In situ inorganic flame retardant modified hemp and its polypropylene composites

A hemp-reinforced polypropylene (PP) composite with excellent flame retarding and mechanical properties was prepared with the melt blending method. Hemp fiber (F) was first simply in situ modified with an inorganic salt solution, which was prepared by mixing 4.32% sodium hydrate (NaOH) and 6% calcium chloride (CaCl2) at room temperature for 0.5 h. The modified hemp (F1) was proved to be modified by the calcium salt through Fourier-transform infrared and scanning electron microscopy (SEM) measurements. The thermal stability, mechanical, and flame retarding properties of the composites were studied by thermogravimetric analysis, mechanical tests, and limiting oxygen index (LOI) measurements, respectively. The LOI of the composites with 50% of F1 and 15% poly(phosphoric acid amine) reached 25.5%, which was much higher than that of PP (18.5%). The tensile strength of the composite with 50% of F1 was 38% higher than that of PP (24 MPa). The composite interface between the F1 and PP matrix was investigated by SEM, and was shown to be significantly improved. This showed that in situ inorganic modification of hemp with a mixture of NaOH and CaCl2 is a promising method to improve both the flame retarding and mechanical properties of the biocomposite.