Use of thermoplastic polyurethanes as carbonisation agents in intumescent blends. Part 3: Modification of the dynamic properties of polypropylene/thermoplastic polyurethane/ammonium polyphosphate formulations with heat and stress

Previous studies have shown the fire retardancy performance and the improve d thermal behaviour of polypropylene (PP)-based materials when a thermoplas tic polyurethane (TPU)/ammonium polyphosphate (APP) system is melt blended with the polypropylene matrix. This work deals with the evaluation of the d ynamic properties of a typical PP/TPU/APP material. The experiments, carrie d out using a rheometer as a thermal reactor, allow a correlation between f ire and thermal behaviour of the material with the change of its high tempe rature dynamic properties. First, it is shown that LOI can he correlated with the viscosity values of the materials. Moreover, the expansion of PP/TPU/APP fire retarded materials, under a norm al force is considered. Then, the study shows that the different steps of i ntumescent formation and degradation can be well evaluated with the measure ment of viscosity. However, the evolution of elastic and viscous modulus al lows comparisons between physical structure of the intumescent shield and i ts efficiency under heating. The intumescence formation leads to a severe i ncrease of the viscosity values of the formulation and implies changes of i ts viscoelastic behaviour.