Note: Descriptions are shown in the official language in which they were submitted.
CA 02322096 2000-10-02
TITLE
THERMOPLASTIC POLYMER PROPELLANT COMPOSITIONS
Field Of The Invention
The present application relates to propellant compositions e.g. rocket
propellant or gas generator compositions, that have a binder formed from
compositions of thermoplastic ethylene co-polymers, especially ethylene/vinyl
acetate copolymers (EVA) and other related polyolefins. In embodiments, the
binder is a composition of cross-linkable thermoplastic ethylene copolymer,
especially cross-linkable ethylene/vinyl acetate copolymer or related cross-
linkable copolymer. An example of a cross-linkable polymer is silane-grafted
EVA.
Backs~round To The Invention
The original black powder rocket propellants were replaced in the early
1900's with propellants based on nitrocellulose and nitroglycerine.
Subsequently propellants were developed that were based on a fuel oil, a
binder e.g. asphalt, and an oxidizer e.g. potassium perchlorate. Polysulphide
fuel binders that could be cast and cured at cool temperatures, mixtures of
ammonium perchlorate, polyester and styrene cured by cumene
hydroperoxide and compositions based on polyvinyl chloride plastisols were
also developed.
A number of polybutadiene materials, including in particular
polybutadiene acrylonitrile, carboxy terminated polybutadiene and hydroxy-
terminated polybutadiene have also been developed and undergone
commercial use. In particular, propellant compositions using polybutadiene
acrylonitrile (PBAN) as binder have been developed and are used in a
number of rocket systems, including the solid rocket boosters for the Space
Shuttle. Propellant compositions using hydroxy-terminated polybutadiene
(HTPB) are also known and in use. It is understood that systems utilizing
thermoset polymers such as PBAN and HTPB exhibit relatively long curing
CA 02322096 2000-10-02
2
times (several days) unless promoted through heat and/or catalysis, and that
pot-life suffers accordingly, and may be as short as about 20 minutes.
In most cases, propellant compositions using the above binder systems and
related systems require the use of toxic chemicals such as epoxides,
diisocyanates or aziridines as curing agents. In addition, plasticizers e.g.
ethyl hexyl acrylate or di-octyl adipate may be used, which are also known to
exhibit toxicological properties. In addition to safety considerations during
manufacture of the propellants, the cost of many of these constituents is
relatively high.
The shelf life of some of these constituents, such as epoxides and
diisocyanates, tends to be short. Special handling e.g. freezing or
refrigeration and/or inert gas blanketing, is required to extend their useful
life,
which further increases the overall cost of the propellants.
Of greater concern is the potential for allergic reactions and the consequent
need for special handling in order to protect persons handling the
compositions.
Propellant compositions offering greater flexibility, less stringent
handling requirements and less lead time in fabrication would be useful.
Summary Of The Invention
Propellants formed from compositions of thermoplastic polymers, and
methods for the manufacture thereof, have now been found.
Accordingly, an aspect of the present invention provides a solid
propellant composition comprising a binder and at least 65% by weight of a
material selected from the group consisting of oxidizer and crystalline high
explosive, said binder being selected from the group consisting of a
thermoplastic ethylene copolymer and a cross-linkable thermoplastic ethylene
copolymer.
In preferred embodiments of the invention, the ethylene copolymer is
ethylene/vinyl acetate copolymer or other ethylene/vinyl alkanoate copolymer
or the copolymer is selected from an ethylene/ethyl acrylate copolymer,
ethylene/methyl acrylate copolymer or ethylene/butyl acrylate copolymer, a
CA 02322096 2000-10-02
3
copolymer of ethylene with acrylic acid or methacrylic acid, an ionomer
thereof and a copolymer of ethylene with an acrylic or methacrylic acid ester.
In other embodiments, the crystalline high explosive is selected from
the group consisting of cyclotetramethylenetetranitramine,
cyclotrimethylenetrinitramine and hexanitrohexaazaisowurtzitane and the
oxidizer is selected from ammonium perchlorate, ammonium nitrate and
potassium perchlorate, especially ammonium perchlorate.
In further embodiments, the binder is cross-linkable ethylene/vinyl
acetate copolymer, said copolymer having a moisture crosslinkable monomer,
especially in which the moisture crosslinkable monomer is selected from vinyl
trimethoxysilane and vinyltriethoxysilane. Alternatively, the binder is a
silane-grafted ethylene/vinyl acetate copolymer.
In additional embodiments, the material is oxidizer.
In preferred embodiments, the composition contains at least 70% by
weight of oxidizer, especially 75-90% by weight of oxidizer.
In other embodiments, there is at least one of an energetic, a ballistic
modifier and a modifier, said energetic being selected from the group
consisting of aluminum, magnesium and aluminum/magnesium alloys, said
ballistic modifier being selected from the group consisting of oxides of iron,
copper, chromium and magnesium and calcium carbonate and said modifier
being selected from the group consisting of a titanate, a zirconate and an
aluminate.
