GB1604197A - Polyester bonding agents for htpb propellants - Google Patents

Description

(54) POLYESTER BONDING AGENTS FOR HTPB PROPELLANTS (71) I, THE MINISTER OF NATIONAL DEFENCE, of Her Majesty's Canadian Government, Ottawa, Ontario, Canada, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to composite solid propellants and is concerned more particularly with an improved hydroxy-terminated polybutadiene (HTPB) binder for such propellants which includes a bonding agent comprising two components.

Castable propellants using polybutadiene-based binders in conjunction with, for example, ammonium perchlorate oxidizer are well known. The mechanical and ballistic properties of such composite solid propellants are very dependent upon the quality of the adhesive bond between the binder and the oxidizer, as discussed in United States Patent Specification 3,745,074. In the prior art, propellants are known which have reasonably good adhesion between the fuel binder and oxidizer, but such propellants have other disadvantages.

One such propellant utilizes carboxyl-terminated polybutadiene (CTPB) binders cured with a curing agent comprising aziridines or epoxides or a mixture thereof. Propellants using such binders have high elongation over a wide temperature range and a relatively high ability to carry solid particles, referred to as solid loading. The binder-oxidizer adhesion is fairly good, as the preferred aziridine curing agent, tris(2 - methylaziridinyl - I)phosphine oxide (known as MAPO) polymerizes around the ammonium perchlorate oxidizer particles to form a strong layer linked to the main portion of the binder by chemical bonds.

However, this type of propellant has the disadvantages of a complicated curing system, poor ageing characteristics when exposed to high temperature and the properties of the polymer binder vary from one batch to another.

Another type of propellant, developed in an attempt to overcome the disadvantages mentioned, is the HTPB-based composite propellant which utilizes hydroxy-terminated polybutadiene prepolymers. These systems are cured with diisocyanates. These propellants require the use of a bonding agent to provide good adhesion of the binder to the oxidizer particles. Aziridine polyesters are efficient bonding agents for these propellants, a preferred aziridine polyester being a reaction product of MAPO with diacids. However, this preferred bonding agent must be synthesized on site in relatively small quantities, as it has a short shelf life at room temperature and must be kept at low temperature to prolong its usefulness.

Also a relatively high concentration is required, namely about 2% to 3% of aziridine polyester in the binder, to provide efficient adhesion between the oxidizer and binder, particularly when the solid loading is high.

It has been established that a combination of two bonding agents, an aziridine polyester (PAZ) and an amine polyester (PAM), with the HTPB-type propellants provides proper adhesion to maintain the stress-strain capability of the propellant, and enables a reduction by a factor of 3 to 4 in the amount of aziridine polyester required as compared to the known systems. Propellant systems according to the present invention have for example enhanced mechanical properties, including improved elongation at maximum load and at rupture with comparable tensile strength and initial moduli, with reference to HTPB propellant systems utilizing only an aziridine polyester bonding agent.

According to German Patent Specification (Offenlegungsschrift) 2,701,494, a curable binder for use in forming a castable propellant comprises: 1. a hydroxy-terminated butadiene polymer, 2. a diisocyanate curing agent, and 3. a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and at least one polycarboxylic acid, and an amine polyester, which is the reaction product of an alkanolamine and a saturated aliphatic polycarboxylic acid. Additionally, an antioxidant such as di tertbutylhydroquinone or phenyl - beta- naphthylamine may be included, together with isodecyl pelargonate as a plasticizer.

The aforesaid patent specification further describes a castable composite propellant utilizing the aforementioned binder and having dispersed therethrough a finely-divided perchlorate oxidizer in di or trimodal distribution, and other optional ingredients such as a metal additive. The disclosure includes a method for the manufacture of the composite propellant, which provides good dispersion of the ingredients and reproducibility in processing, mechanical and ballistic properties.

The HTPB polymers suitable for use in the binder and propellant systems according to the aforesaid specification are, for example, of the type described in Canadian Patent Specification 891,562 and United States Patent Specification 3,792,003. The prepolymers therein described are hydroxy telechelic polybutadienes, particularly hydroxy-terminated polybutadienes which are desirably rich in cis isomer. They are obtained by the reaction of a mono-epoxy compound and the corresponding carboxy polymers. The product is a polymer having hydroxyl groups including primary and secondary such groups attached adjacent the ends of the polymer molecule, and preferably at the ends of the molecule. The mono-epoxy compound is an organic compound containing a single epoxy group, including mono-epoxy resins; particularly preferred are epoxy compounds of the formula:

wherein R and R' are the same or different and each is a hydrogen atom, an aryl group or an alkyl group, preferably a lower alkyl group, and more particularly 1,2 alkylene oxides such as propylene oxide and 1,2 - butylene oxide which produce secondary hydroxyl groups when reacted with the carboxyl telechelic polymer. The carboxy-terminated polybutadienes are suitably those supplied under the trade mark HC 434 by Thiokol Chemical Corporation and under the trade mark HYCAR-CTB by B. F. Goodrich and Company. A typical hydroxy-terminated olybutadiene prepolymer has a molecular weight of about 3200 and is believed to have the following formula:

wherein x=l-75 for cis and trans butadienes and yew30 for vinyl, but y/x should be between 0 and 0.4.

Up to now, a preferred HTPB prepolymer is that sold under the trade mark Poly bd R45M by Arco Chemicals Co., having the following formula:

wherein n=44--60 and the polybutadiene structure is 60% trans - 1,4, 20% cis - 1,4 and 20% vinyl - 1,2.

Over the last six or seven years, R-45M prepolymer has been widely accepted as the best polymer for most of the hydroxyl-terminated polybutadiene (HTPB) propellants currently in use or in development in many countries. Despite this preference for R-45M by the propellant industry, composite propellants accounted for only a small portion of the total demand for R-45M. Recently, major consumers of this product in other industries shifted to a related prepolymer sold under the trade mark R-45HT, also by Arco. As a result, the price of R-45M is now more than double that of R-45HT. Because most propellant compositions comprise less than 10 percent prepolymer, the net effect on the total cost of the propellant is relatively low. It was therefore dffficiilt to justify a shift to another prepolymer, e.g. R-45HT, on cost considerations alone. However, it is possible that R-45M will become difficult to procure in the future due to a very reduced market. In view of the similar structure of R45M and R-45HT, it was decided to experiment with the latter as a possible alternative.

If R45HT was to be employed instead of R45M, a further problem was to find a substitute for N - phenyl - beta - naphthylamine (PBNA), the antioxidant normally used in HTPB propellants based on RASM. Moreover, within a few years, PBNA will probably disappear from the market because it has been Identified as a carcinogenic material and a possible danger to human health.

According to one aspect of the present invention, a curable binder for use in forming a castable composite propellant comprises: (i) a hydroxy-terminated butadiene polymer of the structural formula

wherein n=57--65, (ii) a diisocyanate curing agent, (iii) a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and a polycarboxylic acid, and an amine polyester, which is the reaction product of an alkalolamine and a saturated aliphatic polycarboxylic acid, and (iv) an antioxidant comprising by weight 25%75% of di - tert butylhydroquinone in conjunction with 75%-25% of one or other of N - phenyl N' - (1,3 - dimethylbutyl) - p - phenylene - diamine and N - phenyl - N' cyclohexyl - p- phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer.

