Plastic and nonmetallic article shaping or treating: processes – Explosive or propellant article shaping or treating – Rolling to form sheet or rod
Reexamination Certificate
1998-02-24
2001-01-09
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Explosive or propellant article shaping or treating
Rolling to form sheet or rod
C264S003300, C264S003400, C156S244110
Reexamination Certificate
active
06171530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the processing of high performance gun propellants which use an energetic thermoplastic elastomer (TPE) binder in combination with a high energy oxidizer.
2. Technology Background
There is a continuing need for high performance gun propellants which exceed the performance of currently fielded gun propellants and which are easily processed. As an example, the current Army 120 mm tank round gun propellant is a double base propellant (JA2) containing nitrocellulose, nitroglycerine, and an energetic plasticizer. This composition is gelled. If the gun propellant is processed or gelled improperly, the material cannot be easily reprocessed. The gun propellant JA2 has an impetus of about 1190 J/g and a flame temperature of about 3400° K.
Persons skilled in the art have previously proposed using an energetic thermoplastic elastomeric binder instead of nitrocellulose. For instance, U.S. Pat. No. 4,919,737 to Biddle et al. discloses a gun propellant composition containing an energetic thermoplastic elastomeric (“TPE”) binder and a high-energy oxidizer. Oxetane polymers, such as copoly-BAMO/AMMO (bisazidomethyloxetane/azidomethyl-methyloxetane) and copoly-BAMO/NMMO (bisazidomethyloxetane
itraminomethyl-methyloxetane) are disclosed TPE binders. According to Biddle et al. (column 4, lines 26-32), these gun propellants are prepared mixing at a temperature between 100° C. and 125° C., followed by extrusion at a temperature between 70° C. and 130° C. The high temperatures melt the thermoplastic elastomeric binder and allow the propellant to be processed.
A major disadvantage of Biddle et al.'s processing technique is the need to heat the energetic binder and high energy oxidizer to very high processing temperatures. This creates a substantial hazard to equipment and personnel. It also limits the quantity of gun propellant that can be safely processed at any one time. This batch technique can safely process only about 200 grams due to safety and rheological constraints.
It would be a significant advancement in the art to provide a process for manufacturing a high performance gun propellant containing an energetic thermoplastic elastomeric binder and a high-energy oxidizer which enables the safe processing of large quantities of high performance gun propellant.
Such processes of manufacturing a high performance gun propellant are disclosed and claimed herein.
SUMMARY OF THE INVENTION
The present invention is directed to a process of making high performance gun propellants containing an energetic thermoplastic elastomer binder in combination with a high energy oxidizer. The process includes extrusion of a suitable molding powder consisting of high energy oxidizer particles coated with the energetic binder. The molding powder preferably has a concentration of high-energy oxidizer in the range from 70% to 85%, by weight, and a concentration of the energetic thermoplastic elastomeric binder in the range from 15% to 30%, by weight. More preferably, the molding powder has a concentration of high-energy oxidizer in the range from 76% to 82%, by weight, and a concentration of the energetic thermoplastic elastomeric binder in the range from 18% to 24%, by weight.
Polymer precipitation is used prepare the molding powder. At its simplest, polymer precipitation involves dissolving the energetic polymer in a solvent, adding the solid oxidizer and stirring vigorously, then adding a nonsolvent (relative to the polymer and dry ingredients) to the system to cause precipitation of the polymer. Thus, polymer precipitation is used to uniformly coat the solid oxidizer particles with the precipitated polymer. The coated particles are then extruded into the shape desired for gun propellant.
In a currently preferred embodiment, the thermoplastic elastomeric polymer is dissolved in a solvent to form a lacquer. The high energy oxidizer particles are slurried with water and stirred. The lacquer and slurry are gradually combined, and the polymer precipitates onto the particles. The coated particles are collected and dried. The particle size is preferably in the range from about 200 &mgr;m to 2000 &mgr;m, and more preferably in the range from 200 &mgr;m to 1000 &mgr;m, and most preferably in the range from 500 &mgr;m to 1000 &mgr;m. This process can safely prepare large scale batches of gun propellant at lower cost than previous methods.
The molding powder properties are affected by the solvent concentration, the mixing rate of the polymer and the oxidizer slurry, the agitation rate of the oxidizer slurry, the oxidizer concentration in the slurry, the temperature of the reaction vessel, and the original particle size of the high energy oxidizer. Too much solvent causes the polymer to be sticky and not free flowing. A typical ratio of polymer to solvent is about 1:1.5, by weight. Increased agitation of the oxidizer slurry tends to decrease the particle size of the molding powder. A more dilute oxidizer slurry tends to produce smaller molding powder granules. A typical ratio of oxidizer particles to water is about 5:1, by weight. Colder temperatures also tend to decrease the particle size. Although somewhat solvent dependent, a typical temperature range is from 5° C. to 50° C. Finally, if the original oxidizer particle size is too small and the quantity of polymer is limited, the particles may be poorly coated. For CL-20 oxidizer particles, a typical particle size range is from 3 &mgr;m to 135 &mgr;m.
The molding powder is extruded according to conventional ram or screw extrusion technology. The extruder preferably has a barrel and a die which are jacketed to allow temperature control during the extrusion process. Various die configurations can be used. For instance, solid and perforated dies can be used at various diameters to form a strand of extruded gun propellant. The strand can be cut to a desired length or rolled into sheets. Importantly, because a TPE is used, the inventors have found that extruded material which contains irregularities or imperfections can be chopped up and re-extruded without the use of solvents or processing aids.
The extrusion process is accomplished by adding the molding powder to the extruder. Ram extruders and twin screw extruders can be used. A “thermal soak” step is often performed prior to extrusion to bring the molding powder temperature close to the desired extrusion temperature. The thermal soak can be simple preheating at the desired extrusion temperature. The thermal soak has also been performed by mixing the molding powder in a batch mixer at a temperature above the melt temperature of the TPE while applying a vacuum. The molding powder can be pre-consolidated before extrusion through the die. The extruded gun propellant is cut to the desired length or rolled to form sheets.
Ideally, the extruded gun propellant should have a substantially flat velocity gradient as it exits the die and a smooth surface finish. The extrusion process can be affected by factors such as barrel temperature, die temperature and length, extrusion pressure, conveyor speed, thermal soak time prior to extrusion, and die surface finish.
It will be appreciated that suitable molding powders can be prepared according to the present invention at temperatures significantly lower than those taught by Biddle et al. The present invention also enables safe processing of large scale batches of gun propellant.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to a process for manufacturing a high performance gun propellant containing an energetic thermoplastic elastomeric binder and a high-energy oxidizer. The process includes preparing or obtaining a molding powder of the high-energy oxidizer particles coated with the energetic thermoplastic elastomeric binder and extruding the molding powder into the desired gun propellant configuration.
Suitable molding powders have a concentration of high-energy oxidizer in the range from 70% to 85%, by weight, and a concentration of energetic thermoplastic elastomeric binder in the range
Braithwaite Paul C.
Haaland Andrew C.
Hartwell James A.
Lott Val D.
Rose Michael T.
Cordant Technologies Inc.
Fiorilla Christopher A.
Pillsbury Madison & Sutro LLP
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