Adhesive liner for case bonded solid propellant

Abstract

Bonding of hydroxy terminated polybutadiene based propellant grains to rocket motor liners and of the liner to the motor case is greatly improved by incorporating an aziridine in a urethane cured hydroxy terminated polybutadiene based liner.

Description

BACKGROUND OF THE INVENTION

In general, case bonded solid propellant grains possess greater strength because of the additional support given by their adherence to the rocket motor casing in which they are contained. To insure the maximum strength and to prevent separation of the propellant grain from the motor walls which would create voids and therefore a potential for uneven burning, it is desirable that the propellant grain be bonded to the motor casing uniformly and firmly.

This is usually accomplished by very careful cleaning and descaling of the inside surface of the metal motor casing, followed by coating the inside surface with a liner, normally the binder used for the propellant grain, then casting the propellant grain in place and curing. The liner serves as a bonding agent between the casing and the propellant grain, as an inhibitor preventing burning down the outer surface of the grain and as an insulator, protecting the casing from the heat and gases of combustion when burning has progressed to the point at which the casing would otherwise be exposed.

The present invention relates to case liners containing aziridine compounds, methods for their formulation and use. The liners are formulated from hydroxy terminated polybutadiene based polyurethane binders.

U.S. Pat. No. 3,974,131, issued Aug. 10, 1976, is related to the use of various acrylated aziridinyl monomers and polymers and their use as adhesive promoters in vinyl formulations. The compounds contemplated for use in this invention are intended for use in enhancing the adhesiveness of polyurethanes.

SUMMARY OF THE INVENTION

The invention provides a composition of matter which comprises a liquid hydroxy terminated polybutadiene and an aziridine.

The tangible embodiments of this composition aspect of the invention possess the inherent applied use characteristic of being curable with diisocyanate curing agents to produce solids which are suitable for use as casing liners in case bonded solid fuel rocket motors.

Particularly preferred embodiments of this composition aspect of the invention are those wherein the aziridine is selected from:

Trimesoyl-1-(2-ethyl-aziridine), Bis-isophthaloyl-1-(2-methyl)-aziridine, Bis-isosebacoyl-1-(2-ethyl)-aziridine, Bis-isosebacoyl-1-aziridine, Tri-[-1-(2-methyl)-aziridinyl]-Phosphine oxide, Bis-[1-(2-methyl)-aziridinyl]-ethyl sulfone, or polyfunctional aziridines PFAZ300 or PFAZ301-supplied by Ionac Chemical Company, Birmingham, N.J.

The invention also provides a process for case bonding a hydroxy terminated polybutadiene based polyurethane bound solid propellant grain to a rocket motor casing which comprises:

(a) lining a rocket motor casing to which it is desired to case bond said propellant grain with a cured liner comprising an aziridine and a hydroxy terminated polybutadiene cured with a polyisocyanate curing agent and; (b) casting said hydroxy terminated polybutadiene based propellant grain in an uncured form in the desired configuration into said lined rocket motor; and (c) curing said propellant grain.

The invention also provides an improved case bonded solid propellant rocket motor containing a hydroxy terminated polybutadiene based polyurethane bound propellant grain wherein the improvement comprises the incorporation of an aziridine into the casing liner of said rocket motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The practice of the invention will now be illustrated with reference to a specific embodiment thereof, namely a rocket motor casing liner comprising the hydroxy terminated polybutadiene based polyurethane binder having the trade designation R-45M and the aziridine, trimesoyl-1-(2-ethyl)-aziridine.

The hydroxy terminated polybutadiene polymer R-45M is sold by Arco Chemical Company and its structure may be represented as: ##STR1##

The R-45M is blended with the trimesoyl-1-(2-ethyl)-aziridine, a conventional diisocyanate curing agent, conveniently DDI, a C.sub.36 liquid aliphatic diisocyanate sold by General Mills, and, if desired, one or more fillers such as carbon black. The blended composition is vacuum degassed and cast as a liner into a previously descaled and degreased steel rocket motor casing and cured at elevated temperature from about 145.degree. F. to about 200.degree. F. After cure or partial cure is complete a hydroxy terminated polybutadiene based propellant grain, such as, for example, a propellant consisting of R-45M as a binder ranging from 81/2 to about 24% by weight and including such conventional compounding ingredients as dioctyl adipate as a plasticizer, a polyisocyanate curing agent such as isophorone diisocyanate or the above mentioned DDI up to about 2% by weight, an oxidizer such as, ammonium perchlorate up to 75 to 90% by weight, if desired, a portion of metallic fuel such as aluminum metal, and other conventional combustion stabilizers, flame coolants and the like may be cast and cured in place in the lined motor in conventional fashion.

