A slew of standard lab tests and manufacturers specs can be used to compare and evaluate greases. But all too often, bearings fail when rain, snow, seawater or water based industrial processing fluids contaminate grease.
What happens to water that seeps or condenses in bearings? Do all greases respond the same way to water? Which characteristics of base stocks, thickeners and additives influence how water affects grease consistency, stability and other performance properties?
New findings about water contamination in greases were the subjects of three presentations at the annual meeting of the Society of Tribologists and Lubrication Engineers in May. At the gathering in Atlanta, Georgia, speakers introduced new work on rust inhibitors for greases; unraveled how base oils and thickeners influence base grease response to water; and proposed a fresh perspective on developing rheology guidelines for grease selection.
Can Borates Prevent Rust?
Joseph Kaperick of Afton Chemical in Richmond, Virginia, presented his results on grease formulation and bearing corrosion. In prior work, he recounted, roller bearings were lubricated with simple lithium greases, exposed to water, and stored in plastic jars in a heated oven for 48 hours, per ASTM D1743 (Standard Test Method for Determining Corrosion Preventive Properties of Lubricating Greases). After 48 hours at 52 degrees C (126.5 F), the bearings were cleaned and races were rated (-) for the presence of any rust spot greater than 1.0 millimeter diameter or (+) for no spots. In order to pass D1743, there must be two or three (+) ratings for three replicates.
In these tests, alkaline grease (having excess lithium hydroxide, LiOH) was found to be inherently more corrosive than acidic grease (excess 12-hydroxystearic acid) for three replicates. Unreacted LiOH appeared to interfere with corrosion inhibition by some additive packages, and the greases failed-but greases with other additive chemistries passed.
Continuing this work, Kaperick explored several modifications of D1743. First, the percent of rusted bearing surface area was estimated to compare test greases with regard to the magnitude of failure. Then, the deionized water (DI) specified in D1743 was replaced with synthetic seawater at three concentrations (5, 50 and 100 percent) in accordance with ASTM D5969. Additional tests were then conducted for 24 and 48 hours of immersion.
For a formulated grease, Kaperick observed significantly more surface rust with seawater than the DI, as expected. However, he did not observe clear trends for surface rust with respect to seawater concentration; instead, he saw similar results for the 50 and 100 percent concentrations after 24 and 48 hours. This indicated a plateau was reached for rust formation under these conditions.
Kaperick then used D1743 with DI to test three, simple lithium NLGI grade 2 base greases with different levels of alkalinity (excess LiOH). Average results for three replicates (rated at 30, 35 and 30 percent surface rust) were comparable for three alkalinity levels (0.066, 0.057 and 0.031 weight-percent LiOH), respectively.
Next, base greases were additized with zinc dithiophosphate, antioxidants, sources of sulfur, rust inhibitors (polyester succinimide, succinate ester), and experimental borate additives. In D1743 with DI, results for these two organic rust inhibitors by themselves in base greases A, B and C were 18, 43 and 17 percent rust, versus 20, 25 and 5 percent, respectively, in the fully formulated greases, which showed significant improvement in greases B and C.
Kaperick concluded that ZDDP and sulfur sources improved performance of these two organic rust inhibitors. But why? Was there synergy between these succinates and ZDDP and/or sulfur? Or did ZDDP and/or sulfur inhibit corrosion by reacting with excess LiOH or other corrosive species, thereby allowing these corrosion inhibitors to fulfill their intended purpose?
To investigate, Kaperick formulated the additives in unthickened base stock and observed no rust. The organic rust inhibitors completely prevented corrosion in base oil alone. After further work, he concluded that simple lithium thickener (and not excess LiOH) reduced the effectiveness of the two rust inhibitors.
Kaperick then added LiOH to DI in the corrosion test and found that LiOH did not cause corrosion or have a negative effect on these rust inhibitors. However, Kaperick warned the audience against misinterpreting this result. LiOH is often used to control pH and prevent corrosion in water treatment and reactor facilities. However, his grease study seemed to show that excess LiOH did not have a direct impact on corrosion prevention. The rust inhibitors probably interacted with the lithium grease thickener, which reduced their effectiveness in some way. He felt that alkalinity may affect these interactions and rust inhibitor performance.
