but critical higher-molecularweight PAO application is as oil or as grease for spacecraft (Jones et al., 1998). An important advantage of the lower-molecular-weight PAOs is that they are readily biodegradable. MIL-PRF is a class 1 (the best) and MIL-PRF is a class 2 (second best) in the ASTM D5864 biodegradability test.

15 Decene trimer PAOs are often formulated as a major component of synthetic automobile engine oils, although they are more popular in Europe than in the U.S. Because they are inherently good lubricants, lower viscosity motor oils can be safely used, reducing viscous drag and engine wear. Often automobile operators using synthetic motor oils extend the oil change intervals as well as engine life, greatly offsetting the higher cost of the synthetic oil. Another synthetic hydrocarbon fluid type, with a small but unique niche, is multiply alkylated cyclopentanes (MACs) (Figure 10.7). In most applications, these fluids are too expensive to be commercially viable, but in spacecraft applications, they are being used more and more. Advantages include low volatility and vapor pressure, low pour point, good antiwear properties, and good additive response. The cyclopentane ring acts as an anchor to which alkyl groups can be attached to produce lubricants with good viscosity temperature properties and low pour points. The market FIGURE 10.7 Multiply alkylated for these fluids with unique properties will likely grow. cyclopentanes Esters Hermann Zorn in Germany first researched ester lubricants in the 1930s as a replacement for aircraft engine castor oil lubricants, notorious for sludge formation in higher temperature engines. In the U.S., the Naval Research Laboratory also experimented with esters in the 1940s. MIL-L-7808 first defined the ester gas turbine oil requirements in Revisions of MIL-L-7808 have led to MIL-PRF-7808 K, with Grade 3, a 54 C to 177 C (temperature given refers to the bulk oil temperature) fluid, and Grade 4, a 51 C to 204 C fluid. The U.S. Navy, having less severe low temperature needs, has its own specification, MIL-PRF-23699, with a standard STD type, a 40 C to 177 C oil, and a high temperature HTS type, a 40 C to 204 C oil. Commercial aircraft also use this product as gas turbine engine oil. The first ester lubricants were diesters and, as the temperature demands increased, esters became more complicated, for example, trimethylolpropane, pentaerythritol, and dipentaerythritol esters, as shown in Figure Diesters are the least expensive, often similar in price to PAO fluids. As the ester functionality increases, the costs also increase. Diesters have the best viscosity temperature properties compared to the more complicated esters. The pentaerythritol ester has very high autogenous ignition temperature for a specific viscosity, a property very important in gas turbine engine applications where engine nacelles are extremely hot and fires are disastrous. All esters are considered fire resistant compared to mineral oils of comparable viscosities. Ester lubricants have excellent oxidative stability and can be tailored to new applications. A wide variety of esters are commercially available. Esters are generally readily biodegradable because they react with water as the first step in degradation. One problem with esters in application however, is unwanted hydrolysis, although beta blocking (having an alkyl branch on the second carbon from the oxygen in the acid portion of the ester chain) does stabilize the molecule, both hydrolytically and oxidatively. Esters have many uses besides aircraft engines. Diesters are the base fluids for low temperature greases, e.g., MIL-PRF-23827, gear oils, and instrument oils. Many of the synthetic automotive motor oils are partially ester oils. O O R-O-C(CH 2 ) x C-O-R Diester O CH 3 CH 2 CH 2 -C-[CH 2 -O-C-R ] 3 Trimethyol Propane Ester O C-[CH 2 -O-C-R"] 4 Pentaerythritol Ester O O-[CH 2 -C-(CH 2 OC-R") 3 ] 2 Dipentaerythritol Ester FIGURE 10.8 Gas turbine oil esters.

