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Blowing agent is a substance which is capable of producing a cellular structure in a variety of materials that undergo hardening or phase transition (such as polymers, plastics, metals). They are applied when the blown material is in liquid stage. Blowing agents include compressed gases that expand when pressure is released, soluble solids that leave pores when leached out, liquids that develop cells when they change to gases, and chemical agents that decompose or react under the influence of heat to form a gas. Chemical blowing agents range from simple salts such as ammonium or sodium bicarbonate to complex nitrogen releasing agents.