WO1995009824A1

WO1995009824A1 - Bamo/ammo propellant formulations

Info

Publication number: WO1995009824A1
Application number: PCT/US1994/007945
Authority: WO
Inventors: Robert L. Hatch; Robert B. Wardle; Gary K. Lund; Scott R. Hamilton
Original assignee: Thiokol Corporation
Priority date: 1993-10-06
Filing date: 1994-07-14
Publication date: 1995-04-13
Also published as: AU7952994A

Abstract

A solid rocket motor propellant is provided. The propellant employs a polymeric binder composition which includes a polyoxetane binder such as poly(3,3-bis(azidomethyl)oxetane-co-random-3-azidomethyl-3-methyl oxetane (BAMO/AMMO). A plasticizer is used in conjunction with the polyoxetane binder. The ratio of the plasticizer to the polymer is preferably in the range of from about 0 to about 3.0. The propellant also includes from about 70 % to about 85 % energetic and oxidizing particulates, such as particulate ammonium perchlorate and aluminum.

Description

BAMO/AMMO PROPELLANT FORMULATIONS BACKGROUND 1. The Field of the Invention

The present invention is related to high impulse, high density solid rocket motor propellant formulations. More particularly, the present invention is related to solid rocket motor propellant formulations which include energetic polyoxe¬ tane binders and energetic and oxidizing solid materials.

2. Technical Background

In missile technology it is conventional to employ solid rocket motor propellants for most general applications. Solid rocket motor propellants are energetic compositions similar in many respects to other types of energetic materials such as explosives, pyrotechnics, gasifiers, and illuminants. Conven¬ tionally, these types of formulations are comprised of solid particulates, such as fuel particles and oxidizer particles, which are dispersed and immobilized throughout a binder matrix. The binder matrix generally comprises an elastomeric polymer. Conventional binders used in formulating propellants, explosives, and similar compositions, are "non-energetic" polymers. That is, when the composition is burned, the binder does not contribute significantly to the energy output of the composition. Examples include polycaprolactones, polyethylene- glycols, and polybutadienes. One widely used class of polymers comprises hydroxy-terminated polybutadienes.

In certain types of propellants it is necessary to obtain relatively high burn rates. These types of propellants typically have burn rates on the order of 2 inches per second (ips) at 2000 pounds per square inch (psi) . (As used herein, 1 pound equals 453.593 grams and 1 inch equals 0.0254 meters.) However, in order to achieve these burn rates it has generally been necessary to include burn rate enhancers into the composi¬ tion. Such enhancers include Catocene® and finely divided iron oxide. Catocene® has been determined to be vulnerable to accidental ignition and is less than ideal from the standpoint of safety. In addition, Catocene® is a relatively expensive material. The high cost of this material and safety consider¬ ations reduces the attractiveness of propellants formulated with Catocene®. The use of high levels of iron oxide has led to poor propellant aging characteristics and other similar problems.

Another concern with propellants that incorporate rela¬ tively high percentages of solid materials is processibility. It has been problematic to formulate such materials which are still capable of being easily handled and incorporated into a rocket motor. If a material is not easily processible the resulting handling problems limit its effectiveness in the production of rocket motors.

A related problem is that of achieving acceptable mechani- cal properties. Rocket motor propellants must have stress and strain characteristics within specific ranges. This is difficult to achieve in conventional high solids propellants.

A further problem relates to the types of plasticizers conventionally used in this type of propellant. One commonly used plasticizer is dioctyladipate (DOA) . However, DOA has been observed to migrate into the liner within the rocket motor once the propellant is cast in place. For end-burning propel¬ lant grain configurations, this phenomenon may result in "coning" which increases the surface area of the burning propellant along the liner. This increase in surface area and resulting pressure increase introduces the potential for catastrophic failure of the rocket motor.

Recently there has been an effort to develop binders which are "energetic" such that an increase in specific impulse is provided by the binder. Energetic binders contribute to the overall energy output of the propellant formulation. However, until now energetic binders have not been widely used in high burn rate, high density, high impulse propellants.

