Bio-Based Greases

FEATURE ARTICLE Nancy McGuire / Contributing Editor

Bio-based Greases:

Environmental concerns and improved technology are causing a resurgence for these plant-based lubricants.

36 • MAY 2014 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG AFTER A 150-YEAR HIATUS, lubricating greases made from agricultural products are making a comeback. For some 4,000 years, animal fats, olive oil and other bio- based materials kept wheels and gears moving along smoothly. The shift toward petroleum-based lubricants came about only in the latter half of the 19th century.1 Now uncertainties in petroleum prices and supply sources, coupled with an emerg- ing awareness of environmental factors, are turning a small segment of the market back to lubricants made from renewable resources—mainly plants. Back to the Future

Can bio-based lubricants compete in a world of high-capacity generators, jet aircraft engines and 18-wheel trucks? This question is particularly challenging when the lubricants are greases, which require thickeners and additives and must perform under heavy loads and in hostile environments.

WHY BIO? Bio-based greases are considered a specialty product today, representing between 1 and 5 percent of the market, depending on which source you consult. Performance wise, they lag behind mineral-based greases, but the gap is narrowing according to STLE-member Anoop Kumar, director of R&D and business development at Royal Manufacturing Co. in Tulsa, Okla. The main impetus for buying bio-based grease lubricants is to reduce harm to the environment. Not only are the base oils biodegradable, they tend to be made with fewer heavy metal and chlorinated additives. Even though these products have been on the market in the U.S. since the 1990s (and since the 1980s in Europe), a signiﬁcant amount of development work re- mains to be done before the products can truly be considered mainstream. “This is a learning curve for us,” says Bill Mallory, Royal Manufacturing’s owner and CEO. Bio-based greases are somewhat more expensive in the U.S. than their mineral- based counterparts, notes Mallory. The difference varies with the application and the quantity, but “it’s about 25 percent more expensive for bio,” he says, adding that the differential is less than it used to be because of the rising price of petroleum relative to seed oils.

