Note: Descriptions are shown in the official language in which they were submitted.
- Winer Case 4
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This invention relates to propellant compositions hav-
ing graphite fibers incorporated therein for the purpose of in- `~
creasing the burning rate of the propellant.
The burning rates of propellants have previously been
augmented by incorporating metallic wires, ribbons or chopped
foil into the propellant. The usual metals employed are aluminum,
silver, and zir~onium though many metals have been shown to be
effective to various degrees. The metal may-be introduced as par-
ticles in the form of continuous wires or ribbons, short wires or
ribbons, chopped foil, platelets, flake, and the like. The metal
particles may be purposely oriented or aligned in a-given direc- -
tion or they may be randomly dispersed. The metal particles may
act by providing paths of high thermal diffusivity to transmit
heat from the propellant combustion reaction surface to the pro-
pellant below the reaction surface, by reaction of the metal it-
self or by a combination of the two mechanisms. Depressions are
generally formed in the propellant surrounding the metal particle -
during burning, increasing the propellant burning surface area.
The increased surface area results in an lncrease in the rate of ;~
consumption of the propellant and in increased rate of gas gener-
ation as a result thereof.
Inclusion of metals in propellant can cause several dis-
advantages, dependin~ on the type of propallant and the metal used,
and also depénding on the intended use of the propellant. The
propellant containing metal particles-may be more susceptible to
accidental initiation from impact or frictional forces or from
. .. . .
electrostatic discharge potential. The propellant mechanical
properties may-be degraded due to the introduction of inhomogeni-
ties in the-propellant matrix. The propellant specific impulse
may be reduced due to the low heat of reaction of certain metals.
In some gas generators and in gun propellants, solid particles in
the exhaust may be detrimental to hardware because of abrasive
action. Smokeless propellants may have objectionable-visible ex- ^
hausts due to the products of combustion of the metal. Each of
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these disadvantages can be substantially reduced or overcome in accordance
with the teaching of this invention.
It has no~ been found that tiny graphite fibers can be employed in
all types of propellants such as gun propellants, gas generator propellants,
small rocket propellants and in propellants for large missiles, to augment the
burning rates t~ereof regardless o~ the method of manufacture and regardless
of the orientation of the fibers in the propb~lant.
According to the present invention,there is provided a solid pro-
pellant composition for use as a solid propellant in gas generators, guns, and
rocket motors, said composition being selected from the group consisting of
single base, double base, triple base composite and composite modified double
base composition types, said solid propellant composition containing a multi- ~`
plicity of graphite fibers having diameters of from about 4 microns to about
lO microns substantially uniformly distributed throughout said solid propellant
composition said graphite fibers comprising from about .03% to about 10% by
.. .. .
weight based on the weight of the solid propellant composition. ~ - ~
.:
Whilst graphite fibers are employed as chopped fibers having ` r
diameters of from about 4 to about lO microns, the length o the graphite
fibers employed can be varied over a wide range depending on the particular
application. Lengths of fiber as short as several mils are effective for
increasing burnd~g rates of propellants. Lengths of fibers of from about l/4
inch to about 314 inch are preferably employed. Fibers can be broken during ;
mixing, so mixing is controlled to prevent destruction of the fibers. It has
been found that burning rate augmentation decreases as the lengths of graphite :
fiber employed decreases. ~
It is generally preferred to employ the fibers in an amount of from -~,
about 0.5% to about 6% by weight based on the wei~ght of the propellant
composition.
The graphite fibers should be completely distributed throughout
the propellant for optimum controlled performance. Such distribution is
achieved by thorough mixing in conventional mixing equipment employed in the
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propellant art. Substantially complete distribution can be achieved in most
propellant formulations after about 10 minutes of mixing in conventional
mixers. Suitable distribution of the graphite fibers can usually be evaluated -
~by visual observation of the propellant. Microscopic examination of the
propellant can be made if desired.
