the process described above for the production of flame resistant polyacrylonitrile filaments may be applied equally to various synthetic polymers spun from a spinning solution by a dry or wet spinning process flame resistant shaped products and articles such as films foils and the like may also be produced by the process according to the present invention the process according to the present invention achieves optimum homogenization and distribution of the fire retarding agent over the whole cross section of the spun filament according to electromicrographs the particle diameter of antimony trioxide is below 50 nm preferably from 10 to 20 nm to obtain electron microscopic preparations the fibers were dissolved in dimethyl formamide and the 0 5 by weight solution was drawn out to thin films which are transparent in the electron microscope owing to the small size of the particles only a few particles may be seen under an optical microscope as test method for the flamability and fire characteristics of textile sheet products the measurement according to the limiting oxygen index loi was carried out in this method the proportion by volume of oxygen to the sum of oxygen and nitrogen is determined in that mixture in which the textile only just continues to burn from above downwards after ignition the loi is defined as follows equ1 since the arrangement of the samples and the weight per unit area influence the results only nonwoven webs weighing ca.