INTRODUCTION

Commercial shock tube incorporates a small, continuous charge of loosely bound energetic material in a small diameter plastic tube. When ignited by a hot, high pressure impulse such as a percussion primer or percussive electric match, the energetic material combusts at a typical rate of 6500-7000 feet per second.

At one time in the recent past, the commercial blasting industry relied on electric energy transfer via insulated wires and hot bridgewires for detonator ignition. Over the course of the past 20 years, this method of energy transfer and detonator ignition has been largely displaced by the invention and development of the commercial shock tube. Unlike copper wires which transfer the electrical energy to the hot wire in a blasting cap (e.g. DC current, AC current or capacitive discharge pulse), the shock tube transmits a low pressure shock wave along with hot particles and gases to a pyrotechnic or explosive receptor in a blasting cap. The detonation is confined to the tube along its length and effects output only from the open end. In this way, the shock tube acts as a signal transmission method.

As the use of shock tube has been widely accepted as the new standard in the field of commercial blasting, applications in military demolition, seismic exploration and law enforcement continue to grow. Most recently, shock tube has been adapted for use in the fireworks under the trade name, NOMATCH(r). The primary reasons for the popularity of shock tube in the energetics field has been based on its inherent safety and ease of application.

The purpose of this review is to present a general overview of the common properties of commercial shock tube and provide a review of selected shock tube literature and patents.

Shock Tube Types

Shock tubes have been used for many years as a means for studying the combustion of gases and gas/solid dispersions in both a deflagration and detonation mode(30). These shock tubes are relatively large in diameter e.g. 1 inch inner diameter, and are constructed for analytical purposes with various probes and instrumentation. The invention of the commercial shock tube showed that the gas/solid combustion could reliably react in diameters much smaller than that of the analytical type shock tube and could be contained and propagate in a small diameter tube of plastic construction. This invention coupled with the developing plastics and plastic extrusion technology has led to the proliferation of applications for the commercial shock tube.

There are three primary styles of shock tube presently in commercial use:

Coated Filament Core

The first style of shock tube is based on the Burkdoll patent(1). Another version of this type signal transmission tube is covered in the Janowski patent(2). This design is typified by small diameter plastic tube housing an aluminum/ammonium perchlorate coated fiber. The fiber can be composed of nitrocellulose and/or a strength fiber e.g. kevlar. In this design, the quantity/type of energetic materials can easily be adapted for various end uses by adjusting the coating thickness, coating materials and the fiber materials.

Gas Filled Core

The second type of shock tube uses a gas as the combustible material(3). This design involves charging a plastic tube with the combustible gas e.g. methane/oxygen, in field. The gas is pumped into one tube which connects with other tubes via a trunkline and simple tee connectors. In order to verify that the entire tubing pattern is charged with the gas, a return line back to the gas source location is designed into the pattern. The main advantage of this design is that the tube is totally inert during the pattern setup prior to gas charging of the tube.

Thin Layer Powder Core

The third and most widely used type is the style based on the Persson patent(4). This style tube is composed of 1) a small diameter plastic tube and 2) a light deposit of an explosive composition, typically aluminum and HMX adhered to the inner wall of the plastic tube. The remainder of the text will focus on this style shock tube.


MATERIALS OF CONSTRUCTION - TUBE

The tube construction is typically one to three layers of distinctly different plastic materials(Refer to Figure 1). The first inner layer must have an adhesive quality in the melt stage i.e. as extruded to allow the powder to stick to the tube sides. This first layer and any outer layers must as a whole or individually meet the following typical properties; ability to withstand abrasion, provide good flexibility/handling, prevent water ingress, provide chemical resistance, hot/cold temperature properties, tensile strength for handling and radial strength to contain the shock reaction.