In addition, a significant concern in this art is "sensi¬ tivity." Sensitivity can be defined as the probability of a given formulation igniting, exploding, or detonating when subjected to a standard series of stimuli including impact, friction, electrostatic discharge, projectile impact, shock wave, or thermal heating. It is desirable to prevent acciden¬ tal detonation of rocket motors, flares, and other devices that use propellants or explosives. Thus, there is significant effort in making such devices that will not detonate or ignite if, for example, struck by a stray bullet. This has not been easily achieved with conventional compositions. Most such compositions shatter when struck. This shattering may result in detonation or ignition.

Accordingly, it would be a significant advancement in the art to provide solid rocket motor propellant formulations which include energetic binders, but which are also relatively insensitive. It would be a further advancement in the art to provide such formulations which are easily processed and achieve acceptable mechanical properties. It would also be an advancement in the art to provide such formulations which do not rely on materials which are known to limit propellant desirability such as DOA, Catocene®, and finely divided iron oxide.

Such solid rocket motor propellant formulations are disclosed and claimed herein.

BRIEF SUMMARY OF THE INVENTION The present invention is related to improved solid rocket motor propellant formulations. The formulations employ an energetic binder composition which includes an energetic polyoxetane polymer. One specific example of a polyoxetane that may be employed is a random copolymer of polyp,3- bis(azidomethyl) oxetane and 3-azidomethy1-3-methyl oxetane, sometimes referred to as BAMO/AMMO. Other polyoxetanes such as poly 3-azidoethyl-3-methyloxetane (AMMO) and poly(3,3-bis(azi¬ domethy1) oxetane-co-3-nitratomethyl-3-methyloxetane (BAMO/NMMO) are also usable in the present invention.

It is found that the present invention provides the benefit of high burn rates and stable high pressure combustion, while allowing for a substantial reduction in the solids loading of the propellant. This is achieved by energy parti¬ tioning within the formulation by use of the energetic binder and selected plasticizers. In this manner Catocene® may be eliminated from the composition.

The propellants of the present invention also exhibit reduced sensitivity to impact, cook-off, and friability. This is one of the positive results of limiting solids loading in the compositions. In conventional propellants, reduction in solids loading clearly results in a reduction in performance as well as a reduction in sensitivity. In the propellants of the present invention, however, it is possible to maintain good performance characteristics while reducing solids when using energetic binder systems disclosed herein.

The polyoxetane binder system is coupled with a suitable plasticizer. One such plasticizer is triethyleneglycol dinitrate (TEGDN) . Other plasticizers include glycidyl azide polymer plasticizer (GAP-P) , N-butyl-2-nitratoethyl nitra ine

(BuNENA®) , a mixture of N-methyl-2-nitratoethyl nitramine and

N-ethyl-2-nitratoethyl nitramine (Methyl/Ethyl NENA) , tri- methylol ethane trinitrate (TMETN) , butanetriol trinitrate

(BTTN) , and mixtures thereof. For a discussion of the benefits of using GAP-P as a plasticizer, reference is made to co- pending United States Patent Application Serial Number 08/117,578, filed September 3, 1993, which is incorporated herein by this reference.

The ratio of plasticizer to polyoxetane polymer will generally be in the range of from about 0 to about 3. More particularly, the range will preferably be from about 0.25 to about 3.0, and in the presently preferred embodiments, the range will be from about 0.5 to about 2.5.

In the compositions of the present invention the polymeric binder composition (i.e the combination of polyoxetane, plasticizer, and isocyanate curing agent) comprises from about 15% to about 30% of the overall composition. The bulk of the remainder of the propellant compositions will be comprised of energetic and oxidizing solid particulates. The solid particulates will preferably comprise from about 70% to about 85% of the composition. This level of solids loading is found to result in processible propellant formula- tions with acceptable mechanical properties. The particulates will include oxidizing and energetic materials.

Oxidizing particulates will preferably include ammonium perchlorate particulates, although other similar oxidizing salts also fall within the scope of the present invention. Examples of such oxidizing salts include ammonium nitrate and ammonium nitramide. Other materials that may be added in this respect include cyclotetramethylene tetranitramine (HMX) , cyclo-1, 3, 5-tetranitramine (RDX) , and hexanitro hexaazaiso- wurtzitane (CL-20) .

