The airbag manufacturing. Materials and technology.

Stefanuta, Ioan ; Matran, Cristian

Abstract: The airbag represents one of the most important safety devices of a car - both for the driver and the passengers --(driver airbag, passenger airbag, knee airbag and curtain airbag). Also, because the required safety standards for the functioning of the airbag are very constricted, this paper points out problems regarding the optimal choice of raw materials, of technology and the fabric parameters used for airbags manufacturing, starting from working requirements.

Key words: airbag, technology, weaving, yarn, automotive

1. INTRODUCTION

The airbag, also known as a Supplementary Restraint System (SRS) or the Supplemental Inflatable Restraint (SIR), offers a supplemental protection to the car passengers together with the 'classical' seatbelt. There were devices similar to airbags for airplanes as early as the 1940's, with the first patents filed in 1958.

The airbag was invented by John W. Hetrick in 1952; Ford built the experimental fleet cars with airbags in 1971, followed by General Motors' experimental vehicles in 1973.

Regarding the high standards required by the car passenger protection--"We protect life"--it is necessary to have high quality parameters--"Quality First"--for designing and execution. Usually, the producers from the automotive industry impose their own high quality standards that cause the rejection of the defective products (there is no remedy fabrication phase).

This paper will cover only problems regarding the cushions fabrication (especially the raw materials and the technology).

2. RAW MATERIALS FOR AIRBAGS

The raw material is the fabric obtained from chemical yarns (e.g. polyamide a.k.a. nylon yarns). The 6.6 polyamide, also named nylon, presents different useful characteristics:

--the specific height will vary between 1.14-1.15 g/[cm.sup.3];

--the best frictional resistance compared with other fibres;

--a very high resistance to the traction;

--high waterproof characteristics in the standard atmosphere (at 20[degrees]C and 65% humidity).

2.1. Why the fabric?

Researches in this field have highlighted the characteristics of an airbag for efficient functioning and full safety. These characteristics are:

* high resistance to temperature variation (from -30[degrees]C to +80[degrees]C);

* high durability (more than 15 years);

* good dimensional stability;

* high flexibility;

* good pressure resistance (14.71[+ or -]0.98kPa);

* low air permeability;

* high fireproof;

* low electrical conductivity (ISO/TS16949:2002).

The small permeability is necessary to prevent the leaking of the hot gases, to avoid causing serious injuries during the airbag inflation and also to allow the folds created during the folding and module fixation to open promptly.

The humidity has a small influence on the mechanical properties due to the yarns' high crystalline structure and their waterproof properties.

The torsions' rigidity of the 6.6 polyamide is about 40% in saturated atmosphere compared to 10% for the wool yarns. The effect of the humidity on the breakage elongation is greater than its effect on the breakage loading. The effect of the temperature on the traction resistance is given by the higher temperature.

The electrical conductivity is lower, good voluminous ness and thermo isolated properties. It also presents good bacterial and fungi proof qualities. (Preda C. & Preda Cristian 1995).

It is necessary for the properties of the bag fabric raw material to respect all these requirements.

2.2 Structural characteristics

The structural characteristic of the fabric will be presented in Table 1.

2.3 Physical and mechanical characteristics

The physical and mechanical characteristic of the fabric will be presented in Table 2.

The testing of fabric designed for airbags will respect the automotive standards specifications like those of ASTM (American Society for Testing and Materials) and AKLV (procedures and requirements for air bag and inflator parts).

All airbag specifications are requested by the client or are assessed by current standards. All specifications for each fabric are mentioned in the execution plan.

Some laboratory tests (e.g. breakage elongation, breakage and tear resistance, air permeability) are most important for good air bag performances. The same tests present the influence of material deficiencies on the air bag performance.

3. TECHNOLOGICAL PROCESS OF WEAVING

The weaving process is relatively simple due to the modern equipments. The structure of the technological process is: beaming, weaving, coating.

3.1. Beaming

To have a warp with a high numbers of yarns, it is necessary to use two phases of beaming: sectional beaming and final beaming. The sectional beaming uses the same number of yarns on all sectional beams from a set. The number of yarns on the beam will vary according to the fabric destination (484 yarns/1 sectional beam for a 46x46 fabric, 515 yarns/1 sectional beam for a 49x49 fabric, 491 yarns/1 sectional beam for a 70x70 fabric).

Finally, all yarns from one set of sectional beams will be wrapping up on the final beam. Depending on the fabric destination, the total number of yarns on a final beam is presented in Table 3 (Liute, D. & Liute, Daniela 2002).

3.2. Weaving

The weaving process use water jet looms with 875 rpm working speed. The used structure is linen (plain linen, 4 shafts) (Stefanuta 1997, Matran 2005). Because the obtained fabric is wet, it will be packed to avoid the appearance of mould and the fabric contamination. That fabric will be stored until the coating moment, but no more than 7 days.

