Carbonaceous layer/residue

The majority of plastics, however, produces two zones as illustrated in Figure 2.8 the site of the gas evolution (pyrolysis zone) covered by the carbonaceous layer of the porous solid residue (just below the burning surface). 

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 %. 

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...

Inside the wax residue are small particles covered with a surface layer that looks very similar to that shown in Figure 2. It is indeed remarkable that despite the differences in the method of preparation and initial composition, the primary particles in the catalysts seen in Figures 2 and 1 la look so much alike. The carbide particles (identified by their lattice fringes), are surrounded with a halo of amorphous carbonaceous material which must differ in composition from the wax residue since there is sufficient contrast to distinguish them. In agreement with the XRD results from Run RJO-189 (time on stream 1796 hours) an a-Fe peak is observed in the XRD pattern in... 

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). 

Carbonization Heat treatment between 400 and 700 °C converts the carbonaceous residue into graphite-hke layers. 

Phosphorus is believed to perform most of its flame retardant function in the condensed phase (including both the solid and liquid phases, because various degrees of melting are involved at fire temperatures). Phosphorus-containing compounds increase the amount of carbonaceous residue or char formed by one or both of two mechanisms redirection of the chemical reactions involved in decomposition in favor of reactions yielding carbon rather than carbon monoxide or carbon dioxide and formation of a surface layer of protective char. 

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