Abstract
The effects of particle size and concentration of calcium carbonate fillers on the critical oxygen index and the evolution of hydrogen chloride during pyrolysis of unplasticised PVC has been shown to parallel the effects in plasticised PVC but to differ from them in two respects. Coarser varieties of calcium carbonate (i.e. appreciably greater than 0.1 micron) react with pyrolysis hydrogen chloride more efficiently in UPVC than in plasticised PVC. Plots of critical oxygen index against concentration of calcium carbonate in UPVC show two inflexions, maxima at low concentrations (i.e. < 30 PHR) and minima at high concentration (i.e. > 40 PHR). Similar inflexions are also shown with alumina trihydrate and silica. In both plasticised and unplasticised PVC, calcium oxide produces a steady fall in critical oxygen index with increasing concentration, more marked than with calcium carbonate at equal hydrogen chloride retentions. The complex behaviour is attributed to the number of factors contributing to overall burning behaviour. Of these, char formation, in particular, is not readily assessed numerically, and consequently it has not been possible to quantify the contributions of the individual effects.
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