They can decompose with heat, shock or light and then ignite or explode. They are often diluted with another liquid or solid, or wetted with water to reduce the hazard. Monomers of plastics such as styrene, methyl methacrylate, vinyl acetate and ethylene oxide start polymerization with heat, light, shock or by the addition of a catalyst. This reaction is exothermic and can go out of control, resulting in fire or explosion. Inhibitors are usually added to prevent the onset of spontaneous polymerization.
Polyurethane foam is made by the polymerization of an isocyanate and polyether or polyester.
The reaction is exothermic. Fresh products, when piled up, may cause spontaneous heating. Concluding Statement Better understanding of spontaneous ignition could prevent many fires. The important factors regarding this phenomenon and a number of examples of materials known to ignite spontaneously have been presented. References Materials Subject to Spontaneous Heating, Fire Protection Handbook NFPA, Boston, Mass., Jan. 1976.