Behaviors on Heating and Ignition

Bourbigot et al.85 at Lille have used poly(vinylsilsesquioxane) (POSS) in PP (110 wt%) to melt spin filaments, which were then knitted into fabrics. POSS was thermally stable and no degradation was detected in the processing conditions. They have tested the flammability of the fabrics using cone calorimetry. POSS presence had minimal effect on peak heat and total heat release values of PP fabric, but delayed the TTI. This behavior of POSS is opposite to that of layered silicates, which have minimal effect on TTI, but reduce PHRR. Authors claim that POSS does not act as a FR but only as a heat stabilizer via a decrease of the ignitability. 

It should be emphasized that Oh and Cavendish assumed that the reactions only occur on the surface of the channel wall. This assumption is less realistic for a layer of washcoat (typically y-alumina) dispersed with catalyst applied on to the wall surface. Ramanathan, Balakotaiah, and West showed that the diffusion in the washcoat has a profound influence on the light-off behavior of a monolith converter. They derived an analytical light-off criterion based on a onedimensional two-phase model with position-dependent heat and mass transfer coefficients. The derivation of this criterion is based on the two key assumptions a positive exponential approximation (i.e., the Frank-Kameneskii approximation) and negligible reactant consumption in the fluid phase. The light-off is defined as the occurrence of multiple steady states with the attainment of the ignited steady state. Here, we discuss only the results of their analysis, without going into the details of their derivation. 

The oxidation of ethylene in air on a Pt wire is a good example by which to demonstrate the ignition behavior of exothermic catalytic reactions. The experiment was conducted as follows (Table 4.5.4). A coil consisting of a thin Pt-wire is placed in a tubular reactor. Then an ethylene-air mixture of constant temperature and pressure (303 K, 1 bar) is fed into the tubular reactor. The wire is now electrically heated until ignition (jump in temperature) occurs. The current and the voltage is measured and, thus, also the temperature of the wire as the electrical resistance depends on temperature. 

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