Trends in Extreme Pressure Additives

Tech Beat

By Dr.Neil Canter Contributing Editor Special Report: Trends in extreme pressure additives

ormulation of industrial lubricants performance. The survey indicated that read- F requires the selection of a number of ers wanted to learn more about this additive different additive types such as antioxi- type in order to better understand how to dants, corrosion inhibitors, defoamers, use them in formulations. emulsifiers, extreme pressure (EP) agents, TLT sought interviews with the industry’s lubricity additives, pour point depressants key additive manufacturers to gain insight and viscosity index (VI) improvers. Within on this additive class. The five EP suppliers each category, the formulator is also faced who agreed to talk are, in alphabetical order, with having to select specific chemistries for Arkema, Dover Chemical, Lubrizol, Rhein use in preparation of the lubricant. Chemie and Rhodia. This leads to concern An initial discussion was held to provide Traditional boundary lubrication additives do from a formulation stand- basic information on how EP additives func- point about whether the tion and the various technologies available not remain on the metal surface and cannot pre- additive will not only pro- to the formulator. vent the increasing friction, wear and damage to vide the required per- the machinery seen under these conditions. formance but also how it What is an EP additive? will impact the other Lubricants operate in the boundary components in the for- between moving metal parts to prevent the mulation. This is a major issue that lubri- contact that could lead to an increase in cant compound/blenders must deal with on friction, increase in wear and eventually a daily basis. welding. The classic type of component TLT conducted a readership survey in used to prevent these phenomena is a mid-2007 and asked readers which additive boundary lubricity additive. type has the greatest impact on the perform- The boundary lubricity additive typically ance of their lubricants and which additive functions by adsorbing on the metal surface class they would be most interested in learn- to form a film that will reduce metal-to-metal ing more about. Overwhelmingly, reader contact. This function is achieved because response was that EP additives have the the boundary lubricity additive has a polar greatest impact on head group that can interact with the metal lubricant surface and a tail group that is compatible with the lubricant carrier (mineral oil, synthetic basestock or water). A classic example of a boundary lubricity additive is an ester which could be available as a natural product (canola oil, lard oil, tall oil fatty acid, etc.) or as a functionalized molecule (monobasic ester, diester, polyol ester, complex ester). As the conditions under which metal-to- metal interactions become more severe due to higher temperatures and pressures, the lubricant becomes more stressed. The dis- 10 SEPTEMBER 2007 TRIBOLOGY & LUBRICATION TECHNOLOGY tance between the metal surfaces has decreased to the point where rubbing is occurring and welding becomes more than just a remote possibility. Traditional boundary lubrication additives do not remain on the metal surface and cannot prevent the increasing friction, wear and damage to the machinery seen under these conditions. Extreme pressure additives are required in order to enable the specific application operating under these conditions to continue. There are four main types of extreme pressure additives that are based on chlorinated, phosphorus, sulfur and overbased sulfonate species. The first three EP additive types are activated by reacting with the metal surface within a specific temperature range. In comparison with boundary lubrici- and chlorinated esters. ty additives that operate from ambient tem- In the case of phosphorus compounds, perature up to 200 C, chlorinated additives the main type employed is phosphate are activated at a temperature between 180 esters. But two other functionalities used C and 420 C. Phosphorus operates to a high- are phosphites and phosphonates. er temperature and sulfur at an even higher The following types of sulfurized additives range that ends at 1,000 C. are available: sulfurized greases, sulfurized Figure 1 shows the operating range for esters, sulfurized hydrocarbons and polysul- these three extreme pressure additive types. fides. Depending upon the processing, these In reacting with the metal surface, these additives are available as either dark-colored three additive types form iron chlorides, or light-colored species. Sulfurized additives iron phosphides and iron sulfides, respec- also are differentiated by the concentration tively. The metal salts produce a chemical of active (or free) sulfur present. Active sulfu- film that acts as a barrier to reduce friction, rized additives will stain copper alloys to The operating temperature wear, metal scoring and eliminates the pos- some extent. Formulators are urged to evalu- ranges are shown for chlo- 1 ate these additives in their products for sibility of welding. rinated, phosphorus-based staining by using the ASTM D-130 procedure. The fourth extreme pressure additive and sulfurized EP additives. Overbased calcium and sodium sul- type, overbased sulfonates, operates by a These ranges are compared different mechanism. This species contains fonates are used as EP additives. The total to the operating ranges for a colloidal carbonate salt dispersed within base number (TBN) for these additives traditional boundary lubri- the sulfonate. During the interaction with ranges from 300 to 400. city additives and polymer- iron, the colloidal carbonate will form a film CONTINUED ON PAGE 12 ic esters. that can act as a barrier between metal surfaces. This process is not dependent on tem- Figure 1. Activation Temperataures for Various EP Addititves perature as compared to those with chlorinated, sulfurized and phosphorus-based additives. It is reported that the carbonate film will form at temperatures below 500 C.2

