FEaTuRE aRTicLE Dr. Neil Canter / Contributing Editor Grease additives: Important contributors not to be overlooked

High demand and performance requirements are key factors in selecting the right additives to use in grease applications.

GREaSES aRE uniQuE duE To THEiR coMPoSiTion, which can range kEy concEPTS from a semifl uid to a solid product. This contrasts with most other lubricants, which are fl uid in nature. The two most important • A wide range of additives that are similar components used in greases are the basestock and the thickener. to those used in fl uid lubricants are Greases are a three-dimensional network of thickener particles available to boost the performance of a dispersed in the basestock. In effect, this arrangement enables 1 specifi c grease. greases to act in a similar fashion to a sponge. The application of mechanical or other stress prompts a grease to release oil during • The three-dimensional nature of most an application. Once the stress is removed, the oil is reabsorbed. Much of the focus when discussing grease performance is grease networks can physically trap additive based on the type of thickener used. This is perfectly understand- molecules, which means that formulators able because there are at least 13 different thickeners available to need to use higher treat rates, especially for the grease formulator. The most popular are lithium and lithium- those that function at the metal surface. complex soap thickeners. But what can be overlooked at times is the impact of selecting • There is a continued need for new additives the right type of additives to boost the performance of a specifi c because end-users want to use smaller grease. In a similar fashion to other lubricants, a wide range of ad- ditives are available. Care must be taken to ensure that the right quantities of grease for longer time frames types are used to meet a specifi c application requirement, because under higher operating temperatures. there can also be negative consequences that can hinder the ability of the grease to fulfi ll its function.

28 Basics of MWFs: Register for Metalworking Fluid Management Certifi cate Course, This article focuses on the variety Table 1 | Additives Performing in the Bulk of the Grease of additive types available to grease Antioxidants Extend the life of the lubricant by inhibiting oxida- formulators and how to use them tion, thus minimizing base oil thickening, sludging and properly. deposits. To seek a broad range of opinions, Scavengers Chosen to react with undesirable contaminants such TLT interviewed the following eight as acids or sulfur to render them less harmful. industry experts:

• Joseph Kaperick, customer Table 2 | Additives Performing at a Surface technical service advisor-grease Antiwear Agents Inhibit wear, typically under high-speed, low load technology, Afton Chemical operating conditions. Corp. Corrosion Inhibitors Inhibit the corrosion of metals in contact with the • Dan Vargo, senior research lubricant, protecting equipment and extending the chemist, Functional Products, useful life of the lubricant. Inc. Extreme Pressure (EP) Agents Inhibit seizure under high loads and temperatures. • Chuck Coe, president, Grease Friction Modifiers Reduce the friction between moving parts by surface Technology Solutions LLC. adsorption. • Dr. Maureen Hunter, technical Metal Deactivators Inhibit the metals contacting the lubricant from service manager, King Indus- catalyzing oxidation of the lubricant. tries Inc. Seal Swell Agents Assist elastomer seals and gaskets to perform their function. • Dr. Gareth Fish, CLS, CLGS, technical fellow and industrial technology manager, The Tables 1-2 | The key additives used in greases can be divided into those that operate in the Lubrizol Corp. bulk of the grease (shown in Table 1) and those that perform on the metal surface (shown in Table 2) (Courtesy of King Industries Inc.) • Dr. Stephanie Janeda, grease specialist, Rhein Chemie Rheinau GmbH. Hunter organized the additives into es can. Solids such as molybdenum two categories, as shown in Tables 1 disulfide and graphite can be used as • Dr. Ruiming (Ray) Zhang, and 2. antiwear agents/extreme pressure global grease & industrial oil STLE-member Dr. Ruiming (Ray) agents for special purposes.” technical manager, R.T. Zhang, global grease & industrial oil Dr. Stephanie Janeda, grease spe- Vanderbilt Co., Inc. technical manager for R.T. Vanderbilt cialist for Rhein Chemie Rheinau • Paul Bessette, president, Tribo- Co., Inc., in Norwalk, Conn., indicates GmbH in Mannheim, Germany, con- Science & Engineering Inc. that most thickener systems, in combi- siders zinc dialkyldithiophosphates nation with base oil (i.e., base greas- (ZDDPs) and sulfur carriers to be key TLT asked these reps to address the es), do not possess the desired perfor- additives. She says, “Besides antiwear, issues and provide further guidance in mance properties needed without the EP and secondary antioxidant proper- using grease additives. use of additives. He says, “The only ties, ZDDPs based on long-chain alco- exceptions are calcium-sulfonate com- hols provide additional corrosion inhi- kEy gREASE ADDiTiVES plex greases and, to a lesser degree, bition and friction-reducing properties. Most of the respondents indicated that polytetrafluoroethylene-thickened Sulfur carriers display EP, lubricity, the additives used in greases are simi- perfluoropolyether greases.” antiwear and antioxidant properties. lar to those used in lubricant fluids. STLE-member Chuck Coe, presi- The combination of these potential Dr. Maureen Hunter, technical service dent of Grease Technology Solutions properties is influenced by the design manager for King Industries Inc., in LLC in Manassas, Va., points out that of the sulfur carrier.” Norwalk, Conn., and also STLE’s sec- polymers and solid additives also can STLE-member Joseph Kaperick, retary, says, “Additives may be catego- be incorporated into greases. He says, customer technical service advisor- rized in several ways, including “One type of oil-soluble grease addi- grease technology for Afton Chemical whether they are chemically active or tive has a function different from oil Corp. in Richmond, Va., provides de- chemically inert and by physical inter- additives, namely that of a structure tails on specific antiwear and EP addi- action or whether they perform their modifier that is usually a polymer. tives used. He says, “Properties of indi- function in the bulk of the fluid or at a Whereas fluid lubricants cannot be vidual ZDDPs can be altered by the surface.” formulated with solid additives, greas- chain length (C3 to C8 and higher)

