Performance Degradation Mechanism of Polyurea Grease Under Static Thermal-Oxidative Aging Conditions

Lubricant aging is one of the main causes of rolling bearing failures. Thickener is one of the core components of lubricating greases. Lubricating greases with the different thickeners exit different aging behaviors and mechanisms. Polyurea lubricating grease is commonly used in industrial equipment operating under harsh conditions such as high temperature, high speed, and high load due to its excellent performance. The study of the aging mechanism of polyurea lubricating grease is beneficial for understanding the state of lubricating grease, providing data support and theoretical basis for the practical application and further development of lubricating grease. In this article, the aging behavior of polyurea greases under high temperature conditions were investigated by simulating a static thermal aging circumstance. To investigate the decay process of polyurea grease and the effect of temperature on this process under thermal aging conditions at 160, 180, 200 and 220 ℃, respectively. The aging process of polyurea grease under the thermal aging condition of 180 ℃ was taken as an example. Based on the differences in infrared spectra during the aging process of polyurea lubricating greases, and combined with the microstructures of the thickeners, the oxidation stabilities, thermal stabilities, and rheological properties of the polyurea lubricating greases, their aging and decline mechanism were studied in detailed. The results indicated that in the early stage of thermal aging of polyurea greases, the degree of association between thickener molecules increased with the increasing of aging time which could inhibit the oxidation of the base oil to some extent. With the continuous effect of thermal effect, the microstructures of thickener fibers changed from disordered short fiber structures to spherical particle structures, the base oil precipitated and began to oxidize. At the same time, the structural stabilities of grease became weak and then strengthens, and the structural strength gradually weakened. At the last stage, hydrogen bonds in the microstructures of polyurea greases had begun to be broken and the thickeners undergoes obviously oxidative degradation, which led to a rapid decrease in the oxidation stabilities of polyurea lubricating greases. As a result, there were the loss of a large amount of lubricating oils and hardening of polyurea lubricating greases. It had been found that the aging temperature could change the aging process of lubricating greases, but it would not change their decay law. The higher the temperature was, the faster the aging and deterioration of lubricating grease were, the worse the oxidation stability, the faster the structural change, and the lower the structural strength.