The first extruded shock tube was a single layer(5). The plastic material of choice for this layer was and still is, the ionomer, Surlyn(r). This type of ionomer is a random copolymer of ethylene and methacrylic acid neutralised by the metals, zinc or sodium. The molten plastic has a slight ionic electrical charge or adhesive quality on its surface as it exits the extruder. The powder adheres to this surface during the manufacturing process as a result of electrostatic adhesion and simple tack. Other materials such as ethylene/acrylic acid(EAA) and ethylene vinyl acetate(EVA) also have an adhesive quality as an molten extrudate providing good powder holding properties(6). Another patent(7) reports that several other specific materials also have the necessary adhesive qualities.

Although Surlyn has many useful properties such as tack, radial strength and good fuel resistance, the single layer Surlyn(r) tube is relatively stiff exhibiting poor handling properties. As a means for improving the handling properties, a portion of the surlyn is displaced by an outer layer(s) of other plastics.


The most used material as the outer layer(s) is polyethylene, linear low and medium density types(6,8). The use of these materials can result in a tube with much improved flexibility and handling. The use of nylon, zytel(nylon polyethylene blend), thermoplastic polyesters and other materials are also reported(6,9). It is important to note that the work reported in these patents addresses the possible need for tie layers to allow adhesion of adjacent layers during the coating or overjacket extrusion process.

In a further development(7), the stiffness problem was resolved by incorporating the adhesive material in a mix with other non adhesive materials to form a stable blend. The blend, although containing only a small fraction of the adhesive plastic material, maintains the required adhesive strength necessary to hold the powder in place during handling.

The primary plastic development efforts indicated by the patents center around the ability to withstand high temperature exposure to diesel fuel. This is based on the needs of the blasting industry. One method(10) coats the tubing with a shellac material. The base plastic tube is treated e.g. thermal, chemical to promote good adhesion. The application of coatings of polyvinyl acetate or polyvinyl alcohol to the tube is reported as a means for improving the hot diesel fuel resistance(11). In this development, the tube is also preferably heat treated, chemically treated or coated with a tie layer to promote adhesion of the outer oil barrier layer. Another patent(12) reports the use of melt incompatible materials in the tube melt that act as internal physical barriers to oil diffusion.


MATERIALS OF CONSTRUCTION - ENERGETIC COMPOSITION

The mixture most used as the energetic material is an admixture of HMX and aluminum containing typically 8-10% aluminum. The coreload of the mixture is typically 12-24 milligrams per meter. This translates to a surface loading on the tube inner diameter of approximately 45 mg/cm2. Other materials have been reported in the patents(4) such as the explosives PETN, RDX, TNT and mixture of PETN/aluminum blends and pure aluminum.

The aluminum is a stearic acid coated flake type with an approximate surface area of 40000 cm2/gram. As is typical for aluminum, the flakes have a thin coating of aluminum oxide on the outer surface. This thin coating protects the remainder of the aluminum from oxidation under normal conditions, however, storage at high temperatures in air will slowly cause further oxidation. The aluminum melting point is approximately 660oC and the boiling point is approximately 2470oC. The aluminum oxide layer melting point is approximately 2015oC.
The combustion of the aluminum material presumably is preceded by the melting of the oxide layer.

The explosive, Beta-HMX (Cyclotetramethylene Tetranitramine, Octol), is crystalline and has a typical particle size around 20 microns. The melting point of HMX is approximately 273oC and has an approximate 5 second explosion temperature of 327oC.


The mixture of the HMX and aluminum, both classic fuels, has a negative oxygen balance. Thus the air in the tube and in particular the oxygen content (21% of air) provide the oxygen necessary for combustion. The precise balance is difficult to ascertain due to the flake aluminum surface content of aluminum oxide. However, the approximate oxygen balance without the Al2O3 content in consideration would be approximately 2% aluminum at 18 mg/m coreload. The expected reaction products of the HMX/aluminum mixtures are predominantly gaseous e.g.CO2, NOx, H2O due to the HMX. The predominant aluminum reaction product is expected to be the solid, Al2O3. The resultant output is characterized as a high pressure impulse along with hot burning particles.