Want to send your sales message to some 13,000 lubricant professionals? Talk to Tracy VanEe about a TLT advertising program: [email protected]. 37 STLE-member Lou Honary, profes- tem for regulating biodegradable environment present their own set of sor and founding director of the Uni- products, and product claims are clear challenges. The EPA requires that no versity of Northern Iowa’s National and supportable. Because of this, some visible oil sheen be evident down- Ag-Based Lubricants Center in Water- U.S. suppliers of environmentally ac- stream from facilities located in or loo, Iowa, notes that for large-quantity ceptable lubricants use European stan- close to waterways and that point dis- applications, bio-based greases can ac- dards to benchmark their products.2 charges to waterways must contain tually be price competitive with min- less than 10 ppm mineral-based oil.5 eral greases. He cites the example of This affects not only construction op- rail curve greases used to prevent track erations close to waterways but also and wheel flange wear on curved sec- Bio-based greases ships and other vessels that use metal tions of railroad tracks: “In early 2000, are considered a cables coated with grease as a protec- bio-based rail curve greases were about tion against corrosion. It is almost im- twice the price of petroleum rail curve specialty product today, possible to prevent spills and leakage grease. Today, they are priced the same, representing between completely, so environmentally ac- and at large volumes [bio-based greas- ceptable materials are being substitutes are] even lower priced than the pe- 1 and 5 percent of the ed for their mineral-based counter- troleum version of the same products.” parts. Frank Lochel, research advisor for market. Cargill Industrial Solutions in Plym- HOW TO MAKE GREASE outh, Minn., notes that bio-based greas- To make grease, mix about three parts es are far less expensive than the syn- Honary predicts that, “With the oil to one part soap. “Soap acts like a thetic greases that are currently government’s lead-by-example initia- sponge with its cells filled with oil,” available. The initial purchase price is tive, advocacy by growers associations explains Honary. When the grease is not the only factor in the overall cost, he and advances in lubricant research and subjected to shearing, mechanical says. “A high-cost grease for which the biotechnology of oil seeds, the U.S. shock, loading or other stresses, it re- grease replacement rate is low may be market will progressively see more leases oil, which flows into the gaps less costly to use than a low-cost grease bio-based lubricants.” between mechanical parts and does that needs frequent replacement.” Some progress is being made along the actual lubrication work. Disposal costs are another factor in this front. The USDA has issued a bio- In a chemical context, the word the overall cost of a lubricant. Because preferred list of products3, and the U.S. soap describes any product of a reac- solid and semi-solid lubricants are not military lists manufacturers of biode- tion between a metal hydroxide or easily liquefied, they are not easily re- gradable lubricants among its pre- other strong alkali and a fat or oil—the cycled. Thus, biodegradability is of ferred manufacturers.4 familiar sodium-based bath products, particular interest as an alternative to Even greases that use bio-derived as well as the lithium-, calcium- or recycling. base oils may contain additives that aluminum-based compounds required make them less environmentally be- for making lubricating grease. ENVIRONMENTAL ASPECTS nign. Honary cites the example of Fats are esters that contain saturat- Bio-based products are an attractive greases for high-pressure applications ed or nearly saturated hydrocarbon option for customers who must com- that use zinc or sulfur additives. He chains and are solid at room tempera- ply with regulations on the amount notes that greases of all kinds may be ture. Oils, also esters, have a higher and type of lubricants they can release contaminated with metal particles degree of unsaturation and are liquid into the environment. European envi- during use, and disposing of used at room temperature. (Saturation re- ronmental standards are stricter than greases must be done in compliance fers to the number of carbon-carbon those in the U.S., and the main re- with local rules. Kumar notes that double bonds in the carbon chains: search in bio-based lubricants is oc- Royal Manufacturing substitutes more greater saturation corresponds to few- curring in Europe, according to Mal- benign substances such as bismuth- er double bonds.) lory. “They are making a conscious based additives for ones using heavy Fats are classified by the number of effort because they are more sensitive metals such as antimony, molybdenum fatty acid molecules attached to a mol- to environmental effects,” he says. or lead. Even though bismuth is the ecule of glycerol. Triglycerides, for ex- “The U.S. is coming up to speed on heaviest of these elements, it is more ample, have three attached fatty acid this, as are South America and China.” environmentally benign because it is molecules. When fats or oils react with Part of the difficulty in the U.S. is a less toxic than lead and it does not bio- a strong alkaline solution, they are hy- lack of clear standards on the manu- accumulate. drolyzed into the constituent fatty ac- facture and usage of biodegradable lu- Lubricants that are used close to ids, which then react with the metal in bricants in general. Europe has a sys- waterways or otherwise exposed to the the alkaline compound to form soap.

38 • MAY 2014 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG WHAT GOES INTO GREASE: BASE OILS

BASE OILS eride oils are fairly inexpensive, but they successfully as a partial or full replace- Bio-based lubricants in the 21st Century require additives to prevent them from ment for oleic acid in metalworking fluids. use plant sources rather than sheep tallow oxidizing, hydrolyzing (degrading when This product is now being evaluated for its and whale blubber. A variety of seeds and they come into contact with water) or potential as a low-cost replacement for grains are used, and the products avail- degrading at high temperatures. Increas- the fatty acids used in greases. He notes able in any given market depend largely ing the oleic acid content of the base oil that Cargill uses primarily soybean oil for on what crops are grown in that part of can help prevent oxidation and reduce the its grease applications because it is readily the world. In the U.S., common base oil coefficient of friction. Genetic engineer- available and relatively inexpensive. feedstocks include soybeans, canola, rape- ing has quadrupled the oleic acid content “On the positive side, vegetables oils seeds, corn or castor beans. Palm oil is a of several plant oils over their naturally have intrinsically lower coefficient of fric- common feedstock in Malaysia. Oils that bred counterparts (20 percent in natural tion than their mineral oil counterparts. have a greater hydrogen content (more soybean oil versus more than 80 percent in This allows additives such as antiwear saturation) are desirable, since they tend some genetically engineered soybean oils, and EP [extreme pressure] to be used at to be more stable toward oxidation than for example.) substantially lower levels than for mineral unsaturated oils, and they perform better Cargill has developed a line of in- oils. “This lowers overall product toxicity,” at low temperatures. dustrial base oils using byproducts from says Lochel. “In instances where very low Often, a base oil will be selected vegetable oil refining processes, says Lo- levels of additives are acceptable, these because it strikes the desired balance be- chel. They have isolated a solid fatty acid vegetable oil-based greases are safer for tween oxidative stability and cost. Triglyc- stream (Agri-Pure 138) that was evaluated use in food processing equipment.”

