US3726729A - Solid propellant compositions having a nitrocellulose-hydroxyl-terminated polybutadiene binder and method of preparing the same - Google Patents

Abstract

IMPROVED NITROCELLULOSE-BASE COMPOSITIONS OBTAINED BY INCORPORATING HYDROXYL-TERMINATED POLYBUTADIENE POLYMER IN THE NITROCELLULOSE BINDER. PROPELLANT COMPOSITIONS BASED ON THE RESULTING BINDER EXHIBIT SUBSTANTIALLY IMPROVED STRENGTH AND THERMAL STABILITY.

Description

SOLID PROPELLANT COMPOSITIONS HAVING A NITROCELLULOSE HYDROXYL-TERMINATED POLYBUTADIENE BWDER AND METHOD OF PREPARING THE SAME Everette M. Pierce, Somerville, Ala., assignor to the United States of America as represented by the Secretary f the Army No Drawing. Filed June 30, 1965, Ser. No. 469,975

Int. Cl. (106d /.06

U.S. Cl. 149-49 8 Claims ABSTRACT on THE DISCLOSURE Improved nitrocellulose-base compositions obtained by incorporating hydroxyl-terminated polybutadiene polymer in the nitrocellulose binder. Propellant compositions based on the resulting binder exhibit substantially improved strength and thermal stability.

The invention described herein may be used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to improved nitrocellulose-base compositions and to a method of preparing the same.

Numerous rocket motors now in use or under development use nitrocellulose-base compositions as solid propellants. As used herein, the term nitrocellulose-base composition refers to any composition containing plasticized nitrocellulose. In such compositions nitrocellulose, normally present at a proportion of 5 to 40 weight percent, serves as a polymeric binder or base material, and a plasticizer is used to produce the desired plastic solid structure. Nitrocellulose-base compositions containing an explosive nitrate ester plasticizer such as nitroglycerin are designated double-base compositions. When high energy fuel and oxidizer materials such as powdered aluminum and ammonium perchlorate are provided in a double-base composition, the composition is designated composite double-base.

One of the major problems presented in the use of nitrocellulose-base compositions is attainment of suitable physical properties over a wide temperature range. A satisfactory solid propellant should be flexible enough to withstand handling and firing at low temperatures such as 4() F. without fracturing the grain. The propellant should also have sufiicient thermal stability to avoid melting, deformation or decomposition at temperatures up to at least 140 F., and preferably up to 200 F. Stability of properties over this range of temperatures is required to permit handling and storage of the propellant under a variety of field conditions.

Previously known nitrocellulose-base compositions have not proven fully satisfactory in melting these temperature requirements. Many of these compositions have been too brittle to permit handling at the lower temperatures with out fracture. Improved temperature-dependent properties have been attained by various methods such as using higher proportions of plasticizer and a ferrocene curing agent, but further improvements are desired. In the case of double-base composite compositions, low-temperature strength and high-temperature stability are adversely afiected by the presence of high proportions of fuel and oxidizer additives, the proportions of nitrocellulose binder and plasticizer which form the matrix of the composition being correspondingly decreased. It is desired to provide a means of improving the strength and stability of doublebase composite compositions by improving the nitrocellulose binder component. This approach would allow the use of higher proportions of fuel and oxidizer additives in a given composition.

3,725,729 Patented Apr. 10, 1973 It is therefore an object of this invention to provide improved nitrocellulose-base compositions.

Another object is to provide a method for improving the low-temperature mechanical properties of nitrocellulose-base compositions.

Still another object is to provide a method for increasing the thermal stability of nitrocellulose-base compositions.

Yet another object is to provide a thermally stable double base composite composition containing relatively high proportions of fuel and oxidizer additives.

Other objects and advantages of this invention will be apparent from the following detailed description.

In the present invention improved nitrocellulose-base compositions are obtained by incorporating hydroxylterminated polybutadiene polymer in the nitrocellulose binder. The hydroxyl-terrninated polybutadiene imparts greater strength and thermal stability to the resulting binder component, probably by cross-linking of the nitrocellulose and hydroxyl-terminated polybutadiene polymer chains. Compositions based on this binder, and particularly double-base composite propellants, exhibit substantially improved properties, both at low and high temperatures in the desired operating range.

