Investigation of a Two-Stage Airbag Module with Azide-Free GasGenerators

Helmut Schmid* and Norbert Eisenreich

Fraunhofer-Instiut fuÈr Chemische Technologie (ICT), D-76327 P®nztal (Germany)

Untersuchung eines zweistu®gen Airbagmoduls mit einem azid-freien Gasgenerator

Airbags haben sich inzwischen zur Standardausstattung von Autosentwickelt und steigern die Sicherheit von Fahrern und Beifahrern beiUnfaÈllen betraÈchtlich. Neuere Entwicklungen umfassen azidfreieFormulierungen von Gasgeneratoren und sogenannte ``smart'' Airbag-Verbrennungsmodule, die eine variable Gaserzeugung ermoÈglichen.Guanidiumazotetrazolat (GZT) wird als Brennstoff undCu(NO3)2�2Cu(OH)2 als Oxidator eingesetzt; die resultierendenFormulierungen produzieren Gasmengen von etwa 500 l=kg. DieVerwendung von V6Mo15O60 als Katalysator reduziert die Menge derSchadgaskomponenten unter akzeptierte Grenzwerte. VerschiedeneLadungen wurden hergestellt, in einer ballistischen Bombe angezuÈndetund die Abbrandraten gemessen. Die Untersuchung eines zweistu®genVerbrennungsmoduls, wobei jede Stufe mit einer Squib- und Booster-AnzuÈndeinheit ausgestattet war, zeigte die breite VariabilitaÈt desDruck-Zeit-Verhaltens bei zeitverzoÈgerter AnzuÈndung auf. Mit einervereinfachten Simulation konnte die MoÈglichkeit demonstriert werden,die TemperaturabhaÈngigkeit der Verbrennung durch verzoÈgerteAnzuÈndung der 2. Stufe zu kompensieren. Ein Airbag-Prototyp konntedie Anforderungen in Standversuchen und Schlittentests erfuÈllen.

EÂ tude d'un module de coussin gon¯able aÁ deux eÂtages avec geÂn-eÂrateur de gaz sans azoture

Les coussins gon¯ables (airbag) sont devenus deÂsormais un eÂqui-pement de seÂrie des automobiles et augmentent consideÂrablement laseÂcurite du conducteur et des passagers lors d'accidents. Des deÂve-loppements reÂcents portent sur des formulations de geÂneÂrateurs de gazsans azoture et sur les modules de combustion d'airbag dit`̀ intelligents'', qui permettent une production de gaz variable. LeguanidiumazoteÂtrazolate (GZT) est utilise comme combustible etCu(NO3)2�2Cu(OH)2 comme oxydant. Les formulations qui en reÂsul-tent produisent des quantiteÂs de gaz de l'ordre de 500 l/kg. L'utilisa-tion de V6Mo15O60 en tant que catalyseur reÂduit la quantite de gaznocifs sous la valeur limite accepteÂe. DiffeÂrentes charges ont eÂteÂsyntheÂtiseÂes, allumeÂes dans une bombe balistique et les vitesses decombustion ont eÂte mesureÂes. L'eÂtude d'un module de combustion aÁdeux eÂtages, dans lequel chaque eÂtage est muni d'un in¯ammateur etd'un propulseur auxiliaire, montre le grand nombre de courbespression-temps que l'on peut obtenir avec un allumage retarde dans letemps. Une simulation simpli®eÂe a permis de montrer qu'il eÂtaitpossible de compenser la sensibilite aÁ la tempeÂrature de la combustionpar un allumage retarde du 2eÁme eÂtage. Un prototype de coussingon¯able a reÂpondu aux exigences lors d'essais statiques et sur chariotcoulissant.

