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Home , Muzzle flash, Nitration, Nitrocellulose, Propellant

EXPLOSIVES 374 CHIMIA 2004, 58, No. 6

Chimia 58 (2004) 374–382 © Schweizerische Chemische Gesellschaft ISSN 0009–4293

Propellant Chemistry

Patrick Folly* and Peter Mädera

Abstract: A systematic overview of the different types of , their chemical compositions, charac- teristics and practical applications is given. The classification of propellants, the function of stabilizers and the mod- ification of the ballistic properties are discussed.

Keywords: Composite · Double base · · Single base · Stabilizer

1. Introduction and Double Base (DB) propellants. The 2. Single Base Propellant main ingredient of SB propellants is basi- The names ‘ Powders’, ‘Powders’, and cally , which has been made 2.1. Nitrocellulose ‘Propellants’ are related to solid materials colloidal by the action of solvent. DB pro- The main compound of SB propellant is primarily used to accelerate pellants, contain nitroglycerine (NGL) or nitrocellulose (1, NC), or more exactly the from of all sizes and for propelling other nitroglycol compounds and NC. In of , present in the compo- and missiles. These materials must addition, DB propellants with picrite or ni- sition from 85 up to about 96%. This ener- be of high energy content and must have de- tro-guanidine added to the formulation are getic compound was discovered in 1846 by fined burning characteristics. The energy considered as a separate class and are called Dr. Friedrich Schönbein [1]. In 1868, Abel must be delivered in a very short period of Triple Base (TB) propellants. and later several other authors of various time. Most initial development in the field A third type, the composite propellants, nationalities [2][3] found ways to stabilize of propellant has been activated by military is a more recent development compared NC. The rest of the composition consists of and space exploration needs. with the two others. This is based on an ox- . They exist e.g. in an inert form , a mixture of salpeter, idizing solid, commonly a perchlorate, to- like dibutyl (2), dibutyl sebacate charcoal, and sulfur, also known as ‘black gether with an organic binder, which acts as (3), dibutyl adipate (4), and or as powder’, was first used as a propellant in and gives also adequate mechanical energetic ones like the isomeric mixture of mortars and guns. This old type of propul- strength to the resulting propellant. The ex- 2,4-dinitrotoluene (5) and 2,6-dinitro- sion source had serious disadvantages. It cellent mechanical properties allow propel- (6) (Fig. 1). The role of the plasti- was rather unpredictable in use, it devel- lant grains to be manufactured in dimen- cizers is to act as surface moderants and at oped extremely dirty and residues and sions larger than with SB or DB. This and the same time as coolants. The principle of caused considerable fouling of gun barrels. the high-energy content of composite pro- a surface moderant is to retard the initial The emitted amount of smoke and muzzle- pellants make them favorable for use in burning rate, the initial generation rate, flash immediately disclosed the position of medium and large size solid motors. and the initial flame temperature. Typical the gun to the enemy. The discovery of gelatinized nitrocellu- lose (NC) in France led towards the gener- ation of smokeless powders. These can be divided into two classes: Single Base (SB)

dibutyl phthalate (2)

*Correspondence: Dr. P. Folly armasuisse Science and Technology Feuerwerkerstrasse 39 CH–3602 Thun Tel.: +41 33 228 37 35 Fax: +41 33 228 46 01 E-Mail: [email protected] aAMGS Hardeggweg 24 CH–3612 Steffisburg Tel.: +41 33 437 76 55 Fax: +41 33 437 76 72 Fig. 1. Chemical structure of nitrocellulose and the most commonly used inert and energetic plasti- E-Mail: [email protected] cizers 375 CHIMIA 2004, 58, No. 6 concentrations of plasticizers are between 1 and 10%.

