Polypropylene filled with flame retardant fillers: mechanical and fracture properties

Two grades of isotactic polypropylene (homopolymer and block copolymer) were filled with magnesium and aluminium hydroxides, and studied focusing the mechanical and fracture characteristics of the composites. As expected, dispersion of such fillers in PP resulted in improved stiffness and reduced tensile yield strength. By one hand, the composites fracture resistance was characterised at low strain rate applying the J‐integral concept; the resistance to crack growth initiation (J_IC) was found decreasing as the Mg(OH)₂ concentration was raised in the copolymer PP matrix. By the other hand, the linear‐elastic fracture mechanics (LEFM) parameters were determined by means of instrumented impact tests at 1 m/s on the homopolymer PP filled with uncoated Al(OH)₃ particles. The higher the Al(OH)₃ mean particle size, the lower the composite fracture energy (G_IC). In the opposite, with commercial surface‐coated filler grades it was not possible to achieve LEFM conditions to characterise the fracture toughness of filled PP at 1 m/s, because the Mg(OH)₂ surface coating, which is applied in practice to improve the melt processing, acts increasing the composite plasticity and reducing the tensile yield strength.