Carbonaceous-silicate char

Formation of high performance carbonaceous silicate char on the nanoparticles surface that insulate the underline material and slows the escape of volatile products generated during the decomposition ... 

Polypropylene (PP) has wide acceptance for use in many application areas. However, low thermal resistance complicates its general practice. The new approach in thermal stabilization of PP is based on the synthesis of PP nanocomposites. This paper discusses new advances in the study of the thermo-oxidative degradation of PP nanocomposite. The observed results are interpreted by a proposed kinetic model, and the predominant role of the one-dimensional diffusion type reaction. According to the kinetic analysis, PP nanocomposites had superior thermal and fireproof behavior compared with neat PP. Evidently, the mechanism of nanocomposite flame retardancy is based on shielding role of high-performance carbonaceous-silicate char which insulates the underlying polymeric material and slows down the mass loss rate of decomposition products. 

The positive effect on thermal stability of polymers due to polymers can be attributed to (i) high surface volume, (ii) improving barrier properties due to the clay contribution to tortuosity path, (iii) reduction of polymer molecular mobility, (iv) low permeability and decrease in the rate of evolution of the formed volatile products, (v) formation of high-performance carbonaceous silicate char on the nanoparticles surface that insulate the underline material and slows the escape of volatile products generated during the decomposition, (vi) absorption of formed gases into clay plates. 

The formation of this high-performance carbonaceous-silicate char was first observed by studying the combustion residues using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The structure of the combustion residue of a PE/clay nanocomposite has been observed in scanning electron microscopy (SEM) at nanoscale the residue resulted in a net-like structure of clay platelets and char, while at microscale showed a sponge-like structure, similar to those obtained from combustion of intumescent FR systems. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

Nickel silicate and ferrous silicate are the preferred catalysts in the Smuda process. The Smuda catalyst is a layered silicate clay framework with ordered nickel (or iron) atoms inside. The catalyst is charged at 10 wt% ratio of the plastic feedstock. The catalysts are based on layered silicates with Lewis acid activity [24]. Catalytic cracking results in very little noncondensable gas (<1%) and minimal carbonaceous char. The hfe of the Smuda catalyst is approximately 1 month [24]. 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

Gilman, J. W., Harris, R. H., Shields, J. R., Kashiwagi, T., and Morgan, A. B. 2006. A study of the flammability reduction mechanism of polystyrene-layered silicate nanocomposite Layered silicate reinforced carbonaceous char. Polymers far Advanced Technologies 17 263-271. 

Morgan, A.B. Harris, R.H. Kashiwagi, T. Chyall, L.J. Gilman, J.W. Flammability of polystyrene layered silicate (clay) nanocomposites carbonaceous char formation. Fire Mater. 2002, 26(6), 247. 

As in the case of zeolite, the mechanism of action looks similar. No direct comparison can be made because MMT is a layered silicate compared to the cage structure of zeolite, and also because the carbonization agent is no longer a polyol but a char-forming polymer (PA6). Nevertheless, the main conclusion we can draw is that the action of the synergist (nanoclay or zeolite) is to stabilize in a first step the carbonaceous structure forming aluminophosphates and silicophosphates. With the nanoclay, this effect is only effective up to 310°C, whereas it is still efficient at 560°C with zeolite. To keep its protection efficient at high temperatures, the nanoclay permits the formation of protective ceramiclike material after collapse of the phosphocarbonaceous structure. Note that we did not detect any specific influence of the surfactant of the nanoclays, probably because of its low amount in the formulation. 

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