Energetic Materials: Part 1. Decomposition, Crystal and Molecular Properties

458 Pages·2003·10.398 MB·1-465\458

by  Peter Politzer and Jane S. Murray (Eds.)

PREFACE
Our own involvement in the area of energetic materials stems from
the second half of a New Orleans Saints football game in 1979, which one
of us (P.P.) was attending together with Rod Bartlett (a contributor to these
volumes). The Saints were losing by a large score, and with interest in the
game waning, Rod spent much of the second half expounding, to a very
receptive listener, the opportunities in energetic materials research.
Whether this area has since been advanced or set back as a result of the
Saints' ineptness that afternoon is for others to decide. What is certain,
however, is that during these years we have been extremely fortunate in
having the assistance, encouragement and support of a very fine group of
project officers:
Dick Miller and Judah Goldwasser, ONR;
David Squire, Ron Husk and Bob Shaw, ARO;
Larry Davis, AFOSR;
Jack Alster and Frank Owens, ARDEC;
Bob McKenney, Norm Klausutis and Paul Bolduc, AFATL;
Horst Adolph, NSWC;
and Leonard Caveny, BMDO.
We are grateful to all of them.
In putting together these two volumes, we have greatly appreciated
the enthusiastic encouragement and suggestions that we received from
Betsy Rice and her willingness to prepare the overview that begins each
volume, in addition to her other contribution. Finally, we want to thank
Mick Coleman and Bob Murray for their much-needed help with various
aspects, often tedious, of the editing process.
Peter Politzer and Jane S. Murray
P.A. Politzer and J.S. Murray (Editors)
Energetic Materials, Part :1 Decomposition, Crystal and Molecular Properties
Theoretical and Computational Chemistry, Vol. 21
(cid:14)9 2003 Elsevier B.V. All rights reserved.
Overview of Research in Energetic Materials
Betsy M. Rice
U. .S Army Research Laboratory, Aberdeen Proving Ground, MD
21005-5069
Energetic materials encompass different classes of chemical compositions of
fuel and oxidant that react rapidly upon initiation and release large quantities
of force (through the generation of high-velocity product species) or energy
(in the form of heat and light). Energetic materials are typically classified
as explosives, propellants, pyrotechnics or incendiaries, and distinctions
among the classes are usually in terms of the types of products generated and
rates of reactions. These particular features have been advantageously
employed in a wide variety of industrial and military applications, but often
these utilizations have not been fully optimized, mainly due to the inability
to identify and understand the individual fundamental chemical and physical
steps that control the conversion of the material to its final products.
The conversion of the material is usually not the result of a single-step
reaction, or even a set of a few simple consecutive chemical reactions.
Rather, it is an extremely complex process in which numerous chemical and
physical events occur in a concerted and synergistic fashion, and whose
reaction mechanisms are strongly dependent on a wide variety of factors.
For instance, the performance of a material is a strong function of the
temperature and pressure of its environment and of its mechanical
properties. The rate of energy release can be modified by additives or by
varying the concentrations of fuel and oxidizer. The response of the material
can be affected by the manner in which it was processed. The properties and
behavior of the material can change over time. The rate of the conversion of
the material is strongly dependent on the conditions of the initiation. For
example, an explosive can be ignited to bum, a process that can be thought
of as a reaction wave that proceeds through the material at subsonic speeds.
Or it could be shock-initiated to detonation, where detonation can be
described as a reaction wave that proceeds through the material at supersonic
speeds. For these cases, the product concentrations and species are not the
same. Also, these processes often occur under extreme conditions of
temperature and pressures, making experimental measurement difficult.
These are but a few of the complexities associated with studies of reactions
of energetic materials that make resolving the individual details so difficult.
They are also the reason that research in the field of energetic materials is so
exciting! These difficulties have required the development of a variety of
innovative theoretical methods, models and experiments designed to probe
details of the various phenomena associated with the conversion of energetic
materials to products.
Probably the majority of experimental and theoretical efforts in
energetic materials research to date have been directed toward assessing
performance. The focus is often on quantifying the amount of energy that
is released, identifying key reaction mechanisms, or investigating ways in
which the energy release can be manipulated and controlled. However,
increased environmental and safety concerns are placing new emphases on
understanding other aspects of energetic materials, such as toxicity,
processing emissions, combustion emissions, wear and erosion on
combustion fixtures from reaction products, contamination of ground water
or soil, environmentally-friendly synthesis and processing, destruction or
disposal, life-span (chemical stability), storage, handling, or vulnerability to
a variety of external stimuli. Naturally, models are emerging that can be
used to investigate these aspects of energetic materials.