In further embodiments, there is at least one of an additive selected
from opacifiers; stabilizers; metal de-activators; anti-oxidants; flame
colorants;
or an agent that modifies the processing, performance, mechanical properties,
storage stability or shelf life of solid propellant systems. Preferably, the
stabilizer is selected from zinc oxide, nickel oxide and triacetin, and the
flame
colorant is selected from salts of strontium, barium, sodium and lithium. In
addition, the oxidizer is preferably a mixture of particle sizes selected from
coarse, medium, fine and ultra fine particles, said coarse particles being 400-
600 micron, said medium particles being 50-200 micron, said fine particles
being 5-15 micron and said ultra fine particles being submicron to 5 micron.
CA 02322096 2000-10-02
4
In embodiments, the composition is a gas generator propellant or a
rocket propellant.
In preferred embodiments, at least 75% by weight of the material is
oxidizer.
In further embodiments, the propellant composition additionally
comprising a plasticizer that is solid or semi-solid at 20°C, and an
additive to
increase one or more of elongation, adhesion and tack. The solid or semi-
solid plasticizer is preferably selected from microcrystalline wax,
macrocrystalline wax, an oxidized hydrocarbon polyolefin and a polyketone
wax and the additive selected from a hydrogenated hydrocarbon resin and a
derivative of a rosin.
In embodiments, the binder composition contains 35-65% by weight of
copolymer, 10-30% by weight of solid or semi-solid plasticizer and 20-40% by
weight of said additive. The propellant composition may contain 10-20% of
binder.
In other embodiments, the propellant composition comprises 50-90%
by weight of oxidizer and 5-20% by weight of ethylene/vinyl acetate
copolymer, the remainder of such composition comprising at least one of said
crystalline high explosive, plasticizer, energetic, ballistic modifier and
other
propellant components.
Another aspect of the invention provides a method of manufacture of a
propellant composition comprising:
(a) preparing a pre-propellant composition of ethylene copolymer;
and
(b) admixing the pre-propellant composition with a material selected
from oxidizer and crystalline high explosive such that the resulting
composition has at least 65% by weight of said material.
In preferred embodiments of the method, the propellant composition
obtained in (b) is formed into propellant grains by an extrusion process
and/or
the grain is consolidated into final form under mechanical, pneumatic or
hydraulic pressure or centrifugal force.
CA 02322096 2000-10-02
In embodiments, the extrusion process utilizes a cooling cycle to cool
the propellant grain.
In other embodiments, the pre-propellant composition is prepared by
melt blending.
5 In embodiments, the propellant composition obtained in (b) is formed
into fuel grains in a ram extruder.
In further embodiments, there is a cold cycle to cool the propellant
grain so obtained.
In other embodiments, the pre-propellant contains at least one of an
energetic and ballistic modifier, said energetic being selected from the group
consisting of aluminum, magnesium and aluminum/magnesium alloys, and
said ballistic mod~er is selected from the group consisting of oxides of iron,
copper, chromium and magnesium, and calcium carbonate. The pre-
propellant may contain at least one additive selected from opacifiers;
stabilizers; metal de-activators; anti-oxidants; flame colorants; or an agent
that
modifies the processing, performance, mechanical properties, storage stability
or shelf life of solid propellant systems.
In preferred embodiments, the oxidizer is a mixture of particle sizes
selected from coarse, medium, fine and ultra fine particles, said coarse
particles being 400-600 micron, said medium particles being 50-200 micron,
said fine particles being 5-15 micron and said ultra fine particles being
submicron to 5 micron.
A further aspect of the invention provides a solid propellant
composition comprising a binder and at least 50% by weight of an oxidizer,
said binder being an ethylene-vinyl alkanoate copolymer, especially
ethylene/vinyl acetate copolymer. The copolymer of the binder may have a
moisture crosslinkable monomer, especially vinyl trimethoxysilane or
vinyltriethoxysilane. The binder may be a silane-grafted ethylene/vinyl
acetate copolymer.
CA 02322096 2000-10-02
6
Brief Description of the Drawinsrs
The present invention is illustrated by the embodiments shown in the
drawings, in which:
Figure 1 is a photograph of a control sample of propellant;
Figure 2 is a photograph of a second control sample of propellant; and
Figure 3 is a photograph of a propellant of the invention.
