Resorcinol Stabilizer In Smokeless Powder

Small arms ammunition propellants may be defined as "explosive materials which are formulated, designed, manufactured, and initiated in such a manner as to permit the generation of large volumes of hot gases at highly controlled, predetermined rates."55

Ideally, a propellant would be a single, solid, nontoxic chemical compound that is stable, easy to store, easy to ignite, of compact mass, and so forth, which is cheap and simple to prepare from readily available materials and which on combustion produces no smoke or solid residue, that is, is completely converted into gas or gases. It must contain its own oxygen supply which is necessary for combustion in confined spaces, it must burn very rapidly as opposed to detonation, and it must have a satisfactory energy/weight relationship.

It is not surprising that no single chemical compound fulfills all these specifications. In practice propellants consist of a mixture of substances.

A propellant must fulfill the following general specifications:

It should be capable of being manufactured simply, rapidly, with relative safety, at reasonable cost and from ingredients that are readily obtainable in time of war (military propellants).

It must be easy and safe to load, nonhydroscopic, and free from combustion products that are difficult to remove or injurious to the firearm or cartridge case.

It must give consistent performance under varying conditions of storage and climate, and it must not deteriorate with age (this is especially applicable to propellants for military use which can be stored as a war reserve for a long period of time). It must also not ignite when in the chamber of a very hot firearm for a considerable period of time. (This also applies to priming compositions.)

The energy/weight/bulk relationship of a propellant and the rate of delivery of the energy must be matched to the system, that is, space available within the cartridge case and gun barrel, the bullet weight, pressure requirements, and the required bullet velocity. Consequently, a wide range of propellants is required to satisfy the varying ballistic requirements of a wide range of firearms and ammunition.

The burning rate is extremely important because if the propellant releases hot gases too quickly, it detonates, thereby destroying the gun and possibly causing injury to the firer. If it burns too slowly, it is inefficient, and the bullet will lack sufficient velocity. The burning rate can be controlled by the size and geometrical design of the individual granules. [An individual propellant particle is referred to as a grain (kernel, granule) and grains (kernels, granules) can be very small with simple geometries, or very large with complex geometries. Note that grains in this context should not be confused with the unit of weight; 7,000 grains = 1 lb = 453.59237 grams. In my opinion it is better to use the term granule to avoid confusion].

Apart from the inherent burning characteristics of a propellant the burning rate can also be varied by the use of surface coatings (moderants) on the granules of propellant.

Propellants are frequently referred to as gunpowder, powder charge, or simply as charge or powder. However, they are very rarely a true powder and are manufactured in a wide range of colors, shapes, and sizes. Figure 10.1 illustrates some shapes.

It is critically important that propellant granules not contain non-uniformities such as cracks, pores, and cavities, because this could cause internal granule burning, leading to detonation or excessive pressure.

The relationship between physical shape and burning rate is complex, dependent on the characteristics of the propellant surface, which affect the rate at which decomposition reactions occur, and also on the characteristics of the environment above the propellant, which affects the rate at which heat

Ball

Flattened ball

Ball

Flattened ball

Flake

Flake

Disc

Cylinder

Multi-tube

Disc

Cylinder

Multi-tube

Cord

Ribbon

Slotted tube

Cord

Ribbon

Slotted tube

Figure 10.1 Propellant shapes.

is transferred to the propellant surface to cause chemical breakdown. Both surface and gas phase theories are intimately related.

The process of delivery of propellant gases at a predetermined rate involves the selection of a propellant composition with the required burning rate at the operating pressure of the firearm, and then designing the propellant granules so that the necessary burning surface is available to provide the required mass rate of gas evolution, that is, the necessary time/pressure relationship.

Since the introduction of smokeless powders in the period between 1870 and 1890, the use of black powder as a small arms ammunition propellant has substantially diminished. Black powder is still currently used as a pro-pellant for some specialized purposes, for example, baton guns, punt guns, cable guns, signal flares, and by black powder firearms enthusiasts. It is also used in blank rounds of various types, and in many other ammunition components designed for larger caliber guns. Black powder suffers from several major disadvantages, namely, (a) a large amount of solid residue after combustion which attracts atmospheric moisture causing rusting of the firearm, (b) heavy fouling can also affect the efficient functioning of the firearms mechanism, (c) large amount of smoke formed after combustion can obscure the firer's view for subsequent shots, and (d) the smoke gives away the firing position.

