The Best Base Stocks for Grease?
More than 30 years ago, the National Lubricating Grease Institute published its GC-LB specification. Originally designed for greases used in wheel bearings and automotive chasses, the spec now encompasses greases used in a variety of applications. While the spec remains valid, a new one—the High Performance Multiuse spec—was published in Dec. 2020.
“Due to advancements in materials, technologies and applications, NLGI has recognized that current applications may be better served by updated specifications,” the organization said.
Luca Salvi—product development STP principal at ExxonMobil Technology & Engineering Co.—and Joe Kaperick—senior advisor of grease technology at Afton Chemical and past president of NLGI—studied the difference in performance between finished greases formulated with mineral oil and synthetic base oil. Specifically, they were concerned with different greases’ ability to pass certain tests of the GC-LB and HPM specs.
At the Society of Tribologists and Lubrication Engineers 2022 annual meeting in May, Salvi explained the greases used for the study were not optimized to meet every specification.
“When we put together this project, we didn’t have intention to develop a grease and say, ‘This grease is going to meet all the specific requirements,’” Salvi said. “We really wanted to build a guide for balance in formulation, and maybe this will help some future projects that we have to address each question about feedstock.”
The four greases tested included:
- A mineral oil-based grease
- A metallocene polyalphaolefin grease
- A metallocene polyalphaolefin grease with alkylated naphthalene
- A metallocene polyalphaolefin grease with alkylated naphthalene and an ethylene-propylene polymer
Each grease used a soap complex made by ExxonMobil, and the finished greases were prepared by Afton with an additive package of choice and base stock dilution as needed. The greases used a lithium complex thickener.
The first part of the study looked at NLGI Grade 1 greases using each of the base stocks listed. Then the study compared the finished greases made at both NLGI Grade 1 and Grade 2.
In a test of structural stability, mPAO-based greases generally had better structural stability.
The NLGI Grade 2 mineral oil grease and the grease using polymer both failed to meet the HPM limit in roll stability and worked penetration. “This is kind of unexpected,” Salvi said about the grease with polymer. “We would like to look at this further in the future.”
The mPAO grease generally has better resistance to water washout. Salvi noted the NLGI Grade 1 grease with alkylated naphthalene sans polymer slightly exceeded HPM limits on washout percent due to its polarity. The same grease prepared as an NLGI Grade 2 met the HPM requirements easily. The sample with both alkylated naphthalene and polymer stayed below the limit even at NLGI Grade 1, highlighting the positive contribution of the polymer to improving the mechanical stability of the grease.
For copper corrosion, all three mPAO greases stayed at the HPM limit of copper strip rating, though the base grease formulated with alkylated naphthalene was slightly above. The base grease formulated with mineral oil stayed at the limit, but the addition of additives pushed it beyond the HPM limit.
Each grease shined in the Four-Ball extreme pressure test. All four finished greases met the HPM floor for kilograms of weld load, with the mineral oil-based grease topping the others at 300 kg. Salvi explained this could be due to the additional sulfur coming from the mineral oil itself.
In the four-ball wear test, each finished grease met HPM standards. Here, the mineral oil formulation performance lagged behind the other three greases. Salvi concluded that additives need balance to give more consistent extreme pressure and wear performance.
The oxidative stability test flaunted the strengths of synthetic base stocks in grease formulations. The mineral oil grease had an oxidation induction time of about 70 minutes. All mPAO greases lasted at least two hours, with the alkylated naphthalene grease topping all others at around 130 minutes.
In high-temperature performance, the three mPAO greases outperformed the mineral oil grease in stability and FE9 bearing life.
Additionally, the mPAO alkylated naphthalene formulation of both NLGI grades was the only one to meet HPM standards for low-temperature mobility. Salvi said that formulators must balance alkylated naphthalene, consistency and polymer to meet desired low-temperature performance.
In low-temperature flow pressure, both grades of the mineral oil grease exceeded the HPM limit of 1,500 millibars, while the mPAO greases comfortably stayed under the limit, with the alkylated naphthalene formulation achieving the best results.
“The point you may want to consider for greases targeted for high temperatures is that if you want to go into the low-temperature space, you may need to consider something to address this problem,” Salvi said.
Salvi noted there are further steps to be taken. One is to look at how the difference between the two NLGI grades affects results and to assess manufacturing impact on NLGI Grade 2 grease samples. Another is finding a vital balance of polymer, alkylated naphthalene, NLGI grade and additives needed to achieve high- and low-temperature performance.
Will Beverina is assistant editor for Lubes’n’Greases. Contact him at Will@LubesnGreases.com