16 The U.S. Air Force is now developing the best possible thermally and oxidatively stable ( 40 C to 232 C) ester for advanced engines (Gschwender et al., 2000). Improved and new higher-temperature fuels and structural engine materials, and a desire for improved fuel efficiency are leading the way for highertemperature gas turbine engine oils. Problems in development programs with high-temperature perfluoropolyalkylether candidate gas turbine engine oils, as will be discussed later, reinforced the need to develop the best ester-based gas turbine engine oil possible. A recent advancement in esters is the polymer ester, a copolymer of alphaolefins and unsaturated diesters. While they are more expensive than diesters, they have shown high benefit as antiwear blending fluids in about 20 to 30% concentration with other base fluids. The unpaired electrons on the oxygen in the ester are believed to bond to metal surfaces. Because the polymer esters often replace chlorinated cutting fluids, they offer environmentally safer fluids with good antiwear and viscosity temperature properties Phosphate Esters Phosphate esters are widely used as excellent fire-resistant lubricants. In most flammability tests they are superior to other fireresistant fluids unless significant amounts of halogens (chlorine, fluorine, or bromine) replace the hydrogen in the molecules. The general structure is shown in Figure 10.9, although the triaryl phosphates are also used as fire-resistant fluids. Advantages of phosphate ester fluids, besides their fire resistance, include excellent lubricity and bulk modulus. Phosphate esters must FIGURE 10.9 Phosphate ester general formula. be closely monitored in the application to control hydrolysis (reaction with water) to form phosphoric acid on one or more of the P O R sites. When properly monitored, they are used with great success. Phosphate ester thermal and oxidative stability is fair, with improvements being made as described below. Another issue in using phosphate esters is compatibility of paints, wiring, and elastomeric seals. These materials must be carefully selected, excluding most materials typically used with mineral oils. For this reason, equipment maintainers must use extreme care not to cross-contaminate phosphate ester fluid with mineral oil, or other hydrocarbon fluids, and vice versa, as disastrous consequences will result. In the U.S. Air Force, for example, any aircraft using phosphate ester fluids is maintained in a separate area from aircraft using hydrocarbon-based fluids. Of synthetics, phosphate ester fluids are more expensive than all except the highly halogenated oils and polyphenylethers. Phosphate ester hydraulic fluids were introduced into commercial aircraft in the late 1950s and have gone through many type improvements since then. The fluid is described in the Society of Automotive Engineers document AS Currently Type IV, 54 C to 107 C, is used with low- and high-density versions, Classes 1 and 2 respectively. Type V is a higher thermal stability version for use in newer, highertemperature commercial aircraft and is currently used in some aircraft. Other uses of phosphate ester fluids are as lubricants, compressor fluids, coolants, brake fluids, and greases. When the user properly maintains the fluid, phosphate ester fluids provide excellent fire resistance in applications for which hydrocarbons are too flammable. Lower fire insurance rates may offset the extra expense of phosphate ester initial cost and monitoring. The excellent antiwear properties of phosphate esters, especially compared to water-based fluids, save on component replacement costs Silicon-Containing Fluids Silicate ester and silicone fluids are illustrated by the formula in Figure Both contain Si O bonds that impart unique properties to these fluids, namely very high viscosity temperature index, low bulk modulus, and low surface tension. Many authors attribute these properties to flexibility of the molecule around the Si O bond. These fluids are low volatility and have good fire resistance. The low surface tension of the silicone and silicate ester fluids is generally undesirable in a lubricating application because the oil tends to seep past seals to contaminate other surfaces. A third silicon-containing fluid is the silahydrocarbon, a tetralkylsilane (Figure 10.11). This fluid has the advantages of the other siliconcontaining fluids, while their negative properties are minimized. While silahydrocarbons are not used commercially as yet, they are promising for specialty applications. "

17 FIGURE Silicon-containing fluids. FIGURE Silahydrocarbons Silicate Esters The silicate esters, MIL-C-47220, were once widely used as dielectric and plate coolants in many military radar systems, and as a high-temperature, 54 C to 177 C, hydraulic fluid, MIL-H-8446, in the B-58 aircraft. Silicate esters have excellent thermal stability and wide temperature range, but are very expensive. A disiloxane silicate ester hydraulic fluid is still used in the supersonic Concorde commercial aircraft. Both military specifications are now canceled. The silicate ester coolants have been largely replaced with C 20 PAO-based fluids (MIL-PRF-87252) in the radar cooling application. One major drawback to silicate ester fluids is the hydrolysis of the Si O R bond to form a flammable alcohol and a gel. The alcohol is a safety hazard and the gel is responsible for clogging system filters and, in dielectric applications where the electronics are actually immersed in the coolant, the gel acts as a pathway for electrical arcing, disabling the electronic system. The resulting black strings of decomposed fluid are known in the industry as black plague. Most silicates are beta-blocked, that is, they have an ethyl group on the second carbon from the O atom. This interferes with the hydrolysis reaction, but does not totally stop it. The replacement, hydrolytically stable PAO-based fluids are also considerably less expensive than the silicate esters. While PAOs are not as good in viscosity temperature properties as silicate esters, they are miscible and compatible and have worked well as drain-and-fill replacements wherever tried Silicones The R groups in the silicone fluids (Figure 10.10) can be methyl or other alkyl, phenyl, or alkyl, with some of the hydrogen substituted with either chlorine or fluorine. The greater the ratio of Si and O to hydrocarbon in the molecule, the higher the viscosity index, and the lower the thermal stability and surface tension, and vice versa. When these fluids decompose thermally they create an undesirable gel, similar to the gel from the hydrolytic decomposition of silicate esters. The chlorine or fluorine in some silicone fluids improves their thermal stability and also their antiwear characteristics. Silicone fluids have rather poor performance in wear properties of the base fluid by itself, and few additives are soluble or effective for antiwear benefit. Silicone fluids are now primarily used in greases.

