Note: Descriptions are shown in the official language in which they were submitted.
WO 95/04014 ~ 5 ~ PCT/US94/08781
METHOD FOR PREPARING ANHYDROUS
TETRAZOLE GAS GENERANT COMPOSITIONS
S
15 Field of the Invention
The present invention relates to a method for making
novel gas generating compositions which are useful for
inflating automobile air bags and similar devices. More
particularly, the present invention relates to a method
for making gas generating pyrotechnic compositions based
on anhydrous tetrazole compounds as a primary fuel.
Background of Invention
The art has been seeking an acceptable non-azide gas
generant which has the desired combination of properties
for being a drop-in replacement for the conventional
sodium azide-fueled gas generating composition used in
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WO 95/04014 ~ b PCT/US94/08781
air bags in passenger vehicles.
In addition to properties, such as acceptable rate
of gas generation and reduced or no toxic combustion by-
products, a proposed replacement for the conventional
sodium azide-fueled gas generant must be capable of being
manufactured and formed into a pill, pellet, extruded
cylinder, or other desired shape charge. The desired
shape, typically a pellet, must be capable of retaining
structural integrity.
Various methods have heretofore been proposed for
processing gas generant compositions to obtain shaped
charges such as pellets. However, different gas generant
compositions behave differently during the pelletizing
process, and particular process conditions suitable for
fabricating objects composed of one gas generant are not
necessarily applicable to processing or fabricating
objects, such as pellets, composed of a different gas
generant composition.
We have proposed a non-azide-fueled gas generant
composition, and have made extensive studies on its
preparation and fabrication. We have developed certain
techniques for fabricating the non-azide composition into
pellets or other desired forms. In the course of that
work, we observed, however, that attempts to pelletize
directly anhydrous gas generants based on a fuel of the
tetrazole class, such as aminotetrazole or
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WO 95/04014 a PCT/US94/08781
bistetrazoleamine, produces pellets that crumble and lose
their pellet shape within 24 hours at Rh 45~ at 25°C.
As a result of such efforts, we have determined that
it would, therefore, be a significant advancement in the
art to provide a method for preparing shaped charges,
such as pellets, comprised of non-azide tetrazole-fueled
compositions directly from an anhydrous material composed
of that composition wherein the shaped charges so
produced retain their structural integrity after exposure
at a Rh 45~ at 25°C. It would be an advancement in the
art to provide shaped forms, such as pellets or the like,
which are capable of being combusted to generate large
quantities of gas that would overcome the problems
identified in the existing art. It would be a further
advancement to prepare shaped charges, such as a pellets,
comprised of non-azide tetrazole-fueled gas generating
compositions which are based on substantially nontoxic
starting materials and which produce substantially
nontoxic reaction products. It would be, in particular,
an advancement in the art to produce combustion gases
which primarily consist of nitrogen, with lesser amounts
of carbon dioxide and water vapor so as not to exceed
allowable occupant exposure standards for carbon dioxide
and carbon monoxide. It would be another advancement in
the art to prepare shaped charges, such as pellets,
comprised of gas generating compositions which combust to
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CA 02167386 1999-07-OS
produce limited particulate debris and limited undesirable
4
gaseous products. It would also be an advancement in the art
to prepare pellets comprised of gas generating compositions
which combust and form a readily filterable solid slag upon
reaction.
Methods for making charges, such as pellets, directly
from anhydrous non-azide tetrazole-fueled gas generant
compositions are disclosed and claimed herein.
Summary and Objects of the Invention
The method according to the present invention overcomes
or minimizes processing difficulties encountered in
manufacturing charges, such as pellets, from anhydrous
tetrazole-fueled gas generant compositions.
A method according to the present invention involves
obtaining a desired quantity of gas generating material
comprising particles of at least one oxidizer and particles
of at least one tetrazole as the fuel; preparing a wet mixture
containing the gas generating material; drying the material
to an anhydrous condition having a specified weight average
particle size; and pressing the anhydrous material into
pellets. The gas generating material is preferably pelletized
from anhydrous granules obtained from a wet mixture which can
be agglomerated, such as a granulatable slurry or paste.
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CA 02167386 1999-07-OS
The particle sizes of the oxidizer and the tetrazole fuel can
also be controlled within pre-selected number average particle
size ranges when preparing the slurry or paste.
