Technology to Improve the Grease Making Process

By Gareth Fish, PhD CLS CLGS and Chris Hsu, PhD The Lubrizol Corporation, Wickliffe, Ohio, USA

Abstract

The process to manufacture grease is as important to grease properties as are the base oil, thickener, and the additives used to enhance performance. Grease producers manufacture greases using a wide variety of processes. This can be attributed to the availability of the basic raw materials for grease making and differences in the plant and equipment. Provided that the response of each variable is known, grease making is a controlled acid / base chemical reaction process. Grease makers strive to optimize their process for each thickener type based upon an understanding of the process variables.

This paper will discuss technologies that can enable grease producers to improve and enhance their grease making process. The technologies under discussion are additives to: improve the grease yield without compromising the general properties of the grease; reduce oil bleed at both storage and elevated temperatures; and enhance the dropping points of lithium soap thickened greases. Also discussed are methods to reliably manufacture high dropping point lithium complex greases in a variety of base oils, including vegetable oils and synthetic esters.

Introduction

The process to manufacture grease is as important to grease properties as are the base oil, thickener, and the additives used to enhance the performance properties (1). Grease producers manufacture greases using a wide variety of processes in a wide variety of equipment. This can be attributed to the availability of the basic raw materials for grease making and differences in the plant and equipment. According to the EGLI REACH Consortium (2) >30 types of lithium soap greases were registered by grease makers both within the European Union and by multi-national lubricant companies. The essentials of grease making have not changed since the days of Klemgard (3). One or more fatty acids are reacted with a slight excess of base or bases, to produce a salt or salts and water. The water is stripped out and the saponiἀcation reaction is ἀnished by the application of additional heat. For simple soaps it is typically desirable to fully melt the simple soap followed by recrystallization to achieve optimum properties, and for complex greases that the reaction is driven to or as close to completion as possible, as the more complete the reaction is the better the thickening and typically, the higher the dropping point.

Over the decades there have been numerous NLGI papers and presentations on how to improve the grease making process. Some focused on improved manufacturing plant and equipment, such as Graham, et. al. (4) and Krol (5); while other papers have covered improved processing techniques for lithium greases such as Polishuk (6) and more recently, Morgan et. al. (7).

One of the challenges outlined by Morgan was that it is diἀcult to control both 1-step and 2-step reactions to make high dropping point lithium complex greases.

Several papers have focused on components which can be added to the grease manufacturing process to improve the output. Nolan and Zeitz (8) outlined the use of micronized lithium dispersions to improve the reactivity of the lithium hydroxide and Lorimor (9) discussed dropping point enhancers. Both of these topics will be discussed and updated later in the paper.

What is clear from these papers and the authors’ own experiences is grease makers will optimize their process based on their own equipment. Grease makers strive to optimize their process for each thickener type based on understanding of the process variables. Provided that the response of each variable is known, grease making is a controlled acid / base chemical reaction process. It is known that from a manufacturing perspective, commodity charging of raw materials makes the process easier to manage. If consistent, high quality fatty and complexing acids are used, batch sizes can be defined around a whole number of bags of acid into the cooking vessel. High purity bases also need to be used along with a post-saponification titration to confirm the desired basicity has been achieved. If the acids and bases used are not so consistent, commodity charging typically results in a more variable grease output. Either from supplier certificates or their own laboratory analyses, the saponification numbers of the acids are used to make adjustments to the charging. Many grease makers use calculators to determine the amount of base needed to neutralize the added acids and this improves the consistency of grease batches.

Grease making equipment includes traditional open kettles, autoclaves and other pressurized reactors and continuous grease making units. For each of these types of manufacturing, individual recipes are needed as well as process control plans and procedures to ensure high quality and consistent output.

The largest volume of grease thickeners sold globally are lithium soaps with around 58% based on simple soaps and lithium complexes at around 19% of the global market (10). This is a significant simplification of the actual regional volumes, and to understand the need for improved technology these volumes need to be looked at regionally. In the mature market of North America, lithium complex makes up almost 40% compared to 28% for simple lithium soaps. By contrast, in India, simple lithium soaps dominate the market approaching 85% with complexes only at around 7%. As a developing market, the Chinese market consists of 70% simple lithium and 15% lithium complex thickeners. Europe and Japan match the global average of simple lithium soap greases but manufacture lower percentages of lithium complex greases. In Europe, the difference is made up largely by a higher market share of aluminum complex greases, and in Japan the majority of high temperature greases utilize urea thickeners.

