Metal-rich aluminum–polytetrafluoroethylene reactive composite powders prepared by mechanical milling at different temperatures

Aluminum–polytetrafluoroethylene (Al·PTFE) composite materials with 90 wt% Al are prepared by mechanical milling at both room and cryogenic temperatures. Distribution of PTFE in the material is more homogeneous in the cryogenically milled materials. Thermal analysis in inert atmosphere shows at least three distinct exothermic reaction steps below Al melting as well as evolution of AlF₃ at temperatures above 800 °C. The heat of low-temperature exothermic reactions first increases and then decreases as a function of the milling time. In an oxidizing environment, all materials oxidize qualitatively similar to pure Al, and a composite milled cryogenically for 6 h oxidizes as fast as or faster than nano-sized aluminum powder. When heated at several thousand degrees per second, all composites ignite around 730 °C. Only the powders prepared by cryogenic milling could be ignited by electrostatic discharge (ESD) with energies of 720 mJ, while materials milled at room temperatures could not be ignited with energies as high as 2 J. In the ESD ignition experiments, the optical emission pulse is delayed compared to the pressure pulse, suggesting that a gas-generating PTFE decomposition triggers the ignition. The material cryogenically milled for 6 h is the most attractive, based on the magnitude of both pressure and emission pulses generated upon its ESD ignition, and based on the rate and extent of its oxidation in thermo-analytical experiments.