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Influence of intumescent flame retardant and sepiolite on the mechanical and rheological behavior of polypropylene AIP Conference Proceedings 1736, 020157 (2016) Abstract. Adding the natural fibre thermoplastic composites NFTC to engineering plastics represents a challenge due to the high temperature processing which in turn could result in the natural fibre damage. Finding applications for NFTC depends not only on the mechanical properties but also on the physical ones and the flame retardance property has an utmost importance. However; the implementation of flame retardants FRs materials has to counteract another challenge which is the drop of the corresponding mechanical properties. A comprehensive study is carried out for optimising the compounding process of the three constituents namely the thermoplastic, the natural fibre and the FR. A mixture of halogen-free phosphor based intumescent and inorganic hydrates is used as the FR system. FR mixture does not exceed 20% limit to allow enough binding effect of the thermoplastic with both fibres and FR. After conducting a screening study, it found out that Polybutylene terephthalate (PBT) and Polyamide (PA6) are good candidates due to their relatively low melting point with respect to the candidate natural fibres namely flax and kenaf. Twin screw extruder is used to compound the polymer and the FR with the pellets of the natural fibres. Extruder configuration is adjusted to obtain a stable process at 10, 15 and 22.5 wt.-% fibre. Temperature and speed are optimised in order to avoid the thermal degradation of the natural fibres as much as possible. The prepared composites are subjected to tensile test, Charpy notched and un-notched impact test, UL94 and cone calorimetry. Results are promising especially for PA6 composites. Mechanical testing shows a decrease of 10% in strength and 15-25% in impact strength after the addition of 20% FR.V-0 rating is obtained by 20% FR according to UL94 test. The cone calorimetry show a reduction in heat release rate HRR to 50% after the application of FR.