Reviewing regional volumes it is clear that in North America, the growing volume is lithium complex with simple lithium embracing the commodity market. There is a perceived need to improve the properties of lithium complex greases in the following areas: water resistance; oxidation stability and longer grease life; and improved bleed. Oxidation stability and life improvements have been previously presented (11) and polymer technologies are available to improve water resistance (12). In addition to these properties, dropping points above 280 °C are now common with some customers requesting thickener systems that have consistently high dropping points above 300 °C, trying to match the behavior of calcium sulfonate greases. High quality, high temperature thickeners in sensitive base oils are also required for the growing demand in specialty fluids and for bio-based greases.

Yield improver technologies offer a means to take cost out of simple lithium grease production, by increasing volume manufactured at a given consistency per unit of soap. For markets such as India or China, technologies to improve simple lithium greases are more important today. For a few specialized applications, improved lithium complex greases are also desired.

This paper will discuss technologies that can enable grease producers to improve and enhance their grease making process. For simple lithium greases,

saponiἀcation aides are outlined which improve grease yield without compromising the general properties of the grease.

Additives to control bleed are also discussed. It was previously shown (12) that the incorporation of polymers into grease could reduce both storage bleed and that which occurred in service up to 100 °C. One of the issues with using polymers is that above 100 °C most of the polymers employed start to lose their effectiveness and other solutions are needed.

Technology to improve the dropping point of simple lithium soap greases was previously outlined by Lorimor (9). Two different approaches were illustrated which worked in slightly different ways to enhance the dropping points of lithium soap thickened greases. One ongoing issue is it can be a challenge to consistently manufacture high dropping point lithium complex greases. Both one step and two step processes are used. The one step process can be difficult to control, with batches often giving good yields but with scatter in the resulting dropping points. The two step process reliably will produce dropping points of 260 to 280 °C, but based on current targets of 280 to > 300 °C may not be good enough, and other methods to reliably manufacture high dropping point lithium complex greases in a variety of base oils provide an alternative.

Yield Improvers

Based on the increasing commoditization of lithium greases, cost becomes a very important consideration in the grease making process. One of the features sought by grease producers is the lowest possible soap content to achieve the desired consistency grade. It has been well documented that greases manufactured using naphthenic base oils have lower soap contents compared to those manufactured using paraffinic oils (1), but typically grease quality naphthenic oils are higher cost than paraffinic oils of the same viscosity class. Supply of naphthenic oils is also an issue with only a limited number of suppliers and in some developing markets, quality naphthenic oils are difficult to obtain.

Improved yields can also be obtained by slow cooling from top temperature to below 170 °C. This creates large ἀbers that thicken the oil better but give higher oil bleed and worse shear and roll stability, compared to grease that has undergone a quench or partial quench.

Grease yield improvers are shown to reduce the amount of soap necessary to achieve a penetration target when using purely paraffinic oils. According to the NLGI Grease Guide (1), yield is deἀned as “The amount of grease (of a given consistency) that can be produced from a speciἀc amount of thickening agent.” To the nongrease chemistry professional, this deἀnition is slightly difficult to comprehend. An explanation is given in table 1, with two greases with the same penetration, but different soap contents. Grease B has a 15.4% better yield than grease A.

The basic target was to reduce the amount of soap with an additive or component added at a low treat level, which would have a net treat cost lower than adding additional soap. Reducing the soap content should also improve the pumpability of the grease but give similar shear and roll stability. It was also desired to have no effect on oil bleed and no negative effects on all other grease properties. Based on current grease and patent literature, there are two routes to improving yield. The ἀrst involves the use of polymers to increase the unworked and worked penetration of the grease and the second incorporates structure modiἀers into the soap matrix.

The ἀrst approach was to examine commercially available polymers sold as yield improver additives. These were incorporated into lithium soap greases at the suggested treat rates and tested. The results for the various polymers in group II oil are included in table 2. Based on the data in table 2, polymers can improve grease yield based on reducing the soap content to achieve the same level of worked penetration. Following a review of the data obtained they did not appear to be the complete answer as the cost position was not signiἀcantly improved over the grease without yield improver and some of the “no harms” data collected suggested that the solution may have not been robust.

Looking at general processing for greases, it was thought that better milling could be investigated as part of the study. Three different milling techniques: triple roller with adjustable speeds and gaps, a colloidal mill and a high pressure homogenizer were available. Two slightly different lithium complex soap greases were milled using all three techniques and the milling quality compared in table 3.

