Note: Descriptions are shown in the official language in which they were submitted.
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Description
LOW PERMEABILITY AIRBAG CUSHIONS
HAVING EXTREMELY LOW COATING LEVELS
Technical Field
This invention relates generally to coated inflatable fabrics and more
particularly concerns airbag cushions to which very low add-on amounts of
coating
have been applied and which exhibit extremely low air permeability. The
inventive
inflatable fabrics are primarily for use in automotive restraint cushions that
require low
permeability characteristics (such as side curtain airbags). Traditionally,
heavy, and
thus expensive, coatings of compounds such as neoprene, silicones and the
like, have
been utilized to provide such required low permeability. The inventive fabric
utilizes
an inexpensive, very thin coating to provide such necessarily low permeability
levels.
Thus, the inventive coated inflatable airbag possesses a coating comprising an
elastomeric material (or materials) coated on the target fabric wherein the
elastomeric
material possesses a tensile strength of at least 2,000 and an elongation at
break of at
least 180%. The coating is then applied to the airbag surface in an amount of
at most
2.5 ounces per square yard (and preferably forms a film). The inventive airbag
exhibits
a characteristic leak-down time (defined as the ratio of inflated bag volume
to bag
volumetric leakage rate at 10 psi) of at least 5 seconds after inflation. The
resultant
airbag cushions, particularly low permeability cushions exhibiting very low
rolled
packing volumes, are intended to reside within the scope of this invention.
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Background Art
Airbags for motor vehicles are known and have been used for a substantial
period of time. A typical construction material for airbags has been a
polyester or nylon
fabric, coated with an elastomer such as neoprene, or silicone. The fabric
used in such
bags is typically a woven fabric formed from synthetic yarn by weaving
practices that
are well known in the art.
The coated material has found acceptance because it acts as an impermeable
barrier to the inflation medium. This inflation medium is generally nitrogen,
helium, or
other like gases generated from a gas generator or inflator. Such gas is
conveyed into
the cushion at a relatively warm temperature. The coating obstructs the
permeation of
the fabric by such hot gas, thereby permitting the cushion to rapidly inflate
without
undue decompression during a collision event.
Airbags may also be fonmed from uncoated fabric which has been woven in a
manner that creates a product possessing low permeability or from fabric that
has
undergone treatment such as calendaring to reduce permeability. Fabrics which
reduce
air permeability by calendaring or other mechanical treatments after weaving
are
disclosed in U.S. Patent 4,921,735; U.S. Patent 4,977,016; and U.S. Patent
5,073,418
(all incorporated herein by reference).
Silicone coatings typically utilize either solvent based or complex two
component reaction systems. Silicone coating weight for a traditional driver
side air
bag is usually 0.5 - 1.2 oz/yd2. Very different from driver and passenger side
air bags,
side curtain bags, which emerged in the in late 1990's, are intended to
protect occupants
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during side impact and roll over collisions. A side curtain bag usually has
higher
working pressure and more importantly, has to to stay inflated within a
specific pressure
range for a duration of time at least two orders of magnitude longer than the
duration of
inflation for a driver side or passenger side airbags. Dry coating weights for
silicone
have been in the range of about 3 to 4 ounces per square yard or greater for
both the
front and back panels of side curtain airbags. Lower coating weight for the
side curtain
bags has not been achieved without sacrificing performance. As will be
appreciated by
one of ordinary skill in this art, high add on weights substantially increase
the cost of
the base fabric for the airbag and make packing within small airbag modules
very
difficult. Furthermore, silicone exhibits very tow tensile strength and low
tear
resistance characteristics which do not withstand high pressure inflation
easily without
the utilization of very thick coatings.
The use of certain polyurethanes as coatings as disclosed in U.S. Patent
5,110,666 to Menzel et al. (herein incorporated by reference) permits low add
on
weights reported to be in the range of 0.1 to 1 ounces per square yard but the
material
itself is relatively expensive and is believed to require relatively complex
compounding
and application procedures due to the nature of the coating materials.
Patentees,
however, fails to disclose any pertinent elasticity and/or tensile strength
characteristics
of their particular polyurethane coating materials. Furthermore, there is no
discussion
pertaining to the importance of the coating ability (and thus correlated low
air
permeability or characteristic leak-down time) at low add-on weights of such
polyurethane materials on the new side curtain airbags either only for fabrics
which are
utilized within driver or passenger side cushions. All airbags must be
inflatable
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extremely quickly; upon sensing a collision, in fact, airbags usually reach
peak
pressures within 10 to 20 milliseconds. Regular driver side and passenger side
air bags
are designed to withstand this enormous inflation pressure; however, they also
deflate
very quickly in order to effectively absorb the energy from the vehicle
occupant hitting
the bag. Such driver and passenger side cushions (airbags) are thus made from
low
permeability fabric, but they also deflate quickly at connecting seams and
through vent
holes. Furthermore, the low add-on coatings taught within Menzel, and within
U.S.
Patent 5,945,186 to Li et al., would not provide long-term gas retention; they
would
actually not withstand the prolonged and continuous pressures supplied by
activated
inflators for more than about 2 seconds, at the most. The low permeability of
these
airbag fabrics thus aid in providing a small degree of sustained gas retention
within
driver and passenger airbag cushions to provide the deflating cushioning
effects
necessary for sufficient collision protection. Such airbag fabrics would not
function
well with side curtain airbags, since, at the very least, the connecting seams
which
create the pillowed, cushioned structures within such airbags, as discussed in
greater
detail below, would not be coated. As these areas provide the greatest degree
of
leakage during and after inflation, the aforementioned patented low coating
low
permeability airbag fabrics would not be properly utilized within side curtain
airbags.
