The Chemistry and Function of Lubricant Additives

WEBINARS Debbie Sniderman / Contributing Editor The chemistry and function of lubricant additives

Their effectiveness depends on the base oil and how they interact with other chemicals.

KEY CONCEPTS

• AdditivesAdditives comppensateensate fforor thethe deﬁciencies inherently ppresentesent inin basib c basebase oiloil ssystems.ystems.

• AdditivAdditivee effectiveness deppends on the ooil.il. AnAn additive thathatt works welwelll in one oiloil mighmight notnot inin LUBRICANTS ARE USED TO REDUCE FRICTION AND WEAR, dissipate heat aanothenother. from critical parts of equipment, remove and suspend deposits that may affect performance and protect metal surface damage from deg- • It’sIt’s importantimportant to understaunderstandd radation and corrosion. They serve a diverse range of applications, hhowo addaadditives t es maymay interactinteract each requiring a different combination of base oils and additives. wiwithth each ototheher in With such a complex industry, there are many challenges to devel- oping effective lubricants. synerggisticistic oror antaantaggonisticnistic Base oils themselves perform most of the functions of lubricants. ways whene creatingcreating But they can only do part of the job. Additives are needed when a additive packagges.es. lubricant’s base oil doesn’t provide all the properties the application requires. They’re used to improve the good properties of the

18 Snakes have two sets of eyes—one set to see and the other to detect heat and movement. No eyelids for snakes, just a thin membrane covering the eye. MEET THE PRESENTER

This article is based on a Webinar originally presented by STLE Education on June 17, 2015. The Chemistry and Function of Lubricant Additives is available at www.stle.org: $39 to STLE members, $59 for all others. Vincent Gatto has worked for 30 years as a research, applications, technical service and product de- velopment scientist in the area of lubricant, fuel and polymer additives. He is currently research director for Vanderbilt Chemicals, LLC, in Norwalk, Conn., working to discover and develop new chemicals for use as additives in the lubricant, rubber and plastics industries. He also directs and prioritizes projects in the Petroleum Applications Laboratory. Gatto holds approximately 110 U.S. patents, patent applications and technical publications in the areas of chelation, additives for polymers, lubricants and fuels. He was R&D and technical service manager at Albemarle Corp. for 11 years and a lubricant additive scientist at Ethyl Corp. (now Afton Chemical Co.) for 16 years. He received his doctorate in chemistry from the University of Maryland and held a postdoctoral position at the University of Miami. You can reach Dr. Gatto at [email protected]. Vincent Gatto

base oils and minimize the bad. They diesel engines, which are the most de- compensate for the deficiencies inher- manding and require the most additives EXAMPLES OF ENGINE AND ently present in basic base oil systems (see Figure 1). NON-ENGINE LUBRICANTS (see Examples of Engine and Non-Engine Additives are combined into well- Lubricants). balanced, optimized packages allow- Engine Lubricants Typical lubricants are composed ing lubricants to meet specified perfor- Gasoline engine oils of a base oil, an additive package and, mance criteria in a finished fluid. One Diesel engine oils optionally, a viscosity index (VI) im- example of an additive package is the • Heavy duty prover. Turbine, hydraulic and indus- Dispersant Inhibitor (DI) package used • Off-road trial gears demand much lower treat for engine oils, containing mainly dis- • Automotive rates of additive packages compared to persants, detergents, oxidation inhibi- • Stationary automotive gear, ATF and gasoline and tors, antiwear agents and friction modi- Medium-speed engine oils in: • Railroad • Marine Natural gas engine oils 3-9% Aviation engine oils Gasoline & 10-15% Two-stroke cycle engine oils Diesel Engine 80-88% 3-6% 3-6% Automatic Non-Engine Lubricants 88-94% Transmission Fluid 7% Transmission fluids • Automatic Automotive Gear 93% 2% • Manual Power steering fluids Industrial Gear 98% 1% Shock absorber fluids Gear oils Hydraulic 99% 1% Base oil • Automotive VI Improver • Industrial Additive Package Turbine 99% Hydraulic fluids Metalworking fluids Turbine oils Misc. industrial oils Figure 1 | Composition of base oil, VI improver and additive packages in typical lubricants. Greases (Figure courtesy of Vanderbilt Chemicals, LLC.)