Energetic particulates will most preferably include aluminum. However, other conventional energetic solid materi¬ als such as boron and magnesium may also be added. Typically aluminum or other energetic solids will comprise up to about 25% of the propellant formulation. It is has been observed that it is possible to modify the pressure exponent of the composition by varying the size of the aluminum particles incorporated in the propellant. This is an advantage when formulating propellant for particular applications. It is preferred that the size of said aluminum particles be in the range of from about 3μ to about 60μ.

The characteristics of the propellant may also be tailored by modifying the particle size distribution of the ammonium perchlorate used. Generally finer ammonium perchlorate particle sizes lead to higher burn rates. However, smaller particles lead to poorer processing characteristics. Accord¬ ingly, it is preferable to seek a balance of ammonium perchlo¬ rate particle sizes.

Therefore, one method of adjusting the burn rate of the propellant is to incorporate ammonium perchlorate of various particle sizes. Ammonium perchlorate particles will preferably range in size from about lμ to about 250μ. It is often desirable to incorporate ammonium perchlorate particles of multiple sizes within the propellant. For example particle sizes of 20μ, lOOμ and 200μ have been incorporated into a single propellant with good results. Generally the composition will comprise from about 45% to about 85% ammonium perchlorate particles.

One specific propellant formulation within the scope of the present invention is as follows:

12.5% (by weight) BAMO/AMMO

12.5 TEGDN

48.0 Ammonium Perchlorate

5.0 HMX 22.0 Aluminum

This formulation was found to provide an acceptable propellant while avoiding some of the disadvantages identified above. For example, the propellant formulations of the present invention are relatively insensitive as determined by standard impact, friction, electrostatic discharge, and thermal testing. At the same time they are processible and have acceptable mechanical properties. The formulations eliminate the need for Catocene®, finely divided iron oxide, and DOA which have been found to present limitations in known propellants.

BRIEF DESCRIPTION OF THE DRAWINGS In order to more fully understand the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific data illustrated in the appended drawings. Under¬ standing that these drawings depict only typical data relative to the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1 is a graph plotting burn rate in inches per second against chamber pressure in psi for the exemplary propellant formulation set forth in Example 1.

Figure 2 is a graph plotting burn rate in inches per second against chamber pressure in psi for the exemplary propellant formulation set forth in Example 2. Figure 3 is a graph plotting burn rate in inches per second against chamber pressure in psi for the exemplary propellant formulation set forth in Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides improved solid rocket propellant formulations. The formulations are capable of providing desirable performance, mechanical, and processing characteristics. At the same time the formulations avoid some of the problems encountered with conventional formulations of this type, including the use of DOA as a plasticizer and

Catocene® or finely divided iron oxide as a burn rate modifier.

The present invention relates to high impulse, high density propellant formulations which include energetic polymer binder compositions. The energetic binder compositions are based upon polyoxetanes. Preferred polyoxetanes include random copolymers of poly(3,3-bis(azidomethyl) oxetane (BAMO) and 3- azidomethyl-3-methyl oxetane (AMMO) . Random copolymers of this type will be referred to herein as "poly(3,3-bis(azidomethyl)- oxetane-co-random-3-azidomethy1-3-methyl oxetane" or "BAMO/AMMO." It is preferred that the polymer comprise from about 50 mole% to about 70 mole% BAMO and from about 50 mole% to about 30 mole% AMMO. In one preferred embodiment of the present invention the random copolymer comprises from about 60 mole% BAMO and about 40 mole% AMMO.

Polyoxetanes employed by the present invention may be synthesized by means known in the art. In particular, it is preferred that these materials be synthesized by the methods set forth in United States Patent No. 4,988,797 issued to Wardle, et al. January 29, 1991. Additional discussion of synthesis of these materials is contained in United States Patent 5,099,042 issued to Wardle, et al. March 24, 1992. The above patents are incorporated herein by this reference.