Feb. 19-21, 2013, Philadelphia. Details at www.stle.org or contact [email protected]. 29 Base Grease Long Chain Medium Chain Mixed further on the role of antioxidants in 40,00 greases. He says, “Antioxidants do not Fe‐8 friction moment high load/low speed prevent oxidation under the combined 35,00 conditions ravages of heat and oxygen, but they Evaluation of

pair (Nm) greatly prolong the onset of molecular g 30,00 ZnDTP with various Alcohol chain length degradation of the lubricant.” Dan Vargo, senior research chemist Conditions: 25,00 Treat Level: 3%‐wt. oment of the bearin for Functional Products Inc., in Mace- M Bearing: 7312B. 536050. Hi09 with brass cage Axial load Fa: 80 KN donia, Ohio, discusses the use of water Friction 20,00 duration: 500h Temperature: room temperature spray-off and shear stability improvers speed: 7,5rpm in greases. He says, “Usually, these ad- 15,00 0,00 50,00 100,00 150,00 200,00 250,00 300,00 350,00 400,00 450,00 500,00 ditives are polymers that form an in- running time (h) terpenetrating physical (sometime chemical) network with soap thicken- Figure 1 | Variations within an additive group such as ZDDP can influence performance, as ers that improves the stay-in-place shown in this FAG-FE8 test. (Courtesy of Rhein Chemie Rheinau GmbH) ‘tackiness’ of a grease. The polymers greatly improve water resistance and and the type of alcohol (primary, sec- Specialist), technical fellow and indus- water spray-off/washout performance ondary) utilized in its manufacture. trial technology manager for The Lu- (see Figure 2).” Sulfurized isobutylene (SIB) and poly- brizol Corp. in Wickliffe, Ohio, indi- sulfides are good sources of active sul- cates that there are about 12 different 3D nETWoRk fur that are particularly effective at of- chemistries used as rust inhibitors. He All the respondents indicate that the fering protection under boundary says, “Sulfonates work well in normal three-dimensional nature of grease has lubrication conditions.” water.” an effect on the ability of additives to In differentiating the performance Dr. Fish also discusses specific fulfill their functions. Bessette says, of ZDDP chains, the frictional behavior types of antioxidants and metal deacti- “Effective tribochemistry involves of roller bearings is measured by the vators used. He says, “Aminic and chemical reactivity and mobility. Since FAG-FE8 test. Janeda says, “This spe- phenolic antioxidants are used either the rheology of the grease is assumed cial performance test was developed by as single components or in combina- to be a factor, grease formulators usu- an OEM. The results shown demon- tions. Triazole and mercaptothiadia- ally circumvent the problem by in- strate that variations within the same zole additives are used in metal deacti- creasing the concentration of additives additive group can have a significant vators to reduce yellow metal used in greases.” impact on performance (see Figure 1).” corrosion.” Vargo indicates that the three-di- Kaperick indicates that there are STLE-member Paul Bessette, presi- mensional grease network can physi- also ashless antiwear and EP options dent of TriboScience & Engineering cally trap additive molecules and can available. He adds, “For corrosion pro- Inc., in Dartmouth, Mass., comments be especially effective in preventing tection, imidazoline componentry is often used as an effective inhibitor with low-molecular weight succinates. Amine phosphates also can be very useful in preventing corrosion in the Add polymer presence of saltwater, while low total base number (TBN) calcium sulfo- Heating Mix Cool down nates are often successful in protecting metal surfaces under salt fog condi- tions.” Soap thickener Network Other ashless corrosion inhibitors network dissociation Interpenetrating include amine sulfonates, alkenylsuc- networks cinimides and oleoyl sarcosine. A past Soap thickener study compares the performance of Polymer ash-producing versus ashless corro- sion inhibitors.2 STLE-member Dr. Gareth Fish Figure 2 | Polymers can form an interpenetrating network with soap thickeners that improves (Certified Lubrication Specialist and tackiness, water resistance and water spray-off or washout performance. (Courtesy of Func- NLGI Certified Lubricating Grease tional Products, Inc.)