In addition to the R-45M binder specifically illustrated herein, it will be obvious to one skilled in the art that any liquid hydroxy terminated polybutadiene polymer known to be useful for propellant grain binding will be applicable in the invention. Illustrative of these polymers are, for example, those sold under the trade designations R-45HT, R-15M, CS-15, by Arco Chemical Company, and Butarez HTS by Philips Petroleum Company. It will similarly be evident to one skilled in the art that in addition to the DDI illustrated herein any polyisocyanate may be used to effect cure of the binder. Illustrative of other suitable polyisocyanates are, for example, isophorone diisocyanate, hexamethylene diisocyanate, PAPI polymeric diisocyanates sold by the Upjohn Company and Desmodure N-100 a polyisocyanate sold by I. G. Farben Industries, and also such isocyanates as: 1-methoxyphenyl-2,4-diisocyanate, 1-methyl-4-methoxyphenyl-2,5-diisocyanate, 1-ethoxyphenyl-2,4-diisocyanate, 1,3-dimethoxyphenyl-4,6-diisocyanate, 1,4-dimethoxyphenyl-2,5-diisocyanate, 1-propoxyphenyl-2,4-diisocyanate, 1-isobutoxy-2,4-diisocyanate, 1,4-diethoxyphenyl-2,5-diisocyanate, toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, diphenylether-2,4-diisocyanate, naphthalene-1,4-diisocyanate, 1,1' -dinaphthalene-2,2'-diisocyanate, biphenyl-2,4-diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 3,3'-dimethoxybiphenyl-4,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-2,2'-diisocyanate, 3,3'-dimethoxy diphenylmethane-4,4'-diisocyanate, benzophenone-3,3'-diisocyanate, ethylene diisocyanate, propylene diisocyanate, butylene diisocyanate, pentylene diisocyanate, methylbutylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, dipropyl diisocyanate ether, heptamethylene diisocyanate, 2,2-dimethylpentylene diisocyanate, 3-methoxy-hexamethylene diisocyanate, octamethylene diisocyanate, 2,2,4-trimethylpentylene diisocyanate, 3-butoxyhexamethylene diisocyanate, 1,3-dimethylbenzene diisocyanate, 1,4-dimethyl benzene diisocyanate, 1,2-dimethylcyclohexane diisocyanate, 1,4-dimethylcyclohexane diisocyanate, 1,4-dimethylbenzene diisocyanate, 1,4-dimethylnaphthalene diisocyanate, 1,5-dimethylnaphthalene diisocyanate, cyclohexane-1,3-diisocyanate, cyclohexane-1,4-diisocyanate, 1-methylcyclohexane-2,4-diisocyanate, 1-methylcyclohexane, -2,2-diisocyanate, 1-ethylcyclohexane-2,4-diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, dicyclohexylmethylmethane-4,4-diisocyanate, dicyclohexyldimethylmethane-4,4'-diisocyanate, 2,2-dimethyldicyclohexylmethane-4,4'-diisocyanate, 3,3',5,5'-tetramethyldicyclohexylmethane-4,4'-diisocyanate, 4,4'-methylenebis (cyclohexylisocyanate), ethylidene diisocyanate, 4,4'-diphenyl diisocyanate, dianisidine diisocyanate, 1,5-naphthalene diisocyanate, m-phenylene diisocyanate, isopropylidene bis (phenyl or cyclohexyl isocyanate), 1,3-cyclopentylene diisocyanate, 1,2-cyclohexylene diisocyanate, 1,4-cyclohexylene diisocyanate, 4,4',4"-triphenylmethane triisocyanate, 1,3,5-triisocyanate benzene, phenylethylene diisocyanate, and the like.

It will also be apparent to one skilled in the art that in addition to trimesoyl-1-(2-ethyl) aziridine illustrated herein other aziridines will be full equivalents for the purposes of this invention. Illustrative of these are, for example, tris-2,4,6-(1-aziridinyl)-1,3,5-triazine, bis-isophthaloyl-1-(2-ethyl)-aziridine, bis isosebacoyl-1-aziridine, tri-[-1-(2-methyl)-aziridine] phosphine oxide, bis-[1-(2-methyl)-aziridinyl] ethyl sulfone, polyfunctional aziridines sold under the trade designations PFAZ300, PFAZ301 by Ionac Chemical Company, and the like. It will also be apparent that in addition to the carbon black illustrated herein as being optionally employed as a filler other materials will be suitable for use as fillers and may also optionally be employed singly or in combination with each other or with carbon black. Illustrative of these are: titanium dioxide, silicon dioxide, silica, aluminum oxide, milled glass, and alumino silicate clays.

