Nitramines HMX and RDX

In conclusion, there appears to be some supporting evidence, other than these tracer studies, that the synthesis of HMX and RDX molecules can be accomplished thru a build-up from single methylene-containing spedes or other small molecules, and that this route can also take place via a total degradation and resynthesis from molecules such as Hexamine. However, die development of an economical process for the large scale production of these expl nitramines, in particular HMX, via a method precluding the use of Hexamine, is vet to be accomplished... 

The replacement of amine and amide hydrogen with a nitro group via direct nitration is an important route to A-nitro functionality. However, the cleavage of other bonds is also important. In the case of C-N bond cleavage the process is known as nitrolysis and is an invaluable route to many energetic materials (Section 5.6). The nitrolysis of hexamine and the syntheses of the important explosives HMX and RDX are discussed in Section 5.15. This area of chemistry could easily demand a separate chapter of its own and is the most complex and diverse in the field of nitramine chemistry. 

Nitroguanidine (NQ) is a nitramine compound containing one N-NOj group in its molecular structure. In contrast to cyclic nitramines such as HMX and RDX, its density is low and its heat of explosion is also comparatively low. However, the Mg of its combustion products is low because of the high mass fraction of hydrogen contained within the molecule. Incorporating NQ particles into a double-base propellant forms a composite propellant termed a triple-base propellant, as used in guns. 

Nitramine composite propellants composed of HMX or RDX particles and polymeric materials offer the advantages of low flame temperature and low molecular mass combustion products, as well as reduced infrared emissions. The reduced infrared emissions result from the elimination of COj and H2O from the combustion products. To formulate these composite propellants, crystalline nitramine monopropellants such as HMX or RDX are mixed with a polymeric binder. Since both HMX and RDX are stoichiometrically balanced, the polymeric binder acts as a coolant, producing low-temperature, fuel-rich combustion products. This is in contrast to AP composite propellants, in which the binder surrounding the AP particles acts as a fuel to produce high-temperature combustion products. 

When nitramine particles such as HMX or RDX particles are mixed with a doublebase propellant, nitramine composite-modified double-base propellants are formulated. Since HMX and RDX are stoichiometrically balanced materials, the use of these nitramine particles leads to a somewhat different mode of combustion as compared to AP-CMDB propellants. Since each nitramine particle can burn independently of the base matrix at the burning surface, a monopropellant flamelet is formed in the gas phase from each particle. The monopropellant flamelet diffuses into the reactive gas of the base matrix above the burning surface and a homogeneously mixed gas is formed. 

Since the energy contained within double-base propellants is limited because of the limited energies of nitrocellulose (NC) and nitroglycerin (NG), the addition of ammonium perchlorate or energetic nitramine particles such as HMX and RDX increases the combustion temperature and specific impulse. Extensive experimental studies have been carried out on the combustion characteristics of composite-modified double-base (CMDB) propellants containing AP, RDX or HMX parhclesli- l and several models have been proposed to describe the burning rates of these pro-... 

HMX and RDX are heated, deflagration combustion occurs with a burning rate of about 1 mm s" at 1 MPa. However, when these nitramines are ignited by primers giving rise to shock waves, detonation combustion occurs with a burning rate of more than 7000 m s . The characteristics of combustion wave propagation are determined by the Chapman-Jouguet relationship described in Refs. [1-5]. 

HMX and RDX are energetic materials that produce high-temperature combustion products at about 3000 K. If one assumes that the combustion products at high temperature are HjO, Nj, and CO, rather than COj, both nitramines are considered to be stoichiometricaUy balanced materials and no excess oxidizer or fuel fragments are formed. When HMX or RDX particles are mixed with a polymeric hydrocarbon, a nitramine pyrolant is formed. Each nitramine particle is surrounded by the polymer and hence the physical structure is heterogeneous, similar to that of an AP composite pyrolant... 

Mass spectra of the important explosives RDX, HMX, TNT, TNB and Tetryl were first briefly reported by Meyer (Ref 34) and later investigated in greater detail with high resolution and labeling techniques by Bulusu et al (Ref 45). Mass spectrometric studies of the photodecomposition of labeled dimethyl-nitramine (Ref 56) and the thermal decomposition of HMX and RDX (Refs 27 31) illustrate the application of these techniques to studies of reaction mechanism and bond dissociation processes. Nitroguanidines have only recently been investigated by Beynon (Ref 35)... 

Nitroguanidine (NQ) is a nitramine compound containing one N-N02 group in its molecular structure. Unlike cyclic nitramines such as HMX and RDX, the den-... 

Typical crystalline-energetic-particles are AP and nitramines such as HMX and RDX. The use of AP leads to an AP-CMDB propellant and the use of HMX leads to an HMX-CMDB propellant. These particles burn as a monopropellant at the burning surface of the CMDB propellant, and the combustion products react again with the combustion products of the base matrix. 

A substituted triazaadamantane, 2,4,10-trinitro-2,4,10-triazaadaman-tane, was made a few years ago by Nielsen [18J. Its synthesis showed that a methine (CH) surrounded by nitramines in an adamantane cage is chemically stable, a matter that had previously been the subject of debate. There are four such groupings in HNZADA. Otheivvise. the local connections are much the same as in HMX and RDX. Molecular mechanics model-building shows that the nitramines are no more crowded than in HMX and RDX, so there is reason to expect that this target molecule will not be especially sensitive or readily subject to chemical deterioration. 

Lott et al. [106] report on the successful differentiation between different sources of pentaerythritol tetranitrate (PETN), RDX, cyclotetramethylene tetra-nitramine (HMX), and AN. The effect of using different raw materials and different manufacturing processes on the isotopic composition of the product was evaluated. The batch-to-batch and lot-to-lot variations of these explosives were also assessed. 

From all of the studies on HMX and RDX decomposition, it is still not possible to predict the identity and rate of formation of the pyrolysis products under different heating and pressure conditions. The underlying reason is at in all experiments (with the exception of the IRMPD molecular beam experiments) many different physicochemical processes occur simultaneously and it has not been possible to associate any set of products with the processes leading to their formation. For example, when gas-phase and condensed-phase decomposition occur simultaneously, it has not been possible to quantitatively associate tb measured products with the phase in which they were fornied. Although experiments such as the IRMPD on RDX provide valuable mechanistic information on the decomposition of the isolated molecules, the behavior of the nitramines in a combustion environment may involve more complicated processes. Such processes may include follow-up bimolecular reactions of the initial decomposition products, autocatalytic reactions between decomposition products and the reactant, stress-induced reactions between molecules along slip planes, and preferential reactions at the reactant surfaces. A key to understanding how the nitramines decompose lies in relating the observed species and their rates of fonnation to the different processes that lead to their formation. 

A brief history of the decomposition of HMX and RDX has been presented to iUustrate die development of die mechanisms used to explain the decomposition of these two nitramines. The history spans the initial low temperature thermal decomposition experiments by Robertson up to more recent high heating rate and shock initiated decomposition studies. 

Schroeder, M. A. (1985) "Critical Analysis of Nitramine Decomposition Data Product Distributions From HMX and RDX Decomposition" Technical Report BRL-TR-2659, U.S. Army Ballistic Research Laboratory, Aberdeen Proving Ground, MD, June 1985. 

---