US3386868A - Heat resistant propellants containing organic oxidizers - Google Patents

Description

June 4, 1968 J. R. GIMLER E AL 3,386,368

HEAT RESISTANT PRUPELLANTS CONTAINING ORGANIC OXIDIZERS Filed June 9, 1966 WEIGHT LOSS PERCENT TEMPERATURE "P JOHN R. GIMLER STEPHEN E. JURMAN INVENTORS United States Patent 3,386,868 HEAT RESISTANT PROPELLANTS CGNTAINWG ORGANIC QXIDHZERS John R. Girnler, Succasunna, and Stephen E. .lurman, Ledgewood, N.J., assignors to Hercules Incorporated, Wilmington, DeL, a corporation of Delaware Filed June 9, 1966, Ser. No. 556,432 flairns. (Cl. 149-2) ABSTRACT 0F THE DISCLOS A heat resistant granulated propellant composition containing a crosslinked polyacrylic rubber binder and particulate organic oxidizer. The oxidizer is HMX or RDX and the composition is non-corrosive and substantially free from residue when fired.

This invention relates to the manufacture of propellants and more particularly to an improved heat-resistant propellant which is non-corrosive and substantially free from residue.

The prior art in respect to heat-resistant propellants, generally, Is quite replete. In respect to techniques for obtaining granulated grains of multi-perforated geometry, the commercial technology is quite limited. US. Patent No. 3,152,027 to James H. Godsey discloses a method for the manufacture of granulated propellant having high heat resistance and the products obtained therefrom, which propellant may be manufactured using conventional smokeless powder equipment. The binder and inorganic oxidizer utilized in accordance with this patent, however, lead to the formulation of propellants which produce a certain amount of solid particles and residue upon combustion due to the inorganic oxidizer and, also, under certain conditions produces corrosive hydrogen chloride due to the chlorine contained in the binder. These are characteristics desirable of improvement and particularly where the propellant is to be utilized in devices designed for repeated firing.

Accordingly, a principal object of the present invention is to provide a novel propellant composition of exceptional resistance to heat in respect to thermal stability and physical integrity, and in addition, to provide a composition which is noncorrosive and substantially free from residue. A further object of the present invention is to provide a propellant composition which may be manufactured using equipment such as that conventionally used in the manufacture of smokeless powder and which may be formulated to meet desired ballistic characteristics of smokeless powder.

Other objects of the invention will appear hereinafter, the novel features and combinations being set forth in the appended claims.

Generally described, the present invention contemplates the preparation of a heat-resistant granulated propellant composition comprising by weight from about 10 to about of a cross-linked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of a material of the group consisting of combined N- methylol acrylamide: acrylamide and combined N- methylol acrylamide:methacrylamide in intimate mixture with from about to about of particulate organic oxidizer of the group consisting of cyclotetramethylene tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX), said composition characterized by being noncorrosive and substantially free from residue.

Representative compositions of the invention have been chosen for illustration and description and are given in Table I following as Examples 1 through 4. These examples are prepared using the constituents shown, using TABLE I.-COMPOSITION OF HEAT RESISTANT PROPELLANTS Example 1 2 3 4 Weight percent:

HMX (Class B) (1) 84.00 72.0 84. 00 RDX (Type E) (2)... 72.50 Hycar 2121-X-58"- 15. 84 15. 84 27. 20 15. 84 Salicylic Acid.. 0.16 0.16 0.30 Oxamide 12. 00 Phthalic Anhydride 0. 16

Ultimate Configuration 2 1 Manufactured in non-, mono and multi-perforated gun granule geometries for burning at the 20,000 p.s.i.g. pressure level.

2 Manufactured as a molding powder [or large grain manufacture.

3 Burning rate data at the 1,600 p.s.i.g. pressure level obtained.

For (1) and (2) see the following table:

Sieve Size (mesh) Percent through *Hycar 2121-X-53 is a polyacrylic rubber consisting of 97% ethyl acrylfiite and 3% of combined N-methylol acrylamidezacrylamide by weig t.

