Saving Energy with Polymer GREASES

2017-06-16   来源： 网友评论 0 条

    Metal soaps are the most common thickening technology for lubricat- ing greases, accounting for around 90 percent of grease production. Con- sequently, the majority of studies into the lubricating properties of greases have been performed using soap-based greases.
However, according to Johan Leckner of Axel Christiernsson International AB, greases based on polypropylene thickeners have the potential to save considerable amounts of energy compared to soap-based greases. In a presentation at the European Lubricating Grease Institute Annual General Meeting in Barcelona in April, he reviewed the results of a test program showing the lubricating properties and energy saving potential of polymer greases.

Polymer’s Promise
“Compared to soap-based thicken- ers, the use of polymer thickeners is relatively recent,” said Leckner, “but it is hardly new technology, first being patented in the 1960s.” These products behave very differently, both in laboratory tests and in real life applications, he added.
“In fact, it was probably a bad marketing move to call this type of semisolid lubricant a grease in the first place because that imposes preconceived expectations on how its specifications should look.” He then highlighted some of the major differences between soap-based and polymer greases.
Low-temperature performance: The initial idea behind polymer technology was to create a grease with superior low-temperature performance. “And cold chamber bearing tests demonstrate extremely good low-temperature properties,” said Leckner, “because polymer greases bleed oil at temperatures where most soap-based greases have stopped bleeding.”
Resistance to water and process fluids: The combination of a non- polar thickener and good adhesion makes polymer greases very effective in protecting metal surfaces against corrosion, he noted. “The nonpolar thickener system provides especially good performance in the presence of process fluids containing surface active components.”
In addition, the surfactants that stabilize emulsions can be aggres- sive to soap thickeners, collapsing the thickener structure. Nonpolar thickeners like polypropylene lessen the impact of these components.
Long life and oxidation behavior: According to Leckner, polymer greases have demonstrated L50 values of over 5,000 hours in the SKF R0F rig and over 1,500 hours in the FAG FE9 F50 test at 120 degrees C. Another study of static aging showed that a lithiumcomplex grease became stiffer when exposed to 120 degrees C for five days. “In contrast, although the polymer grease showed signs of oxidation, the grease actually became softer,” Leckner reported.
Low friction: In laboratory tests, polymer greases run at lower temperatures and torques than most other greases. For example, in the SKF R2F-A test, Axel Christiernsson found that running temperatures are normally 10 to 15 degrees C lower than for soap-based greases formulated with the same base oils. This behavior has also been shown in thrust bearings mounted in a modi- fied four-ball machine and in field trials.
Film thickness: “Polymer thick-ened greases have very good capacity for forming a thick lubricating film at low speeds and fully flooded conditions,” said Leckner. Starved lubrication experiments also indicate that polypropylene thickener passes through the contact more often than soap thickeners, improving film thickness.
Additive response: Traditional greases are thickened by an organic salt (the soap) that can react with metal surfaces. “In some cases, the soap adds properties to the lubrication system. These are commonly called functional thickeners and include lithium-bismuth complex soap and calcium-sulfonate complex soap,” Leckner explained.
However, in most cases, the soap has a negative effect on additive function. In addition, most soap- based greases contain a small excess of alkali to prevent grease oxidation and discoloration during manufactur- ing. Alkali is a reactive compound that can affect additive performance negatively.
“The thickener in a polymer grease is very similar to the base oil in its physiochemical properties,” said Leckner. “Therefore, it is less likely to interfere with additive performance. Furthermore, polypropylene has less affinity to metal surfaces compared to soaps and does not compete with additives for access to the surface.” This opens up the possibility of using less polar additives that are normally outcompeted by the soap.
Manufacturing and grease mi-crostructure: In the manufacture of lithium-complex greases, chemical reactions between the metal hydrox- ide and fatty acids are responsible for forming the soap. In the cooling stage, soap fibers form the thickener matrix via crystallization. Both reac- tion and cooling stages are important to the performance of the resulting thickener matrix, and variations in raw materials or process conditions can have a large effect on the result. Leckner related that no chemical reactions occur during the manufac- ture of polymer grease. The polymer and oil blend is simply heated to a temperature above the polymer melt- ing point, and the solution is quench cooled to prevent crystallization and agglomeration of the polymer mol- ecules. The quenched material is then worked to the desired consistency.
“Not surprisingly,” he said, “polymer thickener matrices look very different from soap-based matrices.”