Summary

Airbags are used as a standard equipment of cars to stronglyincrease the safety of drivers and passengers on accidents. Recentdevelopments include azide-free formulations of the gas generator and`smart' airbag combustion modules which enable a variable gas out-put. Guanidium azotetrazolate (GZT) is investigated as a fuel andCu(NO3)2�2Cu(OH)2 as an oxidizer and the resulting azide-free for-mulation produces a gas output of about 500 l=kg. The use ofV6Mo15O60, as a catalyst reduces the amount of harmful gases belowaccepted limits. Various grains were produced, ignited in closed ves-sels, and the burning rates measured. The investigations of a two-stagemodule with igniter units consisting of two squibs and boostersshowed a broad variety of pressure-times curves achievable by timedelayed initiation. A simulation by a simpli®ed model could evendemonstrate the possibility to compensate the temperature dependencyof the combustion by a delayed ignition. In static deployment and sledtests the prototype airbag module ful®lled the requirements.

1. Introduction

The widespread use of airbags as a standard equipment of

cars has strongly increased the safety of passengers and

drivers on accidents. Nearly every new car is actually

equipped with various versions to ®nally obtain full protec-

tion. Recent developments included:

� Reduction of environmental and toxic impacts using

azide-free formulations

� ``Smart'' systems enabling more intelligent and ¯exible

reactions on accidents.

The accident scenarios are de®ned by the type and

heaviness of impacts considering also driver and passenger

situations. With respect to these speci®c conditions the old

airbag systems have only one answer: go or no go!

As a result, in some cases in the past, airbags caused

violations instead of protection(1,2). In addition, the initiation

of the full airbag could induce unreasonable, often unneces-

sary effects on the occupants. Detailed theoretical studies on

required bag unfolding and the dependence on the accidents

were published(3).

Recently, the state of the art of car occupant safety systems

showed that more complex systems taking into account

details of the situation of the drivers and passengers. In

addition details of the accident have been realized. More

complex sensor systems are based on multi-directional

deceleration sensors often at various positions. The signals

of these are analysed to decide at a more sophisticated level

the event to initiate or not(4,5). In the case of new gas

generator concepts with a more ¯exible characteristic,

these sensors have to be adapted to acquire the information

for controlling the action(6).* Corresponding author; e-mail: sd@ict.fhg.de

# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0721-3115/00/0511 ± 0230 $17.50�:50=0

230 Propellants, Explosives, Pyrotechnics 25, 230±235 (2000)

In addition, the principles of the gas production were

re¯ected:

Of physical nature are pressurized gases which have the

disadvantage that the regulations for pressurized containers

have to be accounted for. The gas output has its maximum at

the initiation and decreases exponentially which is contra-

dictory to the requirements. It is accompanied by freezing(8).

An alternative concept uses hybrid systems combining

pyrotechnic igniters and pressurized gases which exhibit

also some disadvantages of pressurized gases. Other con-

cepts use ammonium perchlorate as a heating charge with the

disadvantage of corrosive and toxic gas production. Azide-

free gas generator formulations can be based on N-rich,

C-poor organic compounds: e.g. nitroguanidine (NIGU),

triaminoguanidine nitrate, guanidinium azotetrazolate

(GZT), 5-aminotetrazole etc.(7,9,10).

The N-rich, C-poor organic components GZT and NIGU

had been selected in this work and processed to formulate gas

generators containing in addition oxidizers and catalysts. The

burning behaviour and gas production of the pyrotechnic

materials were investigated. Pellets of the gas generators

were prepared for a two-stage airbag prototype module to

study the variable gas in¯ation.

2. Characterization of the Formulations

The experiments started with the N-rich, C-poor organic

fuels and special catalysts to reduce harmful trace gases on

combustion(6,11 ± 12). Based on thermodynamical calculations

components were selected and synthesized for a formula-

tion consisting of the compounds (see Table 1): GZT,

Cu(NO3)2�3Cu(OH)2 and V6Mo15O60, as catalyst. A

second formulation used NIGU as an oxidizer.

The formulations are chemically stable against heating and

shock loading and do not contain any toxic compounds at

levels exceeding regulations neither in the material nor in the

product gases. They can be easily ignited by standard squibs

and boosters (B=KNO3) and modi®ed to obtain various

burning rates. The gas output is of the order of magnitude

of 500 l=kg and ful®lls the other requirements of airbag gas

generators.