18 The greases are useful as wide temperature range products because of their excellent viscosity temperature properties and good high-temperature performance. Currently, MIL-PRF is an alkyl substituted silicone-fluid grease, and MIL-PRF is a fluoroalkyl dimethyl copolymer-substituted silicone fluid grease. Solubility of additives is less of an issue for greases, as partially soluble or even insoluble additives can be used. Since much less grease is usually used, the higher cost of silicone greases is less of an issue than when a liquid lubricant is used. Other applications of silicone fluids are in synchronous motors, in precision equipment, with plastic and rubber parts of refrigerators and recording tape, as dielectric coolants, as brake fluids, and as damping fluids Silahydrocarbons Silahydrocarbon fluids (Figure 10.11) were first synthesized and characterized in the 1950s (Rosenberg et al., 1960). Their excellent, unique viscosity temperature range and thermal stability were recognized, but these fluids froze at rather high temperatures. In the 1970s, silahydrocarbons were synthesized with good low-temperature properties by using various alkyl group lengths in the starting materials, allowing a 54 C to 316 C missile hydraulic fluid to be developed (Gschwender et al., 1981; Snyder et al., 1982). The fluid was never used, however, because the missile program was canceled. These fluids had one silicon atom that acted as an anchor for the hydrocarbon chains, giving the fluids good viscosity temperature properties and low pour points. In the 1990s, University of Dayton Research Institute under contract to Wright-Patterson Air Force Base first synthesized high-molecular-weight silahydrocarbon spacecraft fluids with three or four silicon atoms. The result was the first candidate space fluid that was unimolecular. These fluids have volatility lower than any other space lubricant, except those based on Fomblin Z perfluoropolyalkylether fluid. In addition, they have lower torque in wear applications than PAOs or multiply alkylated cyclopentane fluids, the newest synthetic space lubricants currently in spacecraft (Sharma et al., 1993b). Other variations of space silahydrocarbons have been synthesized under Air Force sponsorship. Recently NASA Glenn Research Center conducted vacuum wear tests, finding silahydrocarbon space fluids to perform extremely well (Jones et al., 1999). Other advantages of silahydrocarbons are: 1. They are fire resistant. 2. The properties can be tailored to new applications by making them higher or lower molecular weight. 3. They are compatible with other hydrocarbon fluids, an advantage in easy fluid conversion. 4. They have good bulk modulus; not quite as good as PAOs, but much better than other siliconcontaining fluids. 5. They have low traction properties, which means bearings operate with low internal friction. The major disadvantages are they are not commercially available as of this writing and are more expensive than other hydrocarbon synthetics. Potential silahydrocarbon fluid applications include fire-resistant, wide-temperature-range hydraulic fluids, space lubricants, and in niche commercial lubricants especially in precision bearings. Figure shows the typical ASTM D92 flash points of hydraulic fluids. Three fluids with different base fluid chemical classes, all have 54 C viscosities of less than 2500 cst. The flash point of the fourth fluid (MIL- PRF-83282), which has a 40 C viscosity of 2500 cst, is also shown for comparison. The silahydrocarbon has the highest flash point or the greatest safety margin, compared to the mineral oil (MIL-H-5606) and PAO (MIL-PRF-87257) fluids (all 54 C operational fluids). Also, several commercially available additives have been found to be both soluble and effective antiwear additives for silahydrocarbons Chlorotrifluoroethylene Polymers Chlorotrifluoroethylene fluids (CTFEs) were developed in the 1940s for the Manhattan Project and have enjoyed a niche market since then. They are represented by the formula Cl(CClFCCl 2 ) n F. They are nonflammable by any practical definition and can be used in contact with liquid oxygen and other aggressive

19 FIGURE Comparative ASTM D92 flash point of mineral oil, PAO, and SiHC. materials totally unsuitable for hydrocarbon lubricant contact. CTFEs are more expensive than synthetic hydrocarbon fluids but are a less expensive alternative to perfluoropolyalkylethers (PFPAEs) for demanding applications. CTFEs have good stability (up to 177 C) and excellent viscosity temperature range. They have bulk moduli almost as good as that of hydrocarbon fluids and much better than that of the PFPAEs. Another advantage is they solubilize many of the commercially available hydrocarbon additives, although many of those additives are ineffective in CTFE (Gschwender et al., 1992). A major disadvantage of CTFE fluids is their high density, approximately double that of hydrocarbon fluids. Also, they react with copper and low chrome steels, which should be avoided in components. Also, as mentioned earlier, their cost is considerably higher than many people expect to pay for oil. In the 1970s