Lastly, Kaperick discussed two borate amides and a borated dispersant. These compounds are often used to improve high temperature stability of greases, although the borate amides are sometimes used as corrosion inhibitors in water based applications. When blended into a fully formulated grease, all three borates (at 0.67 to 2 wt.-percent) had positive effects on rust inhibition. They also increased the dropping point from 203 C to 299-305 C; reduced fretting wear from 13.1 milligrams to 6.3-8.4 mg; and increased 4-ball weld load from 210 kilograms to 230-260 kg.
Did these borates directly impact grease performance, or did they have an indirect effect, possibly activating other additives or interacting with thickener and allowing other additives to interact with bearing steel surfaces? Further experiments will explore these and other questions, Kaperick said.
Base Greases: All Wet?
From Nol, Sweden, Roland Ardai of Axel Christiernsson presented a paper co-written with his colleague Johanna Larsson that focused on effects of water contamination on environmentally acceptable lubricants. Governments and some international organizations require use of EALs in marine, forestry, earth-moving, agricultural and other equipment operating in environmentally sensitive locations. In these applications, there is significant potential for water contamination of greases.
Ardai reminded listeners that greases must meet biodegradation, bioaccumulation, aquatic toxicity, composition and other specifications in order to be identified with an Ecolabel as environmentally preferable. Ardai tested four NLGI 2 base greases acceptable for use in products compliant with the European Union Ecolabel. Two base stocks, TMP (trimethylolpropane) ester and rapeseed (canola) oil, were formulated with simple lithium thickener and anhydrous calcium thickener. When 50/50 oil/deionized water mixtures were shaken manually to emulsify and then allowed to stand, the vegetable oil and DI separated within five minutes, while the TMP ester/DI emulsion persisted over 24 hours.
Using an 1800 rpm mixer, the DI was blended into grease. Ardai observed that it was easier-took less time-to make homogeneous mixtures of DI in lithium greases than in anhydrous calcium greases. Each sample was deaerated, and free water was decanted prior to testing.
Magnetic resonance imaging of thin slices (190 microns thick) of grease revealed that water was more finely and homogeneously distributed in lithium than in anhydrous calcium greases. On an even smaller scale, diffusion nuclear magnetic resonance data (using a Bruker spectrometer) showed that the organization of water molecules was quite similar for the two thickeners. In the ester, however, water molecules were more restricted in their movement-more tightly bound to the thickener or inside micelles-than in rapeseed oil (seen at left).
Ardais cone penetration (ISO 2137) data showed that 5 wt.-percent DI significantly softened three of the four greases: Li-Vegetable (8 percent), Li-Ester (5 percent) and Ca-Vegetable (14 percent), versus Ca-Ester (1 percent). For wet and dry greases, anhydrous calcium greases had better mechanical stability than lithium greases, that is, much smaller increase in cone penetration after a roll stability test (D1831) that simulated rolling motion inside bearings.
Instrumental data confirmed that the Ca-Ester grease had the best mechanical stability after SRS testing, which employed a rheometer from Anton Parr GmbH to measure flow curves of viscosity versus shear rate.
Ardai also reported that all four greases gave comparable performance in standard water resistance and water spray-off tests.
Flow pressure tests were used to compare grease pumpability at low temperatures. Ardai measured the pressure to push an amount of grease through a specific test nozzle at -10, -20, -30 and -40 C. Vegetable oil formulations failed (flow pressure exceeded 1400 millibar) for both thickeners below -20 C. Water (5 percent DI) was found to reduce flow pressures for rapeseed greases but increase flow pressures for TMP ester greases.
Ardai explained that both base oils and thickeners influenced uptake and distribution of water in these greases. The Ca-Ester grease was the best of these four base greases in terms of ability to maintain structure and mechanical stability. Anhydrous calcium thickener was better (showed greater resistance to incorporation of water) than lithium thickener in this study. And TMP ester base oil was better (formed more stable emulsions) than rapeseed oil. The authors concluded that the combination Ca-Ester showed the most promise for formulating EAL greases with good water resistance.