As alluded to above, there are three primary types of different airbags, each
for
different end uses. For example, driver-side airbags are generally mounted
within
steering columns and exhibit relatively high air permeabilities in order to
act more as a
cushion for the driver upon impact. Passenger-side airbags also comprise
relatively
high air permeability fabrics which permit release of gas either therethrough
or through
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vents integrated therein. Both of these types of airbags are designed to
protect persons
in sudden collisions and generally burst out of packing modules from either a
steering
column or dashboard (and thus have multiple "sides"). Side curtain airbags,
however,
have been designed primarily to protect passengers during rollover crashes by
retaining
S its inflation state for a long duration and generally unroll from packing
containers
stored within the roofline along the side windows of an automobile (and thus
have a
back and front side only). Side curtain airbags therefore not only provide
cushioning
effects but also provide protection from broken glass and other debris. As
such, it is
imperative that side curtain airbags, as noted above, retain large amounts of
gas, as well
as high gas pressures, to remain inflated throughout the longer time periods
of the entire
potential rollover situation. To accomplish this, these side curtains are
generally coated
with very large amounts of sealing materials on both the front and back sides.
Since
most side curtain airbag fabrics comprise woven blanks that are either sewn,
sealed, or
integrally woven together, discrete areas of potentially high leakage of gas
are
prevalent, particularly at and around the seams. It has been accepted as a
requirement
that heavy coatings were necessary to provide the low permeability (and thus
high leak-
down time) necessary for side curtain airbags. Without such heavy coatings,
such
airbags would most likely deflate too quickly and thus would not function
properly
during a rollover collision. As will be well understood by one of ordinary
skill in this
art, such heavy coatings add great cost to the overall manufacture of the
target side
curtain airbags. There is thus a great need to manufacture low permeability
side curtain
airbags with less expensive (preferably Iower coating add-on weight) coatings
without
losing the heat aging, humidity aging, and permeability characteristics
necessary for
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proper functioning upon deployment. To date, there has been little
accomplished, if
anything at all, alleviating the need for such thick and heavy airbag coatings
from side
curtain airbags.
Furthermore, there is a current drive to store such low permeability side
curtain
airbags within cylindrically shaped modules. Since these airbags are generally
stored
within the rooflines of automobiles, and the area available is quite limited,
there is
always a great need to restrict the packing volume of such restraint cushions
to their
absolute minimum. However, the previously practiced low permeability side
curtain
airbags have proven to be very cumbersome to store in such cylindrically
shaped
containers at the target automobile's roofline. The actual time and energy
required to
roll such heavily coated low permeability articles as well as the packing
volume itself,
has been very difficult to reduce. Furthermore, with such heavy coatings
utilized, the
problems of blocking (i.e., adhering together of the different coated portions
of the
cushion) are amplified when such articles are so closely packed together. The
chances
of delayed unrolling during inflation are raised when the potential for
blocking is
present. Thus, a very closely packed, low packing volume, low blocking side
curtain
low permeability airbag is highly desirable. Unfortunately, the prior art has
again not
accorded such an advancement to the airbag industry.
Disclosure of Invention
In light of the background above, it can be readily seen that there exists a
need
for a low permeability, side curtain airbag that utilizes lower, and thus less
expensive,
amounts of coating, and therefore exhibits a substantially reduced packing
volume over
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the standard low permeability type side curtain airbags. Such a coated low
permeability
airbag must provide a necessarily high leak-down time upon inflation and after
long-
term storage. Such a novel airbag and a novel coating formulation provides
marked
improvements over the more expensive, much higher add-on airbag coatings (and
resultant airbag articles) utilized in the past.
It is therefore an object of this invention to provide a coated airbag,
wherein the
coating is present in a very low add-on weight, possessing extremely high leak-
down
time characteristics after inflation and thus complementary low permeability
characteristics. Another object of the invention is to provide an inexpensive
side
curtain airbag cushion. A further object of this invention is to provide an
highly
effective airbag coating formulation which may be applied in very low add-on
amounts
to obtain extremely low permeability airbag structures after inflation. An
additional
object of this invention is to provide an airbag coating formulation which not
only
provides beneficial and long-term low permeability, but also exhibits
excellent long-
term storage stability (through heat aging and humidity aging testing). Yet
another
object of the invention is to provide a low permeability side curtain airbag
possessing a
very low rolled packing volume and non-blocking characteristics for effective
long-
term storage within the roofline of an automobile.
Accordingly, this invention is directed to an airbag cushion comprising a
coated
fabric, wherein said fabric is coated with an elastomeric composition in an
amount of at
most 2.5 ounces per square yard of the fabric; and wherein said airbag
cushion, after
long-term storage, exhibits a characteristic leak-down time of at least 5
seconds. Also,
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this invention concerns an airbag cushion comprising a coated fabric, wherein
said
fabric is coated with an elastomeric composition; wherein said elastomeric
composition
comprises at least one elastomer possessing a tensile strength of at least
1,500 psi and
an elongation of at least 180%; and wherein said airbag cushion, after long-
term
storage, exhibits a characteristics leak-down time of at least 5 seconds.
Additionally,
this invention encompasses a coated airbag cushion which exhibits a rolled
packing
volume factor (measured as unrolled fabric length the rolled diameter of the
airbag
cushion)of at least 17.
The term "characteristic leak-down time" is intended to encompass the
measurement of the pressure decay characteristic of a side curtain bag after
the bag is
inflated to the peak working pressure. The pressure decay curve of a side
curtain airbag
most resembles a mathematical exponential decay curve wherein a simple time
constant
is utilize to characterize the entire curve. The characteristic leak-down time
used in this
invention serves as the time constant in describing the pressure decay of air
bag. The
1 S measurement is made on an already-inflated (to a peak initial pressure
which "opens"
up the areas of weak sealing) and deflated airbag cushion upon subsequent re-
inflation
at a constant pressure at 10 psi. It is well known and well understood within
the airbag
art, and particularly concerning side curtain (low permeability) airbag
cushions, that
retention of inflation gas for long periods of time is of utmost importance
during a
collision. Side curtain airbags are designed to inflate as quickly as driver-
and
passenger-side bags, but they must deflate very slowly to protect the
occupants during
roll over and side impact. Thus, it is imperative that the bag exhibits a very
low leakage
rate after the bag experiences peak pressure during the instantaneous, quick
inflation.