The replacement of the HMX/aluminum mixture with ammonium perchlorate blends with metallic i.e. aluminum or quasi metallic fuels e.g. silicon, boron has also been claimed(14,15). As a result, the use of higher temperature plastic materials are allowed during the extrusion process due to the higher ignition temperatures of the energetic admixture.

Another area of development that uses core compositions other than HMX/Al are the slow reacting shock tubes. Detonation velocities as low as 100 feet per second are attained by using coreloads comprised of pyrotechnic mixtures e.g. red lead / silicon and tungsten/potassium perchlorate(18). This patent also reports the addition of the explosive HMX to the pyrotechnic blends for rate modification. Barium Peroxide, a strong oxidant, is added to the core composition to help sustain the reaction of slow speed burning mixtures(19).

Still another area of research resulting in patents involves the reduction of Al2O3 by replacing aluminum with barium peroxide, potassium perchlorate, sodium azide, barium nitrate or potassium permanganate in an admixture with HMX(13). The result is a tube with detonation velocities greater than 1800 m/sec with reduced ignition in an incendive atmosphere.

As a visual aid, the addition of dye stuffs(16) or ferric oxide(17) to the explosive blend allows improved visual determination of the shock tube condition i.e. combusted or loaded.

Where the HMX material reaction products are very gaseous, it is apparent that the pyrotechnic additives can be added to generate various outputs e.g. cooler gas reaction products, greater condensed reaction products i.e. solids, liquids. Although the intended applications for these various inventions were for non fireworks applications, the fact that the output can be modified by altering the energetic material provides greater significance to the development of other uses particular to the fireworks arena.


MANUFACTURING PROCESS

The original method of shock tube manufacture as described in the Persson patent(4) involved depositing a thin film of an adhesive material e.g. petroleum jelly on a plastic tube inner surface. Then the energetic powder adhered to the inner wall when the powder was poured into the tube ID. A further lab scale improvement involved pulling the energetic composition into the ID via a vacuum source.


The practical application of the shock tube is based on the development of a small tube extrusion process. The standard dimensions for the tube are 0.118(3mm) outside diameter and 0.040-0.050(1.0-1.3mm) inner diameter. The initial extrusion process involved the extrusion of a single layer tube made of the ionomer, Surlyn(r)(5). The tube is extruded using a cross head style die through which the energetic powder is continuously metered into the tube. The process of feeding the powder into the tube is based on a patented method for continuous feeding of an explosive powder into an extruded tube sheath in a safe manner(20). As the powder comes into contact with the molten ionomer, the powder sticks to its surface and remains adhered to the tube inner wall as the tube is subsequently cooled in a water bath(Figure 2). The quantity of the powder in the tube is referred to as the coreload and is commonly measured in milligrams per meter. Typically, coreloads range from 12 to 24 mg/m. Once the tube is cooled and dried, it is wound onto spools for use in the manufacture of the marketed finished products. Although this basic tube won the acceptance of many people in the blasting industry, the single layer Surlyn(r) ionomer plastic tube, as mentioned earlier, was stiff and hard to handle in the field.

Manufacturing Improvements

Since the invention of the shock tube many manufacturing improvements have been developed. Improvements focused on improved tube properties (e.g. powder adhesion, strength, chemical resistance and flexibility), lower material costs and lower manufacturing costs.

The first modern method of manufacture reported in a patent by Kristensen et al(5) made use of Surlyn(r) as well as the use of a second plastic layer for better physical properties. This concept of multiple layers for tailoring the physical properties in this patent along with the methods outlined in the Gladden and Thureson patent(6) form the basis for all the subsequent improvement patents. In this patent, the use of an improved grade of Surlyn(r), EAA or EVA as the inner layer is introduced as well as a potential three layer construction and a tie layer for non adhesive adjacent layers. Also, the use of a vacuum and/or preheating of a first layer tube to cause improved bonding between the first layer and the subsequent over jacketed layer is reported. The most important claim of this patent was the use of tube stretching prior to the application of the second and/or third layers. This technique was found to increase the tube tensile strength while decreasing the layer thickness of the costly ionomer. All the major patents for contemporary shock tubes utilize the stretching technique to improve tube tensile strength while allowing the use of plastics with improved physical properties and lower costs.

