A method for studying nucleating additives in high-density polyethylene via crystallization of micro-droplets in immiscible blends

Typically, the study of additives with nucleating ability towards high-density polyethylene (HDPE) is chal-lenging, because of the exceedingly high intrinsic nucleation density of this polymer, which prevents the detection of mild nucleation effects with standard calorimetric methods. Here we propose a novel method to tackle this issue by making use of the fractionated crystallization, observed when blending HDPE with poly-styrene (PS). In this system, a small amount of HDPE is dispersed into droplets of around 1 mu m size in the amorphous matrix and, due to the presence of droplets ensembles free of nucleating heterogeneity, the achievable undercooling is greatly enlarged with respect to bulk HDPE. As such, it is found that various potentially nucleating additives affect the fractionated crystallization behavior of the blend, forming populations of droplets which crystallize at distinctly higher temperatures with respect to that of neat HDPE/PS. On the contrary, bulk additivated HDPEs do not show meaningful differences in their peak crystallization temperatures. The method was applied to the technological problem of shrinkage and warpage in injection moulded coloured HDPEs, revealing that the critical behavior of a particular coloured masterbatch could be clearly associated with its higher nucleating efficiency with respect to a non-critical masterbatch. Moreover, the calculation of the fraction of crystallization enthalpy in selected temperature ranges allowed to determine in a semi-quantitative way the nucleation efficiency of several additives. Thus, the proposed fractionated crystallization approach proved to be successful in the notoriously difficult task of assessing different HDPE nucleating additives.

Automated summary

This study proposes a novel method to study nucleating additives in high-density polyethylene (HDPE) by observing the fractionated crystallization when blending HDPE with polystyrene (PS). It is found that various nucleating additives affect the fractionated crystallization behavior of the blend, resulting in droplets that crystallize at higher temperatures than neat HDPE/PS.