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Hence, the coating on the bag must be strong enough to withstand the shock and
stresses when the bag is inflated so quickly. Thus, a high characteristic leak-
down time
measurement is paramount in order to retain the maximum amount of beneficial
cushioning gas within the inflated airbag. Airbag leakage after inflation (and
after peak
pressure is reached) is therefore closely related to actual pressure retention
characteristics. The pressure retention characteristics (hereinafter referred
to as "leak-
down time") of already-inflated and deflated side curtain airbags can be
described by a
characteristic leak-down time t, wherein:
Bag volume(ft3)
t (seconds) - ______________________________________________________ X3600
Volumetric leakage rate(SCFH*) at 10 Psi
*SCFH: standard cubic feet per hour.
It is understood that the 10 psi constant is not a limitation to the
invention; but merely
the constant pressure at which the leak-down time measurements are made. Thus,
even
if the pressure is above or below this amount during actual inflation or after
initial
pressurizing of the airbag, the only limitation is that if one of ordinary
skill in the art
were to measure the bag volume and divide that by the volumetric leakage rate
(measured by the amount leaking out of the target airbag during steady state
inflation at
10 psi), the resultant measurement in time would be at least 5 seconds.
Preferably, this
time is greater than about 9 seconds; more preferably, greater than about 15
seconds;
and most preferably, greater than about 20 seconds.
Alternatively, and in a manner of measurement with uninflated side curtain
airbags, the terns "leak-down time" may be measured as the amount of time
required for
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at half of the introduced inflation gas to escape from the target airbag after
initial peak
pressure is reached. Thus, this measurement begins the instant after peak
initial
pressure is reached upon inflation (such as, traditionally, about 30 psi) with
a standard
inflation module. It is well understood that the pressure of gas forced into
the airbag
5 after peak initial pressure is reached will not remain stable (it decreases
during the
subsequent introduction of inflation gas), and that the target airbag will
inevitably
permit escape of a certain amount of inflation gas during that time. The
primary focus
of such side curtain airbags (as noted above) is to remain inflated for as
long as possible
in order to provide sufficient cushioning protection to vehicle occupants
during rollover
10 accidents. The greater amount of gas retained, the better cushioning
effects are
provided the passengers. Thus, the longer the airbag retains a large amount of
inflation
gas, and consequently the greater the characteristic leak-down time, the
better
cushioning results are achieved. At the very least, the inventive airbag must
retain at
least 25%, preferably 50% or higher, of its inflated gas volume 5 seconds
subsequent to
reaching peak initial pressure. Preferably, this time is 9 seconds, more
preferably 15
seconds, and most preferably 20 seconds.
Likewise, the term, "after long-term storage" encompasses either the actual
storage of an inventive airbag cushion within an inflator assembly (module)
within an
automobile, besides in a storage facility awaiting installation. Such a
measurement is
generally accepted, and is well understood and appreciated by the ordinarily
skilled
artisan, to be made through comparable analysis after representative heat and
humidity
aging tests (ASTM D 5427). These tests, adopted by the industry, generally
involve
107° C oven aging for 16 days, followed by 83° C and 95%
relative humidity aging for
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16 days and are generally accepted as proper estimations of the conditions of
long-term
storage stability for airbag cushions. Thus, this term encompasses such
measurement
tests. The inventive airbag fabrics must exhibit proper characteristic leak-
down times
after undergoing such rigorous pseudo-storage testing.
Detailed Description of the Invention
The inventive elastomeric coating composition must comprise at least one
elastomer that possesses a tensile strength of at least 1,500 psi and an
elongation to
break of greater than about 180%. Preferably, the tensile strength is at least
3,000 psi,
more preferably, 4,000, and most preferably at least about 5,000. The high end
is
actually the highest one can produce which can still adhere to a fabric
surface. The
preferred elongation to break is more than about 200%, more preferably more
than
about 300%. These characteristics of the elastomer translate to a coating that
is both
very strong (and thus will withstand enormous pressures both at inflation and
during the
1 S time after inflation and will not easily break) and can stretch to
compensate for such
large inflation, etc., pressures. Thus, when applied at the seams of a side
curtain airbag,
as well as over the rest of the airbag structure, the coating will most
preferably (though
not necessarily) form a continuous film. This coating acts to both fill the
individual
holes between the woven yarns and/or stitches, etc., as well as to "cement"
the
individual yarns in place. During inflation, then, the coating prevents
leakage through
the interstitial spaces between the yarns and aids in preventing yarn shifting
(which may
create larger spaces for possible gas escape).
The utilization of such high tensile strength and high elongation at break
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components permits the consequent utilization, surprisingly, of extremely low
add-on
weight amounts of such coating formulations. Normally, the required coatings
on side
curtain airbags are very high, at least 3.0 ounces per square yard on each
side of the bag
(with the standard actually much higher than that, at about 4.0). The
inventive airbag
cushions require merely at most 2.5 oz/yd2 on each side (preferably less, such
as 2.0,
more preferably 1.8, still more preferably, about 1.5, and most preferably, as
low as 0.8)
ounces per square yard of this inventive coating to effectuate the desired
high leak-
down time (low permeability). Furthenmore, the past coatings were required to
exhibit
excellent heat and humidity aging stability. Unexpectedly, even at such low
add-on
amounts, and particularly with historically questionable coating materials
(polyurethanes, for example), the inventive coatings, and consequently, the
inventive
coated airbag cushions, exhibit excellent heat aging and humidity aging
characteristics.
Thus, the coating compositions and coated airbags are clearly improvements
within
this specific airbag art.
Of particular interest as the elastomer components within the inventive
elastomeric compositions are, specifically, polyamides, polyurethanes, acrylic
elastomers, hydrogenated nitrite rubbers (i.e., hydrogenated NBR), butyl
rubbers,
EPDM rubbers, fluoroelastomers (i.e., fluoropolymers and copolymers containing
fluoro-monomers), ethylene-vinylacetate copolymers, and ethylene acrylate
copolymers.
Also, such elastomers may or may not be cross-linked on the airbag surface.
