(1) Inert and non-toxic;

(2) Compatible with resins;

(3) The diffusion velocity in the resin matrix is small;

(4) When the resin reacts to release heat, or when placed outside for heating, it should be easy to volatilize.

In general, physical blowing agents are divided into three categories: (1) compressed gases, (2) soluble solids, and (3) volatile liquids with a boiling point below 0 degree .

There are many types of physical blowing agents, such as aliphatic hydrocarbons, chlorinated hydrocarbons, hydrochlorofluorocarbons and carbon dioxide gases containing 5~7 carbons, since the 50s of the 20th century, monofluorochloroform (CFC-11) has been widely used as the preferred foaming agent for polyurethane, because of its destructive effect on the atmospheric ozone layer, in order to protect the earth's ecological environment, the use of CFCS compounds must be prohibited. In addition to considering the properties of the foaming agent itself, it is generally necessary to make appropriate adjustments and improvements to the raw materials such as polyether polyols, homogenizers, catalysts, etc., so that the formula system can be optimized, so the key to physical foaming agents lies in the development and application research of alternative products.