Compositions – Compositions containing a single chemical reactant or plural... – Organic reactant
Reexamination Certificate
2001-03-02
2003-10-14
Kopec, Mark (Department: 1751)
Compositions
Compositions containing a single chemical reactant or plural...
Organic reactant
C252S182200, C252S182220, C149S003000, C149S010000, C149S011000, C149S012000, C149S019100, C149S019400, C149S019910, C102S285000, C102S287000, C102S289000, C523S180000, C244S074000, C060S219000
Reexamination Certificate
active
06632378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to nitrate ester plasticized energetic compositions, and in particular a nitrate plasticized energetic composition comprising a binder that is highly compatible with nitrate esters. This invention is further directed to a method of making the energetic composition, and a rocket motor assembly loaded with the propellant comprising the energetic composition.
2. Description of the Related Art
Solid rocket motors typically include an outer case or shell housing a solid propellant grain. The rocket motor case is conventionally manufactured from a rigid, yet durable, material such as steel or filament-wound composite. The solid propellant grain is housed within the case. To protect the rocket motor case from the hot gas and particles streams generated during combustion of the solid propellant grain, solid rocket motors are also provided with a heat insulating layer (insulation) and a protective lining (liner). Typically, the insulation is bonded to the inner surface of the case. Insulated portions of the propellant are bonded to insulation and non-insulated portions of the propellant grain are bonded to the case by use of a lining layer (liner), which is typically an elastomer.
The solid propellant grain is formulated from a composition designed to combust and produce, upon discharge of the combustion products through an associated nozzle, the requisite thrust for attaining rocket motor propulsion. For example, combustion of composite solid rocket propellants generates temperatures inside the rocket motor case that can exceed 2760° C. (5,000° F.), and interior pressures may exceed 1,500 psi. These factors combine to create a high degree of turbulence for particles entrained in the gases produced during the propellant combustion.
Composite solid propellants commonly comprise a metallic fuel and chemical oxidizing agent that react with each other to release large amounts of energy and provide the interior pressures needed to attain rocket motor flight. The fuel and oxidizing agent are immobilized in a polymeric binder. Selection of an appropriate binder can enhance the tensile strength of the propellant, which is important for maintaining the structural integrity of the propellant grain during operation and storage. Other ingredients are added to the composite solid propellant, as are needed or desired, to provide additional energy performance, improve the mechanical properties of the propellant, and/or simplify processing.
Among the additional ingredients commonly found in composite solid propellants are plasticizers. In particular, nitrate ester plasticizers have found wide acceptance due to their abilities to enhance energetic performance of the propellant due their nitrate ester moieties. Nitrate ester plasticizers provided the added benefits of improving rheological properties during processing, preventing crystallization of the binder, and improving low temperature mechanical properties of the propellant. Propellants using nitrate ester plasticizers tend to react less violently during slow cook-off events, which is desirable.
For reasons explained below in connection with the following description of conventional propellants and their drawbacks and problems, it is important that a propellant containing a nitrate ester plasticizer also include a binder system that is highly compatible with the nitrate ester plasticizer. In this regard, several binder systems have been investigated for compatibility with nitrate esters.
An example of a binder system that has been investigated with nitrate ester plasticizers can be found in U.S. Pat. No. 3,004,840 to Pruitt. The Pruitt patent discloses propellants having a binder made of homopolymers of alkylene oxides having from 2 to 3 carbon atoms. The Pruitt patent provides several examples in which polyoxypropylene (also known as poly(ethylene glycol)) was tested. The Pruitt patent cites the higher oxygen content of its binders as enhancing combustion reactions. The present inventors found, however, that ethylene glycol homopolymers and propylene glycol homopolymers often exhibit poor compatibility with nitrate ester plasticizers. For example, ethylene glycol based elastomers tend to crystallize during storage unless large amounts of plasticizer are mixed with the binder. However, the large amount of plasticizer needed to avoid crystallization of poly(ethylene glycol) binders can be so great that the burn rate and shock sensitivity of the propellant can rise to unacceptable levels. On the other hand, propylene glycol homopolymers are incompatible with nitrate ester plasticizers, often causing the nitrate ester plasticizers to be exuded from the propellant so that homogeneity is not achieved.
Another example of a binder system that has been investigated with nitrate ester plasticizers can be found in U.S. Pat. No. 4,799,980 to Reed, which discloses an energetic composition having a poly(alkylene oxide) binder and a nitrate ester plasticizer. According to one embodiment of the Reed patent, a random copolymer of oxyethylene and oxypropylene is used as the binder system. However, oxyethylene present in high levels in the copolymer tends to crystallize during storage unless high levels of nitrate ester plasticizers are present. As mentioned above, high levels of nitrate ester plasticizers can unacceptably lower tensile strength and increase the sensitivity of the propellant. Higher sensitivity, especially shock sensitivity, is inconsistent with insensitive munitions requirements of many of today's propellants.
It has also been attempted to make a nitrate ester plasticized propellant comprising a mixture of poly(propylene glycol) and POLY-G®, which is a di-functional random copolymer of poly(ethylene glycol) and poly(propylene glycol) made by Olin Corporation and having a molecular weight of about 6000. The combination of POLY-G® with poly(propylene glycol) reduces the tendency of the poly(ethylene glycol) (present in the POLY-G®) to crystallize, even with low levels of plasticizers. However, this binder mixture produces a propellant that lacks adequate adhesive bonding properties to rocket motor elastomeric liners.
It would, therefore, be a significant improvement in the art to provide a poly(alkylene glycol)-based binder system that bonds well to a surrounding rocket motor elastomeric liner, does not crystallize in the absence of unacceptably high concentrations of nitrate ester plasticizer, and is sufficiently compatible with nitrate ester plasticizers so as to dissolve the plasticizers and avoid leaching out of the plasticizers during processing.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to fulfill a long-felt need in the art by providing a nitrate ester plasticized energetic composition capable of achieving the above-discussed improvements.
In accordance with the principles of this invention, the above and other objects are attained by a nitrate ester plasticized energetic composition in which the binder comprises, prior to curing, lower alkylene glycol prepolymer blocks end-capped with at least one member selected from the group consisting of ethylene glycol monomers and ethylene glycol oligomers. The end-capped prepolymer blocks are preferably difunctional or trifunctional. As referred to herein, lower alkylene glycol means propylene glycol, butylene glycol, and/or copolymers thereof. The difunctional end-capped alkylene glycol prepolymer blocks are cured by either linking with a diisocyanate or linking and crosslinking with a polyisocyanate. In the case of a trifunctional (or higher functional) end-capped alkylene glycol prepolymer block, preferably a diisocyanate is used to effect crosslinking, since the combination of a trifunctional end-capped prepolymer and polyisocyanate can lead to excess crosslinking.
Where appropriate amounts of nitrate ester plasticizer, curative, and oxidizer, and optionally one or more of a bonding agent, stabilizer, energetic filler and additional binders are added to the binder, and the ethylene gl
Oyler Jeffery
Wallace, II Ingvar A.
Alliant Techsystems Inc.
Kopec Mark
TraskBritt
Vijayakumar Kallambella M
LandOfFree
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