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

SynopsisThe most effective smoke retarders for poly(viny1 chloride) are compounds of transition metals, such as molybdenum trioxide. MOO:% when 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 Moo3 acts 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 crasslinking of polymer chains to form char, rather than the conventional degradation process which gives rise to aromatics and smoke.

Section: Introductionmentioning

confidence: 74%

Section: Introductionmentioning

confidence: 99%

The functional role of molybdenum trioxide as a smoke retarder additive in rigid poly(vinyl chloride)

Lattimer¹,

Kroenke²

1981

“…The amount of carbonaceous residue left from the speciment at the end of the cone calorimeter measurement is another important parameter related to the release of smoke by materials during combustion, simply because one of the main mechanisms for reducing smoke is the fixation of organic matter in the solid phase, retarding/avoiding its pyrolysis and consequent combustion 1, 3, 10–20. A simple check of Table III shows that all of the oxides studied have statistical significance in relation to char residue (CR) quantity values.…”

Section: Resultsmentioning

confidence: 99%

Metallic oxides as fire retardants and smoke suppressants in flexible poly(vinyl chloride)

Rodolfo

Mei

2010

Combustion and smoke emission properties of PVC compounds, modified with copper (II), molybdenum, and zinc oxides, were studied using cone calorimeter, limiting oxygen index (LOI) and thermogravimetry coupled with mass spectrometry (TG/MS). Results showed that the metal oxides have a very significant effect on the combustion and smoke suppression properties of the PVC compounds. The results also confirmed the anticipation mechanisms of the dehydrochlorination reactions, reductive coupling, and elimination of benzene resulting from the presence of copper (II), molybdenum, and zinc, indicated by the increasing content of postcombustion char residue and the significant reduction in benzene production, indicated by the MS measurements performed. The results also provide indications that the combination of the copper (II) and molybdenum oxides is the one with the best balance of combustion properties, as it reduced the heat released and promoted the suppression of smoke more efficiently. The formulations containing ZnO, because of their strong Lewis acid character, indicated a less pronounced reduction of smoke released during the combustion process, when compared with copper (II) and molybdenum oxides.

“…The more active smoke retarders change the thermal degradation pattern of the PVC and promote the formation of char. 13 Several reports relating to the mechanisms by which smoke retarders function in PVC have appeared in the literature. In 1974, Iida, Nakanishi, and Gotb showed that SnOp and %nO promoted the formation of aliphatics rather than benzene and other aromatics.…”

Section: Introductionmentioning

confidence: 99%

Effects of copper and molybdenum oxides on the pyrolysis of model compounds of poly(vinyl chloride)

Lattimer

Kroenke

Getts

1984

Numerous transition metal compounds can be added to poly(vinyl chloride) to decrease the smoke evolved during burning or smoldering conditions. Model compound pyrolysis experiments have been carried out to investigated the mechanisms by which smoke retarder additives function in PVC. Three models were used: 3‐chloropentane, 2,4‐dichloropentane, and 2,4,6‐trichlorohenptane. The additives MoMO3 and Cu2O both enhanced the rate of dehydrochlorination and promoted the formation of crosslinked products (oligomers) during model compound pyrolysis, but their mechanistic pathways were found to be different. The choice of model compounds also was an important influence in determining the observed decomposition pathways. While much of the pyrolysis chemistry can be explained by Lewis acid catalysis, other effects also appear to be important. The pyrolysis results are interpreted in terms of an “early crosslinking” 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.