  Polyimide-based materials, like Kapton, are widely used in flexible cables
and circuitry due to their unique electrical and mechanical characteristics.
This study is aimed at investigating the radiopurity of Kapton for use in
ultralow background, rare-event physics applications by measuring the
$^{238}$U, $^{232}$Th, and $^{nat}$K levels using inductively coupled plasma
mass spectrometry. Commercial-off-the-shelf Kapton varieties, measured at
approximately 950 and 120 pg/g $^{238}$U and $^{232}$Th (1.2$\times$10$^4$ and
490 $\mu$Bq/kg), respectively, can be a significant background source for many
current and next-generation ultralow background detectors. This study has found
that the dominant contamination is due to the use of dicalcium phosphate (DCP),
a nonessential slip additive added during manufacturing. Alternative Kapton
materials were obtained that did not contain DCP and were determined to be
significantly more radiopure than the commercially-available options with 12
and 19 pg/g $^{238}$U and $^{232}$Th (150 and 77 $\mu$Bq/kg), respectively. The
lowest radioactivity version produced (Kapton ELJ, which contains an adhesive)
was found to contain single digit pg/g levels of $^{238}$U and $^{232}$Th,
two-to-three orders of magnitude cleaner than commercial-off-the-shelf options.
Moreover, copper-clad polyimide laminates employing Kapton ELJ as the insulator
were obtained and shown to be very radiopure at 8.6 and 22 pg/g $^{238}$U and
$^{232}$Th (110 and 89 $\mu$Bq/kg), respectively.