Preferably, the elastomer is a polyurethane and most preferably is a
polycarbonate
polyurethane elastomer. Such a compound is available from Bayer Corporation
under
the tradename Impranil~, including Impranil~ 85 UD, ELH, and EHC-O1. Other
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acceptable polyurethanes include Bayhydrol~ 123, also from Bayer; Ru 4I-710,
EX 51-
550, and Ru 40-350, both from Stahl USA. Any polyurethane, or elastomer, for
that
matter, which exhibits the same tensile strength and elongation at break
characteristics
as noted above, however, are potentially available within the inventive
coating
S formulation and thus on the inventive coated airbag cushion. In order to
provide the
desired leak-down times at long-term storage, however, the add-on weights of
other
available elastomers may be greater than others. However, the upper limit of
2.5
ounces per square yard should not be exceeded to meet this invention. The
desired
elastomers may be added in multiple layers if desired as long the required
thickness for
10 the overall coating is not exceeded. Alternatively, the mulitple layer
coating system
may also be utilized as long as at least one elastomer possessing the desired
tensile
strength and elongation at break is utilized.
Other possible components present within the elastomer coating composition
are thickeners, antioxidants, antiblocking agents, crosslinking agents,
surfactants, flame
15 retardants, coalescent agents, adhesion promoters, and colorants. In
accordance with the
potentially preferred practices of the present invention, a dispersion (either
solvent- or
water-borne, depending on the selected elastomer) of finely divided
elastomeric resin or
a resin solution is compounded with a flame retardant to yield a compounded
mix
having a viscosity of about 8000 centipoise or greater. A polyurethane is
potentially
20 preferred, with a polycarbonate polyurethane, such as those noted above
from Bayer
and Stahl, most preferred. Other potential elastomeric resins include other
polyurethanes, such as WitcobondTM 253 (35% solids), from Witco, and Sancure,
from
BFGoodrich, Cleveland, Ohio; hydrogenated NBR, such as ChemisatTM LCH-7335X
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(40% solids), from Goodyear Chemical, Akron, Ohio; EPDM, such as EP-603A
rubber
latex, from Lord Corporation, Erie, Pennsylvania; butyl rubber, such as Butyl
rubber
latex BL-100, from Lord Corporation; and acrylic rubber (elastomers), such as
HyCarTM, from BFGoodrich. This list should not be understood as being all-
inclusive,
only exemplary of potential elastomers. Furthermore, the preferred elastomer
will not
include any silicone, due to the extremely low tensile strength (typically
below about
1,500 psi) characteristics exhibited by such materials. However, in order to
provide
effective aging and non-blocking benefits, such components may be applied to
the
elastomeric composition as a topcoat as long as the add-on weight of the
entire
elastomer and topcoat does not exceed 2.5 ounces per square yard.
Additionally,
elastomers comprising certain polyester or polyether segments (such as
polypropylene
oxide) or other similar components, are undesirable, particularly at very low
add-on
weights (i.e., 0.8-1.2 oz/yd2) due to stability problems in heat and humidity
aging
(polyesters easily hydrolyze in humidity and polyethers easily oxidize in
heat);
however, such elastomers may be utilized in higher add-on amounts as long,
again, as
the 2.5 ounces per square yard on each side is not exceeded.
Among the other additives particularly preferred within this elastomer
composition are heat stabilizers, flame retardants, primer adhesives,
antiblocking agents
and materials for protective topcoats. A potentially preferred thickener is
marketed
under the trade designation NATROSOLTM 250 HHXR by the Aqualon division of
Hercules Corporation which is believed to have a place of business at
Wilmington,
Delaware. In order to meet Federal Motor Vehicle Safety Standard 302 flame
retardant
requirements for the automotive industry, a flame retardant is also preferably
added to
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the compounded mix. One potentially preferred Ilame retardant is AMSPERSE F/R
51
marketed by Amspec Chemical Corporation which is believed to have a place of
business at Gloucester City New Jersey. Primer adhesives may be utilized to
facilitate
adhesion between the surface of the target fabric and the elastomer itself.
Thus,
5 although it is preferable for the elastomer to be the sole component of the
entire
elastomer composition in contact with the fabric surface, it is possible to
utilize
adhesion promoters, such as isocyanates, epoxies; functional silanes, and
other such
resins with adhesive properties, without deleteriously effecting the ability
of the
elastomer to provide the desired low permeability for the target airbag
cushion. An
10 adhesive primer coating may be applied directly to the fabric before
applying the
inventive high strength elastomeric coating to assure great adhesion strength.
A topcoat component, as with potential silicones, as noted above, may also be
utilized to effectuate proper non-blocking characteristics to the target
airbag cushion.
Most elastomers, including certain grades of silicones or polyurethanes,
suitable for
i 5 sealing the side curtain structures exhibit high surface frictions and
tend to block at
elevated temperature. High surface friction would slow down airbag deployment
(unfolding/unrolling) and compromise the safety provided by the airbag. The
current
side curtain airbag uses a nonwoven fabric on top of the silicone coating to
provide the
necessary low friction in addition to providing nonblocking benefits. But the
nonwoven
fabric significantly increases the packing volume and the total cost. It has
now been
found that by using an elastomer with significantly higher hardness and higher
softening point as topcoat, a non-blocking and low friction surface can be
achieved with
lower cost and improved packing volume. The higher hardness and higher
softening
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point can also be achieved by using crosslinking agents in the topcoat.
Suitable
crosslinking agents are, without limitation, melamine-formaldehyde resin,
polyisocyanates (difunctional, trifunctional and polyfunctional), epoxy
crosslinking
resins, polyaziridines, carbodiimide crosslinking resins, phenol formaldehyde
resin,
5 urea formaldehyde resin and the like. A sliding coefficient of friction of
less than about
0.7 (measured according to ASTM D 4518 test method B) can be achieved by using
topcoats possessing significantly higher hardness and softening point
properties. Such
a topcoat may also perform various other functions, including, but not limited
to,
improving aging of the elastomer coating (such as with silicone) or providing
further
reinforcement to the elastomer coating materials (most noticeably with the
preferred
polycarbonate polyurethanes). The topcoat materials thus can be selected from,
besides
silicones, a group of organic polymer resins that have higher softening point
and
hardness upon coating and possible curing. Examples of those materials are,
polyurethanes, polyacrylates, epoxy resins, ethylene-vinyl acetate copolymers,
fluoropolymers, polyamides, and polyesters.
