A comparative study of the mechanism of action of ammonium polyphosphate and expandable graphite in polyurethane

The present work is dedicated to the development of a new method, which enables a better understanding of the intumescence process. Many studies have investigated char formation from a chemical aspect but the foaming and strength of the char have not been well studied before. Thermal scanning measurements using a rheometer as a fire reactor enable a correlation between the fire behaviour and thermal behaviour and the visco-elastic properties of the material. This study investigates the rheological modifications of polyurethane thermoset coatings with or without fire-retardant. The rheological and mechanical destruction properties of the protective char layer have been correlated with their fire retardant performance for two additives: ammonium polyphosphate and expandable graphite. In a first part, the expansion of pure PU and of intumescent materials under normal force is studied. Then, the visco-elastic behaviour is investigated in order to evaluate the different steps of the intumescent process (development, stability and destruction). Finally, mechanical properties of the intumescent chars are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.