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

Abstract

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.

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

“…These results once again corroborate the cone calorimeter observations and the dynamic thermal stability studies in the torque rheometer, as well as the diverse studies conducted by Starnes Jr. and coworkers,10–15 Kroenke and Lattimer16–20 and Li and Wang,21–26 demonstrating the anticipation and acceleration effect of processes tied to dehydrochlorination.…”

“…Thus, all of the formulations can be compared based on the PVC composite's reference formulation. Analysis of the NCR results shows the effect of metallic oxides is significant, corroborating several studies conducted by Starnes Jr. and coworkers, [10][11][12][13][14][15] Kroenke and Lattimer [16][17][18][19][20] and Li and Wang, [21][22][23][24][25][26] confirming that the performance mechanisms of the main smoke suppressor agents used in PVC are based on interference in Diels-Alder condensation reactions and in the formation of benzene and other aromatic compounds. According to the authors, additives based on molybdenum interfere in the dechlorination process and in the formation of polyene sequences, anticipating their occurrence and forcing the formation of char, inducing the formation of structures fundamentally based on trans-polyens to the detriment of cis-polyens.…”

“…Once again, ZnO has a more pronounced anticipation than the other metallic oxides, when it is present in isolation as well as in binary and ternary mixtures. A strong and significant reduction in the formation of benzene can be observed qualitatively in Figure 8, and quantitatively from the integration data of detection rate curves for benzene versus time, shown in Table V, because of the presence of metallic oxides in the formulations, which once again confirms and demonstrates the mechanisms proposed by Starnes Jr. and coworkers,10–15 Kroenke and Lattimer16–20 and Li and Wang 21–26…”

“…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.…”

“…Smoke suppressors in PVC, based on molybdenum and copper, were previously studied by Starnes Jr. and coworkers, [10][11][12][13][14][15] Kroenke and Lattimer,[16][17][18][19][20] and Li and Wang. [21][22][23][24][25][26] In a previous work, 27 the authors presented the combustion and degradation properties of PVC/metallic oxides/O-MMT nanocomposites, and concluded that the effects of a ternary mixture of CuO/MoO 3 /ZnO is significantly more efficient in the reduction of heat release and smoke suppression of a flexible PVC compound.…”