Structural heterogeneity and evolution in ultrahigh-filled polypropylene/flake graphite composites during injection molding.

The structural organization of thermally conductive fillers with large aspect ratios in the polymer matrix is crucial for phonon transmission and heat transfer within polymer composites. However, how asymmetric fillers will response to the flow fields during melt processing especially with ultrahigh loadings is remaining unclear. In this work, the effects of the flow during injection molding on structural development of thermally conductive network in polypropylene/flake graphite (PP/FG) composites were investigated. The results show that an ultrahigh-filled PP/FG composite, namely PP/FG (30/70), exhibit much higher thermal conductivity at the distal end than the proximal end (close to the injection gate). This discrepancy in thermal conductivity of the PP/FG (30/70) composite can be amplified by adding spherical alumina (Al 2 O 3) microparticles with an optimum of 2.5 wt%. The variation of thermal conductivity is attributable to the "heterogeneity" in structural organization of FG along the injection direction. The findings of this work provide new insight that the fountain flow during injection molding could induce structural heterogeneity of FG networks within an ultrahigh-filled PP/FG composites, due to the inhibited relaxation of FG orientation under a crowded packing. [Display omitted] [ABSTRACT FROM AUTHOR]