U.S. patent application number 13/821525 was filed with the patent office on 2013-09-05 for pyrotechnic gas generator compounds.
This patent application is currently assigned to HERAKLES. The applicant listed for this patent is Stephane Besombes, Frederic Marlin. Invention is credited to Stephane Besombes, Frederic Marlin.
Application Number | 20130228254 13/821525 |
Document ID | / |
Family ID | 43927666 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130228254 |
Kind Code |
A1 |
Marlin; Frederic ; et
al. |
September 5, 2013 |
PYROTECHNIC GAS GENERATOR COMPOUNDS
Abstract
The main subject of the present invention is a solid pyrotechnic
gas generator compound, the composition of which contains,
expressed in weight percentages: from 60 to 70% of guanidine
nitrate, from 26 to 33%, advantageously 26 to 30%, of potassium
perchlorate, from 2.5 to 6% of at least one combustion modifier
chosen from transition metal oxides, the precursors of such oxides
and mixtures thereof, from 0 to 6% of at least one additive, and
does not contain an explosive ingredient. Such a compound is
particularly effective for the inflation of side airbags.
Inventors: |
Marlin; Frederic; (Saint
Medard En Jalles, FR) ; Besombes; Stephane;
(Martignas Sur Jalle, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Marlin; Frederic
Besombes; Stephane |
Saint Medard En Jalles
Martignas Sur Jalle |
|
FR
FR |
|
|
Assignee: |
HERAKLES
Le Haillan
FR
|
Family ID: |
43927666 |
Appl. No.: |
13/821525 |
Filed: |
September 15, 2011 |
PCT Filed: |
September 15, 2011 |
PCT NO: |
PCT/FR2011/052125 |
371 Date: |
May 21, 2013 |
Current U.S.
Class: |
149/2 ; 149/45;
149/78 |
Current CPC
Class: |
C06D 5/06 20130101; C06D
5/02 20130101; C06B 25/34 20130101; C06B 29/08 20130101; C06B
23/007 20130101; C06B 21/0066 20130101 |
Class at
Publication: |
149/2 ; 149/78;
149/45 |
International
Class: |
C06B 23/00 20060101
C06B023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2010 |
FR |
1057353 |
Claims
1. A solid pyrotechnic gas generator compound, the composition of
which contains: guanidine nitrate, and potassium perchlorate,
characterized in that its composition, expressed in weight
percentages, contains from: 60 to 70% of guanidine nitrate, 26 to
33%, advantageously 26 to 30%, of potassium perchlorate, 2.5 to 6%
of at least one combustion modifier chosen from transition metal
oxides, the precursors of such oxides and mixtures thereof, 0 to 6%
of at least one additive, and does not contain an explosive
ingredient.
2. The compound as claimed in claim 1, wherein its composition
consists of said guanidine nitrate, potassium perchlorate, at least
one combustion modifier and optionally at least one additive.
3. The compound as claimed in claim 1, wherein its composition is
at least 94% by weight, advantageously at least 98% by weight, or
even 100% by weight, made up of said guanidine nitrate, potassium
perchlorate and at least one combustion modifier.
4. The compound as claimed claim 1, wherein said at least one
combustion modifier is chosen from zinc oxide (ZnO), iron oxide
(Fe.sub.2O.sub.3), chromium oxide (Cr.sub.2O.sub.3), manganese
dioxide (MnO.sub.2), copper oxide (CuO), basic copper nitrate
(Cu(NO.sub.3).sub.2.3Cu(OH).sub.2) and mixtures thereof.
5. The compound as claimed in claim 1, wherein said at least one
combustion modifier consists of copper oxide and/or basic copper
nitrate.
6. The compound as claimed in claim 5, which has a pressure
exponent of less than or equal to 0.1, for a pressure of between 6
and 52 MPa.
7. The compound as claimed in claim 1, wherein said at least one
combustion modifier has a specific surface area of greater than 3
m.sup.2/g, advantageously greater than 10 m.sup.2/g and very
advantageously greater than 25 m.sup.2/g.
8. The compound as claimed in claim 1, wherein its composition
contains silica as additive.
9. The compound as claimed in claim 8, wherein said silica is in
pulverulent form having a high specific surface area,
advantageously of 100 m.sup.2/g or more, and of micrometric size,
advantageously of nanometric size, or in the form of silica fibers
of from 1 to 20 microns in diameter and from 20 to 500 microns in
length.
10. The compound as claimed in claim 1, wherein it is obtained by
means of a dry process, which comprises a step of compacting a
pulverulent mixture containing the constituent ingredients of said
compound in powder form, optionally followed by a granulation step,
itself optionally followed by a step of forming by pelletizing.
11. The compound as claimed in claim 1, wherein it is in the form
of granules, pellets or monolithic blocks.
12. A pulverulent composition, which is a precursor of a compound
as claimed in claim 1, the composition of which corresponds to that
of a compound as claimed in claim 1.
