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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ignition enhancer coating composition for
pyrotechnic propellants, especially those propellants suitable for use in
inflating an inflatable device, such as a vehicle safety restraint.
2. Description of the Prior Art
Various pyrotechnic propellants have been prepared for generating a gas
upon combustion in order to inflate an air bag or similar safety restraint
in a vehicle so as to restrain movement of an occupant in the event of a
sudden deceleration of the vehicle, such as caused by a collision. In
order to be employed as a pyrotechnic gas generating composition for
inflatable occupant restraints, several criteria must be met. The
pyrotechnic must be capable of producing non-toxic, non-flammable and
essentially smokeless gases over a wide variety of temperatures and other
environmental conditions. The gases that are generated must be totally
ignited at a sufficiently low temperature so as not to destroy the
restraint or injure the occupant. The pyrotechnic must also be safe to
handle and must be capable of generating a substantial amount of gas
within a very short period of time, e.g., less than about 100
milliseconds.
In order to achieve total ignition and combustion of the pyrotechnic
formulation various additional ingredients have been employed. Such
ingredients include copper chromite and iron oxide as taught in U.S. Pat.
No. 3,933,543, or a mixture of inorganic oxidizer compounds and an oxygen
bearing organic fuel as taught in U.S. Pat. No. 3,964,255. These
ingredients are conventionally taught as being admixed with the
pyrotechnic propellant before the propellant is pressed into pellets.
Although such admixtures are effective to a degree, there is still a need
for a more effective ignition enhancer as well as a need for a more
effective combination of propellant and ignition enhancer.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided an ignition enhancer
composition, preferably applied as a coating on pyrotechnic propellants
suitable for use in automobile safety restraint devices, comprised of
about 60 to 95 wt. % of an inorganic oxidizer, preferably sodium
perchlorate or potassium perchlorate; about 5 to 20 wt. % of an
oxygen-containing polymeric compound, preferably a polyvinyl acetate
resin; about 0 to 10 wt. % polyethylene having a particle size of equal to
or less than about 100 mesh, preferably about 300 mesh; about 0 to 10 wt.
% of a transition-metal oxide, preferably iron oxide; and about 0 to 5 wt.
% carbon black. The ignition enhancer, especially when used as a coating
for pyrotechnic propellants, is superior in enhancing full ignition of the
propellant within a very short period of time.
In a preferred embodiment of the present invention a pyrotechnic
propellant, in pellet form, is coated with a composition comprised of
about 75 to 85 wt % of an inorganic oxidizer, preferably, potassium
perchlorate; about 9 to 11 wt. % of an oxygen-containing polymeric
compound, preferably, a polyvinyl acetate resin; about 3 to 7 wt. % of a
transition-metal oxide, preferably iron oxide; about 1 to 5 wt. %
polyethylene; and about 0.5 to 2 wt. % carbon black.
DETAILED DESCRIPTION
Pyrotechnic propellant compositions for which the present ignition
enhancers are suitable for use are generally any of those pyrotechnic
propellants conventionally known in the art. Such pyrotechnic propellants
generate a gas upon combustion and are generally comprised of mixtures of
chemical components, such as fuels and oxidizers and optionally binders,
and other propellant adjuvants, which may be activated by, for example, an
electrically energized squib to generate substantial volumes of gas for
inflating such devices as crash bags.
The present leading candidates for commercialization in an all-pyrotechnic
inflation system are sodium azide-based compositions, although the present
ignition enhancer compositions are equally suitable for use on non-azide
pyrotechnic propellants. Such compositions exhibit excellent gas
generating properties and produce a gas which consists almost totally of
nontoxic nitrogen gas.
Preferably, the pyrotechnic propellant is in the form of grains or pellets
which have a geometry which provides a substantially constant surface area
exposed to burning during combustion. By use of the present invention
rapid onset of gas generation is assured preferably by coating the present
ignition enhancer onto the surface of the pyrotechnic propellant.
Oxygen-containing polymeric compounds suitable for use in the present
ignition enhancer compositions are those combustible polymeric materials
containing a substantial amount of oxygen. Non-limiting examples of such
compounds include polyacetal resins including both homopolymers and
copolymers, polyvinyl acetate resins, polyesters, polyurethanes,
polyester-polyurethane copolymers, polycarbonates and polymers based on
cellulose compounds such as cellulose acetate, and the like. Preferred are
the polyacetal resins and polyvinyl acetate, more preferred is polyvinyl
acetate.
Non-limiting examples of inorganic oxidizers suitable for use in the
present invention include the alkali metal oxidizers such as sodium
chlorate, potassium chlorate, sodium perchlorate, potassium perchlorate,
sodium chlorite, sodium nitrate, sodium nitrate, potassium nitrate, as
well as ammonium chlorate, ammonium perchlorate, ammonium nitrate, and the
like. Also, bromates or iodates may be employed instead of the
corresponding chlorates (or perchlorates). Preferred are the sodium and
potassium chlorates and perchlorates, more preferred are sodium and
potassium perchlorate, and most preferred is potassium perchlorate.
Non-limiting examples of transition metal oxides suitable for use in the
present ignition enhancer compositions are iron oxide, copper oxide,
manganese oxide, titanium oxide, and vanadium oxide; preferred are ferric
oxide, ferrous oxide and copper oxide, and more preferred is ferric oxide.
The term "polyethylene" as used herein includes homopolymers of ethylene as
well as copolymers obtained by reacting ethylene with a small amount of a
comonomer. Non-limiting examples of such comonomers include C.sub.3 to
C.sub.8 1-alkenes such as propylene, butene-1,2-methylpropene-1,
4-methylpentene-1, and pentene-1 and the like, as well as mixtures
thereof. Generally the copolymer contains at least 85 weight percent, and
preferably not less than 96 weight percent of polymer units derived from
ethylene. Such copolymers have essentially the same characteristics as the
ethylene homopolymer of the same molecular weight, e.g. the preforming and
sintering characteristics are the same.