Although this invention is not to be understood as limited to a particular theory, it is postulated that the hydroxyl groups of the hydroxyl-terminated polybutadiene are cross-linked with nitrocellulose by attachment at the sites on the nitrocellulose molecule having residual hydroxyl groups. Nitrocellulose is normally not fully nitrated to the theoretical composition cellulose trinitrate, and such residual hydroxyl groups are available for crosslinking. In eifect, the hydroxyl-terminated polybutadiene reinforces and strengthens the nitrocellulose molecular structure.

Hydroxyl-terminated polybutadiene for use in the present invention may be represented by the following formula:

Where n is from 45 to 80. The molecular weight of the polymers included within this formula is from about 2500 to 4000. Commercially available hydroxyl-terminated polybutadiene polymer may be used, or this material can be prepared by reduction of the corresponding carboxylterminated polybutadiene. Carboxyl-terminated polymer is not suitable for the present invention in that it causes excessive gassing during curin' resulting in an unsatisfactory porous product. The amount of hydroxyl-terminated polybutadiene may be varied over a wide range so as to provide a hydroxyl-terminated polybutadiene-tonitrocellulose weight ratio from about 1:8 to 3:1. The amount employed for a particular composition is determined by the properties required in the composition. Higher proportions of hydroxyl-terminated polybutadiene result in better temperature-dependent physical properties, but the total energy of the composition is decreased. For high-energy composite double-base propellants, a ratio as defined above of about 1:4 is preferred to obtain substantially improved properties, consistent with a high energy content.

Nitrocellulose having a nitrogen content from 11 to 13 weight percent may be used in this invention, and about 12.6 weight percent nitrogen content is preferred. The nitrogen content of the nitrocellulose normally used for propellants falls within this range so that no difiiculty is presented by this requirement. Nitrocellulose nitrated to the maximum theoretical extent would not be eifective since no hydroxyl sites would be available for cross-linkmg.

Formation of the nitrocellulose-hydroxyl-terminated polybutadiene binder requires use of a cross-linking agent. Any of the diisocyanates previously used for cross-linking nitrocellulose or hydroxyl-terminated polybutadiene can be used for this purpose. Examples of suitable cross-linking agents are toluene 2,4-diisocyanate and hexamethylene diisocyanate, the latter being preferred. The cross-linking agent is preferably supplied at a proportion by weight of about 1:8 to 1:15 with respect to the total content of nitrocellulose and hydroxyl-terminated polybutadiene.

In its broadest scope, the present invention is applicable to any nitrocellulose-base composition, the novel nitrocellulose-hydroxyl-terminated polybutadiene polymer being used in substantially the same manner as nitrocellulose alone in previous compositions. In general, nitrocellulose-base compositions are prepared by mixing nitrocellulose with a plasticizer to form a gelatinized colloidal mixture and curing the mixture by application of mild heat to produce a tough, rubbery solid. Any desired additives such as additional fuel or oxidizer materials, catalysts and stabilizers are incorporated in the composition prior to curing. The nitrocellulose-polybutadiene binder is compatible with all of the additives employed in previously known nitrocellulose-base compositions. All that is required in the present invention is curing of the nitrocellulose-hydroxyl-terminated polybutadiene containing composition in the presence of a cross-linking agent.

Any of the Well-known plasticizers for nitrocellulose can be used for compositions containing the nitrocellulosehydroxyl-terminated polybutadiene binder. Illustrations of these plasticizers are nitrate esters (e.g., nitroglycerin, hydroxyglycerol trinitrate, butane triol dinitrate, diethylene glycol dinitrate, triethylene glycol dinitrate, trimethylene trinitrate and mixtures of two thereof) dibutyl phthalate, dimethyl sebacate, dibutyl succinate, dibutyl adipate, triacetin, ethyl diphenyl phosphate, tributyl phosphate, tetrazole derivatives as set forth in 1.1.8. Pat. 3,073,731 and dinitriles as set forth in US. Pat. 3,104,190 and mixtures thereof. The nitrate esters are the preferred class of plasticizers. Although not critical to the present invention, the amount of plasticizer employed in nitrocellulose-base compositions is normally about to 50 weight percent.

