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Optimization of Thermal Stability of High-Density Polyethylene Composite Using Antioxidant, Carbon Black and Nanoclay Addition by a Central Composite Design Method: X-ray Diffraction and Rheological Characterization

Abstract The thermal stability of high-density polyethylene, as characterized by an Oxidation Induction Time (OIT) test in the presence of the three additives, antioxidant, carbon black and nanoclay, was investigated. The twenty experiments used were designed by using the Central Composite Design (CCD) method, and the effects of the main parameters and their interactions were analyzed. The results of Analysis of Variance (ANOVA) showed that the antioxidant, nanoclay and carbon black contents, the antioxidant-nanoclay interaction and the antioxidant-carbon black interaction, had significant effects on the thermal stability. By optimization of the percent compositions used, the best composition was obtained, 0.26%, 2.21% and 4.69% for antioxidant, carbon black and nanoclay, respectively, to achieve thermal stability equal to the targeted value of 60 minutes. X-ray diffraction confirmed a weak polymer-nanocaly interaction in the optimal sample structure. The rheology frequency sweep test showed an increase in the elastic and viscous modulus in the optimal sample relative to the control samples. The Melt Flow Index (MFI) test for an optimum sample showed a pseudo-plastic structure with the lowest melt flow index of 0.2614 g/10 min, which was consistent with the results of the complex viscosity, with the optimal sample having the highest value (128500 Pa.s). The density test indicated the most crystalline structure for the optimized sample, with the highest value (0.976 g/cm3) compared to the control samples (0.962 and 0.957 g/cm3 for the polyethylene/nanoclay and the polyethylene/carbon black sample, respectively).