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In a paper that I'm reading, it states that:
The presence of chlorine atoms in polyvinyl chloride make it flame resistant
and I'm just wondering why the presence of chlorine atoms will make it flame resistant. To the best of my knowledge, burning something is just providing a material with enough activation energy such that it can react with oxygen to produce products such as carbon dioxide and water. If so why does the chlorine prevent this process in any way?
PVC is not truly flame resistant, it does decompose on relatively gentle heating (around 150 °C) However, the process produces significant volumes of hydrogen chloride, that is not flammable and isolates the plastic from the flame. This makes makes PVC self-extinguishing or fire-retardant.
Many of the hydrogen halides ($\ce{HCl}$ and $\ce{HBr}$) are able to inhibit the combustion process, as when they are added to a flame they alter the flame chemistry. What will happen is that they react with oxidizing radicals such as hydroxyl radicals to form halogen atoms. The halogen atoms are able to react further but their reaction with fuels such as methane will be less exothermic than the reaction of hyroxyl radicals.
Also, Chemosphere, 1991, 22 (1–2), 67-76 states that alkyl radicals in flames can react with $\ce{HCl}$ to form alkyl chlorides. This will leave behind hydrogen atoms. It has also been stated by M.J. Thomson, A.D. Lawrence and J. Bu (Combustion Science and Technology) that HCl inhibits the combustion of benzene, it increases the carbon monoxide yield and reduces the chain branching which occurs during the combustion.
Combustion reactions are radical reactions, and the chlorine atoms formed during decomposition of the polymer act as radical traps.
My intuitive answer is that Cl has a relatively high electronegativity, so it pulls electrons towards itself, making it harder for the oxygen to react with the molecule. Reactions generally have an oxidizer which attracts electrons, and a reducer that supplies electrons. Oxidation of an organic compound usually means the carbons supply electrons to incoming oxygen atoms, making polarized and thus stable bonds. The bonds with the Chlorine atoms are themselves polarized, making the chlorine somewhat negative and the rest of the molecule slightly positive, making it less eager to give up electrons to an oxygen atom, meaning a resulting bond with oxygen would be less polarized and less stable....