30 • DECEMBER 2012 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG

those that are surface active from reaching their objective. This trapping phenomenon can be seen in Figure 3.3 Vargo says, “This starves the surface for adequate protection, and part fail- ure can occur.” Janeda considers the problem at the surface to be due to the adsorption of the polar thickener on the metal surface. She says, “The ability of addi- tives is also influenced by the grease manufacturing process and the result- ing grease structure. Therefore, a read across of additive response, even be- tween the same thickener and base oil

types, is often not possible.”

Hunter adds, “For an additive that Figure 3 | Additive treat rates are higher in greases than fluid lubricants because they can be functions at the surface, such as a rust physically trapped in the three-dimensional grease network. (Courtesy of Functional Products, Inc.) inhibitor, an excellent performance in a lubricating fluid at a 0.1% treat rate or less may mean that the concentra- blood or rice pudding. Additive interac- sion inhibitors. Janeda adds, “These ad- tion may need to be boosted to 0.5% or tions cannot be discounted.” ditives can reduce the stability and drop more to pass the standard grease cor- point of polyurea and clay greases.” rosion tests. This need for a higher ADDiTiVE inTERAcTionS Zhang believes that formulators concentration is not required for addi- Besides the issues with the thickener, should look at both the short- and tives performing in the bulk of the there are other concerns that must be long-term effects of additive interac- grease. Their treat rates are similar to taken into consideration by the formu- tions with thickeners. He says, “In the what is seen in fluid lubricants.” lator. Coe says, “Some additives can de- short-term, additives selected should One of the characteristics hinder- stabilize or degel certain thickeners. not render the grease thickener less ro- ing movement of additives to the sur- For instance, organo-clay thickened bust, either in the form of a reduced face is the polarity of the thickener. greases can easily be degelled by anti- dropping point and/or increased cone Kaperick comments, “In addition to wear/EP additives. Too much of the penetration. Over the long-term, addi- forming a physical obstruction, many wrong corrosion inhibitor can severely tives should not soften or harden the thickener systems have an inherent impact the water resistance of a grease.” grease slowly over time.” polarity, which further acts to retard Dr. Fish points out that some spe- Hunter indicates that most additive the movement of polar additives cific additives can react with other ad- types can be used with the dominant through the network.” ditives, leading to potentially reducing grease types used globally (lithium Zhang contends that it does not their effectiveness. He says, “Organo- and lithium complex). She says, “We matter what thickener type is used. metallic additives such as dithiophos- frequently see that different surface- The formulator must take interactions phates or dithiocarbamates can ex- active additive treat levels are needed with additives into consideration change ligands and create new species to give the same results with different when preparing a grease for a specific in the lubricated system, which in thickeners. The same level of an addi- application. He says, “Grease additives some applications is beneficial. Acidic tive such as a corrosion inhibitor, anti- can be adsorbed by thickener systems additives can react with basic ones, wear or extreme pressure agent may or have interactions with thickener which, if not carefully controlled, can give different results in different thick- systems, either in the form of soap fi- impact their performance.” eners and even in similarly thickened bers or particle thickeners such as Janeda indicates that additive com- greases from two different manufac- bentonite clay or fumed silica. The ponents can interact, especially if liq- turers. There are no simple answers to grease formulator has to take these in- uid additives are blended into a pack- explain additive interactions in a teractions into account.” age. She says, “The interactions can be grease. It is almost always determined Dr. Fish maintains that not all greas- chemical or physical due to synergistic empirically.” es consist of fibrous three-dimensional or antagonistic effects.” structures. He says, “Some greases are Specific additives that can cause ADDiTiVE EffEcTiVEnESS amorphous dispersions in oil without problems are highly active sulfur carri- One of the challenges facing the for- structure and others are like clotted ers and calcium sulfonate-based corro- mulator is to make sure that the thick-

