The use of manmade fibrous mate-rials on sailing yachts is all-perva-sive: the composites used in thehull, deck and appendages startwith woven and/or uni-direc-

tional glass fibre, aramids or carbon,bonded to itself and to core materials usingresins of various types and sophistications.Composite spars are commonly now madeof uncored uni-directional and wovencarbon as well, and there’s increasing useof composite materials in winches, blocksand other deck hardware. Sails are, ofcourse, constructed using either wovenfibrous materials and stabilising resins, orvarying densities of yarns of fibres bondedto film materials using thermoset resins.

But over the past decade or so much ofthe focus on development and innovationin fibre types has centred upon their appli-cation in running rigging, and, mostrecently, standing rigging. This develop-ment has prompted a vigorous expansionin the rope manufacturing industry as newproducts have been offered that combinethe best qualities of the various fibre typesto suit specific applications on the boat.

Nearly all performance sailors, and notjust riggers, are now at least nominallyconversant on what these qualities are,even though so much of the performanceof these products depends not solely on thematerial qualities of the fibre, but how it istwisted, braided into a rope construction,combined with other fibres and coated.

Nevertheless, and regardless of thesenuances in composition and construction,there have been some recent developmentsamong suppliers of high-modulus fibresthat promise to expand and improve theiruse in current applications and prompt us tostart looking outside the box for their use in completely new contexts. Therefore thetime is right to provide a summary overviewof these properties to compare them againsteach other and engage in some speculationas to their potential for the future.

In this first of a two-part examination ofthe technical properties of fibres and theiruse, particular attention will be given to afibre that is well known and accepted,having proved itself over the past twodecades in multiple running rigging appli-cations, but is still somewhat misunder-stood; it may also be victim to an unde-served bias against its newest incarnationwhich may yet prove to rival carbon as averitable wunderkind of materials. Thisfibre is Dyneema.

Modern synthetic fibres – anoverviewThe three most common manmadesynthetic fibres – nylon, polyester andpolypropylene – have all been producedfor several decades and remain in use formany applications in the marine trade. Atthe time of their introduction in the 1950sthey quickly replaced natural fibrematerials which were generally weaker,less tenacious and prone to degradation inmarine environments.

Nylon, with its low stiffness modulus, isstill the material of choice where highextension is important. Also known by itschemical name polyamide, nylon does,however, suffer a 10% loss in strength infibre form when wet, which can climb toup to a 20% loss in rope form. Wet nylonropes experience this strength loss due to ahigh degree of disorientation of the mole-cules from the moisture absorption. Afterdrying, strength goes back to normal levelsbut the rope shrinks. Nylon is also suscep-tible to internal abrasion during tensioncyclic loading, such as occurs in mooringlines, yet certain types of nylon (such asCordura) have proved excellent for abra-sion resistance.

Ropes and other materials made frompolyester, in contrast, have proved moredurable in cyclic tensile fatigue loading, arehigher in modulus strength, and do not losestrength as dramatically as nylon when wet.

Polypropylene fibre is weaker than both

nylon and polyester, but it has a specificgravity less than one, which allows it tofloat on water. However, its low meltingtemperature means that high-speed cyclicloading of polypropylene rope can make itheat up, lose strength and experience creepunder high loads.

High-modulus fibresUse of the term high-modulus refers tofibre types that have elastic moduli that aresignificantly greater than those of thecommon fibres, and that also have muchhigher breaking strengths.

Aramid was the first high-modulus fibre,introduced as Kevlar-29 by Du Pont in the1970s, and improved on in the 1980s withvariants such as Kevlar-49. Aramid fibresare lyotropic and are produced by solventspinning, and thus have high tensilestrength, but do not melt at high tempera-tures. Their great strength is compromised,however, by susceptibility to weakness from

44 SEAHORSE

The(new)miraclecure? – Part 1Dobbs Davis assesses theultimate potential of the latest incarnation of super-fibre Dyneema… SK78

The(new)miraclecure?– Part 1

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exposure to UV rays (in sunlight), and arather large molecular structure makes thearamid rather dense and hydrophilic (iewater can be absorbed into the fibres).

