Thermoplastic elastomer-based low vulnerability ammunition gun propellants

Abstract: LOVA gun propellants are formed from a thermoplastic elastomer and particulates of high-energy oxidizers, e.g., RDX and HMX.

Description: The present invention is directed to low vulnerability ammunition (LOVA) gun propellants in which the binder is a thermoplastic elastomer. A continuing objective in the design of gun propellants is to provide a gun propellant which is energetic when deliberately ignited, but which exhibits high resistance to accidental ignition from heat, flame, impact, friction, and chemical action. Propellants possessing such resistance to accidental ignition are known as "low vulnerability ammunition" (LOVA) gun propellants. Conventional LOVA gun propellants comprise an elastomeric binder, throughout which are dispersed particulates of high-energy material, particularly oxidizers. The elastomeric binder is generally a cured elastomer, formed, for example, by the urethane reaction of a multi-functional prepolymer with a multifunctional isocyanate. Examples of such LOVA gun propellants are described, for example, in US-A-4,263,070 and 4,456,493. Generally, LOVA propellant grains are formed by extrusion at elevated temperatures whereat substantial curing takes place. Because the grains cure to some extent as they are being formed, control of extrusion conditions is difficult. If cured LOVA propellant is unused, it cannot be recycled, and burning the propellant is generally the only suitable disposal method. Another type of LOVA propellant has a binder of cellulose acetate or a cellulose acetate derivative. An example of this type of propellant is described in US-A-4,570,540. These types of LOVA propellants are solvent processed, a process which entails relatively long processing times and a large number of steps. Also, the use of solvent creates environmental problems. The present invention is directed to LOVA propellants which use thermoplastic elastomers as binders. Thermoplastic elastomers have been previously used in propellants for rocket motors or he like, for example, as described in US-A-4,361,526. Gun propellants, however, are considered to be a different art than rocket motor propellants. Rocket motor propellants typically contain a particulate metal fuel, e.g., particulate aluminum. Gun propellants, on the other hand, should be substantially free of any metal, and for that matter, should be generally free of any material which leaves a solid residue in the barrel of the gun upon burning. Gun propellants should also be substantially free of chlorine, which degrades the gun barrel. Furthermore, rocket motor grains are typically formed in a different manner. Gun propellant grains typically take their shape from the extrusion process and must be sufficiently solid when leaving the extruder to retain their extruded shape. Material for rocket motor propellants may be extruded, but generally large rocket motors assume their shape from a mold, e.g., the rocket motor case; thus, after leaving an extruder or mixer, a propellant composition for a rocket motor should be free-flowing or at least moldable so as to be able to assume the shape of the large mold. EP-A-335499 discloses a method for synthesising block copolymers for use as elastomeric binders for propellants, explosives and the like. The block copolymers comprise at least one B block, which is amorphous at temperatures above -20°C, flanked by polyether A blocks, derived from oxetane or tetrahydrofuran monomers, which are crystalline at temperatures below 60°C. US-A-3,265,543 discloses propellant compositions comprising nitroglycerin, an oxidiser such as ammonium perchlorate and a thermoplastic elastic copolymer containing a multiplicity of urethane structural units connecting, inter alia, various polyether and polyester glycol units. US-A-4,361,526 discloses a composite rocket propellant including, as a binder, a thermoplastic block copolymer comprising amorphous and crystalline segments, preferably a styrene-diene block copolymer. The present