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 applica- tion 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 cour- tesy 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 detrimen- tal 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 un- dergo 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 in- soluble 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 syn- ergism or antagonism with other addi- tives in the oil. Actual treat rates have Lubricant Lubricant Film Film to be based on specifications and per- formance 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 tempera- tures. 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 oxida- tion occurs in hydrocarbons, poly- meric products of oligomerization tend to agglomerate and form soluble particles in the oil. Dispersants’ polar heads attach to these soluble