This development overcomes a problem encountered in our
prior efforts to pelletize anhydrous tetrazole-fueled gas
generant conditions. In our prior efforts, pellets produced
from anhydrous tetrazole-fueled gas generant compositions were
generally observed to crumble and powder, particularly when
exposed to a humid environment, within 24 hours. Pellets
prepared by our present method are, by comparison, robust and
can retain their structural integrity when exposed to humid
environments.
Detailed Description of the Invention
The present method involves preparing charges, such as
pellets" from an anhydrous gas generating composition by
forming a quantity of granulated anhydrous gas generating
material into a charge wherein the gas generating material
comprises an oxidizer and a non-azide fuel which is of the the
tetrazole class. More particularly, a preferred method
involves preparing pellets from the anhydrous gas generating
composition by slurrying a quantity of gas generating material
which comprises oxidizer particles having a number average
particle size greater than 1 micron and fuel particles having
particle sizes greater
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than 1 micron wherein the fuel is selected from the group
consisting of tetrazoles; if necessary, rendering the
slurry capable of being made particulate, such as
granulatable, such as by drying, for instance, the
slurried material; rendering the slurry into particles,
such as granules, having sizes of at least about 100
mesh; drying the granulated material to an anhydrous
condition; and pelletizing the anhydrous granulated
material, i.e., shaping the anhydrous granules into
pellets.
The slurry can be obtained by blending
effective amounts of a fuel from the tetrazole class, and
an oxidizer in sufficient amounts of a medium, such as
water. It is not necessary nor particularly desirable
that the fuel and/or oxidizer be rendered anhydrous prior
to mixing. The slurry can be prepared in one step or in
a series of steps. The number average particle size of
the tetrazole fuel used in preparing the slurry can be in
the range of from about 1 micron to about 100 microns,
although a range of about 10 microns to about 90 microns
is presently preferred. The number average particle size
of the oxidizer, such as CuO, used in preparing the
slurry can be in the range of from about 1 micron to
about 20 microns, although a range of about 3 microns to
about 10 microns is presently preferred, such as a number
average particle size greater than about 5 microns . Sub-
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WO 95/04014 ~ ~, PCT/US94108781
micron sized oxidizer particles are not presently
preferred because the pellets ultimately produced have
been observed to crumble and lose pellet integrity within
24 hours at Rh 45%.
Hy preference, the medium is water. Other solvents
in which the tetrazole exhibits some solubility may be
used such as volatile organic solvents such as, for
instance, alcohols such as methanol, ethanol, and
propanol, and ketones such as acetone or methylethyl
ketone .
In a preferred embodiment, the amount of water is
generally selected to be sufficient to obtain a
granulatable slurry, or a compactable powder which can be
granulated or rendered granulatable. In general, it is
not desired to dissolve all of the fuel or oxidizer.
Consequently, although an excess of water can be used,
its use does not lead to any particular advantages.
Therefore, the slurry can comprise less than about 50 %
by weight water with the gas generating ingredients
comprising the remainder. For instance, the slurry can
comprise about 3% to about 40% by weight water and from
about 60% to about 97% by weight of the gas generating
composition, although it is preferred to use at least
about 20 % by weight and up to about 40 % by weight
water. Predictability of ballistic performance may be
adversely affected by straying substantially beyond the
WO 95/04014 ~ ~ ~, PCT/US94/08781
preferred water concentration.
In a multi-step preparation, a selected amount of
water and the gas generating compositions can be mixed to
obtain a damp compactable powder. The damp powder can,
if desired, be mixed with additional water to obtain a
slurry material having a paste-like consistency.
As evident, obtaining a pulverulent anhydrous gas
generant composition before preparing the slurry is
neither necessary, nor particularly advantageous in the
present process. The particular species of a hydrated
tetrazole fuel and the particular oxidizer selected
should, however, have an average number particle size as
indicated elsewhere herein when preparing the slurry.
The fuel species and oxidizer can be added at once,
alternatively, or in portions to the slurry medium
provided that the materials are in intimate contact, and
sufficient compositional uniformity of the slurry is
achieved.