The composition of Example 1 is a preferred propellant and was developed as a high temperature-resistant substitute for a standard double-base propellant of established ballistic acceptability. The double-base propellant is hereinafter designated as the Control Propellant. A comparison of the thermodynamic properties of the Con-' rol Propellant and Example 1 is shown in Table II. Although the calculated impetus valves appear about equal, closed bomb results indicate that, in Example 1, the impetus or Relative Force runs 10 to 14% higher than the Control Propellant. Under conditions were both propellants were fired using identical hardware at 70 F, the pressure rise times and maximum pressure levels were almost identical.

TABLE II.OALCULATED THERMODYNAMIC P R O P E RTIE S Control Fxample l Propellant Propellant Density, g./cc 1. 618 1. 696

Percent" Formulation-Compound, Wt

Nitrocellulose (13 157 N) TABLE II-Continued NorE.Heat of formation for I-Iycar 2121-X-58 taken as -139,780 caL/gram mole.

Further tests were conducted on the composition of Example 1 to determine its resistance to high temperature and other desirable properties. In this respect reference is made to the accompanying drawing which is a graph having a curve showing the percent weight loss of the composition after four hours exposure to temperature from 325 F. to 400 F. It will be noted that the weight loss after four hours exposure to 350 F. was below 0.5 Wt. percent. These data were obtained on propellant grains 0.095 OD. x 0.015" I.D. X cuts/inch. In addition the composition of Example 1 was subjected to differential thermal analysis (DTA) in an attempt to detect crystalline phase transitions at elevated temperatures which might affect the sensitivity or stability of the propellant. Results of these tests indicated no phase transition at the 350 F. temperature desideratum requirement, but a possible change from the beta to the more sensitive delta form between 388 and 390 F. The DTA studies also indicated that the binder lowers the initial HMX decomposition temperature only 27 F. to 437 F. Commercial No. 8 and Army I-2 cap tests were also run on propellant grains of the composition after four hours exposure to 350 F. with the satisfactory results of having the propellant classified as ICC Class B by the Bureau of Explosives. Moreover, the composition of Example 1 in the form of propellant grains 0.075" OD. x 0.015" ID. x cuts/ inch was loaded into cartridges and was subjected to severe vibrational cycling from 65 to +350 F. with no apparent change in physical or ballistic characteristics. Ballistic stability was demonstrated after this severe vibrational cycling and exposure to 350 F. by actual firing of cartridges.

The compositions of Examples 2, 3 and 4 demonstrate other embodiments of the invention having attributes similar to Example 1 in respect to thermal stability and physical integrity. It will be noted, however, that Example 2 contains oxamide as a coolant for use where a cooler propellant is desired. Example 3 contains RDX in lieu of HMX giving a propellant of good but yet less thermal resistance than HMX, and that Example 4 contains phthalic anhydride in lieu of salicylic acid as the crosslinking rate accelerator.

Although the ballistic performance of all the foregoing examples was extensively evaluated in the 20,000 p.s.i.g. range, the propellants of Examples 1 and 2 were additionally evaluated in the 1000 p.s.i.g. range in the form of 2 /2 diameter x 5" long grains which were compression molded from dried but uncured granules of cut extruded propellant. The pressed charge properties of the propellant of Example 2 which was chosen for the purpose of illustration are given in Table III which follows:

TAB-LE E[ II.-PRESSED CHARGE PROPERTIES Formulation, percent:

4 Uncured propellant:

T.S. (p.s.i.) 225 Percent elong. 4.6 Theoretical flame temp. (T F. 2,354 Sensitivity:

Dry molding powder:

Impact, cm. 26.2 Friction, lb./8 114 E.S.D., joules 6.25 Pressed molded grain:

Impact, cm. 32.7 Friction, lb./8 450 E.S.D., joules 6.25 Temperature cycling High temperature resistance:

Hours at 300 F 8 Hours at 400 F. 3.6

2 0. 1). uninhibited grain, 1A" perf. withstood 3-811'our cycles from 65 F. to F. without visual or apparent physical degradation.