According to another aspect of the invention, a method for the manufacture of a composite propellant comprises: (i) mixing together at atmospheric pressure liquid ingredients comprising a hydroxy-terminated butadiene polymer of the structural formula:

wherein n=5745, a plasticizer, a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and a polycarboxylic acid, and an amine polyester, which is the reaction product of an alkalolamine and a saturated aliphatic polycarboxylic acid, and an antioxidant comprising by weight 25%75% of di - tert - butylhydroquinone in conjunction with 75%25% of one or other of N - phenyl - N' - (1,3 - dimethylbutyl)- p - phenylene - diamine and N phenol - N' - cyclohexyl - p - phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer, (ii) adding solid ingredients comprising finely-divided aluminium, ferric oxide as a burning rate catalyst and ammonium perchlorate oxidizer in an amount comprising approximately 75% by weight of the total amount of such oxidizer required and mixing to give a substantially even dispersion, (iii) heating the mixture at approximately 60"C under vacuum for about 1 hour, (iv) releasing the vacuum, (v) adding and mixing in the remainder of the ammonium perchlorate oxidizer and a diisocyanate curing agent, (vi) reapplying vacuum and heating to about 600C for about 45 minutes, and (vii) casting the resulting mixture in moulds under vacuum.

R-45HT has the same formula as R-45M above, the main difference being in the value of "n" which in the case of RA5HT is 5765. Other differences and similarities will be apparent from the following table.

TABLE I R-45M R-45HT Typical functionality 2.1-2.3 2.6-2.9 OH (m. equiv./g) 0.75 0.83 Typical equivalent weight 1320 1200 Viscosity (poises) at 300C 50 50 Moisture (t/Jw) 0.05 0.05 The R45HT HTPB prepolymer is the major ingredient in the propellant binder of the invention suitably being used in an amount from 50% to 85 by weight, preferably in an amount of 60%65% by weight of the binder composition.

An antioxidant is utilized with the prepolymer and, according to the invention, comprises by weight 25%75% of di - tert - butylhydroquinone in conjunction with 75%25% of one or other of N - phenyl - N' - (1,3 - dimethylbutyl) - p phenylene - diamine and N - phenyl - N' - cyclohexyl -p - phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer.

TABLE II Trade Name Supplier Chemical Name PBNA Uniroyal Phenyl-beta-naphthylamine A02246 Cyanamid Hindered bis-phenol "NAUGARD Q"* Uniroyal Polymerized trimethyl dihydroquinoline Octamine Uniroyal Diphenylamine-diisobutylene reaction product "Flexzone 7L"* Uniroyal N-phenyl-N'-(l,3-dimethyl- butyl)-p-phenylene-diamine "Flexzone 6H"* Uniroyal N-phenyl-N'-cyclohexyl-p- phenylene-diamine UOP-36 Universal Oil N-phenyl-N'-cyclohexyl-p Oil Products phenylene-diamine DTBHQ Eastman Di-tert-butylhydroquinone (*=Regd. Trade Mark).

A synergistic effect on the pot lives of mixes was obtained when a combination of DTBHQ with UOP-36 or "Flexzone 7L" was added to the propellant mix to the extent of 1 percent by weight of the prepolymer.

The diisocyanate curing agent desirably used in the binder composition according to the present invention is for example 2,4-tolylene diisocyanate (TDI), 1,6 - hexamethylene diisocyanate (HMDI), or most preferably DDI, which is a mixture of isomers of diisocyanate containing 36 carbon atoms prepared by dimerization of 18 carbon fatty acids. The preferred diisocyanate is DDI for the purposes of the present propellant binder systems. The isocyanate/hydroxyl (NCO/OH) ratio is adjusted to optimize the mechanical properties of the resulting propellant as recognized with previous systems. For the present purposes, a typical NCO/OH equivalent ratio is in the range of 0.65--0.95. Preferably, this ratio is 0.75.

In order to facilitate mixing of the binder composition with the solid oxidizer in preparing the castable propellants of the invention, a plasticizer is desirably included in the binder composition, preferably in a proportion of 15 to 30 percent by weight based on the total weight of the binder composition. Compounds suitable as plasticizers are well known in the polymer art and, in the present case, may be for example dioctyl adipate (DOA), diethyl hexyl azelate (DEHA) or isodecyl pelargonate (IDP). However, the preferred plasticizer is IDP.

The aziridine polyester component of the bonding agent system is, as indicated, the reaction product of an aziridinyl phosphine oxide with a polycarboxylic acid. These materials are disclosed, for example, in United States Patent Specifications 3,745,074 and 3,762,972. More specifically, they are the reaction products of di- or tri-functional aziridinyl phosphine oxide or its derivatives with organic molecules which are polyfunctional with respect to carboxyl groups and which may contain one or more hydroxyl groups In their structures. The reactants may be represented by the formulae:

where X, represents an aziridine group of the structure

and Q1 and Q2 are either hydrogen or alkyl groups of one to four carbon atoms (Q, and Q2 may be the same or different), X2 may be the same as X, or may be an organic radical such as phenyl, benzyl, methyl or ethyl, R is an alkyl group containing at least one active hydrogen atom or an organic entity of molecules containing one or more hydroxyl groups, and n is 2, 3 or 4. The reaction product is a mixture of compounds, the nominal structure of which may be represented by:

wherein X" X2, Q,. Q2, R and n are as already defined.

The optimum proportions are such that essentially all carboxyl groups in (II) are reacted and nominally one aziridine group in (I) is reacted, i.e., one mole of (I) for each carboxyl equivalent of (II).

The preferred aziridine polyester materials for use in carrying out the present invention are prepared from tris(2 - methyl - aziridinyl - I )phosphine oxide, which is commonly known as MAPO, and diacids, such as adipic acid, malic acid, sebacic acid, succinic acid and tartaric acid.

PAZ is a condensation product of MAPO, tris(2 - methyl - aziridinyl 1 )phosphine oxide, with a straight chain diacid of the general formula HOOC(CH2)XCOOH (x being 2 to 8 and preferably 4, 5 or 6) and a second substituted diacid of the general formula HOOC . CH(R) . (CH2)VCH(R )COOH in which R and R1 are the same or different and each is H or 6H and y=0 to 6 preferably 0, 1 or 2. In a typical synthesis, 3 to 8 moles of MAPO, Preferably 5 to 6 moles, are condensed with 0.5 to 1.0 mole of the substituted diacid and 1.75 to 2.5 moles of the straight chain diacid at 50 to 65"C for 4 hrs under N2.

The proportion of the aziridine polyester bonding agent in the binder according to the present invention is preferably from 0.1 to 1% by weight of the total binder composition.

