Flexible, Self-supporting Explosive Composition

Abstract

A castable composition exhibiting all the desirable properties of flexible xplosives comprising a dispersion of fine particle size explosive filler in a readily curable low-density binder system formed of a liquid curable prepolymer such as a dihydroxy polybutadiene and a coupling curing agent such as a diisocyanate and optionally a low density compatible plasticizer. The composition cures at low temperature to form a flexible, self-supporting, thermally stable, non-thermoplastic high energy explosive.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to flexible, self-supporting explosive compositions and, more particularly, to castable, thermally stable, nonthermoplastic compositions containing very high solids content.

2. Description of the Prior Art

The relative scarcity, but apparent need for readily processable, safe, high energy explosives that have appropriately high rates of detonation and suitably low critical thickness propagation to quality as satisfactory flexible sheet explosives under MIL specification MIL-E-46676 (MV) and BUWEPS Notice 8027, has motivated new research in this field. The qualified sheet explosives are unique because they fulfill certain needs that cannot readily be met by other kinds of explosive material. In service they require toughness, durability, uniform detonation velocity and a high degree of safety. In addition to their flexibility, they are water proof, insensitive to shock, easy to cut to any desired shape and easy to apply. Among the more widely known applications are uses in metal cutting and hardening, underwater and general demolition, line wave generators and safe-arm devices.

Of the explosives that qualify under the cited military specifications, perhaps the most widely known are extrudable compositions containing 63-75 percent pentaerithrytol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX) or cyclotetramethylene tetranitramine (HMX) in a phosphate or carboxylate ester plasticized nitrocellulose binder or in a polyterpene plasticized preformed polymeric rubber binder. These presently available explosive compositions have certain inherent shortcomings when utilized in applications requiring flexible sheet. These application techniques are limited to extrusion because of the thermoplastic nature of the binder. Binder thermoplasticity limits temperature of potential applications or use. Furthermore, the solids content of the explosive filler concentration is limited because at high solids content, the compositions are not readily castable because the resin binder system is incapable of binding large amounts of solids or the composition becomes too brittle to be usable. The low concentration of explosive filler reduces the detonation rate and/or increases the critical thickness required for detonation propagation.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to fabricate castable explosive compositions capable of forming flexible explosives and obviating processing limitations and other shortcomings of presently available, flexible, explosive compositions.

A further object of the invention is the provision of flexible explosive compositions incorporating a high concentration of fine particles of explosive in binder and being provided in lower critical thickness sizes.

A further object of the invention is the provision of castable, thermally stable, non-thermoplastic explosive compositions that are both flexible and self-supporting.

These and other objects and many other attendant advantages of the invention will become apparent as the description proceeds.

The compositions of the present invention comprises a high concentration of substantially fine particle size explosive filler uniformly dispersed in a binder composition formed of a low density prepolymer and appropriate low temperature curing and catalytic agents and optionally and preferably a compatible low density plasticizer. The fine particle size filler insures minimal critical thickness for detonation propagation. The low density prepolymer has a low viscosity in the precured condition and is capable of accommodating the high concentration of explosive filler necessary to attain high rates of detonation. The low temperature prepolymer readily cures under mild conditions to cross-linked, flexible, elastomeric non thermoplastic, polymeric materials that are thermally stable.

The composition of the invention generally includes in parts by weight:

MATERIAL AMOUNT, pbw ______________________________________ Explosive material 60-85 Prepolymer binder resin 10-22 Coupling-curing agent Up to 5% Antioxidant <1% Pigment 0 to 1% Catalyst 0.1 to 1% Plasticizer 5-15% ______________________________________

The explosive filler material is preferably a cap-sensitive, crystalline organic compound suitably having an average particle size below 100 microns and preferably a -325 mesh material having an average particle size between about less than 1 to about 30 microns, preferably 15 to 20 microns. The compounds generally used are organic nitrates such as PETN or cyclonitramines such as HMX or RDX or mixtures thereof. Critical thickness for the composition of the invention are between 0.05 to 0.30 inches. The filler particles may be coated with a suitable desensitizing agent such as a dialkyl ester of a carboxylic acid for example 0.1 to 1 percent of dioctyl adipate based on the weight of the explosive filler.

