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SUMMARYThe study investigates the mechanism of fire retardancy induced by expandable graphite in a polyurethane coating. A complementary investigation to the spectroscopic study performed previously confirms that little interaction occurs between the matrix and the additive. X-ray diffraction study demonstrates that the crystalline structure of the graphite is maintained during the intumescent process. The presence of free radicals in the protective shield is then demonstrated but at low concentration compared with classical intumescent systems, such as for example polyurethane/ammonium polyphosphate. Finally, the physical properties of the char are then investigated. Blowing measurement demonstrates the high degree of expansion of the layer which may partially explain the fire retardant performance of expandable graphite in polyurethane. Surface analyses by optical microscopy and optical profilometry also enable a better understanding of the efficiency of expandable graphite. In the absence of a fire retardant additive the coating displays cracks and holes, providing a pathway for oxygen diffusion as well as heat and mass transfer between the flame and the virgin polymer. Both phenomena are limited when expandable graphite is added to polyurethane. Consequently, fire retardancy of expandable graphite results more from a physical than from a chemical action.

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ABSTRACT:In this work, we studied the mechanism of the fire retardancy of ammonium polyphosphate (APP) in polyurethane (PU). Indeed, according to the limiting oxygen index test, the efficiency of APP in PU coating was proven. On the one hand, thermogravimetric analyses showed that the addition of APP to PU accelerates the decomposition of the matrix but leads to an increase in the amount of high-temperature residue, under an oxidative or inert atmosphere. This stabilized residue acts as a protective thermal barrier during the intumescence-fire retardancy process. On the other hand, spectroscopic analysis of the charring materials using infrared spectroscopy, MAS NMR of the solid state, and ESR enables better understanding of the carbonization process and, consequently, of the intumescence phenomenon. It has been shown that the char resulting from PU consists of an aromatic carbonaceous structure which condenses and oxidizes at high temperature. In the presence of APP, a reaction between the additive and the polymer occurs, which leads to the formation of a phosphocarbonaceous polyaromatic structure. Moreover, this char is strongly paramagnetic. The presence of large radical species, such as a polyaromatic macromolecule trapping free radicals, was demonstrated. Both of these characteristics help to explain the fire-retardant performance of PU/APP.

It is demonstrated that the melting behavior and the morphology of three segmented thermoplastic polyurethane elastomers (TPUs) can be tailored by applying self-nucleation (SN) procedures. The self-nucleating temperature ranges for each of the TPU have been first determined by differential scanning calorimetry (DSC), while their morphology was studied by polarized light optical microscopy (PLOM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). When the samples are cooled at slow to moderate rates after SN, the crystallization temperature of the TPUs increases by up to 45 °C, when the samples are ideally self-nucleated. This large reduction in supercooling increases the melting points of the samples by approximately 20 °C. At the same time, SAXS and AFM experiments demonstrate the growth of thicker lamellae under these slow to moderate cooling conditions as compared to untreated samples. When ideally self-nucleated samples are rapidly quenched (e.g., at rates of 100 °C min–1 or larger) from their self-nucleation temperature to room temperature, the effects of SN described above on the morphology and melting points of the samples disappear for the TPUs that do not crystallize fast enough.