The other component of the bonding agent combination used in binders according to the present invention is the amine polyester material which is as indicated the reaction product of an alkanolamine and a saturated aliphatic polycarboxylic acid. The preferred amine polyester for the present purposes are polymers derived from N - methyldiethanolamine and sebacic acid, known as Polymer N-8. Suitable N-8 polymers have the following specifications: Mn (number average Mw): 12002000; preferred 150W1800 Acidity: < 0.02 eq./100 g Equivalent OH: 1--1.5 meq./g; preferred 1.1-1.3 Humidity: 0.15% wt Other suitable agents have the following formula

wherein R and RtV are the same or different and each is a hydrogen atom or a saturated alkyl chain having one or two carbon atoms; R" is a saturated alkyl chain with one to eighteen carbon atoms; n is an integer from 2 to 16, preferably being 8; and u is an integer from W10, preferably being 5--7. This bonding agent is preferably used in an amount of about 0.1% to 0.5% by weight of the total binder composition.

To form the castable propellant according to the present invention, the binder before curing is admixed according to a preferred process which will be described in detail herein, with finely-divided ammonium perchlorate as oxidizer in dimodal or trimodal distributions of fine powders with average particle sizes of 1100 pm. A preferred particle size distribution of the ammonium perchlorate is 1.7/2.7/1.0 parts by weight of 400 ym size, 200 pm size, and 17 ,um size respectively. For high temperature applications, the ammonium perchlorate may be replaced by potassium perchlorate. Suitably an anticaking agent, e.g. tricalcium phosphate, is included in an amount of about 1% by weight of the 17 ,um perchlorate. A metallic additive may be included in the propellant composition if desired, in a concentration of 0% to 20% by weight of the total propellant. This additive may be finely-divided aluminium or magnesium powder, preferably having an average particle size of 550 Mm. Another solid material suitably included in the over-all composition is a burnmg rate additive or catalyst which may be for example iron oxide, copper chromite or an organometallic compound. The preferred additive or catalyst for the present purposes is ferric oxide in an amount of 0.11% by weight of the propellant composition.

In composite propellant compositions, it is of course desirable to obtain as high a solid loading as possible. With the propellant compositions according to the present invention, the total solids content can be in the range from 85% to 90% by weight of the total composition, of which of course the ammonium perchlorate will constitute the major part and may be from 68% to 88% by weight of the total propellant composition. The other solid components are adjusted accordingly therein. Thus, the polymeric binder preferably constitutes about 10% to 15% by weight of the total propellant composition.

In order to obtain good dispersion of all the ingredients, in preparing the propellant composition, and also reproducibility of properties from one batch to another, the preferred procedure is first to add to a mixer the liquid ingredients, which are the prepolymer, the plasticizer and the two bonding agents. The aluminium or other metal powder when used and the ferric oxide burning rate additive are then added and the mixer contents are mixed for 15 minutes at atmospheric pressure. Then approximately three quarters of the total amount of ammonium perchlorate is added to the mixture and mixing is continued for a further 10 minutes. Vacuum is applied and the contents of the mixer are heated to 60"C for about 60 minutes. The vacuum is then released, the remaining amount of ammonium perchlorate is added and the mixing is continued for a further 5 minutes at atmospheric pressure. The diisocyanate curing agent is then added to the other ingredients in the mixture and mixing is continued for 5 minutes at atmospheric pressure. After this, vacuum is again applied and mixing is continued for 45 minutes at a temperature of 600C. The propellant is then cast in moulds under vacuum.

Certain compositions may be difficult to process. If this problem occurs, the diisocyanate curing agent may be added and admixed prior to addition of the remaining ammonium perchlorate oxidizer.

In the accompanying drawings, which illustrate embodiments of the invention, Figure 1 is a graph of the effect of prepolymer type on the pot life of HTPB propellants, Figure 2 is a graph of the effect of antioxidants on the pot life of HTPB propellants based on R-45HT (prepolymer lot 403245), Figure 3 is a graph of the effect of "Flexzone 6H" (RTM) and DTBHQ antioxidants on the pot life of HTPB propellants (R45HT prepolymer lot 606095), Figure 4 is a graph of the effect of a mixture of antioxidants on the pot life of HTPB propellants (R45HT prepolymer lot 606095), Figure 5 is a graph of the effect of UOP-36 and "Flexzone 6H" (RTM) on the pot life of HTPB propellants (R-45HT prepolymer lots 403245 and 606095), Figure 6 is a graph of the effect of PAZ/PAM bonding agents on the pot life of HTPB propellants (R45HT polymer, lot 606095; antioxidant type D).

The following examples illustrate the use of the binder system according to the present invention in preparing composite solid propellants. The examples are intended to be illustrative of the present invention, but are not to be taken as limiting the scope thereof. The propellant compositions set out in the examples were each prepared according to the method described above.

The mechanical properties the propellant compositions according ta the present invention ascertained by testing were tensile strength (am), elongation at maximum load (Em), elongation at rupture (E,), and initial modulus (E). These tests were carried out on an "Instron" (Regd. Trade Mark) apparatus at a cross-head speed of 2 in/min, corresponding to a strain rate of 0.741 min1 for the ICRPG "dumbell" specimen.

Propellant compositions were prepared having the following proportions of ingredients expressed in weight percent: 1.7 parts ammonium perchlorate (average diameter 400 m) 2.7 parts ammonium perchlorate (average diameter 200 ,um) 69A 1.0 parts ammonium perchlorate (average diameter 17 pm) Aluminium (spheroidal grade (average diameter 22 clam)) 18.0 Ferric oxide catalyst (average diameter 1 jum) 0.6 HTPB binder 12.0 For testing purposes, four binder compositions were used in the foregoing propellant composition. Binders I, II and III contained R45M prepolymer including 1% PBNA and binder IV contained R-45HT including 1% of an equi mixture of DTBHQ and "Flexzone 6H" (RTM). Isodecyl pelargonate was included as plasticizer and DDI diisocyanate as curing agent. In one binder composition, the two bonding agents according to the invention were not utilized but instead only the aziridine polyester for comparative purposes. In the other three compositions, both bonding agents were included.

The binder compositions were as follows: I II III IV Prepolymer(including 1% antioxidant) 62.91 62.91 62.91 62.66 DDI 12.09 12.09 12.09 12.34 Aziridine polyester 2.40 0.9 0.6 0.8 Amine polyester (N-8) - 0.2 0.2 0.2 IDP plasticizer 22.60 23.9 24.2 24.0 The aziridine polyester of this example was prepared by reacting 0.15 mole of d - tartaric acid and 0.3 mole of adipic acid with 1 mole of MAPO (aziridine) in the process previously described herein. The amine polyester (N-8) of this example had the following properties: (i) OH equivalent 1.19 M equivlg (ii) Acidity 0.016 equiv./100 g (iii) Molecular weight (Mn) 1430 determined by VPO The processing and mechanical properties found for the propellant compositions using the aforementioned binders were: I II III IV Aziridine/amine polyester 2.4/- 0.9/0.2 0.6/0.2 0.8/0.2 (% in binder) End-of-mix viscosity (kP/60 C) 2.5 2.7 2.7 3.6 Pot life to 10 kP (hr) 4.2 4.2 4.5 6.6 Mechanical Properties (Initial) am at 22.80C (MN/m2) .58 .79 .63 .65 Em at 22.80C (%) 30.6 36.9 41.6 32.5 Eat 22.8"C (MN/m2) 3.41 4.04 3.24 3.66 Em at45.6 C (%) 48.2 58.8 65.6 50.8 Er at45.6 C (%) 54.5 59.8 68.5 55.6 1 II III IV Mechanical Properties (56 Days at 600C) am at 22.8"C (MN/m2) .58 .79 .73 .81 Em at 22.8"C (%) 35.2 35.9 39.1 25.0 Eat 22.8"C (MN/m2) 3.37 4.82 3.85 5.45 Em at 5.6 C (%) 49.4 48.9 59.5 38.1 Er at 45.60C (%) 50.7 50.4 60.9 40.0 It will be seen from the foregoing results that the processing characteristics, that is end-of-mix viscosity and pot life, for the propellant according to the present invention (IV) were substantially the same as these properties for the comparison propellants (I, II and III). However, the amount of aziridine polyester utilized according to the invention is considerably less than that required for the reference composition I. Also, the propellant according to the invention showed improved elongation at maximum load and at rupture, while the tensile strength and initial modulus are comparable to the reference composition I.

