Extrudable gun propellant composition

Details Extrudable gun propellant composition Extrudable gun propellant composition

1991-06-28

2001-05-08

Miller, Edward A. (Department: 3641)

Explosive and thermic compositions or charges

Structure or arrangement of component or product

Solid particles dispersed in solid solution or matrix

C149S019600

Reexamination Certificate

active

06228190

ABSTRACT:

This invention relates to an extrudable gun propellant composition and to extruded gun propellant prepared therefrom.
A wide range of extrudable propellant formulations are known which are based upon mixtures of nitroglycerine (NG) and nitrocellulose (NC). The manufacture of NC/NG based propellant (also known as colloidal propellant) such as double base propellant (based upon NG and NC alone) and triple base propellant (based on mixtures of NG, NC and picrite), requires many stages of ingredient mixing with a final stage of solvent inclusion, to enable an extrudable dough to be formed. Once extruded into the desired shape such as slotted tubes or longitudinally perforated sticks, the extrudate is stoved to remove the solvent and a fairly brittle propellant results whose physical properties depend essentially on a cellulosic framework encapsulating a solution of NG.
The main requirements for an extrudable gun propellant are that it should possess a high specific energy content compatible with the needs of modern high performance ammunition for artillery and battle tank guns in particular, and it should be processable into the desired propellant shape. The first requirement means that the force constant (F) of the cured propellant should ideally be at least 1100 kJ/kg, where:
F=nRT
o
in which
n=the number of moles of propellant gas products per kg of propellant
R=gas constant
T
o
=the adiabatic flame temperature
The second requirement is met by ensuring that at a typical propellant processing temperature (usually between 30° C. and 70° C.) the extrudable gun propellant possesses a viscosity within a range that permits relative ease of extrusion into the required propellant shape for incorporation into a gun propellant charge and yet also ensures that the extrudate once formed is relatively free from stickiness and maintains its shape without collapsing.
A further requirement of emerging importance is that the gun propellant in its final extruded and cured form should exhibit low vulnerability to attack or accidental ignition. This property is especially important for gun propellant which is to be used in ammunition stored in a confined space, such as the hull of a battle tank, which is likely to be subject to enemy attack, in particular high velocity fragments from shell bursts and/or high velocity jet penetrators from shaped charge warheads.
Known colloidal gun propellants meet some but not all of these requirements. In particular, they tend to exhibit high vulnerability to shaped charge attack. It is known that the force constant of such compositions can be increased, for example, by the addition of high energy particulate explosive filler materials such as the sensitive cyclic nitramines RDX and HMX, but this has tended to increase yet further the vulnerability of the propellant.
It is one object of the present invention to overcome or at least mitigate in part this disadvantage.
Accordingly, the present invention provides an extrudable gun propellant composition comprising from 65 to 85% by weight of particulate explosive filler, from 10 to 30% by weight of a curable mixture of a functionally-terminated poly(nitratoalkyl-substituted)-alkyl ether prepolymer and a cross-linking agent, and from 1 to 12% by weight of an energetic plasticiser.
It has been found that gun propellant compositions according to this invention form extrudable doughs and when extruded into useable forms and cured, typically possess force constants in excess of 1200 kJ/kg. However, even though they may contain large amounts of sensitive particulate explosive such as RDX or HMX, they are unexpectedly significantly less vulnerable to shaped charge attack than colloidal propellants.
The present composition preferably comprises from 70 to 82% by weight of the explosive filler, from 12 to 25% by weight of the prepolymer/cross-linking agent mixture, and from 2 to 10, especially from 4 to 8% by weight of the energetic plasticiser. At prepolymer/cross-linking agent mixture loadings of less than 12% by weight the composition tends to be stiff and difficult to extrude, whereas at loadings in excess of 25% by weight the extrudate tends to collapse under its own weight.
In order to provide the composition with a high force constant, the explosive filler preferably comprises at least one cyclic nitramine, such as RDX or HMX.
Since the present composition contains both an energetic binder prepolymer and an energetic plasticiser, a relatively high force constant can be maintained by increasing to a certain extent the proportions of these ingredients in the composition whilst reducing the loading of explosive filler. Although this has the advantage of reducing the vulnerability of the composition, a reduction in the amount of explosive filler has a marked effect on the processibility of the composition. For example, where the filler comprises a fine particulate cyclic nitramine, the composition must contain a high proportion of filler of typically in excess of 78% otherwise the extrudate is too soft and tends to collapse under its own weight.
It has been discovered however that where up to 40% by weight, and preferably from 10% to 25% by weight, of the filler comprises the relatively low energy, insensitive explosive picrite (nitroguanidine), the total proportion of the filler in the composition can advantageously be reduced to less than 78%, or even less than 75%. The presence of needle-like crystalline particles of picrite in the composition assist in stiffening the extrudate without unduly reducing force constant. The effect of reducing both the total amount of filler and the high energy explosive content of that filler (ie by the inclusion of picrite) is to reduce further the vulnerability of the cured propellant composition.
The prepolymer preferably comprises a hydroxy-terminated poly (nitratoalkyl-substituted cyclic ether), the cyclic ether preferably being an oxetane or an oxirane. The cyclic ether preferably has not more than two, and most preferably has only one, nitratoalkyl substituent group. Suitable examples of cyclic ethers are 3-nitromethyl-3-methyloxetane and glycidyl nitrate. The molecular weight of the prepolymer, which should ideally be a viscous liquid within the temperature range 30-50° C., is preferably in the range 2,000 to 15,000 more preferably 3,000 to 10,000 in order to ensure that the extrudable propellant composition has adequate processability. Suitable hydroxy-terminated prepolymers based on nitratoalkyl-substituted cyclic ethers are disclosed in Applicant's copending applications Ser. No. 07/820,624 filed Jan. 28, 1992 and Ser. No. 07/820,692 filed Jan. 27, 1992.
The average functionality of the prepolymer is preferably between 1.5 and 3.5, more preferably between 1.7 and 3.2. It is desirable that the prepolymer/curing agent combination should not react to produce a highly cross-linked structure in the composition once extruded and cured. It is therefore preferable that when the average functionally of the prepolymer is 2 or less, the curing agent is polyfunctional whereas when the functionality of the prepolymer is greater than 2 the curing agent is difunctional.
The amount of cross-linking agent will normally be selected to ensure that it reacts approximately stoichiometrically with all available terminal groups on the prepolymer. It will not normally comprise more than 15 wt % of the prepolymer/cross-linking agent mixture.
The prepolymer is preferably hydroxy-terminated and the cross-linking agent an isocyanate, so that the prepolymer/cross-linking agent mixture is capable of undergoing a urethane-type curing reaction.
The presence of an energetic plasticiser has been found important to wet the explosive filler, to soften the composition thereby improving its extrudability, and to ensure that the composition possesses a high force constant. The energetic plasticiser preferably comprises at least one nitratoplasticiser such as butane triol trinitrate or, more preferably, at least one nitroplasticiser such as bis-2,2 dinitropropyl formal, bis-2,2 dinit

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