In the preferred embodiments of the present invention, the polyoxetane is coupled with an acceptable plasticizer to form the polymeric binder composition. One such plasticizer is triethyleneglycol nitriate (TEGDN) . However, as mentioned above, other plasticizers are also usable in the present invention including, for example, glycidyl azide polymer plasticizer (GAP-P) , N-butyl-2-nitratoethyl nitramine (BuNENA®) , a mixture of N-methyl-2-nitratoethyl nitramine and N-ethyl-2-nitratoethyl nitramine (Methyl/Ethyl NENA) , tri- methylol ethane trinitrate (TMETN) , butanetriol trinitrate (BTTN) , and mixtures thereof. The plasticizers are used to improve processibility of the polymeric binder composition and to improve the low temperature properties of the propellant formulations.

The ratio of plasticizer to polyoxetane in the polymeric binder composition is of significant interest. It is presently preferred that the plasticizer to polymer ratio (Pl/Po) be in the range of from about 0 to about 3.0. Most formulations falling within the scope of the present invention will have Pl/Po^,s in the range of from about 0.25 to about 3.0. In the presently preferred embodiments, including those discussed below, the Pl/Po is in the range of from about 0.5 to about 2.5. Generally, good mechanical properties have been obtain- able with Pl/Po^,s in the range of about 2.0.

As discussed above, the polymeric binder compositions of the present invention are particularly adaptable for use in propellants which also include energetic and oxidizing particu¬ lates. Total solids in the propellant formulations preferably comprise from about 70% to about 85% of the propellant formula¬ tion. With certain presently preferred compositions, good results are achieved with solids levels in the range of from about 74% to about 80%.

The preferred oxidizing particulate is ammonium perchlo- rate; however, as discussed above other oxidizing salts known in the propellant art could also be used. Ammonium perchlorate will typically comprise from about 45% to about 85% of the propellant composition. Ammonium perchlorate particles of varying sizes may be used. It is possible to tailor the burn rate characteristics of the propellant formulations by select¬ ing the ammonium perchlorate particle size and distribution. In general, however, the propellant compositions will employ ammonium perchlorate particles ranging in size from about lμ to about 250μ. Using ammonium perchlorate in these ranges, high burn rate propellants which are acceptable for end-burning rocket motor applications are produced. Other solids may include HMX and RDX.

The solids incorporated within the propellant formulations may also include energetic solids. The presently preferred energetic solid is aluminum, boron, and magnesium. Energetic solids (such as aluminum particles) will typically comprise up to about 25% of the propellant formulation. The size of the aluminum particles may be varied and can be used as a method of tailoring burn rate. Typically aluminum particles in the range of from about 3μ to about 60μ are preferred.

The propellant formulation also includes an effective quantity of isocyanate curing agent. One acceptable curative is a polyfunctional isocyanate curing agent known as DESMODUR N-100, which is commercially available from Mobay, Inc. It is important to control the amount of curing agent incorporated into the formulation. It is presently preferred that the ratio of isocyanate functional groups in the curative to hydroxyl functional groups in the polymer (NCO/OH) be in the range of from about 1.0 to about 1.6.

It may also be preferred to add a cure catalyst to the propellant formulation. It is possible to adjust the pot life of the propellant formulation by selecting the type or level of catalyst used. Catalysts which are presently preferred include triphenyl bismuth (TPB) and triphenyltin chloride (TPTC) . The catalyst typically comprises on the order of about 0.01% of the overall composition.

Examples The following examples are given to illustrate various embodiments which have been made or may be made in accordance with the present invention. These examples are given by way of example only, and it is to be understood that the following examples are not comprehensive or exhaustive of the many types of embodiments of the present invention which can be prepared in accordance with the present invention.

Examples 1-3 Propellant formulations within the scope of the present invention were made and tested for certain mechanical proper¬ ties. The propellants were formulated using standard mixing techniques. Specifically, the ingredients were mixed in a Baker-Perkins vertical mixer and acceptable end of mix viscosi¬ ties were achieved. The formulations and results of the test are set forth as follows:

Ingredient Weight % of Composition Ex. 1 Ex. . 2 Ex. 3 BAMO/AMMO/N-100 12.38 14. 14 14.14