The aziridines of the invention may be incorporated at from about 1 to 10% by weight of the total weight of the liner mix. Preferably they may be incorporated at from about 4 to 8%, conveniently about 6% by weight of the total.

The following example further illustrates the best mode contemplated by the inventors for the practice of their invention.

EXAMPLE

The following Example illustrates the preparation of a typical liner composition using the adhesion enhancers of the invention.

To R-45M (41.85 weight %) is added DDI (12.15 weight %) trimesoyl-1-(2-ethyl)-aziridine (6.0 weight %) and carbon black (40.0 weight %), the aziridine being melted prior to addition and the entire mixture blended for about 45 minutes and then degassed by conventional techniques such as vacuum mixing or slit deaeration. The mix is then cast by standard procedures and cured at 170.degree. F. for 96 to 168 hours. The normal thickness obtained in one application to a motor casing is from 1/32 to 1/16 inch.

Typical physical properties obtained from cured samples of the mix are as follows:

______________________________________Tensile Strength (ASTM D 412) Test Temp. (.degree.F.) 0 77 150 200______________________________________Stress, Ultimate (psi) 1,043 543 331 258Strain, Ultimate (%) 464 431 324 282Stress-100% Strain (psi) -- 118 -- --Stress-300% Strain (psi) -- 379 -- --Stress-500% Strain (psi) -- 509 -- --Tear Strength, pli -- 111 -- --Hardness (Shore A atAmbient Temperature) 52______________________________________ ______________________________________Ageing Properties (Test at 77.degree. F.) Ageing AgeingAgeing Time Temperature 130.degree. F. Temperature 170.degree. F.(Weeks) Ult. Stress Ult. Strain Ult. Stress Ult. Strain______________________________________0 501 389 501 3892 513 325 781 1464 548 339 847 1218 630 267 878 13612 589 194 861 136______________________________________

Typical Bond Properties

Samples of the above mix were cured on steel surfaces, grit blasted and vapor degreased, or were cured and typical propellant and insulation material formulations cured on the liner surface. Tensile adhesion and peel strengths were then determined with the results shown which may be compared to 6-9 pli peel strength, with thin coat of propellant failure, for the bond at ambient temperature (77.degree. F.) between similar propellant and liner formulations without aziridine where the liner is not fully cured prior to cure of propellants, and 3-6 pli where the liner is fully cured prior to propellant addition.