Large cables and ropes made fromaramid fibres can also experience significantstrength loss due to axial compressionfatigue when tightly woven fibres are forcedinto compression under load. This is whyfor standing rigging applications aramid

fibres are aligned in parallel strand arrays.There are two other principal aramid

fibres: Twaron, made by Teijin, is verysimilar to Kevlar, and Technora, also madeby Teijin, is similar though less prone tofatigue through axial compression loadingand UV light exposure.

Around 1990 Hoechst Celanese intro-duced a fibre made from a liquid crystalpolymer (LCP), or liquid crystal aromaticpolyester (LCAP), called Vectran. This isnow produced by Kuraray America as Vec-tran HT for use in cordage and sails, andVectran UM for use in the reinforcement ofcomposites and electromechanical cables.

Unlike the rather loose chains of conven-tional polyester molecules, Vectran mole-cules are stiff, rod-like structures organisedin ordered domains in the solid and meltstates. These oriented domains lead toanisotropic behaviour in the melt state,thus prompting use of the term ‘liquidcrystal polymer’. Vectran fibre is formed by

melt extrusion of the LCP through finecapillaries, during which the moleculardomains orient parallel to the fibre axis.The structure’s high degree of orientationand thermotropic character account for thefibre’s high melting temperature andproperties of high tensile strength.

High Modulus Polyethylene (HMPE),also sometimes called Ultra High MolecularWeight Polyethylene (UHMWPE) or HighPerformance Polyethylene (HPPE), is athermoplastic made from naptha. The firstgelspun HMPE fibre was given the tradename Dyneema®, from the Greek for‘strong fibre’, and the fibre and process toproduce it was patented by DSM inHolland in 1979.

Since 1990 Dyneema has been producedby DSM at their plant in Heerlen and hasbeen developed into different grades, suchas SK60, SK65, SK75 and now SK78. In themid-1980s DSM granted a licence for pro-duction in the US to Allied Signal Corp,which was more recently acquired byHoneywell and has been producing asimilar fibre known in the US as Spectra.HMPE can also be produced using a lessexpensive melt-spinning and drawingprocess, but the resulting properties areinferior to that produced by gel spinning.

The latest class of high-modulus fibre isa complex molecule called polybenzoxa-zole, or PBO, first developed by the US AirForce, then produced on a limited basis byDow and now in production by Toyobo inJapan. When introduced to the marinetrade a few years ago, it looked as thoughPBO was going to be the new wonderfibre, with strength and modulus proper-ties unmatched by any other syntheticpolymer fibres. However, it was soon dis-covered that exposure to light, and not just

SEAHORSE 45

‘Pirate’ Jerry Kirby (left) and Adam Beashel(above), two men with a daily interest in theperformance of anything claiming to be a‘miracle fibre’. Pirates was one of the firstboats to trial Dyneema SK78 in competition

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UV light, significantly degraded the fibreto having unreliable physical properties.

The ‘strongest fibre in the world’The molecular chemistry as well as the production process to manufactureDyneema contribute to the fibre’s uniquebalance of qualities that lead to its status asthe strongest fibre currently in commercialproduction.

The starting material used is UHMWPE,a type of olefin, which is synthesised frommonomers of ethylene and bonded togetherusing a process based on metallocene cata-lysts into long chain molecules having 100-250,000 monomers each. These moleculeshave extremely high molecular weight,numbering in the millions, due to thisextremely long length. This arrangementallows for excellent packing of the chainsinto the crystal structure, resulting in a toughmaterial with the highest impact strength ofany thermoplastic. (Aramids, in contrast,derive their strength from strong bondsbetween relatively short molecules.)

Production of HMPE fibre involves pre-cision heating of the UHMWPE gel, whichis forced by an extruder through a spinneretas the solvent evaporates. The extrudate isthen drawn through the air. This techniqueproduces a fibre with over 95% orientationof the polymer chains, and a level ofcrystallinity approaching 85%.

