Machinery Lubrication magazine published by Noria Corporation
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sYNthetICs oils. However, unsaturated esters, including vegetable oils, are still limited to lower temperature applications. Saturated esters are required for use at higher temperatures, but there is more to consider. High-temperature oxidative stability depends heavily on the amount and configuration of hydrogen on the beta-carbons in the molecule. The beta- carbon is the second one from the carbon-oxygen bond of the ester group. The beta-hydrogen is very reactive toward oxygen, so esters with no beta-hydrogen are more thermally stable. These are known as neopolyol esters, with their name derived from their structural similarity to neopentane. Neopolyol is shortened to polyol esters and abbreviated as POE. All POEs have good oxidative stability because they have no beta-hydro- gens (see Figure 2). Although unsaturated fatty acids cannot perform at high temperatures, it is not enough to simply substitute saturated fatty acids such as stearic acid. Synthetic short-chain carboxylic acids offer a greater degree of oxidative stability and are much better at low temperatures than saturated fatty acids. Shorter branched fatty acids are used when exceptional thermal stability is required. By eliminating the oxidative weak points, synthetic esters can be designed to operate at high temperatures and will tend to evaporate cleanly before undergoing oxidative polymer- ization so they will not form deposits and varnish. Viscosity Chemists find many examples of the link between viscosity and molecular weight. From linear alkanes to polymers, bigger molecules are expected to be more viscous. However, this simple rule of thumb does not always apply to synthetic esters. The viscosity is strongly dependent on the amount of branching, aromaticity, functionality and ease of rotation of the bonds that make up the molecule. As the structure becomes more branched, it is more difficult for the molecule to bend around and flow over itself. Aromatic esters are extremely viscous because of the rigid aromatic ring. So while it's true that molecular weight is related to viscosity, there are also ways to break this relationship when desired. This is particularly useful when the volatility profile requires a specific molecular weight and the application demands a certain viscosity. Molecular weight is not the only factor that determines the viscosity of a synthetic ester, but it can certainly be used to increase viscosity when necessar y. If the component acids and alcohols each have more than one reactive group, esters can be polymerized to any length. Although the lubricant industr y doesn't employ rigid polyesters that are made into bottles, the same principle can be used to build molecular weight and therefore increase viscosity. These are called complex esters or CPE. Biodegradability and Hydrolytic Stability The rate of the hydrolysis reaction is highly dependent on both the chemistry of the ester bond and the environmental conditions. Synthetic esters can be stable for a few hours or thousands of years, so it is impossible to classify them using words such as "fair" or "good." To manage hydrolysis, it is important to understand the type and purity of the reactants as well as the manufacturing process. Remember that esters are made from alcohols and carbox- ylic acids, and that water is a byproduct of the esterification reaction. All ester reactions are reversible, so water can break ester back into the acid and alcohol components. Once ester is broken into the alcohols and acids, bacteria can complete the digestion of the components. Typically, increasing the amount of natural components such as vegetable-based fatty acids helps biodegradability. When synthetic acids and neopolyol alcohols are used, the ester becomes more inert and the rate of biodegradation is reduced. It is possible to chemically block the hydrolysis pathway using branched carboxylic acids. These esters are extremely stable in water and act like mineral oils in typical hydrolysis tests. In fact, a computer simulation shows that the rate of hydrolytic degra- dation is measured in hundreds of years. Smoke Point, Flash Point, Fire Point and Volatility Synthetic esters are prized for their ability to lubricate at high temperatures. One of the main reasons for this is that they have Figure 2. Polyol ester SYNTHETIC BASE OIL VISCOSITY AT 40°C FLASH POINT PAO 19 cSt 220°C PAG 34 cSt 218°C Alkylated Naphthalene 29 cSt 222°C Diester 14 cSt 231°C Polyol ester (POE) 19 cSt 257°C 40 March - April 2014 | www.machinerylubrication.com