The elastomeric binder is formed by the condensation reaction between a liquid prepolymer of the formula: ##SPC1##

and coupling-curing agents of the formula:

Z--R.sup.2 --(Z).sub.m (II)

where n is an integer from 0-4, m is an integer of at least 2, R.sup.2 is an organic radical containing from 2 to 50 carbon atoms, R is an organic moiety having a molecular weight from 1,000 to 15,000, preferably 1,000-5,000, and Z and Y are coreactive condensible end and/or side groups, which are capable of in-situ reaction to chain extend and crosslink the liquid prepolymer to form a continuous, flexible, thermally stable, high tensile strength explosive filler binder film.

The readily curable liquid prepolymer is of a type compatible with the other components of the explosive composition and is preferably soluble, miscible, or fusible with the other components of the composition. The R or backbone portion of the liquid prepolymer is preferably an elastomer forming prepolymer such as a diene prepolymer or a polyether prepolymer.

The diene prepolymer may be a polymer of a conjugated diene containing from 4 to 12 carbon atoms per molecule and preferably four to eight carbon atoms per molecule, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene (piperylene), 3-methyl-1,3-pentadiene, 1,3-heptadiene, 3-butyl-1,3-octadiene, phenyl-1,3-butadiene and the like. The conjugated diene may also contain hydroxy, carboxyl or lower alkoxy substituents along the chain such as 2 -methoxy-1,3-butadiene, 2-ethoxy-3-ethyl-1,3-butadiene, and 2-ethoxy-3-methyl-1,3-hexadiene.

The co-monomer should not exceed 35 percent of the polymer in order to preserve the elastomeric properties. Suitable co-monomers are vinyl compounds such as vinyl-substituted aromatic and aliphatic compounds. Examples of co-monomers that can be employed in the elastomer forming liquid prepolymers of the invention include acrylonitrile, methacrylonitrile, propylene, butene, isobutylene, styrene, 1-vinylnaphthalene, 2-vinylnapthalene, and alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy, and dialkylamino derivatives thereof.

The equivalent weight of the liquid prepolymer is at least a thousand and not usually more than five thousand. The functionality of the prepolymer is advantageously slightly over 2, but less than 5 to form by cross-linking and chain-extending final polymers of the molecular weight of at least 20,000. With higher molecular weight prepolymers, it may be necessary to apply heat to reduce viscosity. Therefore, the equivalent weight is preferably from 1,000 to 3,000.

Functionality is provided by reactive terminal and side groups which may be at least one of thiol, (--SH) carboxyl, (--C00H) hydroxyl (--OH), isocyanate (--C--N=0), epoxy or amine. Upon addition of polyfunctional reactive coupling-curing agents and suitable catalysts or accelerators, the low molecular weight liquid reacts at ambient temperature to produce a high molecular weight elastomer. The functionality of the prepolymer is preferably maintained within the range of 2.1 to about 2.5 in order that excessive cross-linking does not transform the product into too brittle a state and thus reduce the resilient properties desirable for sheet or shaped forms of the product.

The diene prepolymers preferably contain a minimum amount of 1,2 addition to avoid excessive decrease of elastomeric properties. A suitable material is a butadiene polymer of equivalent weight of about 1,000-2,000 and has a functionality slightly greater than two and comprises 60% cis 1,4 units, 20% trans 1,4 and about 20% 1,2 vinyl units.

Other suitable polyhydroxy elastomer forming prepolymers are polyoxyalkylene glycols such as polyethylene, polypropylene or polybutylene glycols or esters thereof, neopentyl glycol adipate, polyethylene glycol azelate, sorbitol polyethers and polyoxypropylene oxide adducts of trimethylolpropane (TMP).