The following additional examples serve to emphasize the advantages of using R45HT in combination with various novel binder systems. All percentages are by weight unless otherwise indicated.

Preparation of R45HT/Antioxidant Batches Due to marked differences in the pot lives of propellants based on two lots of R45HT, it has been decided to discuss the results for each lot separately.

Premixes of R45HT (99 percent) and antioxidant (1.0 percent) were prepared by heating and agitating the components at 600C during approximately one hour.

Because di - tert - butylhydroquinone (DTBHQ) is not soluble in R-45HT in excess of 0.2 percent, but is soluble in IDP plasticizer, premixes of R 45HTyIDP/DTBHQ were prepared with the following composition:R45HT at 75.0 percent, IDP at 24.25 percent and DTBHQ at 0.75 percent. The relative concentration of DTBHQ with R45HT was thus maintained at 1.0 percent. Unless otherwise stated, all the antioxidants were tested at a concentration of one percent of the prepolymer.

Propellant Mixing and Casting Propellant batches were manufactured in a Cone-Vertical 8-CV mixer operating at a batch size of 6.5 kg (approximately 50 percent of the mixer capacity).

The same mixing and casting procedure was followed during the whole program.

The liquid ingredients (the prepolymer including the antioxidant, the plasticizer and the two bonding agents) were first added to the mixer. The aluminium and the ferric oxide were then added and the contents mixed for 15 minutes at atmospheric pressure with the jacket temperature maintained at 600 C. Then approximately three quarters of the total amount of ammonium perchlorate was added to the mixture and mixing was continued for a further 10 minutes. Vacuum was applied and the contents othe</R period of five days at 600C, the block was X-rayed and cut into slabs 1.27 cm+0.05 thick; ICRPG dogbones were die-cut and measurements taken of the minimum section. Four samples were tested on the "Instron" (RTM) tester at an extention rate of 0.0847 cm/s at each of the three temperatures. For accelerated ageing tests, the machined slabs of propellant were individually wrapped in polyethylene bags.

RESULTS AND DISCUSSION Propellants Based on R-45HT Lot 403245 Effect of Polymer Type on Pot Life The two lower curves of Figure 1 illustrate the effect of the prepolymer (R-45M versus R45HT) on the propellant viscosity variation with the time elapsed after addition of DDI curative agent. With PBNA as the reference antioxidant, the pot lives (to attain a viscosity of 10 kP) are 4.5 and 3.3 hours respectively for R45M and R-45HT.

Effect of the Antioxidants on Pot Life The two upper curves of Figure 1 show the beneficial effect of PBNA antioxidant on the pot life of a typical HTPB/R-45HT propellant. Without antioxidant, the R-45HT propellants would show a pot life of only 2.3 hours. It is therefore important to select an antioxidant which also slows down the initial polymerization rate of the propellant. Each of the three propellants of Figure 1 had two bonding agents at a'concentration of PAZ/PAM of 0.6/0.2 percent of the binder.

Six of the seven antioxidants listed in Table II (excluding "Flexzone 6H" (RTM)) were tested separately in.a similar propellant based on R-45HT lot No.

403245. Three antioxidants yielded a pot life shorter than R-451iTIPBNA propellant; they are, as shown on Figure 2, AO-2246, "NAUGARD Q" (RTM) and Octamine. AO-2246 was the worst one with pot lives of 1.5 and 2.1 hours at concentrations of 0.5 and 0.25 percent in the prepolymer. The three other antioxidants yielded pot lives longer than R-45HT/PBNA propellant (UOP-36 at 3.6 hours, "Flexzone 7L" (RTM) at 4.2 hours and DTBHQ at 4.4 hours).

Effect of Combinations of Antioxidants on Pot Life A synergistic effect was observed (Table III) when a combination of DTBHQ with UOP-36 or "Flexzone 7L" (RTM) was added to the propellant mix to the extent of 1 percent of the prepolymer. Effectively, the pot life of the DTBHQ/UOP-36/R45HT and DTBHQ/"Flexzone 7L"/R45HT propellants were longer than those observed on DTBHQ/RA5HT, UOP-36/R-45HT and "Flexzone"/R-45HT propellants.

Twelve of the 15 mixes prepared with combinations of DTBHQ and UOP-36 or "Flexzone 7L'' (RTM) yielded pot lives longer than 4.5 hours. On the average, pot lives of DTBHQ/UOP-36 propellants were 4.9 hours and those of DTBHQ/"Flexzone 7L" propellants were 4.5 hours.

TABLE III Effect of Combinations of Antioxidants on Pot Life of R-45HT Propellants Combinations of Antioxidants Pot Life at 10 kP Viscosity DTBHQ UOP-36 "Flexzone 7L" (RTM) (hr) 100 - - 4.4 75 25 - 5.0,5.6,M,4.1 50 50 - 5.2,5.1,4.9 25 75 - 4.6 100 3.8 50 - 50 4.3 25 - 75 4.5, 4.8. 4.2,4.6,4.5,5.0 100 4.1 Underlined results were obtained with PAZ/R-45HT formulations.

Screening of Antioxidants Based upon the data presented above, the pot lives of R-45HT propellants appear to be strongly dependent upon the type of antioxidant added to the composition. To reduce the scope of the program, it was decided that the three antioxidants yielding a shorter pot life than R45HT/PBNA propellant (AO-2246, "NAUGARD Q" (RTM) and Octamine) would be dropped from this study. On the other hand, acceirated ageing tests at 600C, would be carried out on propellants containing one or two of three other candidates UOP-36, "Flexzone 7L" RTM) and DTBHQ) to evaluate their effectiveness as an antioxidant.

Mechanical Property Data Eleven propellant mixes based on R45HT Lot 403245 were tested for mechanical properties; their binder compositions are shown in Table IV. The solids composition was as described previously. A reference propellant (mix 29) was formulated without antioxidant. Four mixes had a single antioxidant: mix 26 ("Flexzone 7L"), mix 28 (UOP-36), mixes 25 and 35 (DTBHQ). Their mechanical properties after curing (t=0) and upon ageing (14, 28 and 56 days at 60"C) are shown in Table V. The mechanical properties of six propellant mixes incorporating two antioxidants (UOP-36/DTBHQ or "Flexzone 7L"/DTBHQ are listed in Table VI.

