Research on the Effect of the Molecular Structure of Base Oils on Porous Polyimide with Different Performances of Oil Absorption and Lubrication

Oil-containing porous polyimide (OPPI) with excellent mechanical strength and high-temperature resistance,is normally used as a bearing retainer in certain conditions. The main mechanism of lubrication is primarily based on its internal porous structure, which can provide durable lubrication through the release and re-absorption of stored lubrication oils. Therefore, oil content and oil retention, which are both determined by the porous structure of polyimide have a heavy influence on their long-term lubrication. However, there is a holdback to obtaining OPPI with both higher porosity and smaller pore size according to the present method by cold-pressing and hot sintering. To break through this limitation, researchers have conducted various attempts including design with different molecular structures of polyimide, adjusting internal pore size, and optimizing the lubricants. However, it is easy to ignore the interaction between different base oils and PPI molecular structure, but particularly crucial to oil absorption and release. As well we know, there is a lack of systematic investigations into this field. In this work, we prepared and obtained porous YS-20 (PPI) with a certain pore size (1.46 μm) and porosity (22.63%) by utilizing an established pore-making process. We investigated the effects of the molecular structure of the base oils on the capacity of oil storage and recyclable smart-lubrication of porous YS-20. Three base oils (PAO10, DIOS, PEG400) with different polarity and viscosity were selected. The intrinsic correlation between the molecular structure of the base oil on the oil-containing characteristics and stimulus-response behavior of the porous YS-20 was revealed. The higher polarity base oil PEG400 was found to greatly improve the oil content and oil retention of porous YS-20. Herein, QCM-D was further used to characterize the adsorption properties between YS-20 and different structural base oils, revealing the intrinsic connection between them. It was found that the adsorption of PEG400 on the surface of YS-20 was stronger and the adsorption film was thicker compared with that of PAO10 and DIOS. At the same time, porous YS-20 containing different base oils was used as the friction material to realize the stimulation response of the porous oil-containing polyimide material by varying the load and speed, thus providing a lubrication effect. Porous YS-20 impregnated with PEG400 was found to have excellent friction reduction and anti-wear properties. By characterizing and analyzing the morphology and composition of porous YS-20 and steel ring wear surfaces after friction experiments, the intrinsic lubrication mechanism of porous YS-20 impregnated with base oils of different polarities was elucidated. In conclusion, this study could provide some theoretical support for the design of porous polyimide self-lubricating materials for future high-end equipment.