AbstractThe most effective smoke retarders for poly(vinyl chloride) are compounds of transition metals, such as molybdenum trioxide. MoO3when added to a rigid PVC compound shows three general effects. First, smoke formation is reduced dramatically as measured either by the NBS smoke chamber test or Goodrich smoke–char test. Second, volatile aromatic pyrolyzate formation is also reduced dramatically. The “pure” conjugated aromatic compounds (particularly benzene and naphthalene) are reduced more than the “mixed” aliphatic–aromatic compounds (such as toluene). Third, char formation is effectively promoted. Based on results from studying MoO3(and other metal‐;based additives) in regular, perdeuterated, and syndiotactic PVC, we propose a “reductive coupling” scheme as the dominant mechanism to explain the smoke retarder action. In this mechanism the MoO3acts in the condensed phase to promote extensive crosslinking of the PVC polymer chains very early in the thermal degradation process. Specifically, the metal additive forms a redox catalyst system which promotes intermolecular crosslinking of polymer chains to form char, rather than the conventional degradation process which gives rise to ar
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