Graphite Nanoparticles in Flexible PU Foams

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Graphite is a black powder that lends itself to an infinite number of applications. It is often used as a dry lubricant in brush-based systems but can also be found in coatings for metal surfaces and polymer additives that enhance thermal stability. In addition, the ability to chemically modify graphene with ligands makes it attractive for drug delivery.

We recently demonstrated that layer-by-layer (LbL) assembly of GO nanoplates and chitosan can endow flexible polyurethane (PU) foams with improved flame retardancy [20]. However, the physical properties of these materials, such as their lateral size, play an important role in their performance. To better understand these correlations, we systematically studied the effect of different lateral sizes of the starting GO in the formation and flame retardancy of self-assembled GO/chitosan coatings.

We use TEM microscopy to characterize the surface morphology of the resulting material, including the thickness of the encapsulation layer and the magnetic behavior of the iron core. A TEM image of the ball milled Fe2O3-C powders after an LPCVD process with a holding time of 1 h and subsequent HF leaching shows that all individual nanoparticles are completely encapsulated in graphitic carbon, except for the amorphous regions shown in Figure 5b.

The encapsulated iron nanoparticles have a magnetic moment of 50 emu/g, as confirmed by magnetization experiments. This value is close to the theoretical maximum for a magnetite nanoparticle and indicates that the particles are fully reduced. The encapsulation of the nanoparticles by graphite is triggered by the partial melting of the iron surface and the formation of a carbon-rich region at its interface with water, which in turn leads to the formation of a graphitic overlayer on the particle’s surface.