What are the differences in flame spread between PVC and polypropylene in fires?

When examining fire behavior of thermoplastic materials, significant differences emerge between polyvinyl chloride (PVC) and polypropylene in their flame spread characteristics. PVC, being a chlorinated polymer, exhibits inherent flame retardant properties due to its high chlorine content (approximately 56% by weight). When exposed to fire, PVC first undergoes decomposition releasing hydrogen chloride gas, which dilutes flammable volatiles and creates a char layer that inhibits further flame propagation. This results in relatively slow flame spread rates, typically measured at 5-15 mm/s under standard test conditions.

In contrast, polypropylene – a hydrocarbon polymer – demonstrates notably different combustion behavior. Without inherent flame retardants, polypropylene ignites readily at approximately 350°C and exhibits rapid flame spread rates ranging from 20-40 mm/s. The material melts and drips during combustion, potentially spreading fire to adjacent materials. Polypropylene's heat release rate peaks significantly higher than PVC's, reaching 1,500-2,000 kW/m² versus PVC's 300-500 kW/m².

The smoke production characteristics differ substantially. PVC generates dense, dark smoke containing acidic gases, while polypropylene produces less smoke but with higher rates of complete combustion products. These fundamental differences in chemical composition and decomposition mechanisms make PVC more resistant to flame spread but with greater smoke toxicity concerns, whereas polypropylene spreads flames more readily but with somewhat less toxic combustion products. Understanding these distinctions is crucial for material selection in applications where fire safety is paramount.