Method for preparing insulating varnish

A method for preparing an insulating varnish, the method comprising the following steps:

• a) adding silane coupling agent-modified hexagonal boron nitride (BN) having a particle size of between 200 and 250 nm to acetone to obtain a first solution, stirring the first solution for between 1 and 1.5 h;
• b) mixing an epoxy resin and the first solution to obtain a second solution, stirring, and dispersing the second solution using an ultrasonic cleaning machine at a temperature of between 65 and 70° C. for between 2 and 2.5 h, wherein a weight ratio of the epoxy resin to the acetone is 1:1;
• c) adding as a curing agent low molecular weight polyamide resin to the second solution to obtain a third solution, and stirring the third solution using a magnetic stirrer to allow the curing agent to be dispersed uniformly, wherein an amount of the curing agent is between 20 and 25 wt. % of the epoxy resin;
• d) adding n-butane as a lubricant to the third solution and stirring, cooling to room temperature, then adding di-n-butyl phthalate as a diluent and stirring to obtain a mixture, wherein an amount of the lubricant is between 3 and 5 wt. % of the epoxy resin, and an amount of the diluent is between 10 and 12 wt. % of the epoxy resin;
• and
• e) allowing the mixture to stand in a vacuum drier for removing bubbles to yield the insulating varnish free of bubbles, a temperature of the vacuum drier being controlled at between 20 and 25° C.;
• wherein in a), an amount of the silane coupling agent-modified hexagonal boron nitride is between 25 and 30 wt. % of the epoxy resin used in step b).