For many uses of industrial fabrics, it is crucial for them to possess a controlled air permeability. Examples are fabrics on the underside of parachutes, fabrics for gas filtration, fabrics for manufacturing landing pads, etc.
This is particularly true of air bag fabrics. Air bags usually consist of two fabrics which differ in air permeability. The contact part, which faces the vehicle occupant at the instant of deployment of the air bag, is required to have an extremely low air permeability of &lt;10 l/dm.sup.2 .times.min under a test pressure difference of 500 Pa. It is this fabric which traps the generator-produced gas and causes the air bag to inflate.
However, soft cushioning of the vehicle occupant in the event of the vehicle being involved in a collision is possible only if care has been taken to ensure that some of the gas will be able to escape from the air bag under the impact of the vehicle occupant. For this reason, air bags are made either of single-piece fabrics of varying air permeability or from two sewn-together fabrics whose air permeabilities differ in a controlled way. Such air bags then consist of the above-mentioned so-called contact fabric of very low air permeability and a so-called filter fabric of significantly higher air permeability, permitting controlled escape of the generator gas. The filter fabric usually forms the side parts of the air bag. A filter fabric is required to have air permeabilities ranging between 20-120 l/dm.sup.2 .times.min, measured under a test pressure difference of 500 Pa.
Any material employed in an injury prevention function must, as an absolute requirement, retain its original properties over a prolonged period even under extreme conditions. This requirement particularly applies to air bags, which are intended to protect the vehicle occupants in the event of an accident. For instance, the original air permeability, not only of the contact fabric but also of the filter fabric of an air bag, must remain unchanged if at anytime, and even following an extended period of using the vehicle, protection of the occupants in the event of an accident is to be ensured. If the air permeability of a fabric remains unchanged over a long period, even under extreme conditions, the fabric will normally be said to possess ageing resistance.
Similar to filter fabrics for an air bag, other industrial fabrics having controlled air permeability must meet a maximum ageing resistance requirement, in particular, if they are used to prevent injury. This requirement is also absolutely essential for the above-mentioned fabrics for use in parachutes, for landing pads, etc.
To manufacture contact fabrics for air bags or industrial fabrics which are subject to similar low air permeability requirements, it is possible to employ a process described in EP-A-436,950, which makes it possible to achieve the required low air permeability. This process can also be employed for air bag filter fabrics, which must have a distinctly high air permeability when compared with contact fabrics. This reference does not disclose which process parameters must be observed in order to ensure a high ageing resistance in respect of the original air permeability. The process described in EP-A-436,950 is suitable only for fabrics made of polyamide yarns.