An igniter (18) provides rapid longitudinal and radial propagation of the ignition reaction. The igniter may comprise a pyrotechnic material and an inorganic binder, such as silica, carried on a carrier web (10) which may be fiber-glass. One or more layers of coated web (10) are disposed to provide an igniter (18) of cylindrical configuration and having a hollow core. The coated layers are permeable to the ignition reaction to facilitate radial propagation of ignition. The hollow core and the continuous nature of the pyrotechnic layers promotes longitudinal propagation of ignition. The igniter may consist mostly or entirely of inorganic materials to reduce or eliminate the formation of carbon monoxide upon ignition, and may be contained within a radially perforated sheath (20) to form a radial ignition device (25).

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to surface-initiating deflagrating materials and 
more specifically to elongate igniters which provide both longitudinal and 
radial output of the ignition reaction. 
2. Related Art 
U.S. Pat. No. 3,067,686 to Coover Jr. et al, dated Dec. 11, 1962, discloses 
a carpet-roll type propellant grain utilized for a rocket motor. The web 
is a fabric woven from strands of a poly-alpha-olefin, and serves as the 
reducing agent for the oxidant which may take different forms (column 4, 
lines 3-13) and which may be applied to the web in a mixture exemplified 
by a mixture of aluminum powder and ammonium perchlorate (Example 4, 
column 6). The outer windings of the web are uncoated (column 4, lines 
38-47). 
U.S. Pat. No. 3,763,787 to Schultz, dated Oct. 9, 1973, discloses a rocket 
propellant which may be a composite or modified double base propellant 
applied to a substrate screen. The substrate screen may be a fiberglass 
web (see column 2, lines 23-25). The propellant may include a fuel such as 
powdered aluminum, an inorganic oxidizer such as ammonium perchlorate and 
a rubberized binder (column 3, lines 25-29). 
U.S. Pat. No. 3,213,793 to Dratz, dated Oct. 26, 1965, discloses a solid 
rocket propellant in which a cellulosic web which has been impregnated 
with an oxidizing agent is coated with a dispersion comprising a "fuel" 
(reducing agent) and an oxidant (column 1, lines 48-55). The web, which is 
highly absorptive and may be made from paper (column 1, line 70 to column 
2, line 45), is dried and rolled to serve as a solid propellant charge. 
The oxidant initially impregnated into the web may be ammonium perchlorate 
(column 2, lines 46-52) and the fuel coating may comprise powdered 
aluminum as the reducing agent (column 3, lines 15-20) and an additional 
oxidizer (column 3, lines 15-17. See Example 1, especially column 6, lines 
40-44). 
U.S. Pat. No. 4,838,165 to Gladden et al, dated Jun. 13, 1989, discloses an 
igniter in which pyrotechnic material, which may be aluminum powder mixed 
with potassium perchlorate, is disposed within an elongate sheath. 
SUMMARY OF THE INVENTION 
Generally, the present invention provides an igniter which provides rapid 
radial and longitudinal propagation of the ignition reaction and which may 
be manufactured more economically and efficiently than prior art igniters. 
The reduced content of, or elimination of, carbonaceous materials in 
certain embodiments of the igniter of the present invention results in a 
reduction in the amount of, or precludes the formation of, carbon monoxide 
upon ignition. 
In accordance with one aspect of the present invention, the igniter 
consists essentially of inorganic components comprising an inorganic 
carrier on which is coated a pyrotechnic material to provide a coated 
carrier. The pyrotechnic material comprises an inorganic reductant 
component and an inorganic oxidizer component. 
According to one aspect of the invention, the igniter has a cylindrical 
configuration. In another aspect of the invention, the cylindrical 
configuration is defined by a plurality of radially disposed layers of the 
coated carrier. 
Another aspect of the present invention provides that the igniter is 
configured to have a hollow core extending longitudinally through the 
igniter. 