The coupling-curing systems can include various types of polyfunctional curatives reactive with the end or side chain functional groups. The thiol or hydroxyl substituted liquid prepolymers can be coupled and cured with aliphatic, aromatic cycloaliphatic polyfunctional compounds containing isocyanate, carboxyl, anhydride, amine, hydroxyl or epoxy groups.

Preferably the polyisocyanates are those represented by the general formula R.sup.2 (NCO).sub.m wherein R.sup.2 is a polyvalent organic radical containing from two to 30 carbon atoms and m is 2,3 or 4. R.sup.2 can be aliphatic, cycloaliphatic or aromatic. It is preferred that the organic radical be essentially hydrocarbon in character although the presence of unreactive groups containing elements other than carbon and hydrogen is permissible.

Examples of suitable compounds of this type include benzene 1,3-diisocyanate, hexamethylene 1,6-diisocyanate, (HDI) tolylene 2,4-diisocyanate (TDI), TDI dimer, tolylene 2,3 -diisocyanate, metaphenylene diisocyanate (MDI) diphenylmethane 4,4'-diisocyanate, naphthalene 1,5-diisocyanate, diphenyl 3,3'-dimethyl 4,4'-diisocyanate, diphenyl 3,3'-dimethoxy 4,4'-diisocyanate diethyl ether, 3(diethylamino) pentane 1,5-diisocyanate, butane 1,4-diisocyanate, cyclohexane 1,2-diisocyanate, benzene 1,3,4-triisocyanate, xylene triisocyanate, naphthalene 1,3,5,7-tetraisocyanate, napthalene 1,3,7-triisocyanate, toluidine diisocyanate, isocyanate terminated prepolymers, polyaryl polyisocyanates, and the like. The isocyanate terminated prepolymers are readily formed by reacting a hydroxyl substituted prepolymer with a diisocyanate, or a polyisocyanate.

A suitable, commercially available polyaryl polyisocyanate is known as PAPI--1. This material has an average of 3 isocyanate groups per molecule and an average molecular weight of about 380.

Exemplary polybasic acids reactive with hydroxyl or thiol modified polymers of the invention include maleic acid, pyromellitic acid, succinic acid, phthalic acid, terephthalic acid, trimellitic acid, and the like.

Isocyanate substituted prepolymers are also chain extended and cured with polyamines. Examples of such polyamines include tetraethylenepentamine, ethylenediamine, diethylene triamine, triethylene-triamine, o-phenylenediamine, 1,2-propane-diamine, 1,2-butanediamine, piperazine, 1,2,3-benzenetriamine, 3,3'-biphenyldiamine, methylene dianiline or N,N'bis(1,4-dimethylpentyl)-paraphenylenediamine. The fatty diamines or amine terminated polyamides such as can be produced by condensation of polyamines with polybasic acids can also be used.

Urethane or ester linked polymers are formed when isocyanate or carboxyl substituted prepolymers where from Formula I. Y is --NCO or --COOH, are cured with polyhydroxy compounds. These compounds can be either aliphatic or aromatic polyols or certain polyether products. Examples of such coupling-curing agents include castor oil, ethylene glycol, glycerol, propylene glycol, neopentylglycol, glycerol monoriconoleate, pentaerythritol, trimethanolethane, trimethanolpropane, butanediol or hexanetriol.

It is thus seen that an elastomer is formed of a plurality of prepolymer elastomeric units joined by coupling reagents which condense to form linking urethane, thiourethane, ester, urea, alkyl, or dialkyl, urea, thiourea, aminoalkyl units or combinations thereof.