Detailed Description Of The Invention
The following acronyms are used in this application:
PETN pentaerythritol tetranitrate
TMETN trimethanolethane trinitrate
BTTN butanetriol trinitrate
GAP glycidyl azide polymer
HMX cyclotetramethylenetetranitramine
RDX cyclotrimethylenetrinitramine
PGN propylglycidyl nitrate
BAMO/AMMO bis-azidomethyloxetane/azidomethyl-
methyloxetane copolymer
BAMO/NMMO bis-azidomethyloxetane/nitramethyl-
methyloxetane copolymer
PBAN polybutadiene acrylonitrile
HTPB hydroxy-terminated polybutadiene
AP ammonium perchlorate
CL-20 hexanitrohexaazaisowurtzitane
The propellants of the present invention are particularly intended for
use in rockets, missiles, gas generating devices or similar devices i.e. end-
uses requiring propellants. The propellant comprises, as a binder, a
composition of an ethylene copolymer, especially an ethylene/vinyl acetate
copolymer.
The polymers that are used in the binder compositions of the present
invention are ethylene copolymers. Such polymers include copolymers of
CA 02322096 2000-10-02
7
ethylene and a vinyl alkanoate, especially ethylene/vinyl acetate copolymers
(EVA). Alternatively, the polymer may be a copolymer of ethylene and an
acrylate ester, examples of which are ethylene/ethyl acrylate copolymers,
ethylene/methyl acrylate copolymers and ethylene/butyl acrylate copolymers.
The polymer may also be a copolymer of ethylene with acrylic acid or
methylacrylic acid or the related ionomers viz. copolymers having the acid
groups thereof partially neutralized by metals especially sodium, zinc or
aluminum. The polymers may have other copolymerized monomers e.g.
carbon monoxide.
Compositions formed from EVA tend to exhibit physical properties
equivalent to, or superior to for instance PBAN, and be functional at
temperatures of up to about 50°C. Cross-linked compositions may be used
at
higher temperatures.
The preferred binder is a composition based on ethylene/vinyl acetate
copolymer. It has good overall mechanical properties, which may be further
improved especially at elevated temperatures, through cross-linking.
Propellant compositions may be manufactured for use at temperatures in the
range of -65°C to 130°C. In addition, it has good adhesive
properties, which
has the potential to eliminate the need for bonding agents as are often
required in other systems to improve mechanical adhesion of the binder to the
oxidizer particles. These same adhesive properties also potentially eliminate
the need for tie-coats or other enhancements of the propellant. These
adhesive properties may be enhanced through cross-linking.
The ethyleneNinyl acetate copolymer may have a wide range of melt
index (MI). Melt index is measured by the procedures of ASTM D1238. For
example, melt indices of from about 0.4 dg/min which is a high viscosity
ethylene/vinyl acetate co-polymer, up to at least 2500 dg/min are known, and
may be selected according to the use. In embodiments, the melt index is in
the range of 400-2500 dg/min. Small amounts of plasticizers e.g. polybutene,
may be added to either the binder or formulated propellant system to enhance
processability and/or mechanical properties. Examples of ethylene/vinyl
CA 02322096 2000-10-02
8
acetate copolymers include EIvaxT"" 210, Elvax 220 and Elvax 24.0
ethylene/vinyl acetate copolymers.
The binder is in the form of a composition of the thermoplastic ethylene
copolymer with a plasticizer that is solid or semi-solid at 20°C and an
additive
to increase one or more of elongation, adhesion and tack.
Examples of plasticizers that are solid or semi-solid at 20°C
include
polyhydrocarbons e.g. polybutene, micro crystalline waxes, macro crystalline
waxes, oxidized hydrocarbon polyolefins, which are also known as maleated
polyolefins, and polyketone waxes. The amount of solid or semi-solid
plasticizer may be 10-30% by weight, especially 10-25% by weight.
The binder composition preferably has an additive that increases the
elongation, adhesion and/or the tack of the binder composition. Examples of
such additives include aliphatic hydrocarbon resins, aromatic hydrocarbon
resins, and such resins that have been hydrogenated, and rosin derivatives.
Examples of rosin derivatives include hydrogenated rosin esters. Such
additives may be used in amounts of 20-40% by weight, especially 10-35% by
weight.
In preferred embodiments of the invention, the binder composition
contains 35-65% by weight of copolymer, 10-30% by weight of solid or semi-
solid plasticizer and 20-40% by weight of said additive and especially 45-55%
by weight of copolymer, 15-25% by weight of the plasticizer and 25-35% by
weight of the additive. The propellant composition preferably contains 10-
20% of the binder, and especially 14-18% of binder.
The propellant is comprised of the binder composition i.e. compositions
of ethylene copolymer, and at least 65% by weight of an oxidizer and/or
crystalline high explosive. In embodiments in which the binder is a cross-
linked ethylene copolymer, the propellant may contain at least 50% by weight
of oxidizer, preferably at least 60% by weight and especially at least 70% by
weight of oxidizer. The propellant may also contain an energetic, a ballistic
mod~er and other compounds.
A variety of oxidizers known to the trade may be used, including but not
limited to ammonium perchlorate, potassium perchlorate, lithium perchlorate,
CA 02322096 2000-10-02
9
sodium perchlorate, ammonium nitrate, potassium nitrate, sodium nitrate,
strontium nitrate, guanidine nitrate, ammonium dinitramide. Nonetheless, it is
understood that other oxidizers could be used.
