Road test: among Michel Desjoyeaux’s well-developed sail wardrobe on the last Vendée Globe were several applications of SK78 Dyneema in the D4 laminates from his favourite sail loft, Incidences. He also employed a Cuben Fiber staysail with devastating effect in the Southern Ocean
As Dyneema move more deeply into commercially available laminate sailcloth Dobbs Davis assesses claims that we’ve got a miracle fibre on our hands…
Ever since the day that the Dutch-based chemical company DSM began marketing their high-modulus UHMW-PE (ultra high-molecular weight polyethylene) fibre Dyneema to the recreational marine market nearly 20 years ago, sailmakers have had their beady eyes on it. Possessing qualities that are unmatched by other fibres, it started to look as though this could be a legendary über-fibre that had no equal. In modulus strength, only carbon scores higher, by some 10%, whereas Dyneema is superior to Twaron, Technora and Vectran, outperforming all three by at least 25% in
GILLES MARTIN-RAGET
Uber really?
tenacity, flex fatigue and what is titled as ageing – defined as the ability to resist oxidation and breakdown over time through repeated exposure and flex fatigue. Dyneema is also extremely light in weight and it doesn’t absorb water: in fact, it has a specific gravity of less than 1.0, which in simple language means that it can float! Yet despite these and other admirable qualities, the early manifestations of Dyneema still could not be accepted as the primary load-bearing fibre in performance laminate sailcloth because of two significant and frustratingly persistent limitations: inelastic elongation (better known as creep) and a low melting temperature. For example, an early variant of Dyneema, the HMPE fibre Spectra 900 made by Allied Signal in the US, under licence from DSM, elongates by as much as 14% when a steady 400MPa of load is SEAHORSE 39
ALL TESTED ACCORDING TO ISO 2062
AGED ACCORDING TO ASTM G-155
applied at 30°C for 400 hours – which is clearly unacceptable in almost any performance sail fabric. Even a more recent and improved version of the same fibre, Spectra 1000, elongates 4% under the same environmental conditions in 200 hours. An improvement, but certainly not the answer. The advent of Dyneema SK75 a few years later – a variety now commonly used in running rigging and in a few sailcloths – had dramatically improved the creep problem by exhibiting elongation of only 2% in 600 hours in these same conditions. But it was the 2005 introduction of the latest and lowest-creep variant – Dyneema SK78 – that brought the fibre into line with other rivals such as carbon and Vectran, in matching its load-bearing strength with its resistance to creep. Tried for the first time in running rigging on Paul Cayard’s Volvo 70 Pirates of the Caribbean four years ago, SK78’s performance was found to be excellent; with only 1% elongation noted in 400 hours, and 2% in 900 hours under our standard set of control conditions. Since low stretch is inversely correlated to the modulus of the fibre, the higher the modulus, the lower the stretch. Carbon fibre has the highest modulus, but has issues with flex fatigue. When the sail is folded or when flapping, all fibres – especially carbon – can be damaged, resulting in a decrease in mechanical properties, hence in the durability of the sail. To speed things up the researchers at DP decided to use a quick and dirty method to determine the flex fatigue of the different fibres they 40 SEAHORSE
wished to compare: tie a knot in a singlefibre filament and compare the tenacity of the knotted filament to the original. Now Dyneema outperformed all other fibres, retaining 50% of the initial strength. Another factor influencing durability of sailcloth is the effect of the elements (ageing). In a test performed by DP, the different fibres were exposed to harsh conditions in a climate room and their strength measured after 336 hours. Once again Dyneema outperformed all other yarns, retaining up to 75% of the original strength. The results of these tests endorse claims that the Dyneema SK78 fibre now more closely matches the properties required by our typical end user. And while this fibre is expensive, it soon started turning heads among the sailcloth developers for its tremendous potential to solve problems that plague other loadbearing fibres: the UV sensitivity of aramids like Twaron and Technora and LCPs like Vectran; the lower tenacity of all these and also carbon; and the relatively low index of all these fibres in ageing resistance. So if all this is true, why hasn’t SK78 yet been declared that miracle über-fibre and scooped up by the ton by sailmakers? It’s the one criterion that Dyneema still cannot resolve satisfactorily: high temperature. With a melt point in the region of 144-152°C, Dyneema just cannot take the elevated post-curing temperatures that are now common in the manufacture of the best laminate sailcloths, the temperatures that today’s thermoset and thermoplastic resins need to cure if their full mechanical