Black powder is a mechanical mixture of charcoal, saltpeter (potassium nitrate), and sulfur in the typical proportion 15:75:10, respectively. The charcoal is the fuel, the saltpeter supplies the oxygen necessary for combustion in a confined space, and the sulfur is a binding agent that aids in holding the mixture together and to a much lesser extent also acts as a fuel. Black powder is black and granular in appearance and the burning rate is controlled by granulation size.

When black powder burns, the "initial portion" ignited undergoes a chemical reaction which results in the production of hot gases. The gases expand in all directions, warming the next portion to the "kindling" temperature. This then ignites, producing more hot gases and raising the temperature of the next portion, and so on. As the black powder is confined in the cartridge case the pressure rises and the heat cannot escape; consequently, it is communicated rapidly throughout the mass. In a confined space the combustion becomes extremely rapid; consequently, the pressure rise is also extremely rapid.

Black powder burns to produce a dense white smoke which contains extremely small particles held temporarily in suspension by the hot combustion gases.

Analysis of the combustion products of a particular brand of black powder gave the following results56: 42.98% of its weight as gases, 55.19% solids, and 1.11% water. Analysis of the solid products (percent by weight) and of the gaseous products (percent by volume) is as follows:

Solid Products

Gaseous Products

Potassium carbonate

61.03

Carbon dioxide

49.29

Potassium sulfate

15.10

Carbon monoxide

12.47

Potassium sulfide

14.45

Nitrogen

32.91

Potassium thiocyanate

0.22

Hydrogen sulfide

2.65

Potassium nitrate

0.27

Methane

0.43

Ammonium carbonate

0.08

Hydrogen

2.19

Sulfur

8.74

Carbon

0.08

Black powder can vary from brand to brand. Variations in percentage compositions between manufacturers are small, but different charcoals, types of saltpeter (purity), different moisture content, and so forth can result in different ballistic performances from basically similar mixtures. Owing to a temporary shortage of potassium nitrate during World War I, sodium nitrate was used as a substitute. Ammonium nitrate has also been used as a substitute for potassium nitrate.

Brown powder (cocoa powder) represents the peak of development of black powder and was the most successful form of black powder exhibiting better burning characteristics. It was made in single perforated hexagonal or octagonal prisms. A partially burned brown charcoal made from rye straw, which had colloidal properties and flowed under pressure, cementing the granules together, was used. This made possible the manufacture of slow burning propellant containing little or no sulfur. A typical brown powder was brown charcoal 19%, saltpeter 78%, and sulfur 3%. A sulfur-free brown powder was brown charcoal 20% and saltpeter 80%.

A modern substitute for black powder is "Pyrodex." It is safer to transport, store, and use, and is cleaner burning than conventional black powder. Pyrodex incorporates both charcoal and sulfur but in much smaller proportions than in black powder, and potassium nitrate in addition to other ingredients. Pyrodex also contains potassium perchlorate, sodium benzoate, and dicyandiamide.57 Modern smokeless propellants for small arms ammunition almost exclusively contain plasticized cellulose nitrate (NC) as the major oxidizing ingredient (cellulose hexanitrate, commonly referred to as nitrocellulose). Various other chemicals are added for specific purposes:

High energy oxidizing plasticizers such as nitroglycerine (NG-glyceryl trinitrate) to increase performance.

Fuel type plasticizers such as phthalates, polyester adipate, or urethane to improve physical and processing characteristics.

Organic crystalline chemicals such as nitroguanidine to moderate the ballistic characteristics.

Stabilizers such as diphenylamine, 2-nitrophenylamine, dinitrotoluene, A-methyl-p-nitroaniline, centralites, or acardites (e.g., AW-diphenyl-urea), to increase chemical stability by combining with decomposition products.

