Foaming agent refers to a type of additive that can form a cell structure in plastics, that is, to make foam plastics. They can produce large quantities of gas under certain conditions, forming pores containing continuous or discontinuous pores (i.e., open or closed cells), making plastics a porous structural material with a combination of gases and solids, which can reduce the density of plastics and enhance their acoustic and thermal insulation.
According to the different ways of gas generation, blowing agents can be divided into two categories: physical blowing agents and chemical blowing agents.
Physical blowing agent
Physical blowing agent is mainly through the change of the physical state of the blowing agent to form bubble holes in plastics.
The conditions that an ideal physical blowing agent should have are:
(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°C.
During processing, when the pressure is removed, the compressed gas expands, or the liquid evaporates and expands when heated, or the dissolved soluble solid substance sublimates to produce gas.
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.
Chemical blowing agents
Chemical blowing agents are also known as decomposition blowing agents. They are evenly dispersed in resins and decomposed by heat to produce at least one gas. It can be divided into two categories: inorganic blowing agent and organic blowing agent. Organic blowing agents are the main blowing agents used in plastics, mainly azo, nitroso, and sulfonylhydrazide. There are also blowing agent compositions whose foaming gas is released by an endothermic reaction between the two components.