As already noted, in addition to the adhesives that bond the layers together and the films that cure bias stretch, the other real breakthrough in laminated sails was the discovery that it was no longer necessary for the substrate to be woven, or at least not as tightly woven as had been the case with standard Dacron. The result has been laid-up scrims in which the warp and fill fibers are simply arranged in a loose grid pattern that is then held together by the Mylar film (Figure 3.6). The yarns used in scrims are extruded flat rather than round, and because they are simply placed on top of each other there is none of the over and under that causes crimp in a standard weave. When the scrims are laminated, they are held in tension to remove initial stretch from the fabric, and as soon as they are bonded to the film they remain in place and the initial stretch characteristics of laminated fabrics are always very good as a result. The result-
ing gaps between these yarns have been shown to be the key to good laminated sails since the adhesives used in the laminating process work better when they are allowed to adhere to film rather than the fibers.
Fabric makers have also discovered that in many cases a third layer, for example, a taffeta, helps prevent tearing since the fact that the individual yarns do not interlock as in a woven fabric results in a lower tear strength. The taffetas are usually light, fairly loosely woven polyester fabrics. Their job is not specifically to help with diagonal stretch, although they do contribute a little. Rather the taffetas are there for abrasion resistance and overall strength.
The final piece of the laminate puzzle was the addition of diagonal yarns, which lend some strength to the film when it comes to handling off-threadline stretch. The angles at which the yarns are placed in the fabric have been part of patents, and their number and relative thicknesses are an integral part of the fabric engineering process. It is not as important to have as many diagonal yarns as warp or fill yarns, since if the sail is engineered properly the principal load-bearing yarns will manage most of the loads on the sail. The diagonal fibers come into play when the sail is eased out and the loads no longer travel along predictable load lines or "catenaries." The good news is that once a sail is eased out, the loads are greatly reduced, therefore having as many fibers there to accept them is not that critical.
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