Since its inception, nanomaterials have attracted extensive attention of researchers in many fields due to their unique properties. In the field of lubrication, studies have found that nano-carbonate has a good anti-wear effect. The carbonates used in the current research mainly include CaCO3, MgCO3, BaCO3, Na2CO3, etc. In consideration of environmental friendliness, preparation cost, performance and other factors, nano-CaCO3 is widely used in the field of lubrication. In view of the above analysis, this paper briefly reviews the application of nano-calcium carbonate in the field of lubrication.
1. Application of nano calcium carbonate in the field of lubrication
With the enhancement of people’s awareness of environmental protection and the improvement of environmental regulations, green and environmentally friendly nano-lubricating additives are increasingly favored by people. Nano calcium carbonate particles do not corrode metals and do not pollute the environment, and are a promising lubricant additive. Therefore, more and more researchers have investigated its application performance in lubricating systems. In general, the application of nano-calcium carbonate in the field of lubricating oil is mainly reflected in three aspects: overbased metal detergent, thickener of grease (especially in overbased complex calcium sulfonate based grease) , Efficient extreme pressure anti-wear agent.
1.1 Overbased metal detergents
The earliest application of nano-calcium carbonate in the field of lubrication is the preparation of overbased metal detergents. In the 1950s, due to the high-power supercharged diesel engines and marine diesel engines burning high-sulfur fuels, in order to solve the problems of increased piston coke deposits and cylinder liner corrosion, people began to develop alkaline metals (carbonate colloid systems) that are many times more positive. Overbased metal detergent with metal content in salt. Overbased metal detergents must meet the following conditions: strong alkalinity; good stability after reaction with acid; no adverse effect on oil products after reaction; good fluidity; transparent, stable, and good oil solubility ; cheap. Only the inorganic carbonate colloid system meets these conditions, and the calcium salt in the currently used carbonate develops rapidly.
It is generally believed that the effective portion of alkaline metal detergents consists of stable loaded micelles and free surfactant molecules and their micelles. The loaded micelle is composed of carbonate (containing magnesium, calcium, sodium, etc., with a mass fraction of 15% to 40%) and surfactants adsorbed on the surface (with a mass fraction of 20% to 45%), and the effective components are stabilized by the colloidal system Dispersed in oil, its structure is shown in the figure.
The particle size of the loaded micelle particles must be less than 80 nm, otherwise it cannot be stably dispersed in the oil, making the product turbid and affecting the performance. The preferred particle size should be below 20 nm, and the particle size distribution should be uniform. The particle size distribution and average particle size of the colloid greatly affect the acid neutralization ability, colloidal stability and cleaning and dispersing ability of metal detergents. The higher the temperature, the better the thermal oxidation stability and high temperature cleaning performance. Therefore, when nano-calcium carbonate is used in overbased metal detergents, the important problems to be solved are the particle size control of nano-calcium carbonate and the dispersion stability in lubricating oil.
1.2 Thickener for grease
Nano-calcium carbonate, as a thickener for lubricating grease, is in the research and development stage, and is currently mainly used in complex calcium sulfonate-based greases. Complex calcium sulfonate-based grease has excellent high and low temperature performance (dropping point greater than 330 ℃), mechanical stability, colloidal stability, oxidation stability, corrosion resistance, water resistance, excellent rust resistance and extreme pressure and anti-wear It is called “a new generation of high-efficiency grease” and can be used as a general-purpose grease to replace the existing calcium-based, sodium-based, lithium-based and aluminum-based greases, and its applications have been extended to steel, smelting, power generation, shipping, railways , automotive, construction, food machinery and other fields.
