Lubricant Additives: Chemistry and Applications the Bulk Lubricant
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Copyright Taylor & Francis Group. Do Not Distribute. 3 Dispersants Syed Q.A. Rizvi CONTENTS 3.1 Introduction ...................................................................................................................................................................... 45 3.2 Nature of Deposits and Mode of Their Formation ........................................................................................................... 46 3.3 Deposit Control by Dispersants ........................................................................................................................................ 48 3.4 Desirable Dispersant Properties ....................................................................................................................................... 48 3.5 Dispersant Structure ......................................................................................................................................................... 48 3.6 Dispersant Synthesis ......................................................................................................................................................... 49 3.6.1 The Hydrocarbon Group ....................................................................................................................................... 49 3.6.2 The Connecting Group ......................................................................................................................................... 50 3.6.3 The Polar Moiety .................................................................................................................................................. 51 3.7 Dispersant Properties ........................................................................................................................................................ 57 3.7.1 Dispersancy .......................................................................................................................................................... 57 3.7.2 Thermal and Oxidative Stability .......................................................................................................................... 58 3.7.3 Viscosity Characteristics ...................................................................................................................................... 59 3.7.4 Seal Performance .................................................................................................................................................. 60 3.8 Performance Testing ......................................................................................................................................................... 60 References .................................................................................................................................................................................. 62 3.1 INTRODUCTION 2. Dispersants have little or no acid-neutralizing abil- ity, but detergents do. This is because dispersants Lubricants are composed of a base fluid and additives. have either no basicity, as is the case in ester disper- The base fluid can be mineral, synthetic, or biological in ori- sants, or low basicity, as is the case in imide/amide gin. In terms of use, petroleum-derived (mineral) base fluids dispersants. The basicity of the imide/amide disper- top the list, followed by synthetic fluids. Base oils of biologi- sants is due to the presence of the amine function- cal origin, that is, vegetable and animal oils, have not gained ality. Amines are weak bases and therefore possess much popularity except in environmentally compatible minimal acid-neutralizing ability. Conversely, deter- lubricants. This is because of the inherent drawbacks these gents, especially basic detergents, contain reserve base oils have pertaining to their oxidation stability and low- metal bases as metal hydroxides and metal carbon- temperature properties. Additives are added to the base fluid ates. These are strong bases, with the ability to neu- either to enhance an already-existing property, such as viscos- tralize combustion and oxidation-derived inorganic ity, of a base oil or to impart a new property, such as deter- acids, such as sulfuric and nitric acids, and oxida- gency, lacking in the base oil. The lubricants are designed to tion-derived organic acids. perform a number of functions, including lubrication, cool- 3. Dispersants are much higher in molecular weight, ing, protection against corrosion, and keeping the equipment approximately 4–15 times higher, than the organic components clean by suspending ordinarily insoluble con- portion (soap) of the detergent. Because of this, dis- taminants in the bulk lubricant [1]. Although for automo- persants are more effective in fulfilling the suspend- tive applications all functions are important, suspending the ing and cleaning functions than detergents. insoluble contaminants and keeping the surfaces clean are the most critical. As mentioned in Chapter 4 on “detergents,” this is achieved by the combined action of the detergents and the Dispersants, detergents, and oxidation inhibitors make up the dispersants present in the lubricant. Dispersants differ from general class of additives called stabilizers and deposit con- detergents in three significant ways: trol agents. The goal of oxidation inhibitors is to minimize the formation of deposit precursors, such as hydroperoxides 1. Dispersants are metal-free, but detergents contain and radicals [3,4]. This is because these species are reac- metals, such as magnesium, calcium, and some- tive, and they attack the hydrocarbon base oil and additives, times barium [2]. This means that on combustion which make up the lubricant, to form sludge, resin, varnish, detergents will lead to ash formation and disper- and hard deposits. The goal of the dispersant and the soap sants will not. portion of the detergent is to keep these entities suspended in 45 Copyright Taylor & Francis Group. Do Not Distribute. 46 Lubricant Additives: Chemistry and Applications the bulk lubricant. This not only results in deposit control but inhibitors in engine oils [12,13]. All these acids are neutral- also minimizes particulate-related abrasive wear and viscos- ized by basic detergents to form inorganic metal salts and ity increase. When the lubricant in the equipment is changed, metal carboxylates. These compounds are of low hydrocarbon the deposit precursors and the deposit-forming species are solubility and are likely to fall out of solution. removed with the used oil. The aldehydes and ketones undergo aldol-type condensa- The dispersants suspend deposit precursors in oil in vari- tion in the presence of bases or acids to form oligomeric or ous ways. These comprise the following: polymeric compounds. These can further oxidize to highly oxygenated hydrocarbons, commonly referred to as oxygen- Including the undesirable polar species into micelles. ates. The oxygenates are usually of sticky consistency, and Associating with colloidal particles, thereby preventing the term resin is often used to describe them [14]. Resin is them from agglomerating and falling out of solution. either the basic component in or the precursor to all types Suspending aggregates in the bulk lubricant, if they are of deposits. Common types of deposits include varnish, lac- formed. quer, carbon, and sludge [15,16]. Varnish, lacquer, and carbon Modifying soot particles so as to prevent their aggrega- occur when resin separates on hot surfaces and dehydrates or tion. The aggregation will lead to oil thickening, a polymerizes to make tenacious films. The quantity and the typical problem in heavy-duty diesel engine oils [5,6]. nature of deposits depend on the proximity of the engine parts Lowering the surface/interfacial energy of the polar to the combustion chamber. The parts closer to the combus- species to prevent their adherence to metal surfaces. tion chamber, such as exhaust valve head and stem that experi- ence approximate temperatures of 630°C–730°C [17,18], will 3.2 NATURE OF DEPOSITS AND MODE develop carbon deposits. The same is true of the combustion chamber wall, piston crown, top land, and top groove, which OF THEIR FORMATION are exposed to approximate temperatures of 200°C–300°C. A number of undesirable materials result from the oxidative Carbon deposits are more common in diesel engines than in degradation of various components of the lubricant. These gasoline engines and result from the burning of the liquid are base oil, additives, and the polymeric viscosity modifier, lubricating oil and the high-boiling fractions of the fuel that if present. In engine oils, the starting point for the degrada- adhere to hot surfaces [19]. tion is fuel combustion, which gives rise to hydroperoxides As we move away from these regions to the low-temper- and free radicals [7]. The compounds in the fuel that are ature regions, such as the piston skirt, the deposits are not most likely to form peroxides, hydroperoxides, and radi- heavy and form only a thin film. For diesel engine pistons, this cals include highly branched aliphatics, unstaurates such as type of deposit is referred to as lacquer; for gasoline engine olefins, and aromatics such as alkylbenzenes. All these are pistons, this type of deposit is called varnish. The differ- present in both gasoline and diesel fuels. American Society ence between lacquer and varnish is that lacquer is lubricant- for Testing and Materials (ASTM) test methods D4420 and derived and varnish is largely fuel-derived.