Several GZT- and NIGU propellants were produced to

enable testing and adaptation. More than 5 new formula-

tions ± each for the fast and slow burning mixtures ± were

selected for the laboratory experiments. The formulations

were processed to various shapes including a granulated

powder and pellets of different sizes. Some of these grains

are shown in Figure 1.

All these formulations were tested in the ballistic bomb in

order to characterize the interior ballistic properties. The

ignitability of the formulations was studied. GZT-formula-

tions show a better behaviour than NIGU formulations

because they need a less amount of the booster and have

shorter ignition delay times. In Figure 2 the pressure-time

curves of the combustion in the ballistic bomb are plotted for

powders and pellets. As expected, the conversion time

(maximum pressure) is considerably shorter for the

powders compared to the pellets. Vivacity and linear

burning rates of the gas generator formulations were

derived from the pressure-time curves measured in the

ballistic bomb (volume ± 100 ml). In Figure 3, the depen-

dency of the burning rate of a selected formulation (see

Table 1) on the pressure is drawn. The burning rate r

increases strongly with pressure but does not fully meet

Vieille's law r � a pn (n ± pressure exponent), which is an

Table 1. Gas Generator Compositions

Composition Energy of Formation

Percent Name [kJ=mol]

44.987 Cu(NO3)2* 3Cu(OH)2 ÿ1628.58

4.999 Strontium nitrate ÿ968.9650.014 Nitroguanidine ÿ80.55

Figure 1. Gas generator samples of the granulate and the propellantpellets of various shapes.

Figure 2. Comparison of experimental pressure-time diagrams of thesame GZT=oxidizer mixture measured in a ballistic bomb(V � 100 cm3) used as a powder (*, �) and as cylinders (6). Theseresults illustrate the in¯uence of the shape on the pressure-time curves.

Propellants, Explosives, Pyrotechnics 25, 230±235 (2000) Two-Stage Airbag Module 231

empirical approach to describe this dependency for many

types of solid propellants and gas generators.

3. Investigations on Test Modules

Various versions of two-stage housings of gas generators

were designed and tested. One version was a real double

chamber to be ignited separately and a second one with only a

single burning chamber with the possibility to introduce a

separation and to ignite the combustion by two asymmetri-

cally located ignition units. The results showed that the

second version was suf®cient to realize nearly all desired

pressure-time curves. Therefore, a re®llable single housing

with double asymmetric ignition was prepared and tested in

detail under various conditions for:

� The adaptation of the combustion chamber including

nozzles, ®lter and cooling systems in interaction with

chemistry, pellet size and shaping of the gas generator

mixtures

� Investigation of the pressure-time curves of a two-stage

gas generator module by variation of the time delayed

asymmetric ignition.

Two different types of the propellant with different sizes of

the pellets were prepared by an automated press and tested in

combination with the possibilities of the time delayed

ignition. To design the deployment characteristics the test

module was ®red into a 60 l can which is a standard test and

includes the pressure build-up and the analysis of the product

gases in the airbag development (see Figure 4). The gas

generator test module which contains cylindrical pellets of

two different diameters (5 and 10 mm) and the two igniter

units are depicted in Figure 5. There are described only

results where small pellets and big pellets ®ll a volume of the

same size. The ignition was performed by the two units which

comprise the squibs and the boosters containing 0.5 ± 1 g of a

mixture of B=KNO3. The main experimental variation con-

cerned the ignition sequence of the small and big pellets of

the gas generator grains and the time delay. In some cases,

only one igniting unit was initiated, the second volume region

is also completely ignited by the combustion products of the

®rst volume region after a long delay compared to the other

experiments.

The results show interesting aspects with respect to the

variability of the pressure-time curves or mass ¯ow rates.

Figure 3. The linear burning rate of a GZT formulation as derivedfrom pressure-time curves measured in the ballistic bomb.

Figure 4. The 60 l can as an equipment to perform one type ofstandard tests (pressure build-up as well as the gas product composi-tion) of the gas generator system.

