Blasting cap
A blasting cap is a small sensitive primary explosive device generally used to detonate a larger, more powerful and less sensitive secondary explosive such as TNT, dynamite, or plastic explosive.
Blasting caps come in a variety of types, including non-electric caps, electric caps, and fuse caps. They are used in commercial mining, excavation, and demolition. Electric types are set off by a short burst of current conducted from a blasting machine by a long wire to the cap to ensure safety. Traditional fuse caps have a fuse which is lit by a match.
The need for blasting caps arises from sensitivity issues of an explosive compound. All explosive compounds require a certain amount of energy to detonate. If an explosive is too sensitive, it may go off unexpectedly, so most commercial explosives are formulated to be stable and safe to handle and will not explode if accidentally dropped, mishandled, or exposed to fire. However, such explosives (called secondary explosives) are hard to detonate intentionally as well, and require a small initiating explosion. A blasting cap contains an easy-to-ignite explosive that provides the initial activation energy to start a detonation in a more stable explosive. The blasting cap is stored separately and not inserted into the main explosive charge until just before use, keeping the main charge safe.
Most blasting caps contain what is called a primary explosive, which is a high explosive compound that will explode from flame, heat or shock. A blasting cap may also contain a booster, another less sensitive but more energetic explosive that makes the cap more powerful, and thus more reliable for detonating secondary explosives.
Explosives commonly used in blasting caps include mercury fulminate, lead azide, lead styphnate and tetryl. The recommended primary explosive is DDNP because it is nontoxic, easy to make and not too sensitive.
Because they contain easy-to-detonate primary explosives, blasting caps are hazardous for untrained personnel to handle. They are sometimes not recognized as explosives due to their appearance, leading to injuries.
Types
Pyrotechnic fuse blasting cap
The oldest and simplest type of cap, fuse caps are a metal cylinder, closed at one end. From the open end inwards, there is first an empty space into which a pyrotechnic fuse is inserted and crimped, then a pyrotechnic ignition mix, a primary explosive, and then the main detonating explosive charge.
The primary hazard of pyrotechnic blasting caps is that for proper usage, the fuse must be inserted and then crimped into place by crushing the base of the cap around the fuse. If the tool used to crimp the cap is used too close to the explosives, the primary explosive compound can detonate during crimping. A common hazardous practice is crimping caps with one's teeth; an accidental detonation can cause serious injury to the mouth. Proper recommended procedure is to position the crimping tool and tighten it, then hold it behind oneself slightly below waist level while actually crimping.
Fuse type blasting caps are still in active use today. They are the safest type to use around certain types of electromagnetic interference, and they have a built in time delay as the fuse burns down.
Solid pack electric blasting cap
Solid pack electric blasting caps use a thin bridgewire in direct contact (hence solid pack) with a primary explosive, which is heated by electric current and causes the detonation of the primary explosive. That primary explosive then detonates a larger charge of secondary explosive.
Some solid pack fuses incorporate a small pyrotechnic delay element, up to a few hundred milliseconds, before the cap fires.
Match or fusehead electric blasting cap
Match type blasting caps use an electric match (insulating sheet with electrodes on both sides, a thin bridgewire soldered across the sides, all dipped in ignition and output mixes) to initiate the primary explosive, rather than direct contact between the bridgewire and the primary explosive. The match can be manufactured separately from the rest of the cap and only assembled at the end of the process.
Match type caps are now the most common type found worldwide.
Exploding bridgewire detonator or blasting cap
This type of detonator was invented in the 1940s as part of the Manhattan project to develop nuclear weapons. The design goal was to produce a detonator which acted very rapidly and predictably. Both Match and Solid Pack type electric caps take a few milliseconds to fire, as the bridgewire heats up and heats the explosive to the point of detonation. Explosive bridgewire or EBW detonators use a higher voltage electric charge and a very thin bridgewire (1 mm long, 0.04 mm diameter). Instead of heating up the explosive, the EBW detonator wire is heated so quickly by the high firing current that the wire actually vaporizes and explodes due to electric resistance heating. That electrical driven explosion then fires the detonator's initiator explosive (usually PETN).
Some similar detonators use a thin metal foil instead of a wire, but operate in the same manner as true bridgewire detonators.
In addition to firing very quickly when properly activated, EBW detonators are safe from stray static electricity and other electrical current. Enough current and the bridgewire may melt, but it is small enough that it cannot detonate the initiator explosive unless the full, high voltage high current charge passes through the bridgewire. EBW detonators are used in many civilian applications where radio signals, static electricity, or other electrical hazards might cause accidents with conventional electric detonators.
Slapper detonator or blasting cap
Slapper detonators are an improvement on EBW detonators. Slappers, instead of directly using the exploding foil to detonate the initiator explosive, use the electrical vaporization of the foil to drive a small circle of insulating material such as PET film or kapton down a circular hole in an additional disc of insulating material. At the far end of that hole is a pellet of conventional initiator explosive.
The conversion efficiency of energy from electricity into kinetic energy of the flying disk or slapper can be 20-40%.
Since the slapper impacts a wide area (roughly one mm across) of the explosive, rather than a thin line or point as in an exploding foil or bridgewire detonator, the detonation is more regular and requires less energy. Reliable detonation requires raising a minimum volume of explosive to temperatures and pressures at which detonation starts. If energy is deposited at a single point, it can radiate away in the explosive in all directions in rarefaction or expansion waves, and only a small volume is efficiently heated or compressed. The flier disc loses impact energy at its sides to rarefaction waves, but a conical volume of explosive is efficiently shock compressed.
Slapper detonators are used in nuclear weapons. These components require large quantities of energy to initiate, making them extremely unlikely to accidentally discharge.
Laser ordnance initiators
Another type of initiator that is rarely seen requires the use of a laser to initiate a carbon-doped explosive via a fiber optic. These initiators are highly reliable, and offer a number of advantages, primarily in that unintentional initiation is made very difficult. Without the correct laser initiation system, or a completely independent initiation system, these components are not capable of being controlled remotely.
History
The first blasting cap or detonator was demonstrated in 1745, when a Dr. Watson of the Royal Society showed that the electric spark of a Leyden jar could ignite black powder.
In 1750, Benjamin Franklin in Philadelphia made a commercial blasting cap consisting of a paper tube full of black powder, with wires leading in both sides and wadding sealing up the ends. The two wires came close but did not touch, so a large electric spark discharge between the two wires would fire the cap.
In 1822 the first hot wire detonator was produced by Dr Robert Hare. Using one strand separated out of a multistrand wire as the hot bridgewire, this blasting cap ignited a pyrotechnic mixture (believed to be potassium chlorate/arsenic/sulphur) and then a charge of tamped black powder.
In 1863 Alfred Nobel introduced the first pyrotechnic fuse blasting cap, using mercury fulminate to detonate nitroglycerin.
In 1868, H. Julius Smith introduced a cap that combined a spark gap ignitor and mercury fulminate, the first electric cap able to detonate dynamite.
In 1875, Perry "Pell" Gardiner and Smith independently developed and marketed caps which combined the hot wire detonator with mercury fulminate explosive. These were the first generally modern type blasting caps. Modern caps use different explosives and separate primary and secondary explosive charges, but are generally very similar to the Gardiner and Smith caps.
Electric match caps were developed in the early 1900s in Germany, and spread to the US in the 1950s when ICI International purchased Atlas Powder Co. These match caps have become the predominant world standard cap type.
References
- Cooper, Paul W., Explosives Engineering, New York: Wiley-VCH, 1996. ISBN 0-471-18636-8