For over a century, lubricant manufacturers have reacted fatty acids with bases in oil, forming soaps that thicken the oil and make grease. Its easy to get the impression that thickeners simply provide structure for grease, similar to beams and studs that form the framework of a building. Research is showing, however, that certain types of thickener can pull double duty-providing structure while also lowering friction, torque and temperature and extending grease service life in bearings.
Johan Leckner, Ph.D. group technical manager at Swedish company Axel Christiernsson, has proposed that polypropylene thickener enhances grease performance in two ways. First, polypropylene-thickened greases release polymer fragments that enhance the bearing lubrication provided by base oils. Second, these plastic-thickened greases form a friction modifier in situ in the oil phase in lubricating tracks.
To account for these observations, Leckner suggested modifying the three-phase model of grease lubrication in bearings by adding a fourth phase: the adjustment phase.
Leckner presented these ideas at the Society of Tribologists and Lubrication Engineers late spring meeting in Las Vegas. His research included tribology tests to observe friction and wear in greased bearings, accelerated aging experiments to study grease resistance to heat, and measurements of electricity consumption by bearing test equipment.
Polypropylene serves primarily as a thermoplastic, which becomes soft and malleable when heated and solidifies when cooled. This polymer is used to make packaging materials, textiles and various molded objects, and-for the past decade-has also been used to thicken greases.
To study the behavior of polypropylene greases, Leckner formulated four model greases. Synthetic base oil blends were prepared from 94 percent polyalphaolefin and 6 percent adipate ester, which facilitated the saponification reaction needed for lithium grease. Light and heavy base oil blends had viscosities of approximately 10 and 50 centistokes, respectively, at 100 degrees Celsius.
Two of the greases (LiX-10 and LiX-50) were thickened with traditional lithium 12-hydroxystearate soap and azelate soap and worked to develop consistency. Two more greases (PP-10 and PP-50) were prepared by mixing polypropylene in oil, heating to melt the polymer, quenching and working the mixture. All four greases had NLGI grade 2 consistency and contained no additives.
Advantages of the polypropylene thickener were evident in tribology tests that measured friction and wear of lubricated bearings. Leckner modified the SKF R2F Grease Life Test, in which a pair of roller bearings were run in a test apparatus at ambient temperature at 2,500 rpm until failure occurred. Temperature inside the bearings and electric power consumption were both monitored during the test.
At the start of each test, temperature rose at first and then decreased during the first hours. This was churning, the first phase of the three-phase model of grease lubrication in bearings. During churning, fresh grease distributes itself inside bearings, and contacts are fully flooded with lubricant. Temperature decreases when the oil reduces friction in the contacts. In this study, bearing temperature was lower and steadier for polypropylene-thickened greases than for lithium greases. Leckner explained that PP thickener entered bearing contacts and helped reinforce lubricating films.
During the next 20 days the greases were in the bleeding phase, the second phase of grease lubrication. During bleeding, grease no longer distributes itself but forms reservoirs that release oil into contacts as it is gradually consumed. The lubricating film in each contact breaks down and is replenished repeatedly, and temperature remains relatively low.
For the PP-50 grease, bearing temperature stabilized between 40 and 45 degrees Celsius for both bearings during bleeding. There were occasional temperature spikes to between 60 and 80 degrees C, but the temperature always returned to the 40-45 degrees C range. The polypropylene grease provided consistent protection against friction and prevented overheating.
The performance of the LiX-50 grease was a sharp contrast to the PP-50 grease. During the first week, the temperature fluctuated between 50 and 60 degrees C, and differed between the two bearings. Then, the LiX-50 left the bleeding phase and entered the third and final phase of grease lubrication-severe film breakdown. Bearing temperature soared and fluctuated wildly between 60 and over 100 degrees. This behavior was consistent with breakdown of the lubricating film, metal-to-metal contact and damage due to friction and overheating.
Some R2F tests were terminated after 20 days, and the roller bearings were inspected. For LiX-50 grease, roller bearings were dry, and grease near the raceways appeared baked. For PP-50 grease, oil was present on the roller bearings, and the grease appeared to be in good condition.
Leckner proposed that PP grease softened inside the bearings, released polymer fragments and oil into the contacts, and provided more effective lubrication than LiX-50. These results coincided with results in related literature in which the consistency of lithium complex grease more than doubled (hardened), while PP grease softened when aged at 120 degrees C for 168 hours.