Results from analyzing the data suggested that better milling and dispersing could bring improvements to the yield. From this it was thought that adding a dispersant to the grease would improve the milling process. Several commercial dispersants were tried at the same treat rate and all caused signiἀcant softening of the grease compared to grease without dispersant. Some examples are shown in table 4.

From this, a few other compounds were tested, most of which did not work, but one candidate showed some potential in initial trials. To further explore this, a series of four simple lithium 12-hydroxystearate greases were made up in API group I paraffinic base oil, using an identical saponiἀcation process but with different soap contents, ranging from 5.6 to 9.3%. These samples are plotted in ἀgure 1, showing that for a similarly controlled saponiἀcation in the same base oil, soap content follows worked penetration. Two greases were then made up using the new yield improver treated at 1%. The ἀrst grease had a soap content of 5.5% but gave a worked penetration of 285 and the second had a soap content of 7.5% with a worked penetration of 252. This data is also plotted on ἀgure 1. There are two ways of looking at the data. The grease with 5.5% thickener and yield improver reduced the penetration by approximately 30 points or the soap content by 2% and the grease with 7.5% thickener and yield improver reduced the penetration by approximately 30 points or the soap content by 2%.

Following on from this three series of greases were made up. The ἀrst repeated the initial experiments in API group I oil; the second series investigated the behavior in group II oil; and the ἀnal series looked at group III oils. Testing was carried out on the greases made with and without the yield improver in the three families of oils to check for negative influences. This is typically called “No harms” testing. The test data for the group I base oil (110 mm2/s at 40 °C) grease is in Table 5, group II oil (115 mm2/s at 40 °C) in table 6, and group III oil (8 mm2/s at 100 °C) results are in table 7.

Two larger scale kettle batches were manufactured in a pilot kettle. They used the same Group I base oil as the greases reported in table 4 and identical 7.54% soap. The grease with the yield improver was 28 penetration points stiffer than without it. Scanning electron micrographs of the grease structures were taken to see if there was any difference between the structures of the grease with and without the yield improver. As seen in ἀgure 2, the grease with the yield improver has a much ἀner structure and is better dispersed than the grease sample without the yield improver.

Reviewing all the properties tested, work has shown that by adding a small amount of yield improver additive to simple lithium soap greases, the amount of soap necessary to get the same consistency is reduced without negatively impacting the existing physical properties of the grease. As summarized in table 8 below, the major effects were on yield and this, in turn, improved bleed and washout. The API group I based grease had the best improvement in properties. In the group III based grease, the effects were light to not signiἀcant.

Bleed reducing additives

It had been reported that styrene-diene polymers (11), such as styrene-butadiene (SBR) and styreneisoprene (SIP) showed improvement in reducing bleed in lithium soap thickeners in API group I paraffinic oil, compared to other typical oleἀn co-polymers (OCP) or polyisobutylene (PIB) tackiἀers. The OCP materials are typically marketed as base oil fortiἀers, bleed improvers and components capable of delivering enhanced water resistance. A simple lithium soap base grease with a paraffinic base oil having a viscosity of 120 mm2/s at 40 °C was used to examine the performance of the polymers.

As can be seen from Table 9, the different polymers gave varying results in the three tests with the PIB giving the lowest bleed but the SIP giving the best overall performance. However at temperatures above 120°C, these polymer bleed reduction and control additives did not work and other solutions are needed. Waynick (13) described the use of both organic and inorganic boron containing additives for reducing higher temperature bleed at temperatures up to 175 °C in urea thickened greases. This work was built on by Lorimor (9), who showed that borate additives also work in lithium greases to reduce bleed at higher temperatures. The base grease used in this study was an ISO VG 150 paraffinic base oil blend.

Two new polymers were obtained, which claimed better bleed reduction properties than the original SIP investigated, and incorporated into a variety of different base greases at different treat rates. The group II oil base grease had an oil viscosity of 12 mm2/s at 100 °C, the group II 8 mm2/s at 100 °C. The greases were tested for ASTM D6184 bleed at 150 °C. The results of these tests are shown in table 11.

The data for the two new polymer additives shows clearly that polymer solutions are now available to reduce bleed at temperatures up to 150 °C in group II oils and PAO 6, but nothing is very effective in API group III oils. When initially tested in the group III oils, it was thought that the effect of low viscosity caused the very high bleed, but the lower viscosity PAO 6 gave better bleed characteristics.