When an explosive plasticizer, such as nitroglycerin, is used in nitrocellulose-base compositions contemplated by the present invention, the nitrocellulose and plasticizer contain sufiicient oxidizer to assure substantially complete combustion. To obtain higher energy, however, additional fuel and oxidizer materials can be incorporated in the composition. Organic oxidizers such as pentaerythritol tetranitrate, cyclotetramethylenetetramine, cyclotrimethylenetrinitramine and tetranitrobutane's or inorganic oxidizers such as ammonium perchlorate, ammonium nitrate and alkali and alkaline earth metal chlorates, perchlorates and nitrates can be employed. Up to about 70 weight percent, oxidizer can be used, but less than 60 weight percent is preferred in order to maintain good mechanical properties.

Powdered metal additives can be provided in nitrocellulose to serve as high-energy fuel component; aluminum, zirconium, lithium, magnesium, boron and their hydride's as Well as alloys of these metals can be used for this purpose. Metal additives normally constitute less than 30 weight percent of the cured composition.

Other additives which may be used in nitrocellulosebase compositions within the scope of this invention include, but are not limited to, burning rate modifiers exemplified by iron powder, lead salicylate, lead oxide, lead chromate, lead stearate, lead oxide and ammonium perchlorate; stabilizers exemplified by symmetrical diethyl diphenyl urea, resorcinol and 2-nitrodiphenylamine; pigments or fillers exemplified by zinc oxide, carbon blacks, titanium dioxide, candellila wax and sericite. The amount of such additives may be varied, depending on the specific Permissible Optimum Component proportion proportion Nitrocellulose 540 8-10 Diisocyanate 0. 55 1-1. 5 IIydroxyl-terminated polybutadiene 1-15 24 Plasticizer 20-50 30 35 5-70 30-40 1-30 15-21 Ammonium perchlorate and aluminum are the preferred oxidizer and fuel, respectively, in this embodiment, and nitrate esters are the preferred group of plasticizers. In addition, these compositions normally contain 1 to 5 weight percent of additivies such as the burning rate modifiers, stabilizers, pigments or fillers described above. The low-temperature strength and high-temperature stability of these compositions are substantially improved over prior compositions of this type having nitrocellulose alone as the binder.

The compositions described above are prepared by thoroughly mixing the specified ingredients and curing the resulting mixture to obtain a tough, rubbery solid material. The order of adding the various ingredients is not critical; but, in the case of composite double-base propellants, it is preferred to blend all dry materials except the oxidizer before mixing with the liquid components. Formation of the nitrocellulose-hyclroxyl-terminated polybutadiene copolymer occurs during curing. The mixture is cured by subjecting it to mild heat for an extended period, a suitable curing cycle comprising holding the mixture at a temperature of about 135 F. for five days.

Propellant grains of the desired shape and size can be formed by pouring the mixture into a mold or rocket motor case prior to curing, the mixture preferably being heated to a temperature of to F. for better fluidity in pouring. It is to be understood that this procedure is not critical, and that fabrication methods used for previously known solid propellants can in general be used for the present compositions. The only substantial difference is that the hydroxyl-terminated polybutadiene-containing compositions require a longer curing period, for example, five days as compared with two days for similar nitrocellulose-base compositions without this component.

This invention is further illustrated by the following example.

EXAMPLE I Composite double-base propellants with and without hydroxyl-terminated polybutadiene were prepared to determine the effect of this component on temperaturedependent properties. In each case a one kilogram sample was prepared by mixing the specified ingredients, pouring the mixture into a mold and curing for 5 days at F.

High-temperature thermal stability of each sample was determined by means of a standard 120 C. deflagration test wherein the sample is held at 120 C. until deflagration, or self-ignition at a constant temperature, occurs.

Maximum tensile stress in pounds per square inch and strain in percent elongation were determined at 40 F. and F. by conventional methods. Specific impulse was determined for two samples by firing in a test stand.

The composition in weight percent of each sample and the results obtained may be seen by reference to the following table.

TESTING OF PROPELLANI COMPOSITIONS Sample Number 1 2 3 4 Component:

Ball powder (90% nitrocellulose,

8% nitroglycerin, 2% 2-nitrodiphenylaminc) 16. 7 Resorcinol Aluminum.

