# Flame Retardant Polypropylenes: A Review

## TL;DR

This paper reviews flame retardant additives for polypropylene to help researchers choose the best options for reducing flammability.

## Contribution

A comprehensive classification and evaluation of flame retardants for polypropylene using the Flame Retardancy Index (FRI).

## Key findings

- Flame retardants are categorized as phosphorus-based, nitrogen-based, mineral, carbon-based, bio-based, or hybrid.
- PP composites are classified as Poor, Good, or Excellent based on flame retardancy performance using FRI.
- Correlations between FRI and other criteria like UL-94 and LOI are explored for performance evaluation.

## Abstract

Polypropylene (PP) is a commodity plastic known for high rigidity and crystallinity, which is suitable for a wide range of applications. However, high flammability of PP has always been noticed by users as a constraint; therefore, a variety of additives has been examined to make PP flame-retardant. In this work, research papers on the flame retardancy of PP have been comprehensively reviewed, classified in terms of flame retardancy, and evaluated based on the universal dimensionless criterion of Flame Retardancy Index (FRI). The classification of additives of well-known families, i.e., phosphorus-based, nitrogen-based, mineral, carbon-based, bio-based, and hybrid flame retardants composed of two or more additives, was reflected in FRI mirror calculated from cone calorimetry data, whatever heat flux and sample thickness in a given series of samples. PP composites were categorized in terms of flame retardancy performance as Poor, Good, or Excellent cases. It also attempted to correlate other criteria like UL-94 and limiting oxygen index (LOI) with FRI values, giving a broad view of flame retardancy performance of PP composites. The collected data and the conclusions presented in this survey should help researchers working in the field to select the best additives among possibilities for making the PP sufficiently flame-retardant for advanced applications.