Temperature Resistance

Explosive compounds can suffer in performance if stored under extremely hot or cold conditions. Table 6 lists the temperature resistance of commonly used explosives. Under hot storage conditions, above 90°F, many compounds will slowly decompose or change properties; shelf life will be decreased. Storage of AN blasting agents in temperatures above 90°F can result in cycling, which will affect the performance and safety of the product.

The chemical formula for AN is ??4??3. For its weight, it supplies more gas volume on detonation than any other explosive. In pure form, AN is almost inert and is composed of 60% oxygen by weight, 33% nitrogen, and 7% hydrogen. With the addition of fuel oil, the ideal oxygen balanced reactions for ??4??3 is:

For this reason, the spherical particles, called prills, have a thin protective coating of silica flour (SiO2), which offers some amount of water resistance. The second and most important characteristic is a phenomenon called cycling. Cycling is the ability of a material to change its crystal form with temperature. AN will have one of the five crystal forms depending on temperature:

• Above 257°F, cubic crystals exist.
• Above 184°F and below 257°F, tetragonal crystals exist.
• Above 90°F and below 184°F, orthorhombic crystals exist.
• Above 0°F and below 90°F, pseudo tetragonal crystals exist.
• Below 0°F, tetragonal crystals exist.

The cycling phenomena can seriously affect both the storage and performance of any explosive that contains AN. Most dynamites, both regular nitroglycerine (NG) or permissibles, contain some percentages of AN while blasting agents are composed almost totally of this compound. The two temperatures at which cycling will occur under normal conditions are 0 and 90°F. Therefore, any products that are stored over the winter or for a period of time during the summer most likely will undergo some amount of cycling. During the summer in a poorly ventilated powder magazine or storage bin located in the sun, the cycling temperature may be reached daily. The effect of cycling on AN when isolated from the humidity in the air is that the prills break down into finer and finer particles.

The prills are made up of pseudo tetragonal crystals. When the temperature exceeds 90°F, each crystal breaks into smaller crystals of orthorhombic structure. When the temperature again falls below 90 F, the small crystals break into even finer crystals of the pseudo tetragonal form. This process can continue until the density is no longer near 50 lb/ft³, but can reach a density near 75 lb/ft³. The density increase can make the product more sensitive and contain more energy per unit volume.

To further complicate the situation, some cartridged blasting agents or those stored in bins may not efficiently exclude humidity. After the AN has undergone cycling, the thin water-resistant coating (silica flour) is broken and the water vapor in the air condenses on the particles. As cycling continues water collects on the particles and the mass starts to dissolve. Recrystallizing into large crystals can occur with a reduction of temperature. Therefore, it is evident that a volume of AN after cycling may have very dense areas with decomposed prills and areas of large crystals. The performance of this product may range from that of a very powerful explosive to one that deflagrates or even one that will not shoot at all. Figure 27 shows the effect of this temperature cycling. Compare this to Figure 22, which shows intact prills.

Slurry explosives, which include water gel and emulsions, can have serious detonation problems if stored in cold temperatures and not allowed to warm up before they are detonated. Slurries are quite different from the other products previously mentioned, such as dynamite and blasting agents. The problem comes about because in the past the blaster has been accustomed to using blasting agents from any manufacturer without having any problems due to cold weather. The blaster also has become accustomed to using dynamites from any manufacturer with good results. Today the slurry explosives do not all perform identically. Some can be used immediately if stored at temperatures of 0 °F where others will not detonate if stored at temperatures below 40°F.

The sensitivity of the product can become affected. The priming procedure, which was employed when the produce was stored at 68°F, may cause a misfire if the product is stored at 43°F. It is a good practice to consult the manufacturer’s data sheet whenever any new product is introduced on the job, but it is absolutely essential to consult that data sheet if any new slurry explosives are introduced, since their properties and performance with temperatures can vary greatly.