Graphite fibers consist essentially of carbon atoms arranged in
the crystal form characteristic of graphite. Graphite fibers can be prepared ~
from natural or synthetic organic materials. Illustrative precursor material :
from which carbon fibers are made include, but are not limited to, polyacrylo-
nitrile, cellulose, regenerated cellulose polyvinyl alcohol, polyvinyl
chloride, polyesters, polyamides, pitch and the like.
Propellants containing the graphite fibers can be made by any
suitable method such as by conventional casting, slurry casting, and extrusion.
All of such processing methods are well known in the propellant art. ~he pro-
pellant matrix into which the fibers are incorporated can be of the single
base, double base, triple base, or composite type which term is defined herein
to include composite modified double base propellants.
The use of graphite fibers in preparation of smokeless gun propel-
lants is of particular interest since gun propellant formulations can be
prepared employing composite type propellant in which the characteristics of
certain composite propellants such as low flame temperatures and low molecular ~ ;
weight combustion gases can be taken advantage of, while the burning rate of
the composite propellant is substantially increased by incorporation of
graphite fibers.
Figures 1, 2 and 3 are schematic views of propellant granules con- `~
taining graphite fibers. Figure 4 is a graph showing burning rate versus
pressure for propellants prepared in the examples which follow.
In pr~paring the propellant compositions of this invention, the
graphite fibers may be either randomly dispersed or aligned depending upon
the method employed to manufacture the propellant. If the propellant is
extruded into the shape of a granule having a longitudinal axis such as in ~
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the preparation of base grain for a cast propellant, or in preparation of
gun propellant by conventional extrusion processes, a substantial proportion -
of the graphite fibers will be oriented perpendicular to the end burning
surface of t~e propellant granule, i.e., parallel to the longitudinal axis
of the granule. Orientation of graph~ite fibers in propellant granules is
illustrated in Figs. 1 and 2. The fibers 10 are oriented perpendicular to
end burning surfaces 12, 14, 16, 18. Random orientation of graphite fibers
is shown schematically in Fig. 3. Maximum increase in propellant burning
rate
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as been found to occur when the graphite fibers are oxiented
perpendicular to the burning surface. ~
The propellant compositions of this invention are more --
fully illustrated in the Examples which follow. In the Examples,
parts and percentages are by weight unless otherwise specified.
Example 1
About 13 parts of carboxy terminated polybutadiene rub-
ber, 0~5 part of a curing agent for said rubber and 4.3 parts of
methylisobutylketone, which-is a solvent for the-rubber, are added
lU to a mixer which is~preheated to 120F. These ingredients are
mixed for-five minutes. Then, 24 parts of organic oxidizing agent
and 0.4 part of graphite fibers having a nominal diameter of about
9 microns and having an average length of-about 0.2 inch are added
to the mixture and mixing is continued for ten minutes. The
graphite fibers employed are-available commercially from Hercules -~
Incorporated and are sold as Type HM-S. About 24 parts of the
organic oxidizing agent, 0.4 part of graphite fibers, and 4 3
parts of methylisobutylketone are added-to the mixture a~d mixing l~
is continued for an additional ten minutes. Twenty-four parts of --
,, . ,.,, j .. .... ..
20 the organic oxidizing agent, 0.4 part of graphite fibers, and 4.3 -~
parts of methylisobutylketone are a~ain added to the mixture and -
mixing is continued for an additional ten minutes. The final por-
tion of 0.4 part of graphite fibers is added to the mixer and the
total mixture i~ mixed for 2 hours-at 120F. with the mixer lid
closed. The mixer lid is then opened and the methylisobutylketone
solvent is allowed to evaporate until a propellant dough of extru-
sion consistency results. The douyh is extruded from a 2-7/8 inch
diameter extrusion press through a 0.250 inch diameter die at 900-
1100 p.s.i.g. The extruded propellant strands are cut into six ~ ;
30 inch lengths and cured for four days at 140F. ;
I ~E'~ d
Example 1 is repeated, except the total graphite fiber
content of the propellant is increased from the 2.0% by weight level
to 4.0% by weight (Example 2) and 6.p~ by weight tExample 3). The
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Libers are added in four equal increments as in Example 1.