In preferred embodiments, the propellant contains 75-90% by weight of
oxidizer. Ammonium perchlorate (AP) is the preferred oxidizer for many
applications because it is relatively non-hygroscopic, stable during normal
storage and fabrication procedures and relatively low in cost, in addition to
providing good performance.
A variety of crystalline high explosives may be used. Examples of the
crystalline high explosives include HMX, RDX and CL-20.
As in other solid propellant systems, oxidizers may be incorporated in
various particle sizes as necessitated by burn rate requirements, process
rheology, solids loading, and commercial availability. Typically, propellant
system utilize blends of particle sizes to improve packing density and thus
achieve a high solids loading as is necessary for high performance. Typical
coarse sizes of ammonium perchlorate, for example, would be 400-600
microns, medium sizes would be 50-200 microns, and fine AP e.g. ground
onsite, commonly would be have a mean particle size of 5-15 microns.
Ultrafine and other particle sizes may also be present e.g. particle sizes
from
submicron to 5 microns. A mixture of more than one particle size would
normally be used.
In addition to the above, the propellant may contain one or more of the
following: fuel additives, e.g. metal or non-metal powders or metal hydrides;
energetic materials, liquid high explosives such as TMETN or BTTN; ballistic
modifiers e.g. ferrocene or ferrocene derivatives, borohydrides, copper
chromite, oxamide, oxides of iron, lead, chromium, copper, magnesium, and
others; thermally-conductive burn rate modifiers such as silver wire staples
or
graphite whisker; coupling agents/rheology modifiers such as titanates,
zirconates, aluminates; bonding agents; opacifiers, stabilizers, metal de-
activators, anti-oxidants, or other agents known to the trade to modify
processing, performance, mechanical properties, storage stability or shelf
life.
Examples of stabilizers include zinc oxide, nickel oxide and triacetin.
CA 02322096 2000-10-02
Examples of flame colorants include salts of strontium, barium sodium and
lithium.
Examples of energetic materials are solid high explosives such as
nitroguanidine, nitrocellulose, PETN, lead azide, sodium azide; liquid high
5 explosives such as nitroglycerine, diethylene glycol dintrate, triethylene
glycol
dinitrate, TMETN, BTTN, energetic polymeric binders such as glycidyl azide
polymer, BGN, BAMO/AMMO and BAMO/NMMO.
In order to obtain the propellant, according to one embodiment of the
invention, the co-polymer in the form of pellets or powder may be mixed in a
10 dry state with an oxidizer e.g. ammonium perchlorate.
Fuel additives, energetic materials and other modifiers may be added
at this stage, or in many cases may be pre-blended into the polymer. As
noted herein, a pre-propellant is an important feature of an aspect of the
invention. The dry mixture is then heated e.g. to about 85-130°C, to
form a
mixture that is flowable. The resultant mixture may then be transferred to a
mould and subjected to moderate pressure in order to form a fuel grain.
In preferred embodiments of the method of manufacture of propellant
compositions, a pre-propellant composition is formed. In this method, the
polymer is mixed with some or all of the additives and modifiers required in
the final formulation, excluding oxidizer and energetic materials and any
catalysts. The resultant composition is then pelletized or powdered in an
extruder. To manufacture a propellant composition from this material, it may
be pre-coated with a plasticizer, e.g. polybutene, to assist in heat transfer;
this
step may be eliminated in many instances. The oxidizing and energetic
materials and any other additives are then added, and the subsequent mixture
is mixed at a typical temperature of about 85-130°C, to form a
flowable,
heterogeneous propellant composition.
The composition is then formed into propellant grains. In a typical
process, the propellant is pre-loaded into the case liner using a ram
extruder.
The pre-loaded composition is then transferred to a press, in which the grain
is formed under moderate pressure, conforming to the inner surface of the
liner, and to the core mandrel which defines the profile of the inner core of
the
CA 02322096 2000-10-02
11
propellant grain (if required). The mandrel and/or clamping blocks which
retain the grains may be pre-heated to minimize surface freezing, then
switched to a cold cycle during a pressing operation to speed the overall cure
cycle. Small propellant grains have been formed by this method with a cycle
time of 30-120 seconds, but it is anticipated that shorter cycle times may be
achieved.
The resultant propellant grains have a uniform consistency, showing
excellent distribution of the constituents, especially when compared to non de-
gassed pour-cast propellant compositions, with an absence of significant
bubbles. The propellant shows a high tensile strength and elongation at
break. Such properties, in addition to good adhesive properties, are important
for the finished propellant grain in order to minimize mechanical problems
during storage and handling.