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY NOVEMBER 2017 • 19 ﬁers. Additives may interact with each other and may have multi-functional • Additives are combined to produce well balanced/optimized additive properties. Each additive package is a packages to meet certain performance criteria unique blend whose combination pro- e.g. Dispersant Inhibitor (DI) Packages for Engine Oils duces a complex chemistry with positive Miscellaneous and negative interactions (see Figure 2). e.g. corrosion & Dispersants rust inhibitors 55% THE NEED FOR FRICTION MODIFIER < 2% ADDITIVES

One of the key functions of lubrication Friction is to reduce friction and wear when Modifiers 4% Detergents moving metal parts are in contact. The 20% Anti-wear Agent Oxidation Stribeck curve illustrates how friction 8% Inhibitors reduction takes place in a lubricant. In 12% the boundary regime at low speeds, low • Additives may interact with each other showing synergism or antagonism viscosity or high loads, a lubricant has • Some additives may have multi-functionality high friction due to metal/metal contact in the presence of a very thin film of oil. As the speed or viscosity increases, or load decreases, the system moves Figure 2 | Example of Dispersant Inhibitor (DI) additive package for engine oils. (Figure courtesy of Vanderbilt Chemicals, LLC.) into the mixed regime where friction is significantly reduced in the lubricant mainly due to a thicker oil film and greater separation between the metal surfaces (see Figure 3). Both the boundary and mixed-film lubrication conditions can be detrimental to machinery. Without additives, this contact will lead to significant system wear and premature field fail- ure. Properly selected friction modifier additives can significantly reduce the coefficient of friction in these regimes. Low friction can best manifest itself as an improvement in an engine’s fuel ef- ficiency (see Figure 4).

THE NEED FOR ANTIOXIDANT ADDITIVES Ionic Liquids - New Aspects for the Future, J.-I. Kadokawa ed., CC by 3.0, Chapter 5, Tribological When exposed to heat, oxygen and Properties of Ionic Liquids, Y Kondo, T. Koyama, S. Sasaki, Jan. 23, 2013 metal contamination, lubricants undergo oxidation. Base oils can perform Figure 3 | The Stribeck curve illustrates lubricant friction regimes. (Figure courtesy of Vander- many lubrication functions, but they bilt Chemicals, LLC.) can’t protect against the detrimental effects of oil oxidation. Oxidation creates polymers, oil insoluble high-molecular-weight mol- it remains too long, sludge can plug oil in wear (see Figure 5). ecules that increase the viscosity of the lines and ﬁlters leading to oil starvation There is good news however. Even lubricant, accelerate wear and eventually in the machinery and adhere to metal though lubricant degradation processes leads to varnish. The hard “baked on” surfaces, accelerating wear. Along with are complex, the solution is straightfor- polymeric coating of varnish also accel- other hard accumulations of sludge and ward. Antioxidant additives minimize erates wear and is a long-term detriment. varnish typically on pistons and valves oxidation and deposit formation. De- Oxidation also creates sludge, a soft in engines, deposits due to oxidation tergents and dispersants also play a role oil-insoluble material made of combus- can cause valve and ring sticking, even- in mitigating the effects of sludge and tion byproducts that suspend in oil. If tually leading to catastrophic increase deposits.

20 • NOVEMBER 2017 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG a threshold dosage before becoming active. The dosage is affected by synergism or antagonism with other additives in the oil. Actual treat rates have Lubricant Lubricant Film Film to be based on speciﬁcations and performance requirements. There are two types of additives: FLUID-FILM LUBRICATION BOUNDARY LUBRICATION Film thickness (H) approaches zero Elastohydrodynamic regime: H > R bulk oil additives and surface additives. Hydrodynamic regime: H >>> R Bulk additives affect the rheological, interfacial and chemical properties of the lubricant, and surface additives are Lubricant active at metal surfaces (see Figure 6 on Film Page 22).