Yet another aspect of the present invention provides that the igniter may 
be of cylindrical configuration and comprises a carrier on which is coated 
a pyrotechnic material to provide a coated carrier. The pyrotechnic 
material comprises a reductant component and an oxidizer component. The 
cylindrical configuration of the igniter is defined by one or more 
radially disposed layers of the coated carrier, the layers being permeable 
to ignition of the pyrotechnic material, whereby ignition of the 
pyrotechnic material propagates both longitudinally and radially through 
the igniter. 
In one aspect of the invention, the carrier may have a rolled, i.e., 
convolute configuration. Alternatively, the carrier may have a 
helical-wound configuration. 
Other aspects of the invention are provided by the following features, one 
or more of which may be present in a given embodiment. The carrier may 
comprise a fiberglass web and the web may contain an inorganic sizing or a 
blend of organic and inorganic sizings; the pyrotechnic material may 
further comprise an inorganic binder, for example, colloidal silica. Thus, 
the pyrotechnic material according to the present invention may comprise 
from about 20 to 36% aluminum, from about 55 to 71% ammonium perchlorate, 
from 0 to about 30% potassium perchlorate and from about 2 to 5% binder by 
weight (dry basis) of the pyrotechnic material. 
In another aspect, the present invention provides that the igniter may be 
combined with a radially perforated tubular sheath within which the 
igniter is disposed. The combination provides an ignition device. The 
sheath may be made of any suitable material, e.g., an inorganic material, 
preferably a metal such as steel. 
As used herein and in the claims, the term "inorganic" has a broad meaning 
as indicating that branch of chemistry and chemical compounds other than 
hydrocarbons and their derivatives, i.e., all substances which are not 
compounds of carbon. Although some definitions of "inorganic" do not 
exclude carbon oxides and carbon disulfide, for purposes of this patent 
application, all carbon compounds capable of conversion to carbon monoxide 
upon ignition, and elemental carbon, are excluded from the definition of 
"inorganic". 
As used herein and in the claims the term "organic" has its usual broad 
meaning as indicating that branch of chemistry and chemical compounds 
concerning hydrocarbons and their derivatives, i.e., all substances which 
are compounds of carbon. 
Other aspects of the present invention are described below.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
The present invention provides an igniter which may be used in an ignition 
device to provide both a longitudinally and a radially emanating ignition 
reaction. Such devices are used to initiate the deployment of air bag 
inflators such as are used as automobile safety devices, to ignite the 
ejectors which release munitions from cruise missiles, in artillery as gun 
primers and in other explosives and pyrotechnic devices. The igniter of 
the present invention may be used in these and other applications to 
rapidly propagate an ignition reaction not only longitudinally along the 
length of the line, but radially outwardly of the line as well. 
Generally, an igniter according to the present invention is prepared by 
coating a carrier web with a homogeneous layer of a pyrotechnic material, 
and rolling, winding or otherwise arranging the coated web, while its 
pyrotechnic coating is still wet or sufficiently flexible, into a 
cylindrical configuration having one or more radially disposed layers. As 
used herein and in the claims, a "radially disposed" layer or layers means 
that the layer is, or the layers are, disposed radially about the 
longitudinal axis of the cylindrical igniter, just as the wall of a tube 
or pipe is disposed radially about the longitudinal axis of the tube or 
pipe. The rolled, wound or otherwise arranged web may advantageously be 
configured to define a longitudinal hollow core extending the entire 
length of the igniter. Accordingly, one way to make the igniter is by 
winding the coated web about a mandrel, thereby forming the igniter as a 
hollow cylinder or tube and then removing the igniter from the mandrel (or 
the mandrel from the igniter), to provide a cylindrical, hollow core 
igniter, the diameter of the mandrel determining the diameter of the 
hollow core. The freshly formed igniter is then dried or allowed to dry to 
provide the finished igniter. Only a single layer of coated web may be 
rolled on itself to provide either a rod-like structure or, if a mandrel 
is used, to provide a structure which resembles a tube or pipe, the wall 
of which is provided by a single pyrotechnic-coated web. Usually however, 
a number of overlying layers of coated web will be used. In any case, the 
igniter may, if desired, be provided with a protective outer layer made of 
a suitable thin material such as cellulosic paper, a polymeric film or, if 
inorganic materials are to be used, fiberglass, fiberglass paper or 
aluminum foil. Whether or not a protective outer layer is used, those 
portions of the web which will form the interior hollow core of the 
finished igniter, and those portions which will form the exterior surface 
of the igniter, may be left uncoated or may be covered by a layer of 
suitable material so that the pyrotechnic material is not exposed on the 
exterior of the finished igniter or on the interior core (if the igniter 
is formed to have one) to a degree which will result in loss of the 
pyrotechnic material through abrasion ("dusting"). The igniter may be 
disposed within a perforated sheath or tube to form an ignition device, or 
put to other use. The freshly formed igniter may be placed within the 
sheath while the pyrotechnic material is still wet, and dried while inside 
the sheath. Pyrotechnic material which weeps through the web material may 
have an adhesive effect helping to fix the igniter in place within the 
sheath. 
