Polyurethane foams are made from a chemical reaction of isocyanates with polyols (long chain alcohols) along with catalysts, silicone surfactant and water. These are what we call conventional foams, they are designed to be used in the bedding and furniture industries. Densities range from 1 to 2.5 lbs/ft3. The firmness of the foam can range from very soft to very firm. Speciality foams (Viscoelastic, HR, RBR, etc) are made by adding other process additives but the basic chemistry is similar.