__________________________________________________________________________ Peel (pli)Adjacent Test Temp. Tensile Adhesion (pli) Type ofMaterial (.degree.F.) Load Type of Failure Initial Average Failure__________________________________________________________________________Steel -65 1,333 2-Liner 7197.3 -- 3-Tab 1-Plate Beat BrokeSteel 77 249.3 3-Liner 43.0 35.7 3-Thin Coat LinerSteel 165 184.7 3-Thin Coat 13.8 16.0 3-Thin Coat Liner LinerSteel 200 170.7 3-Thin Coat 27.3 15.7 3-Thin Coat Liner LinerSteel 250 160.0 3-Liner 25.5 14.5 2-Liner Pulled from Shim 1-Thin Coat LinerSteel 300 117.7 3-Liner 17.6 10.6 3-Liner Pulled from ShimAsbestos filled -65 1,586 2-Thin Coat of 75.5 105.0 2-insulationPolyisoprene Liner 1-Thin CoatRubber Based 1-insulation of LinerInsulationAsbestos filled 77 163.0 3-Thin Coat 14.0 16.1 5-BondPolyisoprene of LinerRubber BasedInsulationAsbestos filled 165 103.0 3-Thin Coat 4.0 9.1 3-Thin CoatPolyisoprene of Liner of LinerRubber BasedInsulationAsbestos filled 200 107.5 2-Thin Coat 4.4 7.4 3-Thin CoatPolyisoprene of Liner of LinerRubber BasedInsulationAsbestos filled 250 126.3 3-Thin Coat 12.4 12.4 2-Thin Coats ofPolyisoprene of Liner Liner-1-Thin CoatRubber Based of Liner, and LinerInsulation pulled from ShimAsbestos filled 300 129.0 2-Thin Coat of 12.7 11.1 2-Thin Coat ofPolyisoprene Liner-1-Liner LinerRubber Based 1-Liner pulledInsulation from ShimR45M Based -65 808 2-Thin Coat 75.0 49.7 3-Thin CoatAmmonium Propellant PropellantPerchlorate 1-Thin CoatOxidizer Propellant &Isophorone Linerdiisocyanatecured pro-pellant**R45M Based 77 176 3-Propellant 24.5 46.1 3-LinerAmmoniumPerchlorateOxidizerIsophoronediisocyanatecured pro-pellant**R45M Based 165 114 3-Propellant 25.4 27.7 2-Liner +Ammonium PropellantPerchlorate 1-LinerOxidizerIsophoronediisocyanatecured pro-pellant**__________________________________________________________________________ **Liner precured 24 hours at 145.degree. F. __________________________________________________________________________Typical bond properties on ageing were also determined-Tests were run at77.degree. F. Storage Tensile Adhesion PeelAdjacent Time Temp. (pli) (pli)Material (Weeks) (.degree.F.) Load Failure Initial Average Failure__________________________________________________________________________R-45M Based Propellant Containing 0 111 174.7 5-Propellant 29.6 32.5 5-PropellantAmmonium Perchlorate Oxidizer and LinerCured with DDI, Liner Precured 32Hours at 145.degree. F.R-45M Based Propellant Containing 2 130 215.7 5-Propellant 24.7 31.1 5-PropellantAmmonium Perchlorate OxidizerCured with DDI, Liner Precured 32Hours at 145.degree. F.R-45M Based Propellant Containing 2 170 184.5 5-Propellant 20.8 -- 3-Liner pulled fromAmmonium Perchlorate Oxidizer Shim-2-Liner +Cured with DDI, Liner Precured 32 PropellantHours at 145.degree. F.R-45M Based Propellant Containing 4 130 217.7 5-Propellant 28.9 30.9 5-Propellant andAmmonium Perchlorate Oxidizer linerCured with DDI, Liner Precured 32Hours at 145.degree. F.R-45M Based Propellant Containing 4 170 178.9 5-Propellant 21.0 -- 3-Liner Pulled fromAmmonium Perchlorate Oxidizer Shim-2-Liner +Cured with DDI, Liner Precured 32 PropellantHours at 145.degree. F.R-45M Based Propellant Containing 8 130 225.9 5-Propellant 24.4 27.2 4-Propellant + TheirAmmonium Perchlorate Oxidizer Coat LinerCured with DDI, Liner Precured 32 1-Liner pulled fromHours at 145.degree. F. ShimR-45M Based Propellant Containing 8 170 153.5 5-Propellant 16.0 9.2 4-PropellantAmmonium Perchlorate Oxidizer 1-Liner pulled fromCured with DDI, Liner Precured 32 ShimHours at 145.degree. F.R-45M Based Propellant Containing 12 130 228.8 5-Propellant 24.9 -- 5-Liner pulled from ShimAmmonium Perchlorate OxidizerCured with DDI, Liner Precured 32Hours at 145.degree. F.R-45M Based Propellant Containing 12 170 155.8 5-Propellant 14.5 -- 4-Liner pulled from ShimAmmonium Perchlorate Oxidizer 1-PropellantCured with DDI, Liner Precured 32Hours at 145.degree. F.__________________________________________________________________________

Claims

1. A process for case bonding a hydroxy terminated polybutadiene based polyurethane bound solid propellant grain to a rocket motor casing which comprises:

(a) providing a rocket motor casing to which it is desired to case bond said solid propellant grain with a cured liner comprising an aziridine and a hydroxy terminated polybutadiene cured with a polyisocyanate curing agent;

(b) casting said hydroxy terminated polybutadiene based propellant grain, in an uncured form, into the desired configuration in said rocket motor; and

(c) curing said propellant grain.

2. The process of claim 1, wherein: said cured liner contains from about 1% to 10% of said aziridine.

3. An improved case bonded solid propellant rocket motor having a casing liner and containing a hydroxy terminated polybutadiene based polyurethane bound propellant grain wherein the casing liner for said propellant is based on a hydroxy terminated polybutadiene and wherein the improvement comprises having a precured casing liner having incorporated therein an aziridine into (the) said precured casing liner of said rocket motor.

4. A case bonded solid propellant rocket motor wherein said motor contains a bonded casing liner consisting essentially of hydroxy terminated polybutadiene and from about 1% to 10% of aziridine.