Preparation and properties of sandwich type density layered porous polyimide materials

Porous polyimide materials （PPI） have interconnected pore structures that can effectively store lubricating oil. However，the presence of these pores will reduce the strength and wear resistance of PPI，resulting in a mutual constraint between its oil content and wear resistance，making it difficult to balance. Therefore，a sandwich type density layered porous polyimide material is prepared using the method of layered cold pressing and constant volume sintering. The outer and inner layers of the material can have different porosities. The outer layer uses a high porosity to increase oil content，while the inner layer uses a low porosity to improve material strength and wear resistance，in order to overcome the problem of difficult balance between oil content and wear resistance. Based on the study of uniform PPI performance with different densities，the pore structure of density layered PPI is designed，and two types of layered PPI are prepared. Their mechanical properties，oil content properties，and friction and wear properties are studied. The results show that compared with uniform PPI，density stratified PPI has a higher oil content （>15%） and excellent oil retention rate （94.06%） due to the synergistic effect of different porosity in the inner and outer layers，and maintains excellent mechanical and tribological properties. The tensile strength and compressive strength of the 1.0-1.1-1.0 layered material are 41.7 MPa and 374 MPa，respectively. Its friction coefficient （0.078） is close to that of a uniform PPI density of 1.1 g/cm3 （0.07），but the oil content of the layered material （15.5%） is much higher than that of the uniform material （11.1%）. This sandwich type density layered PPI can balance oil content and wear resistance，providing a new approach for the design of porous materials.