2.1.1. of Cellulose The mechanism of the nitration reaction of cellulose is complex and has only been elucidated a century after its invention and a long time after its industrial production was established. Originally the production of nitrocellulose was very simple, but be- came more scientific with the gradual ad- vance in basic knowledge. Chemically, cellulose contains three al- cohol groups per unit, one primary and two Fig. 2. Chemical structure of sulfate of nitocellulose and of the glucopyranose derivatives orig- secondary. Theoretically, it is therefore pos- inating from hydrolysis of NC sible to obtain the mononitrate (6.76% ni- trogen content), the dinitrate (11.12% nitro- the composition of the nitration acid mix- varying degrees by (a) the presence of gen content), and the trinitrate (14.14% ni- ture consisting of , , residual hydroxyl groups, (b) the fibrous trogen content). The true content and water. The formation of sulfuric acid structure often inhibiting the penetration of results from the statistical distribution of (7) was assumed to be responsible for chemical reagents, as in the case of cellu- the low stability of the nitrocellulose. the NO2 groups on the different lose, and (c) the crystalline microstructure units [4]. It is most probable that the pri- Sulfuric acid, as occlusions or esters, is which also introduces some chemical ac- mary hydroxyl groups are nitrated in pref- not the only reason for the instability of ni- cessibility. erence to the secondary groups and the lat- trocellulose. always contain Nitrocellulose is insoluble in water, ter are denitrated more easily. This has been some hydro- or oxycelluloses. The hemi- which allows its preparation, stabilization, demonstrated by using 15N nitric acid [5]. acetal function, present at the end of the and transport by quenching, using the ap- The nitration of cellulose is a reversible es- chain, is easily esterified and the resulting propriate quantity of water. The solubility terification under equilibrium control. esters are very unstable. This is the case for of NC in organic solvents is essentially a However, as a result of the diffusion phe- the pentanitrate of D-glucopyranose (8), il- function of the nitrogen content and to a nomena of the reactants in the fiber, this re- lustrated in Fig. 2, which is very unstable, small extent to the degree of polymeriza- versibility is particularly slow and thus whereas the methyl D-glucopyranose tion. In case of a like nitrocellu- practically very limited. The extent of de- tetranitrate (9) is stable [8]. To increase the lose, the notion of solubility is more quali- nitration in diluted acid or in pure water is stability these esters have to be broken tative than quantitative. In general, NC is insignificant, which means that NC can be down and removed. soluble in all proportions, but increasing the transported in aqueous solution without ap- In practice, the NC is boiled in water to quantity dissolved increases the viscosity. preciable risk of denitration. Treating NC remove the acid occlusions or to break the In fact, the limit of solubility is often a lim- with boiling pure water for 100 h reduces esters down. During this process, the nitro- it of viscosity. Table 1 illustrates the solu- the nitrogen content by 1% only [6]. The ni- gen content of nitrocellulose decreases si- bility of NC according to [9] as a function tration reaction is highly exothermic with multaneously with the degree of polymer- of the nitrogen content. ization. The hydrolysis of the groups an enthalpy per mol of HNO3 of ∆H = Whereas there are many bacteria capa- –8373 kJ/mol. Increasing the temperature of the polymer occurs simultaneously with ble of destroying cellulose by degradation of nitration does not really modify the the radical cleavage of the acetal bonds of and absorption at the sugar stage, no mi- amount of nitrogen at equilibrium but only the cellulose [6]. croorganism seems to thrive on NC, even slightly accelerates the reaction. when incompletely nitrated. An American Due to the complexity of the nitration 2.1.3. Chemical Properties military study [10] discovered that As- reaction of cellulose, different methods, Nitrocellulose (NC) has the chemical pergillus fumigatus can use the nitrogen of acids and mixtures of acids, were tested: properties of a , complicated to moist NC, not by direct attack, but by ac- pure nitric acid, sulfonitric mixtures, ace- tonitric mixtures, and also phosphonitric mixtures. All these techniques allow specif- Table 1. Solubility of nitrocellulose according to [9] ic reaction profiles of nitration to be estab- lished and it is now possible to produce ni- Nitrogen Acetone Ethyl acetate Diethyl trocellulose in a suitable way [6]. content [%]

