The extrusion of both materials was repeated up to nine cycles.
The screw rotational speed was 200 rpm and an output of about
7 kg/h was achieved. This process was performed at two different
barrel temperature profiles, one within the recommended processing
temperature of SHELFPLUS® O2 2710 (T1:160-220-230-230-
200 C) and one above (T2:210-300-270-300-270 C). Thus we
received four different material/temperature-combinations with a
number of process steps from one to nine. Due to effects that will be
explained in this work, both pure PP and PP_20%SP could not be
extruded nine times at the high temperature profile without major
problems in process stability. This is why pure PP was only extruded
six and PP_20%SP eight times at the high temperature profile (T2).
At high temperature treatment PP_20%SP was noticeably rough on
the surface so that a non-negligible amount of cooling water
remained on the strand. To ensure maximum processability at the
given temperature profile, the wet granulate was dried at 60 C for
2 h. Within one material/temperature-combination a stable melt
temperature and a decreasing melt pressure was noticed along
increasing number of extrusions. All material/temperaturecombinations
developed a smoky smell and discoloration (Fig. 1)
when reprocessed several times.