Greases, oxidative stability

Risdon, T.J. and Sargent, D.J., Comparison of Commercially Available Greases With and Without Molybdenum Disulfide Part II - Oxidation Stability and Ball Joint Tests, NLGI Spokesman, 34, 358, (1971). 

The oxidation induction time can also be used as an indication of oxidation stability. No correlation has been determined between the results of this test and service performance. In the test (ASTM D-5483), a small quantity of grease in a sample pan is placed in a test cell. The cell is heated to a specified temperature and then pressurized with oxygen and is held at a regulated temperature and pressure until an exothermic reaction occurs. The extrapolated onset time is measured and reported as the oxidation... 

The free hydroxyl group in ricinoleic acid is able to interact with polar groups on metal surfaces, thereby giving superior lubricity. The largely mono-ene nature of castor oil gives it low-temperature fluidity without the loss of oxidative stability of polyunsaturates. Castor oil is used in specialist two-stroke applications and can be modified for use in greases. 

Blaine (244) also used pressure DSC to study the oxidative stability of lubricating oils and greases. 

Polyurethanes are sensitive to strong acids, strong alkalis, aromatics, alcohols, hot water, hot moist air and saturated steam. The hydrolytic stability of polyurethanes in applications must be considered carefully. However, polyurethanes are resistant to weak acids, weak alkalis, ozone, oxygen, mineral grease, oils and petroleum. There are doubts for the oxidation stability of polytetramethylene ether glycol based polyurethanes. Polycarbonate urethane is a promising substitute with good oxidation stability. 

According to more recent Russian work, a butoxy type of estersil imj roved the oxidation stability of grease (606). 

Oxidation of grease results in insoluble gum, sludge, and deposits and ultimately leads to decreased metal wettability, sluggish operation, reduced wear protection, and increased corrosion. Excessive temperatures result in accelerated oxidation or even carbonization, where grease hardens or forms a crust as a result of evaporation of base fluid and increased thickener concentration. Therefore, higher evaporation rates require more frequent relubrication. A number of studies have been reported on various aspects of thermo-oxidative stability [45, 46]. The oxidative stability of... 

The oxidation stability of soybean-oil-based greases can be further improved by using a more oxidatively stable oil, such as epoxy soybean oil (ESO). The ESO has low iodine value and high viscosity compared with regular soybean oil (SBO) due to the presence of epoxy rings in place of double bonds. The reduction in double bonds of ESO provides more oxidative stability to its structure. The PDSC onset... 

ASTM International, Standard test method for oxidation stability of lubricating greases by the oxygen pressure vessel method, ASTM Standard ASTM D942-15, ASTM International, West Conshohocken, PA, 2015. 

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