TABLE IV Binder Compositions Based on R45,HT Lot 403245 Mix Antioxidant NCO/OH Composition (% Wt) No. Type Ratio R45HT DDI IDP PAZ PAM 29 nil 0.85 62.03 12.97 24.20 0.6 0.2 26 Flex. 7L 0.875 61.72 13.28 24.00 0.8 0.2 28 UOP-36 0.85 62.03 12.97 24.20 0.6 0.2 25 DTBHQ 0.85 62.03 12.97 24.20 0.6 0.2 35 DTBHQ 0.85 62.03 12.97 24.20 0.6 0.2 75 DTBHQ 07 - 0.85 62.03 12.97 24.20 0.6 0.2 25 UOP-36 75 DTBHQ 08 - 0.85 62.03 12.97 22.60 2.4 0.0 25 UOP-36 25 DTBHQ 09 - 0.875 61.72 13.28 24.20 0.6 0.2 75 Flex-7L 25 DTBHQ 16 - 0.85 62.03 12.97 22.60 2.4 0.0 75 Flex-7L 25 DTBHQ 17 - 0.85 62.03 12.97 22.60 2.4 0.0 75 Flex-7L 25 DTBHQ 18 - 0.85 62.03 12.97 22.60 2.4 0.0 75 Flex-7L TABLE V Mechanical Property Data for Three Antioxidants (R-45HT, Lot 403245) Properties at Temp. Ageing Mixes ( C (Day) 29 26 28 25 35 #m(kP) 22.8 0 703 590 406 681 746 Em (%) 22.8 0 16.5 34.7 37.8 30.5 31.7 E(kPa) 22.8 0 6784 2906 1981 4153 4677 Em (%) 45.6 0 18.9 48.1 56.8 43.9 46.9 Er (%) 45.6 0 20.7 52.0 65.9 50.2 51.1 #m (%) -53.9 0 15.1 42.8 43.9 33.1 39.5 #r(%) -53.9 0 17.1 47.7 47.1 41.1 43.7 #m(kPa) 22.8 14 738 662 479 759 795 #m (%) 22.8 14 9.1 31.0 34.2 33.9 29.8 E(kPa) 22.8 14 10,790 4259 2565 4871 5034 #m(kPa) 22.8 28 492 680 565 769 861 Em Co) 22.8 28 3.4 31.5 33.8 27.6 26.7 E(kPa) 22.8 28 18612 4049 3218 5444 5870 #m (%) -45.6 28 6.2 57.7 50.2 52.5 41.9 Em (%) 45.6 28 6.4 61.0 53.8 55:6 43.7 am (kPa) 22.8 56 438 782 648 857 904 Em(%) 22.8 56 2.2 29.5 30.5 21.7 22.0 E (kPa) 2 56 29582 4890 3656 7443 7423 Em (%) -45.6 56 3.7 43.5 47.1 31.5 33.8 #r(%) 45.6 56 3.8 45.8 48.6 35.1 37.1 Antioxidant Type - - Flex. 7L UOP-36 DTBHQ DTBHQ TABLE VI Mechanical Property Data for Mixtures of Antioxidants (R-45HT, Lot 403245) - Properties at Temp. Ageing Mixes ( C) (Day) 07 08 09 16 17 18 #m(kPa) 22.8 0 622 848 483 611 743 835 Em (%) 22.8 0 27.7 23.0 41.8 29.5 27.4 26.3 E(kPa) 22.8 0 3158 5767 1439 3132 3778 4710 Em (%) 45.6 0 33.2 37.0 55.7 40.9 39.3 38.1 Er (%) 45.6 0 39.2 38.2 60.2 44.8 42.5 41.0 Em (%) -53.9 0 22.8 32.0 27.0 36.3 37.4 35.2 Er (%) -53.9 0 32.7 35.9 48.0 39.1 39.6 37.7 #m(kPa) -53.9 0 660 1083 554 813 946 953 Em (%) -53.9 0 28.6 24.1 38.0 23.7 21.7 22.8 E (kPa) -53.9 0 2659 6986 2431 4873 5933 6302 #m(kPa) 22.8 28 722 1060 594 836 9176 1043 Em (%) 22.8 28 26.0 25.6 36.6 24.8 22.6 23.0 E (kPa) 22.8 28 4500 6339 2467 5273 6575 6583 #m (%) 45.6 28 32.0 36.5 48.6 36.7 34.0 35.1 Er (%) 45.6 28 36.0 37.5 52.7 38.1 35.0 36.3 #m(kPa) 22.8 56 730 1001 608 927 1079 1031 Em (%) 22.8 56 23.4 25.5 32.8 26.7 24.5 23.4 E (kPa) 22.8 56 5371 7466 3492 6407 7760 7220 #m (%) 45.6 56 30.7 35.8 46.4 37.0 33.8 35.5 Er (%) -45.6 56 35.3 36.4 50.7 38.2 34.6 36.0 DTHBQ (% of R-45HT) 0.75 0.75 0.25 0.25 0.25 0.25 UOP-36 (% of R-45HT) 0.25 0.25 - - - - Flex. 7L (% of R-45HT) - - 0.75 0.75 0.75 0.75 R45HT Propellant Without Antioxidant As expected, an unprotected R-45HT propellant loses its mechanical properties very rapidly upon ageing; its stress and strain at room temperature fall from 703 kPa and 16.5% at t=O to 438 kPa and 2.2% only after 56 days of ageing at 600 C. Conversely, the modulus increases from 6784 to 29582 kPa. Obviously, an unprotected R-45HT propellant is very susceptible to oxidation.

Effectiveness of UOP-36, "Flexzone 7L" and DTBHQ The results for mixes 26, 28, 25 and 35 (Table V) tend to indicate that each of the above three candidates provides efficient protection against oxidation. All four mixes, however, show some degree of post-curing as evidenced by the increasing am and E upon ageing.

Effectivenss of Mixtures of Antioxidants It has been shown above that a combination of DTBHQ with UOP-36 or "Flexzone 7L" (RTM) yielded improved pot lives. Ageing results for mixes 07, 08, 09, 16, 17 and 18 (Table VI) indicate that the combinations of two antioxidants also provide efficient protection against oxidation. Good batch-to-batch reproducibility while ageing at 60"C was obtained with mixes 16, 17 and 18.

Propellants Based on R45HT Lot 606095 Thirteen (13) propellant compositions based on R-45HT Lot 606095 were tested for processing characteristics and accelerated ageing properties; their binder composition are listed in Table VII. The main variables under study were: the type of antioxidant(s), the concentration of isocyanate/hydroxyl groups (NCO/OH) and the concentration of bonding agents. Two antioxidants were evaluated alone (DTBHQ and "Flexzone 6H") and three combinations of DTBHQ with "Flexzone 6H" or "'Flexzone 7L" were tested with the solids system described previously.

