1.1 This International Standard specifies a method for determining the mass fraction of a polypropylene homopolymer or copolymer which is soluble in xylene at 25 °C. 1.2 A weighed amount of dried sample is dissolved in xylene under reflux conditions, then cooled under controlled conditions and maintained at 25 °C to ensure controlled crystallization of the insoluble fraction. 1.3 The xylene-soluble fraction is then recovered by evaporation of the xylene and determined by weighing the residue. 1.4 Other materials with solubilities similar to that of the xylene-soluble fraction, such as additives, may interfere with the determination.