Synergistic Effect of Modified Natural Fibres with Halogen-Free Fire Retardants in Reducing Flammability of Composites

Silanization process was used to improve adhesion of hydrophilic hemp fibers with a hydrophobic polypropylene matrix. The modification of fibers was carried out using 3-(diethylenetriamine)propyltrimetoxysilane. Natural fibres reinforced composites (NFRC) were obtained by mixing the polypropylene (PP) with untreated and silane modified hemp fibres using co-rotating twin screw Leistritz extruder. The flame retardants such as multiwalled carbon nanotubes (CNT), ammonium polyphosphate (APP), guanidinium carbonate (GC), pentaerythritol (PEr) and melamine polyphosphate (MPP) were introduced into the NFRC by a Dynisco laboratory extruder. The influence of silanization process on the chemical structure of fibers was studied by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the untreated and silane treated fibres and their impact on the polymer matrix were characterized by scanning electron microscopy (SEM). Interactions between the additives and their impact on the thermal stability of the fibres, polymers and composites was investigated using thermogravimetric analysis (TGA). Combustibility measurements were carried out by pyrolysis combustion flow calorimeter (PCFC). The improvement in the thermal stability of hemp fibres as a result of silanization process and higher efficiency of modified fibers in reducing the flammability of polypropylene were found as compared with the unmodified fibers. The results showed synergistic effect between silanized hemp fibres and carbon nanotubes in reducing flammability and increasing thermal stability of the composites. All of the used compounds in combination with silanized natural fibres decrease flammability of polypropylene with the best results observed for ammonium polyphosphate and melamine polyphosphate.