Just for laughs: 2.4 statute miles of intravenous surgical tubing at Yale University Hospital = 1 IV League. 31 ener used is compatible with the addi- tives selected to boost grease performance. Coe says, “The additive ‘Antioxidants do not prevent oxidation under the combined types and dosages must be optimized ravages of heat and oxygen, but they greatly prolong the to work with the given thickener sys- tem, not vice versa.” onset of molecular degradation of the lubricant.’ Kaperick points out that formula- tors can change the thickener to im- –Paul Bessette, TriboScience & Engineering Inc. prove specific grease properties such as oxidation. He says, “Use of metallic soap thickeners can act as a catalyst for oxidation, so the use of alternative without excessive churning is another oil adjustment and cool-down stage.” thickeners can prolong the oxidative way to reduce the effect.” stability of a grease formulation and Bessette states that processing of the gREASE PRoPERTiES increase the effectiveness of its anti- grease is very important in minimizing Kaperick states that the first objective oxidant system.” thickener problems. “Mill, homogenize for the formulator is to use additives Besides increasing the additive or filter the final product in order to re- that will not harm the grease. He says, treat rate, Kaperick indicates that duce or eliminate thickener agglomer- “The physician’s aphorism primum, non changing the polarity of the thickener ates,” Bessette says. “An excellent nocere—first, do no harm—is an apt may help. He says, “Reducing the po- method to determine the deagglomera- guideline for the use of additives in larity of the thickener system, while tion of the thickener in the oil is FT-IR grease formulations with respect to the often impractical, would lessen the analysis. Lithium 12-hydroxystearate structural stability of the thickener sys- negative impact on additive perfor- has a prominent absorption band at tem. Aggressive additives can soften a mance.” 1579 cm.-1 The intensity of that band grease and destroy its ability to act as a Zhang goes further by indicating increases as a function of the dispersion lubricant in the intended application.” that nonpolar thickeners can be used. of the thickener. When the intensity of A number of different additive types He says, “It is known that nonpolar the 1579 cm-1 remains constant, no ad- can cause grease problems, according thickener systems such as certain ditional milling is required.” to Dr. Fish. He says, “Some additives types of polyolefins will have little or Dr. Fish indicates that the degree of can have an effect on the physical almost no interaction with additive alkalinity built into the thickener has properties of greases. Dropping point systems.” an effect on additive performance. enhancers improve the thermal stabili- Hunter states that additive effec- “Highly basic thickener components ty of greases and increase their drop- tiveness is due in part to whether the can react with additives and neutralize ping points, but others have been application is in motion or static. “An- their effectiveness. This can be an is- shown to cause a significant reduction tiwear and EP additives do their func- sue with gelled overbased calcium sul- in dropping points in both complex- tion when the part is in motion, allow- fonate greases. It is also important and urea-thickened greases.” ing transport and wetting. Corrosion when making soap-thickened greases Odor also can be an issue in using inhibitors, on the other hand, have a that the amount of free alkalinity is extreme pressure additives. Dr. Fish significant role when the grease is stat- within tightly controlled bands,” Dr. continues, “Most high-sulfur, content ic,” Hunter says. “All of the grease cor- Fish says. “In lithium greases, too additives emit sulfurous species. With rosion tests include some time in the much free lithium hydroxide can in- all of these effects, treating the grease static condition, and the treat rate of terfere with additive components and with the right level of additive or se- the rust inhibitor necessary to provide impact their ability to function. How- lecting additives to carefully match the desired performance may be 10 ever, a reasonable excess of lithium hy- thickeners is part of the skilled formu- times the level required where contin- droxide has been shown to increase lator’s armory.” ued wetting of the additive occurs.” grease life in accelerated component Another potential concern is the Zhang also points out that chang- tests.” use of a specific additive to improve a ing the physical properties of the Beyond compatibility, Vargo con- particular property can lead to prob- thickener system can improve additive tends that the procedure used to pro- lems in a second area. Coe explains, effectiveness. He says, “Use of a softer duce the grease is important. He says, “When too much or the wrong type of or semifluid grease can reduce the “To get optimum performance from ad- polymer is utilized to improve oil blocking or hindering effect on addi- ditives and to lessen the chance that ad- bleed, mechanical stability or water re- tive migration from bulk to metal sur- ditives will react with the thickener, it is sistance, the result is that low-temper- faces. Suitable mechanical design that best to add them at the end of the soap- ature performance, particularly pump- will promote enough grease agitation making process, usually during the final ability, is reduced.”