Examples of EP additives Within each category of EP additive, there are several different types of additives. For example, the main chlorinated additive used is chlorinated paraffin, which is available at a number of different percentages of chlorine. Other commercial types include: Corp.) Lubrizol The (Courtesy of chlorinated olefins, chlorinated fatty acids

TRIBOLOGY & LUBRICATION TECHNOLOGY SEPTEMBER 2007 11 CONTINUED FROM PAGE 11 EP additive trends use dilution. But Kroto finds that water-sol- Representatives of the five EP additive man- uble EP additives are effective in severe ufacturers spoke to TLT to provide a feel for applications when formulated into semisyn- how their products are being used and why thetic metalworking formulations, particu- EP additives are so important to the lubri- larly in drawing and stamping. cant formulator. Fessenbecker adds that formulations Achim Fessenbecker, vice president, head based on sulfurized additives can be effec- of technology lubricant oil additives for tive in machining soft metal alloys (e.g. alu- Rhein Chemie Rheinau GmbH (a supplier of minum) that do not reach the activation sulfurized additives, phosphate esters, dithi- temperature. He says, “We have found that ophosphates, sulfur/nitrogen compounds, sulfurized additives are useful in the pro- overbased sulfonates and combinations of cessing of soft aluminum alloys. This can these additives) says, “EP additives can be occur both with neat oils and with water- used in all types of tribological situations to dilutable fluids.” minimize machine wear. Key applications STLE member Curtis Lege, business unit include gear oils, greases, manager, lubricant additives for Dover Figure 2. hydraulic fluids and metal- Chemical Corp. (a supplier of chlorinated working fluids.” additives, sulfurized additives, phosphate STLE member Frank esters & phosphites and overbased sul- Kroto, global technology fonates) says, “One important feature in manager for The Lubrizol working with EP additives is to utilize those Corp. (a supplier of sulfur- chemistries that contain a fatty group. The ized additives, phosphate fatty group provides lubricity and improves esters and overbased sul- the ability of the EP additive to remain fonates) says, “EP additive adsorbed on the metal surface during use. use is formulation and In the case of a chlorinated fatty derivative, application dependent. For chlorine can be released more readily from severe metal removal appli- the carbon atom next to the carbonyl group cations such as tapping, than in chlorinated paraffin. This leads to (Courtesy of Rhein Chemie Rheinau GmbH) reaming and broaching (see improved EP performance.” Figure 2) and metal forming STLE member Charles Aymes, business Broaching of gears is a severe operations, EP additives provide the needed development manager, industrial/metal metal removal operation performance features to minimize metal-to- treatment for Rhodia Inc. (a supplier of that requires the use of met- metal contact and improve tool life.” phosphate esters, phosphites and phospho- alworking fluids formulated Kroto indicated that EP additives may nates) says, “The chemistry of the EP addi- with EP additives. not be required in some cases where the tive can be adjusted to optimize perform- temperature at the point of machining does ance. Phosphate esters are prepared as a not rise sufficiently to activate a specific mixture of mono- and diesters (see Figure 3). additive. An example of where this comes The ratio of these two components can be into play is the use of water-soluble sulfur- adjusted to maximize performance depend-

ized additives. In less severe applications, CONTINUED ON PAGE 14 they would not be activated because of the high level of water Figure 3. The Chemistry of Phosphte Esters present at the (Courtesy of Rhodia Inc.)

