The effects of a synergistic molybdenum-copper smoke retarder additive during the pyrolysis and combustion of rigid poly(vinyl chloride) compounds

The addition of an effective transition metal‐based smoke retarder to rigid poly(vinyl chloride) (PVC) has three general effects during combustion and inert atmosphere pyrolysis: (1) smoke formation is reduced; (2) char formation is enhanced; and (3) volatile aromatic pyrolyzate formation is reduced while aliphatic pyrolyzate formation is enhanced. A very efficient molybdenum—copper smoke retarder additive has been developed for PVC. It consists of an equal weight mixture of melaminium beta‐octamolybdate and copper(II) oxalate. This combination is synergistic in reducing smoke during the combustion of PVC, i.e., the mixture is more effective than either of the two ingredients used individually at the same concentration as the mixture. The molybdenum—copper system reduced smoke effectively in three rigid PVC compounds, two of which resemble commercial formulations. The combustibility and pyrolysis effects of this molybdenum—copper additive can be interpreted in terms of an “early cross‐linking” mechanism of smoke retardation in PVC. In this mechanism the metal smoke retarder works primarily by catalytically promoting early crosslinking of decomposing PVC chains to yield char as a residue.