Table 1 | Greases typically contain about 75 percent oil. This is the component that does the lubricating.

Base Oil Sources Advantages Disadvantages

hydrocarbons petroleum thermal and oxidative stability, low cost non-renewable feedstock, toxicity, (mineral oil) environmental damage, price and supply ﬂuctuations

triglyceride oil soybean, canola, good lubricity, biodegradable thermal and oxidative instability, sunﬂower, corn, cost castor bean, palm oils

diesters and polyol organic acids, thermal, oxidative, hydraulic, and hydrolytic diesters can degrade elastomer esters alcohols stability; wide operating temperature range; seals, high cost for polyol esters (synthetic) wide range of viscosities; low volatility; high lubricity; long service life

polyglycols (e.g., ethylene oxide, biodegradable, good lubricity, high can degrade seals, paints polyethylene glycol, water, catalyst viscosity index, good performance at polypropylene glycol) (synthetic) temperature extremes, hydrolytic stability

poly-alpha-oleﬁn ethylene gas biodegradable (varies), good low-temperature high-viscosity forms have varying (PAO) oils (synthetic) performance, hydrolytic stability, low volatil- degrees of biodegradability ity, good for low-viscosity applications, lower cost than some ester-based oils, hydrocarbon composition similar to mineral oils

(For more information, see Refs. 2, 6)

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY MAY 2014 • 39 to Kumar. They do offer some advantages in lubricity, the lubricant’s ability to reduce friction. Left: glycerol. Right: stearic acid, a common plant-derived fatty acid. Biodegradability is a double-edged sword. The ability of a lubricant to break down in the environment eases disposal problems and reduces the damage from leakage and spills. On Left: a generic ester (R and R' are hydrocarbon chains). Right: A triglyceride fat (a type of the other hand, degradation reduces ester) containing two stearic acid chains (top and middle) and one oleic acid chain (bottom). the shelf life and service life of a grease. Greases used as sealants (metal cables, open gears, gasket protec- tors) must often stand up to harsh environmental conditions—the same environment in which they are ex- Sodium stearate, a type of soap. pected to break down after they have served their purpose. Figure 1 | Chemical components of grease. The food industry would seem to be a natural fit for bio-based greases, but most food-grade lubricants use Lithium and calcium stearate soaps formed from the same or similar fatty non-biodegradable USP White Miner- produce greases that have higher melt- acids as those forming the soap, and al Oil. Kumar explains that the over- ing points than do the sodium and po- they can react along with the fatty ac- riding concern for food-grade lubri- tassium stearates commonly used as ids to produce a complicated soap cants is the safety of materials that washing soaps. Lithium soaps confer structure that performs unpredictably might come into contact with food. anti-corrosion properties, and they are at extreme temperatures. This concern overrides any concern more water-resistant than sodium Most of these issues have been ad- for biodegradability at present. soaps. dressed technologically, says Honary. Honary notes that bio-based greas- When soap and oil are mixed, the Microwave-based processing has been es may be used for any application soap molecules aggregate to form a fi- an important advancement. Exposing that currently uses mineral-based brous network. Oil droplets are con- the starting mixture to microwave ra- greases, as long as they meet National tained in the spaces of the network— diation produces a faster, more uni- Lubricating Grease Institute (NLGI) the holes in the sponge. Under form and complete reaction. specifications for that particular ap- operating conditions, the oil is plication and the manufacturer’s rec- squeezed out of the sponge and into APPLICATIONS AND PERFORMANCE ommendations are followed. For ex- the mechanical parts. A grease with Although bio-based greases may be ample, a bio-based grease containing a good reversibility will re-absorb the oil used for many of the same applications base oil with a high flash point or fire when the stress is removed. that use mineral-based greases, they point could offer an advantage in situ- Bio-based grease manufacturing is tend not to perform as well, according ations involving high temperatures or more complicated than mak- the danger of fire, he says. ing mineral-based greases, ex- For some applications that plains Honary. Conventional require a lubricant to perform grease-making processes re- under extreme conditions, quire reaction temperatures as bio-based greases may actual- high as 220 C (428 F), which ly outperform their mineral- could oxidize many biodegrad- based counterparts. “Lubri- able base oils. cants that can withstand these Also, the soap is formed in more severe operating condi- situ by adding alkali to a mix- tions, plus contaminants such ture of fatty acids and base oils. as dirt, sludge and water, will This isn’t a problem when min- be more valued by the indus- eral oils are used, since they try,” says Dan Deneen, direc- are non-polar and don’t react tor of business development with the alkaline solution. Bio- for lubricants at Solazyme in based oils, however, are esters Figure 2 | Micrograph of a soap matrix. South San Francisco, Calif., in