As noted herein, in more preferred embodiments of the method of
manufacture of the propellant compositions, a pre-propellant composition is
formed using EVA. The EVA is mixed with modfiers e.g. zirconates,
aluminates or titanates, with energetic e.g. aluminum, magnesium,
aluminum/magnesium powders, optionally with zinc powder or chlorinated
hydrocarbons if tracing is required, and any ballistic modifiers to modify the
burn rate e.g. iron oxide, copper oxide, chromium oxide or magnesium oxide,
or calcium carbonate. The resultant composition is pelletized in an extruder.
To add oxidizer to the pellets, the pellets may be coated with polybutene and
then oxidizer e.g. ammonium perchlorate added. The ammonium perchlorate
is typically a mixture of particle sizes which aids in formation of a uniform
composition with high volumetric solids loading. The resultant mixture of
oxidizer and pellets is mixed to a dough-like consistency, which may be
formed into a cylindrical preform for the forming of propellant grains.
Propellant grains are then formed in a ram extruder, under pressure,
using a rapid hot/cold cycle. The use of a ram extruder permits the
fabrication
of propellant grains of complicated or sophisticated shapes without the need
for post-forming operations. The requirement for vacuum de-gassing is
believed to be reduced or eliminated. However, the process may easily be
CA 02322096 2000-10-02
12
run under vacuum to effect removal of trapped and dissolved gases, traces of
volatiles, etc. It is understood that other fabrication techniques may be
used.
Propellant grains formed in the ram extruder have a uniform
consistency, even when viewed under a microscope as shown in the Figures
herein, especially compared to PBAN, with an excellent distribution of
constituents and an absence of significant bubbles. The propellant grains
have high tensile strength and elongation. Such properties are important for
the propellant grain in order to withstand mechanical and thermal stresses
during firing in a rocket without cracking. Cracking could have major adverse
effects on the rocket, including destruction.
Advantages of ethylene/vinyl acetate thermoplastic copolymer-based
solid propellant binders over existing technologies include less critical
limits
on mix ratios of the component, easy processing of the pre-propellant, low to
non-toxic properties of the binder constituents, the absence of shelf or pot-
life
problems, the potential absence of vacuum degassing requirements, the
ability to recycle components and rapid cure times if required. However, the
most significant advantage is likely the lead time required for the production
of
propellants for rocket motors, because either the pre-mixed propellant or the
individual ingredients can be stored indefinitely. Thus rocket motors, or
other
propellant-using systems, could be produced on a just-in-time basis for rapid
deployment in the field of use. In addition, components may be safely stored
near a location where the propellant is to be used, and manufactured as
needed in apparatus that is easy to operate.
The manufacture of the pre-propellant i.e. propellant without oxidizer, is
a rapid continuous process utilizing apparatus known in the plastics industry.
Similarly, the mixing of pre-propellant and oxidizer, and the forming of fuel
grains may also be operated on a continuous basis. In both instances,
recycle of materials may be used, but the amount of recycle material should
be minimal. Nonetheless, the ability to recycle components is an important
characteristic of the present invention as it improves economics of the
process and reduces or eliminates the need to dispose of components of the
propellant by, for example, burning. This is environmentally advantageous.
CA 02322096 2000-10-02
13
It is understood that recycling would normally not be carried out with
cross-linked compositions. However, cross-linkable compositions may be
recycled. For instance, the pre-propellant would normally be prepared without
the cross-linking catalyst, which would permit recycling for a period of time.
In embodiments of the invention, especially where higher temperatures
are involved in deployment and service, especially ambient or storage
temperatures that might exceed about 40°C., the propellant compositions
may
be formed with a crosslinked copolymer. Cross-linked polymer may also be
used at other temperatures including low ambient temperatures, but generally
is not necessary. Cross-linking will improve physical properties, especially
at
elevated temperatures. It will further reduce the likelihood of cracking,
slumping, de-bonding or other mechanical problems of the propellant
compositions. In these embodiments, the propellant compositions is formed
using a polymer that has a moisture-crosslinkable monomer copolymerized
into the polymer or grafted onto the polymer. Examples of moisture-
crosslinkable monomers are vinyl silanes, particularly vinyl trialkoxysilanes,
examples of which are vinyl trimethoxysilane and vinyl triethoxysilane. Such
silanes are available commercially. In addition, compositions containing vinyl
silane, grafting catalyst and crosslinking catalyst are also available
commercially.
Polymers containing vinyl silanes must be maintained in a moisture-
free environment at all times prior to the desired time of crosslinking.
Crosslinking may be effecting by exposure to moisture, especially by merely
exposing the article to atmospheric conditions. Curing by contacting with
water, especially steam, is not preferred in view of effects of water on the
propellants. Crosslinking may take place over a period of a few days in the
presence of atmospheric moisture, it being understood that the shape and
thickness of the fuel grain is a factor in the cross-linking rate, as the
crosslinking reaction is believed to be controlled by the rate of diffusion of
water into the fuel grain. Crosslinking catalysts are normally incorporated
into
the composition. Techniques for the manufacture of moisture-crosslinkable
polymers, and for the curing of such polymers are known.
