Original Article Properties of polypropylene/hemp fibers flame-retardant composites: Effects of different processing methods

Polypropylene/hemp fibers flame retardant composites were prepared by melt compounding. Ammonium polyphosphate and organomodified montmorillonite were used as flame retardants. Maleic anhydride grafted polypropylene was used as compatilizer. Polypropylene/hemp fibers composites with same percentage of ingredients (polypropylene: hemp fibers: ammonium polyphosphate: organomodified montmorillonite: Maleic anhydride grafted polypropylene 35: 35: 30: 5: 5) were prepared by three different processing methods: In Composite A, the ingredients were added continuously into internal mixer and blended one time; In Composite B, the different ingredients were divided into three batch groups and added into internal mixer and blended three times, respectively; In Composite C, hemp fibers were immersed in ammonium polyphosphate aqueous solution, then evaporated water and blended with other ingredients. According to the results of scanning electron microscopy, ammonium polyphosphate and organomodified montmorillonite are distributed differently in the composites prepared by different processing methods. The distribution of ammonium polyphosphate and organomodified montmorillonite was also consistent with dynamic mechanical analysis and differen-tial scanning calorimetry. The limiting oxygen index, thermal gravimetric analysis and mechanical testing were used for the investigation of fire resistance, thermal stability and mechanical properties, respectively. The results show that: limiting oxygen index of composites prepared by different processing methods are same, and the value is 29%; Composite A has the best comprehensive mechanical properties; Composite B exhibits the best thermal stability; Composite C shows the highest tensile modulus and storage modulus, but the worst thermal stability