0 Tiiethylene glycol dinitrate 37. 3 36.

Strain at maximum stress,

140 F., percent elongation. 1 Maximum tensile stress, 40

F., pounds per square inch 579 Maximum tensile stress, 140F.,

poundspersquareinch 45 Specific impulse, pounds thrust per pound of propellant It may be readily seen that thermal stability was greatly improved for the samples containing hydroxyl-terminated polybutadiene, the time to deflagration being increased from 550 and 600 seconds to over 2000.

With respect to physical properties at low and high temperatures, substantial improvement was obtained for the most critical properties, namely, strain at -40 F. and tensile stress at 140 F. The low temperature strain value indicates the extent to which the propellant grain would be subject to brittle fracture. This value was increased from 13 and 7 percent to 22 and 16 percent, respectively, by the addition of hydroxyl-terminated polybutadiene. The stress value at 140 F. indicates the strength and rigidity of the propellant, a relatively high value being required to avoid flowing at high tempera tures. Increases of from 45 to 77 and from 27 to 85 were obtained for this value by addition of hydroxyl-terminated polybutadiene. The values for strain at 140 F. and stress at 40 F. were not improved in all cases, but these values are not nearly as critical to propellant performance, and the values obtained for samples 2 and 4 are satisfactory. The specific impulse of propellant compositions is only slightly decreased by addition of hydroxylterminated polybutadiene, as evidenced by the decrease from 248 for sample 3 to 246 for sample 4.

The above examples are merely illustrative and are not to be understood as limiting the scope of this invention, which is limited only as indicated by the appended claims.

What is claimed is:

1. A binder for plasticized nitrocellulose-base compositions comprising a copolymer of nitrocellulose and hydroxyl-terminated polybutadiene.

2. A propellant composition comprising a cured intimate mixture of nitrocellulose, hydroxyl-terminated polybutadiene, a plasticizer and a cross-linking agent, the weight ratio of hydroxyl-terminated polybutadiene to nitrocellulose being from 1:8 to 3:1.

3. A propellant composition comprising a cured intimate mixture of nitrocellulose, hydroxyl-terminated polybutadiene, a nitrate ester plasticizer and a diisocyanate cross-linking agent, the weight ratio of hydroxyl-terminated polybutadiene to nitrocellulose being from 1:8 to 3:1.

4. A composite double-base propellant composition comprising a cured intimate mixture of 5 to 40 weight percent nitrocellulose, 1 to 15 weight percent hydroxylterminated polybutadiene, 0.5 to 5 weight percent diisocyanate, 20 to 50 weight percent plasticizer, 5 to weight percent oxidizer and 1 to 30 weight percent fuel.

5. A composite double ba'se propellant comprising a cured intimate mixture of 5 to 40 weight percent nitrocellulose, 1 to 15 weight percent hydroxyl-terminated polybutadiene, 0.5 to 5 weight percent diisocyanate, 20 to 50 weight percent nitrate ester plasticizer, 5 to 70 weight percent inorganic oxidizer and 1 to 30 weight percent of a fuel component selected from the group consisting of aluminum, zirconium, lithium, boron, magnesium and alloys and hydrides thereof.

6. A propellant composition comprising a cured intimate mixture of 5 to 40 weight percent nitrocellulose, 0.5 to 5 weight percent diisocyanate cross-linking agent, 1 to 15 weight percent hydroxyl-terminated polybutadiene, 20 to 50 weight percent nitrate ester plasticizer, 5 to 70 weight percent ammonium perchlorate and 1 to 30 weight percent powdered aluminum.

7. A propellant composition comprising a cured intimate mixture of 8 to 10 weight percent nitrocellulose, 1 to 1.5 weight percent diisocyanate cross-linking agent, 2 to 4 weight percent hydroxyl-terminated polybutadiene, 30 to 35 weight percent nitrate ester plasticizer, 30 to 40 weight percent ammonium perchlorate, and 15 to 21 weight percent powdered aluminum.

8. The method of preparing a nitrocellulose-base composition which comprises bringing together in intimate admixture nitrocellulose, hydroxyl-terminated polybutadiene, a diisocyanate cross-linking agent and a plasticizer and curing the resulting mixture, the proportion by weight of hydroxyl-terminated polybutadiene to nitrocellulose in the resulting mixture being from 1:8 to 3: 1.

References Cited UNITED STATES PATENTS BENJAMIN R. PADGETT, Primary Examiner US. Cl. X.R.