The close and efficient packing of the longmolecular chains accounts for the tenacity of

Dyneema, where the strength of the fibrecan be expressed in terms of its free breakinglength. This is the theoretical length of afibre, yarn or rope which breaks under itsown weight when freely hanging, and thus isindependent of thickness. When tested usingISO 2062, Dyneema’s breaking lengthreaches in theory to the orbit of a geostation-ary satellite… 349 km.

The relatively simple structure of theUHMWPE molecule and weak Van derWaals bonds between chains also accountfor other important properties. Theseinclude low density (specific gravity is0.97), the low melting point (152°C) and

significant loss of strength at temperaturesabove 80°C. The slippery, non-adhesivefeel, very low coefficient of friction, lowmoisture absorption and chemically inertnature of the fibre are also due to this sim-ple molecular structure. This is in contrastto the complex aromatic polymers, whichare susceptible to damage from solvents,aggressive chemicals and light radiation.

So does SK78 solve the creepproblem?OK, so if Dyneema has all these wonderfulproperties, why aren’t all load-bearingfibre-based materials made from it? Theanswer is that, like all the high-modulusfibres, Dyneema seems to have creep too.Defined as the fibre deformation or elon-gation that can occur under conditions ofhigh static loads and long periods of time,creep is different from stretch because it isinelastic and non-recoverable – clearly nota desirable quality for our purposes.

Early versions of Spectra and Dyneemaseemed to suffer from noticeable degreesof creep, but this behaviour may actuallyhave been overstated: only when subjectedto high static loading for significantly longperiods of time did SK75 display anymeasurable degree of creep.

Nonetheless, with a mandate to try to solve this vexing problem withoutcompromising the qualities of an otherwisesuperior fibre (ie high strength, UV resis-tance, longevity, etc), scientists at DSM

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Retained strength after ageing 336 hours

Fibre elongation propertiesAbrasion resistance

Fibre tenacity

Fibre density

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started work two years ago on developing aspecific new variant of Dyneema that wouldexhibit improved creep performance. Thisnew fibre, dubbed SK78, was then subjectedto rigorous tests supervised by the firm’ssports applications manager, Dr DanielaRibezzo. After this rope manufacturer Got-tifredi Maffioli tested various lines madefrom SK78 successfully on Pirates of theCaribbean in the last Volvo Ocean Race.

Since the last VOR several manufacturingpartners of DSM Dyneema have been busyincorporating SK78 into their own productlines. For example, Liros in Germany haveconducted their own extensive testingregimen which demonstrated that linesmade with SK78 will show extremely low oreven negligible creep if the load is not held asstatic, but rather there are minor fluctua-tions in the rate and/or frequency of load levels. Using this condition, SK78’selongation measures at about the same levelas those of the other high-modulus fibres.

And keep in mind that creep is a time-

dependent phenomenon and so unnotice-able for most sailing applications, whetherin cordage or sail fabrics, because the lengthof time the materials are subjected to highloads is simply too short. Test data showsthat even under extreme conditions (50°C,500 MPa applied load) Dyneema SK78elongates only 1.5% in 24 hours, a markedimprovement over SK75 which elongates3.3% and Spectra 1000 which elongates10.1% in the same set of conditions.

According to Rolf van Beeck, sportsdirector for DSM, the results were veryencouraging. ‘The creep performance ofSK78 is improved by a minimum factor oftwo times in comparison with SK75. Thismeans the rope will elongate much moreslowly, retaining optimal tension overtime, which improves control of the sailsand overall yachting performance. Andhandling characteristics did not change atall – only the amount of creep.’Next month: out of the lab and into thereal world ❑

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Comparison of different HMPE fibres at the same specific load with huge differences increep rate between HMPE fibre types. Note also significant differences in creep lifetime

Fibre property summary

Creep comparison, Spectra vs Dyneema

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