The structure of phosphate esters can be tailored to meet the requirements of the end-use application. Phosphate esters are a mixture of mono- and diesters.

12 SEPTEMBER 2007 TRIBOLOGY & LUBRICATION TECHNOLOGY CONTINUED FROM PAGE 12

as trisulfides and pentasulfides, respectively. Figure 4. Combining EP Additives: Performances controlled by molecular Court says, “There is a difference in ther- architecture mal activity between trisulfides and pentasulfides as the temperature is increased from 60 C to 250 C (see Figure 4). An increase in temperature leads to the generation of more active sulfur. Trisulfides release active sulfur at a slower rate and at higher temperatures than pentasulfides. Pentasulfides, thus, are well suited to low-speed, high- torque metalworking operations where sulfur must be readily available. Trisulfides are best used in higher speed metalworking operations and also in applications such as automotive and industrial gear oils where (Courtesy of Arkema Inc.) the release of sulfur shall not be detrimen- tal to these systems. Sulfurized additives can be ing upon the application.” prepared with trisulfide Phosphonates can provide excellent per- Interactions with other additives and pentasulfide struc- formance under high-temperature condi- Kroto indicates that determining how to use tures.Thermal stability dif- tions because of their enhanced thermal EP additives in combination with other ferences between these stability. Aymes adds, “Phosphonates are additives is based on the machining opera- two types leads to their use also more hydrolytically stable because they tion, metallurgy, tooling, set-up and the in different applications. contain carbon-phosphorus bonds as com- desired performance. He says, “One of the pared to carbon-oxygen-phosphorus bonds most important aspects for the formulator present in phosphate esters.” to consider is the potential for premature Francois Court, global lubricant market tool failure before the operation reaches the manager for Arkema Inc. (a supplier of sul- proper temperature and the EP additive can furized additives and phosphate esters) be activated. This requires the need for a says, “The consistency of the EP additive, boundary lubricity additive that will work which is linked to the manufacturing synergistically with the EP additive.” process, is very important to ensure opti- Differences in the choice of boundary mized performance. Other factors to consid- lubricity additives require that formulators er are the chemical stability, thermal stabil- evaluate their chemistry and the application ity, solubility in basestocks and odor.” before making a selection. Kroto adds, The structure of a sulfurized additive has “Higher molecular weight polymerized an impact on its use in specific applications. esters display superior thermal and oxida- During the preparation of a sulfurized additive stability compared to other types of tive, the number of sulfur atoms that esters. This characteristic coupled with their become incorporated into the molecule can higher viscosity gradient enables polymeric be controlled. Typically, commercial addi- esters to maintain a film on the metal sur- tives are centered on either three or five sul- face at higher temperatures.” fur atoms that are known Another example of a synergistic combination was cited by Fessenbecker. He says, “We are always looking to find additive combinations that will boost lubricant performance. In laser-guided tool operations, we have found that formulating light-colored sulfurized additives with polymeric esters leads to superior performance without the need to clean the part.” Aymes indicates that it is difficult to select a specific additive because the formulator has an infinite number of choices. He adds, “In formulating lubricants, the chal- lenge is not just to find synergistic combi- 13 SEPTEMBER 2007 TRIBOLOGY & LUBRICATION TECHNOLOGY nations but also to minimize unfavorable interactions that can result in product insta- bility and foam generation. The best approach is to utilize multifunctional additives to simplify formulations.” Lege stressed that the combination of chlorinated and sulfurized additives is very helpful to the formulator, especially in heavy-duty applications. He says, “It is very useful particularly to use both chlorinated and sulfurized additives in heavy- duty drawing operations.” Court indicated that synergisms also can be found with other combinations of EP additives. He says, “We found experimental- ly that synergisms can be obtained between sulfurized additives, phosphorus-based components and overbased sulfonates. Of course, the synergy between sulfurized and EP additives in their products. chlorinated additives also is well document- Court says, “The tough competitive na- ed.” The use of specific additive combina- ture of lubricant applications requires end- tions is dependent upon the application. users to push product performance beyond the limit. The lubricant user will save money Why are EP additives so important? by extending tool life, reducing energy costs In response to the STLE survey, the five rep- and operating its machinery at higher resentatives were asked about why the speeds. All of these technical limits are lubricant industry sees the importance of CONTINUED ON PAGE 16

