  The mass concentrations of Th-232 and U-238 in several strong, unfilled, high
performance polymers are reported as measures of their radiopurity. Highly
radiopure polymers are required as dielectric materials in the construction of
rare event physics detectors, in order to minimize background signals arising
from the detector materials themselves. New data are reported for carefully
sourced samples of polyetheretherketone (PEEK), polyetherketoneketone (PEKK), a
polyamide imide (PAI, branded as Torlon) and polybenzimidazole (PBI). Data for
solid polyetherimide (PEI) are also discussed and new data for PEI in the form
of a commercial 3D printing filament stock are reported. Strong high
performance polymers PEKK, PBI, PAI and PEI were found with levels for Th-232
and U-238 that are below one milliBq/kg, including the PEI 3D printing
filament. Specifically, for Th-232 and U-238 respectively, in microBq/kg, we
found values of 149 and 184 for Arkema Kepstan PEKK 6002 flake; 69 and 2250 for
Solvay Ketaspire PEEK flake; 346 and 291 for PBI Performance Products low
metals grade 100 mesh PBI powder; 66 and 105 for Drake Plastics PAI Torlon
T-4200 pellets; 401 and 285 for Drake Plastics cured PAI rod; 32 and 41 for
Ensinger PEI Ultem 1000 solid; and 15 and 85 microBq/kg, for ThermaX PEI Ultem
1010 filament material. These results were all obtained using a novel dry
ashing method in crucibles constructed of ultra low background (ULB)
electroformed copper. Samples were spiked with T-229h and U-233 as internal
standards prior to ashing and determinations were made by inductively coupled
plasma mass spectrometry (ICP-MS). Radiopurity is displayed graphically
relative to numerical measures of mechanical strength for these and several
other polymers.