MIXED-FILM LUBRICATION DISPERSANTS Film thickness (H) ~ equal to Surface roughness (R) Dispersants are non-metallic, ashless cleaning agents that inhibit sludge from forming at low to moderate temperatures. Dispersants make up between 4%- Figure 4 | Friction modiﬁer additives help reduce friction in boundary and mixed-lubrication 13% of fully formulated oils, and most regimes. (Figure courtesy of Vanderbilt Chemicals, LLC.) are diluted with 50%-75% by weight in mineral oil to lower their viscosity and improve handling properties. Dispersant molecules have a hy- drophilic polar head that attaches to agglomerates and a long-chain hydrophobic hydrocarbon tail that functions as a solubilizer. As oxidation occurs in hydrocarbons, polymeric products of oligomerization tend to agglomerate and form soluble particles in the oil. Dispersants’ polar heads attach to these soluble particles and their tails face into the oil, sus- pending the surrounded particle in the oil, making it look more oil like and preventing it from plugging ﬁlters and screens. In this way, they arrest growth and control agglomeration so particles don’t fall out of the oil and become insoluble. Typical structures are N-substituted Figure 5 | Mechanisms of lubricant degradation motivating the use of antioxidant additives. (Figure courtesy of Vanderbilt Chemicals, LLC.) long-chain alkenyl succinimides, suc- cinate esters, phosphorus dispersants and mannich bases. The many types of dispersant molecules all have polar ADDITIVE USE AND TYPES gives the opportunity to use less overall functionality in the head group and oil An additive’s effectiveness depends on additives. But antagonistic effects may solubilizing tails of either polyisobutyl- where it’s used. For example, an addi- prevent the use of certain types, and ene or polypropylene. The polar head tive may work well in one oil and not in more may be needed to overcome the group usually contains oxygen, nitro- another. Polar additives compete with antagonistic effect. gen, phosphorus or sulfur atoms to aid each other for metal surfaces. Many The dosage or treat rate also de- in dispersion. additives exhibit either synergistic or pends on the system. Usually effective- The main polar head group can con- antagonistic effects with other addi- ness diminishes as dosage increases. tain polyamine or polyol. The linking tives present in the system. Synergism But some additives may need to reach group (succinyl, phenol, phosphate or