40 A ﬂamingo can eat only with its head upside down. WHAT GOES INTO GREASE: THICKENERS

THICKENERS temperatures than calcium soaps, factors Thickening agents provide a spongy matrix which work to offset the higher cost of that stores the lubricating oil, holding it lithium soaps. However, in many cases in place between gears and bearings, the lithium greases have been superseded by ﬁbers of a metal cable or along the bend of other types of lubricants for high-tempera- a railroad track. Soaps are the most com- ture and high-performance applications. mon type of thickeners. The fatty acids for Calcium soaps may be used to make Aluminum Complex Grease the soaps can come from olive, castor, soy- greases for low-speed, low-load applica- (Courtesy of Lou Honary, UNI-NABL) bean or peanut oils, and making them can tions. They offer good water resistance, be as simple as adding an alkaline solution but they do not perform well at elevated to form the metal salts of the fatty acids. temperatures. (38 C). The maximum usable temperature Lithium hydroxide is the go-to alkali for Complex soaps are used for high- for a grease is limited by the ﬂash/igni- many types of greases. Lithium soaps show temperature applications. These soaps tion/evaporation point of the encapsulated better resistance to water than sodium can raise the dropping point of a normal oil — about 350 F, or 180 C.5 soaps, and they perform better at high soap-thickened grease as much as 100 F To make a complex soap, two chemical

Table 2 | Greases typically contain about 20 percent thickeners. These are usually soaps, although some greases use organo-clay, polymer, or silica-based thickeners.

Thickener Sources Advantages Disadvantages

simple soap (e.g., fatty acids from olive, castor, anti-corrosion properties, limited temperature and lithium or aluminum) soybean, peanut or animal/ water resistant; readily performance range; can ﬁsh oils reacted with metal available stiffen after extended use hydroxide

complex soap (commonly simultaneous reaction of an high and low temperature can be water-sensitive; may based on lithium, aluminum, alkali with a fatty acid and applications; wash and harden if overheated or calcium sulfonate) an inorganic or short-chain corrosion resistance; can organic acid carry high loads without requiring EP additives

organo-clay natural or synthetic clays heat resistance may form residues in the (typically bentonite) modiﬁed housing after extended use; with organic amines incompatible with most other grease types

polyurea reaction of amines with little to no metal content; difﬁcult to manufacture; toxic diisocyanates (synthetic) resistance to water and ingredients; some types are oxidation; good for ball incompatible with most other bearings and electric greases motors; newer grease types have improved shear performance, shelf life, pumpability, rust protection, and EP performance

fumed silica ﬂame pyrolysis of SiCl4 or arc high temperatures, polar additives can disrupt the vaporization of quartz sand specialized applications (e.g., silica particle network optical cable ﬁllers)

(For more information, see Refs. 7, 8)

