Finished Lubricants

Making Grease Better


Everyone knows that-even with the best formulation-if you make grease badly, then its going to turn out badly, Gareth Fish told the European Lubricating Grease Institutes annual meeting in May. Getting manufacturing right is very important to grease making.

Fish, strategic technology manager for Lubrizol, and Alexandra Nevskaya of Dow Corning highlighted at the Helsinki gathering some advances in grease-making technology that can be used to produce better quality greases.

During grease production, the more complete the reaction, the better the grease, Fish continued. For complex greases, it is typically desirable that the reaction is driven to or as close to completion as possible. The more complete the reaction, the better the thickening and typically, the higher the dropping point, he stated in a paper submitted to ELGI.

Lithium is the most commonly used grease thickener, in both conventional and complex forms. The National Lubricating Grease Institutes 2016 Grease Production Survey found that 75 percent of the worlds greases are thickened with lithium soaps, and total production edged up from 1.88 billion pounds in 2015 to 1.94 billion pounds in 2016-despite a surge in lithium prices.

Many challenges exist when making high-quality lithium complex grease, Fish noted. The one-step process can be difficult to control, with batches often giving good yields but with scatter in the resulting dropping points, he wrote. The two-step process will reliably produce dropping points of 260 C to 280 C, but based on current targets of 280 C to greater than 300 C, may not be good enough. Dropping point is the temperature at which a grease releases a single drop of oil under controlled conditions.

However, research has shown that micronized dehydrated lithium hydroxide dispersed in oil can improve the speed of the grease-making reaction. Fish decided to investigate whether LiOH dispersions could be used to reliably produce lithium complex greases with high dropping points.

The researchers first tried using a one-step process to make the lithium complex greases but produced inconsistent results using a LiOH monohydrate powder, so they fell back on the two-step process for this thickener. Production of grease with a LiOH dispersion was successful with the one-step process. Both greases were made with ISO viscosity grade 68, API Group I mineral oil.

The LiOH dispersion grease made with the one-step process consistently showed higher dropping points (from 285 C to above 308 C) and better yields with shorter processing time, Fish reported. If the price of lithium continues to soar, using lithium dispersion is more cost effective, he concluded.

Next, Fish addressed ways to reduce bleed, since increasingly common Group II and Group III base oils bleed more than Group I oils at the same viscosity, reducing high-temperature grease performance and shelf life. Polyalphaolefin base stocks bleed even more than mineral oils, he noted.

Polymers are used in greases to perform many functions, including bleed control. Typical additives used for bleed control include styrene-isoprene co-polymer and polyisobutylene, which Fish compared with olefin (ethylene-propylene) copolymers and styrene-butadiene rubber in a limited study.

The styrene-isoprene polymer at 2 percent [treat rate] had a significant improvement in bleed, he found. It also improved tackiness. The biggest problem with PAO greases is controlling bleed, and SIP could significantly improve the life of such greases.

At high temperatures (over 100 C), however, polymers lose their effectiveness. Boron and borate additives have traditionally been used to reduce bleed, in addition to performing as EP additives and complexing agents, in greases for high-temperature applications.

Adding zinc along with borate chemistries can also produce elevated dropping points when mixed into simple lithium grease. Zinc on its own, in our experience, doesnt do anything, said Fish. You get synergy when you take borates and zinc dithiophosphate.

Borate esters have a pungent odor, Fish noted in his paper, so a new, low-odor version has been developed. To investigate how these new borate chemistries perform alongside zinc, Fish tested an ISO VG 100 base lithium grease with and without ZDDP. The new, odorless borate additive matched the behavior of the current borate, he stated.

With anhydrous calcium greases, borate additives couldnt quite push the grease up to an acceptable dropping point. One of the specs that weve seen is a minimum dropping point of 175 C, Fish continued. Taking anhydrous calcium grease and treating it with boron wont get you there. Even when adding zinc, it didnt do anything over and above what adding the boron did.

