AU785509B1 - Double-base propellant - Google Patents

Description

Patents Act WGvMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodeed: Complete Specification Lodged: T Accepted: Published: Priority: Related Art: APPLICANT'S REF.: ICI Case N.27903 Name(s) of Applicant(s): iREIAL CHEMICALL INDUSTRIES LIMITED Address(es) of Applicant(s): Imperial Chemical House, Nillbank, London S.vi.1. England Actual Inventor(s): Gwynn Lewis Address for Service is: PHILLIPS. ORMONDE & FITZPATRICK Patent and Tridc .lark Attornevs 37-41 Oucen Street Mclbourne. Australia. 3000 Complete Specification for the invention entitled: "DOUBLE-BASE PROPELLANT' The following statement is a full description of this invention. including the best nietliod of nerformin T'his i ven. cion reis ctes to double -oP.se nrouellants and more ricu'arly; to suci- proeIllants containing lead salts to modia ct' ei2 ballistic proDerties. A dcuble-b.tse propellant is one containing nitrocellulose plasticised or jelatinsed with a liquid nitric ester, the usual ester being nitroglycerine, although others such as diethylene glycol dinitrate and me tr iol trinitrate may e employed. The inredients may be mixed by a process invol vin mastication and shaped to the final charge by a process such as extrusion. Alternatively, in the case of cast double-base charges, the final charge is usually made b- filling a single (based on nitrocellulose only) or double-base propcllant casting. noter Into a casing, all-owing a 'casting liquid' contain'n. nitroglycerin and a lculd non-cxplosive plasticIser for ni troccllulose to displace all the air in the casing, and curing at about h.0-6,*oC for several days to allow the powder to swell to form a homogeneous rigid or rubbery mass. A cast double-base charge may also be made by filling a container with a reformed slurry of powder in casting liquid with subsequent curing by heat. Conventional propellants having a burning rate at a fIxed ignit ion tepe_.ature in by the expression r =kn where r is Lfhe burnirg rate, p is the Dressure a.nd and n are const.:ts. Nor:all : xponent n has a value of 0.5 to O.f but co:po si.tionz Thy bee ~ n deoe.oped in corporat ing lead salts a:: ballistic modifiers which have the effect of reducing the exponent to zero or nearly so over a substantial range of useful working pressures. The graph of the log r against log p contains a flat portion or plateau and these compositions are therefore termed 'platonised'. It is convenient to refer to the burning rate over the plateau range as the 'plateau burning rate'. These platonised propellants are particularly suited to gas producing and rcoket charges where the desired characteristics are a plateau covering a wide pressure range and, especially for rocket boost charges, a high plateau burning rate. These two requirements are not easily satisfied simu3.tarcously and the composition is usually designed to achieve a compromise of burning rate and plateau characteristic at t -h desired wors:inr pressure. The lead salts currently favoured include the lead salts of salicyli.c, phthalic, P-resorcylic and stearic acids. The effect of such salts is described in the United States Patent Snecification No. 3,088,858 and their effect in conjunction with copper salts is described in the United States Patent Srecification No. 3,138,499. A particularly suitable burning rate is achieved when the ballistic modifier consists of a mixture of lead salicylato and lead I'-resorcylate. However, as described in our core-i-- .- l;;: - aent Application,^ when the moisture content of the .. in poCer is greater than 0.2 % w/', cast double-baF car e2 nlatonised with such mixtures normally show a d ownvare.d drift in burning rate after storage, the drift being raster at elevated temperature. Under normal atmospheric conditions the equilibrium moisture content of such casting powders is of the order of 0.h to 0.60 w/w. It is an object of this invention to provide Iatonised double-base pr-opellant charges of enhanced plateau burning rates, and a further object is to provide cast double-base propellant charges platonised with a mixture of lead salicyl ate and lead r-rescrcylate having enhanced burning rates which are maintained when the charges are stored at elevated temperature. We have now found. that the addition of certain nitrccellul ose crosslin %ing agents to platonised double-base propoliants increases the plateau burning rate without markedly reducing) the plateau pressure range although, in some cases, the pl.teau may occur at a slightly higher pressure level. Thus, in accordance with this invention, a double-base propellant charge comrises at least one platonising lead salt and a nitrocellulose crosslinJ'ing compound having in its molecular structure two or more groups having the formula -N-CH2-LO wherein R is hydrogen or an alkyl group containing I to 8 carbon atoms. Suitable cross slinking compounds include direthvlcl and dialkoxy.