Lubricating greases account for only about 3 to 3.5 percent of total lubricant demand by volume, but they hold a unique position among automotive and industrial lubricants because of their performance characteristics.
In a presentation at the Third Base Oils and Lubes Middle East 2014 Conference in Dubai in April, Sudip Shyam, global head of base oils at Gulf Petrochem, said that grease production has traditionally been viewed an art, but increasingly modern technology has made it more of a science.
Greases can perform all functions expected of any lubricant with the exception of cooling or cleaning. They are very useful in applications where the ability to stay in a mechanism is important. In some instances, they provide lower frictional characteristics than their lubricating oil counterparts.
According to Shyam, the grease market is grappling with two opposing trends that are influencing demand. First, the market is increasing because of industrial and vehicle growth in developing parts of the world. Second, the market is decreasing due to a shift to superior grades that provide longer life.
In Asia and Africa, the first scenario dominates. Within each market, the ratio of automotive to industrial greases varies from country to country, said Shyam. It depends on several factors, including the nature of the automotive sector, geographical size, the economy, overall per capita consumption of lubricants and degree of product sophistication.
Consumption can be classified by segment with on-road vehicles and agricultural equipment accounting for the bulk of usage.
The National Lubricating Grease Institute in the United States has calculated usage patterns based on thickener type with conventional lithium greases accounting for approximately 60 percent of the market, Shyam said. It is followed by lithium complex (15 percent), all calcium (11 percent), nonsoap (9 percent), aluminium (4 percent) and sodium (1 percent).
In the automotive sector, the main grease applications are wheel bearings, chassis, clutch release and constant velocity joints. Shyam explained, There has been a major shift toward lithium greases for wheel bearings, particularly in India, China and Africa. Lithium complex and polyurea greases are used for high-temperature automotive applications similar to those found in the Middle East.
The agricultural and construction markets in Asia and Africa traditionally use low performance greases and multipurpose lithium greases. Similar to the European trend, the use of biodegradable greases is gaining prominence in total loss systems, according to Shyam.
Another important segment is the steel industry, which is a major consumer of high-temperature and high-performance greases, including complex soap and polyurea greases. Shyam added that calcium sulfonate grease is also gaining usage. I have seen these greases gaining popularity for use in steel pipe production in India, he said.
Cement and mining operations are major users of sprayable aluminium complex greases for open tooth gears, particularly where adhesive strength is a consideration. Greases formulated with heavy base oils are preferred in some instances, said Shyam.
Another key application for greases is railroads, specifically where long service life is important. An example is railway axles where long life lithium greases are frequently used. Shyam said, Complex soap greases are also used for traction motor sealed bearings. In recent news, successful trials were carried out on Bullet Trains in Japan using polyurea grades.
Outlook for Asia & Africa
Shyam said that a number of factors will characterize the market going forward. Manufacturing output is adequate for now but may need to increase to meet regional imbalances or produce speciality products. Toll processing will predominate, perhaps even increase in some countries. Overall, the market will replace low performance greases, with lithium and other grades gaining momentum.
For international manufacturers, the majority of markets in Africa will continue to import greases. Shyam added, The continent is not immune to environmental pressures, and we expect eco-friendly greases to gain ground, albeit initially in mature markets.
Changes in the automotive market will drive demand for superior grades, but, Shyam said, We foresee increased use of specialized grease grades for niche applications such as food processing as well as more fill-for-life applications. Lithium and transparent greases are the most widely exported, according to Gulf Petrochem, and developing regions will remain the dominant players on the international stage.
The Persian Gulf is home to a number of toll blenders. With Iran potentially about to re-enter the global economy, the region looks set to become a hub for the supply and re-export of greases. Its proximity to major markets in Africa gives it a unique advantage in supplying the fast growing economies like Nigeria Shyam noted. However, Irans potential re-emergence as a significant manufacturer and supplier of greases poses a competitive threat to nascent Gulf-based toll blenders that have been steadily building capacity.
What Makes a Grease?
The NLGI defines grease as a solid or semisolid product made of a dispersion of a thickener agent in a liquid lubricant base. In addition, greases may contain special additives to enhance specific properties and may also contain solid lubricants.
Shyam noted that because of their structure, greases require dedicated manufacturing facilities. As a result, most marketing companies sell grease produced by third party toll processors.