Example 4
A control propellant composition is prepared in which no
graphite fibers are added. The propellant composition and mixing
procedure is the same as employed in Example 1 with the exception
of the deletion of the graphite fibers.
The effect of graphite fiber strands on the burning
rate of the composite gun propellant composition of Examples 1-4
is evaluated by burning the strands conditioned to 77F. in an
Atlantic Research Corporation Strand Bomb Apparatus. Results-of
the strand burning tests at various test pressures are set forth ;~
in~Table I which follows.
TABLE I
Example 1 2 3 4
Weight ~ Graphite
Fibers 2.0 4.0 6.0 0.0
Bomb Pre~surei ProPellant Burninq Rates
(lbs./in - gauge) ~Inches7secon~
1000 0.255 0.360 0.417 0.154
0.258 0.347 0.417 0.154
~.347 0.419
1500 0.341~ ~.522 0.546 0.192
0.323 0.522 0.551 0.192
0.453 0.563 0.195
2000 0.381 0.537 0.672 0.243
0.391 0.537 0.741 0.238
0.543 0.757
2500 0.420 0.616 0.723 0.301
0.427 0.615 0.743 0.299
0.608 0.754 0.284
3000 0.480 0.688 0.812 0.329
0.469 0.688 0.825 0.325
0.698 0.857 0.358
The effect of the graphite fibers on the burning rate of
the gun propellant compositions is clearly-illustrated by compari-
son of the burning rate data presente~ in Table I. Thus, at 3000
p.s.i.g. for example, the burning rates of the composite propel-
lant prepared in Example 1 (2~ graphite fiber), Example 2 (4%
graphite fiber), and Example 3 (6% graphite fiber) are increased
46%, 108%, and 155% respectively, over the burning rate of the
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ontrol propellant, Example 4. The effect of graphite fibers on
the gun propellant burning rate is graphically presented in Fig. -:
4 in the plot.of burnin~ rate (inches/secona) versus pressure
(lbs./in.2 - gauge). The slope of the curves, n, is seen to be .
less than the.slope of control propellant, Example 4.
Examples 5-10 .
.
The follGwing examples illustrate the increased burning ::
rates achieved by incorporating graphite fibers into composite .-
modified-double base pr~pellants. In these examples the graphite
10 fibers are added to.the propellant matrix during mixing and ex- ;;.
truded into propellant strands. In Example.6, the small amount .:.
of fibers is added in a single.increment. In Example 7 and Exam-
ples-8, 9, 10, the-fibers.are added in two and three equal incre- ~.
ments, respectively. Each of the extruded strands is dried and ~ ;
heat cured for 3 days at 140F. The strands are burned in an ::: :
Atlantic Research Corporation Strand Bomb Apparatus at 2000 p.s.i.,
af~er conditioni~g of the strands to 77F. The basic propellant .. ~
composition and the,effects of the~graphite fiber on propellant .:
burning rate are set forth in Table II. ~xample 5 is a control
propellant.
TABLE II ~
Ingredient .. .
Weight (%j 5 6 7 8 9 10 .;
Nitrocellulose. 16.2 16~3 16.2 15.115.1 15.1 ~-
Nitroglycerin32.3 32.3 32.3 30.1 30Ø 30.0 ,
Triacetin 5.7 5.7 5.7 5.3 5.35.3
Surfactant 0.2 0.2 0.2 - 0.20.2
Stabilizers. 2.3 2.1 2.3 2.1 2.12.1
Ammonium .'
perGhlorate43.3- 43.3 43.0 40.2 40.140.1
Aluminum powder 0 0 0 4.24.2 4.2 . ..
Graphite fiber
(Type HM-5) 0 0.03 0.3 3.0 0 0
Graphite fiber2 `::.