32 • DECEMBER 2012 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG

Bessette gives examples involving the use of too much of one specific ad- ditive, in general, and how specific ad- ditives can affect particular grease types. “Too much antioxidant can be detrimental, since the sterically hin- dered antioxidant radical has a non- zero chemical reactivity,” Bessette says. “Polar additive can disrupt the hydro- gen-bonded network in organo-clay or amorphous silica-thickened greases. Finally, calcium acetate use in poly- urea grease can cause corrosion prob- lems, since this salt contains trace amounts of unreacted acetic acid.” oiL SoLubLE VERSuS SoLiD ADDiTiVES Figure 4 | The ASTM D4049 procedure uses an enclosed booth to direct a spray nozzle on a The unique properties of grease enable panel coated with grease to determine the percent spray-off as a measure of water resis- the formulator to disperse solid addi- tance. (Courtesy of The Lubrizol Corp.) tives that are typically difficult to work with in fluid lubricants in addition to using oil-soluble additives. Bessette particularly under conditions of slid- reach the friction and wear perfor- says, “Solid additives such as boron ni- ing and shock loading. In severe appli- mance necessary in specific applica- tride, graphite, molybdenum disulfide cations such as pivot pins and bush- tions. However, care is needed in for- and polytetrafluoroethylene can readi- ings in mining equipment, grease may mulating, as the performance benefits ly be compounded into grease with be squeezed out of the contact zone, of some solid additives can be reduced substantial tribological benefit for a and a solid additive such as molybde- by interaction with certain oil-soluble wide variety of applications.” num disulfide will provide a reserve of additives.” Dr. Fish feels that the use of an oil protection, even when all or most of soluble or a solid additive is very much the grease is gone.” ADDiTiVE dependent upon the application. “For Zhang considers oil-soluble addi- ASSiSTAncE high-speed coupling greases, solid ad- tives to be more effective than solid ad- There are a number of grease proper- ditives will probably be centrifuged ditives, except in the case of EP agents. ties that can be improved through the out and liquid additives would be a “EP performance depends upon build- use of additives. A case in point is the better choice,” Dr. Fish says. “For ing up an effective EP layer between poor load-carrying capacity of soap- slow-moving, heavily-loaded applica- two rubbing surfaces. Solid EP addi- and urea-thickened grease is improved tions, the addition of a solid lubricant tives may be more efficient in building hugely by the addition of extreme such as molybdenum disulfide may be up a thick physical EP layer, while it pressure and antiwear additives. An- the only way to prevent wear issues.” will take more time for oil-soluble EP other is water washout or spray-off re- Coe draws a distinction between additives to build up a reactive chemi- sistance. Dr. Fish says, “Water spray- the effectiveness of oil soluble and sol- cal EP layer,” Zhang explains. “Under off, as measured by ASTM D4049, is a id additives in a grease. “For oil-solu- extreme load conditions, a reactive problem for lithium greases. Adding ble additives to be effective, they must chemical EP monolayer formed by an the correct polymer into the grease can be efficiently delivered to the mating oil-soluble EP additive may be more reduce the spray-off from >90% to metal surfaces that need lubrication. easily destroyed through metal defor- <25%.” Figure 4 shows the setup for Therefore their effectiveness is impact- mation under extreme load condi- the water spray-off test. ed by the oil release tendency of the tions.” Vargo provides additional examples grease, as well as thickener polarity Kaperick cautions that while solid of how specific polymers boost resis- and additive competition at the metal additives are sometimes more efficient, tance to water washout (ASTM surface,” Coe says. “Solid additives, on it can depend upon the formulation D1264). “Adding 1% of a high-molec- the other hand, are largely unaffected and the application. He says, “For ular weight polyisoprene into a vegeta- by these factors. The primary purpose some formulations, there is a balance ble oil-based grease, the weight loss of most solid additives is to enhance of effectiveness versus cost, and solid percentage of the grease in the water load-carrying performance of a grease, additives are the most efficient way to spray-off test decreased from 83% to