When other R2F tests were run to failure, PP-50 lasted seven times longer than LiX-50 grease. PP-10 had been running more than five times longer than LiX-10 and was still in progress when Leckner gave his STLE presentation. He later told LubesnGreases the test failed after 430 days-also seven times longer for PP-10. He emphasized that grease life was longer with the PP thickener because the polypropylene softened and released oil and polymer fragments to continuously lubricate the contacts in these bearings, while the LiX greases simply bled oil into the contacts.
Leckner also investigated the response of these greases to static heat in the absence of shear. He performed an accelerated aging test by spreading 1.5 mm-thick layers of grease on glass plates and heating them at 120 degrees C in an oven. The plates were removed from the oven and photographed each day. LiX-10 grease was visibly darker on the fifth day of the test. PP-10 grease did not show similar discoloration until the 14th day.
When many lubricants are overheated, their base oils, additives and thickeners undergo oxidation reactions that alter lubricant chemistry, color and performance properties. Leckner used infrared spectroscopy to analyze samples from the test. An infrared spectrum is similar to a fingerprint because it contains peaks or bands that correspond to specific chemical bonds in molecules. A peak centered around 1,716 reciprocal centimeters typically corresponds to carbon-oxygen double bonds that are in carbonyl groups in carboxylic acids, aldehydes and ketones- which typically form when base oils undergo oxidation reactions. The height or strength of this peak is proportional to the number of carbonyl bonds in the sample.
Despite its darker color and a significant increase in consistency, IR spectra showed that oxidation was slower for the LiX-10 samples than PP-10 samples. Leckner explained that lithium thickener, or unreacted lithium hydroxide, seemed to protect the base oils from oxidation. The polypropylene thickener did not have this protective effect, and the PP itself may have been oxidized.
To study how equipment wear might affect grease performance, Leckner mixed 0.1 percent bronze, copper and iron particles into LiX-10 and PP-10 greases. These metal particles modeled wear particles from cages and raceways of in-service bearings. In accelerated aging studies, color changes occurred sooner for LiX-10 than PP-10 greases. IR spectra showed that metal particles accelerated the oxidation of both greases.
Leckner concluded that PP and LiX thickeners had significantly different mechanisms for their response to heat. That is, there was more oxidation and release of base oil in the case of PP grease, whereas LiX resisted oxidation and tended to retain instead of release oil. However, in this case, oxidation of PP produced polymer fragments that improved lubrication.
Leckner further investigated friction and wear by testing greases with a variation of the SRV Wear Test (ASTM D5707) using a ball-on-disk configuration at 200 N load, 50Hz, 1 mm stroke length and 80 degrees C for six hours. Wear scar diameters increased with the age of LiX-10 grease but remained consistent for PP-10 grease. Coefficient of friction clearly decreased for PP-10 but not LiX-10. These results were consistent with R2F data and the release of polymer fragments and oil from PP greases.
Additional SRV data supported Leckners explanation for the performance of PP greases. Oil samples were collected from aging studies and used in SRV tests. Coefficient of friction measurements were similar for aged base oils and oils extracted from aged LiX-10 grease. This indicated that the lithium thickener simply released oil during the aging process.
Coefficient of friction and temperature measurements were much lower for oil extracted from aged PP-10 grease. This provided more evidence that PP-10 thickener reduced friction by releasing polymer fragments into the oil.
To explain his results, Leckner proposed the in situ formation of a friction modifier on bearing tracks for the PP greases. This phenomenon would justify adding a fourth phase to the three-phase model of grease lubrication: An adjustment phase, occurring between the churning and bleeding phases, involving degradation of the thickener and possible in situ formation of a friction modifier on bearing tracks.
Adding to the practical benefits of polypropylene-thickened greases, Leckner reported that the plastic thickener provided significant energy savings. He measured a 330 kilowatt hour savings in electric power over a 60-day SRV test with a single bearing.
As a grease thickener, he emphasized, polypropylene not only provides structure but also a means to reduce friction, wear, temperature and energy consumption-all while extending service life in greased bearings.
Mary Moon, Ph.D., is a physical chemist with R&D and management experience in the lubricating oil, grease and specialty chemicals industries. She is skilled in industrial applications of tribology, electrochemistry and spectroscopy. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.