Creep Properties of Biofiller- and Fire Retardant-Filled Polypropylene Composites

Biofillers, lignocellulosic and cellulosic fillers have the potential to significantly improve the mechanical properties of polypropylene (PP) and reduce its carbon footprint by reducing the amount of petroleum-derived polymer used. In addition, the realization of fire retardancy of biofiller-filled PP composites is an important key topic to enhance their applications; ammonium polyphosphate (APP) is an effective fire retardant (FR). In this study, to ensure the reliability of biofiller- and FR-filled PP composites, the creep properties were investigated in terms of the filler and FR content and filler type. In particular, the influence of APP addition into polymer composites on the creep properties has not been studied thoroughly. Two biofillers, wood flour (WF) and cellulose filler (CeF), with similar particle sizes and aspect ratios were used in this study. The creep test was conducted at a temperature of 80℃ in an accelerated test. Furthermore, the creep strain curves were modelled by the Burgers model of the viscoelastic constitutive equation to analyse the creep deformation behaviour. The incorporation of biofillers into the PP matrix significantly decreased the creep strain and improved the creep-rupture life with increasing filler content. Moreover, the creep-rupture life was longer for WF-filled PP composites than for CeF-filled PP composites. On the other hand, we found that incorporation of FR increased the creep rate at steady state and decreased the creep-rupture life of biofiller-filled PP composites, although the instantaneous creep strain decreased.