Robust, Perchlorate-Free Propellants With Reduced Pollution (PP-1404)SERDP/ESTCP Technical Symposium and Workshop • Washington, DC • Dec 4-6, 2007

The Problem: Ammonium PerchlorateGroundwater Contamination

An estimated 24 million pounds of ammonium perchlorate (AP) are manufactured every year, mostly for DoD use. The perchlorate ion is extremely soluble and stable, percolating into and persisting in surface and groundwater. Perchlorate has been detected in the groundwater at rocket motor manufacturing and testing sites. (C. Hogue, Chem & Eng. News 8/10/03)

The perchlorate ion blocks iodine uptake in the thyroid, and thus is a human health concern

New technologies are needed in order to replace AP in energetic materials formulations. A perfect AP replacement, with all its virtues and none of its environmental problems, is an extremely unlikely discovery

Solid Rocket Propellant Characteristics Solid propellants contain a fuel and an oxidizer in an elastomeric binder

Common fuel is aluminum; common oxidizer is AP; common binder is a polybutadiene rubber

Solid propellants require good mechanical properties to withstand explosive ignition, rough handling etc.

Good mechanical properties require chemical bonding between the solid fillers and the binder system

Special bonding agents have been developed for use with AP. These do not generally work with other oxidizers

Ammonium Perchlorate As A Propellant OxidizerIn many ways, AP is the ideal propellant oxidizer

1. It is inexpensive to manufacture

2. It is stable in shipping, storage and use

3. It is easily and safely ground to any particle size, and processes well

4. It is compatible with most other formulation ingredients and binder cure reactions, developing excellent mechanical properties

5. Bonding agents, specific to AP, have been developed which chemically link the AP crystals and the propellant binder, strengthening the composite into a tough, rubbery material

6. It has very good combustion characteristics, producing oxygen and gaseous byproducts, with a

low burn rate pressure exponent. Burn rates can be tailored via particle sizes and catalysts specific to AP

A “Green” Energetic Oxidizer To Replace Ammonium Perchlorate

A “green” AP replacement must be “like AP, only better.”

1. It must process well with low mix viscosity, allowing for a good pot life and easy casting

2. It must be compatible with binder cure (or vice versa)

3. It must contribute to good mechanical and aging properties. Its particles should chemically bond to the binder

4. It must have no solubility issues, decompositions or unwanted physical or chemical reactions during storage

5. It must have or contribute to good safety properties

6. It must have minimal environmental impact in manufacture and use

7. It must be at least as energetic as AP; the higher the performance the better

Approved for public release; distribution is unlimited.

Coating and Bonding of ADN

Coatings for particulate energetic ingredients are desirable because a coating can reduce sensitivity to unwanted initiation, or reduce reactivity with other formulation ingredients. Coatings make the ingredients safer to handle and prolong the service life of the composite. Coatings for ADN have been applied and evaluated

If the coating compound is also a bonding agent, it allows the particle to chemically interact with the composite polymer matrix, reinforcing and strengthening the composite. Without bonding agents, the particles tend to weaken the tensile properties of the composite

ADN does not have the proper chemistry to utilize the coating and bonding agents which have been developed for AP. Other coating agents and methods of coating and bonding are under development for AP replacements. Dendrimeric molecules having multiple end groups for association or partial solution in the ADN crystal surface have been synthesized and tested. Some of the dendrimer end groups have been derivatized to cure into the binder materials

AP Replacement Candidates

Some Crosslinkers and Binders Synthesized for Evaluation

Varying structures of oligomers and crosslinkers allows tailorability of the properties of the polymer binder

Varying the number of reactive sites on the oligomer and/or the crosslinker influences the crosslink density

Increasing the crosslink density increases stress in the binder

Lengthening chains between crosslinks increases strain in the binder

Varying the polymer chain structure influences stress, strain and glass transition temperature

Trading off these properties gives the propellant the proper balance of hardness, toughness, and resistance to environmental stresses

The pressure exponent of most AP replacement candidates tends to be high. It can sometimes be lowered by additives or physical changes in the ingredients. Only formulation experience with replacement ingredients can reduce the exponent; no theory or road-map exists. Non-AP formulations with moderate pressure exponents have been made