TEGDN 12.37 10. 61 10.61

AP (200 μ) 24. 00

AP (90 μ) 28.80 28.80

AP (20 μ) 19.20 24.00 19.20 HMX (5 μ) 5.00 5.00 5.00

Al 22.00 22.00 22.00

MNA 0.25 0.25 0.25

Rb (2,000 psi, ips) 0.79 0.75 0.77 Pressure exponent 0.29 0.32 0.29

2 ipm Mechanical Properties

Modulus (psi) 560 575 575 Time stain at max. stress (%) 12 17 15 Corrected max. stress 86 104 106 Shore A hardness 59 59 62

MNA is an abbreviation for N-methylnitro aniline. The BAMO/AMMO was a difunctional material having a hydroxyl equivalent weight of 2,445 grams/equivalent.

The formulations set forth above were tested in a standard 60 to 80 gram ballistic test motor. The burning rate (in inches per second) is plotted against the chamber pressure (in pounds per square inch) . The data for Examples 1, 2 and 3 is plotted in Figures 1, 2, and 3 respectively. It will be appreciated from this data that each of the exemplary formula¬ tions comprise usable propellants.

Sensitivity tests were also run on each of the formula¬ tions set forth above. These tests included standard impact, friction, electrostatic discharge, and auto-ignition tests. It was observed that each of the formulations were comparable to currently accepted propellants and were reasonable to low in sensitivity.

Example 4

A propellant was formulated within the scope of the present invention. The propellant contained the following components expressed in weight percent:

Ingredient Weight %

BAMO/AMMO 12 . . 5

TEGDN 12 . . 5

AP 48 . . 0

HMX 5. . 0 Al 22 . . 0

It was found that this formulation resulted in a usable propellant having the general characteristics described above.

Example 5 In this example a propellant within the scope of the present invention was made. The propellant contained the following components expressed in weight percent: Ingredient Weight % BAMO/AMMO 10.74

TEGDN 12.37

MNA 0.25

Al 22.00

AP (200 μ) 24.00 AP (20 μ) 24.00

HMX 5.00

DES N-100 1.64

Example 6 In this example a propellant within the scope of the present invention was made. The propellant contained the following components expressed in weight percent: Ingredient Weight %

BAMO/AMMO 12.59

TEGDN 2.50

Al 25.00

AP 50.00

HMX 5.00

Example 7 In this example a propellant within the scope of the present invention was made. The propellant contained the following components expressed in weight percent: Ingredient Weight %

BAMO/AMMO 12.5

BuNENA 12.5

Al 25.00

AP 50.00

As mentioned above Butyl NENA is an abbreviation for N-butyl-2- nitratoethy1 nitramine.

Example 8-9 Theoretical performance calculations were made for two BAMO/AMMO propellant formulations, as well as for a baseline hydroxy-terminated polybutadiene (HTPB) formulation having DOA as a plasticizer. The results are as follows:

Specific

Impulse Density

Polymer Plasticizer HMX Change Change

HTPB DOA Yes 12.5% BAMO/AMMO 12.5% TEGDN 5% +0.2 -0.0002 12.5% BAMO/AMMO 12.5% BTTN 5% +2.0 +0.0011 The abbreviation BTTN refers to 1,2,4 butanetrioltrinitrate.

It will be appreciated that the theoretical performance characteristics of the BAMO/AMMO propellants compare favorably to the HTPB baseline propellant.

Example 10 In this example a propellant within the scope of the present invention was made. The propellant contained the following components expressed in weight percent: Ingredient Weight %

BAMO/AMMO/N-100 12.38

BTTN 12.37

Al 22.00

AP (200μ) 24.00

AP (20μ) 24.00

HMX 5.00

MNA 0.25

DBTDL 0.002 DBTDL is an abbreviation for dibutyltin dilaurate.

Summary The present invention provides propellant formulations which are usable in solid rocket motors where it is desirable to have a high impulse, high density propellant. The propel¬ lant formulations include energetic binders, but are also relatively insensitive. The formulations are also found to be easily processed with acceptable mechanical properties. These formulations do not rely on materials which are known to limit propellant desirability such as DOA, Catocene® and finely divided iron oxide.

