A transparent, monolithic or laminated sheet material based on a methyl methacrylate homopolymer or copolymer, the laminated material having at least one of its two outer layers made of the methyl methacrylate homopolymer or copolymer, characterized in that the monolithic sheet of methyl methacrylate homopolymer or copolymer or at least one outer layer of methyl methacrylate homopolymer or copolymer of the laminated sheet has, in the region of the surface, over a depth of at least 50 nanometers from the surface, an increase in its average density of 0.1 to 1 g/cm.sup.3, the average density being determined by X-ray reflectometry.