The carrier web material used in preparing the igniter according to this 
invention may be a woven or non-woven material which can be wound into a 
carpet roll-like configuration or can be helically wound, as will be 
described below. Further, the web is permeable to the ignition reaction of 
the pyrotechnic material, so that when pyrotechnic material disposed on 
one side of the web is ignited, the ignition reaction permeates the web to 
a degree sufficient to ignite pyrotechnic material disposed on the other 
side of the web. Preferably, the web is permeable to the coating of 
pyrotechnic material applied thereto so it "weeps" between web layers to 
bridge adjacent layers through the interstices of the web. Once ignited, 
the ignition reaction readily propagates both longitudinally along the 
continuously coated web surfaces, especially along the hollow core, and 
radially across the multiple layers of the web, if such are present. The 
permeability of the web permits the ignition reaction to readily propagate 
radially through the web or through radially disposed multiple layers of 
coated web, and longitudinally along the continuous coating or coatings of 
pyrotechnic material. When the igniter is configured to have a hollow 
core, longitudinal propagation is especially facilitated along the core. 
The carrier web may advantageously be made of a primarily non-carbonaceous 
material, e.g., an inorganic material, which is permeable to the ignition 
reaction. Inorganic materials are preferred because of the resulting 
preclusion of carbon monoxide formation upon combustion, which is 
sometimes desired for reasons discussed below. Fiberglass cloth is a 
preferred web material because it is principally inorganic and the 
conventional carbonaceous additives (e.g., starch sizing) can be removed 
or replaced with inorganic species (as will be disclosed herein). 
Fiberglass fabrics may also be sufficiently porous to allow pyrotechnic 
material to lodge in the interstices of the cloth, thus facilitating the 
transfer of the ignition reaction through the cloth. However, non-woven 
fiberglass matting (or "fiberglass paper") may work as well, and when 
used, is preferably prepared with an inorganic sizing and in a thickness 
of from about 5 to 10 mils (0.127-0.254 mm). In addition, organic 
materials, such as cellulosic paper, or a polymeric material, may be used 
for the web in applications where a reduction or elimination of carbon 
monoxide release upon ignition is not required. 
In a preferred embodiment, a conventional fiberglass web, whether non-woven 
or woven, is heat cleaned, i.e., calcined, to remove any starches or other 
carbonaceous species which may be present. It may then be treated with an 
inorganic sizing rather than a conventional carbonaceous sizing. The 
inorganic sizing may comprise silica and may be applied to the web as a 
water-based colloidal suspension. In an alternative embodiment, the sizing 
may comprise, in addition to silica, a quantity of a carbonaceous 
polymeric material such as acrylic resin, which releases less carbon 
monoxide upon burning than many other carbonaceous binders, and which 
improves handling characteristics, e.g., stiffness and weave set, of the 
carrier web. The acrylic may comprise from about 5 to about 30% by weight 
of the sizing material on a dry basis. The choice of acrylic material and 
the inclusion of an inorganic species in the sizing reduces the carbon 
monoxide production of this cloth in relation to conventional fiberglass 
cloth having primarily carbonaceous sizing. Preferably, the sizing 
comprises from about 3-6% by weight of the uncoated cloth. 