the U.S. Air Force selected a CTFE oligomer as the base fluid for a 54 C to 177 C nonflammable hydraulic fluid, MIL-H Considerable research and development advanced the knowledge of both the fluid and components designed for use with it. In the end, it was not selected for aircraft application beyond test aircraft because it was considered too high a risk. MIL-H contains an additive package with both an antiwear additive and an antirust additive to make it a usable, effective, nonflammable hydraulic fluid. Extensive testing by the U.S. Air Force and others allayed concerns about the potential toxicity and ozone depletion effects of CTFE. Current commercial applications of higher-molecular-weight CTFE fluids are as gyroscope flotation fluids, vacuum pump oils, and metal-working lubricants Polyphenylethers Polyphenylethers (PPEs) (Figure 10.13) were first developed as the gas turbine engine oil for the SR-71 reconnaissance aircraft in the 1960s. MIL-L was written to describe these fluids. PPEs have excellent high-temperature oxidative stability, excellent bulk modulus, low volatility, and good fire resistance. Their major disadvantages are poor low-temperature flow, which is partially improved by isomer mixtures of meta and para linkage of the benzene rings by the oxygen atom, and poor lubricity. The five phenylether with isomer mixture has a pour point of 4 C. In the SR-71, one scheme to keep the PPE liquid at lower temperatures was to dilute it with solvent. After the engine heated from operation, the solvent would boil out, leaving the PPE engine oil. Needless to say, today, such a plan would not be environmentally acceptable. PPEs have poor inherent lubricating ability, and antiwear additives have not been highly successful. Besides these problems, PPEs are quite expensive because of the many synthesis steps required FIGURE Polyphenylethers.

20 FIGURE Perfluoropolyalkylethers. to produce them. Since the SR-71 aircraft was inactivated in the 1980s (except for a few test aircraft used by NASA) PPEs, the 5- and 6-ring fluids, now have only a small market as electronic contact lubricants and as vacuum pump oils Perfluorinated Fluids Perfluoropolyalkylether (PFPAE) fluids were introduced in the 1960s and have been intensely studied ever since, owing to their unique thermal and oxidative stability, nonflammability, and inertness to highly reactive materials. Four classes are generally recognized, as shown in Figure The physical properties and stability of PFPAE fluids depend primarily on the carbon-to-oxygen ratio of the fluid and to a lesser, but significant, extent on the amount of branching and the presence of less stable OCF 2 O linkage. Major limitations of PFPAE fluids are high cost, corrosive wear in the presence of some metals, especially steel and aluminum, and autocatalytic degradation in the presence of metal oxides/lewis acids. Companies that produce PFPAEs, the U.S. Air Force, NASA Glenn Research Center, and, more recently, the recording media industry have led the research on PFPAE fluids. To briefly summarize the experience in the authors laboratory on the types of PFPAE fluids, the K fluids have the highest thermal and oxidative stability, but the poorest viscosity temperature properties and highest volatility. The Y fluids have viscosity temperature and stability properties similar to the K fluids. The Z fluids have exceptional viscosity temperature properties and volatility, but the highest cost and the poorest thermal and oxidative stability of the PFPAE fluids. Z fluids are also the most reactive to metals. M fluids are similar to Z fluids, but cost less and have somewhat higher volatility. The D fluids, the newest commercially available materials, have good viscosity temperature properties and excellent thermal and oxidative stability, almost as good as the K fluids. A further advantage of the D fluids is their lower cost compared to the other three fluids. PFPAEs with functional end groups are used successfully for niche applications, such as by the recording media industry as monolayer surface modifiers to reduce friction and stiction. Soluble additives for PFPAE fluids were first synthesized by the U.S. Air Force (Tamborski and Snyder, 1977). Since then the PFPAE fluid producers and some other groups have expanded the list of additives to combat corrosive wear and catalytic degradation (Srinivasan et al., 1993; Gschwender et al., 1993). PFPAEs are used in contact with reactive materials in wide-temperature-range applications and where a small amount of lubricant is sufficient. Greases based on PFPAE fluids, MIL-PRF-27617, are used in aircraft high-temperature applications and in many commercial applications. References Anonymous (1972), First lubricant-oil cracker has trouble-free record, Oil Gas J., 70, Bui, K. (1999), A defining moment for synthetics part 2, Lubr. World, 9, Flanagan, S.R., Gschwender, L.J., Pekarek, B., Cupples, B., Maeder, A., Bavani, S., Tacco, D., Strong, R., and Letton, G., (1991), PAO coolant conversion workshop proceedings, WL-TR , accession number B Available from DODSOP Subn. Serv. Desk, 700 Robbins Ave., Philadelphia, PA