Assignment B – Flammability of polymers
Combustion and Decomposition of polymers
Most polymers are made up of smaller hydrocarbons repeated to form a long chain molecule. In heat generally the original polymer remains mostly involatile. The large polymer molecules decompose via pyrolysis (Thermochemical decomposition without oxygen) creating highly reactive free radicals which in turn help break down more of the long chain. In the heat the molecules continuously decompose in the condensed phase (solid or liquid) into smaller low molecular weight, highly volatile molecules. These small molecules vaporize and catch fire easily if the temperature rises above ignition temperature. Decomposition will continue as long as the temperature is enough to continue thermal decomposition and in turn feed the flames with volatile compounds. An example of the decomposition of a polymer (polyethene) to smaller molecules is shown below (derived from diagram examples on Sciencedirect.com and Uclan.ac.uk):
Initiation - polymer chain
[pic] + HEAT
Propagation (many possible combinations)
[pic] + [pic]
[pic] + CH2=CH2
Termination- (Gas phase) – Volatile species formed (again many possible forms)
CH2=CH2 CH2=CH-CH3 CH3-CH3 [pic]
These smaller molecules essentially fuel the polymer to burn more once ignited. The flame heats the polymer causing it to decompose further, releasing more fuel for the flames. This creates a self sustaining burn cycle. When the flames burn longer and get hotter larger molecules will reach their ignition temperature and burn creating a fiercer fire.
COMBUSTION CYCLE DIAGRAM
The diagram above shows the combustion cycle of a polymer. The phase boundary shows what state each part happens. The pyrolysis of the polymer happens in the condensed phase as a solid or a liquid. This is the state in which the heat...