Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
The amount of thickener used during manufacture is linked to the desired physical properties of a finished grease. Since the soap or clay is typically more expensive than the base oil, minimizing the soap or clay content while maintaining the physical properties is paramount. The amount of thickener necessary to form the microstructure depends on the interactions between the thickener and the base oil, which depend on the solvency of the base oil. The higher the solvency, the more the interaction and the less thickener required to produce a grease with the targeted NLGI grade, which ultimately lowers the overall formulation cost.
Simple soaps are the most common grease thickeners. A simple soap is the reaction product of an organic acid (long-chain or fatty carboxylic acid) and an alkali metal to form an organic salt. Thus, simple soap is an acid-base reaction product. This reaction is called saponification. Simple soaps are most commonly based on salts of lithium and calcium, and less commonly on salts of sodium, aluminum, and barium. Examples of simple soap thickeners include lithium 12-HSA and calcium stearate.
Complex soaps are also used widely as grease thickeners. The term “complex” refers to the combination of a simple soap and a complexing agent. For example, a lithium complex thickener typically contains lithium 12-HSA (simple soap) and a salt of a shorter chain difunctional carboxylic acid, boric acid, or an aromatic acid (complexing agent). Complex thickeners are usually based on lithium, calcium, or aluminum compounds. Grease can also be thickened with non-soap materials. Common non-soap thickeners include polyurea, clay, fumed silica, fluoropolymers, and others.
Clay thickeners include the minerals bentonite and hectorite. These minerals are purified to remove any non-clay material, ground to the desired particle size distribution, and then chemically treated to make
34 LUBE MAGAZINE NO.144 APRIL 2018
No.115 page 3
the particles organophilic (more compatible with organic chemicals). Clay particles are then dispersed in a fluid lubricant to form grease. Clay particles must be activated with a polar material to stabilize the thickener structure. Clay thickeners have no defined melting point, so they have been used historically in high temperature applications operating up to 200°C, such as kilns and drier ovens.
For this study, we will examine the differences between several naphthenic and paraffinic base oils, a PAO, and a bright stock as they relate to processing and properties of Li 12-HSA, Li complex, Al complex, Ca sulfonate and clay based greases.
Design of experiment In this study, NLGI Grade 2 greases were prepared using five different thickener systems:
1) lithium 12-hydroxystearate (Li 12-HSA) 2) lithium complex (Li complex) 3) aluminum complex (Al complex) 4) calcium sulfonate (Ca sulfonate) 5) clay
Seven base oils with different aniline points were used with each type of thickener:
• 150 N1 - Group V naphthenic base oil, ISO VG 150, 197.5°F (91.9°C)
• 150 N2 - Group V naphthenic base oil, ISO VG 150, 191.3°F (88.5°C)
• 460 N3 - Blend of 150 N1 and 1000 P3, ISO VG 460, 231.5 °F (110.8°C),
• 1000 P3 - Group I bright stock, ISO VG 1000, 248.5°F (120.3°C)
• 100 P1 - Group I paraffinic base oil, ISO VG 100, 248°F (120°C)
• 100 P2 - Group II paraffinic base oil, ISO VG 100, 256.8°F (124.9°C)
• 150 S1 - Group IV PAO base oil, ISO VG 150, >300°F (>148.9°C)
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