DELAY COMPOSITIONS
The purpose of a delay composition is obvious - to provide a time delay between ignition and the delivery of the main effect. Crude delays can be made from loose powder, but a compressed column is capable of much more reproducible performance. The burning rates of delay mixtures range from very fast (millimeters/millisec ond) to slow (millimeters /second).
Black powder was the sole delay mixture available for several centuries. The development and use of "safety fuse" containing a black powder core significantly improved the safety record of the blasting industry. However, the development of modern, long-range, high-altitude projectiles created a requirement for a new generation of delay mixtures. Black powder, under specified conditions, gives reproducible burning rates at ground level. However, it produces a considerable quantity of gas upon ignition (approximately 50%of the reaction products are gaseous), and its burning rate will therefore show a significant dependence on external pressure (faster burning as external pressure increases). To overcome this pressure dependence, researchers set out to develop "gasless" delays - mixtures that evolve heat and burn at reproducible rates with the formation of only solid and liquid products. Such mixtures show little, if any, variation of burning rate with pressure.
One could begin such a project by setting down the requirements for an "ideal" delay mixture:
1. The mixture should be stable during preparation and storage. Materials of low hygroscopicity must be used.
2. The mixture should be readily ignitible from a modest ignition stimulus.
3. There must be minimum variation in the burning rate of the composition with changes in external temperature and pressure. The mixture must readily ignite and reliably burn at low temperature and pressure.
4. There should be a minimum change in the burning rate with small percentage changes in the various ingredients.
5. There must be reproducible burning rates, both within a batch and between batches.
The newer "gasless" delays are usually a combination of a metal oxide or chromate with an elemental fuel. The fuels are metals or high-heat nonmetallic elements such as silicon or boron. If an organic binder (e.g. , nitrocellulose) is used, the resulting mixture will be "low gas" rather than "gasless," due to the carbon dioxide C0 2), carbon monoxide (CO), and nitrogen (N 2) that will form upon combustion of the binder. If a truly "gasless" mixture is required, leave out all organic materials!
If a fast burning rate is desired, a metallic fuel with high heat output per gram should be selected, together with an oxidizer of low decomposition temperature. The oxidizer should also have a small endothermic - or even better, exothermic - heat of decomposition. For slower delay mixtures, metals with less heat output per gram should be selected, and oxidizers with higher decomposition temperatures and more endothermic heats of decomposition should be chosen. By varying the oxidizer and fuel, it is possible to create delay compositions with a wide range of burning rates. Table 1 lists some representative delay mixtures.
TABLE 1. Typical Delay Compositions
Using this approach, lead chromate (melting point 844°C) would be expected to produce faster burning mixtures than barium chromate (higher melting point), and barium peroxide (melting point 450°C) should react more quickly than iron oxide (Fe 2 03 , melting point 1565°C). Similarly, boron (heat of combustion = 14.0 kcal/gram) and aluminum (7.4 kcal/gram) should form quicker delay compositions than tungsten (1.1 kcal/gram) or iron (1.8 kcal/gram). For high reactivity, look for low melting point, exothermic or small endothermic heat of decomposition (in the oxidizer), and high heat of combustion (in the fuel).
The ratio of oxidizer to fuel can be altered for a given binary mixture to achieve substantial changes in the rate of burning. The fastest burning rate should correspond to an oxidizer/fuel ratio near the stoichiometric point, with neither component present in substantial excess. Data have been published for the barium chromate /boron system. Table 2 gives the burn time and heat output per gram for this system.
TABLE 2. The Barium Chromate/Boron System - Effect of % Boron on Burning Timea
We proposed that the maximum in performance centered at approximately 15% boron by weight indicates that the principal pyrotechnic reaction for the BaCrO„/B system is
Although B 20 3 is the expected oxidation product from boron in a room temperature situation, the lower oxide, BO, appears to be more stable at the high reaction temperature of the burning delay mixture.
A small percentage of fuel in excess of the stoichiometric amount increases the burning rate for most delay mixtures, presumably through increased thermal conductivity for the composition. The propagation of burning is enhanced by the additional metal, especially in the absence of substantial quantities of hot gas to aid in the propagation of burning. Air oxidation of the excess metal fuel can also contribute additional heat to increase the reaction rate if the burning composition is exposed to the atmosphere.
The rate of burning of ternary mixtures can similarly be affected by varying the percentages of the components. Table 3 presents data for a three-component delay composition. In this study, a decrease in the burning rate (in cm/second) is observed as the metal percentage is lowered (giving poorer thermal conductivity) and the percentage of higher-melting oxidizer (BaCrO 4 ) is increased at the expense of the lower-melting, more reactive lead chromate, PbCrO 4 .
TABLE 3. A Ternary Delay Mixture - The PbCrO4 /BaCrO4 / Mn System
Table 4 illustrates this same concept for the molybdenum /barium chromate /potassium perchlorate system. Here, KC1O 4 is the better oxidizer.
TABLE 4. The BaCrO4/KCIO 4 /Mo System
Contrary to the behavior expected for "gassy" mixtures, the rate of burning for gasless compositions is expected to increase (in units of grams reacting per second) as the consolidation pressure is increased. "Gasless" delays propagate via heat transfer down the column of pyrotechnic material, and the thermal conductivity of the mixture plays a significant role. As the density of the mixture increases due to increased loading pressure, the components are pressed closer together and better heat transfer occurs.