Bench-scale fire toxicity measurements of polymers and cables

Kaczorek, Katarzyna (2009) Bench-scale fire toxicity measurements of polymers and cables. Masters thesis, University of Central Lancashire.

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This project has been based on obtaining a better understanding of bench-scale techniques and their limitations for fire toxicity assessment including closed box test (NBS Smoke Density Chamber) and flow-through tests such as the steady state tube furnace (ISO TS 19700). Toxicity results for several different polymers under different fire scenarios have been compared using published data. It was apparent that the closed box test could not readily match the various ISO fire conditions whereas it was generally possible to produce conditions similar to these in the steady state tube furnace but with some limitations.
This work has indicated ways of increasing confidence for ISO steady-state tube furnace method by identifying some factors causing variations in steady state burning behaviour and changes to the preset fire test conditions; in particular problems from the possible back flow of secondary air into the furnace tube at low primary flows which alters the preset fire conditions. This can be partially overcome by the use of an orifice plate with appropriate outlet size. Thus Casico power cable has been investigated in the steadystate tube furnace using under-ventilated fire conditions. This gave rise to apparently negative oxygen concentrations inside the furnace tube at low primary airflow, a narrower effluent outlet from the furnace tube is needed, although, this may change the behaviour of fire effluents. Also the apparent reactions between gases and solid residues showed as increasing concentrations of carbon monoxide and decreasing concentrations of carbon dioxide at very high temperatures such as 900°C and in under-ventilated conditions.
A water gas reaction was suggested as a possible cause for this observation. Commercial Casico power cable has been investigated to observe the temperature stability of flaming in the steady state tube furnace apparatus under well-ventilated conditions. It was found that even for very high temperatures such as 900°C this cable did not burn evenly. This has been explained by the flame retardant properties of the Casico material which forms an intumescent structure in a fire.
A study of carbon monoxide yields for LDPE, PVC and a 50:50 LDPE/PVC mixture was undertaken as a function of equivalence ratio and the furnace temperature. For the LDPE/PVC mixture, a high CO yield was obtained, higher than for pure LDPE (about 1.5 times higher) and similar to pure PVC. This may be explained by HCI production interfering in the radical chain mechanism in the gas phase preventing the conversion of CO to CO2 and results in high CO yield even for well-ventilated conditions. However, at high furnace temperatures (950°C) this effect diminishes for both PVC and LDPE/PVC mixture systems.

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