Intumescent flame retardant for polymers. III. The polypropylene–ammonium polyphosphate–polyurethane system

The action of some mixtures of ammonium polyphosphate (APP) and polyurethanes (PUr) as flame retardant (FR) agents for polypropylene (PP) was examined. The APP–PUr systems behave as intumescent flame retardant (IFR) formulations. The IFR action of these systems is described by the comparison of the oxygen index (OI) values measured on molded pellets and on molded rods, by measurements of the polymer temperature just below the burning surface and by visual observations. A strong synergism between APP and some polyurethanes, having piperazine units in the polymer chain, was ascertained with a maximum 3:1 ratio. Our results also indicate that only polyurethanes with specific structure, in combination with APP, are active as IFR agents on PP combustion.