13. A gas generator, suitable for an airbag, which contains at
least one compound as claimed in claim 1.
Description
[0001] A subject of the present invention is pyrotechnic gas
generator compounds, suitable for use in motor vehicle occupant
protection systems, more especially for the inflation of airbags
and most particularly for the inflation of side airbags (see
below).
[0002] The technical field relating to the motor vehicle occupant
protection has experienced a very large expansion over the last
twenty years. The latest generation of vehicles from now on
integrate within the passenger compartment several safety systems,
of airbag type, the operation of which is carried out by the
combustion gases of pyrotechnic compounds. Among the airbag-type
systems, front airbags for front impacts and side airbags for side
impacts are mainly distinguished.
[0003] The side airbag systems differ from the front airbag systems
essentially due to the time required for the deployment and
positioning of the airbag. Typically, this time is shorter for a
side airbag (about 10-20 ms, compared with 40-50 ms for a front
airbag). For a side airbag, the functional requirement of inflation
of the bag over a short time makes it necessary to resort to a
pyrotechnic composition having a sufficiently high combustion rate
(typically equal to or greater than 30 mm/s, or even 35 mm/s, at 20
MPa) over the operating pressure range in the combustion chamber of
the generator, in order to obtain a sufficient value of the
inflation rate per unit area (product
.rho..times.n.times.Tc.times.Vc). Moreover, in order to ensure a
satisfactory start-up of the system, the pyrotechnic composition
must also have good ignitability characteristics. Also, given the
generally tapered surface profile of the charges used (of pellet
type), the composition should ideally have a combustion rate that
is stable and high enough at low pressure. In fact, those skilled
in the art are aware that the specifications for side airbags are
more restrictive than those for front airbags. Of course, any
technological advance in the field of said side airbags can also
advantageously be available in the field of front airbags.
[0004] In the present text, the term "low pressure" is used to
define a pressure P such that: 0.1.ltoreq.P<10 MPa, the term
"medium pressure" is used to define a pressure P such that: 10
MPa.ltoreq.P<30 MPa, and the term "high pressure" is used to
define a pressure P such that: P.gtoreq.30 MPa.
[0005] It is, moreover, customary to compensate for the low
combustion rates of the pyrotechnic compounds used in the current
gas generators for airbags by having recourse to charges composed
of pellets with very small dimensions. Although not cost effective
owing to the low production throughput by weight of pelleting
machines and the tooling costs generated, this makes it possible,
to a certain extent, to partly overcome the lack of combustion
rate. However, this solution accentuates two other drawbacks:
[0006] difficult ignition, which is increased owing to the high
initial surface area of the charge, which then requires the use of
a reinforced igniter or the addition of an additional charge acting
as an ignition relay; [0007] a strong tapering of the combustion
surface owing to the small size of the pellets which creates a long
combustion tail at low pressure. This long combustion tail at low
pressure is the source of the emission of the majority of the toxic
species present in the gases used to inflate the bag.
[0008] It should therefore be noted that the desired increase in
combustion rate of the pyrotechnic compound in question, over the
entire pressure range, including at low pressure, is therefore
necessary not only to increase the gas flow rate in order to
achieve the inflation delay specifications, but also to ensure the
ignitability of the compound without recourse to the use of a relay
charge and the innocuousness of the combustion products.
[0009] Furthermore, constraints exist with reference to the
combustion temperature.
[0010] In general, said combustion temperature must not be too high
(it must at a minimum remain less than 2400 K, more preferably less
than 2350 K) so that the temperature of the gases in the airbag
does not harm the physical integrity of the occupant. A low
combustion temperature makes it possible, on the one hand, to limit
the thickness of the bag and, on the other hand, to simplify the
design of the gas generator by making it possible to reduce the
presence of baffles and of filters within said generator.
[0011] The side airbag systems may call for two types of gas
generators: those which are said to be entirely pyrotechnic (the
gas generation then being exclusively provided by the combustion of
a pyrotechnic charge) and those said to be "hybrid" (the gases then
originating jointly from the combustion of a pyrotechnic charge and
from a volume of neutral gas stored under pressure in a leaktight
reservoir). For "hybrid" generators, the pyrotechnic charge must
not have a too low combustion temperature so that the combustion
gases are hot enough to compensate for the drop in temperature
generated by the volume expansion of the precompressed neutral gas.
Ideally, combustion temperatures above 2000 K are required.
[0012] Thus, those skilled in the art are in search of pyrotechnic
compounds which are suitable for use in entirely pyrotechnic gas
generators or in hybrid generators, more particularly intended for
side airbags, i.e. which simultaneously have a moderate combustion
temperature of about 2000-2400 K, more preferably 2000-2350 K, and
a high combustion rate over the entire operating pressure range (in
particular greater than 30 mm/s at 20 MPa, more preferably greater
than 35 mm/s at 20 MPa), including at low pressure.