Figure 5. Combustion chamber of the two-chamber generator hous-ing with a modular charge of two pellet sizes (diameters 5 and 10 mm).Two igniting units are integrated in order to enable a time delayedinitiation of the combustion.

232 Helmut Schmid and Norbert Eisenreich Propellants, Explosives, Pyrotechnics 25, 230 ± 235 (2000)

However, the possibilities to vary systematically the condi-

tions are very high:

� There are two areas in the chambers (separated or not)

®lled with two differently sized gas generator pellets and

two igniters

� Ignition of the big pellet side

� Ignition on the small pellet side

� Simultaneous ignition

� Time delayed ignition.

In addition, there is a strong in¯uence of the booster

masses of the igniting units. As expected higher amounts of

booster masses lead to a reduction of the ignition delay times

and the complete initiation of the chemical reaction on the

full surface of all gas generator pellets. The latter effect

causes a steeper pressure increase at the beginning of the

combustion reaction. The nozzles connecting the burning

chamber and the bag to be in¯ated or the ®lter chamber affect

also substantially the pressure-time curves. This is caused by

the stronger con®nement of the combustion chamber. The

gas generator pellets burn faster (Vieille's law) at higher

pressures and therefore, produce higher gas volumes per time

compared to the burning at lower pressures.

The experiments were performed with the formulations

where GZT was the fuel. In addition, similar studies con-

cerned the NIGU formulations. The results of the pressure-

time curves when ®ring into the 60 l can are similar to the

®ring into closed vessels and show the broad range of

pressure build-up curves which are possible by varying the

time delay and the side of ignition (small pellets or big

pellets). However, the big volume of the can is immediately

available to the produced gases passing the chamber nozzles,

and therefore, the experiments do not give results which are

very reliable and reproducible. In addition, they do not fully

meet the real situation in an airbag deployment. It is clear that

the ®ring into airbags is essentially different from the can

experiments because the combustion product gases have to

open ®rst the airbag cover and then to in¯ate the folded bag

which induces a resistance to the gas expansion. The different

and variable pressure conditions make it dif®cult to deduce

the pressure in the airbag module especially in the bag from

the pressure-time curves obtained in the can.

In Figures 6 and 7 pressure-time curves of ®ring the two-

stage gas generator with GZT formulations into an airbag

used in SEAT cars are plotted. When igniting the two-stage

gas generator module initially at the big pellet side then

typically broader ®rst pressure peaks are observed com-

pared to the ignition at the small pellet side. The delayed

ignition of the second squib results in second peaks with a

lower height at the expected time. Of practical interest are

mainly short delay times. In static deployment tests and

sled tests the prototype airbag module ful®lled the require-

ments.

Similar experiments were performed with NIGU as a fuel.

The NIGU formulations burn slower than the GZT formula-

tions. They have to be pressed to smaller pellets (diame-

ter< 5 mm) to obtain full conversion of the gas generator in

the speci®ed time of some 10 ms.

Figure 6. Experimental pressure-time diagrams of a GZT-formula-tion, ®ring into a 60 l airbag, double chamber generator with small andbig pellets (5 mm and 10 mm diameter), 2 squibs simultaneously,respectively delayed ignited.* Simultaneous ignition®rst ignition on the big pellet side:� time delay squib 2: 5 msX time delay squib 2: 10 msY time delay squib 2: 15 msZ time delay squib 2: 50 msO no ignition squib 2: max. peak 2: 15 ms

Figure 7. Experimental pressure-time diagrams of a GZT-formula-tion, ®ring into a 60 l airbag, double chamber generator with small andbig pellets (5 mm and 10 mm diameter), 2 squibs simultaneously,respectively delayed ignited.* Simultaneous ignition®rst ignition on the small pellet side:� time delay squib 2: 5 msX time delay squib 2: 10 msY time delay squib 2: 15 msZ time delay squib 2: 50 msO no ignition squib 2: max. peak 2: 15 ms

