Effect of intumescent flame retardants on non-isothermal crystallization behavior of high-density polyethylene

In this study, the influence of the addition of intumescent flame retardants (IFRs) on the non-isothermal crystallization of high-density polyethylene (HDPE) was studied. The investigations were carried out for polyethylene compositions containing two different IFRs, ammonium polyphosphate (APP) and a mixture of copper phosphate and melamine phosphate (CUMP). Both IFRs have been incorporated with a constant amount of 20 wt%. Non-isothermal kinetic parameters were calculated from differential scanning calorimetry (DSC) analysis using the Jeziorny and Mo methods. The research demonstrates the appropriateness of using the latter approach to evaluate the HDPE-based crystallization process changes. The results of the obtained kinetic parameters describing HDPE crystallization were related to the activation energy assessed by the Friedman method, microscopic analysis performed using polarized light microscopy, and wide-angle X-ray scattering (WAXS). Despite the increase in the activation energy (E-X) of the flame retarded polyethylene series, the degree of crystallinity was comparable to that of unmodified HDPE due to a limited dependence effect of E-X on relative crystallinity. Investigations made using polarized light microscopy showed an increase in the density of spherulite impingement for HDPE/IFRs. Moreover, despite introducing crystalline organic compounds, polyethylene did not reveal any polymorphic changes according to WAXS.