[0013] In addition, the pyrotechnic compounds for airbags must also
aim to jointly meet the following requirements: [0014] the gases
generated by the combustion of the pyrotechnic charge (comprising a
compound or n compounds) must be nontoxic, i.e. have a low content
of carbon monoxide, of nitrogen oxides and of chlorinated
compounds; [0015] the gas yield (i.e. the amount of gas generated
by the combustion) must be high in order to result in a high
inflation power; [0016] the amount of solid particles generated by
the combustion, capable of constituting hot spots that may damage
the wall of the airbag, must remain low; [0017] the pressure
exponent must be as low as possible, in particular at medium and
high pressure (typically below 0.35 as described in the prior art)
but also at low pressure. A low pressure exponent in fact makes it
possible to very significantly reduce the operating variability
between the low temperature extreme (around -40.degree. C.) and the
high temperature extreme (around 90.degree. C.) that are required
in the field of use. The operating reproducibility is as a result
improved and the size of the metal structure of the generator can
advantageously be reduced; [0018] their cut-off combustion pressure
(their combustion limit pressure) must also be as close as possible
to atmospheric pressure.
[0019] It is also highly desirable for the basic ingredients of the
compounds not to be dangerous from the pyrotechnic point of view.
The presence of ingredient(s) belonging to the class of explosives,
such as nitroguanidine, hexogen (RDX) or octogen (HMX), is
advantageously avoided. The term "explosive ingredients" means
ingredients classified in risk division 1.1 according to standard
NF T 70-502 (see also UNO--Recommendations relating to the
transport of dangerous goods--manual of tests and criteria, fourth
revised edition, ST/SG/AC.10/11/Rev.4, ISBN 92-1-239083-8155N
1014-7179 and STANAG 4488). Guanidine nitrate and potassium
perchlorate, taken separately, are not ingredients classified in
this risk division. They do not constitute explosive ingredients,
in particular within the meaning of the invention.
[0020] It should at this stage be pointed out that the
incorporation of highly energetic explosive ingredients, such as
nitroguanidine, even at low levels, prejudicially contributes to
increasing the combustion temperature of the compounds, beyond the
cut-off threshold of 2350 K set by the need for technical
improvement desired by the inventors. Thus, compounds as described
in U.S. Pat. No. 6,893,517, consisting mainly of a mixture of a
guanidine derivative (preferentially guanidine nitrate), of an
explosive nitrogenous compound (preferentially nitroguanidine) and
of an inorganic oxidizing agent (such as ammonium perchlorate or
potassium perchlorate), do not meet the requirements of the
specifications of the present invention. These compounds also
include in their composition a low level of a ballistic catalyst,
consisting of an oxygen-containing compound of a transition metal,
advantageously with a high specific surface area, conventionally
used in the field of propellants for increasing the combustion rate
at medium and high pressure (this catalyst accelerates the
decomposition of the oxidizing charge). The gas microgenerators for
seatbelt tensioner devices as described in said U.S. Pat. No.
6,893,517 (and in its priority application EP 1 275 629) operate
via pulses, which requires a high combustion rate at medium and
high pressure. A high pressure exponent at low pressure and
noncombustion at atmospheric pressure of the compounds in question
does not pose a problem, since the pressure does not fall again, in
the context of the use of said compounds, to a low level before the
end of the pyrotechnic operation. This application for seatbelt
tensioner devices does not need, for the gas generator,
requirements as severe as those needed in the context of the
present invention (airbags, and most particularly side airbags),
most particularly a high combustion rate at low pressure, a drop in
the cut-off combustion pressure threshold (as close as possible to
atmospheric pressure) and a low pressure exponent over the entire
combustion range (in particular at low pressure).
[0021] Currently, for front airbags, the pyrotechnic compounds
which offer a good compromise, in terms of gas temperature, gas
yield, level of particles emitted and toxicity, contain, as main
ingredients, guanidine nitrate (GN) as reducing charge and basic
copper nitrate (BCN) as oxidizing charge. U.S. Pat. No. 5,608,183
and U.S. Pat. No. 6,143,102 describe such compounds.
[0022] However, these compounds have relatively low combustion
rates, less than or equal to 20 mm/s at 20 MPa, and also a low gas
yield. They are also difficult to ignite.
[0023] From the perspective of improving the ignitability of
compounds of this type, the addition of perchlorate to their
composition based on guanidine nitrate (GN) and on basic copper
nitrate (BCN) has been proposed according to the prior art. Thus,
patent application EP 1 526 121 describes the addition of a
perchlorate (in particular potassium perchlorate), in a low amount
(less than 5% by weight), for improving the ignition of these
compounds. However, the incorporation of perchlorate in such a low
amount does not make it possible to sufficiently increase the
combustion rate of the compound for satisfactory use in gas
generators for side airbags.