The high time and pecuniary costs associated with the synthesis or
formulation, testing and fielding of a new energetic material has called for
the inclusion of modeling and simulation into the energetic materials design
process. This has resulted in growing demands for accurate models to
predict properties and behavior of notional energetic materials before
committing resources for their development. For example, in earlier times,
extensive testing and modification of proposed candidate materials for
military applications could take decades before the material was actually
fielded, in order to assure the quality and consistent performance of the
material in the desired application. Predictive models that will allow for the
screening and elimination of poor candidates before the expenditure of time
and resources on synthesis and testing of advanced materials promise
significant economic benefit in the development of a new material.
The growing demands for predictive models of energetic materials
have not eliminated the need for experimentation on energetic materials;
rather, they call for increasingly detailed experimental studies to allow
critical assessment, correction and enhancement of the models. This is a
particularly challenging requirement, since the time and length scales of
most experiments are often many orders of magnitude larger than those of
some of the models (i.e. atomistic). Nonetheless, advances are being made
in designing experiments to probe behavior of energetic materials at ever-
decreasing time and spatial scales; some of these are described in these
volumes.
The contributions to these volumes will highlight challenges faced by
researchers in the energetic materials community, as well as describe
research activities directed toward fundamental characterization of
properties and behavior of these interesting materials. An assortment of
theoretical studies of energetic materials processes at different time and
length scales and under different conditions is presented. Accompanying
these are discussions of the complexity, assumptions and levels of
empiricism of the models used in these studies. Also included, in order to
provide perspective, are surveys of experiments designed to investigate the
complex and unique chemical and physical processes occurring in energetic
materials. It is hoped that the works presented in these volumes will
stimulate further investigations and advances in the area of energetic
materials research.
P.A. Politzer and J.S. Murray (Editors)
Energetic Materials, Part :1 Decomposition, Crystal and Molecular Properties
Theoretical and Computational Chemistry, Vol. 21
(cid:14)9 2003 Elsevier B.V. All rights reserved.
Chapter 1
A survey of the thermal stability of energetic materials
Jimmie C. Oxley
Chemistry Dept; University of Rhode Island; Kingston, RI 02881
.1 INTRODUCTION
It is reported that the Chinese discovered black powder as early as 200
AD, but the earliest surviving record is in a Chinese military manual "Wu
Jing Zong Yao" dated 1044 A.D. The Chinese used this new found powder
for warfare, attaching bamboo tubes of gun powder to arrows to make
"rocket arrows" and sometimes bundling many rocket arrows onto launchers
which were hauled into battle on wheelbarrows, also a Chinese invention.
The Chinese formula reached Europe in the 13th century, and the Europeans
were firing bombards by 1327 A.D. Sir Francis Bacon, who published the
formula in the West, was so impressed by its awesome power that he
speculated man would give up making "war when he saw the terror of black
powder 1 .
Black Powder Chinese Bacon present
200 AD 1242 AD day
saltpeter KNO3 72.5% 37.5% 74%
charcoal 19.0% 31.25% 15.6%
sulfur 8.4% 31.25% 10.4%
Man did not give up war, nor did he find a peaceful use for black powder
until the 1600's. Then, for the first time in Hungary, black powder was
used for mining. Tenny L. Davis credits black powder as one of the three
inventions responsible for ending the Middle Ages 1 . (The other two were
the printing press and the discovery of the New World.) The first
commercial production in the United States was by DuPont (1802)2. Worm
War I saw the maximum use of black powder.
To function as good explosives, formulations must maximize expansion
volume (i.e. density and gas formation) and heat release. For a substance to
function as an explosive, its molecules must (a) be broken down to smaller
molecules without high energy costs (weak linkages) so that the net
decomposition reaction is exothermic; (b) contain sufficient oxygen to
quickly oxidize most of the molecule to gas--carbon and hydrogen to CO,
2OC and H20; )c( undergo oxidative breakdown rapidly enough that the
energy release supports the shock wave. It was nearly four hundred years
after the Western world had adopted black powder before explosives using
these principles, the first "high explosives," appeared. By the mid- to late
nineteenth century, Lavoisier had "fathered" modem chemistry and died in
the French revolution; Bertholet had published his famous treatise on
chemistry; and nitration reactions had been discovered: alcohols with mixed
acid (sulfuric and nitric) made highly flammable, if not explosive, species
3.