The polyethylene resin suitable for use as starting material in the present
invention may be prepared by any conventional procedure. One such
procedure is a low pressure ethylene polymerization process using a
chromimum oxide catalyst on a silica or silicaalumina support in
paraffinic or cycloparaffinic solvent thereby forming polyethylene in
solution or as discrete particles in a hydrocarbon slurry. Another
procedure suitable for preparing polyethylene suitable for use herein is
the Ziegler process which teaches the use of an active metal alkyl
catalyst, or by such other processes as described in U.S. Pat. No.
3,050,514 or especially the process outlined in U.S. Pat. No. 3,051,993.
The latter process involves at least intermittently contacting anhydrous
oxygen-free ethylene in the gaseous phase with an inorganic, porous,
frangible, solid contact catalyst prepared from an inorganic compound of
chromium and oxygen and an active metal alkyl.
Generally the polyethylene resins suitable for use as fuel/binders in the
present invention have densities from about 0.92 to 0.97 at 23.degree. C.,
as determined by ASTM Method D792. Their crystalline melting point is in
the order of about 275.degree. F.
The polyethylene suitable for use herein is in granular form wherein the
granules are of a particle size of less than or equal to about 150
(microns), preferably about 15 to 65, more preferably about 25 to 50.
The ignition enhancer compositions of the present invention are comprised
of about 60 to 95 wt. %, preferably about 75 to 85 wt. % inorganic
oxidizer; about 5 to 20 wt. %, preferably about 9 to 11 wt. % combustible
oxygen-containing polymeric compound; about 0 to 10 wt. %, preferably
about 3 to 7 wt. % transition-metal oxide; about 0 to 10 wt. %, preferably
about 1 to 5 wt. % polyethylene; and about 0 to 5 wt. %, preferably 0.5 to
2 wt. % carbon black. All weight percents are based on the total weight of
the ignition enhancer composition.
It is preferred that the ignition enhancer composition of the present
invention be used as a coating for pyrotechnic propellants although it can
also be admixed with the propellant in granules form before the propellant
is compressed into pellets. When these ignition enhancer compositions are
used in coating-form, superior results are obtained over their use as when
admixed with the propellants.
The method of applying the ignition enhancer composition to the propellant
is not critical. One preferred method of coating the propellant is by
first preparing a coating mix. This is accomplished by adding the
oxygen-containing polymeric compound and the polyethylene in an
appropriate solvent such as methylene chloride in a mixing vessel such as
a ball mill jar. The carbon black, inorganic oxidizer and grinding balls
are then placed in the jar and placed on a ball mill for a time sufficient
to put the solids into suspension--generally up to about 12 hours. The
propellant to be coated, in pellet form, is placed in an appropriate
container such as a stainless steel mesh basket and dipped into the
coating with agitation for a time sufficient to completely coat the
propellant, generally about 10 seconds. The basket is then withdrawn from
the mix and suspended over the solution to drip and slightly dry for about
10 to 20 seconds. The coated pellets are then baked in an oven at about
120.degree. to 200.degree. C., preferably 140.degree. to 160.degree. C.
for 0.5 to 2 hours, preferably 0.75 to 1.25 hours.
The propellant is weighed before and after coating to determine the weight
and thickness of the coating which of course can be easily calculated by
one skilled in the art. To decrease the weight of the coating more solvent
can be added and conversely to increase the weight of the coating some
solvent is permitted to evaporate. Generally the coating will constitute
about 1 to 5 wt. % preferably about 2.5 to 3.5 wt. % based on the total
weight of the coated pellet.
In order to further describe the present invention, the following
non-limiting examples are given.
EXAMPLE 1
A pyrotechnic composition consisting of 8.54 weight percent of an acetal
copolymer having a melt index of 9 (sold under the designation "Celcon"),
2.83 weight percent carbon black, 45.5 weight percent sodium chlorate,
20.8 weight percent calcium sulfate sesquihydrate, and 22.3 weight percent
aluminum hydroxide is intimately mixed by ball milling under methylene
chloride, dried, and pressed into pellets. The aforementioned weight
percents are based on the total weight of the pyrotechnic composition.
These pellets are coated with an ignition enchancer composition consisting
of 10 weight percent polyvinyl acetate, 4 weight percent of finely ground
(20 M) polyethylene, 80 weight percent potassium perchlorate, 1 weight
percent carbon black, and 5 weight % ferric oxide, wherein these weight
percents are based on the total weight of the coating composition. The
coating on the pellets should constitute about 3 weight percent based on
the total weight of the coated pellet.
The coated pellets are then placed into a cylindrical steel casing, to
which a nozzle is attached. The pellets are ignited and the burn rate is
found acceptable for automobile safety restraint devices.
EXAMPLE 2
In pyrotechnic composition consisting of 80 weight percent sodium azide and
20 weight percent ferric oxide is intimately mixed by ball milling under
methylene chloride, dried, and pressed into pellets. The pellets are
coated with the ignition enhancer composition of Example 1 wherein the
weight percent of the coating with respect to the pellet plus the coating
is the same as in Example 1.
The coated pellets are then placed into a cylindrical steel casing, to
which a nozzle is attached. The pellets are ignited and the burn rate is
found acceptable for automobile safety restraint devices.
It is to be understood that variations and modifications of the present
invention may be made without departing from the scope thereof. It is also
understood that the present invention is not to be limited by the specific
embodiments disclosed herein but only in accordance with the appended
claims when read in light of the foregoing specification.
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