[0024] Applications WO 2007/042735 and WO 2009/126702 describe
compounds having compositions, of the same type, which contain
guanidine nitrate (GN), as reducing charge, basic copper nitrate
(BCN), as main oxidizing charge, and, in addition, a second
oxidizing charge, which advantageously consists of potassium
perchlorate (KClO.sub.4). These documents associate the good
performances of said compounds, in particular a high combustion
rate at high pressure, with the composition but also with the
specific process for producing said compounds (which process
includes a dry roller compacting step for WO 2007/042735 and two
successive spray-drying and compression steps for WO
2009/126702).
[0025] From the perspective of improving the gas yield and the
combustion rate, compounds based on one (or more) nitrogenous
reducing agent(s) combined with a strong oxidizing agent of
perchlorate type have also been proposed according to the prior
art.
[0026] Patent application US 2006/0137785 describes the combination
of a reducing agent of guanidine type (nitroguanidine or guanidine
nitrate) and of ammonium perchlorate, the latter being necessarily
incorporated in a significantly high amount (30% to 60% by weight).
The incorporation of ammonium perchlorate in such a high amount
results in two major drawbacks which are, firstly, a significant
increase in the combustion temperature (above 2800 K) and,
secondly, the generation of hydrogen chloride (which is a toxic and
highly corrosive gas), said hydrogen chloride then being present in
the gas effluents. In order to overcome this problem, said patent
describes the need to add to the mixture of guanidine+ammonium
perchlorate type a metal compound of iron oxide type in order to
neutralize the hydrochloric acid present in the combustion gases,
which results in a decrease in the gas yield value for the
compound.
[0027] The incorporation of potassium perchlorate in place of
ammonium perchlorate would have the advantage of resulting in the
formation of potassium chloride (KCl) in place of hydrogen chloride
(HCl) (but the drawback of reducing the gas yield). In any event,
the incorporation of KClO.sub.4 in such high amounts (up to 60% by
weight) would produce an increase in the combustion temperature
that is totally unacceptable in the context of the intended
application.
[0028] Logically, those skilled in the art have therefore turned to
compounds consisting of a mixture containing guanidine nitrate (GN,
alone or combined with a coreducing agent) and potassium
perchlorate (KClO.sub.4) in intermediate amounts of approximately
25% to 45% by weight, as described in patent application WO
95/25709 and U.S. Pat. No. 5,854,442 and U.S. Pat. No. 5,997,666,
said mixture making it possible to obtain compounds which partially
meet the essential requirements of the field of application
targeted by the present invention, namely: [0029] a good gas yield;
[0030] a moderate combustion temperature; [0031] an intrinsic
nontoxicity of the particulate effluents; and [0032] a combustion
rate at around 20 MPa which is slightly increased compared with the
compositions formulated on the basis of guanidine nitrate (GN) and
basic copper nitrate (BCN), but which remains insufficient for use
in side airbags.
[0033] The thermodynamic and ballistic characteristics of such a
compound (reference compound 1), the ("binary") composition of
which contains only guanidine nitrate (GN) and potassium
perchlorate (KClO.sub.4), are given in table 1 hereinafter.
TABLE-US-00001 TABLE 1 Ingredients Guanidine nitrate % by weight
68.0 Potassium perchlorate % by weight 32.0 Characteristics Oxygen
balance % -3 Density g/cm.sup.3 1.67 Combustion T at 20 MPa K 2351
Gas yield at 1 bar - 1000 K mol/kg 33.2 Combustion rate at 8 MPa
(low pressure) mm/s 20.6 Combustion rate at 20 MPa (medium
pressure) mm/s 26.3 Combustion rate at 50 MPa (high pressure) mm/s
34.9 Pressure exponent determined between 6 and 0.26 52 MPa Gas
content at 1 bar - 1000 K % 82.5 KCl content % 17.1 Cut-off
combustion pressure (1) MPa 1.7 (1): the value given is a relative
pressure. A zero cut-off combustion pressure corresponds to
atmospheric pressure.
[0034] Reference compound 1 exhibits many advantages among those
expected of a compound for generating gas for an airbag system. The
basic ingredients are simple and readily available, inexpensive,
safe with regard to the pyrotechnic aspects (no constituent
belonging to the explosive compounds class) and nontoxic. The
thermodynamic performance (gas yield, particle content) is good and
the combustion temperature remains moderate and therefore
acceptable. The particles emitted by the combustion are nontoxic
(essentially KCl).
[0035] However, such a compound does not exhibit the entire
performance expected, in particular for a side airbag application.
