One of the advantages that Stratis offers is
that we can select the fibres that will go into
the sail membrane. This means that, with
the experience that we have gained
through research and development and on
the water experience, we can optimise the
make-up of the sail to suit your
Below are some notes from our R&D team about the fibres that
we are using for Stratis sail laminates and the properties that
make them particularly suitable for certain applications.
“Polyester” is a really quite a confusing term; Polyesters are a family of polymers of
which Polyethylene Terephthalate (PET) is just one. Admittedly PET is the most
abundant worldwide, but it is one of many, which include PEN and Vectran.
PET was invented by a couple of British Chemists in Manchester whose bosses
weren’t interested in the material they’d just made, so they took the idea down the
road to Imperial Chemical Industries (ICI), who bought the patent and made a
fortune. ICI used the trade name Terylene for the first commercial material, but it
wasn’t until DuPont started spinning PET fibres under license a few years later that
we got the material we know and love as Dacron(r). Dacron is white/opaque (as all
semi-crystalline polymers are), however a useful property is that when it is biaxially
oriented it becomes transparent ( this is because it has such a high degree of
crystallinity that the gaps between crystallites shrink to less than 350nm which
allows for full scattering by visible light), and in this form we know it as Mylar
(another DuPont trade name).
PEN is the big brother of PET. It was invented very shortly after PET but only
commercialised in the 1990’s after Amoco started the first plant to produce the
NDA needed to make it affordable in commercial quantities. Recently the price
of PEN has been rocketing as almost all the capacity worldwide is being used
for producing tyres; it is now comparable in price to Kevlar.
PEN does provide a reasonably significant increase in modulus over PET, so is
very useful in those classes limited by Class Rules to lower modulus fibres (or
simply “polyester” fibres – though Vectran would actually pass this criteria). One
cosmetic issue with PEN is that the naphthalate in the polymer is prone to
yellowing on exposure to UV light. The yellowed polymer is only a few micron
thick, and if you poke underneath you’ll find the virgin white polymer unharmed
as the yellowed material actually acts as a sunshield. There is an associated
loss in mechanical properties proportional to the amount of yellow sun-shield
material, which is not really that significant, but is pretty ugly – so more
expensive measures are needed to protect it from UV damage.
Polyparaphenylene terephthalamide (PPTA) was another DuPont creation by
Stephanie Kwolek in the late 1960s and branded under the name Kevlar. Akzo
followed suit shortly afterwards naming their product Twaron with Teijin Group
now owning the rights to the product. PPTA is produced in larger volume than all
the other high performance polymeric fibres, and that commoditisation is
reflected in the fact that it’s the cheapest of all the high performance fibres
(though still ten times more expensive than PET). It is produced in a variety of
grades, with the roughly equivalent Twaron 2200 and Kevlar 49 being the most
prevalent throughout the sailcloth market. Twaron and Kevlar are very good
all-rounder fibres at the high performance end of the spectrum, where you might
say it has a good modulus and strength, but lower than average resistance flex
fatigue and pretty crumby UV resistance.
Technora is also made by Teijin, the manufacturers of Twaron, and is a close
relative of PPTA. It is a copolymer, which means it is comprised of two
different building blocks instead of just one. The second component that is
added to the structure introduces a regularly repeating kink in the polymer
backbone. This lowers its modulus compared to PPTA, and significantly
increases its flex fatigue, while leaving the tensile strength largely the same.
Hence it is used to reinforce in those areas where we anticipate a lower initial
load, but require it to keep its strength for longer. Spun dyed black its UV
resistance is greatly increased which makes it a better choice than PPTA
where durability is required.
Before the ink on patents for the first aromatic amides was even dry, work
started on producing wholly aromatic polyesters with the hope that the higher
modulus of PET over Nylon would be mirrored in their aromatic analogues.
Vectran was the commercial result which, although lacking the hoped for
properties, resulted in a polymer whose mechanical properties are almost
identical to Technora. Vectran betters Technora’s flex fatigue for which it also
owes to a kinked copolymer backbone, and only falls down with poor
reaction to UV. It is a very durable fibre and a perfect choice for situations
that demand retained mechanical strength after sustained
The element carbon has two allotropes, diamond and graphite. The
microstructure of carbon fibres is based on the hexagonal layer structure of
graphite, however commercial carbon fibres lack true three-dimensional order
between layer planes. This incompletely ordered graphite structure is termed
graphene. The physical and mechanical properties of carbon fibres are
primarily determined by the axial orientation of the carbon fibre layer planes
and the degree of crystalline perfection. Carbon fibres are generally typed by
precursor such as PAN, pitch, or rayon and classified by tensile modulus and
strength. Tensile modulus classes range from low (<230GPa), to standard
(230-240 GPa), intermediate (280-300 GPa), high (350-500 GPa), and ultrahigh
(500-1000 GPa). Stratis membranes use exclusively PAN based fibres, and in
general standard modulus fibres are used, though some race programs and
superyacht projects require IM fibres.
The thermal conversion of PAN to carbon fibre involves two major actions: firstly
stabilization to thermo-set the fibre, secondly carbonization. Prior to
carbonization the precursor PAN fibre must be chemically thermo-set to lock in
molecular orientation and increase carbon yields. Commercially, stabilization is
accomplished by controlled heating in air at temperatures of 200-300°C C for
periods of 30 minutes to two hours. The molecular orientation developed during
spinning and drawing is locked in place by stretching, or at least restraining
the fibre from shrinking during stabilization.
Stabilised precursor is converted to carbon fibre by controlled heating to
temperatures exceeding 1000°C in a non-oxidizing environment under tension.
Standard-grade, 240 GPa PAN carbon fibres are generally heated to
1200-1400°C. Higher modulus fibres can be produced by increasing
carbonization temperatures. Some very high modulus fibres are produced at
temperatures approaching 3000°C. Fibre composition and structure change
significantly during carbonization as the fibre is converted from a relatively low
strength textile polymer with elongation of approximately 10% to a relatively
brittle ceramic fibre with elongation ranging from 1-2%.
ICE is an entirely new and unique UHMWPE (Ultra High Molecular Weight Polyethelene) fibre, developed by the Doyle Stratis team following market demand for a creep free and highly durable sail membrane.
Exclusively licenced to Doyle for marine use, it exhibits similar properties to high modulus carbon, and is a derivative of Spectra.
All the mechanical properties of the fibres shown above are average values for fibres commonly used in sails. The theoretical modulus and strength of a "perfect" fibre is actually quite a bit higher than those quoted above, it just gets impractical to produce fibres with mechanical properties that get so close to that of the crystal structures. The compromise between higher manufacturing costs and the increase in number of imperfections in the fibres is the reason that there are different grades of fibre. For example, the crystal modulus of PPTA is 220GPa, but Kevlar 49 is produced with a modulus of ~120GPa, and Kevlar 29 a modulus of ~75GPa.
The Tensile Modulus and Strength have been quoted in multiples of SI units as these are more familiar to most, but it should be noted that it is often more suitable to use values which relate to the linear density (denier or decitex) of the fibre. Fibres are bundles of tiny round filaments a few microns in diameter. The total cross-sectional area at any particular point along the fibre can be hard to determine and can vary quite significantly making it more appropriate (and accurate) to measure the modulus and tensile strength (now called tenacity) as an average over a specific length. It is with these values measured with these units (generally gf/denier or cN/dtex) that dpi's for sailcloth and sail membranes are calculated.