Complex calcium sulfonate-based grease is composed of base oil, thickener and additives. The difference from ordinary soap-based grease is the thickener. The thickener of complex calcium sulfonate-based grease is mainly composed of two parts: highly alkaline non-Newtonian calcium sulfonate and complex calcium soap. There are both physical mixing and chemical association between the two. compound system. Non-Newtonian calcium sulfonate is converted from oil-soluble Newtonian body. Newtonian high base number calcium sulfonate is composed of calcium alkylbenzenesulfonate calcium salt and calcium carbonate. Its structure is shown in Figure 2, where R is C20- The alkyl group of C30, m can be as high as 40, and the calcium carbonate particles are generally smaller than 10 nm, which are wrapped by calcium sulfonate molecules to form stable colloidal particles that dissolve in oil. The high alkali value comes from the calcium carbonate dispersed in the colloidal particles.
Usually calcium carbonate accounts for about 30% of Newtonian overbased calcium sulfonate, calcium sulfonate accounts for 20%, and the rest is diluent oil. In addition, according to research, in the Newtonian high basic number calcium sulfonate, the nano-calcium carbonate is in an amorphous state. From the original amorphous to spherical calcite, and the particle size increases. It is worth mentioning that the good low-temperature performance of the complex calcium sulfonate-based grease comes from the large number of spherical nano-calcium carbonate particles in the system, which prevent the tendency of the base oil to coagulate at low temperature. It can be seen that the particle size, crystal form and morphology of nano-calcium carbonate as a thickener of grease have an impact on its performance.
1.3 Efficient extreme pressure antiwear agent
The current application and research of extreme pressure antiwear agents can be divided into extreme pressure antiwear agents containing sulfur, phosphorus and chlorine, organic metal salts, and extreme pressure antiwear agents such as borates and rare earths developed for environmental protection agent. During the development of environmental protection antiwear agents, it was found that sulfonates have a certain antiwear ability. As inert extreme pressure agents, sulfonates can reduce friction and prevent sintering, and can also neutralize acidic pollutants and prevent metal pollution. It produces corrosion without causing environmental pollution, and is a promising extreme pressure antiwear agent. At present, such extreme pressure antiwear agents have been treated by boronization, vulcanization and other methods to improve their extreme pressure antiwear properties.
It is generally believed that the antiwear ability of sulfonates is due to calcium carbonate in the loaded micelles. Zhang Jianrong and others studied the extreme pressure and anti-wear properties of high-base calcium sulfonate. They believed that the friction between metals could generate local high temperature on the friction surface, so that high-base calcium petroleum sulfonate released CaCO3 and formed CaCO3 on the friction surface. Deposition of protective film, so high base number calcium sulfonate has certain extreme pressure properties, but the difference is large. Zhang Jianrong and others believe that the important factor affecting its extreme pressure performance is the structure of CaCO3. When CaCO3 is calcite crystal, a thicker protective film containing CaCO3 can be formed on the friction surface, and the extreme pressure performance is better; when CaCO3 is amorphous, it is A thin protective film containing CaCO3 is formed on the friction surface, and the extreme pressure performance is poor. In addition, some scholars have studied the influence of metal species, organic group structure, carbonate particle size and other factors on extreme pressure wear resistance. Cahoon believes that the influence of metal species and organic group structure on extreme pressure wear resistance is relatively large, while the particle size of carbonate has less effect.
In general, nano-calcium carbonate is more and more widely used in the field of lubrication, and its crystal form and particle size have a great influence on its lubricating properties, so the controllable preparation of nano-calcium carbonate is particularly important.
2. Conclusion
As a kind of green environmental protection nano-lubricating additives, more and more scholars have studied its application performance in overbased metal detergents, thickeners of lubricating greases and high-efficiency extreme pressure and anti-wear agents. After analysis, it is found that the crystal form, particle size and shape of nano-calcium carbonate all affect its lubricating performance. For example, in overbased metal detergents, the important problem to be solved is the particle size control of nano-calcium carbonate; In terms of thickener, it is necessary to control the particle size, crystal form and shape of nano-calcium carbonate; in terms of high-efficiency extreme pressure antiwear agent, its crystal form has a great influence on the extreme pressure performance of sulfonate. Therefore, the controllable preparation of nano-calcium carbonate is particularly important.