Carbonaceous char layer

The PLA/ramie hybrid alone presents no efficient charring to protect matrix, however, the addition of APP in PLA/ramie hybrid enhances the formation of coherence of carbonaceous charring layer as protective shield and thermal barrier (as shown in Fig. 4.37). The TGA test demonstrates the increase of char. When cooperating with APP, ramie fiber being rich in polyhydric compound acts as a charring agent to form an intumescent flame retardant system. This intumescent mechanism has been discussed in the literature. 

It appears that the flame retardant mechanism for the polyamide/clay nanocomposites discussed in this chapter relates to the formation of a continuous protective carbonaceous char layer that acts as a heat shield. The mechanism is similar to that for other kinds of polymer/clay nanocomposites. 

Images of the five pyrolysis samples are shown in Figures 3.5 to 3.9. These images show that initially the clay-carbonaceous char layer is thin, but as the pyrolysis times increase, the samples are comprised of a greater fraction of char until the char dominates the structure in the 400 s and 1150 s samples. Pyrolysis residues were collected from three regions on each of the five... 

Figures 15.8 and 15.9 illustrate examples of how cone calorimeter data can be used in the development of flame-retarded materials. PA 66-GF without Pred showed typical fire behavior for noncharring polymers containing inorganic glass fiber as inert filler,69 when high external heat flux is applied. The shape of the HRR curve is divided in two different parts. In the beginning, the surface layer pyrolysis shows a sharp peak, followed by a reduced pyrolysis rate when the pyrolysis zone is covered by the glass fiber network residue layer. When Pred was added, the PA 66-GF samples were transformed into carbonaceous char-forming materials, which led to a...

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. 

The behavior of the char during flight is pertinent to its success as an ablative material. Once the carbonaceous layer forms, the primary region of pyrolysis gradually shifts from the surface to a substrate zone beneath the char layer. The newly formed char structure is attached to the virgin substrate material and remains thereon for at least a short period of time. Meanwhile, its refractory nature serves to protect the temperature-sensitive substrate from the environment. 

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