<p>The current work explores the synergistic effect of reinforcing dual nanofillers of nanoclay (NC) and alumina in high-density polyethylene (HDPE) to improve thermal properties of the composites. Adding NC fillers into the HDPE affects composite crystallization temperature and impacts the onset crystallization temperature for nano-filled composites. Adding 3% NC to the HDPE matrix provided the Avrami exponent in the range of 1.16–1.35, suggesting a phase transformation involving nucleation and the growth of crystals. The Ozawa plot of polymer composites exhibits a non-linear trend, and fluctuations of the Ozawa component with temperature during crystallization indicate that the two-stage crystallization occurs within the crystallizing temperature span. The lower values of the Mo exponent indicate a slower crystallization for achieving better crystal qualities. The isoconversional approach is considered superior to traditional model-fitting kinetic methods in the context of thermal analysis of materials like polymer composites. The nucleation constants evaluated using isoconversional models are 3.4 × 10⁵, 1.75 × 10⁵, 1.31 × 10⁵, 1.62 × 10⁵ and 3.04 × 10⁵ K² for E, EC2, EC3, ECA2, and ECA3, respectively. The energy required to fold the chain segments into the nucleus surface is low for dual nanofiller composites containing NC and alumina. The fold surface free energy evaluated is 2.21 × 10⁻⁵, 7.16 × 10⁻⁶, 5.73 × 10⁻⁶, 1.19 × 10⁻⁶ and 1.07 × 10⁻⁶ J/m² for E, EC2, EC3, ECA2, and ECA3, respectively. The decreasing order of permeability of polymer composites is EC3 &gt; EA2 &gt; E &gt; ECA2 &gt; EC2. The polymer-free volume and the delamination of silicate layers influence the dispersing ability of fillers in the polymer.</p> Graphical Abstract <p></p>