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TRIBOLOGY & LUBRICATION TECHNOLOGY SEPTEMBER 2007 14 CONTINUED FROM PAGE 15 on gearboxes require the use of EP addi- Figure 5. Non-Chlorinated EP Additive tives in gear oils to ensure long operating intervals.” Kroto summarizes the importance of EP additives: “There are some operations in which EP additives absolutely must be used in order to get the job done.” EP additive trends During at least the past 20 years, there have been many discussions and some regulations enacted to limit the use of chlorinated paraffins. The industry representatives contacted for this article believe in general that the use of chlorinated paraffins will drop, but there is no clear indication about how (Courtesy of Dover Chemical Corp.) (Courtesy of Dover rapid the rate of decline will be. Lege says, “We see that chlorinated In the drilling of 316 stain- ‘extended’ by adding EP additives. EP addi- paraffin use is a little bit at risk. Types such less steel, neat oils formu- tives are required to enable the lubricant to as medium chain and long chain chlorinat- lated with the non-chlori- operate beyond its designed limits.” ed paraffins have been around for a long nated EP additive and Lege adds, “EP additives enable lubri- time and will continue to be used at current chlorinated paraffin dis- cants to exhibit superior load bearing prop- levels for at least the next five years.” played comparable results. erties which leads to extended tool life in Fessenbecker also sees chlorinated Slightly inferior results metalworking operations and gear life in paraffin use in decline and believes there is were found with a neat oil industrial machinery.” Aymes indicates that formulated with a phos- a technical concern. He says, “We see that the importance of EP additives is due in part phate ester. the use of chlorinated paraffin-based metal- to the restricted number of choices avail- working fluids in high-speed machining able to the formulator. “Formulators may be operations is leading to shorter activation willing to sacrifice other lubricant proper- times. This can produce higher quantities of ties such as foaming, but EP is essential hydrogen chloride which will produce corro- because it is directly responsible for ensur- sion problems and reduce tool life.” ing that a specific metalworking operation Kroto sees different regulatory standards will succeed,” he says. governing the use of chlorinated paraffins Fessenbecker says, “Additives formulated worldwide. He says, “Determination of chlo- into lubricants provide different properties rinated paraffin use is made on a country- to enable machinery to function at an opti- by-country basis. Formulators have to mum performance level. While other addi- decide how to use chlorinated paraffins to tives are used to protect the specific lubri- position their product lines from both a cant, EP additives are needed to protect the regional and worldwide perspective.” machinery doing the specific operation. This One driver that will impact the use of all may include the cutting tool, machine tool additives in lubricants is the new regulation and workpiece in metalworking. The system known as REACH that was estab- stronger demands lished in the European Union (EU) in December 2006. The EU considers the most widely used chlorinated paraffin type, known as medium chain chlorinated paraffins, to be toxic to marine life. Appropriate risk phrases (R50/R53) are included on manufacturers’ MSDS. This places chlorinated paraffins in the category of “chemical substances of high concern.” In the future, EU authorities may prompt manufacturers of chlorinated paraffins to commercialize safer alternatives. There is a great deal of uncertainty about 15 SEPTEMBER 2007 TRIBOLOGY & LUBRICATION TECHNOLOGY REACH at present. Further information can be found in a feature article in this issue of TLT.3 All of the EP manufacturers contacted for this article expressed the commitment to register their products in REACH. Research is continuing to learn more about how EP additives are used and to develop new technologies. Lege says, “We have developed a new product that we call a non-chlorinated EP additive. Evaluation of this technology shows good performance on very hard alloys such as stainless steel and hastelloy. We have also observed good results on titanium, which can be very difficult to machine and cannot tolerate chlorinated additives due to concerns about stress cracking.” The non-chlorinated EP additive does not contain any chlorinated, sulfurized or phos- rior results were observed with a fluid con- Editor’s Note: See page phate-based components. Lege indicated taining the same sulfurized additive and a 30 for TLT’s in-depth that this new additive can work in a syner- phosphate ester (see Figure 5). feature article on the EU’s gistic fashion with other EP additives. In a Aymes indicates that Rhodia is looking to REACH regulations. recent study, comparable results were found develop new phosphate ester technologies when using neat oils based on chlorinated that generate lower levels of foam. He adds, paraffin and the non-chlorinated EP additive “We are also looking at customizing our to drill 316 stainless steel. Both fluids also phosphonates to provide good EP proper- contained a sulfurized additive. Slightly infe- CONTINUED ON PAGE 18