A sized, uncoated fiberglass cloth carrier web having a conventional weave 
may have a typical thickness of about 2.3 mils (0.058 millimeters) and may 
be porous or perforated rather than smooth, to allow better adherence of 
the pyrotechnic material onto the web. This thickness allows the ignition 
reaction to pass through the web to ignite pyrotechnic material on the 
other side. Webs made from other suitable materials may likewise be 
dimensioned and configured to provide such permeability. 
A coating material comprising a pyrotechnic material is applied to the 
carrier web. Ordinarily, a sufficient quantity of pyrotechnic material can 
be coated on a single side of the web, but it is possible, in alternative 
embodiments of the invention, to coat both sides of the web. The 
pyrotechnic material is chosen to be any material with suitable 
deflagration properties to serve the needs of the end use, and typically 
comprises a fuel comprising a reductant and an oxidizer. Various mixtures 
and preparations of reductants and oxidizers are known in the art; a 
mixture of aluminum particles, e.g., flake, ammonium perchlorate and, 
optionally, potassium perchlorate is a preferred pyrotechnic material. The 
aluminum flake employed may be of a size which is typical of the kind of 
flake used in aluminum paint, having a particle size distribution such 
that about 99% of the flakes pass through a standard 325 mesh screen. 
Preferably, sufficient pyrotechnic fuel is provided to allow the reaction 
between the reductant and oxidizer to be self-sustaining so that no 
additional source of fuel is needed. Therefore, the carrier need not be 
composed of a material which will serve as part of the fuel in the 
ignition reaction. 
The pyrotechnic material may also include a binder to enhance the adhesion 
of the pyrotechnic material to the carrier web. In contrast to 
conventional binders, which typically comprise carbonaceous polymeric 
materials such as latex or other organic binders, the present invention 
makes use of a binder which is principally, and preferably entirely, 
composed of an inorganic material. Inorganic binders may be preferred over 
carbonaceous binders because they do not produce carbon monoxide when the 
pyrotechnic material is ignited. This is advantageous in a number of 
areas, for example, when the present invention is used as an igniter for 
automotive air bag inflators. In such use, it is desired to reduce the 
quantity of noxious gases such as carbon monoxide produced by the air bag 
inflator to protect the occupants of the automobile, who may be injured 
and unconscious, from exposure to carbon monoxide or other noxious gases 
released by the air bag device. 
A preferred inorganic binder comprises colloidal silica, although other 
inorganic materials such as alumina may work as well. Preferably, the 
binder constitutes from about 2 to 5% by weight, dry basis, calculated as 
silica (SiO.sub.2) of the pyrotechnic material, e.g., about 2.5% by 
weight. The colloidal silica is mixed into the liquid medium to prepare 
the coating material as described below. 
The pyrotechnic coating material may be disposed in a liquid suspension 
known as a "wet mix" which is deposited upon the carrier web. The wet mix 
is a slurry of the pyrotechnic material and the binder in a liquid medium 
which is later removed (e.g., by drying) from the igniter. The liquid 
medium may comprise water and a wetting agent added to assist in 
dispersing the aluminum flakes or particles in the liquid medium. Any 
suitable wetting agent may be used, and among organic wetting agents, 
volatile compounds such as alcohols, which can later be removed from the 
web by evaporation, are preferred over conventional soap-type surfactants, 
which leave a carbonaceous residue. A typical liquid medium comprises from 
about 10 to 100% wetting agent by volume, for example, the liquid medium 
may comprise about 33% isopropyl alcohol by volume, the balance being 
water. Wetting agents comprising fluorinated hydrocarbons of the type sold 
under the trademark Freon by E.I. DuPont de Nemours and Company and which 
are liquid at ambient conditions may be used as, or as a component of, the 
liquid medium. The liquid medium may comprise from about 30 to 90% by 
weight of the wet coating material mix, for example, about 40 to 50%, 
e.g., 47%. The carrier web may be coated with the wet mix by any 
conventional method, e.g., by immersion of the web in a bath station 
containing the wet mix or by depositing the wet mix on the web and 
spreading the wet mix with a doctor blade, or by any other suitable 
methods. The pyrotechnic material may thus be coated upon one or both 
sides of the web. Due to the porosity of the carrier web, the application 
of the wet mix to one side of the web results in a wicking or weeping of 
the wet mix, including some pyrotechnic material, into the interstices of 
the cloth and, to a small extent, onto the opposite surface of the cloth. 