properties are to be achieved. The other related problem with Dyneema, which is actually a strength in many other applications, is its high chemical and environmental resistance. While this inertness is great when you have a Dyneema suture in your body helping hold tendons together while they heal (as I do, from a torn Achilles’ tendon), it makes getting the fibre to adhere during laminate cloth assembly a real problem. So how to solve this problem? Just like all technical challenges: hard work, determination, testing, trial and error, more testing, a bit of luck, some experience… And more testing. Enter D4 This is precisely what the folks at Dimension Polyant have been doing in trying to fit Dyneema into their suite of sailcloth products. While others have been comparatively shy at getting Dyneema into their laminates (with the notable exception of Cuben Fiber, who have been using Dyneema filaments in their secretive process since their inception in the early 1990s), Dimension have been trying since 2005 to incorporate Dyneema into their D4 product line, where the most demanding market resides in being intolerant of poor-performing sail fabrics. D4’s process of assembling the component pieces – film, fibre and adhesive – resembles what many others have been doing in string sail laminations, namely laying down fibres on the film along the load paths and adding tafetta and/or more film to
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Spectra 900 Spectra 1000 Dyneema SK75 Dyneema SK78
elongation (%)
12 10 8 6 4 2 0 0
200
400 time (h)
600
800
1000
Opposite page: tabular comparison of the leading modern fibre materials for performance sail manufacturing displayed in terms of ageing (top left), flex fatigue (top right), modulus (lower left) and tenacity (lower right). Only in terms of ultimate modulus is SK78 Dyneema outperformed, by carbon. Above: improvements in creep resistance with development over time – the performance of the original Spectra 900 looks quite stark seen in this light
then all get bonded together in the final lamination. But what distinguishes D4 from all the others is the application of heavy external pressure in addition to vacuum pressure in the lamination process. This, combined with the use of high molecular weight thermoset resin systems, rather than thermoplastic resins, gives Dimension great confidence that their laminates are well stuck. Lastly, the addition of UV inhibitors and antimicrobial materials to the resins, and top coatings of titanium dioxide plus more UV inhibitors in the taffetas, give the final product additional longevity against structural as well as aesthetic degradation (besides getting weakened by excessive exposure, moisture trapped in a laminate can also promote the growth of mildew, the bane of anyone who cares at all about how their sails appear). But how Dimension Polyant have managed to get the notoriously slippery Dyneema fibres to stay put is a proprietary secret, no doubt related to the external pressure exerted on the laminate, so we can’t discuss that here. But regardless of how they’ve done it, it appears that D4 can now feature the attributes of SK78 with its finest qualities harnessed for sails as well as in more traditional rigging applications. And while this is all very intriguing in theory, does it really work? Just because the laminates may contain this miracle fibre, do the sails last and are they fast? Dimension Polyant say they were especially pleased with the fact that several of the top finishers in the last Vendée Globe were carrying D4 sails, with skippers claiming to have been impressed with the sails’ longevity as well as effective shape retention. Foncia, Brit Air, PRB, Roxy and Safran all carried D4 sails during the race, as did Loïck Peyron (Gitana) and Sébastien Josse (BT). Marc Emig, the first sailor to trial a full D4 Dyneema wardrobe, also reported excellent performance during his most recent transatlantic crossing onboard his Class 40 AST Groupe.
While the long-distance racing market obviously beckons, another important niche for these sails may lie in the world of superyachts, where high strength, light weight, versatility and aesthetics combine to put huge demands on sailcloth. The often elaborate furling and sheeting systems on these large boats can be very hard on the sails, so the taffeta layer helps protect against chafe without adding too much to the laminate. For sails that weigh hundreds of kilos, loading them onto these systems often happens once, and they are not then removed again for years; and if they do need to come off then you can be sure the crew will not be happy about it. A recent sail on a Swan 112 in Palma did not exactly push the sails to the limit, but the experience did help illustrate their potential on these bigger yachts. Flawless performance of the push-button hydraulic systems meant hoisting, trimming, easing and furling operations went without a hitch, and the sails looked similarly bright white and flawless. It also helped give an appreciation of how directly weight savings would translate into ease of handling (and perhaps less visibly into increased righting moment), and how even on relatively heavy craft high-modulus fibres are increasingly now seen as critical for shape retention. Sea change… So will SK78-laden D4 replace all those other laminate sailcloths out there? Probably not any time soon, as carbon does after all still boast the highest modulus. For inshore enthusiasts willing to trade off some longevity for superior shape retention then carbon may for now remain the best… However, for superyachts and passage boats, where the sails have to be white and have to be strong, D4 Dyneema may already be the market favourite. Only more time and miles will tell; but you won’t hear complaints from the winner of the last Vendée Globe… And that is probably a significant leading indicator. ❑ SEAHORSE 41