Use of polyurethanes as char‐forming agents in polypropylene intumescent formulations

This work deals in the first part, with the fire retardancy of polypropylene (PP)/ thermoplastic polyurethane (TPU)/ammonium polyphosphate (APP) intumescent blends, using TPU with different chemical compositions. The influence of the chemical nature of the polyol used in the synthesis of the TPU on the fire resistance (FR) of materials is discussed. Moreover, mechanical properties (elongation/stress relationship) of the polymeric materials are studied and related to the morphologies of the blends. Improved FR materials with very different mechanical properties are formulated. The second part deals with the evaluation of the dynamic properties of a particular FR PP/TPU/APP formulation. The experiments have been carried out using a thermal scanning rheometer as a fire reactor. First, the expansion of intumescent material under normal force is studied. Then, the study shows that the different steps of intumescent formation and degradation can be well evaluated from the measurement of viscosity. The loss of the protective character, ie reduction of heat and mass transfers related to the superficial foam morphology, may be explained by change of the viscosity of the charred material under stress. The influence of an external stress on the surface morphology of the char is then proposed.

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