TABLE VII Binder Compositions Based on R-45HT Lot 606095 Mix Antioxidant NCO/OH Compositions (% Wt) No. Type (1) Ratio R45HT DDI IDP PAZ PAM 41 DTBHQ 0.80 62.03 12.97 24.2 0.6 0.2 47 DTBHQ 0.80 61.98 13.02 24.0 0.8 0.2 64 DTBHQ 0.725 63.01 11.99 24.0 0.8 0.2 71 DTBHQ 0.70 63.36 11.64 24.0 0.8 0.2 72 Flex. 6H 0.75 62.66 12.34 24.0 0.8 0.2 60 C 0.75 62.66 12.34 24.0 0.8 0.2 66 C 0.725 63.01 11.99 24.0 0.8 0.2 73 E 0.75 62.66 12.34 24.0 0.8 0.2 65 D 0.725 63.01 11.99 24.0 0.8 0.2 61 D 0.75 62.66 12.34 24.0 0.8 0.2 68 D 0.75 62.66 12.34 24.4 0.4 0.2 69 D 0.75 62.66 12.34 23.6 1.2 0.2 70 D 0.75 62.66 12.34 23.8 0.8 0.4 (1) Type C is a 50/50 mixture of DTBHQP'Flexzone 7L".

Type D is a 50/50 mixture of DTBHQP'Flexzone 6H".

Type E is a 25/75 mixture of DTBHQP'Flexzone 6H".

Effect of antioxidant on Processing Characteristics As can be seen on Table VIII, very satisfactory EOM viscosities and pot lives were obtained with DTBHQ and combinations of DTBHQ with "Flexzone 7L" or "Flexzone 6H". The pot lives of 12 of the 13 mixes were longer than 5.5 hours for a mix viscosity of 10 kP at 60"C. This is well above the target of 4.5 hours, as the average for 12 mixes is 6.6 hours. This is further illustrated in Figures 3 and 4. The propellant containing "Flexzone 6H" alone as antioxidant (Mix 72) displayed a pot life of 3.9 hours (Figure 5); this compares advantageously with UOP-36, another commercial name for N - phenyl - N' - cyclohexyl - p - phenylene - diamine.

TABLE VIII Processing Characteristics of R-45 HT Propellants Lot 606095 Mix Antioxidant EOM Viscosity Pot Lives (hr) No. Types(l) kP / OC at 10kP(60 C) at20kP(600C) 41 DTBHQ 2.5 / 59 7.5 10.6 47 DTBHQ 3.4 / 57 7.9 10.6 64 DTBHQ 4.4 / 56 5.9 71 DTBHQ 4.5 / 58 6.0 8.6 72 Flex. 6H 5.6 / 59 3.9 6.0 60 C 3.0 / 57 7.8 10.2 66 C 4.1 / 58 6.4 9.2 73 E 4.3 / 59 5.5 7.8 65 D 4.2 / 57 6.2 8.6 61 D 4.1 / 56 6.6 9.2 68 D 4.3 / 58 6.4 8.8 69 D 4.4 / 58 6.7 8;9 70 D 5.6 / 59 6.1 8.2 (1) Type C is a 50/50 mixture of DTBHQ/"Flexzone 7L".

Type D is a 50/50 mixture of DTBHQ/"Flexzone 6H".

Type E is a 25/75 mixture of DTBHQP'Flexzone 6H".

Effect of antioxidants on Mechanical Properties The mechanical properties at t=0 and upon ageing at 600C for four propellant mixes (41, 47, 64 and 71) containing DTBHQ antioxidant are listed in Table IX.

The optimized concentration NCO/OH appears to be 0.725, since am, Em and E at 22.8"C are respectively 686 kPa, 33.4% and 3742 kPa, and, at -53.9 C, Em is as high as 45.8%. A ain, the four mixes show some degree of post-curing. The results in Table X indicate that i) the initial am at room temperature is too low for the "Flexzone 6H"/R-45HT propellant (mix 72); ii) propellants containing combinations of DTBHQ with "Flexzone 7L" and "Flexzone 6H" have satisfactory ageing properties; iii) an NCO/OH ratio of 0.75 yields better propellant than 0.725 (cf. mixes 60 and 66).

TABLE IX Mechanical Properties of R-45HT Propellants Lot 606095; DTHBQ Antioxidant Properties at Temp. Ageing Mixes ( C) (Day) 41 47 64 71 am(kPa) 22.8 0 859 899 686 605 Em (%) 22.8 0 26.4 20.1 33.4 34.9 E (kPa) 22.8 0 5783 8096 3742 3117 Em (%) 45.6 0 37.2 30.3 52.0 54.5 Er(%) 45.6 0 41.7 33.3 58.1 56.7 Em (%) -53.9 0 27.7 27.5 45.8 47.7 Ear(%) -53.9 0 33.6 30.3 48.9 50.6 a (kPa) 22.8 14 895 959 746 693 Em (%) 22.8 14 24.8 20.6 30.0 29.8 E (kPa) 22.8 14 6587 8089 4424 4059 am(kPa) 22.8 28 916 980 794 754 Elm(%) 22.8 28 24.0 21.0 29.8 29.6 E (kPa) 22.8 28 6570 8259 4773 4463 Em (%) 45.6 28 35.9 28.8 48.0 48.4 Er (%) 45.6 28 40.1 31.8 50.5 51.1 a (kPa) 22.8 56 873 996 811 799 Elm(%) 22.8 56 22.1 20.9 24.5 23.5 E(kPa) 22.8 56 6402 9713 5626 5642 Em (%) 45.6 56 27.7 22.9 38.3 32.3 Em (%) 45.6 56 32.5 26.8 40.4 33.6 R (NCO/OH ratio) 0.80 0.80 0.725 0.70 TABLE X Mechanical Properties of R-45HT Propellants Lot 606095; Flex. 6H, Types C and E Antioxidants Properties at Temp. Ageing Mixes ( C) (Day) 72 60 66 73 #m (kPa) 22.8 0 483 674 499 584 #m(%) 22.8 0 35.7 31.4 35.7 28.2 E (kPa) 22.8 0 2461 3663 2641 3835 Em (%) 45.6 0 49.8 47.8 50.9 49.9 #r (%) 45.6 0 56.0 50.8 58.1 52.5 Em (%) -53.9 0 44.0 42.8 45.9 45.4 #r (%) -53.9 0 47.6 43.5 49.0 47.1 am(kPa) 22.8 14 614 720 611 724 Em (%) 22.8 14 29.9 30.6 31.3 28.7 E (kPa) 22.8 14 3510 4207 3490 4517 am(kPa) 22.8 28 654 793 671 814 #m (%) 22.8 28 29.1 29.1 30.0 27.8 E (kPa) 22.8 28 3725 4647 3819 5281 Em (%) 45.6 28 49.6 43.4 49.9 45.2 Er (%) 45.6 28 52.1 46.2 51.7 46.8 am (kPa) 22.8 56 722 849 739 797 Esn (%) 22.8 56 26.8 26.6 28.5 23.9 E (kPa) 22.8 56 4306 5526 4701 6142 E, (%) 45.6 56 43.1 39.4 44.6 34.8 Er (%) 45.6 56 44.7 40.5 46.7 37.3 R NCO/OH ratio 0.75 0.75 0.725 0.75 Flex. 6H (% of R45HT) 1.00 - - 0.75 DTBHQ (O of R-45HT) - 0.50 0.50 0.25 Flex. 7L (/O of R-45HT) - 0.50 0.50 Optimization of Bonding Agents in Propellants Containing Type D antioxidants Two propellants (mixes 65 and 61) containing the 50/50 mixture of DTBHQ exzone 6H (type D antioxidants) performed very well with respect to tllves (Table VIII) and mechanical properties (Table XI). Mix 61, in particular, having an NCO/OH ratio of 0.75 showed a Em and E of 6.50 kPa, 32.5% and 3657 kPa at room temperature and Em of 50.8 and 44.9% at -45.6 and -53.9 C respectively. In most of the previous compositions, the concentration of PAZ/PAM bonding agents was kept at 0.8/0.2 percent of the binder. Three propellants having an NCO/OH of 0.75 (mixes 68, 69 and 70) were formulated to verity the effect of changing the PAZ/PAM concentrations to 0.4/0.2, 1.2/0.2 and 0.8/0.4 percent. All the pot lives were longer than 6 hours (Fig. 6) and the mechanical properties were excellent. The preferred PAZ/PAM ratio is that of mix 70 (0.8/0.4) which yielded an Em Of 45.8 percent at 45.6 C after 56 days of ageing.