A low density plasticizer such as a thermoplastic hydrocarbon resin compatible with the prepolymer and contributing no other properties, suitably a polybutene, in a preferred concentration of 30-50 percent by weight of the prepolymer may be present. The amount of plasticizer is chosen according to the desired critical thickness and in accordance with processing limitations. Optional filler or other additives may be present such as inert inorganic material such as metal or metal oxide particles, for example aluminum or aluminum oxide, or pigments such as lead chromate or carbon black or organic additives such as triacetin pigment or antioxidants, for example sym, di-beta-naphthyl-para-phenylene diamine or other additives useful for improving processing cure or properties of the precured or cured compositions.

All additives and components of the composition are chosen so as to balance processability, extrudability and flexibility with the ability to incorporate maximum explosive filler content with minimum critical thickness. The type and amount of curing agent depends on the functionality and molecular weight of the prepolymer and the amount of plasticizer. The curing agent is added in an amount insufficient to form an inflexible product. If the functionality of the prepolymer is greater than two, a difunctional coupling curing agent in an amount from 50 to 150 percent of stoichiometric based on the functionality of the prepolymer is capable of providing a satisfactory product. Polymers having a functionality of 2 or less the coupling curing agent preferably has a functionality of 2 or more. Crosslinking between prepolymer chains can result whenever the prepolymer or coupling-curing agent has a functionality greater than 2.

Polymerization modifiers can be added to increase or decrease stiffness as desired. For example, in a system in which the functionality of the prepolymer and/or curing agent leads to more than optimum crosslinking a monofunctional modifier such as a monoisocyanate can be added. If it is desired to increase crosslinking a modifier containing at least three functional groups such as triamine or thiol can be added to the explosive composition.

The coupling-curing reaction can be promoted or accelerated by an appropriate curing catalyst such as 0.01 to 1 percent a heavy metal salt of an alkanoic acid, suitably ferric acetylacetonate or stannous octoate.

The composition is simply formed by combining the ingredients, mixing to form a uniform dispersion and curing at a low temperature, suitably from 50.degree.F to about 150.degree.F until a self-supporting thermally stable flexible explosive is formed. The explosive filler may be precoated with the disensitizing agent before addition to the composition. The curable composition can be cast into a film of appropriate thickness and cured or the cured composition can be molded or rolled, sliced or cut into a product of a desired thickness and shape. The material can be utilized in perforate or imperforate form, as is known in the art.

The invention will now become better understood by reference to the following specific example which are presented for illustrative purposes and it being readily understood that alternative ingredients and proportions may be readily utilized to form composition within the scope of this invention.

EXAMPLE I ______________________________________ Material Amount, wt % Explosive filler-HMX 75 TDI Dimer (curing agent) 1 PEG 4000* (prepolymer) 13 Triacetin (Desensitizer) 10.75 PBNA (Antioxidant) 0.25 ______________________________________ * Polyethylene glycol-Mol wt 4000

The filler and desensitizer were premixed before addition to the other ingredients. The total mixture was stirred and cast into a tray with runners and cured at ambient temperature to form a 0.20 inch thick, uniform flexible, tough sheet.

EXAMPLE 2

A 400 gram batch of a flexible explosive was prepared by mixing at 110.degree.-115.degree.F and 65 weight percent of Class E RDX (approximately 77.degree.-325 mesh, average particle size 15-20 micron and containing 0.5 weight percent dioctyl adipate desensitizer based on the weight of RDX), with a prepolymer portion comprised of 100 equivalents of a hydroxy terminated polybutadiene having an equivalent weight of 1150, and a functionality between 2 and 3, 80 equivalents of TDI (equivalent weight 87); plasticized with 40 wt % of a polybutene (Oronite 6) and containing 0.20 wt % of sym. di-beta naphthyl paraphenylene diamine (PBNA) antioxidant. The composition was cast into 0.20 inch thick sheets.

EXAMPLE 3

A composition was prepared according to the procedure of Example 2 containing 73.4 wt % class E RDX (0.5 wt % dioctyl adipate desensitizer) instead of the 65 wt% of Example 2.