Ballistic TailoringSmall Motors for Evaluation of Propellant

A rocket motor needs a specific burning rate at specific pressure. It also needs a low burn rate pressure exponent, otherwise combustion rates can increase disastrously at high pressures. AP propellants are easily tailored for burn rate and typically have low pressure exponents

Mechanical Properties Testing With Instron Test Machine

Mechanical properties of propellant formulation must satisfy missile require-ments. Determinations include stress, strain and elastic modulus at high, low and ambient temperatures

Propellant sample in Instron test apparatus

Below is a recording of the sample being pulled in the Instron. As the propellant stretches (strain; X-axis) it resists with its own force (stress; Y-axis). If the particles of oxidizer (green) are bonded to the binder matrix, they resist pulling away from the particles (dewetting; yellow), and the propellant resists breaking

ConclusionsThe triazole cure has been shown to be a more robust alternative to the more conventional urethane cure

Prilling of ADN can be done to produce round particles of desired sizes

Dendrimers show promise of leading to improved processing and improved mechanical properties in non-AP propellants

ADN (Ammonium Dinitramide)CL-20

(Hexaazahexanitroisowurtzitane)

O2NN NNO2

O2NN NNO2

O2NN NNO2

O2NN NNO

O2NN NNO2

NNO

Ammonium Nitrate

N—NO2NO2N—

NF2

NF2

F2N

F2N

Tetrakis (Difluoramino)Octahydro-Dinitro-Diazocine (HNFX)

NO3-NH4

+NO2

+ NNO2

NH4 N-

Improved Oxidizer Utilization

More RobustRubbery Binders

Better Prilling/Particle Resizing/

Propellant Formulation

New Coatings and Bonding Agents

New Ingredients, Ballistic ModifiersMulti-property Tailoring

Evaluation

Safety/Sensitivity Mechanical Ballistic

AgingEnvironmental

Optimized Propellant

Technical Objective

Develop a Technology Base For AP Replacement Candidates, Which Will Allow Their Use Despite Any Deficiencies, Compared To AP, They Might Possess

1. Develop more robust binders for better compatibility with alternative oxidizers such as ADN, AN, and organics, and advanced fuels. Current binders often exhibit cure problems with these reactive new materials

2. Concurrently, develop modifications of ADN and other energetic ingredients to enhance their compatibility with binders, curatives, and other propellant ingredients. Advanced methods of particle sizing and shaping, as well as coatings and bonding agents will be developed

3. The new ingredients will thus be tailored to the formulation, and vice versa. Other properties will be tailored as needed

Technical Approach

Binder System ImprovementsThe cyclization of polyazides and polyacetylenes to form triazole rings (the “triazole cure”) is a robust reaction which allows use of AP replacements whose reactivity interferes with conventional urethane polymer cures. Both reactions link molecular chains into solid, rubbery networks (see below)

Many oligomers have been derivatized with acetylene and azide terminations and many acetylene crosslinkers have been synthesized. Monitoring of the cure reaction by heat-flow calorimetry, IR and NMR has been done. Determinations of the extent of polymerization, the molecular weights and the polydispersities of the triazole polymers are underway. We are working to mimic the diversity of feedstocks available for the urethane cure in triazole precursors, in order to make the triazole cure as universally applicable as the urethane cure is

Urethane Linkage

R N

C

O

N

C

O

R'OH OH+

DI-ISOCYANATE DIOL

NH

C

O

ROR' NH

C

O

O R'

URETHANE-LINKED POLYMER STRUCTURE

Triazole Linkage

DI-AZIDE

RN

N+

-N

N

+NN-

R'C CCHHC

DIACETYLENE

N

N

RN

TRIAZOLE-LINKED POLYMER STRUCTURE

R'C CH

N

NN

R'CHC

Synthesis of Polyazido and Polyacetylenic Oligomers and Curatives

OOH

HO

n

OOTs

TsO

n

ON3

N3

n

OONO2

O2NO

n

p-TsCl

NaN3

HNO3

NaN3

Direct AZIDE Substitution

PolyolStarting Material

O

n

O

n

N3OH

O

N3O

O

N3

O

O

C OH

OHC

CO

O

HCC

O

O

CH

Azidoesterification

Polyacetylene Derivatization

Photomicrograph and scanning-electron microscope pictures of ADN prills

Prilling is a process where solids are melted and the molten material is sprayed into droplets, which are cooled and quenched into spheres

The spherical shapes process better in formulations than the irregular-shaped crystals of the original solid material

