CA1056984A

CA1056984A - Curable binding systems

Info

Publication number: CA1056984A
Application number: CA243,659A
Authority: CA
Inventors: Guy Perrault; Gonzague L. Duchesne
Original assignee: Minister of National Defence of Canada
Current assignee: Minister of National Defence of Canada
Priority date: 1976-01-16
Filing date: 1976-01-16
Publication date: 1979-06-19
Also published as: SE8207110L; GB1513706A; NO144486C; DE2701494A1; SE437271B; NL180428B; DK154948C; DK15577A; DK154948B; SE460476B; FR2352030B1; JPS5941957B2; SE7700279L; BE850420A; IT1076219B; JPS52110752A; FR2352030A1; NO770011L; NL180428C; NO144486B

Abstract

ABSTRACT OF THE DISCLOSURE
A curable binder for use in forming a castable propellant of the hydroxy terminated polybutadiene type uses two bonding agents, an aziridine polyester and an amine polyester. The amount of aziridine polyester is reduced by a factor of 3 to 4 compared to known systems. Improved pro-pellant properties, including elongation at maximum load, and at rupture while maintaining comparable tensile strength and initial moduli, are obtained. A diisocyanate curing agent, an antioxidant such as phenyl-beta-naphthyl amine and isodecyl pelargonate as a plasticizer may be included.

Description

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1 This invention relates to ~omposite solid propellants and more particularly to an improved hydrbxy-terminated polybutadiene ~TP~) bindor system for such propellants which includQs a mixture of two bonding agents.
Castable propellants using polybutadiene-based binders in conjunction with, for example, ammonium perchlorate oxidizer are well known. The mechanical and ballistic properties of such com-posite solid propellants are very dependent upon the quality of the adhesive bond between the binder and the oxidizer, as discussed in United States Patent 3,745,074 issued July 10, 1973, to Allen. In ~ -~
the prior art, systems are known which have reasonably good adhesion between the fuel binder and oxidizer but such systems have other disadvantages.
One such system utilizes carboxyl-terminated polybutadiene (CTPB) binders cured with a system involving aziridines or epoxides or a mixture thereof. Propellants using such binder systems 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 aziri-dine curing agent, tris(2-methyl aziridinyl-l)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 system has the disadvan-tages that it involves a complicated curing system, the propellants have poor aging characteristics when exposed to high temperature, and the properties of the polymer binder vary from one batch to another. -Another system which was developed in an attempt to overcome the disadvantages of the foregoing system is that of the HTPB-based composite propellants, which utilizes hydroxy-terminated polybuta-diene prepolymers. These systems are cured with diisocyanates.

. ` ~ .

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1 These propellants required 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, and a relatively high concentration is required, about 2 to 3 per cent of aziridine polyester in the binder, to 10 provide efficient adhesion between the oxidizer and binder parti- -~
cularly when the solid loading is high.
The present invention is based on the finding that a combina- -tion of two bonding agents, an aziridine polyester and an amine polyester, 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 aziridine polyester bonding agent.
Thus, the invention provides a curable binder for use in forming a castable propellant which comprises:
1. a hydroxy-terminated butadiene polymer,

2. a diisocyanate curing agent, and

3. a bonding agent system of 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 poly-carboxylic acid. Additionally, an antioxidant such as phenyl-beta--1 naphthyl amine may be included, together with isodecyl pelargonate as a plasticizer.
The invention further provides a castable propellant utilizing the aforementioned binder and having dispersed there-through a finely divided perchlorate oxidizer in di or trimodal distributions, and other optional ingredients such as a metal additive. The invention still further provides a method for pre-paration of the composite propellant which provides good disper-sion of the ingredients, and reproducibility in processing, mechan-ical and ballistic properties.
The HTPB prepolymers suitable for use in the binder andpropellant systems according to the present invention are, for - - example, of the type described in Canadian Patent 891,562 issued January 25, 1972, to Boivin and Tremblay, and United States Patent 3,792,003 issued February 12, 1974, to Duchesne. The prepolymers therein described are hydroxy telechelic polybutadienes, particu-larly 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 com-pound is an organic compound containing a single epoxy group including mono-epoxy resins; particularly preferred are epoxy compounds of the formula:
., O
/\-R - C - C - R
wherein each of R and R' is hydrogen, aryl, or an alkyl preferably lower alkyl group, and more particularly 1,2-alkylene oxides such 30 as propylene oxide and 1,2-butylene oxide which produce secondary hydroxyl groups when reacted with the carboxyl telechelic polymer.