(Type HM-U) 0 0 0 0 3.0 0
L7~f~
TABLE II (Continued)
Ingredient
Weight (%-? 5 6 7 8 9 10 -
Graphite fiber3
(Type HT-S) 0 0 0 3.0
r~ooo (in/sec) 3.0 6.1 6.6 8.6 9.0 6.3
Prepared from polyacrylonitrile (PAN) precursor;
modulus 50-60-x 106 p.s.i.; surface treated
2Prepared from (PAN) precursor; modulus 50-60 x 106 p.s.i.;
no surface treatment
3Prepared from (PAN) precursor; modulus 32-40 x 106 p.s.i~;
surface treated
As can be readily seen from the burning rate data for
Examples 5-10 in Table II, the burning rates of composite modified- ,
double base propellants containing graphite fiber (Examples 6-10)
were all-greatly increased over the control propellant burning
rate (Example 5). In these examples, a substantial proportion-of
the graphite fibers are oriented perpendicular to the end burning
surfaces of the propellant strands during the extrusion of--the
strands.
Exam~s 11-12
The following examples illustrate the use-of graphite-
fibers in preparation of propellants by conventional slurry cast-
ing methods. The-graphite fibers employed in the propellant com-
position of Example 12 is added-to a slurry of the propellant in-
gredientsO Example ll is a control composition. The graphite
fibers employed are chopped and have an average initial length of
1/4 inch. After the propellant ingredients are mixed in the ~`
slurryj blocks of propellant are cast and cured for five days at
140F. Strands 1/4" x 1/4" x 4" are sawed from the cured blocks.
The strands are burned in an Atlantic~Research~Corporation Strand
Bomb Apparatus at 1000 p.s.i., and the burning rates are measured.
The burning rate data of these compositions are set forth in
Table III.
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TABLE III
Ingredient Example ..
Wei~ht (~ 11 12
. .
Nitrocellulose :
(Plastisol type) 11.1 . 11.0
Nitroglycerin 41.4 41.0 .
Crosslinking a~ent 7.4 7.3 .
Plasticizer- 4-9 4 9
Stabilizer 1.0 1.0
~allist}c modifiers 4.0 4.0
Cyclotrimethylenetrinitramine 30.0 29.8 ..
Carbon black (colloidal)0.2 0
Graphite fiber (Type HM-S) 0 1.0
rlO00 (in./sec.) 0-37 ~.48 ~. . .
Burning rate-data show an increased burning rate for
the propellant.of Example 12 of about 30% compared to the-pro- ~ :
pellant composition of Example 11.
The solid propellant-compositions o~ this invention
having graphite fibers uniformly incGrporated therein to augment .
burning rates can be of the single base, double base, triple
base and composite type..composition~ Single.base compositions-
are prepared principally from nitrocellulose.and generally con~
tain.stabilizing agents. Double base propellants are principally
prepared from nitrocellulose and nitroglycerin or a similar type
explosive plasticizer for nitrocellulose. Triple base propel~
lants are.prepared principally from nitrocellulose, nitroglycerin
or similar explosive plasticizer-for nitrocellulose and nitro~
guanidine. Composite type propellants are prepared principally ~.:
from a polymeric binder and anioxidizin~ agent in.solid particulate
form dispersed throughout the:binder. Illustrative polyme.ric
binders employed in preparation of composite.propellants include
carboxy-terminated polybutadiene, hydroxy-terminated polybuta-
dienes, polyethers, polyurethanes and the like. The binders are `
prepared from-liquid polymers which are crossli~ked with curing
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~9117~1
agents to produca the propellant binder. Oxidizing agents are
incorporated in the uncured binder during mixing of the propel-
lants. Illustrative oxidizing agents which can be employed in-
clude inorganic solid oxidizing agents sush as-ammonium perchlor-
ate, and organic solid oxidizing agents such as cyclotrimethylene
trinitramine (R~X), cyclotetramethylene tetranitramine (HMX),
pentaerythritol ~etranitramine, ethylene dinitraminei mixtures
thereof, and the like.
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