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Greasy Times - Part 2

The oils that go into greases.

Courtesy of the Lubrizol Corporation

Click on pics to view larger images


Base Oils

The base stocks used to formulate lubricants are normally of mineral (petroleum) or synthetic origin, although vegetable oils may be used for specialized applications. Synthetics can be made from petroleum or vegetable-oil feedstocks and are tailored for the job they are expected to do.

Lubricating oils are the major components in grease formulations and, as such, exert considerable influence on behaviour of the grease. In formulating a grease, a base-oil viscosity is usually chosen that is similar to that which would normally be chosen if the equipment were oil lubricated. For example, a light, neutral oil might be chosen to formulate a grease lubricant appropriate for a high-speed, lightly-loaded bearing. On the other hand, slow moving, heavily loaded equipment would call for a high-viscosity oil.

To prepare a grease of a given NLGI grade, the relative amounts of oil and thickener must be adjusted according to base-oil viscosity and thickener composition. Most common lithium and calcium greases formulated to NLGI #2 grade contain between 85 and 95% oil. Poor low-temperature properties of a base oil do not always define the behaviour of the finished grease because certain thickeners may themselves function as pour-point depressants. Elastomer-seal compatibility may be influenced by the type of base oil present. Paraffinic oils exert a minimal effect while naphthenic oils may cause some seal materials to swell.

Mineral Oils

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Mineral stocks are produced by a number of processes depending on the crude oil being refined. For this reason, the choice of crude is important. Most favoured are paraffinic crudes, which give good yield of high-VI (HVI) stocks, although they may contain a lot of wax. For certain applications, naphthenic crudes are preferred because they yield high-quality, medium-VI (MVI) and low-VI (LVI) stocks with very little wax and naturally low pour points.

The viscosity of a finished stock is determined by the boiling range of its components. Most refiners settle for three or four stocks from which they blend their range of finished oils. For solvent extracted HVI oils, VI in the range of 90 to 100 is usual. An alternative refining process, which substitutes deep hydrogen treatment for solvent extraction, can yield VI of over 100. An additional advantage of this approach is that such processes can increase the yield of HVI components from almost any crude.

Stock

Specific Gravity at 60 degrees F (15.5 degrees C)

Sulfur (% wt)

VI

Kinematic Viscosity (cSt)

Pour Point (degrees C)

Flash Point (degrees C)

        40 degrees C 100 degrees C    
90 Neutral 0.860 0.005 92 17.40 3.68 -15 190
100 Neutral 0.860 0.065 101 20.29 4.11 -13 192
200 Neutral 0.872 0.096 99 40.74 6.23 -20 226
350 Neutral 0.877 0.126 97 65.59 8.39 -18 252
650 Neutral 0.822 0.155 96 117.90 12.4 -18 272
150 Bright Stock 0.895 0.263 95 438.00 29.46 -18 302

Not all base oils have similar physical or chemical properties or provide equivalent performance. The API has established base oil categories to provide guidance to users when different base oils are used interchangeably.

Base Oil Category Sulphur (%) Saturates (%) Viscosity Index
Group I > 0.03 and/or < 90 80 to 120
Group II < 0.03 and > 90 80 to 120
Group III < 0.03 and > 90 > 120
Group IV

All polyalphaolefins (PAOs)

Group V

All others not included in Groups I, II, III or IV

Synthetic Oils

Synthetic fluids are becoming increasingly important in greases designed for special extreme-temperature applications. These include polyalphaolefins, diesters, polyglycols and halogenated ethers and hydrocarbons:

Type Principal Applications
Olefin Oligomers (PAOs) Automotive and Industrial
Dibasic Acid Esters Aircraft and Automotive
Polyol Esters Aircraft and Automotive
Alkylated Aromatics Automotive and Industrial
Polyalkylene Glycols Industrial
Phosphate Esters Industrial

With the exception of polyglycol fluids, all have viscosities in the range of the lighter HVI Neutral mineral oils. Their viscosity indexes and flash points are higher and their pour points are considerably lower. This makes them valuable blending components when compounding oils for extreme service at both high and low temperatures.

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The main disadvantage of synthetics is that they are more expensive than mineral oils. This limits their use to specialty oils and greases that command premium prices. Esters suffer the additional disadvantage of greater seal-swelling tendencies than hydrocarbons; therefore, caution must be exercised when using them in applications where they may contact elastomers designed for use with mineral oils.

Additives for Grease

Chemical additives can significantly alter the performance of lubricating greases. Factors influencing additive selection are:

  • Performance requirements (product application)
  • Compatibility (synergistic/antagonistic reactions)
  • Environmental considerations (product application, odour, disposal, biodegradability)
  • Colour
  • Cost

Most of the additives described are chemically active; that is, they produce their effect through a chemical reaction either within the lubricant medium or on the metallic surface. Chemically active additives include:

  • Oxidation inhibitors
  • Rust and corrosion preventatives
  • EP/antiwear agents

Structure modifiers and thickeners could also be included in this category, as well as polymers which improve adhesive and water-resistance properties.

