we must distinguish:
- the absolute density, or real density, or specific weight, which is the weight of a certain volume in which the substance is contained without interspaces (also called density of crystal);
- the gravimetric density, or apparent, that is the relationship between the weight of a certain volume of explosive in normal condition and the volume that it occupies. This density is less than absolute density: the difference is small for plastic explosives, larger for those in powder form;
- the density of charge, that is the weight of explosive content in the unit of area of the blasting hole in which will occur the explosion.
All these density are expressed in kilograms/litre.
Increasing the division of grains and the pressure applied increases apparent density but it may never reach the absolute density. With the growth of the density of charge increases the detonation velocity, the explosive wave spreads through the same explosive as a sound wave and the speed of transmission is modified by the generated high pressure, increasing the density, and then declining the compressibility, increases the elasticity modulus and decreases the sensitivity, if we could reach the density of crystal there would not be more detonation.
Starting from small charge density the speed of explosion increases with increasing the density, but up to a certain limit beyond which the speed decreases until to melt away. The density where by speed is maximum is called charge density limit practice and is the one that is appropriate to adopt in mines to have the maximum performance.
is the attitude of an explosive to resist chemical decomposition, this resistance is
characteristic of every explosive, for mixtures of explosives global stability is equal to
that of the substance component that has lower stability.
Some compounds tend to decompose already at room temperature, with formation of products that act as a catalyst and make more rapid the phenomenon in time. Decomposition is accelerated by increase in temperature, moisture and light, we can have dangerous situations in storage and in the use of these explosives.
is its attitude to explode under the action of outside pulse. You can consider sensitivity respect:
shock, friction, heat, to initiation .
Sensitivity to shock: normal explosives are sufficiently resistent to shock and not excessive friction , so they can be pushed and compressed, but not too violently. It may lower the sensitivity of some explosives, which would be too sensitive for the use as they are, by substances these stabilizers or phlegmatizers: the phlegmatizer penetrates in the mass of the explosive which remains divided into tiny particles and protected from shock.
Sensitivity to initiation: each explosive requires an external action that should provide an impetus to do that explode, this action is given by a primer, which must act with greater or lesser intensity that depends on sensitivity of the explosive. For very sensitive explosive is sufficient a small impact; for explosives sensitive, but not much, require a stronger impact or the contribution of a flame; explosives with relatively low sensitivity require a strong impact, it is generated by a small explosive wave generated by a powerful explosive (primer). For certain categories of explosives need only a very small charge contained in a detonator; for little sensitive explosives the detonator start a small cgarge of explosive with intermediate sensitivity and is the shock wave of this charge that is the detonator of the other cherge. Black powder does not require detonator, but explodes for simple inflammation.
Sensitivity to heat: high temperature in the environment of conservation of the explosives may facilitate the decomposition. Any explosive, if subjected to increasing temperature, at a certain time ignites or explodes, temperatures of ignition are generally lower than those which corresponding detonation.
Hygroscopicity: is the ability who have some explosives to absorb more or less water
atmosphere, with the result of losing in a more or less long time the characteristics explosive.
Here certain categories of explosives with decreasing level of sensitivity to humidity: explosives with ammonium nitrate, black powder, explosives with chlorate and perchlorate, dynamite, picric acid, PETN, RDX, TNT.
Plastic explosives resist relative better than powdery explosives, for some hygroscopic powdery explosives the alternation of greater and smaller moisture in the air causes the agglomeration in a solid mass. The problem may be reduced with the addition of special anti-caking.
is a measure of the rapidity with which an explosive develops its maximum
In addition to strength, explosive materials display a second characteristic, which is their shattering effect or brisance (from the French briser, to break), which is distinguished from their total work capacity. This characteristic is of practical importance in determining the effectiveness of an explosion in fragmenting shells, bomb casings, grenades, structures, and the like. The rapidity with which an explosive reaches its peak pressure is a measure of its brisance. A brisant explosive is one in which the maximum pressure is attained so rapidly that a shock wave is formed, and the net effect is to shatter (by shock resonance) the material surrounding or in contact with the supersonic detonation wave created by this explosive. Thus brisance is a measure of the shattering ability of an explosive.
One of the most brisant conventional explosives is cyclotrimethylene trinitramine (also known as RDX).