With respect to the base resin of the matrix, plastic foams may be either thermoplastic or thermosetting. Whereas the chemical and physical nature of the matrix is the principal factor in determining most foam properties, the composition of the gaseous phase has an important effect on selected properties such as thermal resistance. Physical blowing agents undergo only physical change. The most common are low-boiling organic liquids such as hydrocarbons and halogenated hydrocarbons, which develop cells within the plastic material by changing from liquid to gas during foaming under the influence of heat. When physical blowing agents are used in foaming, therefore, the gas phase of the foam is chemically identical with the blowing agent.
Water is a well known blowing agent still used in the production of some polyurethane foams.
Thus water may be considered a sort of chemical blowing agent. Although the gas generated during foaming is originally present in closed-cell foams, it may diffuse slowly in service and gradually be replaced by air, water vapour and CO 2 from the atmosphere. Cell Structure Cell structure determines certain properties, thereby influencing the type of application of the foamed plastic. Open-cell foams offer little resistance to the passage of liquids and gases through them. A general principle is that flexible foams have open-cell structure and rigid foams closed cells.