Abstract
This study experimentally evaluated
the effect of internal bubbles (void fraction) on
the combustion behavior of polypropylene (PP)
spheres under microgravity. In previous
research, it has been demonstrated that bubble
generation during the burning of polymethyl
methacrylate (PMMA) spheres clearly affected
the burning rate constant. However, because
PMMA exhibits unique pyrolysis characteristics —being composed predominantly of monomers— the
observed void effect may not necessarily apply to other polymers. In this study, PP spheres with various
initial internal void fractions were used as test specimens and burned under microgravity conditions
generated using a drop tower test facility. A total of thirteen productive runs were successfully conducted,
showing that the burning rate constant of PP spheres exhibited a positive correlation with the initial void
fraction in the range of 0–40%, while remaining nearly constant above 40%. This trend indicates that the
material-dependent physical properties play a dominant role in the process from bubble growth to
bursting. Extrapolation to zero initial void fraction yielded an ideal (with the least initial void effect)
burning rate constant of 0.60–0.72 mm²/s (indicating the potential uncertainty range), with the
corresponding B-number ranging from 0.42–0.53. These findings suggest that bubble dynamics still
significantly influence the burning behavior of PP and confirm the validity of void-fraction-based
evaluation methods for identifying the intrinsic burning properties of polymers.