Dropping point enhancers

Dropping point enhancers are used in one of two ways. They can be applied to simple lithium soap greases to increase the dropping point by up to 100 °C or they can be added to a “failed” low dropping point lithium complex grease to restore the dropping point. Traditionally, low molecular weight borate esters were used, but other components to do this are now available. Lorimor (9) described and reviewed various boron chemistries that are available. Based on the data presented, borate esters are the most effective way to deliver dropping point enhancement. However, borate esters are not without issue. Hydrolytic stability can be an issue and they can release alcohols and boric acid, which is an environmental concern in some markets. The alcohols give rise to subjective odors, which may be undesirable. A new odorless borate additive has now been developed that also has enhanced hydrolytic stability and responds similarly to the incorporation of zinc dithiophosphate. Comparative data for this and the other borates is summarized in table 12.

Lithium hydroxide dispersions

Many grease end-users now specify dropping points for lithium complex greases above 280 °C. This target is difficult to achieve with conventional processes. It was previously demonstrated that by using anhydrous micronized dispersions of lithium hydroxide in oil, the grease making process can be improved (8). Further work has shown that consistently high dropping points above 300 °C can be achieved in conventional base fluids using this technology to make complex 12-hydroxystearate soaps, with both azelaic and sebacic acids as the complexing agent. A series of greases were made using an ISO VG 220 group I paraffinic oil blend test in a laboratory open kettle system. The target alkalinity was 0.05 to 0.06% LiOH and the recipes were designed to make 1 kg of grease.

A lithium complex grease using a 1-step process with lithium hydroxide monohydrate powder provided inconsistent results compared to making the grease using the same base oil and acids with the lithium dispersion. The data summarized in table 13 shows that the complex grease made using the lithium hydroxide dispersion gave higher dropping points than the complex grease made using lithium hydroxide monohydrate powder and a 2-step process. This outcome provides the grease manufacturer with a tool to improve the dropping point consistency of lithium complex greases and potentially extract greater value from the complex grease process.

Sensitive base oils

Historically, it was only possible to use pre-formed simple calcium or lithium soaps to thicken water sensitive base oils. Preformed calcium anhydrous (12-hydroxystearate) soaps will give grease dropping points of 140 – 150 °C and those of pre-formed lithium stearate or 12-hydroxystearate will give grease dropping points of 180 to 200 °C, but these ranges are somewhat dependent on the base oils, especially if exotic fluids with signiἀcantly different polarities and solvencies compared to mineral oils are used. At the NLGI Annual Meeting in 2000, Polishuk (6) revisited using preformed soaps and later Bessette (14) outlined how to use preformed soaps “Dry Technology” to make greases with preformed simple thickeners in a variety of different base oils.

Honary (15) outlined several manufacturing techniques methods that could be used to develop greases in temperature and water sensitive vegetable oils, including the use of preformed soaps, soap concentrates, lithium hydroxide dispersions and heating by microwave technology. Honary explored the various issues with the different ways of making grease in sensitive base oils. Preformed soaps still have to be heated to melt the soap and are not readily available as complexes. Soap concentrates and the lithium hydroxide dispersions worked well but introduced mineral oils or nonbiodegradable synthetic fluids into the grease. The microwave heating technology was reported as being able to drive the saponiἀcation reaction quickly and heat the grease up to melting temperatures without degradation. At the time, microwave heating technology was chosen but later work showed that there were issues with the microwave heating technology (16). Further development of the anhydrous lithium hydroxide dispersion technology showed that it could make high quality lithium complex greases directly in water sensitive base oils (17), and that the only viable option to make high quality greases in sensitive base oils was to use anhydrous lithium dispersions.

As part of the development of the technology, a series of sensitive base oils was selected to see if the lithium dispersion technology was capable of making high quality lithium thickeners directly in the base fluid. The properties of the ἀve base fluids chosen soybean oil, canola oil, high oleic sunflower oil, estolide and synthetic diester are compared in table 14 along with those of a 600N API group I mineral oil, which was used as the control.

The anhydrous lithium hydroxide dispersion technology, as outlined by Nolan and Zeitz (8), is ideally suited to make grease in water and thermally sensitive base oils. The small particle size (<10 µm) allows the lithium hydroxide to react very quickly with the two acids. The only water present is from the saponiἀcation reaction and it is easily removed with minimal hydrolysis of the base oil. The process is also much quicker than conventional lithium complex grease manufacturing. In normal or traditional manufacturing, as the lithium complex structure is formed, the thickener traps unreacted alkali and acids components in the matrix and prevents complete reaction. The micronized lithium hydroxide dispersion technology involves small, more reactive particles of lithium hydroxide in a homogeneous reaction phase. As a result, a more complete saponiἀcation of lithium complex bases is observed. The net result of both of these effects is a high quality complex thickener. The absence of water normally used as co-medium of the soap forming mixture minimizes potential of hydrolysis of base fluid ester function and eliminates foaming associated with long water removal.