ADN Priller

• Melt zone

• Droplet formation

zone

• Solidification zone

ADN Prilling For Processibility

Publications Sponsored by PP-1404

“Further Developments in Triazole Binders and Propellants” D. A. Ciaramitaro, T. Jacks, J. M. Hitner and T. Bui. JANNAF PEDCS-S&EPS Meeting, Seattle, WA July 26-29, 2004. CPIA Publication JSC CD 34 (2004)

“Triazole Crosslinked Polymers in Recyclable Energetic Compositions and Method of Preparing the Same” D. A. Ciaramitaro. U. S. Patent No. 6,872,266, March 29, 2005

“High-Energy Propellant with Reduced Pollution” D. A. Ciaramitaro and R. Reed. US Patent 6,805,760B1, Oct 19, 2004.

“Monitoring Binder Cure Kinetics with Microcalorimetry” L. Lusk, D. A. Ciaramitaro, H. M. Matheke and A. Chafin. JANNAF PCDS-S&EPS Meeting, Seattle WA July 26-29, 2004. CPIA Pub JSC CD 34 (2004).

“Advanced Rocket Motor Formulations with Alternative Binder Cures” D. A. Ciaramitaro, T. Jacks and A. Lieux. JANNAF PEDCS-S&EPS Meeting, Destin, FL, March 6-9, 2006. JSC CD 42 (2006).

“Ammonium Dinitramide Prilling Process” K. P. Ford, D. L. Dean, C. P. Waltz, D. P. Pate, and J. J. Hosto. JANNAF PEDCS-S&EPS Meeting, Destin, FL, March 6-9, 2006. JSC CD 42 March 2006

“Robust, Insensitive Propellant for Air Launched Missile Systems” D. L. Dean, D. A. Ciaramitaro, S. Nguyen, F. J. Dodson, R. W. Pritchard, T. S. Ward and K. P Ford. JANNAF PEDCS-S&EPS Meeting, Destin, FL, March 6-9, 2006. JSC CD 42 March 2006

“Triazole-Oligomers by 1,3-Dipolar Cycloaddition” A. R. Katritzky, S. K. Singh, N. K. Meher, J. Doskocz, K. Suzuki, R. Jiang, G. L. Sommers, D. A. Ciaramitaro and P. J. Steel. ARKIVOC 2006, (v), 43-62.

“Dendrimers for Improved Mechanical Properties of Composite Propellants” Peter Zarras, David Ciaramitaro, David Dean, Samantha Hawkins, Kara D. Lormand and Lawrence Baldwin. 231st ACS National Meeting, Polymeric Materials Science & Engineer-ing 94, 258 (2006).

“New Polymerization Reaction Promises Less Toxic, More Robust, More Environmentally Friendly Composite Materials” D. A. Ciaramitaro. Currents, the Navy’s Environmental Magazine Spring Issue, April 2006

“Triazole-Cured Binder Structure/Property Correlations” D. A. Ciaramitaro, K. Baum, W. Lin, A. R. Katritzky, R. Duran and N. K. Meher. JANNAF PEDCS-S&EPS Meeting, Reno NV, August 13-16, 2007. In press.

“Getting the AP Out: What it Means to Make a Greener Propellant” D. L. Dean, F. J. Dodson, S. Nguyen. JANNAF PEDCS-S&EPS Meeting, Reno NV, August 13-16, 2007. In press.

“Preparation and Characterization of 1,2,3-Triazole-Cured Polymers from Endcapped Azides and Alkynes” A. R. Katritzky, N. K. Meher, S. Hanci, R. Gyanda, S. K. Tala, S. Mathai, R. S. Duran, S. Bernard, F. Sabri, S. K. Singh, J. Doskocz, D. A. Ciaramitaro. J. Polym. Sci.: Chemistry 2007. In Press.

“Dendritic-Based Bonding Agents for High Density Insensitive Munitions (IM) Propellant Formulations” Peter Zarras, David Dean, David Ciaramitaro, Suong Nguyen, Fred J. Dodson, Lee R. Cambrea and Lawrence Baldwin. 235th ACS National Meeting, Polymeric Materials Division, New Orleans, LA. April 6-10, 2008. ACS Polymer Preprints. In Press.

