Ignition sensitivity

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

Inerting is rarely used for powder storage, although its use is increasing for pneumatic conveying, especially for ignition sensitive dusts. 

It is therefore usual to combine the evaluation of the ignition sensitivity (MIE) of the dusts with the type and extent of protective measures in accordance with Table 23-9. 

An analysis of static ignition hazard should start with data on the ignition sensitivity of the particular flammable material at its most flammable concentration in air, i.e., its MIE. This is especially important for dusts. It is prudent to determine this value on fines of the specific dust of interest, rather than to rely on published data. Hybrid mixtures, i.e., mixtures of dust and vapor for which vapor concentrations may be below their lower flammable limit, can be ignited by smaller discharge energies than might be expected. 

Work on the deflagration hazards of organic peroxides has been done using a revised Time-Pressure test, to determine the characteristics of ignition sensitivity and violence of deflagration. Some correlation is evident between these characteristics and the AO content within each structurally based peroxide type. Also, for the same AO content, the nature of the characteristics appears to decrease hi the order diacyl peroxides, peroxyesters, dialkyl peroxides, alkylhydroperoxides [18],... 

Therefore, the initial choice for an oxidizer is one with an exothermic heat of decomposition such as potassium chlorate (KCIO 3). However, mixtures of both chlorate and perchlorate salts with active metal fuels are too ignition-sensitive for commercial use, and the less-reactive - but safer - nitrate compormds are usually selected. Potassium perchlorate is used with aluminum and magnesium in some "photoflash" mixtures these are extremely reactive compositions, with velocities in the explosive range. 

One must also keep in mind that these experiments are measuring the temperature sensitivity of a particular composition, in which the entire sample is heated to the experimental temperature. Ignition sensitivity can also be discussed in terms of the relative ease of ignition due to other types of potential stimuli, including static spark, impact, friction, and flame. 

Test No 1 Same test as Fuse Test described under Ignition Sensitiveness... 

Test No 3 If the explosive withstands the Fuse Test, it is subjected to the Red Hot Iron Test described under Ignition Sensitiveness Tests Test No 4 Wood Fire Test In some cases, larger quantities, ie a pound or two, are burned in a wood fire to ascertain the degree of danger the explosive may offer in case of fire. Tne observations should be made under precautions at a safe distance. (Compare with Iron Box Test, described under Ignition Sensitiveness)... 

The BurMines criteria for racing the potential explosion hazard of dust by the standard electric spark is defined by the ignition sensitivity, explosion severity and the overall explosibility indices or the correlative adjectives, weak, moderate, strong and severe (Ref 10)... 

L. Schmitz, Chem Ing Tech 21, 386-89 (1949) CA44, 1259 (1950) (Measurement of electrostatic charge and ignition sensitivity of various primary and initiating explosives) 7) R.F. Wilkerson "Research and Development in the Field... 

A lab method of prepn is described in Ref 6. Its toxicology, fire hazard, storage and handling are discussed in Ref 7. It was claimed (Ref 3) that the ignition sensitivity life of igniter compds contg cuprous acery-lide(or other metallic acetylides) is improved by the addition of small amounts of abietic acid... 

The importance of good housekeeping must be emphasized. Any accumulation of Zr dust or powder on work benches, floors or clothes of personnel must be removed promptly. The pyrophoric properties of Zr dust or powder requires the selection of either wet or dry methods of dust collection A limited amount of moisture can make Zr powder ignition sensitive, but attempts to collect the dust dry by suction or convection flow risk dust expins which are easily initiated by static discharge or spontaneous combustion. Because nitrogen will also react with Zr,... 

The compd is ignitable with an open flame. The hydride is much safer to handle and ship in a dry, finely divided state than the metal itself. This greater inertness also shows itself in a lower ignition sensitivity and a slower burning rate than powdered Zr (Ref 7)... 

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