Experimental Study on Reducing the Degradation of Mechanical Properties in Huntite-Hydromagnesite-Filled Flame-Retardant Polymer Composites Containing Calcite and Clay for Engineering Applications

In this work, the possibility of ensuring sufficient fire resistance while preserving the mechanical properties of composites based on polypropylene with the use of hybrid systems of mineral fillers, including huntite/hydromagnesite, calcite, and clay, was studied. Composites with different polypropylene/mineral filler ratios were produced using twin-screw extruder and injection molding methods. The tensile properties, Limiting Oxygen Index, UL-94 vertical combustion performance, and fracture surface morphologies of the samples were evaluated. The results showed that the elastic modulus increased with increasing total mineral filler ratio, while the maximum tensile stress and elongation at break decreased. Using huntite/hydromagnesite in a hybrid with calcite and clay at the same total filler ratio resulted in a partial improvement in mechanical properties compared to composites containing only huntite/hydromagnesite. The highest flame-retardancy performance was obtained in composites containing 70% total mineral filler; all samples in this group met the UL-94 V0 class, and the highest Limiting Oxygen Index value of 42.6% was determined in the sample with a polypropylene:huntite/hydromagnesite:clay ratio of 30%:60%:10%. The developed composites exhibit the highest fire-resistance class. The findings reveal that huntite/hydromagnesite/calcite/clay hybrid mineral systems are a promising alternative that can contribute to balancing mechanical performance while providing halogen-free flame retardancy in polypropylene composites.