Airbag fabrics must pass certain tests in order to be utilized within
restraint
systems. One such test is called a blocking test which indicates the force
required to
separate two portions of coated fabric from one another after prolonged
storage in
contact with each other (such as an airbag is stored). Laboratory analysis for
blocking
20 entails pressing together coated sides of two 2 inch by 2 inch swatches of
airbag fabric
at 5 psi at 100°C for 7 days. If the force required to pull the two
swatches apart after
this time is greater than 50 grams, or the time required to separate the
fabrics utilizing a
50 gram weight suspended from the bottom fabric layer is greater than 10
seconds, the
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coating fails the blocking test. Clearly, the lower the required separating
shear force,
the more favorable the coating. For improved blocking resistance (and thus the
reduced
chance of improper adhesion between the packed fabric portions), topcoat
components
may be utilized, such as talc, silica, silicate clays, starch powders and
topcoat polymer
resins mentioned earlier, as long as the add-on weight of the entire elastomer
composition (including the topcoat) does not exceed 2.5 ounces per square yard
(and
preferably exists at a much lower level, about 1.5, for instance).
Two other tests which the specific coated airbag cushion must pass are the
oven
(heat) aging and humidity aging tests. Such tests also simulate the storage of
an airbag
fabric over a long period of time upon exposure at high temperatures and at
relatively
high humidities. These tests are actually used to analyze alterations of
various different
fabric properties after such a prolonged storage in a hot ventilated oven
(>100°C) (with
or without humid conditions) for 2 or more weeks. For the purposes of this
invention,
this test was used basically to analyze the air permeability of the coated
side curtain
airbag by measuring the characteristic leak-down time (as discussed above, in
detail).
The initially produced and aged inventive airbag cushion should exhibit a
characteristic
leak-down time of greater than about 5 seconds (upon re-inflation at 10 psi
gas pressure
after the bag had previously been inflated to a peak pressure above about 15
psi and
allowed to fully deflate) under such harsh aging conditions. Since
polyurethanes, the
preferred elastomers in this invention, may be deleteriously affected by high
heat and
humidity (though not as deleteriously as certain polyester and polyether-
containing
elastomers), it may be prudent to add certain components within a topcoat
layer and/or
within the elastomer itself. Antioxidants, antidegradants, and metal
deactivators may
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I8
be utilized for this purpose. Examples include, and are not intended to be
limited to,
Irganox~ 1010 and Irganox~ 565, both available from CIBA Specialty Chemicals.
This topcoat may also provide additional protection against aging and thus may
include
topcoat aging improvement materials, such as, and not limited to, polyamides,
NBR
rubbers, EPDM rubbers, polyurethanes, melamine-formaldehyde resin, urea
formaldehyde resin, polyacrylate, silicones, polyacrylates, fluoropolymers and
the like,
as long as the elastomer composition (including the topcoat) does not exceed
the 2.5
ounces per square yard (preferably much less than that, about 1.5 at the most)
of the
add-on weight to the target fabric.
Other additives may be present within the elastomer composition, including,
and
not limited to, colorants, LJV stabilizers, fillers, pigments, and
crosslinking/curing
agents, as are well known within this art.
It is further noted that silicones may be applied on certain airbags within
this
invention as long as the construction of the airbag permits long
characteristic leak-down
times with such silicones.
Scrape coating methods are typically utilized to apply standard silicone
coatings
on regular air bag fabrics (driver and passenger side air bags). Since the
scrape-coating
knife remains in constant contact with the high points of the yarns on the
target fabric,
the resultant coating exhibits large thickness variations on the fabric
surface or forms a
discontinous film. The thin points in the resultant coating then become the
weak point
for potential failure during inflation and contribute to high leakage rate.
However, due to the unevenness of the fabric surface topology, a coating
method that allows for production of a relatively uniform continuous film on
the target
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19
fabric surface with good adhesion is most preferred. Fixed gap coating
procedures
provide the best results. Such coating procedures include knife-over-roll,
roll-over-roll,
and the like. Transfer roll coating methods (such as reverse roll, calender
roll ,and
gravure roll) may also be used since it can provide a continuous and uniform
coating on
the fabric. Extrusion coating and slot die coating methods are also possible
as long as
they provide good adhesion. Resin solutions or dispersions are preferred in
fixed gap
coating process. For the same dry coating weight, significantly larger gap
resetting is
used for resin solutions or dispersions than for 100% resin systems. A higher
gap
setting allows for the production of a film exhibiting a more uniform coating
thickness.
Most preferred is the utilization of a resin solution since it provides a
better film
formation process (a resin dispersion requires complete resin particle
coalescence to
form a good filrn).
The substrate to which the inventive elastomeric coatings are applied to form
the airbag base fabric in accordance with the present invention is preferably
a woven
1 S fabric formed from yarns comprising synthetic fibers, such as polyamides
or polyesters.
Such yarn preferably has a linear density of about 105 denier to about 840
denier, more
preferably from about 210 to about 630 denier. Such yarns are preferably
formed from
multiple filaments wherein the filaments have linear densities of about 6
denier per
filaments or less and most preferably about 4 denier per filament or less. In
the more
preferred embodiment such substrate fabric will be formed from fibers of
nylon, and
most preferred is nylon 6,6. It has been found that such polyamide materials
exhibit
particularly good adhesion and maintenance of resistance to hydrolysis when
used in
combination with the coating according to the present invention. Such
substrate fabrics
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are preferably woven using fluid jet weaving machines as disclosed in U.S.
Patents
5,503,197 and 5,421,378 to Bower et al. (incorporated herein by reference).
