Trihydrazinotriazine (THT) and biurea are both used industrially as blowing agents for expanding certain types of polymers. The term 'blowing agent' is applied to any substance which can produce pores or cells in a polymeric material. There are two classes of blowing agent, physical and chemical. Physical blowing agents depend on a change of state of the substance to produce the cells e.g. the expansion of a compressed gas or evaporation of a liquid. Chemical. blowing agents depend on the evolution of gases by thermal decomposition to produce the. cells. It is to this latter class that THT and biurea belong.