Various techniques are known to produce rapid infla- tion of an air bag utilised in motor vehicles. It is essential that gas fills the air bag extremely quickly to allow the bag sufficient time to inflate into a satis- factory impact protection means in an imminent crash situation. For example, an air bag stored in deflated condition in the hub of the vehicle steering-wheel must be filled in 30-40 msec in the case of a head-on colli- sion. Air bags stored in deflated condition laterally of a vehicle seat or in a vehicle door must be filled at an even higher rate.

In accordance with one prior-art solution, a gas generator is used of a kind comprising a propellant which upon ignition rapidly develops a hot, air-bag inflating gas. The substance generally used is sodium azide (NaN3), which forms nitrogen gas (N2).

The advantage of this substance is that it develops large quantities of gas in a brief space of time. A disadvantage is, however, that sodium azide is a toxic substance.

For this reason various substances have been tested in order to find another suitable gas generator which while likewise developing large quantities of gas upon ignition is non-toxic. The tested propellants have, however, proved to develop gases that are even hotter than the above, well established sodium azide. Hotter gases do, however, risk to burst the air bag because they may cause fusion damages to the air bag fabric.

Tests have also been carried out with a so-called hybrid generator, which is a gas cartridge containing pressurised gas (argon), a gas which is not hot when gushing out of the air bag. However, other disadvantages are connected with this solution. The gas cartridge is a pressurised vessel and as such submitted to specific restrictions, and occupies comparatively large space, for example in the module of a vehicle steering-wheel.

The invention provides a fabric by means of which all problems referred to above are solved. The charac- teristic feature of the invention is that the fabric face forming the inner face of the air bag has a coating applied thereon. The coating consists of one or several layers of polymer with admixture thereto of a pigment having heat-reflecting properties. A suitable coating for this purpose has been found to be a silicone elastomer with admixture thereto of an aluminium paste or aluminium pigments.

Measurements have been performed on non-coated fab- rics, fabrics coated with paste having coloured pigments admixed thereinto, and fabrics coated with aluminium pigments. These measurements have shown that the non- coated fabric and a coated fabric of the first-mentioned kind possess the same heat-reflection index whereas in the fabric of the last-mentioned kind the heat losses through thermal reflection from the aluminium-coated surface is reduced by 10%. In conformity with the general law on gases, this results in a higher pressure inside the air bag.

The indicated value of reduced heat losses has been established by means of a method for determining the heat insulating properties of fabrics in accordance with ISO 5085-1:1989, known as the single plate method. Measure- ments were carried out on a fabric sample mounted on a spacer ring, with the aluminium-coated surface facing downwards, towards a heated measurement plate and spaced 15 mm from the plate. The environmental climate condi-

tions were as follows: temperature 20°, relative humidity 65%, air velocity 0.5 m/s.

The invention provides several important advantages.

The fabric may be thin, yet strong at the same time, and therefore requires only a small space inside the module.

A fabric with said coating applied thereon is able to resist also high-temperature gases. Consequently, a propellant may be used that develops a higher-temperature gas than does sodium azide but that is non-toxic, and this without involving risks for fusion damages to the fabric.

As already mentioned, the thermal reflection in- creases the pressure inside the bag. As a result, the power of the gas generator may be reduced and a smaller amount of propellant need be used. In consequence thereof, the air bag is exposed to less stress while at the same time cost savings are made.

Obviously some other coating substance than the one referred to above is conceivable, provided it possesses the characteristic of having heat-reflecting properties.