Research Progress in Bio-Based Thickeners for Lubricating Grease

Lubricating grease is a solid or semi fluid substance formed by uniformly dispersing a thickener into the base oil. As an important branch of lubricants, lubricating grease has a wide range of applications in fields such as automotive manufacturing, mechanical manufacturing, power equipment, and metallurgical equipment. The correct and reasonable use of lubricating grease can effectively reduce the friction and wear of mechanical equipment, reduce faults and damage, thereby achieving the objective of energy conservation and extending the service life of equipment. Compared to lubricating oil, lubricating grease has the following performance advantages: (1) good pressure resistance, lubricating grease can form a solid lubricating film in areas that cannot be lubricated by lubricating oil, and can withstand great pressure. (2) Good buffering performance, lubricating grease used in reciprocating machinery can play a buffering role in shock and vibration. (3) Not easily lost, lubricating grease is not easily lost under centrifugal force, ensuring reliable lubrication. (4) Good sealing performance, the sealing performance of lubricating grease is better than that of lubricating oil, and lubricating grease can prevent moisture, ash, impurities, and corrosive substances from entering the friction surface. (5) Good viscosity-temperature performance, lubricating grease is less affected by temperature, and is suitable for situations with large movement speed and temperature changes. However, as a typical petrochemical product, lubricating grease is facing the problem of increasingly depleted petrochemical resources. Moreover, with the improvement of people’s environmental awareness, the environmental pollution caused by lubricating products has also attracted widespread attention. With the proposal of the national strategic goals of “carbon neutrality” and “carbon peaking”, bio-based materials are increasingly attracting attention from people. Bio-based materials have advantages such as environmentally friendly, renewable and degradable, abundant reserves, which are in line with the national sustainable development strategy. Therefore, the use of natural bio-based materials to prepare lubricating grease is an important direction for achieving the sustainable development of lubricating grease materials. At present, the technology of developing bio-based lubricating grease using vegetable oil as the base oil is very mature, but research on bio-based thickeners is still in its infancy. The most widely used thickeners of lubricating grease on the market are still metal soaps and polyurea based thickeners. Thickeners can exist as dispersed phases in the base oil and forming a “three-dimensional network” or “sponge” skeleton. They bind the base oil through physical bonds such as hydrogen bonds or van der Waals forces, fixing the oil in the skeleton structure, thereby endowing the lubricating grease with appropriate rheological and tribological behavior. Bio-based materials have diverse structures and contain rich functional structures such as amino groups, hydroxyl groups, and unsaturated bonds. They can be modified through isocyanation, acylation, and epoxidation to improve the affinity with the base oil and achieving the goal of thickening the base oil. This article focuses on the research and development status of three typical bio-based materials, cellulose, lignin, and chitosan, as thickeners for lubricating grease from the perspective of selection of bio-based materials, and prospects the application prospects of bio-based materials as thickeners.