Pressed charges as aforementioned were made by compressing the uncured granules 0.038" OD. x 100 cuts/ inch in a suitable mold at a pressure of 17,000 lbs./sq. in. and allowing the charge to cure for 16 hours at a temperature of F. The pressed charges were readily processed and the finished products thereof were found to give good ballistic and physical reproducibility.

From the foregoing, it is evident that there are numerous factors which will influence conditions for the most satisfactory operation of the invention, the requirements of which are determined by the intermediate and the finished products desired. For example, representative grain geometries which were prepared using the propellant of this invention are as follows:

Die O. D., inch 0.202 0.202 0.038 0.053 0.095 0.075 Pin D1a,ineh 0.014 0.018 0.015 0.015 0.015 Granulation, cuts/inch 2 40 100 20 10 15 The physical characteristics of the propellant have demonstrated that it is readily processible in existing commercial smokeless powder equipment and that it can be cut into extremely fine granulations cuts/inch) as required by fine grain ballistic specifications.

The binder utilized in accordance with this invention is one that is free from corrosiveness. Specifically, as used in the examples it is a crosslinked polyacrylic rubber consisting of about 97% ethyl acrylate and about 3% of combined N-methylol acrylamide: acrylamide by weight. However, a similar material identified as Hycar 2121-x-66 is equally suitable as the binder. This material is a crosslinked polyacrylic rubber consisting of about 97% ethyl acrylate and about 3% of combined N-methylol acrylamide:methacrylamide by weight. The acrylamide or the methacrylamide is usually present in these physical blends in an amount of from about 1 to about 1.5% by weight of the binder composition. The binder compositions are used in an amount of from about 10 to about 30% and preferably from about 10 to about 20% by weight of the composition. A crosslinking rate accelerator such as salicylic acid, benzoic acid, and phthalic anhydride may or may not be present. Elimination of the accelerator requires a longer cure cycle, but the resulting propellant is essentially the same. From about 0.1 to about 0.5% by weight of the accelerator has been found satisfactory and salicylic acid is preferred for effecting a fast cure. The oxidizers utilized are I-IMX and RDX which are organic oxidizers which burn completely, are non-corrosive, and have no noticeable residue. These oxidizers are used in an amount of from about 70 to about 90% and preferably from about 80 to about 90% by weight of the composition. The oxidizers are incorporated in the composition in particulate form with the particle sizes used in the examples being exemplary of preferred ranges. In preparing the doughy mixture of binder and oxidizer MEK is the preferred desensitizing volatile solvent although other desensitizing agents may be employed such as ether, acetone, methyl isobutyl ketone, ethyl acetate and tetrahydrofuran. Generally, the amount of solvent utilized will be about by weight of the propellant formulation but this may be varied from 7 to about depending more particularly upon the exact formula utilized, oxidizer particle size, and mix temperature. In selecting the type and amount of solvent, it will be appreciated that the volatile solvent serves to densensitize the oxidizer and the propellant mixture as well as to make the propellant mixture more dough-like and thereby easier to mix and extrude.

As previously set forth, the propellants of this invention may be manufactured utilizing conventional smokeless powder equipment and in granule or grain size and with perforations similar thereto. Moreover, exceedingly large grain sizes may be manufactured simply by compression molding of the uncured granules. Conventional additives, such as, aluminum, boron, lead carbon black and coolant such as oxamide, ammonium oxalate and the like may be simply incorporated in particulate form and small amount, ordinarily not exceeding from about 5 to about 15 of the propellant weight. Coating agents compatible with the propellant may be employed, if desired.

The utility of the present invention pertains to many applications requiring low and high temperature resistance and includes small gas generators, cartridge actuated devices, thrusters and rockets; oil well perforating guns; squibs, primers and igniters; gun propellants for high rate of fire automatic weapons where chamber heating is a problem; and molding powder for gas generator rocket grains and shaped charges.

