Operating conditions for many types of machinery are becoming more severe, and this places rising demands on lubricants, including greases. Fortunately, the lubes industry has provided a long list of high-performance greases.
As Wilfried J. Bartz noted, though, performance characteristics of these greases vary significantly. The well-known academic tribologist told the South African Institute of Tribologys 10th International Tribology Conference in April that high-performance greases clearly have strengths, but that most also have weaknesses. It is therefore important, he said, to understand the specific capabilities of different greases in order to know which are best suited for particular applications.
Lubricating greases are dispersions of thickening systems and base stocks, the former suspending the latter until pressure is applied, at which point the base stock is released to lubricate. Chemical additives are added to enhance performance properties. Bartz, a professor in the Tribology Department of the Technische Akademie Esslingen in Ostfildern, Germany, said base stocks can constitute between 65 percent and 95 percent of the grease by weight. Thickeners can account for 5 percent to 35 percent and additives up to 10 percent.
Bartz recognized that some greases made with mineral base oil can provide high performance, but he restricted his presentation to synthetic greases, which he defined as those made with synthetic base stocks. He identified several synthetic base stocks used in greases: polyalphaolefins, esters, silicones, polyphenylethers, perfluoropolyethers, polyglycols and phosphate esters.
The type of base stock selected influences several characteristics of the grease. Its viscosity and viscosity index are the major factors determining the greases pumping and flow behavior, especially at low temperatures, Bartz said. Base stock oxidation stability governs service life, reaction to temperature and storage stability. The amount of aromatics in the grease dictates the greases compatibility with elastomers.
Thickening systems, on the other hand, affect physical properties and physical behavior of the grease. By binding or absorbing the base stock, they provide a structure for the grease. They also affect flow behavior and support the manner in which the grease forms lubricating films. And they determine properties such as water resistance.
Thickener systems divide into two main categories. The first is soap thickeners, which include lithium, lithium complex, sodium complex, barium complex, calcium complex and aluminum complex. Non-soaps include organic thickeners such as di- and tetraureas and PTFE (polytetrafluoroethylene) and inorganics such as clays and highly dispersed silicic acid.
Formulas may include several categories of additives, and their amounts vary depending on the other ingredients and the degree to which formulators want to improve performance properties, Bartz said. Solid lubricant additives may constitute between 1.5 percent and 3 percent of the greases weight. Extreme pressure and antiwear additives could be anywhere from 0.5 percent to 5 percent. Most products contain between 0.5 percent and 3 percent corrosion inhibitors and 0.1 percent to 1 percent of both antioxidants and tackifiers.
Most synthetic base stocks work better with some thickeners than others, so certain combinations are more common. Esters, for example, can be used with most soap thickeners and with bentonite. PAOs are less versatile, Bartz said. They are only used with bentonite, lithium and barium complex soaps.
Silicone is used with lithium and natrium complex soaps and also with fumed silica, polyurea and PTFE thickeners. Polyphenylether is more exclusive, used mostly with polyurea. Polyfluoropolyether is used with PTFE and fluorinated ethylene propylene.
High-performance greases may excel in a number of areas, Bartz said, the foremost probably being the range of temperature that they tolerate.
The most important property of lubricating greases is their temperature range of application, he said. Whereas for conventional greases this range is between minus 30 and 180 degrees C, some synthetic greases can be applied at temperatures as low as minus 60 and as high as 250 degrees.
Among the other advantages that synthetic greases may offer over conventional counterparts are:
higher oxidative and thermal stability;
lower volatility at high temperatures;
better chemical and electrical properties.
Greases made with soap thickeners and ester, PAO or silicone base stocks generally provide good anti-wear and low-temperature performance and are well-suited for roller bearings. Greases made with non-soap thickeners tend to exhibit excellent resistance to water. Those containing bentonite usually have good low-temperature performance, while those made with polyurea perform well at high temperatures. Products thickened by PTFE or fumed silica do well at both low and high temperatures.
Unfortunately, synthetic greases also tend to come with a variety of disadvantages, including:
incompatibility with seals;
poor resistance to steam;
tendency to harden or soften at high temperatures;
higher noise levels in roller bearings.
It has to be taken into account that despite advantages in some areas, synthetic greases also have some disadvantages, Bartz said. The trick with synthetics, he added, is to choose a product with strengths that are needed for a particular application and whose weaknesses will be irrelevant.
In order to realize an optimized balance between advantages and disadvantages … the combination of base oil, thickener and additives has to be closely adapted to the application of the grease.
For example, Bartz said, greases made with silicone base stocks have superior viscosity indices and excellent low-temperature fluidity, but they do a poor job of protecting steel surfaces that come into direct contact with other steel surfaces.
Ester-based products bring several advantages, Bartz said. They have good thermal stability and maintain flow properties at low temperatures, and, when combined with lithium soap thickeners, can be used across a temperature range of minus 50 to 150 degrees.
Often they are applied in sealed roller bearings in electric motors, consumer electronics and car electrical equipment, Bartz said. They are also known for their low-noise behavior in small sealed roller bearings. Being plant products, they also biodegrade quickly and can meet requirements for Europes Ecolabel.
PAOs come in a broad range of viscosities and therefore make versatile base stocks, Bartz said. They also offer long service life, function across a wide temperature range and are non-polar, meaning they do not react negatively with organic ingredients. They have a drawback, however, in that they cause high noise in sealed roller bearings. Nevertheless, their main use is in roller bearings for applications such as audio-visual equipment and office machines and in actuators for car mirrors and switches.
Polyphenylethers have good thermal and oxidative stability but low viscosity indices and high pour-points. They also cost 100 times as much as mineral base oils. These characteristics restrict their usefulness to niche applications, Bartz explained.
Alkyldiphenylether oils thickened with diureas make greases that are used for long service life under high temperatures, he said. For instance, they are used in sealed bearings in iron and steel mills allowing casting machines to operate continuously and maintenance-free for a year and even more.
In automotive applications, these greases are applied in bearings running at speeds of 18,000 revolutions per minute and more under high temperatures. Because of the potential for ingress of saltwater from streets, Bartz added, greases used in these applications need rust inhibitors that do not facilitate oxidation at high temperatures.
Perfluoroalkylpolyether-based greases also offer long life at high temperatures. They can last six months in copy machine heat bearings, corrugating machines and electromagnetic clutches, applications where greases made with conventional oils might last no more than a day or week. Once again, cost is a drawback. Perfluoroalkylpolyethers cost roughly eight times as much as mineral base oils.
Applications exposed to electrical current usually require greases that are electrical insulators – as most conventional products are. Occasionally, though, greases that conduct are needed, and these can be formulated with polyglycols and carbon black thickeners.
According to Bartz, high-performance and synthetic greases are gaining more and more popularity, and this despite their high costs and performance disadvantages. The reason, he said, is clear: As operating conditions of machines continue to become more severe, manufacturers need these greases performance advantages. As the trend in machines continues, one can expect interest in synthetic greases tocontinue growing.