invention consists in a low vulnerability ammunition gun propellant composition comprising from 60 to 85 wt. percent of particulates of a high-energy oxidiser and between 15 wt. percent and 40 wt. percent of a thermoplastic elastomeric binder system, said binder system being substantially free of metallic particulates and materials which leave a solid residue, said binder system comprising a thermoplastic elastomeric polymer in which at least one pair of crystalline A blocks flanks at least one amorphous B block and from 0 to 80 wt. percent of a plasticizer, wherein said elastomeric polymer comprises a mixed polyester or a polyester-polyether. The thermoplastic elastomer of the binder system has at least one block which is amorphous at room temperature, e.g., in the range of 20°C to 25°C, flanked by at least one pair of blocks which are crystalline at room temperature, whereby a thermoplastic network may be formed. The crystalline hard blocks preferably melt in a temperature range of between 70°C and 105°C. This temperature range allows processing at temperatures which do not decompose nitramine fillers. At the same time, in this temperature range, the binder retains good mechanical properties at about 63°C, considered to be the upper use temperature of LOVA gun propellants. The binder system may contain up to 80 wt. percent of an energetic or non-energetic plasticizer, the plasticizer comprising up to 35 wt. percent of the LOVA gun propellant composition as a whole. The two most common oxidizer particulates are tetramethylenetetranitramine (HMX) and trimethylenetrinitramine (RDX). Mixtures of these oxidizers may be used. Various configurations of thermoplastic elastomers are suitable, including (AB) n polymers, ABA polymers, and A n B star polymers, wherein the A blocks are crystalline and B blocks are amorphous at room temperature. In each of these structures, at least two A blocks flank at least one B block, allowing the crystalline A blocks to define a cross-linked structure at lower temperatures, while the amorphous B blocks give the polymer its elastomeric properties. A wide variety of thermoplastic elastomers may be used in accordance with the present invention. One preferred class of thermoplastic elastomers is polyethylene succinate/poly diethyleneglycol adipate (PES/PEDGA) block polymers. Currently preferred thermoplastic polymers are (AB) n type polyesters having short chain crystalline ester units and long chain amorphous ester units. Examples of such polymers are: Polyester Number Short Chain Ester Units Long Chain Ester Units 1 4GI PTMEG 2 4GI PEG 3 6GT/4GT PTMEG 4 6GT/6GI PTMEG 5 6GT/4GT PEG 6 4GT/4GI PEG 4GI 1,4-butylene isophthalate 4GT 1,4-butylene terephthalate 6GI 1,6-butylene terephthalate 6GT 1,6-butylene terephthalate PTMEG polytetramethylene ether glycol PEG polyethylene ether glycol The plasticizer, if used, may be non-energetic, e.g., dioctyl phthalate (DOP), dioctyl adipate (DOA), Santicizer 8 polyester by Monsanto, butanetriol trinitrate (BTTN), trimethylolethane trinitrate (TMETN), polyglycidal nitrate, or nitroglycerine (NG). Generally, if an energetic plasticizer is used, it is used at a low level in order to maintain the low vulnerability properties of the propellant. Other suitable plasticizers include, but are not limited to dibutoxyethyl phthalate (DBEP), dibutoxyethyl adipate (DBEA), chlorinated paraffin, methyl abietate, methyl dihydro-abietate, n-ethyl- o and p -toluene sulfonamide, polypropylene glycol sebacate, dipropylene glycol dibenzoate, di(2-ethyl-hexyl) phthalate, 2-ethyl-hexyl-diphenyl phosphate, tri(2-ethyl-hexyl) phosphate, di(2-ethyl-hexyl)sebacate, Santicizer 409 polyester by Monsanto, tetra-ethylene glycol-di(2-ethyl hexoate), dibutoxyethoxyethyl adipate (DBEEA), oleamide, dibutoxyethyl azelate (DBEZ), dioctyl azelate (DOZ), dibutoxyethoxyethyl glutarate (DBEEG), dibutoxyethyl glutarate (DBEG), polyethylene glycol 400 dilaurate, polyethylene glycol 400 dioleate, dibutoxyethoxyethyl sebacate, dibutoxyethyl sebacate, and trioctyl trimellitate (TOTM). The thermoplastic