Fresh Take on Grease Selection
Piet Lugt of SKF Research & Technology in Nieuwegein, Netherlands, examined the impact of water on grease lubrication in rolling bearings. Also a professor at the University of Twente, Lugt studied effects of water on grease rheology and discussed how to develop guidelines for selecting greases subject to water contamination in applications such as steel, food and paper manufacturing.
Lugts first task was to build a grease worker from a round, 24 mm diameter plate, with eight holes (2.5 mm diameter) attached to a vertical piston inside a cylinder with a 30 mm stroke length. Samples of six greases, dry and wet (mixed with DI), were worked 1,000 strokes in this apparatus prior to testing.
To measure the maximum amount of water that each grease could absorb (saturation point), Lugt added DI and worked the grease until droplets of free water were observed with the unaided eye. The six greases, shown below, absorbed relatively large volumes of water due primarily to the polar nature of their thickeners. Two greases, formulated with calcium sulfonate complex thickeners, absorbed enough water to almost double their weight!
Steady-state (flow curves) and dynamic (oscillatory strain curves) rheology tests were carried out at four temperatures (10, 25, 50 and 75 C) with an MCR 501 parallel plate rheometer (Anton Parr). Water caused calcium sulfonate complex and lithium complex greases to stiffen, simple lithium grease to soften, and polyurea grease to soften at lower temperatures but stiffen at higher temperatures.
Lugt also studied the effects of water on low-temperature grease properties. He reminded the audience that start-up torque is often used to measure the low temperature limit of a grease. As temperature decreases, start-up torque increases gradually, reaches a break point, and then increases sharply. At its low temperature limit, the start-up torque of a grease exceeds a specified value in a standard test. It is unwise to operate a bearing below this limit due to the risk of hindered motion of rolling elements, skidding and damage to the bearing.
In his study, Lugt used the rheometer to measure start-up torques for axially loaded contact ball bearings (IP 186) at temperatures between -30 and 50 C. The rheometer was also used to measure the yield stress of grease sandwiched between two parallel plates. There were similar temperature dependence and break points for yield point and start-up torque for each grease. However, the break points for grease start-up torque were lower than those for the yield stress. Breakpoints were related to oil pour point, or temperature where oil loses its fluid-like characteristics.
Yield point, Lugt told the Atlanta audience, gives useful information about the behavior of a grease at low temperatures and can be used as a guideline for grease selection. He noted that in this study, water had relatively little effect on start-up torque.
In the final portion of his presentation, Lugt presented film thickness measurements for bearings under starved and fully flooded lubrication conditions. Film thickness measurements for a single contact were performed with a ball-on-flat-disk test apparatus with optical interferometry, from PCS Instruments. The film thickness measurements in a full bearing were done with a bearing test rig developed at the University of Twente.
Although these six greases absorbed large amounts of water, he observed that water was squeezed out of the contact under fully flooded conditions (grease supplied continuously to the contact). Water contamination had little effect on film thickness under these conditions.
Under starved conditions, grease forms a reservoir outside the contact zone and gradually bleeds or releases oil into the contact. In this study, Lugt observed small differences in film thickness related to oil bleed. Oil bleed increased and films were thicker in the presence of water than in its absence for simple lithium, lithium complex and polyurea grease. In contrast, water contamination slightly reduced oil bleed and film thickness for calcium sulfonate complex greases.
Water affected the formation of grease reservoirs and lubricating films under these realistic starved conditions. These laboratory results help to explain bearing failures in applications where water contaminates greases.
Mary Moon, Ph.D., is a chemist with hands-on R&D and management experience formulating, testing and manufacturing lubricating oils and greases and specialty chemicals. She is skilled in industrial applications of tribology, electrochemistry and spectroscopy. Contact: firstname.lastname@example.org or (267) 567-7234.