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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 specific requirements.
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 (PET)
“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
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. ARAMIDS (ARomatic AMIDes) 1. Kevlar and Twaron
2. Technora
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. Vectran
Carbon Fibre
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%.
EXPANATORY NOTES
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. |