2.1.2. Stabilization of Nitrocellulose One of the most important problems in 14.14 soluble soluble insoluble insoluble nitrocellulose technology concerns the pu- 13.00 soluble soluble insoluble insoluble rification process which is responsible for increasing the stability of the product as far 12.75 soluble soluble increased solubility insoluble as possible. Numerous catastrophic explo- 12.00 soluble soluble increased solubility increased solubility sions took place due to insufficient purifi- 11.00 soluble soluble increased solubility partially soluble cation of the product. Abel discovered in 1865 that the amount of acid retained in the 10.00 soluble soluble decreased solubility decreased solubility NC fibers was responsible for the lack of 9.00 decreased solubility decreased solubility insoluble insoluble stability [7]. It was experimentally demon- strated that the resistance of nitrocellulose 8.00 hardly soluble hardly soluble insoluble insoluble to temperature elevation was dependent on EXPLOSIVES 376 CHIMIA 2004, 58, No. 6 celerating its hydrolysis and recovering the Table 2. Heat of explosion and other properties according to [11][12] resulting nitrogen. Because NC is non-tox- ic, it is often used in food packaging. Nitrogen content [%] 13.5 13.3 12.5 12.5 Observations Nitrate esters have poor resistance to acid and are more stable in basic medium. 9475 – 10069 10660 CO2 and H2O In the case of nitrocellulose, the polymeric (constant volume) structure must also be taken into account; [J/g] these exhibit the following properties [6]: Heat of formation Determined from elements – The action of concentrated or very [J/g] at standard state slightly diluted acids or bases usually – constant volume 2386 2482 2641 2901 to denitration of the polymer, of- – constant 2294 2206 2549 2805 ten leading to destruction of the prod- uct. Heat of explosion Measured in a calorimetric – Concentrated cold sulfuric acid dis- [J/g] bomb 4601 4053 3559 – (H O vapor) solves nitrocellulose and hydrolyses the 2 – 4409 – – (H2O liquid) NO2 groups liberating nitric acid. – Concentrated bases like sodium hy- Detonation velocity 7300 – 2630 – d: density for the measurement droxide, potassium hydroxide or ammo- [m/s] (d=1.2) (d=0.3) – nia have strong saponification action ac- companied by more or less rapid de- Specific gas volume 920 841 880 – struction of the polymeric bonds. by detonation [l/kg] – Relatively weak and diluted bases, like Shock sensitivity [J] – 3 1.2 – Julius Peters impact machine, sodium carbonate solution, are fairly Bruceton method compatible with nitrocellulose (allow- ing an alkaline digestion treatment to be Friction sensitivity [N] no no 10% – Julius Peters friction machine, used for stabilization). reaction reaction Bruceton method – However the nitrocellulose should not (353) (353) (353) be treated with medium to concentrated solutions of sodium carbonate, because a saponification of the product may oc- storage life – SB propellants show all these ammunition for can also contain cur, even when cold. positive characteristics. cylindrical grains or short tubes. – Other products denitrate nitrocellulose Used in rifles and guns during the me- – Flake powders are restricted to ammu- easily, e.g. aqueous solutions of alkaline dieval times, SB propellants are still found nition for antitank guns and mortars. hydrosulfites. today in the same types of weapons, of – For aircraft- and antiaircraft guns, course, tailored to the new requirements. grains in the geometry of short tubes or 2.1.4. Explosive Properties In nearly all calibers of handgun, , hollow cylinders are used. The energetic performances of NC are machine gun, aircraft- and antiaircraft gun, – In modern aircraft- and antiaircraft guns given by the nitrogen content. The higher and howitzers, SB propellant the use of the more progressive burning the nitrogen content, the higher the heat of grains or sticks of any geometric shape can 7-perforated grains is normal. explosion. More nitrogen is present in the be found. In bigger caliber cannons and – Tank guns use the even faster burning and more can be used to howitzers, like those illustrated in Fig. 3, 19-perforated grains. oxidize the reducing agents ( and hy- SB propellant grains or sticks can be part of – Long stripe- and cross-stick propellants drogen). This leads to a better oxygen bal- the whole charge. are found in ammunition for cannons ance in the molecule. The heat of explosion The following list shows different and howitzers. and other explosive properties according to geometries of SB propellant and their main The use of SB propellant in caseless am- [11][12] are shown in Table 2. application: munition for small caliber guns, NC with For military purposes, the propellants – Ball powders can be found in ammuni- low nitrogen content in combustible car- with the highest nitrogen content are used tion for pistols, rifles and machine guns; tridge cases and in combustible containers because the highest performances are re- quested. Therefore two main products are employed: – Grade A ‘pyrocellulose’: nitrogen con- tent between 12.3 and 12.8% – Grade B ‘guncotton’: nitrogen content higher than 13.35%.

2.2. Application of Single Base Propellants The disadvantages of black powder – un- reliability in use, smoke together with acidic residues and therefore corrosion of the gun barrels together with the low energy content – demanded ‘better’powder. This was estab- lished with the discovery of gelatinized NC and the thereafter developed Single Base (SB) propellant. No smoke, little muzzle Fig. 3. Firing of SB ammunition with large caliber . One can see the flash, more energy, reliability in use, long muzzle flash but not the smoke. EXPLOSIVES 377 CHIMIA 2004, 58, No. 6 for modular-charges for big caliber guns must also be mentioned.