According to Shyam, typical grease comprises 70 to 95 percent base oil, 6 to 25 percent thickener and 0.5 to 10 percent additives. The additives are very similar to the ones used in finished lubricants, he said. Additives impart special properties and include oxidation inhibitors, corrosion inhibitors, antiwear agents, solid lubricants and adhesive additives. Nevertheless, most lithium greases contain around 90 percent base oil, Shyam added.
The grease manufacturing process entails several stages. The first requires cooking soap in an open kettle, pressure vessel or contractor. Thereafter, the grease is cooled in a finishing vessel, and additives are incorporated. Milling and deaeration takes place before the finished product is packaged.
The base oils used in grease provide hydrodynamic lubrication in the contact, and the type of base oil will depend on the application. Mineral base oils account for about 90 percent of the market and perform well in applications that range from minus 30 to +150 degrees C. Synthetic oils with a temperature range of minus 70 to +300 degrees C can be based on polyalphaolefin, ester, silicone or perfluoropolyether (PFPE).
In terms of cost, mineral base oils are the least expensive. In contrast, a synthetic hydrocarbon is about four to five times more expensive, said Shyam. Esters are 7.5 times more expensive, silicones 13 times and PFPEs a staggering 380 times.
The role of thickening agents is similar to that of a sponge, said Shyam. Its function is to retain the lubricating oil and liberate it gradually according to the demands of the application. For example, low pressure squeezes out a small amount of oil, while higher pressure produces a higher outflow.
Several types of thickeners are used, including lithium, calcium, aluminium and sodium. Nonsoap thickeners are silica, clay and polytetrafluoroethylene. Lithium accounts for the bulk of the grease market. Shyam explained that lithium soap-based greases dominate the market and typically have dropping points of 180 to185 degrees C and operate efficiently in temperatures around 145 degrees C.
Shyam noted that viscosity is an important performance characteristic of grease. Usually, light loads require base oils with a lower viscosity, and higher loads need thicker base oils. High-speed applications require lighter viscosity base oils, and slow speed applications require thicker viscosity. Similarly lower temperature applications require lower viscosity base oils.
The high use of lithium compared to other thickeners is due to the properties it produces in a grease, particularly smooth buttery appearance and high dropping point. It is used in automotive chassis, wheel bearings and general industrial applications.
Calcium thickeners produce a smooth buttery grease and good water resistance, Shyam said, but with a lower dropping point compared to lithium thickeners. Calcium is widely used in bearings in wet applications, railways and automotive chassis.
Sodium produces rough fibrous grease with a moderate to high dropping point and poor water resistance. Its use is confined to older industrial equipment where frequent lubrication is required, Shyam noted.
Aluminium thickeners produce smooth gel-like greases with a low dropping point and excellent water resistance. They are used in wet applications or bearings operating at slow speeds.
Lithium, aluminium and calcium complex greases have higher dropping points, greater than 260 degrees C. Except for lithium complex, these thickeners provide good water resistance, Shyam said. They are used in high-temperature industrial and automotive bearing applications. Typically, their appearance is smooth and buttery or gel-like.
PTFE thickeners produce smooth, white nonmelting grease with good water resistance, low coefficient of friction and good load/wear properties. They are used in aircraft applications, vacuum oxygen systems, high temperature industrial, chemical plants and food industries, said Shyam.
Carbon black thickeners produce grease suitable for load bearing applications in high temperature conditions such as open gears. Other thickeners include polyurea, organo clays and silica.
Testing and Classification
Because greases are semisolid, most tests used for lubricating oils are not suitable for greases, Shyam explained. Instead, three tests are used on greases. The first measures dropping point. The second assesses penetration and consistency. The third measures penetration after working the grease in a grease worker.
NLGI has standardized a method to classify greases into nine categories on the basis of penetration. The most commonly used categories are NLGI 2 and 3, with NLGI 2 accounting for 85 percent of the grease market, according to Shyam. Generally, the lower the penetration (measured in tenths of a millimeter), the thicker the grease.
Laboratory evaluation of lubricating greases involves the simulation of actual working conditions in standardized test equipment. The tests include oxidation stability, corrosion resistance, shear stability, water resistance and extreme pressure properties to evaluate the load carrying ability of greases. Testing also assesses oil separation tendency, which is a key property, said Shyam. Test standards have been set down by three international bodies: ASTM, ISO and International Petroleum.