TRIBOLOGY & LUBRICATION TECHNOLOGY SEPTEMBER 2007 16 CONTINUED FROM PAGE 17 ties and stain-free surfaces.” Court believes there will be a move Fessenbecker says research efforts are toward more efficient metalworking fluids in directed toward more environmentally part because of regulatory drivers such as friendly EP additives. He says, “We are look- REACH. He adds, “This will prompt further ing to develop a portfolio of biodegradable interest in the use of high-performing addi- products. Other areas of emphasis include tives because of the need to reduce waste. We boosting the solubility of EP additives in see sulfurized technology as a key contributor the latest generation of base oils (Group II, (or cornerstone) to this evolution, together Group III and Group IV) and improving their with the development of dry lubrication.” thermal stability.” Aymes agrees that more efficient lubri- Kroto indicates that Lubrizol will continue cant and EP additives will be required in the to broaden its product line. He says, “We future. He says, “Incorporation of more than added sulfurized vegetable oils one EP additive chemistry on a molecule two years ago, and we are look- (hybrid) and the use of gemini surfactant ‘EP additives can be used in all types ing to develop new products technology could be promising in the devel- of tribological situations to minimize with better aesthetics (color opment of new products. But EP additive machine wear. Key applications and odor) in combination with use could be reduced with the greater use of include gear oils, greases, hydraulic superior performance.” wear-resistant coatings on cutting tools.” fluids and metalworking fluids.’ Court says, “We have re- Lege says, “It is tough to determine how cently developed a water-solu- EP additive technology will evolve in the ble sulfurized additive that can future. There are too many variables involved be used in semisynthetic and synthetic met- such as workpiece alloys, cutting tool tech- alworking fluids to boost performance in nologies (coatings) and wastewater treat- specific applications. In the future, additives ment. EP additive manufacturers must quick- that will provide flexibility to the formulator ly react to these changes in order to have will be desirable.” products available to meet these needs.” All industry representatives interviewed Kroto believes that EP additive suppliers feel that regulatory drivers will have a major will be challenged to develop new products impact on the future use of EP additives. But that can be used to machine a wide variety making further predictions about changes in of alloys. He says, “Formulators are faced the offerings of EP additive technologies with developing products for more difficult- either in the near or long term is difficult. to-machine alloys. This will place a premi- Fessenbecker says, “The lubricant indus- um on using the right types of EP additives try is very conservative, and we do not see in the metalworking fluid formulation to any major changes over the long term. There machine parts that meet increasingly tight will be a steady evolution in which EP addi- specifications.” << tive technology will be adjusted. Solid EP Neil Canter heads his own consulting company, additive use will increase, and this will Chemical Solutions, in Willow Grove, Pa. Ideas for include finely dispersed nanoparticles that Tech Beat items can be sent to him at neilcanter are being developed in the relatively new @comcast.net. field of nanotechnology. There also will be a greater need for EP additives with better References thermal stability.” 1. Canter, N. (1997), “Additives for Metal- working Fluids,” Tribology Data Handbook Edit- ed by E. Richard Booser, pp. 862–871.

2. Riga, A., Hong, H., Kornbrekke, R., Cahoon, J. and Vinci, J. (1993), “Reactions of Overbased Sulfonates and Sulfurized Com- pounds with Ferric Oxide,” Lubrication Engi- neering, 49 (1), pp. 65–71.

3. Canter, N. (2007), “REACH: The Time to Act is Now!” Tribology and Lubrication Technolo- gy, 63 (9), pp. 30-39.

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