3. Double Base Propellant

Due to the demand of the military for long distance shooting and therefore more Scheme 1. Chemical equation describing the explosive decomposition of nitroglycerine velocity of the and more energy in the propellant, the search for new energetic materials continued. With the addition of nitroglycerine (NG), or other ‘nitrogly- cols’, as energetic plasticizers to nitrocellu- lose (NC), these goals could be met. Due to the solventless production technology for the Double Base (DB) propellants, bigger Scheme 2. The commercial synthesis of dinitrate (11) consists of the conversion of size propellant grains with large wall thick- glycol and ethylene oxide into diethylene glycol, following by a nitration process ness (web) and big blocks of propellant with different and complicated geometries became possible.

3.1. Nitroglycerine Nitroglycerine (10, NG), strictly glyc- erol trinitrate, is one of the most commonly used explosives. It is the main component of high explosives such as dynamite as well as an ingredient of most mining explosives and it is an essential ingredient of smoke- less, so-called double base propellants. NG is prepared by nitration of glycerin. Scheme 3. Semi-commercial scale synthesis of -1,2,4-triol trinitrate (13) In the process, glycerin is slowly tipped in- to a mixture of concentrated nitric and sul- furic acids. The solution is slowly and care- fully mixed and the temperature must never exceed 30°C. When the reaction is finished, NC in the NG and on the viscosity of the glycol [15]. The nitration reaction is then the mixture is poured into a large amount NC. For practical reasons, e.g. in the manu- performed using a mixture of nitric acid of water where the NG settles to the facture of blasting gelatin, pyrocellulose with . The compound obtained, di- ground. The separated NG is washed with that gives high viscosity solutions of jelly ethylene glycol dinitrate (11), is much less water and sodium carbonate until it be- consistency should be used, even if they sensitive to shock than NG and its heat of comes neutral. contain only a small quantity of NC. This explosion amounts to only 4480 J/g. Ad- principle applies also for the elaboration of vantage is taken of this low heat of explo- 3.1.1. Chemical Properties double base propellant formulations. Typi- sion in the manufacture of flashless and There are two modifications of NG, dif- cally, double base propellants contain non-erosive low calorific smokeless DB fering in freezing point and crystalline 50–60% nitrocellulose and 30–49% nitro- propellants. By using a triglycol like trieth- forms. With respect to crystal structure the glycerine. ylene glycol dinitrate (12), it is possible to modification melting at the lower tempera- obtain a compound that is insensitive to ture represents a labile form, which can be 3.1.2. Explosive Properties shock and delivers an even lower heat of ex- transformed spontaneously into the higher The explosive decomposition of NG is plosion (3140 J/g) [6]. melting and stable modification of NG. The generally expressed in Scheme 1. The heat Another compound, a polyhydroxylic melting point and the freezing point, re- of explosion for this reaction is about 6100 ester called ‘nitrobutanetriol’, is a spectively, is about 2 °C for the labile form J/g. This figure is 1.5 times higher than that good solvent for pyrocellulose. It is less and 13 °C for the stable form [13]. With of NC. This gain of energy is positive and volatile than NG and is a chemically stable such a low freezing temperature, it is not fulfills the military demand for increased compound. Its explosive strength is not convenient to work with pure NG. By velocity and prolonged range of much inferior to that of NG and its heat of adding other explosive compounds, it is shooting. The high shock sensitivity of NG explosion of about 6000 J/g is quite similar possible to obtain mixtures with a higher and the high flame temperature, which fa- to that of NG [15]. freezing temperature. In respect to obtain- vors erosion, was reason to eval- Butane-1,2,4-triol trinitrate (13) was ing mixtures with higher freezing points the uate and introduce other nitroglycols. produced in Germany during World War II capacity to dissolve other nitro compounds on a semi-commercial scale according to in NG was investigated [14]. NG is able to 3.2. Other Plasticizers Used in the sequence of chemical changes outlined dissolve NC containing a relatively low per- Double Base Propellants in the Scheme 3 [16]. centage of nitrogen, as Another nitroglycol introduced into To further lower the flame temperature (‘pyrocellulose’). Dissolving of NC at double base propellant formulations is di- and the stress in the loading chamber and room temperature takes a very long time, ethylene glycol dinitrate (11). The commer- barrel during the ballistic cycle, a certain whereas at 60–65 °C it is completed in cial synthesis of this compound, illustrated percentage of nitroguanidine (14) is added 15–20 min. The consistency of the solution in Scheme 2, consists of the conversion of to DB formulations. The resulting propel- obtained depends on the concentration of glycol and ethylene oxide into diethylene lant is called Triple Base [17] (Fig. 4). EXPLOSIVES 378 CHIMIA 2004, 58, No. 6

