Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/290460
www.machinerylubrication.com | March - April 2014 41 a much lower volatility than other lubricant base oils at a given viscosity. Volatility is strongly related to smoke point, flash point and fire point, which are part of ASTM D-92. As the temperature rises, the amount of evaporation increases until there is visible smoke and eventually enough smoke to support a flash or fire in the presence of a flame. The table on page 40 shows the relation- ship between flash point and viscosity for several common types of synthetic lubricants. Volatility is also dependent on the distribution of molecular weight in a lubricant. It has been proven that a small amount of flammable solvent will still be flammable even if mixed with other inert components. The mixture will ignite as long as there is enough flammable vapor in the air. Likewise, the most volatile components of a lubricant base oil determine flash point. Esters can be designed to have a very pure composition so there are few small molecules to smoke and flash. An added benefit is that viscosity stays in grade because no light ends evaporate from the lubricant. Volatility and Deposits From a chemical standpoint, volatility is related to molecular weight, polarity and chemical stability. While molecular weight and polarity are well-known effects, chemical stability is often overlooked because it considers only small organic molecules. However, a high-temperature lubricant is made from larger molecules that do not evap- orate readily, so stability becomes important. Oxidative and thermal degradation begin to occur between 200 to 300 degrees C. At these temperatures, base oil evaporation is a slow process. However, oxidation can break the molecule into small, volatile fractions. A large percentage of the weight loss in evapo- ration tests such as ASTM D-2595 comes from oxidation. Not only does oxidation cause weight loss, but it also causes varnish. The decomposition products in the vapor phase are often free radicals or reactive molecules. Deposits and varnish can form as the radical groups in the vapor condense and create a polymer varnish on metal surfaces. These polymers can also form sludge if they reach a high enough concentration to be insoluble in the bulk oil. Synthetic esters reduce varnish and other deposits because they have outstanding oxida- tive stability and do not form many radical decomposition products. Furthermore, they are good high-temperature solvents and tend to dissolve the varnish back into the liquid phase so it can be filtered out. Lubricity, Polarity and Additives The key property of a lubricant is that it is expected to lubricate. Lubricity has to do with how easily the molecule flows over itself and how well it competes for and coats the metal surface. Esters are generally considered good boundar y lubricants because they associate with metal surfaces and reduce the amount of metal-to-metal contact during sliding motion. Structural factors that impact lubricity include the chain length, the amount of branching and the location of linkages within the molecule. Longer carbon chains, less branching and good polarity all favor boundary lubrication. Ester linkages are polar but can be less surface active if they are shielded by carbon chains. Synthetic esters are designed from different acid and alcohol feed stocks, so the location of ester groups and type of carbon chains can be selected independently. The lubricity of the ester base stock depends on the interaction of the ester with the metal surface. Esters have good lubricity, but under severe conditions, anti- wear and extreme-pressure additives are used to carry the bulk of the load. Some say esters compete so vigorously for the metal surfaces that they crowd out necessary additives. However, many additives are active enough to displace an ester from a surface. Expertise and experience are important here, as some additives do not work well with synthetic esters. It is also important to choose an ester that is appropriate for the application. If the application involves boundary lubrication SGS Herguth leads the industry in high quality innovative condition monitoring analysis for industrial users. If a machine contains oil, grease or operates with fuel SGS Herguth is involved with condition monitoring somewhere on the globe. With 28 condition monitoring laboratories around the world we are the leaders in industry. SGS Herguth provides custom or tailored testing packages and special analytical services designed for your plant or application. 1-800-OIL-LABS www.sgsherguth.com ContaCt us today for all your testing needs! ContaCt us today for all your testing needs!