Another significant invention claimed the ability to use a blend of plastics which included the adhesive material e.g. ionomer while maintaining the required tack for powder adhesion(20,7). This allows the use of a simple single layer extrusion process as opposed to the more complicated and expensive multi-extruder process used for multiple discreet layer coatings. In addition, this patent is the first to note the use of stress relief after stretching. This process allows the tube to have dimensional stability at high temperatures as well as improved radial and tensile strength(8).

Coextrusion of a two layer tube with a subsequent over jacketed third layer and then stretching the tube at a precise temperature to effect control over the polymer orientation offers a means for obtaining the individual benefits of each unique plastic layer and better control of the physical properties e.g. strength, oil permeation resistance(8).


As an option to stretching to improve the tube strength, a method of incorporating a thread into the side wall of the tube during extrusion of the tube is patented(21). Another strengthening fiber design(22)is accomplished by placing the filament between to independently extruded layers.


GENERAL THEORY OF SHOCK TUBE MECHANISM

The actual mechanism of shock tube ignition is not well documented. In the blasting industry, ignition is typically achieved from the detonation of a blasting cap or similarly strong shock waves. In practice, however, the shock tube can be ignited from non explosive i.e. pyrotechnic sources(27,29). Once ignited the reaction will reach a steady state propagation mode. As a basis for shock tube understanding, the following propagation mechanism based on a shock ignition is provided.

The ignition of the reactive material coated on the inner surface of the shock tube results in the formation of a shock front. As a result of the shock front streaming past the reactive material on the tube walls, the reactive material is assumed to undergo a turbulent dispersion towards the center of the tube. The shock front also heats up the gases in the tube. The dispersed energetic material is heated and then combusts to release energy which supports the shock front(Refer to Figure 3). The combustion reaction thus would resemble that of a dust explosion.

As with any self sustaining reaction, without the proper amount of energetic material, the energy losses to the tube side walls due to friction, mass movement and heat loss would cause extinction of the reaction.

Work carried out by Sutton et al.(23) provides interesting insight into the shock tube reaction. Based on experimental observations, the shock front, characterized by a bright band of light is closely followed by the low light emitting, pressure pulse producing combustion front. The duration of the pressure exceeds a millisecond while the shock front flash diminishes in approximately 100 microseconds. Of particular interest in the experimental portion of the study is the finding that there is a lower pressure output( 2 vs.17 MPa) and longer duration light emission(300 vs.100us) of the combustion front when the core energetics are pyrotechnics (Al/KMnO4) as opposed to aluminum/HMX. Thus, as a result of a modified coreload composition, a lower pressure impulse(i.e. less gases) and longer duration output can be attained.


PERFORMANCE

Propagation Velocity
The most common shock tube containing the HMX/aluminum powder typically propagate at velocities from 5000-7000 feet per second. Pyrotechnic additives can slow the rate of the combustion propagation to between 100 and 5000 feet per second(18). The invention methods employed the use of pyrotechnic compounds such as red lead/silicon, zirconium/ferric oxide and tungsten/potassium perchlorate.


The addition of barium peroxide allows for stable burning of slow burning reactive core materials(19).

The use of restrictions in the tube can also be used to impede the shock tube reaction as a means for reducing and controlling the velocity(24).

Propagation
The propagation of the shock wave passes through most knots, restrictions, bends and kinks in the tube. Only a very tight knot or a series of knots will extinguish the reaction. The propagation of the reaction has been shown to reliably propagate through an eighteen inch empty section of tube. In handling the shock tube in the field, the user should take care to avoid allowing water to enter the tube end. Only a small drop will effectively stop the propagation reaction or inhibit ignition.