In a preferred embodiment the water has a pH in the
range of about 5 to about 11 prior to being combined with
the fuel particles and oxidizer particles. After the
tetrazole fuel, such as BTA, is added the pH decreases to
about pH < 3. A pH substantially outside the preferred
ranges is undesired owing to dissolution of the oxidizer
and to avoid complex formation. Poor pH control can be
evident in even the final anhydrous product.
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li'O 95!04014 PCT/US94/08781
It is preferable to avoid allowing the slurry or
suspension of gas generant particles to remain wet, in
water or other solvent, for an extended period of time.
Complexes of the tetrazole may form, or complexes of the
tetrazole and oxidizer may form. For instance, a BTA-Cu
complex is dark green, and a Cu-SAT complex is green.
Uncontrolled complex formation during the slurrying
step may result in less predictable ballistic properties
of the final anhydrous product.
The slurry can be dried, if necessary, to obtain a
partially dried granulatable material, although it is not
desired to render the material anhydrous before
granulation.
Next, the material is rendered into particulate form
having a weight average particle size from about 100 mesh
and to about 14 mesh. Crumbs, prills, extruded
cylinders, disks, pills, or granules of the appropriate
size distribution can be used. General techniques are
adaptable to agglomerating, i.e., increasing particle
size, include, granulating, extruding, bricketting,
pelleting, tableting, and spray drying, and are described
in Perry's Chemical Engineers's Handbook, Section 16
(3rd. Ed. 1950) provided that the desired particle sizes
are obtained. For instance, while the agglomeratable
material is still ~~:et or moist, the
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WO 95/04014 PCT/US94/08781
material can be wet-meshed or wet-extruded to obtain
granules. The granules can have particle sizes in the
range of, for instance, from about 14 to about 100 mesh.
It is presently preferred that the particles, such
granules, have a weight average particle size in the
range of from about 14 to about 30 mesh.
The material which has been rendered into
particulate form is dried to remove solvent to achieve an
anhydrous state. In general, with respect to water, the
essential absence of free water, including water measured
as hydrate or unbound but occluded or adventitious
moisture, is indicative of the anhydrous state. By
preference the material is rendered water-free. In this
regard, it is known that a dried tetrazole fueled
composition consisting of 22.9 wt~ BTA monohydrate and 77.1 wt~
Cu0 can still lose about 3 to 4 wt$ when further dried,
and that the additional weight loss reflects removal of
principally hydrate and small amounts of unbound but
occluded or adventitious moisture. Hence, the drying
contemplated herein involves removal of the water of
hydration and any occluded or unbound or adventitious
water. The precise temperature and length of time of
drying are not critical to the practice of the invention,
as long as anhydrous granules are obtained. For
instance, drying the agglomeratable material to constant
weight at less than 75°C, generally less than 45°C,
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~~ WO 95/04014 PCT/US94/08781
2167~$D
followed by further drying, such as at a temperature of
from about 110°C to about 140°C, for a sufficient
duration of time to remove hydrate, occluded or unbound,
and adventitious moisture. The just mentioned sufficient
time will be a function of the temperature and pressure
conditions prevailing during the drying step. For
example, in a drying oven at 1 bar, 12 to 24 hours can be
sufficient when the temperature is in the range of about
110°C to about 120°C. It is presently preferred that the
materials not be subjected to temperatures in excess of
about 150 °C for extended periods of time.
Adequate achievement of the anhydrous state as
specified or contemplated herein can be readily
determined by subsequently exposing an anhydrous
composition to a relative humidity of at least 45~ for a
minimum of 24 hours. A gain in weight of the composition
resulting from this treatment to within about 0.5$ of the
theoretical amount due to fuel hydration is indicative of
sufficient dehydration of the composition.
When an organic solvent other than water is used,
anhydrous means removal of the solvent residue as well as
water of hydration and any occluded or unbound or
adventitious water.
Other drying techniques can be used such as freeze
drying, vacuum drying, convection drying, dielectric or
high frequency drying, spray drying, and, for instance,
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fluidized bed drying as described in Perry's Chemical
Engineer's Handbook, Section 13 (3rd. Ed. 1950). The
slurry can, if desired, be converted directly to the desired
size and anhydrous particles, such as granules, by, for
instance, extruding the slurry in a heated and vented extruder
or spray drying the slurry. Other means for converting the
slurry directly to the desired sized particles, such as
granules, can also be used.
