Nitrocellulose Plastisol Propellants - Advances in Chemistry (ACS

2 Nitrocellulose Plastisol Propellants

Downloaded by PENNSYLVANIA STATE UNIV on June 7, 2012 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0088.ch002

ALBERT T. CAMP Development and Technology Department, Naval Ordnance Station, Indian Head, Md.

Slurry-cast, filled nitrocellulose plastisols offer a variety of easily processed compositions with high performance potential as solid propellants. As a fully available vehicle for beryllium fuel, which offers the highest potential of any metal in an oxygen oxidizing environment, these plastisols are unexcelled. Processing hazards have justified the development of a non-mechanical, inert-carrier process for safer mixing. High quality, low cost, fine-particle spheroidal nitrocellulose is the key ingredient for successful application of this system on a large scale. Many different nitrate ester plasticizers and conventional stabilizers have been used successfully. Fillers consist of perchlorate salts, nitramine compounds, and certain light metals and hydrides.

/ ^ o l l o i d e d nitrocellulose-base compositions which attain their final form ^ essentially without the aid of volatile solvents meet the general definition for plastisols. In this sense the history of nitrocellulose plastisols dates back to the early work of the last century on blasting gelatins. Here, a small proportion of fibrous nitrocellulose is dispersed i n nitroglycerin, and the mixture is gelatinized and partially desensitized by diffusion of the nitroglycerin into the nitrocellulose fibers. This diffusion tends to be too rapid, even at room temperature, to allow the practical use of fibrous nitrocellulose i n a highly filled pourable slurry. Therefore, a great deal of effort has been directed since 1945 toward developing approaches which would allow real control over the rate of diffusion of high energy plasticizers into the nitrocellulose macrostructure. The chapter by R. Steinberger and Paul Drechsel (12) describes 20 years of progress with the extruded casting powder and casting solvent, or "interstitial-filling," approach. 29 In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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The present paper deals with the intrinsically simpler, pourable slurry approach which has been studied for about as long but applied to only a limited extent in flight-type rockets. The key to the successful application of high performance, pourable nitrocellulose plastisols lies in a reasonably priced, high quality source of fine-particle, at least partially colloided, spheroidal nitrocellulose. Here we are speaking of particles much finer than the well-known ball powder, produced by the O l i n Mathieson Chemical Co. for small arms for over 30 years (7). Actually, particles on the order of 5-50/a diameter appear to be required to assure a reasonable continuum of uniformly plasticized nitrocellulose binder in a propellant containing 45% or more of combined crystalline oxidizer and powdered metal fuel. Such a continuum of binder is necessary to assure acceptable mechanical properties and reproducible burning characteristics of the finished propellant. Preincorporation of a certain content of the water-insoluble solids within the nitrocellulose microspheres is an effective means of helping to assure this continuum of binder and alleviates the requirements for extremely small ball size. The use of a total of 45% or more of crystalline oxidizer and (generally) metal fuel is essential if the propellant is to be competitive with other modern propellants now i n service. Discussion Table I gives a range of typical ingredient types and percentages for potentially useful nitrocellulose plastisol propellants. Table I.

Typical Ingredients for Nitrocellulose Plastisol Propellants Ingredients

Weight %

Spheroidal nitrocellulose (12.2 or 12.6% nitrogen) Nitrate ester plasticizers Desensitizing plasticizers Stabilizers Oxidizers Metallic fuels

5-20 25-40 0-10 0.5-2.0 40-50 0-20

N i t r o g l y c e r i n , p e n t a e r y t h r i t o l t r i n i t r a t e , 1,1,1-trimethylolethane (metriol) trinitrate, 1,2,4-butanetriol trinitrate, diethylene glycol dinitrate, and triethylene glycol dinitrate are typical energetic plasticizers. Glyceryl triacetate is a typical desensitizing plasticizer for nitroglycerin. Stabilizers include st/m-diethyldiphenylurea (ethyl centralite), 2-nitrodiphenylamine, and other phenyl compounds capable of scavenging — N 0 radicals evolved in the slow decomposition of nitrate esters. Crystalline oxidizers 2

