The subject of this invention is a process for the preparation of lightweight, planar molded articles, e.g. those having a thickness from 0.1 to 40 mm and having a density of 0.015 to 0.25 g/cm.sup.3, by treating porous materials, preferably open celled or flexible or semi-rigid polyurethane foams, with an effective amount of an aqueous dispersion or emulsion of organic compounds containing NCO groups and allowing the isocyanate compound to cure, optionally while molding, at a temperature from 25.degree. C. to 180.degree. C. and optionally at a pressure of 1 to 50 bar. The invention also relates to molded articles prepared according to this process which have special mechanical properties.