The most significant factors in the lifespan of a hang glider sail are environment conditions. Sails that are exposed to the most severe environments (for example, the Owens Valley) tend to deteriorate at about 3 times the rate of places like New England and Europe. Also, we see a significant variation in the lifespan of gliders even in the same geographic areas since most UV / dirt exposure, wear and tear is related to the way a pilot cares for his or her glider. Gliders often wear out more from parking them in the LZ or even from too much time on car racks than from flight hours.
See also: General Sailcloth Options Discussion
Let’s start by comparing woven fabric with the most common laminate construction of polyester film, scrim, x-ply, film and taffeta. The most significant structural difference between a conventional polyester fabric (like your glider bag) and sailcloth is the finishing and resin that stabilizes the bias direction of the fabric. This process binds the fill (fabric width) and warp (fabric length) fibers to more effectively support non-uniform and off-axis loads. However, conventional sailcloth is limited by the structural properties of the resin system which isn’t nearly as strong or stiff as yarn (fiber) in the direction of the applied load, and heavier finishes that increase fabric stiffness and bias stability also reduce tear strength. Fabrics like Polyant 205/245 square and Hydranet optimize the fabric stability and tear strength by combining a smaller denier, tightly-woven base fabric with a square pattern of heavy or very strong yarns.
Laminate sailcloth approaches these constraints by applying an optimized material to each structural requirement. The film layer replaces the resin system, is isotropic (uniform strength in all directions) and more structurally efficient than fiber based construction. It sounds like the perfect material except that film has poor tear strength and is easily damaged from surface abrasion. Laminates add to the film with heavy fibers oriented in the directions of the highest loads. These fibers in the scrim and X-ply layers have a coarse weave, no twist and very little crimp (they don’t have to go over and under other fibers) so they are more efficient than yarn in a woven fabric and also provide exceptional tear strength. The second film layer encapsulates the scrim and X-ply layers and is more protected than the surface film. The taffeta layer is a fine woven polyester that adds very little strength and about 1 oz (20%) weight to the fabric weight but protects the second film layer and adds reinforcing for stitching.
The most vulnerable component of laminates is the outer film layer. With increasing exposure to UV, it becomes brittle and cracks, particularly in areas that are scratched or have higher stress concentrations. Once the film starts cracking, the failure progression accelerates. As with most materials, polyester resistance to UV deterioration is proportional to material thickness which is one reason why thicker woven fabrics retain strength better than films. Also, the multi-filament yarn construction in woven fabrics is more robust than film.
All modern sailcloth and laminates include UV stabilizers and coatings in the construction. For laminates, supplemental coatings are applied between the layers below the upper film ply and are somewhat effective at protecting the scrim and lower film ply. Unfortunately, they don’t protect the outer film layer.
Dimension-Polyant develops and produces an incredible variety of high-tech fabrics for the industrial, sporting goods and sailcloth markets. Last year, my technical representative mentioned an industrial polyester film that is molecularly altered to be ‘impervious’ to UV deterioration. Also, it stops 90% of UV thru-transmission so it’s more effective than coatings at protecting the other laminate plies. The only problem was that it seemed prohibitively expensive—20x more than untreated polyester film. Fortunately, the film price is a fraction of the laminate price and I decided that, even if the film was only 50% better than standard film, I could easily justify the extra cost. It’s important to note that film color is not related to structural properties. The common ‘smoke’ laminate is conventional polyester film. Our UVM material is grey because the minimum order quantity for a different color (like white) of this special-purpose film is 20,000 meters. According to Polyant, the Wills Wing UVM laminate is the first application of this film in sailcloth.
When we introduced UVM laminates, I projected a 30% durability advantage over conventional laminates. I also asked Polyant to run accelerated lab UV tests to compare the new UVM material with a laminate of the same construction with conventional polyester film to confirm my expectations. Polyant performed structural tests of both fabrics at 0, 100, 200 and 300 hours. Each test measures the fabric resistance to elongation from an increasing load and is performed separately for the bias, fill and warp fabric orientations. The bias orientation is the most important of these tests because it has the highest elongation under load and it isolates the performance of the film layer from the composite fabric. I produced two composite images to illustrate the results of theses tests. The first compares (conventional) PM05T laminate at 0 and 300 hours. The results show a 30% reduction in load capacity at 300 hours at failure.(when the slope of the elongation vs. load curve goes vertical.). The second graph compares UVM05T at 0 and 300 hours. The bias elongation of the PM05T and UVM05T are identical at 0 hours (from separate graphs). At 300 hours of UV exposure, the UVM05T has better performance—it is both stiffer and has a higher elongation at failure than the unexposed fabric. These results are far better than I hoped for and support my expectations for a significantly longer sail life from our UVM material.
I expect a PXT laminate sail to remain airworthy for 450 hours +/- 150 hours. Based on these lab results and our initial field tests, I expect a UVM sail to last up to 50% longer than the PXT material. Even though it appears to be relatively unaffected by UV exposure, the film/laminate construction is still not as durable as a good woven fabric. Historically, woven fabric sails last at least 100% longer than a PXT laminate.
The two remaining factors are performance and behavior in rain. We’ve never been able to demonstrate a performance advantage of a Mylar sail over a woven sail when comparing new gliders with equivalent tuning in side-by-side performance evaluations. Of course we speculate that the structurally stiffer laminates improve performance at tight VG settings through better twist control. In any case, as I think you’ve experienced, flying a conventional fabric T2 in a contest back-to-back with a new Mylar sail, the difference is not very significant. Also, conventional fabric in the leading edge panel in most cases reduces the magnitude of controllability problems, elevated stall speeds and trim change that often affect gliders encountering rain, although these factors are strongly influenced by airfoil shape. For these reasons, we recommend that recreational pilots choose conventional Dacron sails and comp pilots choose Mylar.