Diffusion of the pyrotechnic material through the entire thickness of the 
web in this way promotes radial permeability of the web to the ignition 
reaction by causing the pyrotechnic material in one layer of the multiple 
ply igniter to bridge the web thickness to contact the pyrotechnic 
material in each radially adjacent layer. The bridging of the web layers 
by the pyrotechnic material insures that the ignition of one layer of 
material will ignite the radially adjacent layer, to provide reliable 
radial, as well as longitudinal, propagation of the ignition reaction. The 
"weep-through" of a wet mix applied only to one side of the web does not 
deposit nearly as much pyrotechnic material on the opposite surface of the 
web as is disposed on the coated surface but a sufficient amount of binder 
weeps through the cloth to help bind adjacent windings of the web together 
when the igniter is dried, as will be described below. Thus, the uncoated 
surface can be considered as being effectively "masked" or free of 
pyrotechnic material as described herein. One-sided coating of the web is 
preferred as being a simpler manufacturing procedure. To provide 
additional protection against the loss of pyrotechnic material, the 
igniter may, as noted above, optionally be wrapped with thin aluminum foil 
or a similarly suitable covering. 
Typically, the wet mix contains enough pyrotechnic material to provide 
about 1 gram of pyrotechnic material (dry basis) per linear inch (0.394 
gram per linear cm) of the finished igniter. After the wet mix is applied 
to the web, the web may be partially dried to remove some, but preferably 
not all, of the liquid medium of the wet mix, because the handling 
characteristics of the web in the wet state are preferred to those of a 
dry-coated web. When dry, the pryotechnic material is brittle, and 
subsequent handling is difficult and leads to loss of pyrotechnic 
material, referred to as "dusting". In addition, dry pyrotechnic material 
may be susceptible to accidental ignition from static electricity which is 
often produced in handling web materials. 
To produce an ignition device according to the present invention, the 
wet-coated carrier web may be disposed in a configuration having one layer 
or a plurality of concentric, radially disposed layers to form an igniter 
by, for example, winding the web around a mandrel. The wound web is 
removed from the mandrel, leaving an open, longitudinally extending hollow 
interior core in the igniter. The carrier is thus disposed in a "carpet 
roll" configuration, referred to herein and in the claims as a "convolute" 
configuration. Winding the coated web in a convolute configuration 
disposes one layer upon the next in radial succession, beginning with an 
innermost winding and ending with an outermost winding. 
A section of the carrier web corresponding to the outer surface of the 
outermost winding of the igniter is masked so that pyrotechnic material is 
not prominently exposed on the outer surface of the igniter. Likewise, 
another portion of the carrier material is masked so that, upon removal of 
the mandrel, pyrotechnic material is not exposed to the open interior core 
of the igniter. Masking may be accomplished by providing an uncoated 
portion of the exposed surface of the carrier web or by affixing a swatch 
of a suitable material over a coated portion of the carrier web to cover 
the coated areas which are to be masked. When a web is coated on one side 
only, the entire uncoated side of the web can be considered to be masked, 
and part of the masked side will be exposed as the surface of the inner 
core or as the outer surface of the igniter. The result of masking is that 
the pyrotechnic material is protected from dusting, i.e., from having 
particles of the pyrotechnic material dislodged from the exterior surface 
or from the interior hollow core. 
