Dispersed polypropylene fibrils improve the foaming ability of a polyethylene matrix

We compare the foaming ability of metallocene catalysed polyethylene (mPE) with that of mPE containing fibrillated polypropylene (PP) domains (mPE/fibrillated-PP) and mPE containing spherical PP domains (mPE/spherical-PP). We observe that mPE/fibrillated-PP shows the best foam morphology with the highest number of bubbles per unit volume. We explain this enhancement in foaming ability through rheological and crystallization studies. We identify that the fibrils form a percolated network at a fibril content of 4.5 wt% using the Winter-Chambon analysis. The network of entangled fibrils results in strain hardening in uniaxial extension. Shear thickening responses are observed in shear flow. Such responses are not observed for mPE or mPE/spherical-PP. Oscillatory shear flow investigation of the isothermal melt crystallization reveals a two decade decrease in the time for the onset of crystallization in the mPE/fibrillated-PP and a decade decrease in mPE/spherical-PP relative to neat mPE. We attribute the enhancement in foaming ability of mPE/fibrillated-PP to the concurrent increase in strain-induced hardening response and improved crystallization kinetics.