34 Tribo-dictionary: Thixotropy – The tendency of grease or other material to soften or flow when subjected to shearing action. 17%,” Vargo says. “By adding 1% of an ethylene/propylene copolymer into a mineral oil-based grease, the weight loss percentage decreased from 73% to 15%. The water resistance improve- ment is due to the formation of inter- penetration networks by the polymer and the soap thickener. The polymer networks can be formed by several ways such as the physical crosslinking via crystalline phase, hydrogen bond- ing or chain entanglement.” Kaperick believes that two key grease problems addressed by additives are fretting wear and corrosion. He says, “Fretting wear has been shown to respond positively to phosphorus- based antiwear components, especially Figure 5 | Fretting wear reduction as a function of the concentration of an amine sulfurized in more lightly-loaded applications or phosphite in a fully formulated aluminum grease is shown. (Courtesy of Afton Chemical Corp.) those that might be accurately modeled by the Fafnir Fretting tester.” To illustrate this point, Figure 5 re- are a number of different polymer types EVALuATion TESTS veals data generated by the Fafnir that retard this effect.” “Although some methods used to evalu- Fretting tester (ASTM D4170) that Zhang indicates that there is an ad- ate greases are similar to those used in shows a reduction in wear as the per- ditive that will affect the dropping testing fluid lubricants (e.g., EMCOR centage of an amine-sulfurized phos- point of a grease. He says, “A certain corrosion, 4-ball wear/weld, Timken), phite is increased. The relationship ap- boron ester can complex with the many are unique to the study of greases pears to be linear in nature for the thickener soap structure to further in- and are employed to assess such proper- aluminum complex grease tested. crease the dropping point of or the ties as consistency, oil separation, pump- Corrosion problems are difficult to consistency of a grease.” ability and high- or low-temperature correct with additives because they are Janeda points out the problem that performance,” Kaperick says. “Evalua- very dependent on the application. can occur if a highly active sulfur car- tion of the importance of tests is a matter Kaperick says, “Often the final solu- rier is used to provide EP performance. of opinion which can vary widely, espe- tion is not a single additive, but a syn- She says, “These additives can cause cially between those with a bent toward ergistic combination of componentry copper corrosion, wear and can also fundamental research and those who that is more effective than a single in- negatively influence stability. Long- need to meet specifications set by cus- hibitor.” term copper corrosion, even at higher tomers or equipment builders.” This point is illustrated by showing temperatures, often cannot be con- He continues, “For evaluation of results from a corrosion study on a trolled by additional dosage of a yel- friction and wear under a variety of fully formulated lithium grease. The low metal deactivator.” conditions (temperature, load, speed, study used the standard industry test known as the EMCOR corrosion method (ASTM D6138). The results shown in Table 3 indicate that a com- bination of a succinimide with a suc- cinate ester and an imidazoline pro- vide better performance than either an amine phosphate or the succinimide or a combination of the two. Polymers also can be used to control the release of oil from the grease, which is known as bleed control. Coe says, “If a grease bleeds too much oil, it can dry Table 3 | A synergistic combination of additives such as the corrosion inhibitors shown in this out, leaving mostly thickener behind, study often provide more effective performance than a single additive. (Courtesy of Afton which can cause many problems. There Chemical Corp.)