Output
General
The output of the HMX/aluminum tube is characterized by a short duration shock pulse (approximately 20-50 microseconds) accompanied by hot gases and particles. The pressure will increase significantly as a result of higher coreloads in the range from 10 to 30 milligrams per meter. The pulse strength has been shown to be on the order of 7(1000)-27(4000) Mpa(psi)(25,26). The variation in reported pressures is likely due to analytical instrumentation and coreload. The hot particles for the HMX/aluminum system are Al2O3 while the hot gases are predominantly the resultant combustion products of the HMX(26).

The temperature of the HMX/aluminum reaction has been estimated to be ca 2000-2500oK.

Length of Tube Effects
The build up from ignition to steady state velocity is generally recognized to be approximately 6-8 inches. A length of 15-18 inches is generally used to ensure a consistent tube output when necessary.

Coreload Effects
The coreload adhering to the tube inner surface effects the output strength in terms of shock pressure and duration. However, the propagation velocity and output strength are relatively similar in strength over the range of 12-24 mg/m coreload.

Ignitability
Open End
For open end ignition, a low strength shock wave with associated hot gases and burning particles appears to be the minimum means for ignition. Confinement of the tube end during ignition is also critical.

The use of shot gun primer No. 209 is the most common source for ignition of the open end. This is typically accomplished using a small hand held fixture which houses the primer and directs the primer output into the captured tube end.

Other materials such as hot, violent pyrotechnic mixtures will also initiate the open end tube. Quantities vary according to the material, the confinement and the fixturing for transfer but can be as little as a five to ten milligrams.

Still another open end ignition method is electrical. A spark formed as a result of an electrical discharge across a small gap between two conductors can ignite the shock tube. The potential between the two electrodes is primarily dependent on the distance between the electrodes(4).

Through the Tube Wall
Ignition through the wall of the tube is generally accomplished via a strong shock wave such as that of a blasting cap or mini detonator. Other exotic methods using high power lasers and spark generators for shock formation are not practical for the field at this time.


SAFETY CONSIDERATIONS

Electrostatic Sensitivity
Powder
The HMX/aluminum powder is insensitive to ignition from a 30KV, 500pf capacitive discharge. This exceeds the electrostatic charge found carried by humans.

Devices
Similar to the use of electric devices, the shock tube coupled devices must also consider the potential of static electricity in the design. Although the shock tube system does not have the lead wires of electric devices as a means of conduction to the device, the plastic tube can carry an electrostatic charge. The tube design with an aluminum content at 8% will virtually negate the ability of the any electrostatic charge to conduct. In addition, when coupling the tube or tube bearing device into a charge or container enclosing an electrically sensitive substance, care should be taken to isolate the substance from the tube or provide a static shunt. In the NOMATCH(r) system, the use of insenstive energetics as well as a shunt to the shell negates the risks. As in all cases, whether it be electric or shock tube, it is best to minimize the use of materials that are electrostatically sensitive and build into the design a static shunt.

Impact Resistance
The HMX/aluminum shock tube is extremely insensitive. The impact of a common bullet cannot ignite the tube. The impact of a tank rolling over the tube cannot ignite (or desensitize) the tube. As mentioned earlier, the ignition of the shock tube through the side wall requires a relatively strong shock wave e.g. blasting cap to cause ignition.

Adhesion

The powder is lightly adhered to the inner wall of the tube. Subjecting the tube to extreme vibrations can loosen the powder from the tube walls allowing the powder to migrate. An accumulation of powder as a result of powder migration and collection in a small section of the tube can result in a rupture of the tube during propagation. Although the occurrence of a rupture will not typically result in a shock wave propagation shut down, a large enough accumulation and rupture can inhibit propagation. With the development of improved processes and plastic materials, this phenomena has been greatly reduced. Periodic testing of the shock tube as manufactured is carried out to ensure proper adhesion. The vibratory testing simulates worst case vibrations as a result of transportation and handling. The purpose for noting the adhesion/rupture phenomena is to draw attention to the fact that the reaction in the tube is a detonation and that the tube should never be held or positioned in close proximity to the eyes when fired.