4.1. The Oxidizers The two main oxidizing compounds used in composite propellant manufacture are (AP) and am- monium nitrate (AN). The most frequently used is AP. The explosive salt of perchloric acid is considerably more interesting than the explosive chlorate, because it is more stable and safer to handle. The explosive decomposition, occurring during the burn- ing process, is described in Scheme 4 [18]. One can see that during the burning process a lot of (HCl) is devel- oped. With the moisture in the air, this leads to an intense white smoke, so the trajectory Fig. 4. Chemical structure of nitroglycerine and other common nitroglycols, used as substitutes and the starting point of the missile can eas- ily be observed. Adding HCl-suppressant substances, like magnesium-aluminum al- loys or sodium nitrate, the formation of this white smoke will be reduced. This problem is very important for tactical missiles, but is less important for space exploration. The heat of explosion of AP is 1115 J/g, which is less than that of NC or NG. Because com- posite propellant must burn for a relatively long time, the burning rate has to be low and a high heat of explosion is not at the fore- front.

Scheme 4. Chemical equation describing the ex- plosive decomposition of ammonium perchlo- rate

AN on the other hand, does not show the problem of hydrochloric acid production, but its main drawback are the different crys- Fig. 5. Antiaircraft missile Rapier. Its launch and flight motor consist both tal modifications. A crystal modification of double-base propellant. change in a rocket motor is fatal. It will in- fluence the physical properties, cracks in the propellant could occur enlarging the burning surface. Under operational condi- 3.3. Application of Double Base 4. Composite Propellant tions the propellant produces more gases Propellants than the nozzle can manage and the rocket Large caliber cannons and howitzers The size, dimension, and geometry of or missile finally explodes. The use of AN use DB (or Triple Base) propellant in the propellant grains based on SB and DB pro- for rocket motor technology also needs the shape of tubes or slotted tubes. pellant is limited. For rockets or missiles addition of a crystal structure stabilizer, e.g. Flake powders are found in ammunition with a longer range as guns or long-range . for pistols and grenade launchers. missiles, other types of propellants were Rockets and missiles: the antitank mis- required. During World War II, the devel- 4.2. The Reducing Agents sile Dragon has rolled up DB propellant opment of the composite propellants be- The first used in composite pro- sheets in its numerous small motors. The gan. In the beginning, ingredients used pellants were tar or rubber. Later, with the ejection of the missile from the launcher is were tar or rubber and an oxidizer. Today, evolution of polymer technology, more done by a charge of short tubes of DB pro- a curable polymeric binder is loaded with modern polymeric fuels were introduced pellant. The launch motor of the antitank an oxygen-rich, crystalline solid (mostly like polymethacrylates or polybutadienes. missile TOW is constructed of big size perchlorates), and a metal (mainly alu- The used in tubes. The launch motor of the Russian an- minum). After mixing, the highly viscous technology are quite similar to those used in titank missile RPG-7 consists of DB sticks mass is cast into big and complicated high explosives technology (PBX). They and the flight motor of a hollow DB cylin- molds. After curing either a case-bonded will not be described in this paper. der. To produce bigger DB rocket motors, or free-standing solid rocket motor is ob- As the polymer is only partly responsi- the casting technology is used. An example tained. Under a case-bonded motor one ble for the reducing part of the reac- of this propellant type is the antiaircraft understands that case and cast-cured mo- tion in the formulation, some metals are missile Rapier with its launch and flight tor are one unit, whereas a free-standing added in the composition. The most popu- phase propellant cast into the same motor, motor is a propellant block which is later lar ones are aluminum, zirconium (high illustrated in Fig. 5. applied into a case. density), beryllium (very energetic but tox- EXPLOSIVES 379 CHIMIA 2004, 58, No. 6