-etyl derivatives of aides, urea and cyclic areas, and net-lcA and alkoxeethyl derivEtIves of melamine. Preferre. crossln<:n cor-pounds are iN'-bis(methoxymethyl )uron and hexamethoxymethyl melarr.ine. To achieve a substantial increase in the propellant plateau burning rate the crosslinking agent should preferably be present in an amount in the range from 0.5 to 2.0% by weight of the composition. This amount of crosslinking agent has no effect on the chemical stability or crack life of the propellant charge and generally improves the tensile strength. Although the specified crosslinking agent has the effect of increasing the plateau burning rate of all double-base propellants containing platonisir ead salts, the effect is osneciallv beneficial in cast double-base propellants containing platonising mixtures of lead salicylate and lead P-resorcylate (lead 2,).-dihydroxybenzoate). Such mixtures generally contain lead sal icylate and lead P-resorcylate in the weight ratio of from b-:1 to 1 :L. These platonised propellants have high plateau burning rates but, unless the casting powder is dried to an abnormally low moisture content, the burning rate decreases on storage at elevated temperature to a minimum value about 5 to 15% lower than the rate immediately after curing. Using the specified crosslinking compouiids, noL only are the initial plateau burning rates increased but there is no subsecuent downward drift in burning rate when the propellant is stored at 5 0 1C. In addition to the nitrocellulose, liquid nitric ester and ballistic rodifier, the. pcmr:pllant charge may include plasticisers, s.tab'lisers, glazing agents and the like as are conventional in such charges. Metal powders such as aluminium may also be incorporated, if desired, to raise the heat of combustion.(and consequently the specific impulse). Carbon black is also commonly added to increase the burning rate. The preferred double-base propellant composition comprises as basic ingredients from about 35 to 551 by weight of nitro cellulose, from about 30 to 50% by weight of nitroglycerine or other liquid nitric ester, from 0.5 to 2.0% by weight of the nitrocellu lo se crosslinking compound, from about 1 to 5% by weight of lead salt ballistic modifier and from about' 1 to 3/ by weight of a stabiliser such as. for example, 2-nitrodiphenyl amTine or N-methyi-p-ni tro aniline. The composition preferably also comprises from about 5 to 12% by weight of' a non-explosive plasticlser such as, for example, triacetin or dibutyl phthalate. In the case of cast double-base propellant charges, the nitrocellulose crosslinking agent may be introduced in either the casting powder or casting liquid as an extra additive or to reOlace an equivalent weight of another non-explosive ingredient of a conventional propellant composition. An increased cur_ period may be required to complete the reaction of the cross.inking agent and obtain the maximum plateau burning rat., This may be conveniently achieved by exposing the propol lant to a short period -of storage at elevated temperature after a relatively low temperature cure. or alternatively, by the use lc-e at i 1-L-1 of a higher cure temperature. The invention is further illustrated by the following Examples wherein all parts and percentages are by weight. Examples 1 and 6 do not contain the nitrocellulose crosslinking agent are are not in accordance with the invention-but are included for comparison. The Examples were cast double-base propellant charges having the composition and calculated heats of exPlosion as shown in Table 1 and prepared by the following procedure. In the preparation of the casting powder all the ingred ents of the casting powder shown in Table 1 , together with a ufficient quantity of' solvent for the nitrocellulose, were mixed in an incorporator mixer to form a homogeneous rubbery dough. Th- dough was extruded nto cords and the cords cut on a rotary cutting machine to produce granules. The solvent was removed from the result ing powder granules by hot stoving and the finishod powder was glazed by tumbling with graphite in a rotating pan glazer. The casting powder was filled into a cellulose acetate beaker in a casing and a casting liquid as shown in Table 1 which had been evacuated under high vacuum for about 2 hours to remove !:ater and volatile impurities, was added to displace the air in the beaker. The charge was cured at ' 3 4Cc under nitrogen at a pressure of' 23 psi for h days to allow the powder toswell t f0orn a :c-eZeous rigid. or rubbery- mass. Those ExNamples containing .itrOcllulose cross li n king agent were given an additional cure at 500C in air at atmospheric pressure for 1 to 2 weeks until the maximum burning rate was achieved. The final propellant compositions shown in Table 1 were cal culated from the measured powder to liquid (weight to weight ) ratios of the cast propellant charges. The heats of explosion (expressed in Table 1 as Kilojoules/ K ioram) were calculated in the conventional manner. The ballisti data auotd in the Examnles and illustrated in the accompanying grapes was obtained by burning a 122 mm x 61 mm x 20 mm slab of nro-ellant in a motor at an ignition temperature of 200C and obta in i n. the pressure v time curve. This method of ballistic proof is well known in the art. Exnmrle 1 The cast double-base propellant charge of this Example contained lead salicylate (anhydrous normal salt) and lead p--resorcylate (normal salt monohydrate) and was prepared from a casti.ng pow,!der having, equilibrium water content of 0.53%. The charge had the following burning rates when fresh and after storage at ' 4C. Tt will be noted that there was a significant downward drift in the plateau burning rate after storage.

