Grease Technology - Part II

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MolaKule

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Grease Technology – Part II
by Molakule


General
We discussed in Part I that all lubricating greases, whether mineral or synthetic, are composed of a base fluid and additives (for lubrication), a thickening agent (to immobilize the fluid), and an optional tackifier. The concentration of thickener determines the consistency and general properties of the finished product. Additives can be any number of antiwear, extreme pressure, and anticorrosion chemistries.

Greases not only reduce friction and wear, but absorb shock and vibration, seal the bearings against water, serve as oil reservoirs, and release oil as conditions warrant.


Thickeners


Thickener types are Metallic Soap Thickeners, Complexed Metallic Soap Thickeners, and Non-Soap Thickeners. The Non-Soap thickeners are organo-clay, polyurea, and PTFE.

Since Metallic Soap Thickeners are rarely used, we will concentrate on the Complex metallic soaps and other thickeners.

Complex Metallic Soap Thickeners gives the grease a higher temperature boost with better oxidation and high drop points. We will discuss advantages and disadvantages of each grease below.

The Metallic Soap Thickeners are:

Aluminum Complex - Higher drop point than the noncomplex aluminum grease. If the grease is subjected to higher temperatures than designed, it can turn rubbery on cooling. Low temperature characteristics are fair to good. Shear stability is good to excellent. Water washout and some other tests show only good result, while water spray tests show excellent resistance. Often mixed with calcium for applications in pharmaceutical and food processing where USDA H-1 specifications are required.

Calcium Complex - This grease is touted as a general purpose grease for those not wanting a clay-based grease, but have been obsoleted by other types of high temperature, general purpose greases. This grease requires a major amount of thickener for proper consistency. Low temperature and water resistance operation is satisfactory. Sometimes used alone or with aluminum for applications in pharmaceutical and food processing where USDA H-1 specifications are required.

Barium Complex – a complex barium soap grease using acetic acid as a complexing agent. At one time it was a popular grease for factory wheel bearing packing applications since it was firm and stringy, or fibrous. Has since lost its appeal and is rarely used in aftermarket applications.

Lithium Complex – Lithium greases, especially the EP fortified greases, are the most popular high temperature, multipurpose greases today. They have some of the highest dropping points (350 F) and with good synthetic base oils, sustain 325 F temps with up to 450 F intermittent temperature peaks. They have good water resistance and excellent mechanical stability. Lithium greases can absorb up to 50% of its weight in water and continue to lubricate and protect metal surfaces from rust.

The Non-Soap thickeners are organo-clay, polyurea, PTFE, and silica gel.

The organo-clay greases requires thorough dispersion of the oil in the clay and require a polar dispersant to force the grease to gel. Organo-clay greases are used where temperatures hit 500 F, such as ovens, steel rolling mills, etc. Of course, the base oil in this case should be a synthetic oil of viscosity between ISO 220 and 680. A clay is usually a alkylaryl ammonium hectorate or equivalent. All clays have to have a surface treatment in order to act as an oil absorber, because they are normally hydrophillic. Clay thickened greases are less expensive to produce than the complex greases because they take less energy to heat and mix. Although clay-based greases are nonmelting, and are often recommended for high temperature applications, sustained high temperature operation is highly dependent on the viscosity and quality of the base oil, which implies mineral-synthetic blends or full synthetic base oils.

Polyurea Greases are made by mixing oils with suitable amines and isocyanates or diisocynanates. Polurea greases are also good high temperatures greases. These greases are NOT currently compatible with any other grease. They do perform well in low shear applications such as hot ball bearings, but not as well in high-shear applications such as roller bearings. Polyurea greases are being developed to be more compatible with other greases and to increase their water washout resistance.

PTFE (Teflon) micropowders do made a good grease thickener. PTFE can be classified as a universal gellant, and can be used with any synthetic or mineral oil, but excels with synthetic oils. It thickens because of its low surface tension, surface area, and dispersability in most organic fluids. Only 20 to 40% of thickener is needed for PTFE greases, so most of the grease package contains oil.

Sila gel is used as the thickening agent for high temperature greases which find application above 500 F to 1200 F and may contain copper, moly MoS2 powder, graphite, and titanium dioxide. The EP add is generally one of the high temperature phosphate esters. The base fluid is usually Polyethylene Glycol (PEG). Used mainly for furnaces and other extremely high temperature “refractory” applications.

Additives:

Additive packages are usually added to the base oil(s) before thickening, and are similar to Gear Lubricant oils. These may consist of various EP additives such as Moly disulfide powder, special forms of ZDDP, Phosphorus-Sulfur (S-P) compounds, and other organometallic EP additives such as SbDTC and Cerium DTC. Nonorganometallic EP and AW additives may include Borate esters or suspend borates and calcium carbonates. Friction reducers may be Molybdenum dithiocarbamates (MoTDC), esters of fatty acids, and synthetic base oils. Antioxidants may be ZDDP, MoTDC, and SbDTC. Rust inhibitors and metal deactivators are also included to reduce oxygen interaction with metals and to prevent rusting of ferrous surfaces.

Grease Classifications and Testing as Defined by the NLGI and ASTM.

Testing is according to ASTM testing protocols such as (not a comprehensive list):

ASTM D-217 – Unworked penetration. This test measures the consistency of grease prior to the input of mechanical energy. For pumping, this is what the customers pump is required to transfer.
ASTM D-217 – Worked penetration for 60 strokes. This test measures the mechanical stability of the grease.
ASTM D-2265 – This test determines the highest temperature attainable before a drop of oil separates from the grease.

There are about two dozen other tests made as well, and one can see these by looking at a manufacturer’s specifications sheet for the grease.

The grease consistency is classified according to the ASTM D-217 tests for worked penetration. From softer to harder greases:
NGLI
#000
#00
#0
#1
#2
#3
#4
#5
#6

Greases 000 to 00 are in reality “semi-fluid” greases, with only base oils and olefin copolymers or polyisobutylene thickeners and additives. The hardest of greases are the 4-6 greases with 6 being the hardest.

For most ball bearing applications in automotive use, NLGI 0 to 2 is used, with 1 being for moderate temperature applications, and #2 being the most popular.

Let’s make a Synthetic-Based Aluminum Complex Grease.

We assume that the batch size we are making is about 100 kg of grease. The thickener content will be about 7.5% or 7.5 kg of aluminum hydroxide, and benzoic and stearic acid will be 0.8% or 800 grams. The reaction of the aluminum hydroxide, with the benzoic and stearic acid, is done with a small portion of the synthetic oil (PAO in this case). The remaining oil and additives are added after the reaction takes place or when the reaction temperature is below 100 C. The reaction is “exothermic” which means that heat is given off, with a small amount of alcohol liberated as well. The viscosity of the base oil and the rate of cooling will determine the final consistency of the grease.

Dyes and fillers are added for cosmetic purposes and for identifying product lines.

A tackifier, composed of polyisobutylene or ethylene and propylene copolymers, are sometimes included to assist cling or increase the adhesiveness of the grease.
 
Those are interesting articles, MolaKule, thanks. I wonder if anyone has made a grease out of the really heavy SuperSyn PAOs sold by ExxonMobil? They make one that's 2800cSt @ 100C. That's practically grease without any thickeners.
 
Jay,
they make a Mobilith SHC1500, which is a lithium base, with 1500cst synthetic oil, and Mobilgear SHC6800 TM (8031cst @40C, and 357cst@100C)

Molakule, thanks as always for your articles.
 
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