In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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include ammonium perchlorate, potassium perchlorate, and cyclic methylene nitramines. Aluminum, beryllium, and compatible metal hydrides are typical energetic fuels. Since 1958 Picatinny Arsenal has reported moderate success with high density-impulse, smokeless-burning, nitrocellulose plastisols containing crystalline nitramines (1). However, by far the greatest effort in the United States and Great Britain has been directed toward the most energetic versions of this system containing, in the double-base matrix, ammonium perchlorate, aluminum or beryllium, and often a nitramine oxidizer. Most U.S. propellant research and development groups have worked on these high energy systems at some time since 1956 because of their high performance potential and simplicity of manufacture. Patent applications have been filed by some of these organizations, and two or more have been granted to the respective inventors (4, 14). During the past 10 years great strides have been made in the development and service application of highly loaded, synthetic rubber-base and interstitially-cast double-base composite-modified propellants. Largely for this reason and for economic and logistic reasons as well, the slurry-cast nitrocellulose plastisols have been relegated to a secondary role. The foregoing trends have placed increasing demands on the quality of spheroidal nitrocellulose and have, for some applications, necessitated serious attention to chemical crosslinking of the nitrocellulose. More than 10 years ago investigators at the Naval Ordnance Test Station attempted to crosslink nitrocellulose with pentaerythritol trinitrate ( P E T r i N ) through the use of a diisocyanate. These attempts proved impractical because the reaction rate of isocyanate with the hydroxyl group in P E T r i N was much faster than that with the nitrocellulose hydroxyl groups. The resultant dimer of P E T r i N , formed with diisocyanate, further proved to be a poor plasticizer for nitrocellulose. Eventually it was discovered that P E T r i N alone was an effective gelatinizing agent for spheroidal (10/x) nitrocellulose and imparted a degree of mechanical strength and dimensional stability to the resultant binder, which has apparently not been duplicated by other castable combinations of nitrocellulose and plasticizers. It was postulated that opportunities for hydrogen bonding existed in the PETriN-nitrocellulose system which could account for its remarkable dimensional stability and physical strength. H i g h viscosity of the slurry, poor low temperature properties of the binder, and certain economic and logistic considerations have combined to prevent serious exploitation of this scientifically interesting discovery. In more recent years investigators at Picatinny Arsenal, Hercules, Inc., and elsewhere have succeeded in developing practical crosslinked


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nitrocellulose binders using diisocyanates and other bifunctional reactants. These are attractive where the nitrocellulose content must be less than about 10% to permit high solids loading and high content of more energetic nitrate ester plasticizer. Reactivity of isocyanates with residual moisture in the propellant ingredients has contributed to the difficulty of using this route for crosslinking.


Sources of Spheroidal Nitrocellulose Olin Mathieson ball powders of various particle-size distributions have been evaluated since about 1947 as the source of nitrocellulose for interstitially cast, as well as slurry-cast, propellants (3, 9). By 1959 the Naval Ordnance Test Station ( N O T S ) and the Naval Ordanance Section at Indian Head, M d . ( N O S I H ) , applying earlier work by the Atlantic Research Corp. (11), had developed a reasonable process for producing an essentially pure, stabilized spherical nitrocellulose with an average particle size of about 10/x. Though expensive, this product has proved to be a good standard for comparison with spheroidal nitrocelluloses developed by the du Pont Co., Hercules, Inc. (13), and O l i n Mathieson. The N O T S - N O S I H - A R C process involves complete solution of the nitrocellulose in nitromethane and dispersion i n water with the aid of a colloid mill and an emulsifying agent. The O l i n process is also a lacquer emulsion-in-water approach described for gun granulations in the literature in 1946 ( 6) and again in 1956 (8). The relatively small 50-60/x "fluid-ball" size used as a base for rocket propellants is achieved in the emulsion and solvent distillation stage (3, 9). The du Pont process (2) and the Hercules process appear to differ from the other two in that they do not involve complete solution of the nitrocellulose. The du Pont patent reveals the use of special emulsifying agents and a crosslinked coating of neutralized polyacrylic acid and allyl sucrose. A l l four sources of spheroidal nitrocellulose share the important property of having a resistant shell which is not penetrated quickly at room temperature by the commonly used energetic and conventional plasticizers already mentioned. Only Olin Mathieson provides both single- and double-base commercial forms of spheroidal nitrocellulose, but both Olin and Hercules have provided extensive tailoring services to government and industry by incorporating oxidizers and burning rate modifiers to meet special requirements. A l l four sources are still under consideration by U.S. propellant development groups, choice being governed by solids loading required (higher solids loading generally requiring the finer, average particle size), cost, propellant mixing equipment available, other ingredients to be employed, and preference of the user.


