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:
The Government has rights in this invention pursuant to Contract No. J243229D, awarded by the Department of the Army to Thiokol Corporation. 
    
    
     This is a continuation of application Ser. No. 888,058, filed Mar. 20, 1978, which in turn is a continuation of application Ser. No. 757,260, filed Jan. 6, 1977, both now abandoned. 
    
    
     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 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° F. to about 200° 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&#39; -dinaphthalene-2,2&#39;-diisocyanate, biphenyl-2,4-diisocyanate, 3,3&#39;-dimethylbiphenyl-4,4&#39;-diisocyanate, 3,3&#39;-dimethoxybiphenyl-4,4&#39;-diisocyanate, diphenylmethane-4,4&#39;-diisocyanate, diphenylmethane-2,4&#39;-diisocyanate, diphenylmethane-2,2&#39;-diisocyanate, 3,3&#39;-dimethoxy diphenylmethane-4,4&#39;-diisocyanate, benzophenone-3,3&#39;-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&#39;-diisocyanate, dicyclohexylmethylmethane-4,4-diisocyanate, dicyclohexyldimethylmethane-4,4&#39;-diisocyanate, 2,2-dimethyldicyclohexylmethane-4,4&#39;-diisocyanate, 3,3&#39;,5,5&#39;-tetramethyldicyclohexylmethane-4,4&#39;-diisocyanate, 4,4&#39;-methylenebis (cyclohexylisocyanate), ethylidene diisocyanate, 4,4&#39;-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&#39;,4&#34;-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° 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. (°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° F.)    Ageing          AgeingAgeing Time    Temperature 130° F.                    Temperature 170° 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° 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   (°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 &amp;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° F. 
    
     
         __________________________________________________________________________Typical bond properties on ageing were also determined-Tests were run at77° F.             Storage   Tensile Adhesion                                  PeelAdjacent          Time Temp.                       (pli)      (pli)Material          (Weeks)                  (°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° 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° 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° 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° 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° 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° 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° 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° 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° F.__________________________________________________________________________