Evaluation of Zinc Borate and Melamine Polyphosphate Synergy on the Rheological, Thermal, Mechanical, Dimensional, and Flame Retardant Performance of <scp>rLLDPE</scp> / <scp>CaCO</scp> ₃ Composites for Pipes Applications

ABSTRACT The use of recycled thermoplastics is crucial for addressing environmental challenges such as waste management and sustainability. Recent upcycling trends have focused on enhancing recycled materials for advanced applications, such as flame‐retardant pipes used in underground mining. To improve the flame‐retardant properties of recycled LLDPE (rLLDPE) and expand its potential applications, flame‐resistant rLLDPE/CaCO 3 composites containing melamine polyphosphate (MPP) and zinc borate (ZB) were developed using a melt‐mixing method. The contents of MPP and ZB in the polymer were varied at 5 and 10 wt% respectively, to evaluate their individual and combined effects. The results showed improvements in flame retardancy, with higher char content, leading to a considerable improvement in the Limiting Oxygen Index (LOI) from 18.2% for neat rLLDPE to 23.7% for the MPP/ZB combination, achieving a UL‐94 V‐1 rating. The rLLDPE/CaCO 3 /MPP5/ZB10 composite exhibited a 52% increase in elastic modulus and a 10% increase in tensile strength, along with low water absorption (&lt; 0.07%) and good dimensional stability. The synergistic effect of MPP and ZB improved the flame‐retardant performance by forming a dense and compact physical barrier that stopped heat transmission and suppressed flame spread. This research contributes to the development of upcycled materials with enhanced flame‐retardant properties for industrial applications including pipe fabrication.