The four-eyed fish has its eyes raised above the top of its head so it can see both above and below water simultaneously. 21 none) attaches the head to the tail and is an artifact from how the dispersant is made. Caps are a term for a final group of chemistries added to alter overall Additive Package properties of dispersants. Different types can offer supplemental corrosion and wear properties, act as antioxidants Bulk Oil Additives Surface Additives or improve seal compatibility. Most dispersants contain amine functions that are aggressive to Viton® seals. Glycolic Interfacial Rheological Chemical EP/Anti-Wear acid, boric acid, molybdenum oxide and formaldehyde are frequently added caps to improve compatibility and Corrosion/Rust Inhibitor properties. Foam inhibitor VII Antioxidant Demulsifier PPD Detergent Friction Modifier DETERGENTS Dispersant Detergents are cleaning agents that contain metals. They work at high temperatures to reduce or remove de- Figure 6 | Lubricant additive types. (Figure courtesy of Vanderbilt Chemicals, LLC.) posits on surfaces and also in the bulk of the oil. They also neutralize acids and provide rust protection. Between 1% and 2% by weight are used in fully ample of a highly overbased detergent, evated temperatures, some hydrogen formulated oils, and like dispersants, with calcium carbonate molecules in peroxides are unstable and detrimental. most also are diluted with mineral oil the structure’s core available to neutral- Secondary antioxidants are peroxide by approximately 50% by weight to im- ize acids. In the non-overbased portion decomposers that consume unstable prove handling. of the same molecule, neutral calcium hydrogen peroxides and form stable They have a similar structure to dis- sulfonate molecules provide the surface alcohols. persants, with a polar head and a long, activity to prohibit gum buildup. The two main primary antioxidants non-polar hydrophobic tail, but deter- Neutral detergents most commonly are hindered phenolics and aminics, gents contain a metal salt on an acidic used in lubricants have structures in- and the application determines which organic molecule in the polar head. volving neutral calcium. Salicylates to use. Di-tert-butylphenol (DTBP) and The other main difference between are extensively used in engine oils. butylated hydroxytoluene (BHT), two the two is where they function—dis- Naphthalene sulfonates are used more hindered phenolics, are inexpensive but persants function in the bulk of the oil in industrial fluids, grease and metal- are too volatile for certain applications. and detergents clean at the surface of working fluids. Phenates, sulfonates Methylenebis(2,6-di-tert-butylphenol) metal parts. and sulfurized phenates also are used. (MBDTBP) (a hindered phenolic) and In hydrocarbons, insoluble gum alkylated phenyl-alpha-naphthylamine particles fall out of the oil. Since they ANTIOXIDANTS (APANA) (an aminic) are solids and are polar, they are attracted and stick Antioxidants inhibit oil oxidation and can be difficult to handle in blending to polar metal surfaces. Effective de- are the primary defense against lubri- operations. Iso-octyl-3,5-di-tert-butyl- tergents act as surfactants when their cant degradation. They prevent vis- 4-hydroxy-hydrocinnamate (BHC) and polar ends stick on polar surfaces and cosity increase and suppress deposit, alkylated diphenylamine (ADPA) are produce a film that displaces gum sludge and varnish formation. They are the antioxidants most commonly used deposits from the metal surface. The usually low molecular weight liquids, because they are fully liquid materials cleaning effect depends on the oil and solids or oil dilutions and are used be- with very low volatility and excellent metal surface temperatures and how tween 0.1% and 2% by weight in fully performance properties. quickly gum particles bake on the formulated oils. Secondary antioxidants either con- metal surface. Antioxidants contain primary and tain sulfur or phosphorus. Selection is Depending on the application, over- secondary types working synergisti- based on thermal stability levels and based and neutral detergents are used. cally to suppress lubricant degradation whether sulfur is to function as an anti- Overbasing gives a detergent a way to when combined. Primary antioxidants oxidant, a corrosion inhibitor or a cor- neutralize acids formed during com- are radical scavengers that turn reac- rosion promoter. Sulfurized alkylphe- bustion or lubricant oxidation. A calci- tive peroxy radicals into more stable nols contain both sulfur, which acts as um sulfonate micelle structure is an ex- hydrogen peroxide. However, at el- a secondary antioxidant, and phenolic, ÎÎÎÎÎÎÎ

22 Owls cannot move their eyeballs, so they have evolved a special blood-pooling system that allows them to rotate their heads 270 degrees. ÎÎÎÎÎÎÎ which acts as a primary antioxidant providing multi-functional behavior. Dithiocarbamates, sulfurized oleﬁns and phosphites are examples of secondary antioxidants.

SURFACE ACTIVE ADDITIVES Surface active additives reduce friction and wear at the interface of rub- bing surfaces. Friction modiﬁers (FM), antiwear (AW) and extreme pressure (EP) additives all form adsorbed lay- ers of surface ﬁlms that prevent surface asperity contact, reducing fatigue and corrosive wear each in different ways. Generally FM additives function by

reversible or pseudo-reversible adsorp- Lubricants 2013, 1, 132-148; doi:10.3390/lubricants1040132, www.mdpi.com/2075-4442/1/4/132/pdf tion processes. AW and EP additives chemically alter metal surfaces, forming strong, glass-like elastic materials Figure 7 | Mechanism of ZDDP wear protection. (Figure courtesy of Vanderbilt Chemicals, LLC.) that perform AW or EP functions.