Pressure (bar) 80 100 120 140 160 180 Burnir Pate nm/sec Fresh (after cure) 21 .42 23.0.2 23.08 22.95 22.51 22-12 After storage at 50'C 5 2 weeks 18.24 19.73 20.47 21.10 21.52 21 .28 5 weeks 16.76 18.08 19 31 19-88 20.02 20.11 24 weeks 16.72 18.12 19.28 19.75 20.05 20.06 The rate of burning v pressure curves at 200. are also shown graphically in Fig. 1 . The plateau burning rate reached 10 a minimum value after 2 to 5 weeks and showed no significant change on further storage at 50*C. Example 2 This Fxaple had the same composition as Example 1 c.xcept5 that the casting powder contained additionally 1 .5 parts by 15 weight of U,N'-bis(rmethoxymethyl )uron. The equilibrium moistur content of the casting powder was 0.525. The charge had the following burning rates which are shown graphically in Fig. 2. Pressure (bar) 100 120 140 160 180 20 20 B-rnn - rnte mm/s e c Fresh (after cure) 25.47 26.91 27.42 27.28 27.15 27.65 After storas ; at C:0c 12 S.c

.

SI i 27 .LO 2-1 32 - It will be seen that, the plateau burning rate was signif icantly higher than that of Examrple 1 and the rate did not ditainish on storage at 500C. but the plateau occurred at a somewhat higher pressure level. The heat of explosion did 5 not differ significantly from that of Example 1. Example 3 The composition of the cast double-base propellant char2;e of this Exampiole was the same as Examole 1 , except that the casting liquid contained 3.0 parts of N,N'-bis(methoxymethyl) 10 uron instead of the same weight of the triacetin. The equilibriumn water content of the casting powder was 0.53%. The propellant had an increased plateau burning rate compared wih Example 1 and showed essential _ally stable ballistic properties when stored at 50 0 C, The burning rates are given below and 15 are shown r-raphically in Fig. 2. Pressure (bar) 120 140 160 180 200 220 Fresh (after cure) 27.31 28.23 28.62 28.51 28.149 29.14I After storaEe at 500C 20 12 wee s 27.20 28.19 28.60 28.5 28.8.7 29.16 The plateau burning rate was similar to that .of Exaple 2 and there . no reduction of the burning rate after storage at 500c. 2 5 of .Examp.le was the same as hanple 2 exe.t that the casting powder contaIrne3d only 2.0 arts of lead P-resorcylate. The equilibrium water content of the casting powder was 0.51/. The propellant had simiilar plateau characteristics to Example 5 1 (fresh after cure) but there was no reduction in the burning rate after storage at 500C. The burning rates are given below and are shown graphically in Fig. 2. Pressure (bar) 80 100 120 140 160 180 urnin :. -_.7./SeC 10 Fresh (after cure) 21 .35 2 2 .94 23.59 23.75 23.03 21. After storage at 50%C 12 weeks |21 .31 22.87 23.55 23.79 23.05 21.0 Exanrle The comnosition of the cast double-base procellant of this 15 Exawme .as the same as Example 1 except that the casting liquid cont ined 3 parts of hexamethoxymmthyl melamine instead of 3 parts of trIcetin. The ecuilibrium water content of the cast ing powder was 0.56 . The propellnt had a plateau burning rate mar'.ec-; higher than that of Example 1 and slightly higher 20 than those of Examples 2 and 3, the plateau occurring at a highr press~~ re leel. The burning rate did not reduce on storage at 50Cc. The burning rates are given below and shown 'raaCa: 7 . 7 ig 7 Pressur (bar) 120 140 160 180 200 220 Burni'- rate :-.n/sec Fresh (after cure) 29.16 30.55 31 .48 31 .66 31 .49 31 .43 After storage at 50 C 5 12 weeks 29.15 30.52 31.45 31.61 31 .52 31 -4.8 Example 6 The cast double-base .propellant of this Example was prepared with a lower energy casting powder containing as ballistic modi fier lead stearnte, (an-:-drou:, norrial salt) in addition to lead 10 P-resorcylate and lead salicylate, and no aluminium. The equilibrir 1:ater content of the casting powder was 0.