.

lOStj984 The carbo~ terminated ~olybutaclienes are suitably thosc supplied under the trade mark IIC 434 by Thiokol Chemical Corporation and ~-under the trade ~ark HYCAR-CTB by B.F. Goodrich and Company.
Thus a typical hydroxy-terminated polybutadiene has a molecular weight of about 3~00 and is believed to have the following for-mula:
9 ~ O
HO-CH-C112-O-C-(CH2-CII=CHCH2)X - C~12-CH -C-OCH2-CHOH
Cll3 CH CH3 C1~2 Y ~.
wherein x and y each vary between 1 and 67.
A preferred IITPB prepolymer is that sold under the trade mark Poly bd R45-M by Arco Chemicals Co. and has the following formula HO ~ CH2-CH=CH-CH ~ (C~ H~ CH2-CH=C~-c~2 ~ --OH
CH-C~2 _ n wherein n=44-60 and the polybutadiene structure is 60% trans-1,4, 20~ cis-1,4, and 20% vinyl-1,2.
The prepolymer is used as the major ingredient in the propellant binder system, suitably in an amount of about 50-85 wt. %, preferably in an amount of about 60-65 wt. ~ of the binder composition. Suitably, an antioxidant is utilized with the pre-polymer and may be for example the phenyl-beta-naphthylamine 2~ (PBNA), the bisphenyl AO-2246 tAmerican Cyanamid) or various com-pounds having the chemical group p-phenylenediamine. The pref-erred antioxidant for the purposes of the present invention is phenyl-beta-naphthylamine in a concentration of about 1 per cent by weight of the prepolymer.
The diisocyanate curing agent for use in the binder compo-sition accordin~ to the present invention is for example 2,4-toly-lene diisocyanate (TDI), 1,6-hexamethylene diisocyanate (HMDI)or~DI

_ ..... . . . . . .. . .. . .

105t;984 1 which is a mixture of isomers of diisocyanate containing 36 carbon atoms prepared by dimerization of 18 carbon fatty acids. The pre-ferred diisocyanate is DDI for the purposes of the present propel-lant 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 0.75-0.95.
In order to facilitate mixing of the binder composition with the solid oxidizer in preparing the castable propellants of the invention, a plasticizer is suitably included in the binder composition in a proportion of about 15 to 30 per cent based on total weight of the binder composition. Compounds suitable as plasticizers are well known in the polymer art, and in the present compositions 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 in - for example United S*ates Patents 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 are X - P - X (I) and ~ C - OH (II) where Xl represents an aziridine group of the structure lOS69B4 --N / ¦
C~

and Ql and Q2 are either hydrogen or alkyl groups of one to four carbon atoms (Ql and Q2 may be the same or different), X2 may be the same as Xl or may be an organic radical such as phenyl, benzyl, methyl, ethyl, etc., R is an alkyl that contains at least one active hydrogen atom or an organic entity of molecules that contain 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 C - O - C-~ R (III) 2 H H n where Xl, X2, Ql' 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 accord-ing to the present invention are prepared from tris-1-(2-methyl ; aziridinyl) 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 aziridimyl-l) phosphine oxide with a straight chain diacid of general formula HOOC (CH2 ) X COOH (x being 2 to 8 and preferably

4, 5 or 6) and a second substituted diacid of general formula HOOC CH (R) (CH2)y CH (Rl) COOH in which R and Rl can be similar lC)S~984 1 or different being H or OH and y = 0 to 6 being preferably 0, 1 or 2. In a typical synthesis, 3 to 8 moles of MAPO,- and preferably