cohol, but abandoned it after 1905 for diphenylamine (14), considered a better stabilizer [20]. It is a colorless crystalline product in which the bound to the nitrogen is sufficiently mobile to readily permit substitution reactions, but which al- so confers a sufficient basicity to promote the hydrolysis of labile nitrate esters like NG. Diphenylamine fixes nitrogen oxides by a series of nitrosation and nitration reac- tions or transnitrations, which are both se- quential and competitive (Scheme 6). A study of the different nitro derivatives of diphenylamine gives precise information about the ageing state of a propellant and of ammunition, respectively. Chemical analy- sis using HPLC gives a fingerprint of the ageing of the propellant. An example to il- lustrate the fingerprint of a DB propellant is given in Fig. 7. One can see the main peak of the first degradation product N-ni- trosodiphenylamine (16), but also the degradation products of the following step: 2-nitro-diphenylamine (15) and 4-ni- Fig. 6. Firing of with the huge smoke cloud typical for composite propellant motors trodiphenylamine (17). ic), boron, and magnesium. The pyrotech- nic properties of these reducing agents are not the purpose of this article.

4.3. Application of Composite Propellants Most of today’s air-to-air missiles (Sidewinder, AMRAAM) or ground-to-air (Mistral, Stinger, 9K38 Igla or SA-18) are propelled by solid rocket motors based on Scheme 5. The mechanism of the homolytic scission of the nitrate ester of nitrocellulose composite propellant. Very large solid rocket motors are used as boosters for the European Ariane Space mechanical and ballistic properties or in- According to the work of Marqueyrol Missile (Fig. 6) or the US Space Shuttle. crease the risk of auto-ignition during stor- [21], if the quantity of diphenylamine intro- The weight of the composite propellant of a age. duced into DB propellants reaches or ex- single booster motor for the Space Shuttle The problematic nature in NC propel- ceeds 3–4%, an incompatibility problem is 480 tons! lants is the ageing reaction that liberates occurs. Since 2-nitrodiphenylamine (15) The only use of composite propellant in NOx into the medium. The primary part of still possesses stabilizing properties and is guns is restricted to the so-called Base the mechanism of this reaction is the ho- compatible with NG, it can consequently be Bleed grains. For extended long range molytic scission of the nitrate ester of NC, used in some double base propellants. The shooting, mostly with 155 mm caliber which is illustrated in Scheme 5 [19]. This elaboration of more recent stabilizers (Fig. Howitzers, a composite propellant grain is part of the process is an unimolecular reac- 8) completely compatible with NG and oth- burned off during the flight of the . The tion and, according to the kinetic equation er nitroglycols, allows new DB and triple emitting gases of the slow burning Base of Arrhenius, only the temperature is an in- base propellants to be produced which may Bleed grain do not deliver thrust, but limit fluencing factor. The radicals formed dur- be better stabilized and may be stored for a the drag at the end of the . This re- ing the first step of the reaction exponen- longer period [20]. This also leads to a pro- duction of drag results in a significantly tially activate the oxidative degradation of longation of the shelf life of ammunition in longer reach of the projectile. NC, which may to an auto-catalytic a fully safe manner. degradation of the propellant. To avoid such an autocatalytic process, stabilizers are in- 5. Stabilizers of Powders troduced in the propellant formulation, to 6. Combustion Modifying Agents and Propellants catch the nitrogen-oxides (NOx) formed during the first part of the degradation reac- The velocity of a burning reaction, also Powders and propellants must contain tion. By introducing stabilizers, the first called combustion, is given by the steady one or several additives called ‘stabilizers’, part of the degradation is not influenced but state of heat production and the efficiency the function of which is to prevent chemical the second part is broken down. of heat transfer to reach ignition tempera- change in the energetic constituents over a The origin of stabilizers goes back to the ture within the material. The combustion of reasonable period of time. Any changes invention of powder by Paul Vieille in 1884, this material is linked to the superficial py- could lead to unacceptable variations in the who first recommended the use of amyl al- rolysis reaction of the constituents. This re- EXPLOSIVES 380 CHIMIA 2004, 58, No. 6

Scheme 6. Sequential and competitive degrada- tion mechanism of diphenylamine stabilizer Absorbance [AU]

Fig. 7. HPLC chromatogram of a double base propellant, stabilized with diphenylamine. The presence of many degradation products is a in- Retention Time [min] dication for a propellant which was manufac- tured more than 10 years ago.