42 Who you gonna call? A complete list of STLE HQ staffers is available at www.stle.org. reactions are carried out simultaneously. Fatty acids are reacted with an alkali, as in a normal soap, while the alkali also reacts with an inorganic acid or a short-chain organic acid to form a metallic salt called a complexing agent. The base oil is present during this process, and it becomes encapsulated in the complex soap matrix. Organic complexing agents are commonly made using acetic, lactic or adipic acids. Carbonates, borates or chlorides may be used to form inorganic complexing agents, although chlorides produce a less environmentally benign product. Multi-colored soy greases (Courtesy of Lou Honary, UNI-NABL) Complex soaps commonly contain lithium or aluminum as the metal component, and calcium sulfonates are used in some applications. Aluminum complex grease has a very a recent survey.9 “For these value-added applications, polyol fine fiber structure and a high dropping point. It provides esters derived from fatty acid feedstocks between C8 and C18 good mechanical and thermal stability, along with good have worked well, because they exhibit good cost perfor- water resistance. mance, a wide temperature operating range and good oxida- For specialized applications, non-soap thickeners offer tive and thermal stability.” performance features that offset their greater cost. Poly- Kumar warns that generally speaking mineral-based urea thickeners are often used for ball bearings, providing greases perform better in harsh environments (marine ap- resistance to water and oxidation. Polyurea is metal-free, plications, for example), but some bio-esters come close to but manufacturing it requires organic amines and toxic matching this performance. diisocyanate compounds. COMPATIBILITY Mixing lubricating greases is a tricky business. Mixing incompatible greases results in performance that is significant- ly inferior to that of either grease used alone—for example, a marked increase in softening or bleeding. Incompatible products can cause blockages and degradation. Base oil can migrate out of one grease and into the other, leaving behind a plug of thickener and additive material. Manufacturers generally recommend against mixing dif- Polyurea structure ferent types of greases, especially when one is mineral-based and the other is bio-based. However, they sometimes provide R and R’ are hydrocarbon chains of various lengths, and information on the compatibility between specific types of n is the number of times the basic monomer unit is re- grease. (Ref. 5 shows one example of a compatibility chart.) peated in the polymer chain. Incompatibility can occur when two types of grease have differing metallic soap structures, explains Lochel. The soap matrix can contract, causing the base oil to be expelled, a Organo-clay thickeners are made by treating the phenomenon called synerisis. He recommends that bio- surfaces of dispersed clay particles (typically, bentonite) based and mineral-based greases not be mixed. with organic amines or ammonia compounds. The result- Compatibility requires case-by-case evaluation, states Ku- ing thickener has an amorphous, gel-like structure, rather mar. He has done extensive work in this area, and he has than the fibrous structure typical of soap thickeners. The developed a reference chart for specific types of greases. Why clay component provides heat resistance, and the organic not put compatibility data on product packaging? Mallory compounds aid in compatibility between the clay and the notes that it’s impractical to issue this type of general state- organic components of the grease. ment because of the wide variety of grease formulations, and Fumed silica, made from branch-like aggregates of that compatibility really must be determined for specific amorphous silica microparticles, is used as a thickener for pairings of greases. high-temperature applications. This form of silica has a high Honary states, “Most bio-based greases can be mixed with surface area and a very low density. It makes an effective mineral-based greases without negative impact as long as gelling agent, and it can be used to make greases for cable they are made of the same soaps.” For example, two greases fillers. that both use a lithium soap might work well together. He

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY MAY 2014 • 43 WHAT GOES INTO GREASE: ADDITIVES

ADDITIVES Metal-free additives, referred to as each other easily. These additives are used Grease manufacturers add a host of other ashless additives, can provide corrosion for lubricants meant for high-temperature materials, many of them proprietary, to protection and EP/antiwear properties with- and high-pressure applications. Greases achieve a set of desired properties: heat out sacrificing safety to the environment with solid additives must be changed more resistance, corrosion resistance, EP/anti- and personnel. These materials include frequently to prevent the solids from accu- wear characteristics and so on. The choice sulfurized fatty materials and succinic acid mulating in components and causing wear. of additives can lessen the biodegradability derivatives. Accumulated wear particles must be or increase the toxicity of a grease; thus, Solid lubricants, such as molybde- taken into account when evaluating the performance factors must be weighed num disulfide or graphite, can be used environmental effects of disposing of used against environmental factors. The physical to further reduce friction, especially for greases. Even the most environmentally be- and chemical properties of bio-based oils high-temperature applications. These com- nign grease product can become a pollutant are very different from those of mineral pounds have layered crystal structures, if it contains metal particles or other sub- oils, and they require different additives. and the solids form flakes that slide past stances that have eroded into the grease.

