Synthetic oils are used for the manufacture of grease when lubricating properties are required which cannot be achieved with a mineral oil. Traditional polyalkylene glycols (PAGs) used in the industry are either water soluble or water insoluble. Recently, new Oil Soluble Polyalkylene Glycols (OSPs) have been introduced for the lubricants market  . These new base fluids are very promising since they are soluble in hydrocarbon oils as well as other natural and synthetic esters. OSPs have low friction coefficient, higher thermo- oxidative nature and exhibit polar behavior. These characteristics make them suitable for high quality gear oil, hydraulic oil and many types of metalworking fluid formulations . The objective of the present communication is to describe new application of OSPs in making high performance lubricant-greases. Lithium Complex Grease was chosen due to its wide use in the various types of industries including automotive wheel bearings and heavy duty industrial machineries.
As is well known in the industry, grease can’t be any better than its base oil. Consequently, before selecting an oil, its characteristics are to be examined in detail for the making of a grease for proper application. The tribological properties of oil soluble PAGs can be very helpful for grease formulators who have specific application requirement from their customers. The main advantages of OSPs are shown in Table 1. The OSP polymer is shown in Figure 1.
Table 1: Important Characteristics of OSPs for Grease Formulation
Figure 1: Oxygen Rich OSP Molecule
As discussed in many research papers, synthetics like poly-alpha-olefins and other mineral oils have lower solvating power for additives except naphthenic base oils . Solvency power of the fluid is indicated by aniline point which can be measured by ASTM D 611. A lower aniline point of a fluid prompts grease formulator to use such base fluids as a blending component in their grease formulation. It helps them to plan solubility of sparingly soluble additives in the grease structure. Figure 2 displays the aniline point of different fluids used by grease formulators. OSPs have the lowest aniline point of about -30° C compared to currently use base stocks for grease formulations. The effect of lower aniline point of OSP gives the following advantages with respect to grease making and the properties of a Lithium Complex grease.
- Improved soap yield
- Lower temperature for HAS (hydroxy stearic acid) solubility
- Lower risk of soap oxidation
- Lower energy consumption
- Soap fibers more uniform in shape and size
- A homogeneous soap structure confers smooth appearance
- Higher solvency oils dissolve additives more easily
- OSP based Li-Complex grease with high drop point (>300°C)
Figure 2: OSPs Significantly Boost Solvent Power
Three base fluids were chosen and Li-complex greases NLGI grade 2 were made using the same additive package. The data for soap content and drop point are compared in Table 2. The OSP based grease has been tested per ASTM D 4950 for GC-LB specification and has given very low torque values, high drop point, high PDSC times and bearing life hours .
Table 2: Drop Point and Soap Content Comparison
Govind Khemchandani, Ph.D., works in Technical Services at The Dow Chemical Company. You can find his contact information in our membership directory.
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