Solid charges are prepared from the anhydrous
material. In a presently preferred embodiment, the
anhydrous granulated material. is typically pelletized,
i.e. pressed into pellet form to meet the specific
requirements for use in automotive safety restraint
systems.
~ The solid charges produced-according to the method
of the present invention have at least one compound of
the tetrazole class (sometimes referred to herein as
simply "tetrazole") as a fuel and at least one
appropriate oxidizer. In particular, the pellets of the
gas generant composition are based on anhydrous
tetrazoles, such as 5-aminotetrazole and
bitetrazoleamines, or a salt or a complex thereof or
mixtures thereof. One presently preferred bitetrazoleam-
ine is bis-(1(2)H-tetrazole-5-yl)-amine (hereinafter
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- WO 95/04014 ~ PCT/US94/08781
sometimes referred to as "BTA"). The shaped charges are
useful in supplemental restraint systems, such as
automobile air bags.
One group of suitable tetrazoles for use in the
present invention are bitetrazole-amines such as those
having the following structure:
NON I NON
A y~N~~R 2
NON NiN
wherein X, R1 and RZ, each independently, represent
hydrogen, methyl, ethyl, cyano, nitro, amino, tetrazolyl,
a metal from Group Ia, Ib, IIa, IIb, IIIa, IVb, VIb, VIIb
or VIII of the Periodic Table (Merck Index (11th Edition
1989)), or a nonmetallic cation of a high nitrogen-
content base.
Other tetrazoles include tetrazole, 5-aminotetrazole
(hereinafter sometimes referred to as "5AT"),
bitetrazole, the n-substituted derivatives of
aminotetrazole such as nitro, cyano, guanyl, and the
like, and c-substituted tetrazoles such as cyano, nitro,
hydrazino, and the like.
Salts or complexes of any of these tetrazoles
including those of transition metals such as copper,
cobalt, iron, titanium, and zinc; alkali metals such as
potassium and sodium; alkaline earth metals such as
strontium, magnesium, and calcium; boron; aluminum; and
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WO 95/04014 PCT/US94/08781
nonmetallic cations such as ammonium, hydroxylammonium,
hydrazinium, guanidinium, aminoguanidinium,
diaminoguanidinium, triaminoguanidinium,or biguanidinium
can also serve as the fuel in the pellets produced in
accordance with the present invention.
An appropriate oxidizer is included in the
composition. Inorganic oxidizing agents are preferred
because they produce a lower flame temperature and an
improved filterable slag. Such oxidizers include metal
oxides and metal hydroxides, such as transition metal
oxides and transition metal hydroxides. Other oxidizers
include a metal nitrate such as, for instance, an alkali
metal nitrate or strontium nitrate, a metal nitrite such
as, for instance, an alkali metal nitrite or a nitrite
of, for instance, strontium, cobalt or chromium, a metal
chlorate such as, for instance, KC10" a metal
perchlorate such as, for instance, NaClO" KC10, and the
like, a metal peroxide such as, ~ for instance, an alkaline
earth peroxide, ammonium nitrate; ammonium perchlorate
and the like. The use of metal oxides or hydroxides as
oxidizers is particularly useful and such materials
include for instance, the oxides and hydroxides of
copper, coba2t, manganese, tungsten, bismuth, molybdenum,
and iron, such as CuO, CozO" Fez03, Mo0" Bi2Mo06, Bi20"
and Cu(OH)2. The oxide and hydroxide oxidizing agents
mentioned above can, if desired, be combined with other
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WO 95/04014 PCTIUS94/08781
conventional oxidizers such as Sr (N03) 2, NH4C109, and KN03,
for a particular application, such as, for instance, to
provide increased flame temperature or to modify the gas
product yields.
The tetrazole fuel is combined, in a fuel-effective
amount, with an appropriate oxidizing agent to obtain a
gas generating composition. In a typical formulation,
the tetrazole fuel comprises from about 10 to about 50
weight percent of the composition and the oxidizer
comprises from about 50 to about 90 weight percent
thereof. More particularly, a composition can comprise
from about 15 to about 35 weight percent fuel and from
about 65 to about 85 weight percent oxidizer.