First of all, the combustion rate of about 26 mm/s at 20 MPa is
increased only by 20 to 30% compared with that of a compound based
on guanidine nitrate (GN) and on basic copper nitrate (BCN), and
remains low with respect to the set specifications. Next, during
tests with this reference formulation, it emerged that, while the
total replacement of the basic copper nitrate (BCN) oxidizing agent
with potassium perchlorate (KClO.sub.4) makes it possible to
increase the combustion rate above 5 MPa and therefore to improve
the ignitability, it in return and highly prejudicially induces a
very high pressure exponent at low pressure (greater than 0.55 over
the range 6 to 10 MPa) and noncombustion at atmospheric pressure
(additional tests showed that the cut-off operating pressure is
around 1.7 MPa, whereas a compound formulated on the basis of
guanidine nitrate (GN) and of basic copper nitrate (BCN)
advantageously has a nonzero combustion at atmospheric
pressure).
[0036] Starting from the known performance of the guanidine nitrate
(GN)/potassium perchlorate (KClO.sub.4) mixture, the inventors
wished to propose improved pyrotechnic gas generator compounds
which are suitable most particularly for use in side airbags. They
more particularly set themselves the objective, while retaining or
improving the other characteristics, of significantly improving the
following three points: [0037] decrease in the cut-off combustion
pressure, [0038] decrease in the pressure exponent (<0.26),
advantageously a large decrease (.ltoreq.0.2), very advantageously
a very large decrease (.ltoreq.0.1) from 6 MPa, [0039] increase in
the combustion rate over the entire pressure range, in particular
at low pressure.
[0040] Completely unexpectedly, it proved to be the case that the
presence, in the composition of the compounds of the invention, of
a low content of (at least) one oxygen-containing compound of a
transition metal (a transition metal oxide or a compound which is a
precursor of such an oxide), advantageously with a high specific
surface area (conventionally used as a ballistic catalyst in the
field of propellants for increasing the combustion rate at high
pressure (by accelerating the decomposition of the oxidizing
charge)), (also) has major effects on the desired three points of
improvement above (namely, an increase in the combustion rate
(also) at low pressure, a decrease in the cut-off combustion
pressure and a decrease in the pressure exponent over the entire
pressure range).
[0041] The compositions of the pyrotechnic gas generator compounds
of the invention (which are most particularly suitable for airbag,
in particular side airbag, applications) contain: [0042] guanidine
nitrate, and [0043] potassium perchlorate.
[0044] They are characterized in that they contain, expressed as
weight percentages, from: [0045] 60 to 70% of guanidine nitrate,
[0046] 26 to 33%, advantageously 26 to 30%, of potassium
perchlorate, [0047] 2.5 to 6% of at least one combustion modifier
chosen from transition metal oxides, the precursors of such oxides
and mixtures thereof, [0048] 0 to 6% of at least one additive, and
do not contain an explosive ingredient.
[0049] According to one variant, compositions of compounds of the
invention consist (exclusively) of the ingredients listed above
(GN+KClO.sub.4+at least one combustion modifier+optionally at least
one additive), taken in the contents indicated above.
[0050] The ingredients of the first three types above (guanidine
nitrate, potassium perchlorate and specific combustion modifier)
generally represent more than 90% by weight of the total weight (of
the composition) of the compounds of the invention, very generally
at least 94% by weight, or even more than 98% by weight. The
optional presence of additive(s), such as manufacturing aids
(calcium stearate, silica, for example), is expressly envisioned.
The ingredients of the three types above can absolutely represent
100% by weight of the total weight of the compounds of the
invention.
[0051] The guanidine nitrate, representing from 60 to 70% of the
total weight, is in addition selected for pyrotechnic safety
reasons and for its rheoplastic behavior, suitable for carrying out
the compacting and optional pelletizing phases of the dry process
(see hereinafter), ensuring a good densification of the starting
pulverulent pyrotechnic composition while limiting the compressive
load to be applied. The manufacture of the compounds via the dry
process comprises up to four main steps (see hereinafter), which
have in particular been described in patent application WO
2006/134311.
[0052] The potassium perchlorate is present, in the composition of
the compounds of the invention, in a moderate intermediate content
(from 26 to 33% by weight, advantageously from 26 to 30% by
weight), very particularly with reference to the combustion
temperature, the "ignitability" and the combustion rate at high
pressure that are targeted.
[0053] Within the GN+KClO.sub.4 mixtures, the combustion modifiers,
selected by the inventors, develop particularly advantageous
(unexpected) properties with reference to the desired three points
of improvement (see above).
[0054] Said at least one combustion modifier is chosen from
transition metal oxides, the precursors of such oxides and mixtures
thereof. A precursor of such an oxide results in the formation of
such an oxide (generates such an oxide) at the time of its
decomposition at temperature during the combustion of the
pyrotechnic compound. Thus, the basic copper nitrate
(Cu(NO.sub.3).sub.2.3Cu(OH).sub.2) decomposes to copper oxide (CuO)
(see hereinafter).
[0055] Said at least one combustion modifier is present in an
amount which is sufficient (.gtoreq.2.5% by weight) to be effective
(with reference to the three points of improvement above), and not
excessive (.ltoreq.6% by weight) so as not to harm the gas yield.
