132 HIGH PERFORMANCE POLYMERS
The wet cake is then dried for 13 h in an oven at
105
◦
C in order to recover t he polymer. The dried par-
ticles exhibit an NMP concentration of 200 ppm, and
Na
+
concentration of 1300 ppm. The amount of PPS
recovered by a twofold continuous washing operation
is 95% of the PPS originally in the polymer slurry. The
process does not include a commercially problematic
sieving step as essential, but can effectively recover
PPS particles from the polymer slurry. As a method for
raising the crystallization temperature, the polymer is
treated in a strong acid solution of a pH less than 2 [8].
PPS with high whiteness and high melt viscosity are
obtained by adding small amounts of a zinc compound
to the polymerization feed [8]. The zinc ...