Pyrolysis, surface phenomena

Particle Encapsulation. A unique capability of the p-xylylene vapor deposition process has been uncovered in the area of encapsulation (12) of particulate solids. The particles or granules to be encapsulated are placed in a container which in turn is placed in the deposition chamber, and the nozzle from the pyrolysis tube is inserted in the mouth of the bottle. During the run the monomers pass from the pyrolysis zone through a nozzle into the bottle and polymerize on the surface of the tumbling particles or granules. Polymer is also formed on the inner surface of the bottle which is rotated at 50-150 r.p.m. Relatively simple equipment (Figure 4) has been used to study this phenomenon. 

Phosphorus is known as an effective char promoter. Charring limits the release of fuels to the flame and therefore reduces the heat released. Moreover, the char accumulates on the surface of the material and can act as a protective layer which limits the heat transfer from the flame to the condensed phase and the gas transfer from the pyrolysis zone to the flame. This phenomenon is called the barrier effect. Nevertheless, it should be noted that the presence of char is not a sufficient condition to observe an effective barrier effect. Its structure (cohesion, porosity, thickness) is also veiy important but rarely studied. Thermal stability is another important parameter to assess the reaction to fire of a material. Indeed, the higher is the degradation temperature of a polymer, the greater is the heat required to start its pyrolysis. Table 12.2 lists the effects of phosphorus-containing groups on the thermal stability and charring for a variety of polymers. 

Results presented in Fig. 20.20 show that polysiloxanes, obtained by ordinary sol-gel processing of TEOS, are also effective as nanometric size stabilizers. The role of the SiO covering in the stability of the SnO grains was discussed before. However, since the current interpretation of the phenomenon requires the absorption of hydrophilic species on the solid particles, the temperature treatment reduces the Si-OH population and favors the segregation of SnO this event parallels an important increase in the dimensions of metallic oxide crystallites. In contrast, pyrolysis of glucosidic moieties after treatment produces carboxylic groups able to coordinate metallic ions at the particle surface, and the related size stabilization, prolonged up to 600 °C, appears exclusive of starch. 

Whisker carbon is formed as characteristic fibres (nanotubes) [505] from CO, CH4 and higher hydrocarbons and may result in breakdown of the catalyst pellet (Section 5.2.2). The low-temperature phenomenon gum formation involves blockage of the metal surface by a film of polymerised carbonaceous material (Section 5.3.3). Pyrolytic carbon is a result of thermal reactions (pyrolysis) as experienced in steam crackers... 

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