ANTIWEAR ADDITIVES AW additives are effective at moderate loads and temperatures. They are used between 0.25% and 3% by weight in fully formulated oil. Depending on the amounts of oxygen present, the applied load and oil temperature, they form durable films mainly composed of sulfate, sulfide and polyphosphates of zinc, iron or other cation species. These films are replenished as long as the additive is present. Most AW additives are phosphates, and many are supplemented with sulfur or molydenum-containing compounds or other metals to alter performance. Tricresyl phosphates (TCP), amine phosphates, molybdenum dithiocarbamates (MoDTC) and molybdate esters are typical AW additives, but zinc dialkyl dithiophosphate (ZDDP) is the most widely used because of its superior performance in many applications. There are two types of ZDDP, primary formed from primary alcohols that have greater thermal stability and less volatile phosphorus, which benefits engine oil applications but less effective antiwear Figure 8 | Yellow lines show circulation path of oil in a typical internal combustion engine to action. Secondary ZDDP comes from lubricate critical components. The oil pump passes oil through a screen sump strainer and secondary alcohols and have more effec- filter for coarse and fine cleaning, then through passages in the engine block and back to the tive antiwear action but lower thermal sump where the cycle repeats. Bypass valves ensure oil will reach the engine in case of block- stability and more volatile phosphorus, age and ensures continued lubrication. Lubrication also is accomplished by splashing caused which is detrimental in engine oils. by rotation of the crankshaft. (Figure courtesy of Machinery Lubrication magazine.)

24 • NOVEMBER 2017 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG formulated oil and are typically used at higher levels in modern engines Activation Temperature because they improve fuel economy. Their effectiveness varies greatly with engine design and type of lubricant for- EP1 EP2 mulation used. Surface active FMs are organic friction modifiers whose polar heads contain oxygen, nitrogen or boron. Many T1 T2 are derivatives of oleic or laurel acids. Reaction Rate Chemically reactive FMs are typi- 50100 150 200 cally molybdate esters, molybdenum Temperature, °C dithiocarbamates or molybdenum dithiophosphates. Metallic FMs are mo- Highly reactive EP additives/high concentrations may lybdenum based and are well known as cause corrosive wear one of the most effective in the industry. FM lubricants also are available in solid molybdenum disulfide, graphite and PTFE types. Figure 9 | Activation temperature in EP additives. (Figure courtesy of Vanderbilt Chemicals, LLC.) VISCOSITY INDEX IMPROVERS VI improvers minimize relative viscosity changes with temperature, increas- The complex zinc and iron-based tions. Those containing amines are less ing it at high temperatures and reduc- polyphosphates and glassy films that compatible with elastomer seals. ing it at low. They show solubility and form on metal surfaces from ZDDP conformational changes with changes increases modulus under high loads EP ADDITIVES in temperature and extend the operat- and temperatures allowing the film EP additives are used under highly ing range of oils to higher temperatures to maintain integrity and have ‘smart’ loaded conditions to prevent weld- without adversely affecting their low- behavior. The film becomes stronger ing. There are two types, sulfur based temperature fluidity. These oil-soluble when load increases, like during engine (DMTD derivatives, sulfurized olefins, organic polymers with high molecular startup, to enhance the antiwear ben- sulfurized fats and oils and dithiocarba- weights (10,000-1,000,000) are used efits when they are needed most. For mates) and molybdenum based (dithio- to produce multi-grade engine oils and this reason ZDDP’s effectiveness at pro- carbamates, dithiophosphates). automatic transmission fluids. Com- tecting machinery of all types against Typical EP additives contain sulfur mon treating doses range from 3%-10% wear is superior to most other options compounds, molybdenum derivatives by weight. (see Figure 7). and chlorine compounds. The key fea- These additives work by thicken- Even though ZDDP is the work- ture between the different types is the ing. At low temperatures their polymer horse and industry standard for trans- activation temperature. chains are compressed and occupy a mission and internal combustion en- Selection depends on the reactivity low-molecular volume. They behave gines, limits on its use in engine oils in an application. Using a highly reac- like low-molecular weight molecules in has been mandated because of concerns tive substance may result in corrosion, an oil system, having the effect to lower around phosphorus as a catalyst poison so it is important to balance corrosion viscosity. As the temperature increases, for emission control systems, and zinc, and high concentrations against maxi- the chains expand and incorporate oil which contributes to ash contents and mizing the EP additive’s effect. Chlori- molecules to give a higher volume and has detrimental effects on diesel engine nated structures are the least reactive, higher apparent molecular weight, diagnostic systems (see Figure 8). followed by phosphorous compounds. which has the effect of increasing the For this reason, there is growing Sulfur compounds have the highest re- oil’s viscosity. interest in ashless AW additives. Two activity (see Figure 9). When high molecular weight poly- types are thiophosphate based, which mers undergo shearing conditions, they contain phosphorus and sulfur, and FRICTION MODIFIERS decompose to low-molecular weight phosphate based, which contain only FM additives form slippery physical fragments. They can show tempo- phosphorus. Selection depends on the or chemical boundaries between metal rary viscosity loss at moderate-shear application. Those with higher thermal surfaces and oil to reduce friction. They stresses and permanent viscosity loss stability are used in aviation applica- are used up to 1% by weight in fully at high-shear stresses. When choos-

