Expanded coating, preparation method and use thereof, and permanent magnet comprising same

An expanded coating, comprising spheroid pores and a filler resin arranged among the pores, wherein:

• the filler resin is a thermosetting resin;
• each spheroid pore has a cross section with a long diameter (R2) and a short diameter (R1), based on the total amount of the spheroid pores,
• a percentage of spheroid pores having a ratio of 0.7<R2/R1≤1 is 60%-80%,
• a percentage of the spheroid pores having a ratio of R2/R1≤0.5 is less than 8%,
• a percentage of spheroid pores having a ratio of 0.9<R2/R1≤1 is 25%-30%,
• a percentage of spheroid pores having 20 μm<R1≤30 μm is 15%-20%, and
• a percentage of spheroid pores having R1>80 μm is less than 5%;
• a sum of cross-sectional areas of the spheroid pores in a cross section of the expanded coating accounts for 50%-60% of the cross-sectional area of the expanded coating;
• for each spheroid pore, a cross section of the spheroid pore is a quasi-cross section that passes through a central point of the spheroid pore, the long diameter is the longest distance passing through the central point on the quasi-circular cross section and the short diameter is the shortest distance passing through the center point on the quasi-circular cross section;
• wherein the expanded coating is prepared by heating a raw material mixture comprising, by parts by weight, 60-80 parts of the thermosetting resin, 5-20 parts of an expandable microsphere, 0.2-1.5 parts of a chemical foaming agent, 0.3-1.5 parts of a curing agent, and 15-30 parts of a pigment and filler;
• the amount ratio of the expandable microsphere to the chemical foaming agent is (5-20):(0.1-1.5);
• the thermosetting resin is at least one selected from bisphenol A-based epoxy resins;
• the expandable microsphere is selected from an expandable microsphere with an average particle size of 5-50 μm;
• the chemical foaming agent is at least one selected from azodicarbonamide, azobisisobutyronitrile, and p-toluenesulfonyl hydrazide;
• the curing agent is at least one selected from imidazole, an epoxy resin adduct of imidazole, and an epoxy resin adduct of polyamine;
• the pigment and filler is at least one selected from insulating carbon black, mica powder, calcium carbonate, and a nano-aluminosilicate fiber.