Such one combustion modifier is generally present, but the presence
of at least two such additives is expressly envisioned in the scope
of the present invention.
[0056] Preferably, said at least one combustion modifier is chosen
from zinc oxide (ZnO), iron oxide (Fe.sub.2O.sub.3), chromium oxide
(Cr.sub.2O.sub.3), manganese dioxide (MnO.sub.2), copper oxide
(CuO), basic copper nitrate (Cu(NO.sub.3).sub.2.3Cu(OH).sub.2) and
mixtures thereof. The copper oxide and the basic copper nitrate,
which is a precursor of said copper oxide (in the sense that BCN
results in the formation of copper oxide CuO at the time of its
decomposition at temperature), are particularly effective.
Particularly preferably, the compounds of the invention therefore
contain, as combustion modifier, copper oxide and/or basic copper
nitrate. The use of these combustion modifiers makes it possible to
obtain compounds of the invention which have a pressure exponent
value of less than or equal to 0.1 over the pressure range 6-52
MPa.
[0057] Preferably, said at least one combustion modifier according
to the invention has a specific surface area of greater than 3
m.sup.2/g, advantageously greater than 10 m.sup.2/g and very
advantageously greater than 25 m.sup.2/g.
[0058] It is understood that the function of said at least one
specific combustion modifier (chosen from transition metal oxides,
precursors thereof and mixtures thereof) within the composition of
the compounds of the invention is not only, as in the prior art
(see in particular the teaching of U.S. Pat. No. 6,893,517 recalled
above), to increase the combustion rate at high and medium pressure
but also, surprisingly, to confer on the pyrotechnic compounds:
[0059] a stable and self-sustaining combustion at low pressure (or
even at a pressure virtually equal to atmospheric pressure), [0060]
a combustion rate at low pressure that is higher than that of the
prior art compositions, [0061] a low, or even virtually zero,
pressure exponent at low, medium and high pressure, that is
significantly lower than that of the prior art compositions, this
being with "good ignitability" of said compounds, without
generating too many solid particles at combustion, and a combustion
temperature of around 2300 K.
[0062] It may be indicated here that the compounds of the
invention, the composition of which was specified above, have:
[0063] a combustion temperature of less than 2350 K, [0064] a
(relative, i.e. with reference to atmospheric pressure) cut-off
combustion pressure of less than or equal to 1.5 MPa,
advantageously less than 0.2 MPa and very advantageously equal to
0.1 MPa, [0065] a pressure exponent of less than or equal to 0.25,
advantageously less than or equal to 0.2 and very advantageously
less than or equal to 0.1, for a pressure between 6 and 52 MPa,
[0066] a combustion rate: [0067] greater than 24 mm/s,
advantageously greater than 36 mm/s, at low pressure, [0068]
greater than 30 mm/s, advantageously greater than 35 mm/s, at
medium pressure, [0069] greater than 37 mm/s, advantageously
greater than 45 mm/s, at high pressure.
[0070] The low, or even very low, pressure exponent values of the
compounds of the invention must be emphasized here.
[0071] The best results indicated above (advantageous variants and
very advantageous variants) were in particular obtained with copper
oxide and basic copper nitrate as combustion modifier. In support
of this assertion, reference may be made to the examples
hereinafter.
[0072] In the context of the present invention, an original use
(most particularly with reference to the above parameters) is
therefore proposed for the oxides and oxide precursors in question,
in the composition of the compounds of the invention (said use
being original with respect to the known conventional use of a
ballistic catalyst in different compositions).
[0073] In addition to the above constituents (GN+KClO.sub.4+at
least one combustion modifier of the specified type), the
pyrotechnic compounds of the invention can contain, at a low
content by weight (less than or equal to 6%, generally at least
0.1%), at least one additive, in particular at least one additive
that facilitates the obtaining of said compounds (the forming
during the obtaining thereof), such as calcium stearate or
magnesium stearate, graphite and/or at least one additive for
improving the aggregation of the solid products of their
combustion, chosen from refractory oxides with a softening or
melting point adapted to the composition, such as silica or
alumina. This is advantageously silica, generally introduced in
fine pulverulent form (advantageously of micrometric size, very
advantageously of nanometric size) having a high specific surface
area (advantageously of 100 m.sup.2/g or more), or in the form of
silica fibers of small diameter (1 to 20 microns) and some tens or
hundreds of microns (20 to 500 microns) in length. Surprisingly, it
proved to be the case that the presence, in the pyrotechnic
compounds of the invention, of silica at contents between 0.5 and
6% by weight, advantageously between 0.5 and 3.5% by weight, also
has a very significant effect of drop in the cut-off combustion
pressure.