Owls’ pupils can’t dilate. To reduce light they lower their eyelids halfway, giving them their famous sleepy look. 25 ing VI improvers, it’s important to take shear stability into account as well as viscosity-controlling performance. Problem: Solution:

Oil H O Oil O H2O O2 2 2 O CORROSION AND RUST INHIBITORS O2 2 Corrosion inhibitors react with metal and Cathode form protective coatings on their surfaces Anode to protect from harmful effects of water, e- e- polar impurities and oxygen. They are Iron Iron primarily used for copper and iron sur- Rust Formation Protective Rust faces, and are used between 0.01% and Inhibitor Layer 0.3% by weight in formulated oils. Cathodic Reaction Corrosion is an electrochemical - - ½ O2 + H2O + 2 e 2 OH reaction between the metal surface functioning as anode and the interface Anodic Reaction 2+ - between the metal surface and the lu- Fe Fe + 2 e bricant as the cathode. When a metal surface is in contact with oxygen and water, cathodic and anodic reactions take place, releasing metal into the oil. Figure 10 | Mechanism of corrosion and rust inhibitors. (Figure courtesy of Vanderbilt Chemicals, Corrosion inhibitors prevent this LLC.) process from taking place by chemically insulating the metallic anode from the cathode. When they are introduced into a system, they coat the metallic PDSC Bulk Oil Oxidation surfaces due to their high affinity to (OIT @ 175oC) (% Vis. Inc. after 72h @ 160oC) the metal and form metal deactivating 120 600 Synergistic films. These separate the metal surfaces effect 100 500 from oxygen and water and shut off the Predicted reactions, preventing corrosion. Both AOX Predicted 80 400 Both AOX Many types exist: those that under- 60 300 go physical adsorption (fatty amines Synergistic effect and acid derivatives), chemical reac- 40 200 tions (phosphorus and sulfur compounds), acid neutralization (some 20 100 dispersants and detergents) and metal 0 0 chelation (N-salicylidene-propylene- No AOX 0.1% 0.27% Both AOX No AOX 0.1% 0.27% Both AOX diamine used mostly in fuels). To- ADPA Mo Ester ADPA Mo Ester lutriazoles, thiadiazoles, triazoles and US Patent 5,650,381 mercaptobenzothiazoles are used, with varying nitrogen and sulfur materials. Some are more effective depending on Figure 11 | Example of antioxidant synergism—observed effects of alkylated diphenylamine (ADPA) and molybdate ester (mo ester). (Figure courtesy of Vanderbilt Chemicals, LLC.) the molecular weight and their ability to get close to a metal surface and form a strong film on the surface. The main difference between rust additives (FM, AW, detergents and EP) phosphates offer wear and corrosion inhibitors and corrosion inhibitors is for the same metal surfaces, an exam- protection. Molybdenum compounds where they function. Corrosion inhibi- ple of an antagonistic combination (see give effective wear, EP and oxidation tors work to protect copper surfaces, Figure 10). protection plus powerful friction re- and rust inhibitors protect iron surfac- duction. Sulfur compounds provide es. They both have polar head groups MULTI-FUNCTIONAL ADDITIVES wear, EP, corrosion and oxidation pro- that attach to the metal surfaces and AND INTERACTIONS tection. Diphenylamine derivatives of hydrocarbon tails that form a film to Many additives perform more than one tolutriazole provide oxidation and cor- protect metal surfaces against chemical function, making the need for other rosion protection. And ZDDP is an AW attack. They compete with other polar types of additives much lower. Amine additive that provides very good oxida- ÎÎÎÎÎÎÎ