Measuring Performance
Leckner observed, “We have often been surprised by the differences in performance when replacing a soap-thickened grease in industrial applications. These differences range from significantly lower running tem- peratures and reduced vibrations to much longer relubrication intervals.”
Axel researchers concluded that to provide these advantages the lubricating mechanism for polypro- pylene thickened greases must differ significantly from that of soap-based greases. They ran a test program to highlight some of these differences by comparing five polymer greases and five lithium-complex greases with identical base oil compositions. Leckner admitted, “This might be comparing apples with oranges because the two thickener types are very different, but we made the grease samples as similar as possible. The greases were evaluated in rolling element bearing tests, using different bearing designs, contact pressures and speeds, including SKF R2F-A lubricity, SRV4 oscillating wear and Anton Paar rheometer.
“A common cause of lubricant failure in bearings is deterioration of the lubricating film, leading to a rapid temperature increase that promotes excess oil bleed,” Leckner related. On the other hand, rising temperatures also cause oxidation, polymerization and evaporation that increase stiffness and reduce oil bleed rate in soap-based greases. These conditions can also lead to the formation of a so called grease skin that can effectively stop oil bleed in soap-based greases, he added.
In contrast, thickener degradation in polymer greases offsets the tem- perature effects somewhat. “Severe shear forces close to the contact zone mechanically degrade the thickener, resulting in oil release,” said Leckner. This helps improve track replenish- ment and keep running temperature down. “Friction curves from the SRV wear test showed that the polymer grease reached a low steady-state friction after about 40 minutes, while the lithium-complex grease showed increasing friction with time, indicat- ing a less stable tribofilm,” Leckner reported.
In addition, the speed ramps for the two thickener technologies showed considerable differences. Rolling resistance for the lithium-complex greases started at lower speeds and was significantly higher than for the polymer greases. However, once the bearings start rolling, there are no appreciable differences between the two greases.
“The cause of the difference in rolling resistance may be related to the characteristics of the lubricant in the running track,” Leckner said. Because the base oil composition is identical, thickener in the raceway probably caused the difference. “A layer of soap absorbed to the metal surface could be one explanation.”

Performance Differences
Polymer and lithium-complex grease behave differently in a number of ways. For example, Leckner noted that churning is higher for polymer grease than for lithium-complex grease with the same base oil compo- sition.
During the first few hours after starting a newly lubricated bearing, the grease is redistributed inside the bearing, causing drag losses and increased temperature. This period is known as the churning phase. “After this phase, the temperature normally stabilizes quite fast,” said Leckner. “But the polymer grease continues to experience friction spikes that can only be explained by additional churning caused by grease entering the contact.”
Polymer grease also exhibits signifi- cantly longer running-in times, which Axel researchers attribute to its physiochemical properties. “Further studies are needed to understand the mechanism that draws the polymer thickener into the contact to a much larger extent than the lithium-com- plex soap,” Leckner reported. But the longer running-in period is not an is- sue in the field because temperatures do not reach levels that damage the grease. “Also, a few days at slightly elevated temperatures is not a long time in the perspective of a 6 to 12 month relubrication interval.”
In grades with high-viscosity base oil, polymer grease is much less likely to generate severe starvation and metal-to-metal contact. “One factor that promotes oil bleed and reduces the risk of starvation is shear degradation of the thickener,” said Leckner. “For the polymer grease, it is likely that shear, or thermal, degradation of the thickener close to the running track significantly improves track replenishment, in effect reducing thickener content in the shear zone.” This prevents the starvation seen in soap-based greases of the same viscosity grade.
As the polymer thickener shears down, a mixture of oil and degraded polymer replenishes the track. This mechanism is quite different from soap-based greases where oil bleed is the main source of replenishment. Occasionally, fresh lumps of grease will enter the contact and replenish it, but this causes additional churning and temperature events.
From the end user’s perspective, the possibility of using a higher viscosity oil than in a soap-based grease will be advantageous because it ensures adequate lubricating films at low running speeds. “This would be beneficial in systems that run at high speeds during production hours but at lower speeds to conserve energy when production is not running,” Leckner said.
“Probably the most notable benefit of polymer grease is that it allows maintenance intervals to be increased by at least 30 percent,” Leckner con- tended. “Polymer grease has demon- strated very long life both in bearing tests and in the field. A plausible explanation for this is that polymer grease has less thickener. Also, poly- propylene can be considered a very thick oil, and sheared polymer in or close to the contact zone acts like a grease with no thickener content.” Conventional grease is essentially a two-phase system, consisting of a liquid oil phase and a solid thickener phase. “Polymer grease is more of a quasi-one-phase system with low- viscosity hydrocarbon dissolved in an extremely high-viscosity hydrocar- bon,” Leckner explained. “When the polymer is sheared down close to the contact zone, the grease essentially becomes 100 percent oil, providing more lubricating capacity than a soap-based grease, thereby increasing grease life.”
Leckner concluded, “The main differences between polymer and soap-based grease are oil bleed at low temperatures, additive response, higher churning, longer running-in times, longer grease life and reduced risk of dry running, especially for high viscosity grades.” From an end user’s perspective, this translates into the ability to use higher viscosity base oils, longer maintenance and relubrication intervals and lower (2 to 5 degrees C) running temperatures.
“Reduced labor, material and downtime are the dominant cost benefits for most industrial users,” he said. Added to these benefits is the fact that lower friction losses result in reduced power consumption, which can be directly related to energy savings.