Analysis of Flame Retardancy in Polymer Blends by Synchrotron X‑ray K‑edge Tomography and Interferometric Phase Contrast Movies

Underwriters Laboratories 94 test
bars have been imaged with X-ray
K-edge tomography between 12 and 32 keV to assess the bromine and
antimony concentration gradient across char layers of partially burnt
samples. Phase contrast tomography on partially burnt samples showed
gas bubbles and dark-field scattering ascribed to residual blend inhomogeneity.
In addition, single-shot grating interferometry was used to record
X-ray movies of test samples during heating (IR and flame) intended
to mimic the UL 94 plastics flammability test. The UL 94 test bars
were formulated with varying concentrations of a brominated flame
retardant, Saytex 8010, and a synergist, Sb2O3, blended into high-impact polystyrene (HIPS). Depending on the sample
composition, samples will pass or fail the UL 94 plastics flammability
test. Tomography and interferometry imaging show differences that
correlate with UL 94 performance. Key features such as char layer,
gas bubble formation, microcracks, and dissolution of the flame retardant
in the char layer regions are used in understanding the efficiency
of the flame retardant and synergist. The samples that pass the UL
94 test have a thick, highly visible char layer as well as an interior
rich in gas bubbles. Growth of gas bubbles from flame-retardant thermal
decomposition is noted in the X-ray phase contrast movies. Also noteworthy
is an absence of bubbles near the burning surface of the polymer;
dark-field images after burning suggest a microcrack structure between
interior bubbles and the surface. The accepted mechanism for flame
retardant activity includes free radical quenching in the flame by
bromine and antimony species. The imaging supports this as well as
provides a fast inspection of other parameters, such as viscosity
and surface tension.