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Synthetic sails

Jan 1, 2003
From Ocean Navigator #58
January/February 1994
Most sailors would be hard-pressed to remember a time when sails were actually made of cotton. Since the late forties, Dacron polyester has held sway as the foremost sail fabric. Now, of course, we have a slew of other brand names to choose from, as well: Mylar (DuPont), Kevlar (DuPont), Spectra (Allied-Signal), Technora (Teijin), Vectran (Hoechst-Celanese), and the ultimate exotic material, carbon fiber.

Brand names are great, of course, but they don't answer a simple question: What are these materials?

The first, most obvious answer to this question is that they are all synthetic fibers. And that description can be narrowed further: they all are derived from petroleum products. (It may seem strange to imagine Dacron as having much to do with gasoline, but the processing of crude oil produces a myriad of products ranging from asphalt, grease, and waxes to heating oil, kerosene, and gasoline.)

One of the earliest synthetic fibers, however, was not based on petroleum. Rayon, originally devised to mimic silk, was produced using cellulose derived from cotton or wood pulp. Later, various compounds were synthetically produced and used to make fibers. In 1932, for example, nylon was first produced using synthesized products derived from coal.

Coal and petroleum are similar in that they are both forms of hydrocarbons. That classification is, however, not much of a distinction because there are a staggering number of compounds that fall into the category of hydrocarbons. One reason for this is carbon's tendency to form a wide variety of chain-like and ring-like molecules.

This property of carbon can be used to advantage by chemically doctoring hydrocarbons and inducing something called polymerization. When this happens, molecules link up in long chains or, in the case of some compounds, tight rings, perfect for making synthetic yarn. Polyester (first produced in 1941), can be polymerized by combining terephthalic acid with ethylene glycol (the essential element in anti-freeze).

After polymerization, the next step is to form the material into fibers. In the case of polyester and nylon, this is done via a process called melt-spinning. The polymer is heated above its melting point and then extruded through holes in a metal die. When the strands formed by this process cool, they are combined into yarns and then woven into fabric.

The polyester fabric used in sails is widely referred to using DuPont's brand name of Dacron, but other companies also make polyester for sailcloth. In addition to Dacron, DuPont also makes Mylar, which is polyester formed into a film, rather than a fiber.Concocting polymers

There are a tremendous number of petrochemicals that can be used for concocting polymers. Added to that is an equally wide array of techniques for tweaking the mix to get a fiber with enhanced qualities. The various types of high-tech fabrics used in sails are a result of extensive research and experimentation by chemical firms, all trying to find the "perfect" fabric.

In 1965, a research chemist at DuPont stumbled over a new fiber that later came to be called Kevlar. Known for its remarkable tensile strength, Kevlar is an aromatic polyamide (aramid) that is similar in composition to nylon. The aromatic part of its chemical classification doesn't describe Kevlar's pleasant smell, but, rather, indicates its molecular structure. In an aromatic compound, carbon molecules circle to form stable, six-sided molecular rings. It is this structure that helps to give Kevlar its ability to resist tensile loads.

No formulation is perfect, however. Kevlar has its downside in its susceptibility to ultraviolet (UV) degradation, abrasion and fatigue.

A product similar to Kevlar is an aramid fiber called Technora, manufactured by Teijin Ltd., a Japanese company. "Technora has a black pigment introduced into the yarns when they are being formed," says Rich McGhee vice president of Dimension/Polyant, the U.S. distributor for Technora. "This gives Technora good resistance to UV degradation." Technora also stands up better to abrasion than Kevlar.

Spectra is a product of Allied-Signal Corp. This fiber is composed of polyethylene, the same material used for making milk jugs. There is obviously a difference, however, between the two materials. The polyethylene used for Spectra is of much higher molecular weight and high molecular weights translate into materials with large molecules. In the case of Spectra, those molecules are in the form of extended chains.

"Milk jug polyethylene might have a molecular weight of 40 to 80,000," says Chris Phillips, in technical marketing at Allied-Signal. "But Spectra has a molecular weight of more than 1,000,000. When a polymer has this high a weight, it doesn't want to flow. We use an elaborate chemical process to work with this material."

To extrude Spectra, the material is first dissolved into a gel using a solvent. This material is then drawn out into fibers. Finally, the fibers are washed clean of the original solvent.

Spectra's properties give it an excellent strength to weight ratio. It also resists abrasion better than Kevlar. However, while Spectra has higher tensile strength than Kevlar, it does experience "creep" (it permanently deforms under high constant load).Improved polyester

Polyester, of course, is one of the oldest synthetic fabrics used for sails, as witnessed by the fact that Dacron has become almost a generic name for standard sail cloth. A new cloth from Hoechst-Celanese called Vectran is also a polyester cloth, but it is formulated using a different chemical base. Where most polyester uses terephthalic acid and ethylene glycol, Vectran is synthesized from naphthalene. This difference makes Vectran an aromatic polyester. Remember that an aromatic material (like Kevlar or Technora) has carbon linked in six-atom rings, lending it stability and strength. "We produce Vectran using a two-step process," says Peter Adams, business manager of advanced fiber materials at Hoechst-Celanese. "After melt spinning the material, we make the molecules longer by subjecting them to certain temperatures and pressures. This gives Vectran its superior structure."

While Vectran is more expensive than Kevlar or Spectra, it has the advantages of virtually no creep, very good fatigue resistance and high strength.

In the high-tech sails area, some sailcloth has been dubbed "liquid-crystal" fabric. This refers to the molecular structure of the yarns. During the polymerization process (when they are still liquid), these compounds are structurally rearranged so that a higher percentage of their molecules form crystalline structures. (A crystalline structure can be defined as a defined geometric pattern that provides stability and allows molecules to pack together more tightly. The molecules in most metals are arranged in crystalline fashion.) This crystalline structure is another factor in the great strength of these compounds.

The ultimate high-tech material for sails has to be carbon fiber. Because of its excellent tensile strength, carbon fiber has been used as a reinforcement for both boat hulls and for sails. The experiments in sailcloth have had mixed results, however. While it is strong, carbon fiber is also extremely brittle and breaks easily when folded. On top of that, it is a high-cost material.

A3 Technologies, Inc., an offspring of the America3 America's Cup campaign, is introducing sailcloth built with carbon fiber called Filament Fortified Film. By blending long-chain polymers with the carbon monofilaments, A3 claims to have gotten around the problem of carbon fiber's brittle nature.

Producing these carbon fiber yarns is done by polymerizing a hydrocarbon mix and then extruding it while subjecting it to high temperature and pressure. Under these conditions, the other elements of the mix are burned off leaving only strands of carbon. The carbon fibers are then blended with other yarns for abrasion resistance.