Table 3 | Greases typically contain about 5-10 percent additives to provide stability, extend the operating temperature range, enable the grease to perform under high pressure, or otherwise enhance its performance.

Additive Sources Advantages Disadvantages

Zn dialkyl dithiophosphates synthetic high pressure applications, toxicity, environmental (ZDDP); Mo, Sb, or Pb dialkyl anticorrosion applications effects, can accumulate dithiocarbamates (DTC); Pb in mechanisms and cause naphthenate wear

Bi naphthenate synthetic, used in less toxic, doesn’t active sulfur co-EP agents conjunction with bioaccumulate; similar can increase wear scarring organosulfur compounds EP properties to Pb at high speeds naphthenate

MoS2, graphite natural (graphite) or friction reduction, particles can accumulate

synthetic (graphite, MoS2) high temperature and in components and cause high pressure applications wear

dodecylene succinic acid synthetic corrosion protection, not as effective in and other succinate metal-free (ashless), preventing corrosion on derivatives inhibits sludge and varnish cast iron or lead formation

notes that several NLGI papers have etable oil basestocks are based on or mixtures of bio-based oils with explored the mixing of bio-based and oxygen-containing esters rather than mineral synthetic oils. mineral-based greases. hydrocarbons, it can be difﬁcult to Honary foresees a new generation ﬁnd additives that are soluble in these of bio-based greases moving into ex- THE FUTURE oils. The next generation of bio-based treme applications, including aviation Lochel predicts that as regulations greases may use complex esters, oils greases and specialized military appli- promoting bio-based lubricants in- derived using new chemical processes. cations, as well as a growing market crease, the demand for compatible ad- New feedstocks may come into play, for bio-based greases in more main- ditives also will increase. Because veg- using oils from exotic plants or algae, stream markets.

46 A bird’s heart beats 400 times per minute while resting and up to 1,000 beats per minute when flying. Bringing bio-based lubricants, in- REFERENCES cluding greases, into the U.S. market 1. Carnes, K. (2005), “The Ten Great- 6. Schaefer, T.G., “Esters in Synthetic mainstream will require well-defined, est Events in Tribology History,” Lubricants,” Bob Is The Oil Guy enforceable standards for the perfor- TLT, 61 (6), pp. 38-47. (blog), http://www.bobistheoilguy. mance claims and terminology that 2. Rostro, B.-C. (2007), “Everything com/esters-in-synthetic-lubricants. manufacturers use. Standards set by Old Is New Again,” TLT, 63 (12), 7. Johnson, M. (2008), “Understanding the NLGI and the European markets pp. 26-37. Grease Construction and Function,” are a start, and customer demand can 3. USDA Bio-Preferred Catalog: http:// TLT, 64 (6), pp. 32-38. drive further progress. Environmental www.biopreferred.gov/bioPreferred- regulations will continue to drive the 8. “Grease Bases used in the Manu- Catalog/faces/jsp/catalogLanding.jsp. facture of Lubricants,” Techni-Tips move toward bio-based products, but No. 56, Lubrication Engineers Inc., petroleum price fluctuations and bet- 4. Military Specification Lubri- cants Suppliers, ThomasNet. http://www.le-international.com/ ter-performing bio-based products will com, http://www.thomasnet.com/ uploads/documents/056_Grease%20 add further incentives for going green products/lubricants-military-specifica- Bases.pdf. with grease. tion-44951507-1.html. 9. Challener, C., “Base oils supplement: 5. U.S. Army Corps of Engineers En- Betting on bio for better base oils,” gineering and Design Manual 1110- ICIS Chemical Business, Feb. 20, 2-1424: Lubricants and Hydraulic 2012, http://www.icis.com/resources/ Fluids—http://www.publications. news/2012/02/20/9533166/base-oils- Nancy McGuire is a free-lance usace.army.mil/Portals/76/Publica- supplement-betting-on-bio-for-better- writer based in Silver Spring, tions/EngineerManuals/EM_1110-2-1424. base-oils. Md. You can contact her at pdf. [email protected].

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WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY MAY 2014 • 47