5 to 6 moles, are cor.densed with 0.5 to l.0 mole of the sub~tituted diacid and 1.75 to 2.5 moles of the straight chain diacid at 50 to 65C for 4 hrs. under N2.
The proportion of the aziridine polyester bonding agent in the binder system according to the present invention is between about 0.1 and 1% by weight of 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 alkanol-amine and a saturated aliphatic polycarboxylic acid. The preferred amine polyester for the present purposes is a polymer derived from N-methyldiethanolamine and sebacic acid which is known as Polymer N-8. Suitable N-8 Polymers have the following specifications:
Mn (number average Mw) : 1200-2000; preferred 1500-1800 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 formulas 2 I CH2 FHO [co(cH2)ncoocH-cH2-N-cH2-cHo~ H
~ R''' R'V R'' R''' R~v wherein R'' and R~v can be the same or different and can be an hydrogen or a saturated alkyl chain having one or two carbon atoms;
R''' can be a saturated alkyl chain with one to eighteen carbon atoms; n is an integer from 2 to 16, the preferred one being 8;
and ~ is an integer between 3-10, the preferred ones being 5-7.
This bonding agent is 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 pre-105f~984 1 ferred 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 sizes of 1-400fcm. A preferred particle size distribution of the ammonium perchlorate is 1.7/2.7/1.0 parts of 400~m size, 200f~m size, and 17~m size respectively. For high temperature applications the ammonium perchlorate may be substituted by potassium perchlor-ate. Suitably an anticaking agent, e.g. tricalcium phosphate, is included in an amount of about 1% by weight of the 17~m 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 aluminum or magnesium powder, preferably having an average particle diameter of 5-50~ m. Another solid material suitably included in the over-all composition is a burning rate additive or catalyst which may be for example iron oxide, copper chromite, or an organo-metallic compound. The preferred additive or catalyst for the present purposes is ferric oxide in an amount of 0.1-1% by weight - of propellant composition.
In composite propellant compositions it is of course desir-able to obtain as high solid loading as possible, and with the propellant compositions according to the present invention the total solids content can vary between about 85 to 90% by weight of total composition, of which of course the ammonium perchlorate will -` constitute the major part and may be between 68 and 88% by weight of total propellant composition. The other solid components are adjusted accordingly therein. Thus, the polymeric binder will constitute about 10 to 15~ by weight of the total propellant com-position.
In order to obtain good dispersion of all the ingredients in preparation of the propellant composition and also reproducibility ~....
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105~984 1 of properties from one batch to another, the preferred procedure is to first add to a mixer the liquid ingredients which are the prepolymer, the plasticizer, and the two bonding agents. The aluminum or other metal powder when used and the ferric oxide burning rate additive are then added and the contents 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 continued for a further 10 minutes. Vacuum is applied and the contents of the mixer heated to 60C for about 60 minutes. The vacuum is then released and the remaining amount of ammonium perchlorate added and mixing 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 continued for 5 minutes at atmospheric pressure. After this vacuum is again applied and mixing continued for 45 minutes at a temperature of 60C. The propellant is then cast into molds under vacuum.
The following example illustrates the use of the binder system according to the present invention in preparing composite solid propellants. This example is intended to be illustrative of~
the present invention but is not to be taken as limiting to the scope thereof. The propellant compositions set out in the example were each prepared according to the method described in the previous paragraph herein.
The mechanical properties tested for the propellant composi-; tions according to the present invention were tensile strength (~m)~
elongation at maximum load (~m)' elongation at rupture (~ , and initial moduli (E). These tests were carried out on an Instron apparatus at a cross-head speed of 2 in./min. corresponding to a - 30 strain rate of 0.741 min. 1 for the ICRPG "dumbell" specimen.

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105~984 Propellant compositions were prepared having the following proportions of ingredients expressed in weight per cent:
Ammonium perchlorate (400~m) Ammonium perchlorate (200fLm) ) 69.4 Ammonium perchlorate (17~ m) Aluminum (spheroidal grade 22~um) 18.0 Ferric Oxide Catalyst 0.6 HTPB Binder 12.0 For testing three binder compositions were used in the ~
foregoing propellant composition. The prepolymer used in each ;
binder was the R45-M (trade mark) type of HTPB binder sold by Arco Chemical Co. previously described herein. Phenyl-beta-naphthyl amine as antioxidant was included, isode~yl pelargonate as plasticizer, and DDI diisocyanate as curing agent. In one ~-binder composition the two bonding agents according to the inven- ~;
tion were not utilized but rather only the aziridine polyester for comparative purposes. In the other two compositions both bonding agents were included.
The binder compositions were as fQllows:
I II III
R45-M polymer (including 1~ PBNA) 62.91 62.91 62.91 ~
DDI 12.09 12.09 12.09 ~ -- Aziridine polyester 2.40 0.9 0.6 Amine polyester (N-8) - 0.2 0.2 IDP plasticizer 22.60 23.9 24.2 , 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 follow-ing properties:
~.

~................ .. . - .

lOS6g84 1 (i) OH equivalent 1.19 M equiv./g.
(ii) Acidity 0.016 equiv./100 g.
(iii) Molecular weight (Mn) 1430 determined by V~0.
The processing and mechanical properties obtained for the propellant compositions using the aforementioned binders were:
I II III
Aziridine/amine polyester (% in binder)2.4/- 0.9/0.2 0.6/0.2 End-of-mix viscosity (kP/60C) 2.5 2.7 2.7 Pot life to 10 kP (hr) 4.2 4.2 4.5 Mechanical Properties (initial) at 22 8C ~MN/m2) .58 .79 .63 ~m at " (%) 30.6 36.9 41.6 E at " (~IN/m2) 3.41 4.04 3.24 : m at -45.6C (%) 48.2 58.8 65.6 ~r at " (%) 54.5 59.8 68.5 Mechanical Properties (56 days at 60C) ~m at 22.8C (MN/m2) .58 .79 .73 C m at " (%) 35.2 35.9 39.1 E at " (MN/m2) 3.37 4.82 3.85 ~m at -45.6C (%) 49.4 48.9 59.5 ~r at " (%) 50.7 50.4 60.9 It-will be seen from the foregoing results that the process-ing characteristics, that is end-of-mix viscosity and pot life, for the propellants according to the present invention (II and III) were substantially the same as these properties for the reference propellant. However, the amount of aziridine polyester utilized according to the invention is considerably less than that required - for the reference composition. Also, the propellants according to the invention show improved elongations at maximum load and at rupture while the tensile strength and initial moduli are comparable - to the reference composition.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follow:

1. A curable binder system for use in forming a castable composite propellant which comprises (i) a hydroxy-terminated butadiene polymer, (ii) a diisocyanate curing agent, and (iii) a bonding agent system of an aziridine polyester which is the reaction product of an aziridinyl phosphine oxide and a poly-carboxylic acid, and an amine polyester which is the reaction pro-duct of an alkanolamine and a saturated aliphatic polycarboxylic acid.

