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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. 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. The xylene-soluble fraction is then recovered by evaporation of the xylene and determined by weighing the residue. Other materials with solubilities similar to that of the xylene-soluble fraction, such as additives, may interfere with the determination.