CA 02322096 2000-10-02
14
An advantage of the use of ethylene/vinyl acetate copolymers as the
binder in propellants is the increased safety that may be achieved during the
manufacturing process. Energetic materials such as aluminium powder must
be handled carefully during traditional fuel-grain manufacturing processes, as
air-borne metallic powders can be extremely volatile under certain conditions.
According to the present invention, the energetic materials may be
compounded with the thermoplastic polymer in compounding extruders. In
such a method, the energetic materials would be inhibited and would no
longer pose a hazard during shipping or the manufacture of fuel-grains. It is
understood that such powders are frequently compounded with thermoplastic
polymers as colorants or dyes for such polymers.
Shelf life of the components, including in particular the compounds of
the pre-propellant that is formed, is generally indefinite. Thus, pot-life is
not a
consideration. However, if crosslinked compositions are prepared, steps must
be taken to protect the vinyl compounds from moisture, as discussed above.
The present invention provides a fuel grain with uniform properties, and
a method of manufacture that is versatile and easy to operate. Components
may be recycled.
The propellant compositions may be used in a variety of end uses,
including as rocket propellants or in gas generators. The latter use a
propellant composition to produce gas for mechanical work such as start a
turbine engine, drive a piston, or inflate an airbag, in contrast to a rocket
motor which exhausts the gas through a nozzle to generate a reaction force.
One of the most desirable features of the present invention is that
unlike most conventional propellant systems and processes this invention
provides a propellant system and process which exhibits the combination of a
very long or nearly indefinite pot-life, with a very short casting and curing
time.
The present invention is illustrated by the following examples.
Example 1
A control sample (Sample #1 ) of a first typical AP/AI/HTPB propellant
composition was prepared. This composition employed a tri-modal blend of
CA 02322096 2000-10-02
400, 200 and 90 micron AP, 3 micron spherical aluminum, and HTPB binder,
in addition to HX-878 bonding agent and carbon black opacifier in minor
percentages. This sample was processed under atmospheric pressure, to
show the severe porosity of propellants processed in this manner.
5 The total time required for preparation, including weighing, mixing and
cast cycle time was 6 hours. The cure time was 2 days.
Figure 1 shows a photograph of a cross-section of the propellant. The
dark pockets are voids of approximately 250-340 micron in diameter. Porosity
of this magnitude would at least result in erratic performance and low density
10 of the propellant, and likely result in catastrophic motor failure from
pressure
fluctuations or adiabatic compression. In addition, the AP particles show
weak bonding in the binder matrix as evidenced by the crescent shaped air
pockets created during sample preparation.
A second propellant composition (Sample #2) nearly identical to the
15 first sample was prepared, with the major exception that the process was
run
under high vacuum for 1.5 hour mix cycle. In addition, this composition
employed state of the art bonding agents to enhance adhesion of the binder
to the AP particles.
The total time required for preparation, including weighing, mixing and
cast cycle time was 6 hours. The cure time was 6 days.
Figure 2 shows a photograph of a cross-section of the propellant. The
surface shows clean cleavage of the AP particles and lack of de-bonding
under the shear force of cutting, in addition to an excellent surface
distribution
profile. Some tearing and pocketing is evident at the perimeter of the sample,
as expected during sample preparation. The measured density of this sample
is 0.0589 pounds per cubic inch (1.630 g/cc).
A sample (Sample #3) of propellant of the invention was prepared, with
identical solids composition to the second sample except that ethylene/vinyl
acetate co-polymer was used as binder. The propellant was formed from pre-
propellant, using the ram extruder, as described. The total mix cycle for this
batch was 20 minutes, under atmospheric pressure (no vacuum).
CA 02322096 2000-10-02
16
The total time required for preparation, including weighing, mixing and
mix cycle time was 1.5 hours. The time for extrusion, pressing, cooling and
curing in non-continuous prototype equipment was 4 minutes/grain.
Figure 3 shows a photograph of a cross-section of the propellant. This
sample exhibits the same excellent surface distribution as the second sample
above, as well as aggressive bonding of the AP particles to the binder matrix.
No voids are visible other than at the periphery of the sample. The measured
density of this sample is 0.05962 pounds per cubic inch (1.650 g/cc), an
increase of 1.2% over the second sample.
Example II
Propellant compositions were prepared by the following procedure. A
mixing bowl was heated to 210-215°F (99-102°C). Fine to medium
particulate
ammonium perchlorate (AP) was fed to the bowl, together with any bum rate
catalysts, and mixed for two minutes. All remaining AP was added, and dry
mixed for 3-5 minutes at high speed.
