The kinetics of intumescent flame retardant foaming

Introduction. Intumescent flame retardants are intensively used as passive fire protection means. Under fire conditions, these coatings foam and turn into coke, which turns into ash. These products have various fire resistant properties. These transformations are possible due to the foaming process, whose kinetics determines the fire protective characteristics of the compositions used. The paper considers the kinetics of the foaming process in the course of the pyrolysis of four different foaming compositions. The classical triad was used as a thermally expanding agent for the three of them, it includes ammonium polyphosphate, pentaerythritol, and melamine, and the fourth one has intercalated graphite.

Research methods. Thermal analysis is widely used to identify and study various materials, substances and fire retardants. However, we have not found any kinetic studies performed using methods of thermal analysis in the literature. In this work, methods of non-isothermal kinetics are used to identify the mechanism of foaming. For this purpose, four series of thermogravimetric tests were carried out at different heating rates for each composition under study. The results of the experiment made it possible to solve inverse and direct kinetic problems and identify mechanisms of the processes.

Results and discussion. Methods of non-isothermal kinetics were employed to show that low-temperature stages of thermolysis can be considered as gross-one-stage processes for all samples. The solution of the direct kinetic problem has helped to identify that the limiting foaming stage is described by the Avrami – Erofeev equation for all compositions under study, while the values of kinetic parameters differ significantly. Consequently, foaming proceeds are similar for samples having different compositions. The foaming of the sample containing intercalated graphite depends on heating conditions.

Conclusions. It was identified that the studied compositions transform into the viscous-fluid state at high temperatures. In this case, the limiting stage of the foaming process is the nucleation of primary bubbles in the volume of the liquid phase. This process determines the kinetics of foaming, coke properties and its thermophysical characteristics.

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