The advantages of the present invention are multifold and include: (1) Processibility completely compatible with existing smokeless powder manufacturing equipment in high rate production runs. (2) No corrosive or erosive particles upon combustion and little if any residue after burning. The residue is equal to or less than its smokeless powder counterpart. (3) Temperature resistance and ballistic stability after four hours exposure to 350 F. Resistance without autoignition in gun granulations for more than four hours at 400 F. (4) Ease of ignition over the range 65 F. to 350 F. (5) By virtue of the wide variation in granulation possible with the propellant of the invention, a controlled release of energy is possible to fit many needs reproducibly. (6) The processing solvent desensitizes the oxidizers and propellant during all phases of manufacture. (7) The propellant granulated but uncured can be used as compression molding powder for still larger grains, and (8) In addition to controlling the burn rate by particle size distribution of HMX or RDX, lot to lot variation in ballistic properties can be minimized by blending several lots into one final lot as commonly practiced in smokeless powder processing.

Since further modifications of the invention will be apparent to those skilled in the art, it is intended that the scope of the invention be limited only by the appended claims.

What we claim and desire to protect by Letters Patent 1. A heat-resistant, granulated propellant composition comprising by weight from about 10 to about 30% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of a material of the group consisting of combined N-methylol acryl amidezacrylamide and combined N-methylol acrylamidezmethacrylamide in intimate mixture with from about to about 90% of particulate organic oxidizer of the group consisting of cyclotetramethylene tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX), said composition characterized by being non-corrosive and substantially free from residue.

2. A heat-resistant, granulated propellant composition comprising by weight from about 10 to about 30% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of combined N-methylol acrylamidezacrylamide in intimate mixture with from about 70 to about 90% of particulate organic oxidizer of the group consisting of cyclotetramethylene tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX), said composition characterized by being non-corrosive and substantially free from residue.

3. A heat-resistant, granulated propellant composition comprising by weight from about 10 to about 30% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylafe and about 3% of combined N-methylol acrylamide:methacrylamide in intimate mixture with from about 70 to about 90% of particulate organic oxidizer of the group consisting of cyclotetramethylene tetrauitramine (HMX) and cyclotrimethylene trinitramine (RDX), said composition characterized by being non-corrosive and substantially free from residue.

4. The composition of claim 1 in which the granulated propellant is cut small grain propellant.

5. The composition of claim 4 in which the cut small grain propellant is perforated.

6. The composition of claim 1 in which the composition is compression molded large grain propellant.

7. The composition of claim 1 in which the composition contains from about 0.1 to about 0.5% of a crosslinking rate accelerator and from about 5.0 to about 15.0% of a coolant.

8. A heat-resistant granulated propellant composition comprising by weight from about 10 to about 20% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of a material of the group consisting of combined N-methylol acrylamidezacrylamide and combined N-methylol acrylamidezmethacrylamide in intimate mixture with from about to about of particulate HMX and from about 0.1 to about 0.5% of particulate salicylic acid.

9. A heat-resistant granulated propellant composition comprising by weight from about 10 to about 20% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of combined N-methylol acrylamide:acrylamide in intimate mixture with from about 80 to 90% of particulate HMX and from about 0.1 to about 0.5% of particulate salicylic acid.

10. A heat-resistant granulated propellant composition comprising by weight from about 10 to about 20% of a crosslinked polyacrylic rubber binder consisting essentially of about 97% ethyl acrylate and about 3% of combined N-methylol acrylamide:methacrylamide in intimate mixture with from about 80 to about 90% of particulate HMX and from about 0.1 to about 0.5% of particulate salicylic acid.

References Cited UNITED STATES PATENTS 3,152,027 10/1964 Godsey 14919 X 3,269,880 8/1966 Visnov et a1. 149-19 X 3,322,583 5/1967 Guthrie et al. 149-19 BENJAMIN R. PADGETT, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,386,868 June 4, 1968 John R. Gimler et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, TABLE 11, first column, line 6 thereof,

"VafiNO should read Ba(NO same column, line 23 thereof, "VaO" should read BaO Column 3, same TABLE II, Second and third columns, line 1 thereof, "1.000", each occurrence, should read 1,000 same second and third columns, line 2 thereof, "2.573" and "2.127" should read 2,573 and 2,127

Signed and sealed this 21st day of October 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Mtesting Officer Commissioner of Patents

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