The igniter may be inserted into a radially perforated tubular sheath to 
form an ignition device. After the igniter is disposed within the sheath, 
the igniter is dried to remove the remaining liquid medium. Upon drying, 
the pyrotechnic material fuses adjoining windings of the carrier web (if 
such there be) into an integral structure, those portions of the wet mix 
which wept through the web thickness serving to secure adjacent layers 
together. In addition, drying fixes the wound igniter within the sheath 
because the binder which wept through the outermost winding of the web 
contacts the interior surface of the sheath. 
The sheath is preferably formed from inorganic material and may be of any 
suitable cross section such as circular, ovoidal or polygonal cross 
section. In addition to an axial opening at one or each longitudinal end 
of the tubular sheath, the wall of the sheath has perforations formed 
therein (radially through the wall) to allow the ignition reaction of the 
pyrotechnic material to spread radially outwardly of the sheath once the 
ignition device is ignited. The wall of the sheath, where it is not 
perforated, contains the ignition reaction of the pyrotechnic material, 
thus preventing the energy of the deflagration reaction from being 
excessively dissipated. Preferably, the sheath is made from an inorganic 
material, typically, steel. 
The sheath should be sufficiently strong to withstand the deflagration of 
the igniter without being ruptured. Therefore, as is known in the art, the 
size and number of perforations must be chosen to balance mechanical 
strength of the sheath with the need to provide radial propagation of the 
ignition reaction without allowing the energy of the reaction to dissipate 
excessively. The perforations may be conventionally circular, or may have 
any other geometric configuration. 
As noted above, a preferred embodiment of the present invention makes use 
of materials which, upon ignition of the igniter, produce no carbon 
monoxide or less carbon monoxide than ignition devices of the prior art. 
For this reason, the web material, the pyrotechnic material and the sheath 
are, preferably, each composed of inorganic material, or at least contain 
less organic material, i.e., carbonaceous matter, than conventional 
ignition devices. As discussed with respect to the sizing, a limited 
amount of carbonaceous material may be incorporated into the present 
invention, if necessary or convenient, without defeating the overall 
benefit of reduced carbon monoxide production. As discussed above, there 
are uses of the present invention in which carbon monoxide production is 
not viewed as a problem, so that carbonaceous components may be used 
without constraint. 
In a specific embodiment, pyrotechnic material is mixed with an inorganic 
binder in a liquid medium to produce a wet mix. The pyrotechnic material 
comprises a fuel comprising a reductant such as flaked aluminum and an 
oxidizer such as ammonium perchlorate and, optionally, potassium 
perchlorate, in amounts which favor high energy output and a fast 
deflagration rate. A preferred mixture comprises 7% potassium perchlorate, 
61.5% ammonium perchlorate, 29% aluminum flake and 2.5% colloidal silica 
by weight, in a liquid medium comprising 33% isopropyl alcohol and 67% 
water, by volume of the liquid medium. The solids content of the wet mix 
is about 47% by weight. 
Referring now to FIG. 1, a web 10 of fiberglass cloth prepared with an 
inorganic sizing and measuring about 8.5 inches (21.6 cm) in length 1 and 
from about 5.7 inches (14.5 cm) to about 10.3 inches (26.2 cm) in width w, 
for example, 8.3 inches (21.1 cm) wide, is placed on a flat surface (not 
shown) and a quantity of the wet mix is placed thereon and spread over the 
surface of web 10 using a doctor blade or other suitable device (not 
shown) to provide a pyrotechnic material coating 12. The coating procedure 
is conducted to deposit sufficient pyrotechnic material to provide about 1 
gram (dry basis) of pyrotechnic material per linear inch (0.39 g per 
linear cm) of the finished igniter. One edge portion of web 10 provides a 
masked region 14 having a width M of about 1 inch (2.5 cm). While the 
pyrotechnic material coating 12 is still damp, coated web 10, as 
schematically illustrated in FIG. 2, is wound around a mandrel 16 with the 
uncoated side of web 10 being disposed outwardly of the igniter being 
formed on the mandrel. Mandrel 16 may typically have a diameter of about 
0.27 inches (0.68 cm). The rolled-up carrier web may typically have an 
outer diameter of about 0.43 inches, (1.1 cm). Masked region 14 is the 
first part of web 10 which is wound about mandrel 16, to provide the 
innermost winding and to assure that little pyrotechnic material will come 
into contact with mandrel 16 or will be exposed to the interior core when 
the igniter is later removed from mandrel 16. The remainder of web 10 is 
wound in successive layers on top of the innermost winding until a last, 
outermost winding is made, forming the igniter 18 (FIG. 3). Since the 
coated side of web 10 was disposed inwardly during winding, the outermost 
winding is disposed with the uncoated side of web 10 providing the 
exterior surface of igniter 18. 