As may be expected, for calcium-lithium mixed soap greases, the dropping point increases along with the lithium content. For products with lower lithium content, borate esters can help boost the dropping point back up 180 C, stated Fish. In addition, the researchers successfully used borate ester, along with a performance package, to boost the dropping point of a low-performing lithium complex grease from 217 C to over 280 C.

The researchers wondered whether the new borate ester might be able to replace diacid as a complexing agent, since the relative cost in North America of adding the borate ester is 5 percent above simple lithium grease, as compared to a 9 percent rise in cost for diacid-containing grease. They made greases with ISO VG 220 mixed paraffinic and PIB base oil, along with a zinc-containing additive package at a 4 percent treat rate. One grease contained diacid, and one contained the new borate ester, while the third was a simple lithium grease.

Fish pointed to the key finding that flow pressure was much higher for the lithium complex grease with diacid, while the flow pressure for the borate ester treated grease was close to that of simple lithium, with similar performance. The borate ester grease also had significantly improved oxidation resistance.

No data was available yet for the FE9 test-the arbiter of high-temperature performance in greases-but the tests should be complete in January, Fish regretted. From the data available so far, there has been no loss of performance when the new borate ester is used as a complexing agent. Its a work in progress, but this suggests that the grease has promise.

Dow Cornings Nevskaya also pointed to polymers as a novel way to improve grease performance, but as base oils rather than additives.

Historically, polysiloxanes (silicones) have had limited use in lubricants because of their limited load-carrying ability, especially in metal-to-metal contacts, Nevskaya acknowledged to the audience in Helsinki. However, newly developed phenyl/fluoro siloxane fluids, which Dow Corning has patented, show significantly improved lubricity and high-temperature performance, along with high thermal stability and enhanced wear resistance.

Three types of silicones are used as base oil for lubricants: polydimethyl silicone (PDMS); phenylmethyl silicone (PMPS), which gives additional thermal and oxidation stability above PDMS; and fluoro silicone (FS), which has excellent chemical resistance and better load-carrying capacity and wear resistance, but does not provide the same thermal stability as other silicones, Nevskaya reminded.

Phenyl/fluoro siloxane copolymer combines the thermal stability of phenylmethyl silicone and the wear resistance of fluoro silicone, she explained. Different phenyl-to-fluoro ratios can balance these two properties, and chain lengths can be varied to change viscosity.

For example, increasing the fluoro content improves wear performance. While traditional PMPS silicone cant withstand the load of a four-ball wear test (DIN 51350-3), Ph/F siloxane can be measured in certain ratios. The wear scar for a 50-50 phenyl-fluoro copolymer is 1.48 millimeters at 400 Newton load and 2.82 mm at 800 N. For a 25-75 phenyl-fluoro fluid, the wear scar is reduced to 0.55 mm at 400 N and 1.81 at 800 N.

Compatibility is another barrier for traditional silicones. Polysiloxane fluids have limited miscibility with additives, and fluorosiloxanes are completely immiscible. Ph/F copolymers, on the other hand, show good acceptance with many commercially available additives. When combined with antiwear additives, in particular, wear resistance can be boosted by 50 percent above neat fluid performance, Nevskaya boasted.

Silicones have a particularly bad rap in the automotive market because of paint incompatibility. The new Ph/F copolymer grease performed better than PDMS and PFPE in paintability tests and can be coated with paint, but still caused some defects after being dry-wiped from a surface that was then painted. The copolymers are, however, compatible with plastics, reported Nevskaya.

Ph/F copolymer greases can be prepared using simple and complex thickener systems, such as lithium and lithium complex soaps, as well as non-soap thickeners like polyurea or polytetrafluorethylene. Preparation is similar to that of current polysiloxane greases. Dow Corning plans to launch its own pre-formulated grease made with the copolymer.

Potential applications include high-temperature bearings and equipment, metal processing, automotive under-hood components, paper corrugating bearings, textile equipment, tire molding, injecting molding equipment and heat treatment furnaces. The fluid could be a lower cost alternative to PFPE greases in applications that do not require the ultimate high-temperature performance provided by PFPE, Nevskaya hopes.

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