A range of inorganic additives such as chalk, graphite, potassium sulfate, potassium nitrate, barium nitrate, to improve ignitability, facilitate handling, and minimize muzzle flash. (Graphite acts as a lubricant to cover the granules and prevent them from sticking together and it also helps to dissipate static electricity).

Powdered metals are sometimes added to change thermal characteristics such as conductivity.

Some manufacturers also add colored taggants to aid in identifying their product.

Propellants that contain nitrocellulose as the only oxidizer are referred to as single base and propellants that contain both nitrocellulose and nitroglycerine (or some other explosive plasticizer) as double base. Triple-based propellants are produced when substantial quantities of an organic, energy-producing, crystalline compound such as nitroguanidine are incorporated in double-based propellants. Triple-based propellants are unlikely to be encountered in small arms ammunition.

Stabilizers are necessary because nitrocellulose decomposes with age. The decomposition reaction yields dinitrogen tetraoxide which acts as an autocatalyst and accelerates the decomposition.58 Stabilizers act as dinitro-gen tetraoxide scavengers; consequently shelf life is increased. Stabilizers are normally added in the region of 0.5 to 2.0%. To neutralize the decomposition products, which could cause corrosion of the firearm, calcium carbonate is added to some propellants. A common stabilizer is diphenylamine or its nitro derivatives (Figure 10.2).

diphenylamine N-nitrosodiphenylamine (diphenylnitrosamine)

diphenylamine N-nitrosodiphenylamine (diphenylnitrosamine)

2-nitrodiphenylamine 4-nitrodiphenylamine

Figure 10.2 Stabilizers.

2-nitrodiphenylamine 4-nitrodiphenylamine

Figure 10.2 Stabilizers.

C2H5 C2H5

C2H5 C2H5

Figure10.3 Ethyl centralite (smydiethyl diphenylurea).

Figure10.3 Ethyl centralite (smydiethyl diphenylurea).

Diphenylamine is the most common stabilizer especially in single-based powders. It has been suggested that diphenylamine is not a good stabilizer for double-based propellants as it may hydrolyze NG.59 2-Nitrodi-phenylamine is used for double- and triple-based propellants.

Another common stabilizer is ethyl centralite (Figure 10.3) Figure 10.4 Resorcinol. although methyl centralite is sometimes used.60 Methyl centralite (Sym-dimethyl diphenylurea; Centralite II) is also used as a moderant to reduce the burning rate. Ethyl centralite is usually found in double-based propellants. Resorcinol (Figure 10.4) is also used as a stabilizer.

Plasticizers add strength and flexibility to the propellant granules. Examples of some plasticizers used are shown in Figure 10.5 and Figure 10.6.61,62

Muzzle flash suppressors (flash reducers) include dinitrotoluene (Figure 10.7). Dinitrotoluene acts as a flash suppressor by reducing the heat of explosion. Nitroguanidine (picrite) is another flash suppressor which acts by producing nitrogen, thereby diluting the combustible muzzle gases. Potassium nitrate and potassium sulfate are also used as flash suppressors but both have the disadvantage of producing smoke.

Wear reduction additives include wax, talc, and titanium dioxide.

Binders (to hold the granule shape) include ethyl acetate, and rosin (also called colophony; the sap or sticky substance from pine or spruce trees).

Decoppering additives used to decrease the buildup of copper residues in the barrel rifling include tin metal and compounds such as tin dioxide; bismuth metal and compounds such as bismuth trioxide, bismuth subcarbonate, bismuth nitrate, bismuth antimonide. The bismuth compounds are

Resorcinol

CH2-O-COCH3 2 3

CH — O — COCH3 Triacetin (Glyceryl triacetate)

CH2 O COCH3

Figure 10.5 Triacetin (glyceryl triacetate).

R is CH3 R is C2H5

dimethyl phthalate diethyl phthalate

Figure 10.6 Dimethyl phthalate, diethyl phthalate, and dibutyl phthalate.

preferred as copper "dissolves" in molten bismuth, forming brittle and easily removable alloy. Lead foil and compounds were also used but due to toxicity they are being phased out.

Examples of single- and double-based propellant compositions are given in Table 10.1 and Table 10.2.

