pressure such as at least 2000 psi (13.8 MPa) or greater, to prevent cell formation and form a dense product. A molded polyurethane may be cured in a mold until it has developed enough green strength to be demolded without damage or permanent distortion; in such cases the polyurethane may be post-cured at ambient and/or elevated temperatures as just described to further complete the cure and develop mechanical and other properties. The reaction mixture in the final curing step may contain various optional ingredients. These include, for example, plasticizers (both reactive and non‹ reactive), diluents, dispersing agents, flame retardants, colorants such as pigments and dyes; particulate reinforcing agents and fillers such as various days, calrium carbonate, silica, quartz, alumina, talc, wollastonite, mica, carbon black, graphite and other particulate carbonaceous materials, titanium dioxide, ferric oxide, chromium oxide, ground thermoset polymers and the like (it being noted that certain of these reinforcing agents will also function as colorants); short reinforcing fibers (having lengths, for example of about 2 to about 150 mm) antifoaming agents; mold release agents; antimicrobials and other preservatives; adhesion promoters such as reactive silanes; antioxidants, and the like. When making a molded or shaped cast elastomer product, curing may take place in a mold or other form, which may be open or closed. It may be performed on a belt or other apparatus. The reaction mixture may be formed into a layer on a substrate and cured thereon to form a composite. The reaction mixture might be used to impregnate a fibrous material and then be cured in place to form a fiber-reinforced composite. If desired, the reaction mixture in the final curing step can be applied to a substrate to form a multilayer structure. It is also within the scope of the invention to impregnate a fibrous substrate such as a mat of reinforcing fibers with the reaction mixture in the final curing step and then to cure the reaction mixture to form a fiber-reinforced composite. Polyurethanes made in accordance with the invention are in certain embodiments characterized in being flexible materials having an elongation (when dry) to break of at least 40% as measured according to ASTM D1708. The elongation may be at least 100%, at least 150% or at least 200%. Its tensile strength may be at least 5 KPa, at least 10 KPa or at least 20 KPa. In some embodiments the polyurethane may have a glass transition temperature (T$_{g}$) of