As illustrated in FIG. 4, the mandrel 16 carrying web 10 now having been 
wound into a rolled convolute configuration to provide an igniter 18, is 
inserted into a perforated tube or sheath 20 from an end opening thereof. 
Sheath 20 has a longitudinal axis L--L and is comprised of a sheath wall 
22 having formed therein a plurality of radial perforations 24. Sheath 20 
may be made from any suitable material, e.g., steel, and may have an inner 
diameter of, e.g., 0.5 inches (1.3 cm) and an outer diameter of about 
0.562 inches 1.4 cm). A plurality of pyrotechnic-coated layers of web 10 
is thus disposed radially along radius r (FIG. 3A) of igniter 18. Once 
igniter 18 is fully inserted within sheath 20, mandrel 16 is removed, 
leaving sheath 20 behind. The igniter 18, encased within sheath 20, is 
then dried to evaporate the liquid medium from pyrotechnic material wet 
mix 12 and provide the ignition device 25 (FIG. 5). Drying of pyrotechnic 
material wet mix 12 fuses the individual windings of web 10 together and 
binds igniter 18 within sheath 20 to produce an ignition device 25 (FIG. 
5) according to an embodiment of the present invention and having a 
plurality of radially disposed layers and a longitudinally extending 
hollow core 26 (FIG. 3A). As well known to those skilled in the art, 
sheath 20 is adapted to receive at one end thereof an explosive squib or 
other suitable device (not shown) which is positioned against or in 
proximity to one end of igniter 18 and is used to ignite the ignition 
device. 
In another embodiment of the present invention, the entire surface of web 
10 may be coated with a wet mix of pyrotechnic material 12, as shown in 
FIG. 6, without leaving a masked area corresponding to area 14 of FIG. 1. 
In such case, a ribbon of mask material 28 is applied along one or both 
edges of the coated web to provide a masked region of width M. Mask 
material 28 is preferably an uncoated ribbon of the same material as web 
10. Alternatively, the mask material may be a sheet of thin aluminum foil 
or a similarly suitable covering. 
An alternative method of producing an ignition device having a plurality of 
concentric radially discrete layers of a coated carrier web is illustrated 
in FIG. 7. According to this method, a first ribbon of pyrotechnic 
material-coated carrier web 10a is wound in a helical fashion about a 
mandrel 16' which is carried on a support 16a'. While adjacent windings of 
web 10a may overlap each other, they are preferably disposed in a 
butt-seamed configuration so that coated web 10a lies in a continuous, 
single radial layer of uniform thickness. The interior surface of web 10a 
is free of pyrotechnic material, web 10a having a coating of pyrotechnic 
material only on the outward-facing surface thereof wound upon mandrel 
16'. Optionally, one or more additional radial layers of pyrotechnic 
material-coated carrier web 10b are similarly wound around mandrel 16', 
the second layer being applied on top of web 10a and subsequent ones on 
top of each preceding layer. The outermost winding of web material, 
provided in the illustrated embodiment by web 10b, is free of a 
pyrotechnic material coating, at least on its outwardly-facing surface and 
may comprise a protective outer layer as described above with reference to 
the embodiment of FIGS. 3 and 8. The helically-wound webs provide an 
igniter. 18' which is fed off mandrel 16' continuously, as indicated by 
arrow p, as it is produced. The directions of travel of webs 10a and 10b 
are indicated by the arrows associated therewith. Lengths of igniter 18' 
can be stored or cut to length before being dried, or may be passed 
directly to an oven for drying and subsequent processing, as desired. 