4 8,. The following:; b rning rates, which are graphically shown In Fig. 3 were observed. There was no si inificant reduction in burninr i'ate on storage at 50 3 C. Pressure (bar) 60 8o 1 160 Burnin- rate :rm/.sec Fresh (after cure) 12.81 13.75 13.68 13.46 15.20 17.0 4 After storage at 5i 0 C 12 weeks 12.78 13.68 13.65 13.143 15.09 16.98 20 Examole 7 The propellant of this Examcle had the same composit icn as Etxazple :e st licuid contairod 3 parts of No replace 3 arts triacetin. The ec~uil: .cu zc:n; of ne c::~r\ar~ The folloi:ing buring rates, hh are shown graphically in Fig. 3, were observed. PreSsure (bar) 60 180 100 120 1)10 160 Burninri rate mi/sec Fresh (after cure) 114.16 16.21 16.85 16.79 15.87 17. 0 After storage at 50 0 C 12 weeks 114..09 16.20 16.81 16.76 15.83 17.

4 0 Example 7 had a significantIy higher plateau burning ratc than Exampole 6 which also was not reduced by storage at 50 0.

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Claims (10)

1. A eee1l bc nrorellaft char-ee7zr a atc~ ,J.La ti-js-Fc,--ad sal-t and a nitroC ) ulos 0 UO,' ross 1r.nkiflg como~n harin i E t, Iocla structllr' tor orore group,- bavir~ the ir~n'-C -N~ -, -- o~In F. is hydrogen or a.aci gro-ap contafinflg I to carbon atom-s.

2. A rclat hr>as caime>.di Claiam 1 WCriflthe prplAt ropO. 1Y2 omrSC a-baicigrliftl fo 35 to 55:! w/u nitrocclilos, from. 30 t,) :50' of Jainul5d nl-trlic ester, fr om 0.5 to 2.0C u/ of the nitrocellul ose, c r)sr 1~ n",ic1- . coimpounds specified in Cim1 ,frc'r:' I to 51< 1,/w o,";a saltbcl stc, -od aie an Frr: o 3 1/w of

3. - -r) 1 - Ia I It c " 1,- - a s clIa 17d" I ., 2 ;.'ne-jo in t he -11 01tonn so co!rI-:r-sS frori 5to 1 2'- by ce-h o a rion exul osive u 1 n ~stic-1-er.

L~. A p ropel] c- anT t cr -e rF.e as clmed in Claim 3 -'herein the plasicisr' omon--ises t-riacetin oi- dibu,,"Yl ptTaa

5. A Propollu-nt charrae a.,s c1 nirled in an;one of Clis2 toliwhren hestab'oiiseyr ccin-rises 2-nitrod-I~enfl e-fIn n or N--.eth-yIT1P-TfItr'o aniline.

6. Apo:lart ha oas cla'r-3o In any; one c- Cais WIo 0, 1hri t e o E crccl -cs ci'~ _'~ cno, nl. comr, e a an1E rh'I .0 aloI'1 ru K I . C & i ! C3fl L. ,. 02 ( 0 c. >'. 0 e " . ). . .

7. A propellant charge as claimed in any one of Claims 1 to 6 wherein the crosslinking agent is present in an amount in the range from 0.5 to 2.01 w/w of the composition.

8. A cast double-base propellant charge in accordance with any one of Claims 1 to 7 containing a platonisirg mixture of lead salicylate and lead B-resorcylate.

9. A propellant charge as claimed in Claim 8 wherein the platonisin.g mixture comprises lead salic- late and lead p-rezorcylate in the weight ratio of from 4:1 to 1 :1L.

10. A double-base propellant charge substantially as heroin docribed with particular reference to any one of Examples 2 to usive and3 Exmple 7 DATED: 7th Hay, 1976 IMPERIAL CHEMICAL INDUSTRIES LIMITED By its Patent Attorneys PHILLIPS ORMONDE & FITZPATRICK