measured by x ray fluorescence it is expressed in table 2 as ppm by weight of sodium with respect to the pvdf as regards the potassium alkylsulphonate r u2014 so 3 k the proportion by weight with respect to the pvdf manufactured can be between 0 and 300 ppm the alkyl groups r of this sulphonate are linear or branched and have from 1 to 11 carbon atoms ethyl methyl isopropyl and n propyl are preferred the proportion of alkylsulphonate in the pvdf is determined by nmr analysis evaluation of the heat stability a 260 u00d7 20 u00d7 4 mm plaque is formed from 40 g of pvdf powder by compression moulding under 30 bar and at 205 u00b0 c for 6 minutes and is subjected to quenching in water at 20 u00b0 c the plaque is subsequently reheated in a metrastat u00ae psd 260 oven at 265 u00b0 c for 1 h after this heat treatment the plaque may be more or less coloured the colour is determined by a yellowing measurement the plaque is placed on a calibrated white ceramic and the yellowing index is measured with the help of a minolta u00ae cr 200 calorimeter using the astm d 1925 standard for the calculation of the yellow index the heat stability is also evaluated by a plaque moulding the powder resulting from the drying of the latex is granulated in a clextrale u00ae bc 21 twin screw corotating extruder at a temperature of 230 u00b0 c and a rotational speed of 120 revolutions per minute a rod die makes it possible to prepare granules with a size of 4 mm using these granules plaques with a diameter of 65 mm and.