elastomer must be selected so that the filled propellant has a strain (elongation) of at least 1 percent, preferably at least 3 percent, and preferably less than 10. The modulus must be high enough so that the propellant grain maintains its shape during firing, i.e., so that it does not compress into a blob, and sufficiently low so as not to be brittle. A relatively broad range of moduli are acceptable, i.e., a range of between 5,000 and 50,000, preferably below about 35,000. Propellant compositions are generally required to operate over a wide temperature range and gun propellant grains should be stable at least to a temperature of 165°F (74°C). In order for the gun propellants to be used in low temperature environments, it is preferred that the thermoplastic elastomers incorporate soft blocks which retain their amorphous characteristics at low temperatures, i.e., down to -20°C and, preferably, even down to -40°C. Gun propellant grains are generally intended to operate in high pressure ranges, i.e., 206-8 MPa (30,000 psi) or above. In addition to the binder system and the oxidizer particulates, the LOVA gun propellant composition may contain minor amounts of other materials, such as processing aids, lubricants, colorants, etc. An important difference between rocket motor propellants and gun propellants is that gun propellants are fired through a barrel which is used multiple times, requiring that the gun propellants be substantially free of materials which would either corrode the barrel or leave deposits in the barrel. Gun propellants are substantially free of metallic particulates and other materials which leave a solid residue. Generally, metal-containing compounds are avoided as these tend to leave deposits; however, metal in compound form may comprise up to 0.5 wt. percent of the total weight of the propellant composition. For example, potassium sulfate may be incorporated as a flame suppressant. To avoid gun barrel corrosion, corrosive materials or materials which become corrosive upon firing are avoided. Gun propellants should be substantially free of chlorine. The propellants are processed by blending the ingredients at a temperature of between 100°C and 125°C in a mixer, such as a horizontal sigma blade mixer, planetary vertical mixer or twin screw mixer. The mix is then extruded and cut into a predetermined shape. Extrusion temperatures typically range from 70°C to 130°C. A typical shape for a gun propellant is a cylinder having a plurality of axially-directed perforations. In one typical embodiment, the propellant is cylindrical having a perforation running along the cylindrical axis and six additional perforations arranged along a circle halfway between the central perforation and the outside cylindrical wall. One general feature of thermoplastic elastomers which makes them particularly suitable for LOVA gun propellant applications is their endothermic melting characteristics. The fact that they absorb thermal energy as they begin to melt makes the LOVA gun propellants more capable of withstanding high temperatures. The invention will now be described in greater detail by way of specific examples. EXAMPLE 1 Table 1 below summarizes various properties of LOVA gun propellants prepared using different thermoplastic elastomeric binder systems, including mixing conditions, extrusion conditions, mechanical and physical properties and burn rates. In each case, the composition is 78% RDX, 22% binder system. The third composition from the left has a binder system which includes 20% by weight of a non-energetic plasticizer, dioctyl phthalate (DOP). Polymer PES PDEGA Polyester #1 (4GI/PTMEG) Santicizer 8 Polyester #1 (4GI/PTMEG) DOP(4:1) Rheocord 40 Test (78% RDX) LT035 LT033 LT051 Peak Torque, m-g 590 416 1255 Peak Temperature, °C 116° 114° 128° Extrusion (EX87) 0707-2 0629 0930-2 4.14MPa (600 psi) Barrel T, °C 89° (5.17MPa, 750psi) 95° 112° Die T, °C 80° 85° 99° DSC (10°C/min,N ₂ Tg, °C -44° -54° -35° Tm, °C +79° +93° +120° 63°C Slump Compressibility, % 2.2 19 1.9 60 Min Creep, % 1.6 17 0.3 DMA (5°C/Min) Tg,°C -33° -39° -64° E' @ -40° C, MPa 568 508 343 0° 224 89 201 +20° 151 55 162 +40° 55 9 99 Tensiles @ 25°C (2.54 mm/min, 0.1 in/min) Modulus MPa(psi) 96.5(14,000) 41.4(6000) 174(25,300) Stress KPa(psi) 1613(234) 407(59) 3172(460) Strain, % 2.2 1.1 2.0 Burn Rate @ 75.84 MPa, cm/sec 2.16 2.79 1.93 (11,000psi, in/sec 0.85 1.10 0.76) 179 MPa, cm/sec 7.34 10.39 5.31 (26,000psi, in/sec 2.89 4.09 2.09) Thermoplastic elastomers of the (AB) n type suitable for forming gun propellants in accordance with the present invention may be made from joining hard blocks and soft blocks from the following lists in the manner taught in the above-referenced EP-A-335499: Soft Blocks poly ethylene glycol (PEG) polycaprolactone (PCP) polytetrahydrofuran (PolyTHF) polypropylene glycol (PPG) amorphous polyoxetanes poly(ethylene oxide-tetrahydrofuran) poly(diethylene glycol adipate) polyglycidzyl nitrate polyglycidyl azide (GAP) Hard Blocks polyallyl acrylate polyisobutyl acrylate poly 1,4-cyclohexylenedimethylene formal, trans poly 1,2-cyclopropanedimethylene isophthalate poly decamethylene adipate poly decamethylene azelaate poly decamethylene oxalate poly decamethylene sebacate polyethylene sebacate polyethylene succinate poly hexamethylene sebacate poly 10-hydroxydecanoic acid poly tert-butyl-isotactic poly nonamethylene terephthalate poly octadecamethylene terephthalate poly pentamethylene terephthalate poly B-propiolactone poly tetramethylene p-phenylenediacetate poly trimethylene oxalate polyethyl vinyl ether poly p -xylylene adipate poly p -xylylene sebacate.

Claims: A low vulnerability ammunition gun propellant composition comprising from 60 to 85 wt. percent of particulates of a high-energy oxidizer and between 15 wt. percent and 40 wt. percent of a thermoplastic elastomeric binder system, said binder system being substantially free of metallic particulates and materials which leave a solid residue, said binder system comprising a thermoplastic elastomeric polymer in which at least one pair of crystalline A blocks flanks at least one amorphous B block and from 0 to 80 wt. percent of a plasticizer, wherein said elastomeric polymer comprises a mixed polyester or a polyester-polyether. A propellant composition according to claim 1 wherein a non-energetic plasticizer is used. A propellant composition according to claim 2 wherein said non-energetic plasticizer is dioctyl phthalate. A propellant composition according to claim 1 wherein an energetic plasticizer is used. A propellant composition according to claim 4 wherein said plasticizer is selected from butanetriol trinitrate, trimethylolethane trinitrate and nitroglycerine. A propellant composition according to any preceding claim wherein the oxidizer from which said oxidizer particulates are formed is selected from tetramethylenetetranitramine, trimethylenetrinitramine, and mixtures thereof. A propellant composition according to any preceding claim, wherein said thermoplastic elastomeric polymer is a block polymer having polyethylene succinate blocks and polydiethyleneglycoladipate blocks. A propellant composition according to any one of claims 1 to 6 wherein the crystalline A blocks of said thermoplastic elastomeric polymer are selected from 1,4-butylene isophthalate, 1,4-butylene terephthalate, 1,6-butylene isophthalate, 1,6-butylene terephthalate and mixtures thereof and wherein the amorphous blocks of said thermoplastic elastomeric polymer are selected from polytetramethylene ether glycol, polyethylene ether glycol and mixtures thereof. A propellant composition according to any preceding claim wherein said propellant is substantially free of chlorine. A propellant composition according to any preceding claim, wherein said crystalline blocks of said thermoplastic elastomeric polymer melt in a temperature range of between 70°C and 105°C.

Inventors: BIDDLE RICHARD ALBERT, WILLER RODNEY LEE, BIDDLE, RICHARD ALBERT, WILLER, RODNEY LEE
Applicants: THIOKOL CORP, THIOKOL CORPORATION
Application year: 1989
Main classification: C06B45
IPC codes: C06B45, C06B23, C06B25, F42B5