Oxidative stability is an important indicator for evaluating the quality of lubricating grease, and it is also the main factor affecting the service life of lubricating grease, which is of great significance to the storage and use of lubricating grease. After the oxidative deterioration of the grease, the content of free organic acids will increase, the dropping point will decrease, the appearance color will become darker, the consistency, strength limit, similar viscosity will decrease, and the generation of corrosive products and substances that destroy the colloidal structure of the grease will occur. The physical and chemical properties of the lubricating grease decrease rapidly, which has adverse consequences for the continued storage and use of the lubricating grease. So what are the factors that affect the oxidative stability of grease?
The effect of thickeners Thickener constitutes the basic structural skeleton of lubricating grease, and its content accounts for about 5% to 30%. Its main function is to adsorb and fix the Base Oil to form a stable colloidal system. Differences in thickeners often have a significant impact on the fat-forming effect and physical and chemical properties of greases. Among different metal soap-based greases, lithium soap has the worst oxidation stability, followed by sodium soap, calcium soap, barium soap and aluminum soap. The oxidative stability of lipids is worse. Compared with metal soap-based greases, non-soap-based greases such as organic or inorganic thickeners have better oxidation stability. In addition, the polyurea thickener obtained by compounding cycloalkylamine, aliphatic amine and ethylenediamine has better thickening ability than that obtained by compounding aromatic amine, aliphatic amine and ethylenediamine. The prepared polyurea grease has better oxidation stability and longer service life.
The effect of base oil Base oil is the most abundant component in lubricating grease, accounting for more than 80%. The oxidation stability of lubricating grease is consistent with that of base oil. Various indicators of base oil will have a significant impact on the physical and chemical properties of lubricating grease. Mineral base oils with different sources or different degrees of refinement also have different oxidative stability of the prepared lubricating greases. The initial oxidation temperature of the grease prepared from the mineral base oil with high viscosity index is higher, and the higher the viscosity index of the base oil, the better the oxidation stability of the grease prepared accordingly. Greases prepared with moderately refined base oils have better oxidation stability than greases prepared with unrefined or over-refined base oils. Unrefined base oils are prone to oxidation under the action of the atmosphere, and excessive refining will remove the natural antioxidants contained in the lubricating oil, resulting in poor oxidation stability. The effect of antioxidants Grease additives are chemical substances added to grease to enhance its physical and chemical properties. The additive content is basically below 5%, and most of the grease additives are oil-soluble. Different types of antioxidants have different antioxidant effects on the same grease. The synergistic use of phenolamine and bisphenol antioxidants can make various types of greases show good oxidative stability and thermal stability. Three typical phenolic antioxidants, thiobisphenol antioxidant (L115) has the best antioxidant performance, macromolecular mass liquid phenolic antioxidant (L135) is second, common monophenolic antioxidant ( T501) the worst.
