C.1.1 50 to 99, preferably 60 to 80 parts by weight of aromatic vinyl compounds and/or of aromatic vinyl compounds comprising substituted nuclei, such as styrene, ??-methyl-styrene, p-methylstyrene, p-chlorostyrene, and/or (C1 to C8) alkyl esters of (meth)acrylic acid such as methyl methacrylate or ethyl methacrylate), and C.1.2 1 to 50, preferably 20 to 40 parts by weight of vinyl cyanides (unsat