Propellants, Explosives, Pyrotechnics 25, 230±235 (2000) Two-Stage Airbag Module 233

The pressure-time curves in a closed vessel can be

simulated by a simpli®ed interior ballistics calculation

compared to Ref. 13. It was assumed a separated combustion

of two volumes ®lled with pellets of identical shape and size

which are ignited independently and time delayed and burn

independently with the same burning rate r (see Eq. (1), Ts ±

temperature of the burning surface). Using a burning rate by

modifying Vieille's law for taking into account the depen-

dency of the burning rate on initial temperature T0(14 ± 16) and

the heat loss (see Eq. (2), a ± heat transfer coef®cient) to the

combustion chamber(13) the temperature effect can be par-

tially compensated which would solve an actual problem in

air bag applications.

r T0; p� � � Apn

Ts ÿ T0

�1�

p t� � / pmaxeÿat �2�

This means that the pressure-time curves can be similar at

different initial temperatures of the gas generator. Figure 8

demonstrates this effect. The simulated pressure-time curve

on simultaneous ignition at a higher initial temperature is

strongly shifted to shorter times, increases more rapidly and

gives a higher maximum pressure. An ignition delay of the

second stage at a higher initial temperature produces a

pressure-time curve close to the curve obtained by a simulta-

neous ignition of the two stages at a lower initial temperature.

At low pressures (> 10 MPa) such a more or less indepen-

dent combustion can be realized in one combustion chamber

as shown by the experimental results at least when using a

separating wall.

4. Conclusions

Several GZT and NIGU propellants were investigated to

enable testing and adaptation whereas the GZT formulations

show best results concerning airbag applications. The for-

mulations are chemically stable against heating and shock

loading, and do not contain any toxic compounds at levels

exceeding regulations neither in the solid material nor in the

product gases. They can be ignited easily and modi®ed to

obtain various burning rates. The gas output is of the order of

magnitude of 500 l=kg and ful®ls the other requirements of

airbag in¯ators. The concept of the described dual stage gas

generator system enables a wide range of possibilities for

pressure build-up and in¯ation characteristics of airbags. For

low pressure combustion modules a version with asymme-

trical ignition meets the requirements of aggressive or soft

in¯ation. The possibility to compensate the temperature

effect is pointed out. Therefore, a broad variability to meet

requirements of airbag deployment correlated to the crash

situation is obtained.

5. References

(1) R. Mattern, `̀ Accident Injuries ± Present Status and Future Prio-rities'', Airbag 2000, 1st Int. Symp. on Sophisticated Car Occu-pant Safety Systems, Karlsruhe 1992.

(2) K. Langwieder, `̀ Passive Safety and Occupant Injuries ± PresentStatus and Future Priorities'', Airbag 2000, 1st Int. Symp. onSophisticated Car Occupant Safety Systems, Karlsruhe 1992.

(3) R. SchoÈneburg, `̀ Numerical Simulation in Airbag DevelopmentState of the Art, Developments, Trends'', Airbag 2000, 1st Int.Symp. on Sophisticated Car Occupant Safety Systems, Karlsruhe1992.

(4) H. Spies, `̀ Sensor Technology and Computation Algorithmen forthe Future'', Airbag 2000, 1st Int. Symp. on Sophisticated CarOccupant Safety Systems, Karlsruhe 1992.

(5) H. Spies, `̀ The Evolution of a Single Point Airbag Sensor into anIntelligent Safety Management System'', Airbag 2000, 2nd Int.Symp. on Sophisticated Car Occupant Safety Systems, Karlsruhe1994.

(6) X. Agostin, A. Anthofer, N. Eisenreich, E. Eibensteiner, H.Fabing, R. Gantner, M. Hurtado ArtozoÂn, H. Schmid, and R.Trouselle, `̀ Intelligent Airbag Model System Based on Envir-onmental Friendly Gasgenerators'', Airbag 2000�, 4th Int.Symposium and Exhibition on Sophisticated Car Occupant SafetySystems, Karlsruhe (Germany), 1998, Nov. 30 ± Dec.2; pp.45=1 ± 45=6.

