Ignition polymer decomposition products

Using the Le Chatelier principle, one may calculate the lower (L) and upper (U) concentration limits of the ignition of polymer decomposition products in any oxidative medium... 

Tign for PMMA and unsaturated polyester is 310 and 390 °C, respectively. From these results it follows that PMMA and similar products are more easy to ignite and to burn than unsaturated polyesters, under a given external heat flux qe. It is interesting to note that q0 ign values, obtained from flame spread experiments in this manner, are consistent with the q0 ign values determined from direct ignitability tests. This is an evidence of the validity of the treatment of flame spread on a surface as a continuous gas-phase ignition of polymer decomposition products. 

Substances applied to or incorporated in a combustible material (e.g. organic polymers, nylon, vinyl and rubber, etc.) to reduce flammability. Act by retarding ignition, control/douse burning, reduce smoke evolution. Slow down or interrupt the self-sustained combustion cycle when the heat-flux is limited. Flame retardants (FRs) improve the combustion behaviour and alter the combustion process (cool, shield, dilute, react) so that decomposition products will differ from nonflame retarded articles. FRs are usually divided into three classes ... 

The decomposition products formed by cleavage of primary bonds in the polymeric molecule may be subjected to secondary degradation or oxidation reactions leading to new compounds. These secondary reactions control the composition of the gas phase in the vicinity of the polymer they may be responsible, therefore, for the system reaching the concentration limit of ignition. 

There are three essential conditions to be met if a polymer, once ignited, is to continue burning. There must be a supply of heat to the bulk polymer, a generation of fuel (typically volatile decomposition products) and there must be a flame. Halogen-based systems act by a well-documented flame poisoning mechanism in the vapour phase. The alternative halogen-free systems, which encompass a wide variety of additives, tend to act by mechanisms which disrupt heat flow and the supply of fuel to the flame. Here the mechanisms are not always understood in great detail but two broad types of flame retardant action can be defined. 

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

Combustibles must be produced before the ignition of polymers to flame. By heating a plastic it eventnally reaches a temperature at which the weakest bonds start to break, and little change occurs such as discoloration. At higher temperature pyrolysis an irreversible chemical decomposition due to an increased temperature without oxidation plays the main role in the production of combustible gases in the burning of plastics [119]. 

Fluorine is more effective as an inherent flame retardant in the basic polymer than as a reactive flame retardant or additive flame retardant employed in later stages of the product. As part of the basic polymer, it functions essentially in the condensed phase. It is effective in the stages of decomposition and ignition, and increases the possibility of arresting the burning process at an... 

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