Factors
The ignition process initiates a self-propagating, high-temperature chemical reaction at the surface of the mixture. The rate at which the reaction then proceeds through the remainder of the composition will depend on the nature of the oxidizer and fuel, as well as on a variety of other factors. "Rate" can be expressed in two ways - mass reacting per unit time or length burned per unit time. The loading pressure used, and the resulting density of the composition, will determine the relationship between these two rate expressions.
Reaction velocity is primarily determined by the selection of the oxidizer and fuel. The rate-determining step in many high- energy reactions appears to be an endothermic process, with decomposition of the oxidizer frequently the key step. The higher the decomposition temperature of the oxidizer, and the more endothermic the decomposition, the slower the burning rate will be (with all other factors held constant).
Shimizu reports the following reactivity sequence for the most- common of the fireworks oxidizers.
Shimizu notes that potassium nitrate is not slow when used in black powder and metal-containing compositions in which a "hot" fuel is present. Sodium nitrate is quite similar to potassium nitrate in reactivity.
Shidlovskiy has gathered data on burning rates for some of the common oxidizers. Table 5 contains data for oxidizers with a variety of fuels. Again, note the high reactivity of potassium chlorate.
TABLE 5. Burning Rates of Stoichiometric Binary
For the ferric oxide/aluminum (Fe 20 3 /Al), manganese dioxide / aluminum (MnO 2 /Al), and chromic oxide/magnesium (Cr 2O 3 /Mg) systems, slight gas phase involvement is indicated by the 3-4 fold rate increase observed as the external pressure is raised from 1 to 150 atm. The chromic oxide /aluminum system, however, reportedly burns at exactly the same rate - 2.4 millimeters /sec - at 1 and 100 atm ; suggesting that it is a true "gasless" system. Data for the burning rate of a delay system as a function of external pressure (a nitrogen atmosphere was used) are given in Table 6. Another matter to consider is whether or not pyrotechnic compositions will burn, and at what rate, at very low pressures. For reactions that use oxygen from the air as an important part of their functioning, a substantial drop in performance is expected at low pressure. Mixtures high in fuel (such as the magnesiumrich illuminating compositions) will not burn well at low pressures. Stoichiometric mixtures - in which all the oxygen needed to burn the fuel is provided by the oxidizer - should be the least affected by pressure variations.
TABLE 6. Burning Rate of a Delay Mixture as a Function of External Pressure