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY DECEMBER 2012 • 35 materials), both the High Frequency Reciprocating Rig (HFRR) and the SRV are critical tools helpful in gaining understanding of the effects of friction modifier and antiwear components.” Dr. Fish comments on regional test preferences for the evaluation of ex- treme pressure additive performance and automotive grease testing. He says, “In North America, there is a bias toward using the Timken OK test (ASTM D2509) as a means of deter- mining the load-carrying capacity of greases, whereas in Europe the 4-ball extreme pressure test is preferred. In Europe, the SRV test is preferred to de- termine sliding wear, but the rest of the world still largely uses the 4-ball wear test (ASTM D2266).” Dr. Fish continues, “Automotive greases need to pass the 10 tests speci- Figure 6 | The tribometer shown is used to evaluate the extreme pressure (EP) properties of fied in the ASTM D4950 method used a grease. (Courtesy of TriboScience & Engineering, Inc.) in North America, but most of those tests are not recognized or are used under different conditions in Europe Cone Bleed (ASTM D6184). Oxidation simply through improved additives, and Japan. In terms of additives for resistance is most often reported by but through new combinations of ad- long-life greases, every major bearing the Static Oxygen Bomb test (ASTM ditive chemistries with specialized company has its own bearing tests that D942) but is better measured by a dy- thickener chemistries.” need to be passed. There is some stan- namic life test such as the FAG FE9 Janeda sees that new additives are dardization with the adoption of the Bearing Life Test (DIN 51821).” needed for the most recently devel- FAG FE9 as the internationally recog- oped thickeners and synthetic base- nized method to determine the high- PERfoRmAncE imPRoVEmEnTS stocks. She says, “There is a strong temperature performance of greases. Feedback from the respondents varied need for improved additive perfor- All other major requirements have re- on where improved additive perfor- mance for greases based on modern gional variations.” mance is required. One area that was functionalized thickeners and/or based Bessette shows results from testing mentioned more than once is fretting on modern synthetic basestocks (e.g., a grease using ASTM D2596. He says, wear. Kaperick says, “Fretting wear is polyglycols, silicones, perfluoropoly- “We evaluated the EP properties of a a form of oscillatory wear that can be a ethers and PAOs) to fulfill the very new grease under development using problem for a broad array of applica- high requirements placed on them the tribometer (shown in Figure 6). A tions from automotive wheel bearings during use. Choice and performance good result where the grease prevents to wind turbines. Due to the wide of additives suitable to formulate H1 the 52100 steel balls from welding is range of loads, vibrational frequencies greases or environmentally friendly provided in Figure 7. In contrast, fric- and environmental conditions associ- grease also has to be improved.” tional heat causes the balls to weld in a ated with different forms of fretting In a consistent fashion with more poor result (shown in Figure 8).” wear, there is neither a single screen- demanding end-user applications, Dr. Other important grease tests are ing test nor one additive solution to Fish believes there is need for addi- used to measure such properties as this often vexing grease formulation tives that can operate at elevated tem- mechanical stability, oil separation and challenge.” peratures. He says, “High temperature oxidation resistance. Coe says, “Me- Coe also included fretting wear as stability is a growing need. More users chanical stability is usually measured one of several properties that need to and grease formulators are specifying by either Extended Work Penetration be upgraded. He says, “High tempera- tests to be run at temperatures of 150 (ASTM D217) or by Roll Stability ture life, fretting wear and saltwater C and above, and this is hotter than (ASTM D1831). The two most com- corrosion are some of the key areas most additives have traditionally been mon ways to measure oil separation where improvements are desired. used at.” are Pressure Bleed (ASTM D1742) and These challenges will likely be met not Bessette agrees and specifically

36 Tribo-dictionary: PNA (polynuclear aromatic) – any of numerous complex hydrocarbon mentions the need for better antioxi- oils and greater stress on the additives dants in applications run above 200 C. to extend the functional life. He adds, “Boron additives are needed Hunter continues, “There may be a ‘Besides antiwear, EP and to increase the dropping point of lithi- tendency to move away from metal- secondary antioxidant um 12-hydroxystearate grease from based additives. It is increasingly diffi- approximately 200 C to 260 C without cult and more expensive to replace properties, ZDDPs based the evolution of alcohol.” metal-based surface-active additives on long-chain alcohols Zhang points out that an additive with ashless counterparts, as the ex- combining excellent antiwear and EP pectations of the grease increase.” provide additional corrosion characteristics would be very helpful. Janeda agrees, “Applications and inhibition and friction- He says, “The current problem is that the requirements for grease perfor- in the competition for positioning on mance are becoming more severe. Ad- reducing properties.’ the metal surface, a dominant EP addi- ditive development will have to follow tive may render an antiwear additive this trend and will play a big role in –Dr. Stephanie Janeda, Rhein Chemie less effective and vice versa.” grease development.” Rheinau GmbH The theme of needing greases to fuTuRE TREnDS perform at higher temperatures for The demands placed on greases in the longer operating time frames is also future are similar to fluid lubricants. expressed by Dr. Fish. He says, “Equip- gy-efficient machinery, while still able Hunter says, “The trends are not really ment manufacturers are producing to provide higher power output. The changing much. There is a continued new designs with smaller, lighter com- latter will force people to seek long-life desire for longer life under more ponentry to provide increased power or sealed-for-life greases,” Zhang says. stress—higher operating temperatures density. Higher power density equals “This will increase the need for newer with smaller quantities. This leads to higher operating temperatures, which additives to achieve the desired perfor- more stable thickeners, synthetic base also has a significant impact on the mance in all grease thickener types, type and quality of greases used. except for calcium-sulfonate complex Grease additives with improved ther- grease that provide most of the grease mal stability are being developed to performance functionalities through support these requirements.” its unique thickener system, though “Bearing users want longer life for certain applications, even calcium- greases that offer the advantages of re- sulfonate complex grease may need duced grease consumption and waste, additional additives to make it suitable less frequent regreasing and lower en- for extreme conditions.” vironmental impact. By using in- Bessette lists one future trend that creased but balanced levels of antiwear is already being undertaken by one and antioxidant additives, significant- grease manufacturer. He says, “More ly improved grease life can be ob- polymer-thickened greases, eliminat- Figure 7 | Good EP performance means that tained,” Dr. Fish says. “At the same ing discrete solid particles of thicken- a grease will prevent the 52100 steel balls time, these additives need to have low er, will reduce bearing noise and pro- from welding. (Courtesy of TriboScience & En- human and eco-toxicity. In many ar- mote improved lubricant entrainment.” gineering, Inc.) eas, legislation already is restricting Coe’s comments reflect those of the the choice of additives available, but other respondents. He lists the follow- competent formulators are only using ing four trends: additives that will be available for the long-term.” 1. Higher-performing food grade and Zhang provides his perspective on biodegradable greases. New and the future of grease development. improved food grade and biode- “High-performance greases such as gradable additives are critical to lithium complex, aluminum complex, meeting the needs of these calcium sulfonate and polyurea will growing sectors. gradually take over for greases with less desirable attributes. Two driving 2. Fill-for-life sealed bearings. Im- Figure 8 | Frictional heat causes the 52100 forces for this trend are modern me- proved antioxidants will play an steel balls to weld in a poor result. (Courtesy chanical design and sustainability. The important role along with long- of TriboScience & Engineering, Inc.) former favors more compact and ener- life thickeners.