A mixture of ethylene/vinyl acetate copolymer, polybutene or other
plasticizer, rheology modifiers (if present) was prepared. This mixture was
added to the mixer. Mixing was continued until the polymer had melted, and
then for a further 10 minutes. The resultant mixture was then extruded into
propellant grains in the ram extruder.
Example III
The procedure used for preparing pre-propellant blends was as follows.
Using a Ross LDM-4 double planetary mixture, the temperature was set at
265°F (129°C), when the ethylene-vinyl acetate copolymer was
EIvaxTM 205
or 210 polymer. All fine dry materials of the composition were added to the
mixing bowl, with any dust being allowed to settle. The mixing head was then
wiped to remove settled material. The Elvax ethylene-vinyl acetate copolymer
and all other medium to coarse materials were added, including fluid
ingredients if any. A vacuum was then applied to the mixture.
CA 02322096 2000-10-02
17
The dry mixing cycle was commenced, at low speed mixing. The
mixing speed was increased when the temperature passed the melting point
of the composition, typically about 100°C, and mixing was continued for
20
minutes. The mixing bowl was scraped down once after about 10 minutes.
The mixing head was then lifted, leaving the blades at the surface of
the composition for 1-2 minutes to allow adhered material to flow into the
bowl
with minimal trapped air. The contents of the bowl were then discharged. A
similar procedure may be used with different types of mixer, including mixes
that do not operate under vacuum.
Examine IV
A composition was prepared in a K-5 air mixer. The mixing bowl
temperature was set at 99-102°C, and all fine to medium sized
particulate
ammonium perchlorate and any burn rate catalyst was added to the mix bowl.
The mixture was then mixed at high speed for 2 minutes. Any remaining
ammonium perchlorate was added and the resultant mixture was preheated
for 3-5 minutes under high speed mixing. A mixture of Elvax ethylene/vinyl
acetate copolymer with any polybutene or other plasticizer, rheology modifiers
or other ingredients was then added, and mixed at a moderate mixing speed.
After the mixture had reached the melting point, the mixture was mixed for a
further 10 minutes. The mixture was subsequently extruded into grains.
Using this procedure a composition was prepared as follows:
Material: Weigiht Chargied (a)
AP-400* 500.5
AP-200* 115.5
AP-90* 153.5
Pre-blend** 215.5
Polybutene 6 (Soltex) 10
NZ-33 neo-alkoxy zirconate*** 5
Batch total 1000 grams
* Ammonium perchlorate, with 400, 200 or 90 micron particulate size.
** Pre-blend of Example VII, below.
*** Kenrich Petrochemicals, Inc.
CA 02322096 2000-10-02
18
Processing was difficult in that the composition exhibited high initial
tack, requiring more frequent scraping.
The procedure was repeated, using a pre-blend of binder PB-6 and
NZ-33. All ammonium perchlorate was added at the same time. The mixture
dispersed easily as it heated, and suddenly transitioned into a crumbly,
mobile
blend. Extrusion in the ram extruder was good.
Example V
The procedure of Example IV was repeated, using the following
composition:
Material: Wei4ht Charged (a)
AP-400 479.45
AP-200 122.1
AP-90 212.3
Pre-blend* 66.3
ElvaxT"" 210 EVA 103.85
Polybutene-6 10
NZ-33 neo-alkoxy zirconate 5
Iron oxide 1
Batch Total 1000 grams
* Pre-blend of Example VII, below.
This formulation has reduced aluminum content (2%), an increased
amount of 90 micron AP (from 25 to 30% by weight), and iron oxide, with a
proportional reduction in ammonium perchlorate of 400 micron size. These
changes were intended to reduce nozzle erosion and increase ignition
response.
CA 02322096 2000-10-02
19
Example VI
A pre-blend batch was prepared with the following composition:
Material: Weight Charged la)
Elvax 210 EVA 719.28
Carbon Black 2.40
Zinc, 6 N powder 1678.32
Batch Total 2400 grams
No problems were experienced in the mixing cycle, the extrusion cycle
or pelletising of the pre-blend.
The pre-blend was then formulated into a propellant grain using
50.05% by weight of the pre-blend and 49.95% by weight of 200 micron
particulate ammonium perchlorate.
Example VII
A pre-blend was formed of the following composition:
Material: Weight Char ec~d (4)
Elvax 210 EVA 2088.17
Carbon Black 6.96
Aluminum, spherical 904.87
Batch Total 3000 grams
The pre-blend was blended with ammonium perchlorate, to form
propellant grain, with the resultant composition containing 21.55% by weight
of the pre-blend.
No problems were encountered in the processing of the composition.
Example VIII
Samples similar to Sample #2 and #3 of Example I were subjected to
Martin-Marietta PEPcode analysis to determine characteristics of the
CA 02322096 2000-10-02
compositions as propellants. Sample #2 is a HTPB control sample and
Sample #3 is an EVA composition of the invention.