[0074] It is therefore also to the credit of the inventors to have
demonstrated this effect of silica in compositions of GN+KClO.sub.4
type (see table 3 hereinafter) and therefore in the compositions of
the invention (of the type GN+KClO.sub.4+at least one combustion
modifier), where said effect comes on top of those (encompassing
that of the drop in the cut-off combustion pressure) of the at
least one combustion modifier present.
[0075] The at least one additive intervenes with the constituent
ingredients (GN, KClO.sub.4+at least one combustion modifier of the
above-mentioned type) (at the beginning of the manufacturing
process) or is added, further downstream, in the process for
manufacturing the compounds of the invention.
[0076] It is recalled that the compositions of the compounds of the
invention do not contain an explosive ingredient (see the NF
standard and the UNO recommendations specified above), this being
in particular with reference to the parameters: pyrotechnic safety
and combustion temperature. It is, moreover, noted that the weights
of pyrotechnic compounds required for the inflation of an airbag,
in particular of a side airbag, are greater than those required for
the inflation of a seatbelt tensioner device according to U.S. Pat.
No. 6,893,517 (said inflations not being of the same type:
inflation time greater than 10-20 ms/per pulse).
[0077] The pyrotechnic compounds of the invention can be obtained
according to a wet process. According to one variant, said process
comprises the extrusion of a paste containing the constituents of
the compound. According to another variant, said process includes a
step of placing all the (or some of the) main constituents in
aqueous solution, comprising solubilization of at least one of said
main constituents (oxidizing agent and/or reducing agent), and then
the production of a powder by spray drying, the addition to the
powder produced of the constituent(s) that were not placed in
solution, and then the forming of the powder in the form of objects
via the usual dry processes.
[0078] The pyrotechnic compounds of the invention can also be
obtained by dry process, for example by simple pelletizing of the
powder obtained by mixing of their constituents.
[0079] The preferential process for obtaining the pyrotechnic
compounds of the invention includes a step of dry compacting of a
mixture of the constituent ingredients in powder form of said
compounds (except for said at least one additive which can be added
during the process). The dry compacting is generally carried out,
in a manner known per se, in a roll compacter, at a compacting
pressure of between 10.sup.8 and 6.times.10.sup.8 Pa. It can be
carried out according to different variants (with a characteristic
step of "simple" compacting followed by at least one additional
step, with a characteristic step of compacting coupled to a forming
step). Thus, the pyrotechnic compounds of the invention are capable
of existing in various forms (in particular along the manufacturing
process resulting in the final compounds): [0080] at the end of dry
compacting coupled with forming (by using at least one compacting
roll, the external surface of which has cavities), plates are
obtained with relief patterns, which can be broken to directly
obtain formed pyrotechnic objects; [0081] at the end of dry
compacting followed by granulation, granules are obtained; [0082]
at the end of dry compacting followed by granulation then
pelletizing (dry compression), pellets are obtained; [0083] at the
end of dry compacting followed by granulation and then mixing of
the granules obtained with an extrudable binder and the extrusion
of said binder loaded with said granules, extruded monolithic
blocks (loaded with said granules) are obtained.
[0084] The pyrotechnic compounds of the invention are therefore in
particular capable of existing in the form of objects of the
following type: [0085] granules, [0086] pellets, [0087] monolith
blocks.
[0088] In a manner which is in no way limiting, it may be indicated
here that: [0089] the granules of the invention generally have a
particle size (a median diameter) of between 200 and 1400 .mu.m
(and also an apparent density of between 0.8 and 1.2 cm.sup.3/g);
[0090] the pellets of the invention generally have a thickness of
between 1 and 3 mm.
[0091] When the compounds of the invention are obtained by a dry
process, the constituent ingredients of the compounds of the
invention advantageously have a fine particle size, of less than or
equal to 20 .mu.m. Said particle size (value of the median
diameter) is generally between 3 and 20 .mu.m. The compounds
described in the present invention express all their potential if
they are obtained by a dry process from powders having a median
diameter of between 10 and 20 .mu.m for KClO.sub.4 and 5 to 15
.mu.m for guanidine nitrate.
[0092] According to another of its subjects, the present invention
relates to a pulverulent composition (mixture of powders), which is
a precursor of a compound of the invention, the composition of
which therefore corresponds to that of a compound of the invention
(see above).
[0093] According to another of its subjects, the present invention
relates to the gas generators containing at least one pyrotechnic
compound of the invention. Said generators are perfectly suitable
for airbags, in particular side airbags (see above).
[0094] It is now proposed to illustrate the invention in a manner
that is in no way limiting.
[0095] Table 2 hereinafter gives examples of compositions of
compounds of the present invention, and also the performances of
said compounds compared with those of the reference prior art
compound 1. The compounds were evaluated by means of thermodynamic
calculations or on the basis of physical measurements carried out
on granules or pellets manufactured from the compositions via the
dry process of mixing of powders--compacting--granulation--and
optionally pelletizing.
