Patent ID: 7816456

Claim:
A biaxially stretched film having a longitudinal stretching ratio of at least 1:3 and a transverse stretching ratio of at least 1:5 obtained by biaxially stretching a semicrystalline propylene polymer composition with a melting point T M of from 135 to 165° C., where the melting point T M is measured by differential scanning calorimetry (DSC) to ISO 3146 by heating a previously melted specimen at a heating rate of 20° C/min, and is the maximum of the resultant curve, where the semicrystalline propylene polymer composition can be broken down into from 65 to 85% by weight of a principal component A, from 10 to 35% by weight of an ancillary component B and from 0 to 25% by weight of an ancillary component C, where the proportions of component A, B and C are determined by carrying out TREF (temperature rising elution fractionation) in which the polymers are firstly dissolved in boiling xylene and the solution is then cooled at a cooling rate of 10° C/h to 25° C., and then, as the temperature rises, that fraction of the propylene polymer composition which is soluble in xylene at (T M /2)+7.5° C. is firstly dissolved and separated off from the remaining solid, and then, as the temperature rises, at all of the higher temperatures 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 98° C., 102° C., 107° C., 112° C., 117° C., 122° C. and 125° C., the fractions soluble within the temperature range between this elution temperature and the preceding elution temperature are eluted, and the fractions taken into consideration during the evaluation which follows are those whose proportion by weight is at least 1% by weight of the initial weight of the propylene polymer composition, and gel permeation chromatography (GPC) at 145° C. in 1,2,4-trichlorobenzene is used to measure the molar mass distribution of all of the fractions to be taken into consideration, and the principal component A is formed by all of the fractions which are to be taken into consideration and are eluted at above (T M /2)+7.5° C. and have an average molar mass M N (number average) ≧120,000 g/mol, the ancillary component B is formed by the fraction which is eluted at (T M /2)+7.5° C., and the ancillary component C is formed by all of the fractions which are to be taken into consideration and are eluted at above (T M /2)+7.5° C. and have an average molar mass M N (number average) <120,000 g/mol, and where at least 50 mol % of the resultant monomer units stem from the polymerization of propylene, which process comprises polymerizing propylene, alone or together with ethylene and/or C 4 -C 18 -1-alkenes, using a catalyst system with only one metallocene catalyst, and conducting the polymerization in a reactor cascade composed of at least two reactors.