Note: Descriptions are shown in the official language in which they were submitted.
~o5~i~9~a~
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.
. ` ~ .
105t~
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-
-
1056984
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.
.
--3--
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
1 / Q 1
--N / ¦
C~
H Q2
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
--6--
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
-8-
~....
;
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.
_9_
.- .
105~984
1 EXAMPLE
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:
~.
--10--
~................ .. . - .
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.
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