The so-called foaming agent is a substance that makes a target substance pore. It can be divided into three major categories: chemical foaming agent, physical foaming agent and surfactant. Chemical foaming agents are those which release carbon dioxide and nitrogen gas after being decomposed by heating and form fine pores in the polymer composition; physical foaming agents are foam pores which are changed by the physical form of a certain substance. , which is formed by the expansion of compressed gas, the volatilization of liquid or the dissolution of solid; the foaming agent has high surface activity, can effectively reduce the surface tension of the liquid, and surrounds the air in the double electron layer on the surface of the liquid film. , forming a bubble, and then consisting of a single bubble.