Most military explosives satisfy requirements (a) and (b) by containing
only carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). The oxygen
is found usually in nitro groups since it is relatively easy to break the 2ON-X
bond, and the nitrogen can convert to nitrogen gas, while the oxygen aids
the gasification of carbon and hydrogen. Chronologically, nitrate esters (O-
2ON e.g. nitrocellulose, nitroglycerin) were the first high explosives
discovered in the mid-nineteenth century. Nitroarenes (C-NO2, e.g. picric
acid, TNT) came into use at the end of the nineteenth century. Nitramines
,2ON-N( e.g. RDX), though discovered about the same time as TNT, did not
come into their own until World War II and later. In modem explosive
design requirements (a) and (b) still dictate that compounds be designed to
be dense and have positive heats of formation. High oxygen content is less
important in high nitrogen/low carbon containing materials.
In the United States about five billion pounds of explosives are used
annually. Most are used for commercial purposes and are ammonium-nitrate
based formulations 2,4. There are less than a dozen chemical explosives
that are manufactured in bulk quantities. These were "discovered" in the
fifty-year period between 1850 and 1900 5,6,7,8,9,10. New explosives
have been synthesized, yet optimization of synthesis, study of stability and
sensitivity, and optimizing the formulations takes decades. This problem is
not new; the development of nitrocellulose and nitroglycerin as useful
explosives also took decades. Thermal stability must be evaluated and
quantified, not only under the intended parameters of manufacture or
storage, but outside these limits where a catastrophic upset might occur.
Often, as in the case of nitrocellulose, the intrinsic thermal stability is low or
easily deteriorated by contamination so that a stability enhancement agent is
necessary. To find a stabilizing agent, the mechanism of decomposition must
be thoroughly understood. Thus, thermal stability must be evaluated over a
wide range of temperatures and the decomposition mechanism(s) determined
before stabilizing agents can be chosen rationally.
2. NITRATE ESTERS
Nitrocellulose or "gun cotton" was discovered by Braconnot (France,
1833) and patented by Schonbein (1846) (Fig. 1). It is speculated that in
order to determine whether the action of nitric acid was a reaction or merely
a sorption into fibrous material, Sobrero (1846) treated glycerin, a liquid,
with nitric acid and found a true reaction took place. Hence, nitrocellulose
and nitroglycerin were discovered within a decade of each other, but neither
found widespread use until the 1860's when methods of stabilizing them
were devised. Between 1865 and 1868, Abel patented improved preparations
of nitrocellulose. He found that pulping allowed impurities, such as residual
acid, to be more easily washed out by "poaching" and resulted in improved
stability. The Abel stability test is named after him5.
2ON
O2NG
I
2ONO- /X 2ONO O2~ ONO2
nitroglycerin NTEP
nitrocellulose
9NO2 ONO2 CHiONO 2
H2cH CH2~H2 O2NO "CH2CH CH 3 C "~("3H CH 2"ONO 2
0 2 ONO 2
CH 2 -ONO 2
BTTN PGDN NTEMT
O2NO CH2cH20 CH2cH20 CH2cH2ONO 2 NDGET
.giF 1 Nitrate Esters
While complex nitrate esters, such as nitrocellulose and nitroglycerin, were
the first to find application as explosives, understanding the mechanisms of
nitrate ester decomposition was accomplished through the study of simpler
compounds 11-16. Ethanol nitrate was examiaed by numerous researchers,
and they concluded the first, and rate-determining, step was revers~le loss of
NO2.