action produces gases, which react with ing characteristics to the propellant at cer- known to be efficient burning rate accelera- each other, producing flames with high tain , and reduce the sensitivity of tors. It seems that the ferrocene group acti- temperature, not well localized and with a the propellant to temperature. vates the decomposition of perchloric acid bad reproducibility. This phenomenon has in the first step of the burning reaction. Dif- different influences [20], (a) the ballistic 6.1. Burning Accelerators or ferent ferrocenic compounds have been stability of the projectile or the rocket is no Regulators tested [22] because ferrocene is a solid with longer optimal and the performance of the The use of inorganic and metallic salts a high sublimation tendency. This property propellant is not constant; (b) the high tem- may enhance the combustion velocity of leads to a migration of ferrocene to the sur- perature of the flame favors erosion effects propellants. For example potassium cryo- face of the propellant. An interesting solu- in gun barrels or in rocket motor nozzles; lite, an aluminum fluoride compound tion has been found by fixing the ferrocene and (c) the muzzle flash, due to the flame, (K3AlF6), is able to accelerate the burn- group to a polymeric chain to avoid this mi- would immediately disclose the position of ing rate of propellant. Copper chromite gration phenomenon. The commercial ® the gun to the enemy. To avoid such prob- (CuCr2O4), which is used as catalyst for the compound Butacene (33) from the French lems, it is possible to act on the pyrolysis re- degradation of perchlorate, also accelerates company SME , is often used in action or on the flame itself to modify the the burning behavior of propellants. The composite propellants. Catocene (32) is a combustion reaction. Addition of special mechanism of this process is not yet known. ferrocene derivative often used in the con- chemicals will influence the propellant For composite propellants, ferrocene temporary propellants. The liquid catalysts burning rate, impart plateau or mesa burn- (31) and ferrocene derivatives (Fig. 9), are have a negative influence on the rheological EXPLOSIVES 381 CHIMIA 2004, 58, No. 6

p

Fig. 8. Chemical structure of the most popular and modern stabilizers

Fig. 9. Burning accelerators or regulators based on ferrocenic or borane compounds

properties of the propellants. To avoid this tanoates (37), and lead β-resorcylate (35) problem, a new generation of catalyst, v = a + b Pn (Fig. 10). based on carborane have been in- Today, the general use of lead must be troduced. An example is n-hexylcarborane by introducing a plateau effect or mesa ef- avoided. Many research projects are now (NHC, 34) (Fig. 9) [23]. fect. The mechanism of these compounds concentrating on the replacement of lead The introduction in the propellant for- on the burning process is not well under- compounds with a new generation of ballis- mulation of carbon black, obtained by cal- stood. The formation of a large number of tic modifiers. Nevertheless, in rocket pro- cination of hydrocarbons, regulates the free radicals in the burning zone is able to pellants lead and copper compounds have burning rate of propellant, leading to a influence the combustion reaction. It is pos- proven to produce better storage stability, more efficient thermal exchange. Some- sible that the increase of the pressure at the ballistic reproducibility, burning behavior, times carbon black may catalyze the de- burning surface is responsible for the ob- aging characteristics and temperature in- composition of nitric esters by the influ- served modifications. The compounds that sensitivity than propellants containing bal- ence of free radicals liberated into the mi- are used to modify the combustion are com- listic modifyers free of heavy metal com- crofibers. binations of lead and copper salts. The fol- pounds. lowing compounds are usually used for 6.2. Ballistic Modifying Agents such compositions: lead oxide (Pb3O4), 6.3. Muzzle-Flash Regulators Ballistic modifying agents are able to copper oxide (CuO), lead stearate (36), The muzzle-flash is due to a post-com- modify the general law of combustion copper salicylate (39), lead or copper oc- bustion of gases, , and hy- EXPLOSIVES 382 CHIMIA 2004, 58, No. 6

[18] P. Naoum,R. Aufschläger, Z. ges. Schiess- u. Sprengstoffw. 1924, 19, 121. [19] H. Andres,B. Vogelsanger, (Nitrochemie AG), Bericht Nr. 2858, 2003. [20] J. Quinchon,J. Trinchant, M. Nicolas,in ‘Les poudres, propergols et explosifs: 3. Les poudres pour armes’, Technique et Documentation (Lavoisier), Paris, 1986. [21] M. Marqueyrol, Mém. Poudres 1928, 23, 158. [22] A.T. Nielson, J. Norris, J. Org. Chem. 1976, 41, 655. [23] S.M. Vuga, Propellants, Explosives, Py- rotechnics 1991, 16, 293.

Fig. 10. Chemical structure of the most popular modifying agents

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