Such
woven fabric will be hereinafter referred to as an airbag base fabric. As
noted above,
the inventive airbag must exhibit extremely low permeability and thus must be
what is
5 termed a "side curtain" airbag. As noted previously and extensively, such
side curtain
airbags (a.k.a., cushions) must retain a large amount of inflation gas during
a collision
in order to accord proper long-duration cushioning protection to passengers
during
rollover accidents. Any standard side curtain airbag may be utilized in
combination
with the low add-on coating to provide a product which exhibits the desired
leak-down
10 times as noted above. Some side curtain airbags are produced through labor-
intensive
sewing or stitching (or other manner) together two separate woven fabric
blanks to form
an inflatable structure. Furthermore, as is well understood by the ordinarily
skilled
artisan, such sewing, etc., is performed in strategic locations to form seams
(connection
points between fabric layers) which in turn produce discrete open areas into
which
15 inflation gasses may flow during inflation. Such open areas thus produce
pillowed
structures within the final inflated airbag cushion to provide more surface
area during a
collision, as well as provide strength to the bag itself in order to withstand
the very high
initial inflation pressures (and thus not explode during such an inflation
event). For
sewn side curtain airbags, this inventive coating, applied on the flat fabric
and sewn
20 seams, provides excellent comb-out resistance at the sewn seam and provides
low air
leakage throughout both the seam and the fabric. Other side curtain airbag
cushions
exist which are of the one-piece woven variety. Basically, some inflatable
airbags are
produced through the simultaneous weaving of two separate layers of fabric
which are
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21
joined together at certain strategic locations (again, to form the desired
pillowed
structures). Such cushions thus present seams of connection between the two
layers. It
is the presence of so many seams (in both multiple-piece and one-piece woven
bags)
which create the aforementioned problems of gas loss during and after
inflation. The
possibility of yarn shifting, particularly where the yarns shift in and at
many different
ways and amounts, thus creates the quick deflation of the bag through quick
escaping of
inflation gasses. Thus, the base airbag fabrics do not provide much help in
reducing
permeability (and correlated leak-down times, particularly at relatively high
pressures).
It is this seam problem which has primarily created the need for the
utilization of very
thick, and thus expensive, coatings to provide necessarily Iow pernieability
in the past.
Recently, a move has been made away from both the multiple-piece side curtain
airbags (which require great amounts of labor-intensive sewing to attached
woven
fabric blanks) and the traditionally produced one-piece woven cushions, to
more
specific one-piece woven fabrics which exhibit substantially reduced floats
between
woven yarns to substantially reduce the unbalanced shifting of yarns upon
inflation,
such as in Ser. No. 09/406,264, to Sollars, Jr., the specification of which is
completely
incorporated herein. These one-piece woven bags are generally produced on
dobby or
jacquard fluid jet looms, preferably the utilized one-piece airbag is made
from a
jacquard weaving process. With such an improvement, the possibility of high
leakage
at seams is substantially reduced. These airbags provide balanced weave
constructions
at and around attachment points between two layers of fabrics such that the
ability of
the yarns to become displaced upon inflation at high pressures is reduced as
compared
with the traditional one-piece woven airbags. Due its greatly improved
virtually float
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22
free and balanced seam structure, such one-piece woven structures permit
extremely
low add-on amounts of elastomeric coatings for low permeability effects. In
fact, these
inventive airbags function extremely well with low add-on coatings below 1.5
and as
low as about 0.5 ounces per square yard.
Furthermore, although it is not preferred in this invention, it has been found
that
the inventive coating composition provides similar low permeability benefits
to
standard one-piece woven airbags, particularly with the inventive low add-on
amounts
of high tensile strength, high elongation, non-silicone coatings; however, the
amount of
coating required to permit high leak-down times is much higher than for the
aforementioned Sollars, Jr. inventive one-piece woven structure. Thus, add-on
amounts
of as much as 1.2 and even up to about 2.2 ounces per square yard may be
necessary to
effectuate the proper low level of air permeability for these other one-piece
woven
airbags. Even with such higher add-on coatings, the inventive coatings
themselves
clearly provide a marked improvement over the standard, commercial, prior art
silicone,
etc., coatings (which must be present in amounts of at least 3.0 ounces per
square
yard).
Additionally, it has also been found that the inventive coating compositions,
at
the inventive add-on amounts, etc., provide the same types of benefits with
the
aforementioned sewn, stitched, etc., side curtain airbags. Although such
structures are
highly undesirable due to the high potential for leakage at these attachment
seams, it
has been found that the inventive coating provides a substantial reduction in
permeability (to acceptable leak-down time levels, in fact) with correlative
lower add-
on amounts than with standard silicone and neoprene rubber coating
formulations.
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23
Such add-on amounts will approach the 2.5 ounces per square yard, but lower
amounts
have proven effective (1.5 ounces per square yard, for example) depending on
the
utilization of a sufficiently high tensile strength and sufficiently
stretchable elastomeric
component within the coating composition on the target fabric surface. Again,
with the
ability to reduce the amount of coating materials (which are generally always
quite
expensive), while simultaneously providing a substantial reduction in
permeability to
the target airbag structure, as well as high resistance to humidity and
extremely
effective aging stability, the inventive coating composition, and the
inventive coated
airbag itself is clearly a vast improvement over the prior airbag coating art.
Surprisingly, the inventive coating may also be beneficially applied to a side
curtain bag with less tight construction. Traditional woven side curtain bags
utilize
very tightly woven fabric to provide reduced seam combing possibilities and
less stress
on the applied coating. For example, a typical 420 denier Jacquard one-piece
woven
bag has a construction of 54 -57 yarns per inch. In comparison, a typical
driver side
and passenger side air bag fabric has a construction of 39 - 49 yarns per inch
using the
same 420 denier yarn. The inventive coating actually provides a very low
permeability
on a fabric using 420 denier yarn at construction less tight than 54 yarns per
inch. The
combination of this inventive high strength coating with lower construction
results in
faster weaving speed, lower fiber usage, more flexible fabric, better packing
volume of
coated fabric, lower package weight and lower total cost. For fabric with
lower
construction, a higher strength material or higher coating weight may be
necessary to
achieve the required low overall permeability or characteristic leak-down
time.
