Aromatic char formation, effect

Table IV. Effect of Inorganic Additives on Aromatic Char Formation...

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

Ballistreri et a/. found that the principal mechanism of smoke inhibition was a cross-linking process catalyzed by metallic species. In the mechanism of PVC decomposition, as shown in Figure 5.8, mainly the production of non-substituted aromatic hydrocarbons (benzene, naphthalene, anthracene) was suppressed by the catalytic effect of metal oxides on the intermolecular cross-linking reaction, at the expense of the intramolecular rearrangement of the polyene. Smoke production was reduced due to the formation of less fuel. In the presence of certain metal oxides, a diminished char yield was also observed. It was concluded, however, that oxidation occurred at a later stage of the combustion, i.e. after the evolution of smoke in the pyrolytic process. 

Molybdenum compounds are regarded as the standard for low-smoke formulations, especially ammonium octamolybdates (AOM) for vinyl applications. Many studies claim that molybdates work in the solid phase, as most of the molybdenum is found in the char residue. Molybdates are said to reduce smoke by catalyzing the formation of trans polymer fragments so that it cannot form aromatic species, characteristic of soot. One producer uses an inert filler (talc) as the core support media onto which a zinc molybdate layer is precipitated on the surface. This coated core stmcture effectively provides smoke suppression at a lower manufacturing cost than neat molybdates. 

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