University Of Florida, GainesvilleAlan R. Katritzky Heterocyclic chemistry expertiseRandolph S. Duran Polymer engineering Nabin K. Meher Triazole reaction mechanism Sandeep K. Singh Small samples Srinivasa R. Tala Reactivity studiesSureyya Hanci Analysis techniques Chunming Cai Direct azido-substitutionYuming SongReena Gyanda Firouzeh SabriDiana GomezLing WangSophie Bernard

Fluorochem, Inc.Kurt Baum Synthesis scaleupWendy Lin Samples, Analyses ATK Thiokol, Inc.Alex Paraskos Energetic materialsScott Lusk Synthesis and samples CD Systems, INC.Victor Crainich Particle coating samples of materials

Air Force Research Lab, Edwards AFBTom Hawkins Rocket motor firings

SERDP SponsorsCharles Pellerin and Bruce Sartwell

NAWCWPNS China Lake David A. Ciaramitaro Project managementDavid L. Dean Sample evaluationPeter Zarras Formulation, evaluationFred Dodson Evaluation Alan Turner ADN prilling, sizing Terrie Jacks Dendrimer coating synthesis Suong Nguyen Coating evaluationsTrent Ward Methodology Kevin WardCristina LovernAndrew Lieux

conc. HClHO

C

OH

OH

HO

H2C CH

CN

THF

NaN3

DMF

C O NH

O OH

OHOH

C O NH

O N3

N3N3

C O CN C O COOH

C OOH

C OOMs

C ON3

C ONH2

C O CN

C O CN C ONH2

H2C CH

CN

C ON

CN

CN

conc. HClC O

N

COOH

COOH

C ON

OH

OH

C ON

N3

N3

acrylonitrileDI water

pentaerythritol

+

Tetrakis(5-cyano-2-oxabutyl)methane (Tetranitrile)

1hr, 70-75oCTetrakis(5-carboxy-2-oxabutyl)methane,

Triton B

(Tetraacid)

Tetrakis(5-hydroxy-2-oxapentyl)methane (Tetraol)

BH3*THF, dry THF

25oC, 24 hours

MsCl/Et3N

Tetrakis(5-mesyloxy-2-oxapentyl)methane (Tetramesylate)

Tetrakis(5-azido-2-oxapentyl)methane (Tetraazide)

1. MeOH, dry HCl, 2 hours

2. Tris 6

4

Dodecylol (12-mer)

1. MsCl/Et3N

2. NaN3/DMF

4

Dodecylazide, 12-mer)

4 4

4 4

4

Pd/C, H2

25oC, 7 hrs4

Tetrakis(5-amino-2-oxapentyl)methane (Tetraamine)

4

Tetranitrile

4

Tetranitrile

BH3*THF, dry THF

70oC, 24 hours 4Tetraamine

MeOH/H2O

0-80oC

Octanitrile

1hr, 70-75oC

Octanitrile

BH3*THF, dry THF

25oC, 24 hours

4

4

4Octaol

1. MsCl/Et3N

2. NaN3/DMF

4Octaazide

SERDP Dendrimer Synthesis

NH4+N-

N

N

O

O O

O

CO

OO

OH

OH

HO

OHO

Structural Correlations With Triazole Reaction Rates

1. The activation of the acetylenic function by an ester group allows lower reaction temperatures for faster cure

2. Lack of this activation requires longer cure times and higher temperatures

3. Steric hindrance of the acetylenic function slows the reaction rate

4. The azido-oligomer structure also influences the rate of cure

5. These differences in reactivity cause changes in the mechanical properties of the polymer, which will enable tailoring as needed for any application

A propellant formulation for a specific missile needs a specific burning rate at specific pressure. AP propellants are easily tailored for burn rate

The firing of small motors on static fixtures (test stands) allows burn rate and other data to be developed with a minimum expenditure of material. Such a facility is valuable when the ingredients and binders are in short supply, as they invariably are early in their development

The picture above is a schematic of a small motor assembly, with the cured formulation in green. The one at right shows a small motor being fired. The test stand can be instrumented to take pressure and temperature data, as well as thrust readings

Propellant Burning Rate Versus Pressure

1 0 0 0 1 0 0 0 0

P re s s u re - P s ia

P r o p e l l a n t w i t h L o w P r e s s u r e E x p o n e n t

P r o p e l l a n t w i t h H i g h P r e s s u r e E x p o n e n t

Schematic of Dendrimer Bonding Agent with ADN