Smokeless powders leave relatively little solid residue on combustion and produce much less smoke than black powder. Combustion of smokeless powders produces primarily nitrogen, carbon monoxide, carbon dioxide, hydrogen, and water vapor. The quantity of smokeless powder varies depending on the caliber, bullet weight/type, required pressure/velocity, space available within the cartridge case/chamber, and so forth. Ammunition for use in rifles contains propellant varying in weight from ~0.45 g (6.9 grains) for a .22" caliber to ~6.45 g (99.5 grains) for a .378" caliber. For pistols/revolvers the range can vary from ~0.06 g (0.9 grains) for a .25" caliber to ~1.72 g (26.5 grains) for a .44" Magnum caliber. For shotguns the range can vary from ~1.10 g (17.0 grains) for a 20-bore caliber to ~2.0 g (30.9 grains) for a 12-bore caliber.

Generally, about 700 to 1,100 cm3 of gas per gram is produced and flame temperature can range from, for example, 2,000 K for a cool propellant to 4,000 K for very hot propellants. Typical gas composition from double-based ch3

R is C4H9 dibutyl phthalate

Figure 10.7 2,4-Dinitrotoluene and 2,6-dinitrotoluene.

Table 10.1 Single-Based Propellants (% composition)

Barium nitrate 6.0

Potassium nitrate 3.0

Starch 0.75

Paraffin oil

Dinitrotoluene 8.0 10.0 Methyl centralite

Dibutyl phthalate 1.0 4.0

Glyceryl triacetate 5.0

Tin 0.75

Graphite 0.2

Potassium sulfate 0.75

Dye (aurine) 0.25

Trinitrotoluene 15.0

87.0

96.25

94.25

94.0

98.0

99.4

92.4

6.0

2.0

.25

4.0

1.0

1.0

1.0

1.0

1.0

0.6

0.6

6.5

2.0

2.0

1.75

2.0

1.75

0.8

With NC

With NC

0.5

0.5

0.75

0.75

0.75

cc o hd

Table 10.2 Double-Based Propellants (% composition)

Nitrocellulose

77.45

Nitroglycerine

19.50

Diethylphthalate

Dibutylphthalate

Diphenylphthalate

Dinitrotoluene

Potassium sulfate

Potassium nitrate

0.75

Ethyl centralite

0.60

Graphite

0.30

Barium nitrate

1.40

Candelilla wax

Methyl cellulose

Sodium sulfate

Calcium carbonate

Diphenylamine

Water

Methyl centralite

Added to basic composition.

0.40

1.05

85.45 9.00

59.40 36.00

0.40

0.55

0.25

1.00

0.60

0.60

0.10

0.10

0.10

0.10

0.40

0.40

0.10

0.10

1.00

1.00

0.50

0.50

0.90

0.55

0.40

propellants is carbon dioxide 28%, carbon monoxide 23%, hydrogen 8%, nitrogen 15%, and water 26%.

Other ingredients that may be found in smokeless powders include camphor, carbazole, cresol, diethyleneglycoldinitrate (DEGDN), dimethylse-bacate, dinitrocresol, di-normal-propyl adipate, 2.4-dinitrodiphenylamine, PETN, TNT, RDX, acaroid resin, gum arabic, synthetic resins, aluminum, ammonium chlorate/oxalate/perchlorate, pentaerythritol dioleate, oxamide, lead carbonate/salicylate/stearate, magnesium oxide, sodium aluminum fluoride, sodium carbonate/bicarbonate, petrolatum, dioctylphthalate, stannic oxide, potassium cyrolate, triphenyl bismuth.

The percentage of NG in double-based propellants can range from as low as 5% to as high as 44%.

Apart from firearms ammunition other propellant-activated devices have numerous uses, for example, to drive turbines, to move pistons, to eject pilots from jet planes, to shear bolts and wires, to operate vanes in rockets, to act as sources of heat in special operations, to operate pumps in missiles, to clear blocked drill bits underground, to start aircraft engines, to jettison stores from aircraft, and generally for systems that require well-controlled sources of high force applied over relatively short periods of time. Propel-lants are also used in some blank cartridges.

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