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In spite of inherent low flammability, wool fibres have not been thoroughly investigated in fire retardant performance of natural fibre polymer composites. In the present work, polypropylene (PP)-short wool fibre composites were produced by compounding in a twin screw extruder and compression moulding. The enhancement of fire retardancy of the composites by adding wool and ammonium polyphosphate (APP) was confirmed by flammability tests. The cone calorimeter and vertical burn tests showed a significant decrease in heat release rate and a direct flame self-extinguishment of composites, respectively. Furthermore, the cone calorimeter experiments and char morphology have revealed that different polymer viscosities in the composites can influence the APP dispersion, leading to different flammability characteristics. The charring tendency of wool contributed to the formation of a rigid char layer and an …