What Is The Chemical Makeup Of Grease

Key Concepts

• Soap is the principal thickener for 90% of all greases.
• Simple soaps have a metallic base of operations with a single fatty acid; complex soaps have a metallic base and multiple fatty acids as complexing agents.
• These soaps self-gather into micelles that allow a chemical compound that is commonly insoluble, such as lather, to deliquesce.

Grease was first used on chariot axles more 3000 years ago. Today more than than fourscore% of bearings are lubricated with grease. Lithium soap greases, the near prevalent, were introduced in the early 1940s. Lithium complex greases, introduced in the 1960s, are condign the most prevalent in Due north America.

A soap is, by definition, a metal salt of a fatty acid. The National Lubricating Grease Institute defines grease as, "a solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included (1)."

Grease making is a relatively simple time-temperature process: a ane-pot batch method. For a soap grease, fatty acids are added; if it is non-soap, the other constituents are put into base of operations oil. Mutual acids include the high-molecular-weight fatty acids, stearic acrid and 12-hydroxystearic acid and short-chain complexing acids such as tallow, azelaic acid and sebacic acid. Once the acrid gets up to temperature (i.eastward., the fat acid melts) the metal base of operations is added. The procedure is called saponification or soap making. So basically acid + base = soap + water (run across Figure 1).

Figure ane. Greasing-making process: basically acrid + base = soap + water. Courtesy: STLE

Then, because there needs to be very trivial water in lubricants, all the water is removed. Once that is done, the material is cooled and gelatinous—this is the point where the mixture becomes a grease. Next the mixture is adjusted for consistency by adding base oil (additives might be added, likewise). It may have to be reheated, recooled and tested several times to go the consistency that is required for the product. Well-nigh people call back grease is primarily thickener, but in actuality it is generally oil. Lather concentration in oil is typically 10%-20%.

Types of thickeners

The thickener defines the type of grease. There are iii or four different types of materials that get into thickeners. The focus in this article is on organic thickeners such equally lithium stearate, sodium dodecylsulfate and diurea. At that place are simple greases and complex greases, depending on the types of fatty acids used.

• Elementary soaps. The main thickener used in grease is a metal soap. These metals include lithium, aluminum, sodium and calcium.
• Complex soaps. Greases with complex soap thickeners are becoming more than popular considering of higher operating temperature and superior load-carrying abilities. Complex greases are fabricated by combining the metal soap with a complexing agent. The well-nigh widely used circuitous grease is lithium based, made with a conventional lithium soap and low-molecular-weight organic acid as the complexing agent.
• Non-soaps. Non-lather thickeners make sense in special applications such as bentonite clay for high temperatures where it does not melt (two).

Common thickeners include:

Soaps (comprising about 90% of all greases used)

• Lithium. Because lithium soaps are very efficient thickeners, lithium 12-hydroxystearate greases are the most prevalent. Lithium greases provide practiced lubricity and have swell shear stability, thermal resistance and relatively low oil separation. Antioxidants are added to better oxidative resistance (Run across Effigy 2).
• Calcium. These greases accept better water resistance than lithium greases. They besides have skilful shear stability. However, they have low-dropping points, do non have practiced operating temperature range and can only exist used in operating conditions up to 110 C (230 F).
• Sodium. These greases offer high-operating temperature, up to 175 C (347 F) only are confined to operating atmospheric condition no higher than 120 C (248 F) because of poor oxidative stability and high oil bleed. They also are not very water resistant. Yet, they exercise provide good lubricity and shear stability.
• Aluminum. These accept excellent oxidative resistance and good h2o resistance. Just they accept a low-dropping point of only 110-115 C (230-239 F). Their usage is by and large limited to operating weather condition less than 80 C (176 F). When these greases overheat in bearings, they cause sharp torque increases.

Figure two. Thickener fiber/micelle construction of ii grease compounds. Courtesy: STLE

Not-Soaps

• Urea. A polyurea thickener is a reaction production of a diisocyanate with monoamines and/or diamines. This class includes diurea, tetraurea, urea-urethane and others. The ratios of the ingredients make up one's mind the characteristics of the thickener. Since polyurea thickeners do not incorporate metallic elements, they are ashless and, thus, more than oxidatively stable.
• Organophilic dirt. These thickeners include minerals bentonite and hectorite. The minerals are purified to remove non-clay material—ground to the desired particle size—and chemically treated to brand the particles more than compatible with organic chemicals. Dirt thickeners take no defined melting point, and so they can exist used in high-temperature conditions.
• Other. Other non-soap greases include teflon, mica and silica gel.
• Calcium sulfonate. This is not strictly a soap but is a metal salt of a sulfonic acid detergent. These greases have a loftier operating temperature and good water resistance.