RCH20-NO2 <=> RCH20" + 2ON"
Griffiths, Gilligan, and Gray studied 2-propanol nitrate pyrolysis and found it
exhibited more 3-cleavage than primary nitrate esters 17. It yielded nearly
equal amounts of 2-propanol nitrite and acetaldehyde, as well as small
amounts of acetone, nitromethane, and methyl nitrite. Homolytic cleavage of
the RO-NOz bond formed the 2-propoxy radical, which subsequently reacted
with nitric oxide to produce 2-propanol nitrite, or it could elimiaate methyl
radical to form acetaldehyde and methyl-derived products; or it could be
converted to acetone by oxidization or loss of a hydrogen atom. Dinilrates of
butanediol were investigated by Powling and Smith 18. They found that 1,4-
butanediol dinitrate, following cleavage of one NO2 moiety, decomposed to
formaldehyde, nitrogen dioxide, and ethylene, whereas 2,3-butanediol dinitrate
gave acetaldehyde ni place of formaldehyde and ethylene. A simihr
intramolecular decomposition to gaseous products has been proposed for
nitroglycerin 19. Ng, Field, and Hauser used time-of-flight mass
spectrometry to identify the decomposition products of pentaerythritol
tetranitrate (PETN); based on their results they posttdated the decomposition
involved the formation of the tert~y tris-(nitroxymethyl)methyl radical 20.
A comprehensive study examined the thermal stability and decomposition
mechanism of a large number of primary, secondary, and tertiary
mononitrates: n-pentanol, 3-buten- 1-ol, ethanol, neopentanol, 2-
phenylethanol, 2-propanol, cyclohexanol, 2-methyl-2-propanol, and 2-methyl-
2-butanol nitrates. Hydrogen-donating solvents, capable of cappiag radical
intermediates, were employed to stabilize intermediates for identification and
to divert the oxides of nitrogen from further complicating the course of
degradation. The reversibility of 2ON homolysis was demonstrated by solvent
cage effects and isotopic labeling experiments. To shed fight on the timing of
the loss of NO2 and CH20 from primary alkanol nitrates, certain nitrate esters
leading to stabilized alkyl radicals were designed to favor concerted
fragmentation. The effect of -substitution was studied by using nitrate esters
of varyiag degrees of substitution. Nitrate esters with multiple nitrate ester
moieties were also examiaed: 1,4-butanediol dinitrate; 1,5-pentanediol dinitrate;
2, 2- dimethyl- 1,3-propanediol dinitrate; 1,1,1-tris(hydroxymethyl)- ethane
trinitrate; and PETN. These thermal decomposition studies illustrated three
general principles 21 .
a) The rate-determining step in nitrate ester thermolysis si usually homolytic
cleavage of the RO-NO2 bond but tertiary nitrates can undergo 1E elimiaation.
b) The presence of radical-stabilizing substituents on the l-carbon determines
the rate and extent of ~3-scission and elimiaation of formaldehyde.
c) In compounds containing more than one nitrate ester, the structural
orientation has a marked effect on the reaction products. The thermolysis of
compounds of this sort can result in ring closure or sequential elimination of
NO2 and CH20 if the nitrate esters are in close proxinity (Fig. 2).
Most nitrate esters are in the same physical state at ambient temperature as
their parent alcohol. With few exceptions (e.g. highly symmetric PETN and
the polymeric nitrocellulose), most are liquids. Many nitrate esters find use in
propellants. Present-day studies on nitrocellulose primarily focus on its
reactions during combustions 22,23. The Navy uses a variety of liquid
nitrate esters as plasticizers and propellants trimethylolethane trinitrate
(TMETN), triethylene glycol dinitrate (TEGDN), 1,2-propylene glycol
dinitrate (PGDN), 1,2,4 butanetriol trinitrate (BTTN). Brill et al have
examined the thermal decomposition of some of these nitrate esters using T-
jump (heating at 2000~ FT-IR. They reported the relationship between
the nitrate ester and its pyrolysis products was straightforward. The
amounts of NO2 and CH20 produced were proportional to the number of
-NO2 and-CH20- groups in the starting material 24. Less conventional
uses of nitrate esters include the use of ethyl hexyl nitrate ester as a cetane
modifier in diesel fuel 25 and isopropyl nitrate ester as the principle
ingredient in a number of Russian enhanced-blasted explosive devices, such
as the RPO-A 26.
01
2ONO/
\ON2O I ~ON\ 0 2 ON ~O (cid:12)9
2ON-
N2O O/ ,ON2O /'~',
-CH20
ON2O ~ .~ 2ONO
/,v O 671~
-NO 2
O2N)
2ONO
_No~
O /\. 0
/ k.i-~o. -CH20
LNO2 LO N 20
-NO 2
//
O
,
I
0
X 0
Fig. 2 Proposed Decomposition Pathway of PETN (Ref. 21)