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

This work deals in the first part, with the fire retardancy of polypropylen e (PP)/thermoplastic polyurethane (TPU)/ammonium polyphosphate (APP) intume scent blends, using TPU with different chemical compositions. The influence of the chemical nature of the polyol used in the synthesis of the TPU on t he fire resistance (FR) of materials is discussed. Moreover, mechanical pro perties (elongation/stress relationship) of the polymeric materials are stu died and related to the morphologies of the blends. Improved FR materials w ith very different mechanical properties are formulated. The second part deals with the evaluation of the dynamic properties of a pa rticular FR PP/TPU/APP formulation. The experiments have been carried out u sing a thermal scanning rheometer as a fire reactor. First, the expansion o f intumescent material under normal force is studied. Then, the study shows that the different steps of intumescent formation and degradation can be w ell evaluated from the measurement of viscosity. The loss of the protective character, ie reduction of heat and mass transfers related to the superfic ial foam morphology, may be explained by change of the viscosity of the cha rred material under stress. The influence of an external stress on the surf ace morphology of the char is then proposed. (C) 2000 society of Chemical I ndustry.