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The oxygen index is established by ASTM as a standard measurement for the comparison of the flammability of plastics in Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (D 2863). The index is measured at atmospheric pressure. Measurements of oxygen index and burning rate at absolute pressures to 2 MPa (20 atm) have been made at 25°C for six materials: polytetrafluoroethylene, Viton®, neoprene, nylon, glass-filled nylon, and polymethylmethacrylate. The oxygen index was found to decrease with increasing pressure but to be independent of Reynolds number up to 1500. At the oxygen index, the burning rate was found to be independent of pressure. The data are correlated by a model that defines the oxygen index as a point of constant oxygen diffusion rate. It is concluded that with the exception of nylon, the ranking of materials for oxygen service at elevated pressures by their atmospheric pressure oxygen index is not changed. The index for nylon ranks it on the high side because, at lower pressures, hot liquid is lost from the test specimen and does not burn.
oxygen index, flammability (plastics), burning rate, Viton®, neoprene, nylon (glass-filled), polymethylmethacrylate, polytetrafluoroethylene, acrylic, fluoroelastomer, gaseous diffusion, hazards, fire
Air Products and Chemicals, Inc., Allentown, PA