The critical thickness processability and curing properties of the sheet explosives of Example 2 and 3 are listed in Table 1 below.

TABLE I __________________________________________________________________________ Critical Terminal EX- RDX RDX-E Thickness (1), in. Shore AMPLE Wt. % Wt. % Castability No Go Go Hardness __________________________________________________________________________ 2 65.0 65.0 Excellent <0.41 28 3 73.0 73.0 Very Good 0.19 0.21 35 __________________________________________________________________________ (1) Initiated with a No. 8 blasting cap perpendicular to top surface near end of specimen.

A number of properties of the explosives of Example 2 and 3 as delineated in MIL-E--46676 (MU) were measured and are summarized in Table II, below

TABLE 2 ______________________________________ COMPARISON OF PHYSICAL MECHANICAL, AND EXPLOSIVE CHARACTERISTICS OF FLEXIBLE EXPLOSIVE FORMULATIONS ______________________________________ EXAMPLE 1 2 ______________________________________ Composition Variables Total RDX 65.0 73.0 Class E RDX 65.0 73.0 Detonation Vel. m/sec (meas) -- 7230 Density, gm/cc 1,390 1.460 Min. Propagation* Thickness, in. <0.41 0.20 Mechanical Properties Tensile, psi, 80.degree.F 47 46 Elongation, %, 80.degree.F 155 72 Flexibility, after -40.degree.** expos. No cracks No cracks Flexibility, after 24 hrs., in 160.degree. water Flexibility Range, .degree.F -65 to -65 to +160 +160 Sensitivity BuMines Impact, cm/2 kg wt*** 78 84 Electrostatic (joules)+ 2.10 1.70 Rotary Friction Insensitive Insensitive Flame Test, dur. secs. ++ -- 17 sec. Vacuum Stability, ml/gm/100.degree.C/48 hrs. 0.33 0.25 ______________________________________ * No. 8 blasting cap, unconfined sheet ** No cracks when bent 90.degree. or 180.degree. around 1/4" dowel *** RDX = 32 cm + Dry RDX = 0.025 to 0.15 joules ++ All burned smoothly

The compositions were easily processable even at 73 wt% explosive filler. The sheet materials of both Examples 2 and 3 were smooth, homogenous, very rubbery and retained the explosive RDX when cut or broken. They were cast into sheet or block but can be readily cast into any shape.

Example 3 gave a detonation velocity of 7,230 m/sec which is above the 7,000 m/sec value considered as satisfactory. The tensile strengths were 47 and 46 psi and the elongations at 77.degree., 155 and 72 percent, respectively. Flexibility from -65.degree. to +160.degree.F is good and no cracks occur either at -40.degree.F or after 24 hours of immersion in water at 160.degree.F when bent 90.degree. around a 0.25-in. dowel. In fact, no cracks occur when bent 180.degree. around the dowel although a 1/16-in. crack is specified as tolerable.

The sensitivities as measured by Bureau of Mines impact sensitivity, rotary friction, and electrostatic sensitivity appear satisfactory. The vacuum stabilities are of the order of 0.25 to 0.33 ml/gm/100.degree.C/48 hrs compared to 5.0 ml considered adequate. Based on previous experience, these formulations would be expected to have long shelf lives.

It is thus seen that the invention provides a castable, thermally stable, non-thermoplastic, flexible, explosive composition with low critical thickness propagation to detonation, containing an explosive filler, a low density diluent or plasticizer, and a binder comprising a low density, readily curable prepolymer and a curing agent. The composition will readily find use in sheet, or ribbon and a variety of other cast molded or extruded shapes. Typical applications are in destruct and anti-personnel devices, field demolition, underwater energy generation and in metal hardening. The compositions of this invention will find substantial use as flexible thin sheet explosives having a critical thickness from about 0.05 to about 0.5 inches, a detonation velocity between 6,600 and 8,000 m/sec and a density from 1.4 to 1.6 g/cc. The sheet may be perforated to form a line-wave generator as a triangular section of the sheet. A detonation initiated at any apex of the triangle will proceed as a straight-line detonation zone to the opposite edge. The line-wave generator can be used to initiate cylindrical explosion charges or to fabricate plane-wave generators.

