Bulk vs. Nano ZnO: Influence of fire retardant behavior on sisal fibre yarn

•Fire retardant sisal yarn has been developed using Nano ZnO based formulations.•1% Nano ZnO provides almost similar thermal stability to sisal as like 12% bulk.•Nano ZnO treated sisal yarn maintained physical properties after the treatment.•Burning, thermal degradation, char morphology and the mechanism of fire retardancy. Flame retardant functionality was imparted in sisal (leaf fibre) yarn whereas a strong scientific finding has been established between fire retardant efficacy of bulk and nano zinc oxide based formulations. Bulk and nano ZnO treated sisal yarns have been compared on the basis of their flame retardant efficacy, weight add-on% and tensile strength. Limiting oxygen index and char length of the 12% ZnO treated sisal was found to be well comparable with the 1% nano ZnO treated yarn. Further, add-on% and the tensile strength of the nano ZnO treated sample is 70–80% lower and 20% more, respectively, compared to the 12% bulk ZnO treated sisal yarn. Besides, thermo-gravimetry and char morphology of the control and both the formulation treated sisal yarn were compared and analysed to understand the pyrolysis path of the sisal yarn. The possible mechanism of attachment of ZnO to the microstructure of sisal has also been established.