Reviewing the European LuSC (17), lithium 12-hydroxystearate is useable as a greases thickener for bio-greases as is dilithium sebacate, up to a prescribed limit. Dilithium azelate can be used to thicken sensitive and bio-based oils, but it is listed as having aquatic toxicity and is not useable in Ecolabel greases and therefore vessel general permit (VGP) greases. From this it is clear that making a grease with 12-hydroxystearic acid and sebacic acid below the permitted limit could potentially be used to make high dropping point Ecolabel and VGP greases.

Using the lithium hydroxide dispersion route, base greases were manufactured using the base oils listed in table 14. The thickener was either simple lithium or lithium complex utilizing a 3:1 12-hydroxystearic acid to sebacic acid ratio. The greases were made slightly stiff so that they could be cut back to an NLGI grade 2 using the corresponding base fluid. The properties of the base greases are included in table 15.

This paper has shown that there are now several technologies available to help the grease producer manufacture cost effective lithium soap greases and high quality lithium complex greases. The key technologies outlined are: yield improvers for simple lithium greases; bleed improvers for both simple and lithium complex greases in a variety of different base oils; and enhancers which can be used to improve the thermal stability and dropping points of simple lithium greases

This paper also demonstrates that lithium complex lubricating greases that utilize biobased synthetic esters as base oils can be readily produced. The technology has resulted in the development of environmentally considerate higher performance lubricating greases.

Acknowledgements

The authors wish to acknowledge many co-workers and departments within The Lubrizol Corporation for their contributions to this work.

References

(1) NLGI Grease Guide, 5th Edition, NLGI 2006

(2) ELGI Reach consortium presentation, 18th ELGI Annual Meeting 2006, Prague, Czech Republic

(3) Klemgard, E.N., “Lubricating Greases”, (1927) The Chemical Catalog Company, Inc.

(4) Graham, D.S., Masters, K. and Scott, “Grease Manufacturing Methods”, NLGI Spokesman (1992) Volume 56 page 363

(5) Krol, R., “Planning, Design and Construction of Specialty Lubricants Plant”, NLGI Spokesman (1993) Volume 57, page 490

(6) Polishuk, A. T., “Lubricating Greases from Preformed Soaps” NLGI Spokesman (2001) Volume 65 (1) pages 12-15

(7) Morgan, D., Kay, J.S. and Coe, C., “Critical Variables in Lithium Complex Grease Manufacturing” 80th NLGI Annual General Meeting paper #1313, Tucson, Arizona, 2013 (9) Lorimor, J.J., “Improving the Heat Resistance of Simple Lithium Soaps Using Borated Additives”, 77th NLGI Annual Meeting, Bonita Springs, FL, 14th June 2010

(10) Grease Production Survey Report 2012, NLGI, Kansas City, Missouri 64112 (www.NLGI.org)

(11) Ward, Jr., W. C., and Fish, G., “Development of Greases with Extended Grease and Bearing Life Using Pressure Differential Scanning Calorimetry and Wheel Bearing Life Testing.” NLGI Spokesman (2010) Volume 74(5) pages1427

(12) Ward, Jr., W. C., and Qureshi, F.S. “Influence of components blended to a target base oil viscosity on liquid phase and lithium grease properties” NLGI Spokesman (2009) Volume 74 (1) pages 21-31

(13) Waynick, J.A. “Polyurea Grease with Reduced Oil Separation”, US Patent 4,759,859, Jul. 26, 1988, USPTO

(14) Bessette, P.A., “Manufacturing Grease Using Dry Technology”, NLGI Spokesman (2002) Volume 65 (11) pages 14-17

(15) Honary, L; “Market Opportunities in Biobased Lubricating Greases”, 76th NLGI Annual Meeting Address 2009, Loews Ventana Canyon, Tucson, AZ, June 15th 2009

(16) Honary, L; “An Update on the Use of Microwaves in Manufacturing Grease” paper #1302 80th NLGI Annual Meeting Loews Ventana Canyon, Tucson, AZ, June 16th 2013

(17) Fish, G., Robinson. P, and McSkimming, N. “Understanding Component Requirements for Formulating High Performance Environmentally Acceptable Greases” ASTM symposium on Environmentally Considerate Lubricants, December 9, 2013, Tampa, FL