A Study on the Foaming
Behaviors of PP Resins with Talc as Nucleating Agent
Hani E. Naguib*,
Chul B. Park, Patrick C. Lee, and Donglai
Xu
Microcellular Plastics Manufacturing Laboratory
Department of Mechanical and Industrial Engineering
This paper elucidates the effects of
nucleating agents on the foaming behavior of polypropylene (PP) foams blown
with butane in extrusion. The cell density was controlled by varying the talc
content, and foam expansion was observed at a fixed blowing-agent content while
varying the melt and die temperatures. As observed in our previous studies, the
curve of the final expansion ratio of PP foam versus temperature showed a
typical mountain shape for each talc content, indicating the two governing
expansion mechanisms which are gas loss and stiffening of melt. As the talc
content increased, the expansion curve skewed towards the lower temperature,
which showed that the expanded foams of high talc content were more susceptible
to gas loss. In order to analyze this change, the early-stage expansion
behavior of extruded PP foams was investigated using a CCD camera. The
expansion-profile images captured at the die exit show that the expansion rate
of extruded foams was observed to be faster with a higher talc content because
of the reduced diffusion distance of gas molecules to the nearest stabilized
cell. The higher growth rate promoted the formation of an initial hump in the
expansion profile which is known to be detrimental to large expansion. In order
to decrease the expansion rate and thereby remove the initial hump, the
temperature had to be further decreased, and consequently, the optimum
temperature to maximize expansion decreased at a higher talc content. On the
other hand, the increased cell density with a high talc content increased the
number of cell layers in the cross section of the extruded foam, and
consequently the gas loss was localized to the cells on the surface which acted
favorably for the final expansion ratio to a certain degree.
Keywords: Foam Extrusion, Polypropylene,
Blowing Agent, Nucleating Agent, Crystallization, Volume Expansion, Gas
Diffusivity, Foam Density, Cell Density