# Analysis of the Effect of Transition-Metal Oxide Content on Reducing the Flammability of Polypropylene

## TL;DR

Adding small amounts of zinc oxide and manganese oxide significantly improves the fire resistance of polypropylene without severely compromising its mechanical properties.

## Contribution

This study demonstrates the effectiveness of low concentrations of ZnO and MnO as synergists in enhancing flame retardancy in PP/IFR composites.

## Key findings

- 0.25 wt.% of ZnO or MnO increased the LOI to 30% and achieved UL-94 V-0 classification.
- ZnO at 1.5 wt.% achieved the highest LOI of 43.7%, while MnO reached 38.6% at 2 wt.%.
- Mechanical properties like stiffness and flexural strength improved, but impact strength decreased.

## Abstract

Polypropylene (PP) exhibits high flammability (LOI ≈ 17.5%), which limits its industrial applications. Previous studies have primarily focused on the flame-retardant mechanisms of intumescent flame-retardant (IFR) systems, while less attention has been given to the role of inorganic synergistic additives in balancing flame retardancy with mechanical performance—an aspect crucial for commercial applications This study investigated the effect of small additions of zinc oxide (ZnO) and manganese oxide (MnO) on the flame-retardant, mechanical, and thermal properties of PP/IFR (APP + PER) composites. The oxide content was varied between 0 and 2 wt.%. LOI and UL-94 tests showed that as little as 0.25 wt.% increased LOI to 30% and enabled all materials to achieve a UL-94 V-0 classification. The highest performance was observed for ZnO (LOI = 43.7% at 1.5 wt.%), while MnO induced a linear increase up to 38.6% at 2 wt.%. SEM analysis confirmed the formation of a compact, foamed char layer. Mechanical testing revealed improved stiffness (~15%) and flexural strength (~20%), with unchanged tensile strength but reduced impact strength (−50% for ZnO, −30% for MnO). The HDT increased from 55 °C to 65 °C. These findings demonstrate that small amounts of ZnO and MnO act as effective and economically viable IFR synergists in PP composites.