compounds consisting of three or more benzene rings in a compact molecular arrangement. 37 3. Higher temperatures. As with fill- the recent steep increases in prices to provide an important role in meet- for-life bearings, higher temper- may stimulate interest in alternative ing this objective now and in the fu- atures require improved antiox- chemistries. The biggest obstacle to ture. Further background information idants in combination with this change is the reluctance of build- on greases and additives can be found high-temperature thickeners. ers to move away from a component in a text published by Polishuk.4 4. Higher loads. As the industry that, in their experience, is field-prov- moves away from antimony- en.” Neil Canter heads his based chemistries to meet high The growing demands for superior own consulting compa- loads, new environmentally grease performance over longer oper- ny, Chemical Solutions, friendly EP additives are needed. ating intervals will continue to drive in Willow Grove, Pa. the need for formulators to upgrade You can reach him at Kaperick sees the need for new ad- their products. Additives will continue [email protected]. ditives to replace some key raw mate- rials such as lithium and molybdenum disulfide that are rising in cost. He says, “Greases based on thickeners REfEREncES such as calcium sulfonates, that offer 1. Kaperick, J. (2010), “Grease – The ‘Other Lubricant,”’ STLE Philadelphia Section better inherent extreme pressure and presentation, April 15, 2010. corrosion protection, may increase in 2. Hunter, M., Rizvi, S. and Baker, R. (2001), “Ashless Rust Inhibitors for Greases,” popularity as the historical cost benefit NLGI Spokesman, 65 (3), pp. 24-32. of lithium thickeners is reduced.” 3. (2004), “The Chemistry and Physics of Grease-The Advantages of Grease,” As for molybdenum disulfide, Ka- Axel Christiernsson White Paper, 2004 (01) http://www.axelch.com/composite-74. perick adds, “Although the use of this htm?catid=4. raw material is specified at certain lev- 4. Polishuk, A. (1998), “A Brief History of Lubricating Greases,” Grease Technology, els (typically 3 and 5 wt. %) in many Lima, Pa. greases by heavy equipment builders,

It’s here – true ASTM D445 automation at a price guaranteed to make your lab manager smile. Small and Affordable Low Solvent/Sample Volume Faster Through-Put ! 10 or 100-fold tube ranges KinVis up CUSTOMIZED to 5000 cSt FOR USED ! Selectable temperature 20° to 100°C OIL ANALYSIS ! Smart table technology – save space, save time, save money!

2139 High Tech Road • State College •PA•16803 814-353-8000•800-676-6232•Fax 814-353-8007

[email protected] • www.cannoninstrument.com 38 Science quiz: Name the scientist who first discovered that the earth revolves round the sun? (Copernicus)