What is claimed is:

Claims

1. A propellant composition comprising: from about from about 70% to about 85% by weight solids selected from the group consisting of energet¬ ic solids and solid oxidizers; and from about 15% to about 30% by weight polymeric binder composition, said polymeric binder composition comprising poly(3,3-bis(azidomethyl) oxetane-co- random-3-azidomethyl-3-methyl oxetane, a plasticizer, and an isocyanate curative.

2. A propellant composition as defined in claim 1 wherein the ratio of poly(3,3-bis(azidomethyl) oxetane to 3- azidomethy1-3-methyl oxetane in the polymer is in the range of from about 50 mole% to 50 mole% to about 70 mole% to 30 mole%.

3. A propellant composition as defined in claim 1 wherein the plasticizer is selected from the group consisting of triethylene glycol dinitrate (TEGDN) , N-butyl-2-nitratoethyl nitramine (BuNENA®) , a mixture of N-methyl-2-nitratoethyl nitramine and N-ethyl-2-nitratoethyl nitramine (Methyl/Ethyl NENA) , trimethylol ethane trinitrate (TMETN) , butanetriol trinitrate (BTTN) , and mixtures thereof.

4. A propellant composition as defined in claim 1 wherein the ratio of plasticizer to polyoxetane in the propellant is in the range of from about 0.25 to about 3.0.

5. A propellant composition as defined in claim 1 comprising up to about 25% aluminum particles.

6. A propellant composition as defined in claim 5 wherein the size of said aluminum particles is in the range of from about 3μ to about 60μ.

7. A propellant composition as defined in claim 1 comprising from about 45% to about 85% ammonium perchlorate particles.

8. A propellant composition as defined in claim 7 wherein the size of said ammonium perchlorate particles is in the range of from about lμ to about 250μ.

9. A propellant composition as defined in claim 7 wherein said ammonium perchlorate particles are of multiple sizes.

10. A propellant composition as defined in claim 1 wherein the ratio of isocyanate functional groups in the curative to hydroxyl groups in the polymer is in the range of from about 1.0 to about 1.6.

11. A propellant composition comprising: from about 45% to about 85% by weight particulate ammonium perchlorate; up to about 25% by weight aluminum; from about 15% to about by weight 30% polymeric binder composition, said polymeric binder composition comprising plasticizer, an isocyanate curative, and poly(3,3-bis(azidomethyl) oxetane-co-random-3- azidomethy1-3-methy1 oxetane.

12. A propellant composition as defined in claim 11 wherein the ratio of plasticizer to poly(3,3-bis(azidomethyl) oxetane-co-3-azidomethy1-3-methyl oxetane in the propellant is in the range of from about 0.25 to about 3.0.

13. A propellant compositions as defined in claim 11 wherein said ammonium perchlorate consists of multiple particle sizes in the range of from about lμ to about 250μ.

14. A propellant composition as defined in claim 11 wherein the size of said aluminum particles is in the range of from about 3μ to about 60μ.

15. A propellant composition as defined in claim 11 wherein the ratio of poly(3,3-bis(azidomethyl) oxetane to 3- azidomethy1-3-methyl oxetane in the polymer is in the range of from about 50 mole% to 50 mole% to about 70 mole% to 30 mole%.

16. A propellant composition as defined in claim 11 wherein the plasticizer is selected from the group consisting of triethylene glycol dinitrate (TEGDN) , N-butyl-2- nitratoethyl nitramine (BuNENA®) , a mixture of N-methyl-2- nitratoethyl nitramine and N-ethyl-2-nitratoethyl nitramine (Methyl/Ethyl NENA) , trimethylol ethane trinitrate (TMETN) , butanetriol trinitrate (BTTN) , and mixtures thereof.

17. A propellant composition comprising: from about from about 74% to about 80% by weight solids selected from the group consisting of particulate ammonium perchlorate and particulate aluminum; from about 18% to about 22% by weight polymeric binder composition, said polymeric binder composition comprising plasticizer, isocyanate curative, and poly(3,3-bis(azidomethyl) oxetane-co-random-3- azidomethy1-3-methyl oxetane (BAMO/AMMO) , wherein the ratio of plasticizer to BAMO/AMMO is in the range of from about 0.5 to about 2.5.