3.3. Coating

The wet fabric will be fed into the coating machine. The fabric will go through the machine furnace for drying and for dimensional stabilization.

The dried fabric will be coated with a thin layer of silicone (max 40 g/[m.sup.2]) to reduce the air permeability. The fabric will go again through the other machine furnace for drying and for a full joining of the silicone to the fabric. The drying temperature will be set to avoid the fabric overburn. Finally, the fabric will be rolled on the A-frame device (used for layers).

The coating parameters are very important. The silicone is a three component mixture. The first component of the mixture is an elastomer (a viscous liquid, with a white to light yellow colour, and no smell).

The second component is a rubber silicone (viscous liquid clear white, no smell). The last component is a pigment (red colour, water insoluble, no smell). The temperature will be set at 190-200 [degrees]C to dry the fabric/silicone without debasing it.

4. CONCLUSION

The passenger safety norms require a rigorous approach to airbag manufacturing, seen as an indispensable product of the automobile industry.

The airbag design and the choice of optimal fabric--like a special destination device--require high quality control of technological processes. The airbag characteristics and safety standards require a fabric based on filamentary chemical yarns. The weaving parameters needs to guarantee optimal

values of some fabric characteristics like: air permeability, breakage and tear resistance, high flexibility.

Even the fabric's defects doesn't have a negative influence on the tests' result (the fabric parameters respect fabric specifications), all these defects can have a negative influence in working process of the airbag.

The airbag manufacturing processes are simple, short and use few finishing processes.

The coating ensures three important characteristics of the airbag:

--a good coefficient of air permeability;

--high temperature resistance at the inflation moment.

--a reduced flammability coefficient.

5. FURTHER RESEARCH

The authors will enlarge upon the area of research including the influence of fabric fault on airbag properties, the influence of other raw materials and other manufacturing parameters on airbag characteristics.

Also, we will study the influence of the thickness of the silicone and other parameters of coating process on the airbag working parameters.

On the other hand, the authors will study the kinematics and the dynamics of the different mechanisms of the beaming, weaving and coating machines.

6. REFERENCES

Ionescu-Muscel, I. (1990). The Textile Fibres at the End of the Century, Technical Publishing House, Bucharest, ISBN 973-31-0198-2

Liute, D.; Liute, Daniela (2002). The basics of the textile yarn preparation, Iasi: "Gh. Asachi" Publishing House, ISBN 973-8292-22-0

Matran, Cristian (2005). Researches concerning the improvement of kinematic and dynamic weaving conditions of the textile yarns, Ph. D. Thesis, "Gh. Asachi" Technical University of Iasi, Romania

Preda, C.; Preda, Cristian (1995). Methods and tests for the fabrics quality control, Iasi: BIT Publishing House, ISBN 973-97907-2-0

Stefanuta, Ioan (1997). Weaving technology, Sibiu: "Lucian Blaga" University Publishing House

ISBN 973-9280-73-0

The ISO/TS16949:2002 Standard retrieved May 29, 2007 from DQS Romania

http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/Esv/esv16/98S8P12, accessed on 2007-05-29

http://en.wikipedia.org/wiki/Airbag, Accessed: 2007-05-29

http://en.wikipedia.org/wiki/Polyamide, Accessed: 2007-05-29

http://en.wikipedia.org/wiki/Nylon, Accessed: 2007-05-29

Table 1. The fabric characteristics.

Destination Yarn Yarn picks Silicone
 gauge yarns/inch] Design thickness

 [dtex] -- [g/[m.sup.2]]

Drivers' air bag 470 46x46 cloth 40 [+ or -] 5
Passengers' air bag 49x49 30 [+ or -] 5
Knees air bag
Head side air bag 235 70x70

Table 2. The physical and mechanical properties of the fabric.

 Breakage
Fabric Yarn picks resistance

 [yarns/10cm] [N/cm]

46x46 181 [+ or -] 6 [greater than or equal to]400
49x49 193 [+ or -] 8 [greater than or equal to]400
70x70 276 [+ or -] 10 [greater than or equal to]330

 Breakage Tear
Fabric elongation resistance

 [%] [N/cm]

46x46 [greater than or equal to]18 [greater than or equal to]150
49x49 [greater than or equal to]18 [greater than or equal to]150
70x70 [greater than or equal to]18 [greater than or equal to]90

 Air
Fabric permeability

 [kPa]

46x46 [greater than or equal to]190
49x49 [greater than or equal to]190
70x70 [greater than or equal to]190

Table 3. The number of warp yarns on the final beam.

 Number of Number of yarns/ 1 Number of yarns/1
Standard sectional beams sectional beam final beam

46x46 8 484 3872
49x49 8 515 4120
70x70 12 491 5892