Anomalous behavior of molybdenum oxide as a fire retardant for polyvinyl chloride

Abstract

The widespread application of polyvinyl chloride (PVC) as wire and cable insulation in the communication and electrical industries is due in part to its inherent fire retardant properties, which are improved by the addition of antimony oxide. This addition also increases somewhat the already high rate of smoke formation. Molybdenum trioxide, used either with or instead of Sb 2O 3, has been reported to be a smoke suppressant as well as a flame retardant. This is corroborated by laboratory-scale measurements reported here. However, in a large-scale test under high enthalpy input conditions, MoO 3 appeared to be ineffective in limiting the flame spread. A detailed investigation of the pyrolysis of PVC by thermogravimetric analysis and mass spectroscopy showed that the addition of MoO 3 results in the reduction of the temperature at which HCl is evolved, an increase in the temperature at which the organic pyrolyzate is produced, and a change in compostion of this pyrolyzate from aromatic to aliphatic. The underlying chemical mechanism is explained by the action of MoO 3 as a Lewis acid: 1. 1. The dehydrochlorination of PVC is catalyzed and occurs at a lower temperature; 2. 2. This process yields trans polyenes, either directly or via rapid isomerization of cis polyenes; 3. 3. The trans polyenes, being unable to cyclize and split off benzene, are stable to higher temperatures, at which a different mechanism obtains to give aliphatic products. The ineffectiveness of MoO 3 is ascribed to the difference in flame characteristics between aliphatic and aromatic fuels. The disagreement between laboratory and large-scale tests is attributed to the higher temperature at which the aliphatic pyrolyzate appears, which is reached only under conditions of higher enthalpy input that prevail in the large-scale tests.