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Classification and Characteristics of Grease
Apr 21, 2022
Soap Grease |
Calcium Soap Grease In a typical manufacturing process of calcium soap grease, mineral oil, fatty acid, calcium hydroxide (hydrated lime) and water are mixed and cooked to bring to saponification. The process is complete after adjustment of water content. Grease made with tallow contains a small amount of water as structural stabilizer, and, when heated above 80°C, it loses its associated water, resulting in structure breakdown and separation of oil from thickener. Due to this poor heat resistance, it is used for general plain bearings operating under rather low speed and low load where temperature would not rise above 70°C in service. The water resistance is good on the other hand, and thus the grease performs well in applications exposed to water. Use of caster oil fatty acid results in a grease which contains no water. Because the structure is stabilized without water, it can be used up to about 100°C. Lithium Soap Grease Lithium soap grease is the most widely used multi-purpose grease, from general industrial uses to automobiles, bearings and home electric products. It consists of mineral or synthetic oil and lithium stearate or lithium salt, a hardened fatty acid derived from castor oil. Usable in a wide range of temperature, and has excellent water resistance and mechanical stability. Aluminum Complex Soap Grease Aluminum complex grease is made from a complex soap which is formed by reaction of aromatic carboxylic acid and stearic acid on aluminum hydroxide. Aluminum complex grease is characterized by a very fine fiber structure, high dropping point (200°C or above) , excellent heat and water resistance and mechanical stability. Lithium Complex Soap Grease The soap is formed by reaction of lithium hydroxide with the mixture of fatty acid and dibasic acid. The dropping point of the finished grease is 250°C or higher. Lithium complex grease has excellent heat and water resistance and rust preventing property, as well as longer life at high temperature than lithium soap grease. |
Non-Soap Greases |
Urea Grease Typical urea grease formulation uses organic compound containing more than two urea groups (-NH-CO-NH-) as thickening agent. Because of its excellent heat and water resistance, urea grease is an optimum choice for continuous casting lines and iron mills, and actually is the most widely used nonsoap grease. Urea grease is also widely used for automotive electrical components. For high temperature condition, synthetic oil based urea grease is preferred. Bentonite Grease This grease thickened by organic bentonite is often called "grease without dropping point" or "grease without melting point," because it does not lose the grease structure even at extremely high temperature. The grease can offer other advantages like good shear stability, but its application is limited because of rather poor rust prevention, hardening tendency when exposed to high temperature condition (200°C or above) for a long time, and poor ability to keep oil film on the bearing race surface during high speed rotation. Other Non-Soap Greases Other nonsoap greases include Na terephthalamate Grease, Copper Phthalocyanine Grease, Teflon (PTFE) Grease, Mica Grease and Silica Gel Grease. |
Comparison of properties of grease due to thickener
Soap Thickener
| Thickener type | Maximum temperature limit | Water resistance | Shear Stability | Remarks | |
|---|---|---|---|---|---|
| Metalic soap | Calcium soap (stearate) | 70°C | F | F | Contains water (1%) as structural stabilizer. |
| Calcium soap (hydroxystearate) | 100°C | G | G | Contains no water. | |
| Aluminum soap | 80°C | G | P | Excellent adhesive characteristics. | |
| Sodium soap | 120°C | P | F | Emulsifies with water. | |
| Lithium soap (stearate) | 130°C | G | G | All-purpose with least weak points. | |
| Lithium soap (hydroxystearate) | 130°C | G | E | All-purpose with least weak points. | |
| Complex soap | Calcium complex | 150°C | G | G | Tends to harden by time and heat. |
| Aluminum complex | 150°C | E | E | Water-repellent type: good pumpability | |
| Lithium complex | 150°C | G | E | Lithium soap grease with better heat resistance. |
O: Outstanding E: Excellent G: Good F: Fair P: Poor
Non-Soap Thickener
| Thickener type | Maximum temperature limit | Water resistance | Shear Stability | Remarks | ||
|---|---|---|---|---|---|---|
| Urea | Diurea | Aromatic diurea | 180°C | O | O | Most stable urea; optimum for sealed application. |
| Aliphatic diurea | 180°C | E | E | All-purpose shear - softening type; optimum for centralized system. | ||
| Alicyclic diurea | 180°C | E | E | All-purpose but some tends to harden by shear. | ||
| Triurea | 180°C | G | F | Hardens by heat. | ||
| Tetraurea (Polyurea) | 180°C | G | F | Softens by shear; wide batch to batch variation. | ||
| Organic | Sodium terephthalamate | 180°C | G | G | High oil separation tendency, susceptible to oxidation due to its constituent metal group. | |
| PTFE | 250°C | O | O | Most stable but costly and requires high usage. | ||
| Inorganic | Organic bentonite | 200°C | F | G | Carbonized if used at high temperature for prolonged periods. | |
| Silica gel | 200°C | P | P | Susceptible to rusting in the presence of moisture. | ||
O: Outstanding E: Excellent G: Good F: Fair P: Poor
Electron Micrograph (x104) of Thickener Fiber Structure
| Soap Thickener | Soap Thickener | Soap Thickener |
Calcium soap (stearate) |
Sodium soap (stearate) |
Lithium soap (stearate) |
| Soap Thickener | Non-Soap Thickener | Non-Soap Thickener |
Lithium soap (hydroxystearate) |
|
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