(7) K.-M. Bucerius and H. Schmid, `̀ New Stable Nitrogen-RichCompounds as Sodium Azide Alternative'', Airbag 2000, 1st Int.Symp. on Sophisticated Car Occupant Safety Systems, Karlsruhe1992.

(8) J. C. Stansel and J. L. Blumenthal, `̀ Heated Gas In¯ator Tech-nology'', Airbag 2000, 2nd Int. Symp. on Sophisticated CarOccupant Safety Systems, Karlsruhe 1994.

(9) K.-M. Bucerius and H. Schmid, `̀ Nonpoisonous Crash Bag GasGenerants ± From Technical Issue to Market Introduction'', Air-bag 2000, 2nd Int. Symp. on Sophisticated Car Occupant SafetySystems, Karlsruhe 1994.

(10) M. Hermann, W. Engel, N. Eisenreich, V. Kolarik, and K.-D.Thiel, `̀ Stabilized Ammonium Nitrate for Applications in GasGenerators'', Airbag 2000, 2nd Int. Symp. on Sophisticated CarOccupant Safety Systems, Karlsruhe 1994.

(11) J. Neutz, V. Weiser, A. Baier, and H. Schmid, `̀ Charakterisie-rung der Flammenstruktur eines Gasgenerators'', Airbag 2000�,4th Int. Symposium and Exhibition on Sophisticated Car Occu-

Figure 8. Simulated pressure-time curves showing the temperaturedependence of the pressure build-up of a solid propellant, pressureexponent n � 2y3, assuming a temperature dependent burning rate anda heat transfer to the airbag housing.- - - - simultaneous ignition of the two volumes of the gas generator at350 K initial temperature. . . . simultaneous ignition of the two volumes of the gas generator at250 K initial temperatureÐÐ delayed ignition of the two volumes of the gas generator at350 K

234 Helmut Schmid and Norbert Eisenreich Propellants, Explosives, Pyrotechnics 25, 230 ± 235 (2000)

pant Safety Systems, Karlsruhe (Germany), 1998, Nov. 30 ±Dec.2; pp. 53=1 ± 53=6.

(12) H. Schmid, N. Eisenreich, A. Baier, J. Neutz, D. SchroÈter, and V.Weiser, `̀ Development of Gasgenerators for Fire Extinguishers'',Propellants, Explosives, Pyrotechnics 24, 144 ± 148 (1999).

(13) M. Hund, N. Eisenreich, and F. Volk, `̀ Determination of InteriorBallistic Parameters of Solid Propellants by Different Methods'',Proc. 6th Int. Symp. on Ballistics, Orlando 1981, pp. 77 ± 84.

(14) N. Eisenreich, `̀ Vergleich theoretischer und experimentellerUntersuchungen uÈber die AnfangstemperaturabhaÈngigkeit vonFesttreibstoffen'', ICT-Bericht 8=77, (1977), Fraunhofer-InstitutfuÈr Chemische Technologie ICT, P®nztal, Germany.

(15) W. Eckl, S. Kelzenberg, V. Weiser, and N. Eisenreich, `̀ EinfacheModelle der AnzuÈndung von Festtreibstoffen'', 29th Int. Annual

Conference of ICT, Karlsruhe, June 30 ± July 3, 1998, Germany,pp. 154.

(16) N. Eisenreich, T. Fischer, and G. Langer, `̀ Burning Rate Modelsof Gun Propellants'', European Forum on Ballistics of Pro-jectiles, Saint-Louis, France, April 11 ± 14, 2000, French-GermanResearch Institute of Saint-Louis (ISL), pp. 117 ± 127.

AcknowledgementThe project was funded by the European Community under the

Industrial and Materials Technologies Programme (BRITE-EURAMIII), Contract No: BRPR-CT96-0165.

(Received August 21, 2000; Ms 2000/032)

Propellants, Explosives, Pyrotechnics 25, 230±235 (2000) Two-Stage Airbag Module 235