Of particular importance within this invention is the ability to pack the
coated
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24
airbag cushions within cylindrical storage containers at the roof line of a
target
automobile in as small a volume as possible. In a rolled configuration (in
order to best
fit within the cylindrical container itself, and thus in order to best inflate
upon a
collision event downward to accord the passengers sufficient protection), the
inventive
S airbag may be constructed to a cylindrical shape having a diameter of at
most 23
millimeters with an unrolled fabric length of ~43 cm. In such an instance,
with a 2
meter long cylindrical roofline storage container, the necessary volume of
such a
container would equal about 830 cm3 (with the volume calculated as
length[n]radius2).
Standard rolled packing diameters are at least 25 millimeters for commercially
available side curtain airbag cushions (due to the thickness of the required
coating to
provide low permeability characteristics). Thus, the required cylindrical
container
volume would be at least 980 cm3. Preferably, the rolled diameter of the
inventive
airbag cushion during storage is at most 20 millimeters (giving a packed
volume of
about 628 cm3) which is clearly well below the standard packing volume. In
relation,
then, to the depth of the airbag cushion upon inflation (i.e., the length the
airbag
extends from the roofline down to its lowest point along the inside of the
target
automobile, such as at the windows), the quotient of the inventive airbag
cushion's
depth (which is standard at approximately 17 inches or 431.8 millimeters) to
its rolled
packed diameter should be at least about 18.8. Preferably this quotient should
be about
21.6 (20 millimeter diameter), and, at its maximum, should be about 24 (with a
minimum diameter of about 18 millimeters). Of course, this range of quotients
does
not require the depth to be at a standard of 17 inches, and is primarily a
function of
coating thickness, and thus add-on weight.
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A further benefit derived from the utilization of the inventive side curtain
airbag
is the ability to utilize low pressure inflators therewith. In the past, the
coatings applied
(i.e., relatively thick, 4.0 ounces per square yard, for example, silicone-
based
formulations) provided effective sealing and thus sufficient gas retention for
side
5 curtain airbags, but only when the inflation pressure was extremely high.
Since a high
initial peak pressure introduced a large amount of inflation gas within the
target airbag
very quickly, an amount of time the target airbag remained inflated to a level
which
provided sufficient cushioning could be achieved. Unfortunately, although
desired
levels of inflation time and retained gas volume were met, these were
basically very low
10 and at the bare minimum on the scale of such desired characteristics. The
inventive
side curtain airbags provide definite improvements in gas retention and
inflation times
(i.e., characteristic leak-down times) over such traditionally silicone-coated
airbags.
The ultimate user may utilize much lower inflation pressures (i.e., 15-20 psi,
and
possibly lower) and still provide an inflated side curtain airbag which will
remain
15 sufficiently inflated to provide maximum cushioning benefits during long-
duration
rollover collisions. Such an ability to utilize a smaller inflator translates
into better
safety (lower the power output, the safer for the vehicle occupants upon
inflation due to
a lower likelihood to cause serious injury), less expensive inflators, lower
volume
inflators, and bags and fabrics which need to withstand lower physical demands
upon
20 inflation.
While the invention will be described and disclosed in connection with certain
preferred embodiments and practices, it is in no way intended to limit the
invention to
those specific embodiments, rather it is intended to cover equivalent
structures
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26
structural equivalents and all alternative embodiments and modifications as
may be
defined by the scope of the appended claims and equivalence thereto.
Detailed Description and Preferred Embodiments of the Invention
Surprisingly, it has been discovered that any elastomer with a tensile
strength of
at least 1,500 psi and an elongation at break of at least 180% coated onto and
over both
sides of a side curtain airbag fabric surface at a weight of at most 2.5
ounces per square
yard, and preferably between 0.8 and 2.0, more preferably from 0.8 to about
1.5, still
more preferably from 0.8 to about 1.2, and most preferably about 0.8 ounces
per square
yard, provides a coated airbag cushion which passes both the long-term
blocking test
and long-term oven aging test with very low, and extended permeability upon
and after
inflation. This unexpectedly beneficial type and amount of coating thus
provides an
airbag cushion which will easily inflate after prolonged storage and will
remain inflated
for a sufficient amount of time to ensure an optimum level of safety within a
restraint
system. Furthermore, it goes without saying that the less coating composition
required,
the less expensive the final product. Additionally, the less coating
composition
required will translate into a decrease in the packaging volume of the airbag
fabric
within an airbag device. This benefit thus improves the packability for the
airbag
fabric.
The elastomer composition of this invention was preferably produced in
accordance with the following Table:
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27
TABLE 1
Standard Water-Borne Elastomer Composition
Component Parts per entire composition)
Resin (30-40% solids content in water) 100
Natrosol~ 250 HHXR (thickener) 10
Irganox~ 1010 (stabilizer) 0.5
DE-83 R (flame retardant) 10
(The particular resins are listed below in Table 2 and thus are merely added
within this
standard composition in the amount listed to form preferred embodiments of the
inventive coating formulation).
The compounded composition's viscosity measured about 15,000 centipoise by
a Brookfield viscometer. Once compounding was complete, the formulation was
applied to both sides of a one-piece Jacquard woven airbag (having 420 denier
nylon
6,6 yarns therein) as discussed within the Sollars, Jr. application noted
above through a
fixed gap procedure (with the gap between the coater and the bag surface at
its greatest
distance being approximately 100 microns). The bag was then dried at an
elevated
temperature (about 300°F for about 3 minutes) to cure and thus form to
form the
necessarily thin coating. As noted above, scrape coating may also be followed
to
provide the desired film coating; however, fixed gap coating provides the
desired film
width uniformity on the bag surface and thus is preferred. Scrape coating, in
this sense,
includes, and is not limited to, knife coating, in particular knife-over-gap
table, floating
knife, and knife-over-foam pad methods. The final dry weight of the coating is
preferably from about 0.6-2.5 ounces per square yard or less and most
preferably 0.8-
1.2 ounces per square yard or less. The resultant airbag cushion is
substantially
impermeable to air when measured according to ASTM Test D737, "Air
Permeability
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28
of Textile Fabrics," standards.