Additives conventionally used in gas generating
compositions, propellants, and explosives, such as
binders, burn rate modifiers, slag formers, release
agents, and additives which effectively remove NOx can,
if desired, be included in the anhydrous compositions
obtained in accordance with the present invention. For
instance, the additives can be introduced when the slurry
is being prepared or at another step in the present
process. Typical binders include lactose, boric acid,
silicates including magnesium silicate, polypropylene
carbonate, polyethylene glycol, and other conventional
polymeric binders. The binder can be added at any
convenient stage of the process. Typical burn rate
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WO 95/04014 PCT/US94/08781
Zlb7~~a
modifiers include FezO" KZBI2Hlz. Bi2Mo06, and graphite
carbon fibers. A number of slag forming agents are known
and include, for example, clays, talcs, silicon oxides,
alkaline earth oxides, hydroxides, oxalates, of which
magnesium carbonate, and magnesium hydroxide are exempla-
ry. A number of additives and/or agents are also known
to reduce or eliminate the oxides of nitrogen from the
combustion products of a gas generant composition,
including alkali metal salts and complexes of tetrazoles,
aminotetrazoles, triazoles and related nitrogen
heterocycles of which potassium aminotetrazole, sodium
carbonate and potassium carbonate are exemplary. The
composition can also include materials which facilitate
the release of the composition from a mold such as
graphite, molybdenum sulfide, calcium stearate, or boron
nitride.
Tetrazoles are commercially available or can be
readily synthesized. A synthesis of BTA is disclosed by
Norris, et al., Cyanoguanyl Azide Chemistry, Journal of
Organic Chemistry, 29: 650 (1964).
Substituted tetrazole derivatives, such as substi-
tuted 5AT and BTA derivatives, can be prepared from
suitable starting materials, such as substituted
tetrazoles, according to techniques available to those
skilled in the art. For instance, derivatives containing
lower alkyl, such as methyl or ethyl, cyano, or
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_,
tetrazoyl can be prepared by adapting the procedures described
in Journal of Organic Chemistry, 29: 650 (1964). Amino-
containing derivatives can be prepared by adapting the
procedures described in Canadian Journal of Chemistry, 47:3677
(1969). Nitro-containing derivatives can be prepared by
adapting the procedures described in Journal of the American
Chemical Society, 73:2327 (1951). Other radical-containing
derivatives such as those containing ammonium,
hydroxylammonium, hydrazinium, guanidinium, amino-guanidinium,
diaminoguanidinium, triaminoguanidinium or biguanidinium
radicals, can be prepared by adapting the procedures detailed
in Boyer, Nitroazoles, Organic Nitro Chemistry (1986).
An embodiment of the present invention relates
specifically to preparing anhydrous gas generant compositions
in the form of pellets. Anhydrous tetrazole compositions
produce advantages over the hydrated forms. For example, a
higher (more acceptable) burn rate is generally observed. At
the same time, the methods of the present invention allow for
pressing the composition in the anhydrous form such that
pellets with good integrity
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WO 95/04014 PCT/US94/08781
~1 ~~3g6
are produced.
Following pellet formation, it may be preferable to
protect the material from exposure to moisture or water,
even though the material in this form may not be unduly
hygroscopic at humidities below 20~ Rh at room
temperature. Thus, the pellet may be placed within a
sealed container, or coated with a water impermeable
material.
The burn rate performance of an anhydrous tetrazole
fueled gas generant composition is good. Burn rates
above 0.5 inch per second (ipso are preferred. Ideally,
burn rates are in the range of from about 1.0 ips to
about 1.2 ips at 1,000 psi. Burn rates in these ranges
can be achieved. The burn rates compare favorably with
the burn rates observed for sodium azide compositions.
In general, pellets prepared by a preferred method
are capable of exhibiting crush strengths in excess of 10
lb load in a typical configuration (3/8 inch diameter by
0.07 inches thick). This compares favorably to those
obtained with commercial sodium azide generant pellets of
the same dimensions, which typically yield crush
strengths of 5 lb to 15 lb load.
This is important because gas generants in pellet
form are generally used for placement in gas generating
devices, such as automobile supplemental restraint
systems. Gas generant pellets should have sufficient
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w WO 95/04014 ~ ~ PCT/US94/08781
crush strength to maintain their shape and configuration
during normal use and withstand loads produced upon
ignition since pellet failure results in uncontrollable
internal ballistics.
