Reflections on migration of clay and structural changes in nanocomposites

The profound effect on flame retardancy of the blending of small amounts of clay with polymers is believed to be due to the formation of a protective clay‐rich char barrier on the surface of the pyrolyzing polymer during combustion. The mode of formation of this barrier is discussed, in view of recent results. Evidence is reviewed, showing that the organically layered silicon particles migrate to, and accumulate on the surface of the molten polymer at temperatures far below pyrolysis temperature. The causes of the migration are: temperature and viscosity gradients of the melt with regard to heat source; propelling the organically layered silicon to the surface by gas bubbles from the decomposing surfactant and polymer; and by the difference in surface free energy between the polymer and the polymer/clay aggregate. The interfacial tension of the clay/polymer aggregates is calculated for a number of polymers, and shown to be much lower than the surface tension of the low surface free energy ingredient. It is suggested that these aggregates migrate to the surface. The decomposition of the surfactant in the clay above 200°C brings about, in the case of polypropylene/nanocomposite, two simultaneous and opposing phenomena: the migration of the clay; and the conversion of the nanocomposite into a non‐colloidal microcomposite. The decomposition of the surfactant is accompanied by an increase in interlayer distance, and internal pressure in the clay particles, culminating either in a separation of the external layers (i.e. exfoliation), or in their collapse into each other, forming a microcomposite. Mechanistic considerations are presented.