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IN THIS CHAPTER WE DESCRIBE THE CHEMISTRY, manufacture, and properties of the chemicals that are used in lubricants as additives. The function of these chemicals is either to enhance the already existing properties of the base fluids or to impart new properties that they lack. Discussion also includes the mechanism by which each additive type performs its functions, multifunctional nature of some additives, and their impact on the environment. The need and the development of a new additive and the process by which it is approved for use in a particular application are also explained. All mechanical equipment must be lubricated. The primary purpose is to reduce friction and wear. If not controlled, these can lead to inefficiencies, damage, and ultimately to equipment seizure. Pictorial records depicting the use of lubricants date as far back as 1650 B.C. Analysis of the residue from chariot axle hubs suggests the use of animal fat as a lubricant, as early as 1400 B.C. This practice continued until 1859 when petroleum-based lubricants became available. While the claimed use of additives in lubricants dates far back into the 1860s, the modern history of lubricant additives began in the early 20th century, with the use of fatty oils and sulfur in mineral oils to improve lubrication under high loads. The use of additives became common only after the 1930s when more compact and faster engines were developed and the OEMs stalled to specify oils for use in their equipment [181,182]. However, these were the military needs during and after the Second World War that were the major driving force behind the development of the lubricant additives as we know them today. This is because the fast-running engines had a complex design, which placed a heavy demand on the lubricants. At that time, besides the SAE viscosity classification system, there were no established performance criteria and the end-user had to depend on the lubricant supplier's claims regarding the suitability of the lubricants . Additives are not only critical to the manufacture and use of automotive fuels and lubricants; they are also extensively used in petroleum recovery as drilling additives and refining as refinery chemicals . This was discussed in Chapter 2 on Mineral Base Stocks. Some of the highlights of the additives development are as follows. 1. Sodium soap greases were developed in the 1930s. They did not suffer from water sensitivity and had better resistance to high temperatures than the lime or calcium soap (calcium carboxylate) greases, the use of which dates back to the 1880s. 2. In 1937, the lubricants containing a lead soap-active sulfur additive combination, known as an extreme-pressure or EP additive system, was found to be effective. When GM in its 1937 models introduced hypoid axles, it tested and recommended the use of a number of such lubricants. 3. Complex calcium soap greases were invented in 1940, lithium soap greases were invented in 1942/1943, and barium greases were invented in the 1950s. These greases retain their semi-solid morphology at much higher temperatures than their sodium and calcium analogues. They contain sodium nitrite to inhibit corrosion and oxidation. 4. Until 1947, the performance of the engine oils was defined solely by their viscosity grades, without due consideration to the engine design, its operating environments, and the fuel type and quality. In 1947, API introduced three performance categories, regular, premium, and heavy-duty, based on the severity of service. Regular oils were straight mineral oils with or without viscosity modifiers and corresponded to oils defined previously by the viscosity grades. These oils were for both gasoline and diesel engines operating under mild to moderate service conditions. Premium oils, designed for somewhat more severe operating conditions, generally contained oxidation and corrosion inhibitors, and, in some cases, mild detergents. Heavy-duty oils possessed better oxidation and corrosion resistance and detergency than the premium oils and were designed to withstand the most severe service. 5. In the early 1950s, multi-grade oils were an outcome of the development of the viscosity modifiers. These were made by dissolving polymeric materials to low viscosity, or thin, oils.