TABLE XI Mechanical Properties of R45HT Propellants Lot 606095; Type D"' Antioxidant Properties at Temp. Ageing Mixes ( C) (Day) 65 61 68 69 70 am(kPa) 22.8 0 529 650 616 714 578 Em (%) 22.8 0 32.1 32.5 37.7 30.8 35.4 E (kPa) 22.8 0 2929 3657 3210 4101 3078 Em (%) 45.6 0 52.5 50.8 55.9 48.9 52.5 Er (%) 45.6 0 57.0 55.6 58.8 51.5 57.2 Em (%) -53.9 0 46.7 44.9 48.2 43.1 48.7 E,(%) -53.9 0 51.0 48.1 49.8 45.8 50.9 am(kPa) 22.8 14 635 724 676 800 673 Em (%) 22.8 14 29.7 30.2 33.1 27.0 32.5 E (kPa) 22.8 14 3940 4297 3840 4836 3577 am (kPa) 22.8 28 700 819 731 818 713 Em (%) 22.8 28 29.7 28.6 30.1 27.7 30.9 E(kPa) 22.8 28 4451 5089 4286 4781 3932 Elm(%) -45.6 28 51.0 44.1 45.9 45.3 49.3 Ear(%) 45.6 28 54.8 45.9 48.4 48.1 54.7 am(kPa) 22.8 56 766 816 718 825 779 Em (%) 22.8 56 26.2 25.0 24.6 25.6 279 E (kPa) 22.8 56 5428 5445 5173 5260 4768 Elm(%) 45.6 56 44.0 38.1 33.7 40.1 45.8 Er(%) 45.6 56 47.2 40.0 35.6 41.9 48.0 R NCO/OH ratio 0.725 0.75 0.75 0.75 0.75 PAZ ( /0 wt of binder) 0.8 0.8 0.4 1.2 0.8 PAM (% wt of binder) 0.2 0.2 0.2 0.2 0.4 (1 Type D antioxidant is a 50/50 mixture of DTBHQ/Flexzone 6H.

Comparison Between the Two Lots of R-45HT The Longer Pot Lives of Propellants Based on Lot 606095 The longer pot lives obtained with Lot 606095 are not easily explicable.

Various laboratory tests were conducted on the R45HT prepolymers Lots 403245 and 606095. No significant difference could be found to date with such tests as the equivalent weight (OH groups) the water content, the GPC and VPO. Viscosity measurements over the temperature range of 20 to 60C led to almost identical results for both lots. It is possible, however, that the twoyear period which elapsed between the manufacture of Lot 403245 and the addition of an antioxidant could have contributed to a certain deterioration of the prepolymer by homopolymerization. In an attempt to minimize the degradation of the product, the second lot of R-45HT was purchased with the additional specification"that the material must be delivered within six months after its manufacture".

Mechanical Properties Very little difference could be found between the mechanical properties 'of propellants based on R-45HT Lot 403245 and those from Lot 606095. The properties of two propellants containing DTBHQ, mix 25 (Lot 403245) and mix 64 (Lot 606095) are compared in Table XII. For nearly identical am (681 versus 686 kPa), mix 64 shows slightly higher elongations at low temperature. The reduced values, which are an indication of the stability of the two propellants upon accelerated ageing at 600 C, are also nearly the same. Some post-curing is obvious with increases of am, E and reduction in the elongation.

TABLE XII Mechanical Properties of R-45HT Propellants Containing 1% DTBHQ (Lots 403245 and 606095) Reduced Value A) Mix 25, Lot 403245 t=0(1) t=56 t 56/to em (kPa) o 22.8 C 681 857 1.20 #m (%)# 22.8 C 30.5 21.7 0.71 E (kPa)# 22.8 C 4154 7443 1.79 #m (%)# -45.6 C 43.9 31.5 0.72 #r (%)#-45.6 C 50.2 35.1 0.70 Em (%) e53.9 C 33.1 1 #r(%)#-53.9 C 41.1 B) Mix 64, Lot 606095 em (kPa) o 22.8 C 686 811 1.18 Em (%) e 22.80C 33.4 24.5 0.73 E (kPa) # 22.8 C 3742 5626 1.50 #m (%)# -45.6 C 52.0 38.3 0.74 #r (%) # --45.6 C 58.1 40.2 0.70 #m (%)# --53.9 C 45.8 - #(%)#-53.9 C 48.9 (1) t=0 for initial properties.

t=56 for properties after 56 days of aging at 600 C.

WHAT I CLAIM IS: 1. A curable binder for use in forming a castable propellant, which comprises: (i) a hydroxy-terminated butadiene polymer of the structural formula

wherein n=57--65, (ii) a diisocyanate curing agent, (iii) a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and a polycarboxylic acid, and an amine polyester, which is the reaction product of a alkanolamine and a saturated aliphatic polycarboxylic acid, and (iv) an antioxidant comprising by weight 25%--75% of di - terrbutylhydroquinone in conjunction with 75%-25% of one or other of N - phenyl N' - (1,3 - dimethylbutyl) - p - phenylene - diamine and N - phenyl - N' cyclohexyl - p- phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer.

2. A binder as claimed in Claim 1, wherein the antioxidant comprises by weight approximately 50 /O of di-tert - butylhydroquinone and approximately 50 tO of N phenyl - N' - cyclohexyl - p - phenylene - diamine.

3. A binder as claimed in Claim I, wherein the antioxidant comprises by weight approximately 509/O bf di-tert - butylhydroquinone and approximately 50 of N phenyl - N' - (1,3 - dimethyl - butyl) - p - phenylene - diamine.

4. A binder as claimed in any preceding claim, wherein the aziridine polyester is the reaction product of tris - (2 - methylaziridinyl - I )phosphine oxide, a straight chain diacid of the general formula HOOC(CH2)xCOOH, x being 2 to 8, and a substituted diacid of the general formula HOOCCH(R) . (CH2)yCH(R1)COOH, in which R and R1 are the same or different and each is H or OH and y is 0 to 6, the phosphine oxide being reacted in an amount of 1 mole per carboxyl equivalent of the acid.

5. A binder as claimed in Claim 4, wherein the diacids are d-tartaric acid and adipic acid in the respective proportions of 0.15 and 0.3 mole per mole of the aziridinyl phosphine oxide.