[0096] The reference prior art compound 1 (see table 1 above)
contains guanidine nitrate and potassium perchlorate and does not
contain any combustion modifier within the meaning of the
invention. The compounds of examples 1 to 7 contain such a
combustion modifier in their composition, in addition to the two
constituents of the reference compound 1.
[0097] The amounts of the major constituents were adjusted in order
to preserve an oxygen balance value close to -3%, so as to be able
to directly compare the performances of the compounds of table
1.
[0098] The results of table 2 show, as expected according to the
teaching of the prior art (teaching of U.S. Pat. No. 6,893,517 and
that of the propellant field), that the addition of a combustion
modifier within the meaning of the invention to a composition of
the type of that of the reference compound 1 results in an increase
in the combustion rate at medium and high pressure without any
significant modification of the combustion temperature.
[0099] Surprisingly, said addition results jointly in a very large
drop in the pressure exponent, which pressure exponent is very low
over the entire operating pressure range (beyond 6 MPa) and in the
cut-off combustion pressure and in a considerable increase in the
combustion rate at low pressure.
[0100] CuO is the compound which, when added to the reference
composition 1, provides the most significant improvements (see
example 2). The pressure exponent is virtually zero over the whole
of the operating range, the cut-off operating pressure is virtually
equal to atmospheric pressure.
[0101] Insofar as a metal complex such as BCN decomposes during
exothermic combustion reactions, generating, in situ, CuO with a
high specific surface area (which was verified experimentally), CuO
can therefore be replaced with BCN as combustion modifier, with
results which are equivalent to those of CuO (see example 7).
[0102] CuO and BCN make it possible, when they are incorporated in
a low amount (5% in the examples), to preserve an advantageous gas
yield value (>32 g/mol) and result, in the end, in a very
significant improvement in the inflation rate per unit area value
(of more than 40%) compared with the GN/KClO.sub.4 reference
composition of the reference compound 1.
TABLE-US-00002 TABLE 2 Examples Ref. 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex.
5 Ex. 6 Ex 7 Ingredients Guanidine nitrate % 68.0 65.0 65.7 65.7
65.0 65.0 67.3 66.7 Potassium perchlorate % 32.0 30.0 29.3 29.3
30.0 30.0 29.7 28.3 Zinc oxide (ZnO) % -- 5.0 -- -- -- -- -- --
Copper oxide (CuO) % -- -- 5.0 -- -- -- -- -- Manganese oxide
(MnO.sub.2) % -- -- -- 5.0 -- -- -- -- Chromium oxide
(Cr.sub.2O.sub.3) % -- -- -- 5.0 -- -- -- Iron oxide
(Fe.sub.2O.sub.3) % -- -- -- -- -- 5.0 -- Basic copper nitrate
(BCN) % -- -- -- -- -- -- 3.0 5.0 Characteristics Combustion rate
at 8 MPa (low pressure) mm/s 20.6 26.7 36.2 24.9 27.0 25.8 34.5
38.1 Combustion rate at 20 MPa (medium pressure) mm/s 26.3 31.9
38.0 31.5 31.1 31.2 36.4 39.3 Combustion rate at 50 MPa (high
pressure) mm/s 34.9 38.0 45.7 39.5 37.8 41.5 44.7 47.8 Combustion
temperature K 2351 2237 2303 2285 2304 2294 2312 2296 Pressure
exponent determined over range -- 0.26 0.16 0.07 0.19 0.17 0.25
0.09 0.10 6-52 MPa Cut-off combustion pressure (relative) MPa 1.7
1.5 0.1 1 0.5 0.25 0.1 0.1 Oxygen balance % -3.0 -2.8 -3.1 -3.1
-2.8 -2.8 -3.1 -3.1 Density g/cm.sup.3 1.67 1.73 1.72 1.72 1.73
1.73 1.69 1.70 Gas yield at 1 bar - 1000 K mol/kg 33.2 31.6 32.1
32.1 31.6 31.6 33.1 33.0 Inflation rate per unit area (.rho.. n, T,
V.sub.c) mol K/cm.sup.2 s 344 391 485 397 391 390 470 505 at 20
MPa
[0103] Table 3 hereinafter shows the second surprising effect
demonstrated by the inventors, namely the very significant decrease
in the cut-off combustion pressure (measured on granules) when
silica is introduced at a moderate content into the composition of
the compounds of the invention. This same effect, obtained with
another refractory metal oxide such as alumina, is not of a
sufficient size to be of interest.
TABLE-US-00003 TABLE 3 Examples unit Ref. 1 Ex. 8 Ex. 9 Ingredients
Guanidine nitrate % 68.0 65.0 65.0 Potassium perchlorate % 32.0
30.0 30.0 Alumina % -- 5.0 -- Silica % -- -- 5.0 Characteristic
Cut-off combustion pressure (relative) MPa 1.7 1.5 0.5
* * * * *