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Processing Techniques and Equipment The processes and equipment used for making pourable nitrocellulose plastisol rocket propellants are generally simpler and of lighter construction, respectively, than those used for other propellant types. In 1959 Grand Central Rocket Co. (now Lockheed Propulsion Co.) introduced a process based on the use of open polyethylene mixing vessels and air-powered turbine agitators. The process is still used and offers a low investment cost for remote, pilot-scale operations with especially hazardous and/or toxic compositions. More conventional vertical planetary mixers, produced by several U.S. and British manufacturers, have also proved effective. Mixers with submerged bearings are not recommended and have proved dangerous. Vacuum mixing is unnecessary because of the ease of deaeration in a subsequent transfer step. It is also potentially much more hazardous than atmospheric pressure mixing because of the extremely rapid deflagration characteristics of the foamed propellant during deaeration if it should become ignited by friction or some malfunction in the mixing apparatus. In the slurry process, as conventionally conducted, spheroidal nitrocellulose is first gently dispersed in a solution of plasticizers and stabilizers; this is followed by dispersion of metal powder (if used), burning rate modifiers, and oxidizers in the same vessel. M i x temperature is generally maintained below 85 °F. to avoid excessive gelatinization and viscosity increase. M i x viscosity is generally from l/10th to 1/100th that of typical rubber-based composite propellants. This accounts for the low power requirements during mixing and the feasibility of efficient, rapid deaeration during the subsequent vacuum transfer and/or casting steps. Curing is accomplished at 100°-140°F. over a few days, time and temperature depending on composition and size of the propellant charge. Pressure curing is commonly used to overcome the effects of shrinkage. Inert Diluent Process The unusual sensitivity of some composite-modified double-phase propellants before curing has justified intensive effort to exploit a nonmechanical mixing process. First introduced in about 1959 as the "quickmix" process by Rocketdyne Division of North American Aviation (5, 10), the inert diluent process has been developed at the Naval Ordnance Station, Indian Head, M d . for application to a variety of propellant compositions. Separate streams of solids, slurried in heptane, and an emulsion of plasticizers in heptane, are combined in a non-mechanical mixing chamber. The complete propellant slurry is allowed to settle, and the heptane is separated and recycled in a continuous operation. Figure 1


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shows the various elements and steps i n the process. Various double-base and composite modified double-base compositions have been successfully processed at the 300-lb./hr. scale i n a demonstration plant. A larger, fully continuous, remotely controlled, closed-cycle plant is under construction at the Naval Ordnance Station (Indian Head, M d . ) . It is expected that fluorocarbon-based composites, as well as the complete family of slurry cast, plastisol double-base propellants, w i l l be processible in this facility.

Figure 1.

Inert diluent process flow diagram

Adhesives for Case Bonding The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used i n the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps


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to assure satisfactory bonding; forces to produce hydraulic pressures of 30-70 p.s.i.g. are commonly employed during the early stages of curing.


Literature

Cited

(1) Baumann, Robert, Picatinny Arsenal Tech. Rept. 2601 (March 1959) (classified). (2) Bergman, R. C., U. S. Patent 3,200,092 (Aug. 10, 1965). (3) Cook, R. L., Andrew, E. A., U. S. Patent 2,888,713 (1959). (4) Godfrey, J. N., U. S. Patent 3,290,190 (Dec. 1966). (5) Kramer, Frank B., U. S. Patent 3,022,149 (Feb. 20, 1962). (6) Olive, Theodore, Chem. Eng. 53 (12), 92, 136 (1946). (7) Olsen, Fred, Tibbitts, G. C., Kerone, E. B. W., U. S. Patent 2,027,114 (Jan. 7, 1936). (8) O'Neil, J. J., Ordnance (Sept.-Oct. 1956). (9) Reinhardt, C. M., U. S. Patent 2,919,181 (1959). (10) Sheeline, R. D., Chem. Eng. Progr. (Feb. 1965). (11) Sloan, A. W., Mann, D. J., U. S. Patent 2,891,055 (June 16, 1959). (12) Steinberger, R., Drechsel, P., ADVAN. CHEM. SER. 88, 1 (1969). (13) Voris, R. S., U. S. Patent 2,843,582 (1958). (14) Weil, L. L., U. S. Patent 2,967,098 (Jan. 3, 1961). RECEIVED

March 17, 1967.


Nitrocellulose Plastisol Propellants - Advances in Chemistry (ACS

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