6. A binder as claimed in any preceding claim, wherein the amine polyester is the reaction product of N - methyldiethanolamine and sebacic acid.

7. A binder as claimed in any preceding claim, wherein the aziridine polyester

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (23)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   TABLE XII Mechanical Properties of R-45HT Propellants Containing 1% DTBHQ (Lots

   403245 and 606095) Reduced Value A) Mix 25, Lot 403245 t=0(1) t=56 t 56/to em (kPa) o 22.8 C 681 857 1.20 #m (%)# 22.8 C 30.5 21.7 0.71 E (kPa)# 22.8 C 4154 7443 1.79 #m (%)# -45.6 C 43.9 31.5 0.72 #r (%)#-45.6 C 50.2 35.1 0.70 Em (%) e53.9 C 33.1 1 #r(%)#-53.9 C 41.1 B) Mix 64, Lot 606095 em (kPa) o 22.8 C 686 811 1.18 Em (%) e 22.80C 33.4 24.5 0.73 E (kPa) # 22.8 C 3742 5626 1.50 #m (%)# -45.6 C 52.0 38.3 0.74 #r (%) # --45.6 C 58.1 40.2 0.70 #m (%)# --53.9 C 45.8 - #(%)#-53.9 C 48.9 (1) t=0 for initial properties.

   t=56 for properties after 56 days of aging at 600 C.

   WHAT I CLAIM IS: 1. A curable binder for use in forming a castable propellant, which comprises: (i) a hydroxy-terminated butadiene polymer of the structural formula

   wherein n=57--65, (ii) a diisocyanate curing agent, (iii) a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and a polycarboxylic acid, and an amine polyester, which is the reaction product of a alkanolamine and a saturated aliphatic polycarboxylic acid, and (iv) an antioxidant comprising by weight 25%--75% of di - terrbutylhydroquinone in conjunction with 75%-25% of one or other of N - phenyl N' - (1,3 - dimethylbutyl) - p - phenylene - diamine and N - phenyl - N' cyclohexyl - p- phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer.
2. 2. A binder as claimed in Claim 1, wherein the antioxidant comprises by weight approximately 50 /O of di-tert - butylhydroquinone and approximately 50 tO of N phenyl - N' - cyclohexyl - p - phenylene - diamine.
3. 3. A binder as claimed in Claim I, wherein the antioxidant comprises by weight approximately 509/O bf di-tert - butylhydroquinone and approximately 50 Ó of N phenyl - N' - (1,3 - dimethyl - butyl) - p - phenylene - diamine.
4. 4. A binder as claimed in any preceding claim, wherein the aziridine polyester is the reaction product of tris - (2 - methylaziridinyl - I )phosphine oxide, a straight chain diacid of the general formula HOOC(CH2)xCOOH, x being 2 to 8, and a substituted diacid of the general formula HOOCCH(R) . (CH2)yCH(R1)COOH, in which R and R1 are the same or different and each is H or OH and y is 0 to 6, the phosphine oxide being reacted in an amount of 1 mole per carboxyl equivalent of the acid.
5. 5. A binder as claimed in Claim 4, wherein the diacids are d-tartaric acid and adipic acid in the respective proportions of 0.15 and 0.3 mole per mole of the aziridinyl phosphine oxide.
6. 6. A binder as claimed in any preceding claim, wherein the amine polyester is the reaction product of N - methyldiethanolamine and sebacic acid.
7. 7. A binder as claimed in any preceding claim, wherein the aziridine polyester

   is present in an amount of 0.1% to 1% by weight of the total binder and the amine polyester is present in an amount of 0.1% to 0.5% by weight of the total binder.
8. 8. A binder as claimed in Claim 7, wherein the aziridine polyester is present in an amount of 0.4% to 0.8% by weight of the total binder and the amine polyester is present in an amount of 0.2% to 0.4% by weight of the total binder.
9. 9. A binder as claimed in Claim 8, wherein the aziridine polyester is present in an amount of 0.8% by weight of the total binder and the amine polyester is present in an amount of 0.4% by weight of total binder.
10. 10. A binder as claimed in an preceding claim, wherein the diisocyanate curing agent is DDI (as herein defined), in a proportion so that the NCO/OH equivalent ratio is in the range from 0.65--0.95.
11. 11. A binder as claimed in Claim 10, wherein NCO/OH ratio is 0.75.
12. 12. A binder as claimed in any preceding claim, which contains isodecyl pelargonate as plasticizer.
13. 13. A curable binder as claimed in Claim 1, substantially as described hereinbefore.
14. 14. A castable propellant, comprising a binder according to any preceding claim, having dispersed therethrough finely-divided ammonium or potassium perchlorate as oxidizer, in an amount from 68%88% by weight of the total propellant and optionally a metallic additive in an amount from 0% to 20% by weight of the total propellant.
15. 15. A propellant as claimed in Claim 14, wherein the metallic additive is finelydivided aluminium or magnesium.
16. 16. A propellant as claimed in Claim 14 or 15, further comprising a burning rate catalyst consisting of 0.1% to 1% of ferric oxide by weight of the propellant composition.
17. 17. A propellant as claimed in Claim 14, 15 or 16, wherein the oxidizer is present as a di or trimodal distribution of average particle size in the range from I to 400 m.
18. 18. A propellant as claimed in any of Claims 14 to 17, wherein the metallic additive has an average particle size in the range from 5-50 ,um.
19. 19. A castable propellant according to Claim 14, substantially as hereinbefore described.
20. 20. A method for the manufacture of a castable propellant which compfises: (i) mixing together at atmospheric pressure liquid ingredients comprising a hydroxy-terminated butadiene polymer of the structural formula:

    wherein n=5745, a plasticizer, a bonding agent comprising an aziridine polyester, which is the reaction product of an aziridinyl phosphine oxide and a polycarboxylic acid, and an amine polyester, which is the reaction product of an alkalolamine and a saturated aliphatic polycarboxylic acid, and an antioxidant comprising by weight 25%75% of di - tert - butylhydroquinone in conjunction with 75%25% of one or other of N - phenyl - N' - (1,3 - dimethylbutyl) - p - phenylene - diamine and N phenyl - N' - cyclohexyl - p - phenylene - diamine, the amount of antioxidant being approximately 1% by weight of the polybutadiene polymer, (ii) adding solid ingredients comprising finely-divided aluminium, ferric oxide as a burning rate catalyst and ammonium perchlorate oxidizer in an amount comprising approximately 75% by weight of the total amount of such oxidizer required and mixing to give a substantially even dispersion, (iii) heating the mixture at approximately 600C under vacuum for about 1 hour, (iv) releasing the vacuum, (v) adding and mixing in the remainder of the ammonium perchlorate oxidizer and a diisocyanate curing agent (vi) reapplying vacuum and heating to about 60"C for about 45 minutes, and (vii) casting the resulting mixture in moulds under vacuum.
21. 21. A method according to Claim 20, wherein the diisocyanate curing agent is added and admixed prior to addition of the remainder of the ammonium perchlorate oxidizer.
22. 22. A method for the manufacture of a castable propellant according to Claim 20, substantially as hereinbefore described.
23. 23. A castable propellant when made by a method as claimed in any of Claims 20 to 22.