The invention provides a multi-layer porous polyimide film and a preparation method of the porous
polyimide film. The film comprises a large-hole layer and a small-hole layer or is formed in the way that one small-hole layer is sandwiched between two large-hole
layers, wherein the large-hole layer is 0.15-6 microns in hole
diameter and 5-100 microns in thickness, and the small-hole layer is 0.01-0.15 microns in hole
diameter and 1-5 microns in thickness. The preparation method of the film comprises the following steps of: synthesizing two or three kinds of composite
polyamide resin solution, by using dianhydride,
diamine and two or three kinds of pore-forming materials with different granularities; carrying out composite salivating to form a two / three-layer film, carrying out two-way stretch and imidization,
pickling the film for removing the pore-forming materials, thus obtaining the multi-layer porous
polyimide film. The
molar ratio of dianhydride to
diamine is 1:1, and the
mass of the pore-forming materials is 0.25-2 times that of dianhydride and
diamine. Various media can conveniently pass through the large-hole layer of the film and the strength of the film is ensured; the small-hole layer can be thinned to 1 micron and the
transmittance is good. According to the method, the
porosity and hole
diameter of all
layers in the film can be easily controlled, and the method is suitable for preparing
porous film products.