Degradation combustion cycle

This scheme may now be used to understand how different flame-retardant strategies may be designed and adopted to break the combustion cycle. For a given polymer, the strategy to be adopted will be largely dictated by its particular thermal degradative chemistry, which has been briefly reviewed in Section 2.3 earlier, for all the significant polymer types. 

Ignition of poI)aners occurs either spontaneously or from an external source if the concentration of volatile combustible products evolved by pyrolysis or thermo-oxidative degradation of the polymer is within the flammability limits. A self-susfeiined combustion cycle is then triggered. 

This is driven by the heat of the flame promoting the pyrolysis of the polymer. The process will continue as long as the heat transmitted to the polymer is sufficient to keep its rate of thermal degradation above the level required to feed the flame, otherwise it will extinguish. The self-sustained combustion cycle occurs in both the condensed and gas phase. This means that in order to extinguish the flame by depressing the rate of chemical and/or physical processes taking place in one or both phases, polymers have to contain a variety of additives that may act as fire retardants or they have to be modified (chemical or physical modifications) to resist fire [2]. 

Implementation of advanced performance degradation models, necessitate the inclusion of advanced instrumentation and sensors such as pyrometers for monitoring hot section components, dynamic pressure transducers for detection of surge and other flow instabilities such as combustion especially in the new dry low NO combustors. To fully round out a condition monitoring system the use of expert systems in determining fault and life cycle of various components is a necessity. 

Berthon, L., Carries, B. 2000. Precede DIAMEX Degradation hydrolytique et radio-lytique de l extractant diamide DMDOHEMA, in Rapport Scienlifique 1999. CEA, Direction du Cycle du Combustible. Eds. Report CEA-R-5892 206-211. 

Chemically, the strategies are clear from consideration of the fire cycle. We can modify the degradation chemistry, reducing volatile production, or quench the combustion process. Alternatively we can restrict the heat flow back into the polymer. 

As shown in the burning cycle, a polymer burns via the vapour phase combustion of the volatile products produced during its thermal oxidative degradation. The vapour phase combustion is a free radical process which can be simplified and expressed as follows, where RCH3 is representative of the hydrocarbon undergoing combustion ... 

As is shown in the burning cycle, the burning of a polymer occurs by the combustion of the volatile products emitted during the oxidative thermal degradation of the polymer. If the degradation mechanism could be altered to produce more char and less volatiles the flame retardancy of the formulation should be decreased. Many flame retardants act in this way, promoting low energy, solid-state reactions which lead to the carbonization of the polymer and a carbonaceous char on the surface. 

If the heat transferred back to the polymer from the combustion of the volatiles is sufficient to continue degradation, the burning cycle will be self-sustaining. If some of this heat is removed, the cycle will be broken and the combustion stopped because of a slowing of the rate of pyrolysis of the polymer. 

---