Feasibility of Mini-Scale Injection Molding for Resource-Efficient Screening of PP-Based Cable Insulation Nanocomposites

This study presents a critical evaluation of the effect of two different sample manufacturing techniques on the morphological and dielectric properties of polypropylene (PP)-based nanocomposites, namely mini-scale injection molding (IM) vs. pilot-scale cast film extrusion. Polarized light microscopy revealed that the IM specimen morphology exhibited a layered "skin-core" type morphology, largely differing from the spherulitic morphology of the corresponding extruded cast films. Higher degree of crystallinity in the IM specimens was evidenced by calorimetric and X-ray diffraction methods. The processing-dependent morphological differences were found to affect the isothermal charging current (ICC) and thermally stimulated depolarization current (TSDC) characteristics due to differences in charge mobility and trapping, thus making direct comparison of IM and cast film specimens non-straightforward. Nevertheless, mini-scale injection molding can be seen as a resource-efficient sample manufacturing method for facilitating early-stage screening of the best-performing material candidates, given that the morphological features are carefully taken into account. I. INTRODUCTION The engineering dielectric properties of semi-crystalline polymers are known to be closely related with their crystalline structure and bulk morphological features. For instance, the processing-dependent morphology evolution at different scales of structures ranging from small (lamellar) to large (e.g. spherulitic) scale dictates the formation and dielectric properties of the crystalline-amorphous interface [1], thus making processing and specimen preparation non-trivial factors when evaluating and comparing dielectric performance of new materials [2]-[4]. PP based nanocomposite materials are currently being developed for high voltage direct current (HVDC) cable insulation applications in the EU project GRIDABLE, involving material optimization, compounding and sample manufacturing at several different batch sizes at VTT. Recently, mini-scale compounding and injection molding have been identified as a potential resource-efficient alternative for pilot-scale compounding and cast film extrusion to facilitate early-stage screening of the best-performing material candidates. This paper presents a critical comparison of the morphological and dielectric features of PP-based mini-injection molded and extruded cast film specimens, and evaluates the feasibility of the mini-injection molding technique for assessing the effect of nanosilica on the charge trapping and transport properties of PP based nanocomposites.