Micelles

Thickeners all self-get together into threadlike molecular structures, chosen micelles, that let a compound that is unremarkably insoluble to dissolve. Micelles form when soaps and detergents are added to water. The individual molecule has a strongly polar caput and a non-polar hydrocarbon chain tail. When this type of molecule is added to water, the non-polar tails aggregate into the center to grade a ball-like structure (micelle). This aggregation is due to Van der Waals Forces between the molecules (encounter Van der Waals Forces). Considering they are hydrophobic, the polar caput of the molecule faces outward for interaction with the water molecules on the outside of the micelle, while the tail faces inward. (See figure three)

Effigy 3. Micelle concept showing ionic head/non-polar tail formation. Not-polar tails aggregate into the middle to class the ball-like micelle structure. Courtesy: STLE

Van Der Waals forces

According to Wikipedia, Van der Waals Forces are relatively weak, short-range attractive forces that human action on neutral atoms and molecules and arise because of the electric polarization induced in each of the particles by the presence of other particles. These forces include attraction and repulsion between atoms, molecules and surfaces, every bit well as other intermolecular forces. The forces event from a transient shift in electron density. As the electrons orbit the nucleus inside an atom, the electron density may tend to shift more than to one side. This generates a transient accuse (i.east., an induced dipole) to which a nearby cantlet tin either be attracted or repelled. When the inter-atomic distance of two atoms is greater than 0.half dozen nm, the force is so weak that it is non strong enough to be observed. When the inter-atomic altitude is below 0.4 nm, the force becomes repulsive.

Since lubricants are not-polar environments, these micelles are contrary or changed micelles. The non-polar tails face up outward and the ionic head faces inward. And then information technology is the micelle structures in thickeners that permit the thickener to hold onto the lubricant (See effigy 4).

Figure 4. Reverse micelle concept. Since lubricants are not-polar environments, these micelles are reverse or inverse micelles. The non-polar tails face outward and the ionic caput faces inward. Courtesy: STLE

Darcy'southward constabulary and oil bleed

In 1856 Henry Darcy first studied and published work on flow through porous media. Darcy's law is an equation that describes the phenomenon. The law is based on the results of Darcy's experiments on the flow of water through sand beds. It is a simple mathematical argument that summarizes the following properties of water flow:

• If at that place is no pressure gradient over a distance, the conditions are hydrostatic (no menstruum occurs).
• If there is a pressure level slope, flow occurs from high pressure to depression pressure.
• The greater the pressure gradient, the greater the charge per unit of discharge.
• The discharge charge per unit of fluid will ofttimes exist unlike—through different formation materials or the same material in a different direction—even if the aforementioned pressure gradient exists in both cases (three).

Darcy'south law can be used to understand oil bleed—a term to depict how grease is released from the thickener for the purposes of lubrication. One of the primary criteria for selecting lubricating grease is its ability to bleed oil—which is dependent on the microstructure of the thickener.

There are a few ways to measure oil bleed. For the cone drain test, the grease is put in a cone where information technology resides for thirty hours at 100 C (212 F) before measuring how much oil has dripped. The log of cone bleed is a linear function of the log of the percent soap.

Grease aging

The aging of grease follows closely with the aging of lubricating oils with the improver of the thickener reactions. The thickener structure contributes to the crumbling of the lubricants through oxidation and hydrolysis. When the thickener oxidizes, the hydroxy group can be oxidized to a ketone then ultimately to an acid group severing the chain. Thickener hydrolysis occurs when soap thickeners hydrolyze to the fatty acid and the metal base. The electrostatic effects that keep the contrary micelles stable are lost in this reaction.

Reprinted with permission from Tribology & Lubrication Technology (TLT), the monthly magazine of the Social club of Tribologists and Lubrication Engineers (STLE), an international not-for-profit professional person society headquartered in Park Ridge, Ill. STLE is a CFE Media content partner.

Meet the presenter bio

Effigy 5. Paul Shiller, University of Akron. Courtesy: STLE

This feature was based on information independent in a webinar presented by Paul Shiller. Shiller received his doctorate in physical chemical science from Case Western Reserve Academy in Cleveland, studying the surface reactions at fuel cell electrodes. He holds a Master of Scientific discipline degree in chemic engineering also from Case Western Reserve University, where he studied the characteristics of diamond-like films. Shiller holds a Master of Science caste in chemistry studying the spectro-electrochemistry of surface reactions, and he received a Bachelor of Engineering degree in chemical applied science from Youngstown State University.

Shiller joined The Timken Co. as a product development specialist for lubricants and lubrication in 2004. He then became a tribological specialist with the Tribology Fundamentals Group at the Timken Applied science Center in Northward Canton, Ohio. In 2011 he moved to the Academy of Akron in an industrial innovation/collaboration endeavor every bit a research scientist. He is currently a research scientist at the academy working in the Civil Technology-Timken Engineered Surface Laboratory.

You lot can accomplish Shiller at jps70@uakron.edu.

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Source: https://www.plantengineering.com/articles/grease-chemistry-is-governed-by-thickener-structure/

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