It is to be understood that only preferred embodiments of the invention have been described and that numerous substitutions, alterations and modifications are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed is:

1. A castable composition for forming a self-supporting, thermally stable, flexible explosive comprising 60-85 weight percent of fine particulate explosive filler selected from organic nitrates and organic nitramines dispersed in a binder formed of a low temperature curable liquid prepolymer of the formula ##SPC2##

and a coupling curing agent of the formula

Z--R.sup.2 --(Z).sub.m (II)

where n is an integer from 0 to 4, m is an integer of at least 2, R is an organic moiety having a molecular weight from 1,000 to 15,000, R.sup.2 is an organic radical containing 2-50 carbon atoms and Z and Y are coreactive, condensible groups, capable of reaction to form ZY links which chain extend and crosslink the liquid prepolymer to form a continuous, flexible, thermally stable, high tensile strength explosive composition with high detonation velocity and low critical thickness, wherein Y is selected from the group consisting of thiol, hydroxyl, isocyanate, epoxy and amine and Z is a group coreactive and condensible with Y selected from the group consisting of isocyanate, carboxyl, amine, anhydride, hydroxyl or epoxy.

2. A composition according to claim 1 in which the explosive filler has a particle size below 100 microns and is selected from the group consisting of pentaerythritol, tetranitrate, cyclotetramethylenetetranitramino, and cyclotrimethylenetrinitramine.

3. A composition according to claim 2 in which the explosive filler comprises particles of cyclotrimethylene-trinitramine having an average particle size of less than 1 to about 30 microns.

4. A composition according to claim 3 in which the explosive filler is of average particle size between 15 to 20 microns, and contains for 0.1 to 1 percent parts by weight of a desensitizing agent.

5. A composition according to claim 4 in which the liquid prepolymer is a polyhydroxy polybutadiene having an equivalent weight from 1,000 to 3,000 and a functionality from 2.0 to 2.5 and the coupling-curing agent is a triisocyanate.

6. A composition according to claim 1 containing about 10 to 22 parts by weight of the prepolymer and 1 to 5 parts by weight of the coupling-curing agent.

7. A composition according to claim 6 further including 5 to 15 percent by weight of a low density, compatible plasticizer.

8. A composition according to claim 1 in sheet form having a thickness from 0.05 to about 0.40 inches and a detonation velocity from 6,600 to 7,500 meters per second.

9. A composition according to claim 8 in which said sheet is perforated.

10. A method of forming a castable, thermally stable, non-thermoplastic explosive composition that is both flexible and self-supporting comprising the steps of:

dispersing 60 to 85 percent by weight of a fine particulate explosive material in a binder portion comprising a curable liquid prepolymer of the formula: ##SPC3##

and a coupling curing agent of the formula:

Z--R.sup.2 --(Z).sub.m (II)

where n is an integer from 0 to 4, m is an integer of at least 2, R is an organic moiety having a molecular weight from 1,000 to 15,000, R.sup.2 is an organic radical containing 2-50 carbon atoms and Z and Y are coreactive, condensible groups, capable of reaction to form ZY links which chain extend and crosslink the liquid prepolymer,

curing the composition at a temperature from about 50.degree.F to about 150.degree.F to form a continuous, flexible, thermally stable, high tensile strength explosive composition wherein Y is selected from the group consisting of thiol, hydroxyl, isocyanate, epoxy and amine and Z is a group coreactive and condensible with Y selected from the group consisting of isocyanate, carboxyl, amine, anhydride, hydroxyl or epoxy.