Chemically inert additives, on the other hand, affect a physical property of the grease such as structure, rheology or water tolerance. Chemically inert additives include:

  • Viscosity modifiers
  • Pour-point depressants
  • Antifoam agents
  • Emulsifiers
  • Demulsifiers
Surface Protective Additives
Additive Type Purpose Typical Compounds Functions
Antiwear and EP Agent Reduce friction and wear and prevent scoring and seizure Zinc dithiophosphates, organic phosphates, acid phosphates, organic sulphur and chlorine compounds, sulphurised fats, sulphides and disulfides Chemical reaction with metal surface to form a film with lower shear strength than the metal, thereby preventing metal-to-metal contact
Corrosion and Rust Inhibitor Prevent corrosion and rusting of metal parts in contact with the lubricant Zinc dithiophosphates, metal phenolates, basic metal sulphonates, fatty acids and amines Preferential adsorption of polar constituent on metal surface to provide protective film, or neutralize corrosive acids
Friction Modifier Alter coefficient of friction Organic fatty acids and amides, lard oil, high molecular weight organic phosphorus and phosphoric acid esters Preferential adsorption of surface-active materials
Performance Additives
Viscosity Modifier Reduce the rate of viscosity change with temperature Polymers and copolymers of methacrylates, butadiene, olefins or alkylated styrenes Polymers expand with increasing temperature to counteract oil thinning
Protective Additives
Antioxidant Retard oxidative decomposition Zinc dithiophosphates, hindered phenols, aromatic amines, sulphurised phenols Decompose peroxides and terminate free-radical reactions
Metal Deactivator Reduce catalytic effect of metals on oxidation rate Organic complexes containing nitrogen or sulphur, amines, sulphides and phosphites Form inactive film on metal surfaces by complexing with metallic ions

Oxidation Inhibitors

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Like lubricating oils, greases under oxidizing conditions yield unstable materials called peroxides. Once formed, peroxides quickly decompose to form other materials which are even more susceptible to oxidation. The process is a chain reaction which is accelerated by increased temperatures and which is catalysed by certain metals - particularly those present in soap-based thickening agents.

The final products of oxidation are gums, lacquers and acidic materials. In a grease composition, oxidation is manifested by any one or a combination of symptoms:

  • Drying and cracking
  • Increase in penetration
  • Lowering of dropping point
  • Increased uptake of oxygen
  • Increased acid number

Deposits on bearings are the most obvious sign of oxidation in service.

Typical rust inhibitors include:

  • Fatty amines
  • Fatty amides
  • Carboxylic acids
  • Sodium sulphonates
  • Barium sulphonates
  • Lead naphthenate

Extreme Pressure/Antiwear Agents

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Greases are formulated with extreme-pressure (EP) agents to prevent seizure under conditions of high temperature, heavy loading or extended periods of operation. Chemicals which serve as extreme-pressure agents usually contain sulphur, chlorine, phosphorus, metals, or combinations of these elements.

Function: Extreme-pressure agents function under boundary conditions where metal surfaces are in intimate contact. As the surfaces move against one another, collision of surface asperities produces localized temperature rises which activate the EP agents. Distinct chemical compounds form and immediately plate out on the metal surface as a thin film. Sulphide, chloride and phosphide films shear more easily than the metal itself; consequently, less frictional heat is generated and the potential for severe welding is reduced.

Types: Some of the materials which function as EP agents in greases include:

  • Sulphurised olefins
  • Lead naphthenate
  • Sulphurised esters
  • Phosphate and thiophosphate esters
  • Metal dithiocarbamates
  • Metal dialkyldithiophosphates
  • Chlorinated paraffins

These materials are also widely used as components in gear-oil formulations, although treatment amounts in greases tend to be higher.

Solid Fillers

Fillers, often referred to as "physical additives" or "dry lubricants," are organic or polymeric solid materials which are intended to impart EP protection and durability to greases. Examples include:

  • Molybdenum disulfide
  • Graphite
  • Zinc oxide
  • Cerium fluoride
  • Fluorinated polymers
Click for larger image

These materials are generally most useful for protecting heavily loaded bearings from galling and seizing.

Unlike conventional organic EP agents, solid fillers do not produce their effect by reacting chemically with metal surfaces. Under boundary conditions, suspended solids physically plate out on the metal surface to produce low shear-strength films. In the case of molybdenum disulfide, for example, sulphur atoms adhere tenaciously to the metal surface while weak bonds form between sulphur atoms in adjacent molecules. As shear commences, the weak sulphur-sulphur bonds are broken and molecules easily slide over one another. After a hydrodynamic lubricant film has collapsed, a solid film remains in the contact area and prevents frictional heat build-up. Solid fillers and organic EP agents produce the same ultimate effect by physical and chemical means, respectively.





Viscosity Modifiers

Chemical structure and molecular size are the most important elements of the molecular architecture of viscosity modifiers. Many types of viscosity modifiers are available and choice depends on the particular circumstances and requirements.

Viscosity modifiers consist of aliphatic carbon-to-carbon backbones. The major structural differences are in the side groups, which differ both chemically and in size. These variations in chemical structure are responsible for the various properties of viscosity modifiers such as oil-thickening ability, viscosity-temperature dependency and oxidation stability.

Synergistic Additive Effects

Frequently, combinations of two or more additives show enhanced performance over that of the individual components. The occurrence and magnitude of synergistic behaviour involving specific compounds very likely depends on the nature of the base grease. For example, zinc dialkyldithiophosphates are good oxidation inhibitors and mild EP agents in grease media. However, they appear to deliver stronger EP performance when combined with sulphurised olefins or sulphur-phosphorus packages.

Synergistic Effect of a Secondary Zinc Dialkyldithiophosphate/Sulfurized Olefin Combination:

Test Method Lithium NLGI #2 Base Grease with
Nothing 2.0% LZ 1360 2.0% Ang 33 1.0% LZ 1360 + 1.0% Ang 33
Timken OK Load (lb) 10 40 35 55
4 Ball EP Weld (kg) 160 160 200 250

http://www.lubrizol.com

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