The results obtained were as follows:
Sample #2 Sample #3
Density Ib/in3 0.05946 0.06017
g/cm3 1.6457 1.6654
Specific Impulse
Frozen Flow 233.9 233.3
Shifting Flow 236.3 236.6
Density Impulse
Frozen Flow 384.9 388.5
Shifting Flow 388.8 394.0
5
The composition of the invention showed a small increase in density
and density impulse indicating that the composition is at least equivalent to
a
HTPB composition, and possibly superior for some uses.
10 Example VIIII
A series of Elvax~ ethylene/vinyl acetate resins having a melt index of
at least 150 dg/min were selected, and characterized by physical properties,
as follows:
Table
I
Type Melt VA* TensileTensile SofteningElongationAdhesion
Index StrengthModulus Point At
Content
(dg/min) (%) (psi) (psi) (C) Break
(%)
SOOW 2500 14 725 5200 98 90 Poor
200W 2500 28 230 1000 81 90 Poor
205W 800 28 375 1700 80 S00 Fair
410 500 8 675 4800 88 750 Fair
210W 400 28 400 1700 82 900 Good
310 400 25 475 2300 88 900 Fair
220W 150 28 800 2300 88 900 Excellent
15 VA inyl acetate
= v
CA 02322096 2000-10-02
21
Three resins were selected for further testing viz. Elvax 500W, Elvax
210W and Elvax 220W. These resins were blended with either Foral 105
rosin ester from Hercules, or Regalrez 1094 hydrogenated hydrocarbon resin,
also from Hercules. The ethylene/vinyl acetate resins were blended with each
of the rosin ester or hydrogenated hydrocarbon resin in ratios of 70:30, which
preliminary tests had indicated was a preferred ratio.
A series of tests were conducted, and the results were rated on a scale
of from 1-5, with 5 being excellent.
The results obtained are summarized in Table II.
Table II
A. Rosin Ester
Melt Adhesion Tensile Tensile
Elongation Overall
Viscosity Strength Modulus
Elvax SOOW 5 1 1 2 2 4.2
/Foral 1 O5
Elvax 210W 4 4 4 4 4 4
/Foral 1 O5
Elvax 220W 2 4 4 5 3 3.6
/Floral 105
B. H vdros~enatedydrocarbon in
H Res
Melt Adhesion Tensile Overall
Elongation
Tensile
Viscosity Strength Modulus
Elvax SOOW 5 1 1 2 2 4.2
/Regalrez 1094
Elvax 210W 4 4 4 4 4 4
/ Regalrez 1094
Elvax 220W 2 4 4 S 3 3.6
/ Regalrez 1094
Elvax 210W ethylene/vinyl acetate resin was selected for further
testing. This resin was blended with microcrystalline wax and with
polyethylene oxide wax. A series of blends were prepared and evaluated.
The results from evaluation of blends of ethylene/vinyl acetate and
polyethylene oxide wax, shown as general trends, were as follows:
CA 02322096 2000-10-02
22
Table III
Ratio Melt Tensile Adhesion Overall
Index Strength
80/20 2 4 4 3.3
70/30 3 3 3 3.0
60/40 3 3 3 3.0
50/50 4 2 2 2.7
40/60 4 2 1 2.3
Similar results were obtained using the ethylene/vinyl acetate resin and
microcrystalline wax.
Example X
A variety of propellant formulations were prepared using 14-18% by
weight of a binder of 45-55% by weight of Elvax 210W ethylene/vinyl acetate
resin, 30% of either Regalrez 1094 hydrogenated hydrocarbon resin or Foral
105 rosin ester and 15-25% by weight of polyethylene oxide wax. The
propellant formulations were found to have acceptable processability, good
mechanical properties and the expected ballistic performance for the
formulations.
Example XI
The formulation of Example II was modified by addition of a silane
grafting/moisture curing composition. It was found that addition of 1-5 phr,
based on the ethylene/vinyl acetate resin, of vinyl trimethoxy silane/dialkyl
peroxide/dibutyl tin dilaurate e.g. SiIcatRT"" gave grafting and subsequent
cross-linking. The melt index was significantly reduced and thermal stability
was improved, both of which improved properties of the propellant
formulations.
In one example, the formulation was as follows:
CA 02322096 2000-10-02
23
wt.
Ammonium erchlorate AP-400 48.32
Ammonium erchlorate AP-200 12.27
Ammonium erchlorate AP-90 16.11
Elvax 210 / 3% SiIcatR 8.90
Regalrez 1094 hydrogenated 5.34
h drocarbon resin
Pol eth lene oxide wax 3.56
Iron oxide 0.25
NZ-33 neo-alko zirconate 0.20
Carbon black, Monarch 8 0.05
~ Magnalium (Mg/AI), fine I 5 00
The composition had a burn rate pressure exponent of 0.47, and burn
rate at 1000 psi of 0.28 inches/second and a delivered speck impulse at
1000 psi of 224 seconds.