A cross-sectional view of a conventional prior art ignition device 28 is 
illustrated in FIG. 8 in a view corresponding to that of FIG. 3A showing 
an embodiment of the present invention. The prior art device comprises a 
detonating cord 30 disposed within a radially perforated (perforations 
24') tubular sheath 20'. Detonating cord 30 is packed within a 
longitudinal granular bed 32 of a relatively stable pyrotechnic charge, 
e.g., BKNO.sub.3. Conventional pyrotechnics such as granular bed 32 do not 
linearly propagate fast enough to provide uniform radial output along the 
length of the igniter, thereby requiring that the presence of detonating 
cord 30 to ignite granular bed 32. A detonator or squib (not shown) is 
positioned to ignite detonating cord 30, which in turn releases sufficient 
energy to ignite the more stable pyrotechnic granular bed 32. So 
configured, ignition device 28 of the prior art suffers from several 
significant disadvantages. First, it is difficult, time-consuming and 
expensive to manufacture compared to the igniters of the present invention 
because the cord 30 must be embedded within the granular bed 32 and 
accurately centered therein. If detonating cord 30 is not accurately 
centered, uniform radial deflagration of granular bed 32 will not be 
attained. Accurate centering can be accomplished, for example, by 
accurately centering the detonating cord 30 within the perforated sheath 
20' and, while holding its centered position, filling the remaining volume 
of sheath 20' with the granular pyrotechnic material. In addition to 
manufacturing difficulties, special provisions must be made to prevent the 
pyrotechnic material of granular bed 32 from escaping from ignition device 
28 through the perforations 24' in the sheath 20' during manufacture and 
handling. This requires the use of a barrier layer (not shown) on the 
inner surface of perforated sheath 20' or other expedients. In addition, 
detonating cord 30 provides a very small target for a squib or other 
detonator. Since detonating cord 30 is so highly reactive, it cannot be 
made significantly bigger in diameter because upon ignition it could 
rupture the ignition device and disperse the granular bed 32 without 
igniting it. 
An igniter according to the present invention, e.g., used in ignition 
device 25, overcomes the aforesaid problems of the prior art. As 
previously described, igniter 18 is easily manufactured with masked 
surfaces forming its hollow core 26 and its outermost surface, which 
reduces dusting because relatively little, if any, pyrotechnic material is 
disposed directly upon the exposed surfaces. Further, since the 
concentric, radially disposed layers of the carrier web are radially 
permeable to the ignition of the pyrotechnic material disposed thereon, 
the initial ignition of any part of the cross-sectional area of igniter 18 
will serve to initiate and propagate the pyrotechnic action both 
longitudinally and radially. Therefore, an initiator may effectively 
ignite any part of the axial end of igniter 18 and therefore need not be 
designed to such stringent specifications as prior art devices. Thus, the 
reliability of the igniter is improved and the cost of its manufacture 
reduced. When the exposed interior surface is ignited, the ignition 
reaction proceeds along the interior surface of the igniter at a rate much 
faster than the rate of radial propagation. It is preferred, but not 
necessary, to ignite the igniter as close to its center or the interior 
core surface as possible, so that the slower radial propagation is 
initiated from the center or interior core surface. The rapid longitudinal 
propagation provides radial propagation nearly concurrently along the 
length of the relatively short igniter. The rate of radial propagation 
increases smoothly as the internal pressure of the igniter rises due to 
the burning pyrotechnic material. The use of a sheath such as illustrated 
sheath 20 is optional, as the igniter may be used without the sheath in 
certain applications. 
While the invention has been described in detail with reference to a 
particular embodiment thereof, it will be apparent that upon a reading and 
understanding of the foregoing, numerous alterations to the described 
embodiment will occur to those skilled in the art and it is intended to 
include such alterations within the scope of the appended claims.