2. A binder as claimed in Claim 1, wherein the aziridine polyester is the reaction product of tris-(2-methyl aziridinyl-1) phosphine oxide and a diacid selected from the group comprising a straight chain diacid of general formula HOOC(CH2)xCOOH, x being 2 to 8, and a substituted diacid of general formula HOOC CH(R)(CH2)yCH(R1)COOH in which R and R1 may be similar or different being H or OH, and y = 0 to 6, the phosphine oxide being reacted in an amount of 1 mole per carboxyl equivalent of the acid.

3. A binder as in Claim 2 wherein the diacids are d-tartaric and adipic in the respective proportions of 0.15 and 0.3 mole per mole of aziridine.

4. A binder as claimed in Claim 1, wherein the amine polyester is a polymer derived from N-methyldiethanolamine and sebacic acid.

5. A binder as claimed in Claim 1, 2 or 3, wherein the aziridine polyester is present in an amount of 0.1-1% by weight of the total binder, and the amine polyester is present in an amount of 0.1 to 0.5% by weight of total binder.

6. A binder as claimed in Claim 1, 2 or 3, wherein the diisocyanate curing agent is a mixture of isomers of diisocyanate con-taining 36 carbon atoms, used in a proportion so that the NCO/OH ratio is in the range of 0.75-0.95.

7. A binder as claimed in Claim 1, wherein the hydroxy-terminated butadiene polymer is wherein n = 44-60.

8. A binder as claimed in Claim 1 or 7, which contains isodecyl pelargonate as plasticizer.

9. A castable propellant comprising the binder of Claim 1 having dispersed therethrough finely divided ammonium perchlor-ate as oxidizer and finely divided aluminum, the latter being present in an amount of 0-20% by weight of total propellant.

10. The propellant of Claim 9, wherein the oxidizer is present in an amount of 68-88% by weight of total propellant com-position.

11. The propellant of Claim 10, wherein the oxidizer is present as a di or trimodal distribution of particle sizes between 1 and 4000µm.

12. The propellant of Claim 9, wherein the aluminum has an average particle diameter in the range 5-50µm.

13. The propellant of Claim 9, 10 or 11 containing isodecyl pelargonate as plasticizer.

14. The propellant of Claim 9, 10 or 11, wherein the curing agent is a mixture of isomers of diisocyanate containing 36 carbonates.

15. The propellant of Claim 9, wherein the aziridine polyester and amine polyester are present in amounts of 0.1-1%
and 0.1-0.5% respectively by weight of binder.

16. The propellant of Claim 15, wherein the aziridine polyester is the reaction product of tris-(2-methyl aziridinyl-l) phosphine oxide and d-tartaric and adipic diacids.

17. The propellant of Claim 15, wherein the amine poly-ester is a polymer derived from N-methyldiethanolamine and sebacic acid.

18. The binder of Claim 1, 2 or 3 wherein phenyl-beta-naphthylamine is included as anti-oxidant.

19. The binder of Claim 1, 2 or 3 wherein phenyl-beta-napthylamine is included as anti-oxidant in an amount of about 1 percent by weight of the hydroxy-terminated butadiene polymer.

20. The propellant of Claim 9, 10 or 11 wherein ferric oxide is included as catalyst.

21. The propellant of Claim 9, 10 or 11 wherein ferric oxide is included as catalyst in an amount of 0.1 to 1% by weight of propellant composition.

22. A method for the manufacture of a composite propell-ant which comprises (i) mixing together liquid ingredients comprising a hydroxyterminated butadiene prepolymer, a plasticizer and an aziridine polyester and an amine polyester bonding agent at atmospheric temperature and pressure, (ii) adding any finely divided aluminum metal used and mixing to even dispersion, (iii) adding about 75% of the total amount of ammonium perchlorate oxidizer to be dispersed, (iv) heating at about 60°C under vacuum for about 1 hour, (v) releasing the vacuum and adding the remainder of the ammonium perchlorate oxidizer and mixing, (vi) adding a diisocyanate curing agent, and mixing for a short time, (vii) reapplying vacuum and heating to about 60°C for about 45 minutes, (viii) casting the resulting mixture into molds.