TABLE 2
Standard Solvent-Borne Elastomer Composition
10
Component Parts per entire composition)
Resin (25-40% solids content in solvent) 100
Irganox~ 1010 (stabilizer) 0.5
DE-83 R (flame retardant) 10
The resultant coatings were applied in the same manner as noted above for the
water-
borne elastomers.
In order to further describe the present invention the following non-limiting
examples are set forth. These examples are provided for the sole purpose of
illustrating
some preferred embodiments of the invention and are not to be construed as
limiting the
scope of the invention in any manner. These examples involve the incorporation
of the
below-noted preferred elastomers within the coating formulations of TABLES l
and 2,
above.
Each coated bag was first subjected to quick inflation to a peak pressure of
30 Psi. Air
leakage(SCFH) of the inflated bag was then measured at 10 Psi pressure. The
characteristic leak-down time t(sec) was calculated based on the leakage rate
and bag
volume.
Example Tensile Elongatit (sec). T (sec.) Coating add-
Before
Numberl Strength on at aging Post- on weight
Elastomer (Psi) break ging* (ozlyd2)
28
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29
(%)
1. Impranil 6000 400 18.1 16.3 0.8
85
UD
2. Ex 51-550 3100 320 110.2 105 0.8
3.Impranil 7200 300 120.2 125 0.9
ELH
4. Ru 41-710 7000 600 27.3 26.4 0.8
5. Ru 40 -3507000 500 34.4 36.2 0.8
6.Bayhydrol 6000 300 8.6 5.7 0.8
123
7.Dow Corning700 90 < 2 < 2 2.1
3625***
8. Silastic 1400 580 < 2 < 2 1.8
94-
595-HC**
9. Ru 40-415 5000 180 < 2 < 2 0.8
10.Sancure 3000 580 25.2 < 2 0.8
861
11. Witcobond6000 600 28.4 < 2 0.8
290H
-~-: Hgtng commons: m ~ c: oven aging fbr 16 days, followed by 83 C and 95%
relative
humidity aging for 16 days.
**: The resins are silicone rubbers.
As noted above, Examples 1-6 work extremely well and are thus within the scope
of
this invention. Examples 10 and 11 show some limitations, polyester based
elastomers
(Witcobond 290H) exhibit excellent heat aging (oxidation) stability but tend
to
hydrolyze easily at high humidity; polyether based elastomers (Sancure 861)
have
excellent hydrolysis resistance, but poor oxidation performance. However,
these
elastomers have proven to be acceptable permeability reducers at higher add-on
weights
below the maximum of 2.5 ounces per square yard. Furthermore, although
silicones
a- q
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show excellent resistance to heat aging and hydrolysis (humidity aging), they,
however,
possess limited tensile strength and tear resistance resistance. Natural
rubber, SBR,
chloroprene rubbers and others containing unsaturated carbon double bonds have
excellent hydrolysis resistance. But the unsaturated carbon double bond that
gives their
S elasticity oxidizes readily and the .properties of the rubber change after
heat aging.
Elastomers that have good physical properties and excellent resistance to
hydrolysis and
oxidation are preferred for this application. Polyurethanes based on
polycarbonate soft
segments are the preferred materials for this application.
The airbag of Example 3 exhibited a sliding coefficient of friction constant
of
10 roughly 0.6. A comparative thick silicone-coated side curtain airbag which
included a
non-woven layer, exhibited a constant of about 0.8.
Description of the Drawin",gs
FIG. 1 depicts the side, inside view of a vehicle prior to deployment of the
inventive side curtain airbag.
FIG. 2 depicts the side, inside view of a vehicle after deployment of the
inventive side curtain airbag.
FIG. 3 depicts a side view of a side curtain airbag.
FIG. 4 provides a side view of a side curtain airbag container.
FIG. 5 provides a cross-sectional perspective of the stored airbag within the
container of FIG. 4.
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31
Detailed Description of the Drawings
As depicted in FIG. l, an interior of a vehicle 10 prior to inflation of a
side
curtain airbag (not illustrated) is shown. The vehicle 10 includes a front
seat 12 and a
back seat 14, a front side window 16 and a back-side window 18, a roofline 20,
within
which is stored a cylindrically shaped container 22 comprising the inventive
side
curtain airbag (not illustrated). Also present within the roofline 20 is an
inflator
assembly 24 which ignites and forces gas into the side curtain airbag (26 of
FIG. 2)
upon a collision event.
FIG. 2 shows the inflated side curtain airbag 26. As noted above, the airbag
26
is coated with at most 2.5 ounces per square of a coating formulation (not
illustrated),
preferably polyurethane polycarbonate. The inventive airbag 26 will remain
sufficiently
inflated for at least 5 seconds, and preferably more, as high as at least 20
seconds, most
preferably.
FIG. 3 shows the side curtain airbag 26 prior to storage in its uninflated
state
within the roofline cylindrically shaped container 22. The thickness of the
airbag 26,
measured as the rolled packing diameter (as in FIG. 5, below) as compared with
the
depth of the airbag measured from the roofline cylindrically shaped container
22 to the
bottom most point 28 of the airbag 26 either in its uninflated or inflated
state will be at
least 17 and at most 29, as noted above.
FIGs. 4 and 5 aid in understanding this concept through the viewing of the
rolled airbag 26 as stored within the container 22 along line 2. The diameter
measurement of the airbag 26 of Example 3, above, is roughly 20 millimeters.
The
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32
standard depth of side curtain airbags is roughly 17 inches, or about 431.8
millimeters.
Thus, the preferred packing volume factor is about 21.6. A comparative
silicone-based
thick coating add-on weight of about 4.0 ounces per square yard provided a
diameter of
about 25 millimeters for a factor of about 17.3.
There are, of course, many alternative embodiments and modifications of the
present invention which are intended to be included within the spirit and
scope of the
following claims.
15
CLAIMS
WHAT IS CLAIMED IS:
1. An airbag cushion comprising a coated fabric, wherein said fabric is coated
with
an