The compositions are capable of generating large
quantities of gas while overcoming various problems
associated with conventional gas generating compositions.
The compositions produce substantially nontoxic reaction
products. The compositions are particularly useful for
generating large quantities of a nontoxic gas, such as
nitrogen gas. Significantly, the compositions also avoid
the use of azides, produce no sodium hydroxide by-
products, and generate no sulfur compounds such as
hydrogen sulfide and sulfur oxides.
The compositions also produce only limited
particulate debris, provide good slag formation and
substantially avoid, if not avoid, the formation of
nonfilterable particulate debris. At the same time, the
compositions achieve a relatively high burn rate, while
producing a reasonably low temperature gas. Thus, the
gas produced by the present invention is readily
adaptable for use in deploying supplemental restraint
systems, such as automobile air bags.
An inflatable restraining device, such as an
automobile air bag system comprises a collapsed, inflat
able air bag, a means for generating gas connected to
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WO 95/04014 PCTIUS94108781
~~~13~,
that air bag for inflating the air bag wherein the gas
generating means contains a nontoxic gas generating
composition which comprises a fuel and an oxidizer
therefor wherein the fuel comprises an anhydrous
tetrazole or a salt or complex thereof, such as 5AT or
BTA.
Suitable means for generating gas include gas
generating devices which are used in supplemental safety
restraint systems used in the automotive industry. The
supplemental safety restraint system may, if desired,
include conventional screen packs to remove particulates,
if any, formed while the gas generant is combusted.
The present invention is further described in the
following non-limiting example.
Examples
Example 1
A non-azide fuel, BTA monohydrate (274 .8 grams)
having a nominal particle size of approximately 100
microns, is blended in a muller mixer with an oxidizer,
Cu0 (925.0 grams) having a nominal particle size of 6
microns, and water (30.0 grams) for about one hour to
obtain a compactable powder. The compactable powder is
blended in a Hobart mixer with water (400.0 grams) for
about 15 minutes to obtain a paste . The paste is allowed
to air dry at about 40°C until it achieves a consistency
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suitable for agglomeration, such as by granulation,
corresponding to about 20 to 25 $ by weight water. The
partially dried paste is suitable for granulation, and is
forced through a 16 mesh screen to produce small
granules . The granules are then dried to constant weight
at a temperature of about 31°C. An amount of the dried
granules is removed and split into two portions of equal
amounts. Both portions are further dried at about 120°C
for about 24 hours to remove remaining water. To one
portion is added calcium stearate (0.20 ~ by wt. ) . Pill-
shaped pellets (3/8" diameter) are pressed from each of
the further dried portions . The pellets are subj ected to
a Rh 45~ treatment for 24 hours at 25°C, and the pellet
condition is monitored. The pellets retain their pellet
shape after 24 hours exposure to Rh 45~.
Comparative Example
A non-azide gas generating composition is prepared
by blending 274.8 grams of the non-azide fuel, BTA
monohydrate, having a nominal particle size of about 100
microns with 925.4 grams of copper oxide (Cu0) having a
nominal particle size of about 6 microns and 480 grams of
water for about 90 minutes in a Hobart blender/mixer to
obtain a past. The paste is dried at 90°C to a
consistency suitable for granulation. The mixture is
then meshed through a 18-mesh screen to produce granules
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WO 95/04014 216 7 ,~ ~ 6 PCT/US94/08781
which were then allowed to dry in the air at ambient
conditions. The granules are then pulverized for 30
minutes in a muller mixer to obtain a powder having an
average particle size of substantially less than 100
microns.
A portion of the resultant powder is further dried
at 120°C for an additional 24 hours to achieve an
anhydrous composition. The anhydrous powder is then
pressed to produce 3/8-inch diameter pellets which are
subsequently exposed to a Rh of 45$ at 25 °C. The
pellets lose all integrity within four hours.
The present invention may be embodied in other
specific forms without departing from its spirit or
essential characteristics. The described embodiments are
to be considered in all respects only as illustrative and
not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than
by the foregoing description. All changes which come
within the meaning and range of equivalency of the claims
are to be embraced within their scope.
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