the bi elastic fabric with an elastomeric gas tight layer on its back face the following procedure as illustrated in fig2 is used a first film 17 of polyurethane is produced on the back of the stabilized preferably fixed bi elastic fabric 14 by solution coating using a conventional technique such as a roller doctor blade or the like for example one can employ a solution of the one component polyurethane available commercially under the trade mark 34 impranil c 34 from farbenfabriken bayer of west germany such as a 30 weight percent solution of impranil c in ethyl acetate after application this coating is dried preliminarily at in this example about 80 u00b0 to 90 u00b0 c the coating thickness is sufficient to produce a layer after such a drying operation of about 15 to 25 microns in thickness though thicker and thinner layers can be used this first film 17 is directly adhered to the fabric 14 and functions to produce a smooth surface for a second film 18 independently though conveniently concurrently with the production of the first film 17 there is produced a second film 18 of a polyurethane film 18 can be conveniently produced by solution coating a sheet of release paper or the like for example one can use as the release paper a siliconized paper sheet and as the coating solution a 30 weight percent of a polyurethane such as one available commercially under the trade designation enb 01 or under the trade designation enb 02 from farbenfabriken bayer.