26 A dragonﬂy has 30,000 lenses in its eyes to assist with motion detection and make them difﬁcult for predators to kill. ÎÎÎÎÎÎÎ Additive PCMO HDDEO Aviation Natural Gas 2-cycle

Dispersants Detergents

Antiwear/EP

Antioxidants

Corrosion/Rust Inhibitor Friction Modifier

PPD

Anti Foam

VII

Others

Others includes coupler, dye and emulsifiers

Figure 12 | Additives used in engine oil formulations. (Figure courtesy of Vanderbilt Chemicals, LLC.)

tion and corrosion protection, requir- and EP additives contribute to corro- equipment and severity of the ap- ing much less corrosion inhibitors and sion under certain conditions and are plication’s temperature, load and antioxidants in a system. incompatible with nitrile seals. Aminic exposure to moisture, atmosphere When additives interact, they can antioxidants promote deposits, sludge and combustion products can be a either undergo synergism, where the and varnish. guide toward specific chemicals to total effect is greater than the sum of Under certain conditions, ZDDP solve specific problems. Drain inter- the effects independently, or antago- promotes deposit formation at elevat- val and desired lubricant life matters. nism, where the observed total effect ed temperatures (e.g., TEOSTTM 33C). Lubricant going through frequent oil is less than the sum of measuring each It also promotes wear under certain changes would not require as much additive individually. conditions (e.g., timing chains). And additive as extended life or fill-for-life Examples of synergistic combina- the phosphorus from blow-by poi- applications. tions involve amine phosphates and sons automotive catalysts. In the early Some additives have a very high sulfurized EP, amine antioxidants and 2000s, the level of ZDDP was reduced solubility in oil, and some of their organo-molybdenum compounds, and in gasoline engine oils. But it is one of degradation products may have low amine and phenolic antioxidants. An- the most effective and inexpensive AW solubility, which may impact additive tagonistic combinations occur when agents available, so in recent years the selection. The chemistry of the base oil, combining AW and corrosion inhibi- industry has been reluctant to drop it whether it is mineral or synthetic, and tors, friction modifiers and AW and further, fearing that using alternatives the type and degree of refining it under- dispersants and AW (see Figure 11 on could lead to warranty problems long goes also can impact selection. Page 26). term. Discussions on both sides will Finished oils must meet specified Additives exhibiting detrimental continue because it is critical in the properties and performance character- properties must be understood and automotive segment. istics outlined by OEMs and trade and controlled. A proper balance of addi- standards organizations. Environmen- tives in a finished lubricant formulation SELECTING ADDITIVES tal and toxicological restrictions may is critical for minimizing the harmful Whether additives are included as part impact whether an additive is includ- effects observed with many additives. of a fully formulated additive package ed in a formulation that may have a Molybdenum compounds can be or by top treating existing packages, global reach. Material cost is a critical corrosive to copper and lead at high there are many factors affecting addi- factor. levels or in the wrong application. tive selection. The type of oil produced Amine-based dispersants and antiwear determines the types of additives used Debbie Sniderman is an engineer and CEO of VI additives can be incompatible with (see Figures 12 and 13). Ventures, LLC, an engineering consulting company. Viton® seals. Sulfurized antioxidants Geometry and metallurgy of the You can reach her at [email protected].

28 • NOVEMBER 2017 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG