Industrial Fibers and Films Used in the Sailcloth Industry
With the development of polyester fiber in the 1950s, the sailcloth industry started down the road of synthetic fiber use and has not turned back. From polyester (Dacron®), the next step was to a family of nylon fibers known as the Aramids: including Kevlar®, Twaron® and Technora®.
Most recently, Carbon Fiber, in a family of its own, has emerged as the newest and hottest fiber in the industry.
A manufactured fiber introduced in the early 1950s, polyester is the most common fiber used in sailcloth. It is most commonly referred to by its DuPont trade name Dacron®, the name given to the Type 52 high modulus fiber made specifically for sailcloth. Recent advancements by Allied Signal have produced a fiber called 1W70 polyester that has a 27% higher tenacity than Type 52. Other polyester trade names include Terylene®, Tetoron®, Trevira® and Diolen®. Polyester’s desirable properties include excellent resiliency, high abrasion resistance, high UV resistance, high flex strength and low cost. Low absorbency allows the fiber to dry quickly. Although polyester has been replaced by higher modulus fibers for most racing applications, its proven durability makes it the most common cruising sailcloth fiber.
First produced in 1938, nylon is the first completely synthetic fiber developed. In the sailcloth industry, it is used in full radial and asymmetrical spinnakers because of its light weight, high strength, superior abrasion resistance and flexibility. It does not, however, have good resistance to stretch, which is not a big factor in downwind sails, but makes it unsuitable for mains and headsails. Nylon is more susceptible to UV and chemical degradation than polyester and should never be washed with chlorine bleach. Also, its physical properties can change by as must as 100% due to moisture absorption.
An aramid fiber introduced by DuPont in 1971, Kevlar has become the predominant fiber in the racing sector of the sailcloth industry. It is stronger than steel for its weight and has a modulus that is five times greater than polyester. When Kevlar was first introduced there were two types: Type 29 and Type 49 (High Modulus). Type 49 has a 50% higher initial modulus than Type 29 and lower flex strength. Other Kevlar styles (Types 129, 149 and 159) have also been developed, but have seen limited use in sails, since the higher strength goes hand in hand with lower flex strength. Most recently, DuPont has introduced Kevlar Edge, a fiber developed specifically for the sailcloth industry that has a 25% higher tenacity and better modulus than Kevlar 49. However, the benefits of Kevlar 49 vs. Kevlar Edge are still being debated. Additional drawbacks of Kevlar include poor UV resistance (Kevlar loses strength roughly twice as quickly in sunlight as polyester and, when affected, the gold Kevlar fibers turn brown) and rapid loss of strength with flexing, folding and flogging. Minimal flogging and careful handling can greatly extend the life of a Kevlar sail.
Produced in Japan by Teijin, Technora exhibits a similar or somewhat lower initial modulus than Kevlar 29 and slightly higher resistance to flex fatigue. The fiber’s lower UV resistance is enhanced by dying the naturally gold fiber black. Technora is most often used as bias support (X-ply) in laminate sailcloth.
Produced by Teijin, Twaron is chemically and physically similar to DuPont’s Kevlar®. High modulus Twaron (HMT) has similar stretch properties to Kevlar 49, greater tensile strength and better UV resistance. Like Kevlar, the fiber is a bright gold color. As the demand for Kevlar increases beyond supply capabilities, more and more sailcloth manufacturers are switching to Twaron.
Hoechst Celanese produces this polyethylene fiber as a lower modulus version of Spectra. It has similar properties to Spectra including superior resistance to flex fatigue and UV degradation and a tendency to creep (permanently and continuously stretch under a sustained load).
Equivalent to Spectra®, Dyneema® is produced by the Dutch company DSM. It is often used by European sailcloth manufacturers, is available in a wider variety of yarn sizes than Spectra, and is growing in popularity in North America.
PEN Fiber (polyethylene naphthalate, Pentex®)
A modified polyester fiber made my Honeywell, Pentex has 2.5 times less stretch than standard polyester fibers (between polyester and Kevlar 29). Because it does not shrink as much as a good Dacron fiber (5% vs. 15%), Pentex weaves cannot be as tight as a Dacron weave. Thus, woven Pentex must be highly resinated and requires great care to retain the shape and extend the life of a sail. Pentex is much better suited to laminated sailcloth, in which the fibers are laid straight for strength and are sandwiched between sheets of film for stability. Pentex laminates are a lower-cost option for the racing sailor who is looking for a slightly higher performance sail but does not want to spend the money for a Kevlar or other exotic-fiber sail.
Spectra® is a highly processed ultra high molecular weight polyethylene (UHMWPE) made by Honeywell. Spectra offers good UV resistance (on par with polyester), very high initial modulus numbers (second only to high modulus Carbon Fiber), superior breaking strength, and high flex strength. More careful testing, however, reveals that it also exhibits a property known as “creep” (permanent, continuous elongation under a sustained load) that results in a change in shape as the sail ages. Creep makes Spectra unfit for high performance racing applications and more suited for large cruising boats where strength, UV resistance, light weight and durability are paramount. Spectra yarns are white, but by dying other laminate elements, it has been offered in additional colors, particularly light blue.
LIQUID CRYSTAL POLYMERS
PBO (Poly (p-phenylene-2, 6-benzobisoxazole)) is a rigid-rod lyotropic liquid crystal polymer developed by Japan-based Toyobo under the trade name Zylon®. PBO is a gold fiber with an initial modulus that is significantly higher than other high modulus yarns, including aramids. Among PBO’s desirable properties are high thermal stability, low creep, high chemical resistance, high cut and abrasion resistance, and excellent resistance to stretch after repeated folding. PBO is also quite flexible and has a soft hand. Less desirable is PBO’s poor resistance to both UV and visible light, which, unless a UV film is used, greatly reduces the working life of the sail.
Vectran is a polyester-based high performance LCP (liquid crystal polymer) produced by Hoechst Celanese. It is naturally gold in color and has a modulus similar to Kevlar 29, but suffers negligible strength loss with flex. This is a benefit in endurance applications and for cruising sails where durability is key. Additional advantages of Vectran fiber include zero creep, high chemical and abrasion resistance and high tensile strength. Although it was initially though that Vectran’s UV resistance was lower than those of the Aramids or of Spectra, it has been shown that, over an extended test period, the UV degradation of Vectran levels off after roughly 400 hours of exposure, while the Aramids and Spectra® continue to degrade.
Carbon fiber is a high modulus synthetic fiber made from an acrylic containing carbon, hydrogen and nitrogen atoms that is heated in three successive stages to eliminate all but the carbon atoms. It is naturally black in color and is essentially unaffected by UV exposure. First used successfully in the America’s Cup, carbon fiber laminates provide exceptionally low stretch for their weight. Recent manufacturing advances have led to improved fiber flexibility, which translates to longer sail life in exchange for lower modulus numbers. The balance between low stretch and high flexibility means that, depending on the production method, carbon fiber sailcloth can be extremely brittle and damage intolerant, or can be extremely durable and slightly stretchier than the comparable aramid sail. Recently developed carbon sailcloth styles have paired high modulus carbon fiber with more durable aramid, Spectra, and/or Vectran fibers to achieve durability without sacrificing low stretch.
An extruded version of DuPont’s polyester fiber, Mylar is the most common film used in laminated sailcloth.
PEN film is an extruded version of PEN fiber. Just as PEN fiber is stronger than polyester fiber, PEN film is stronger than polyester (Mylar®) film. However, PEN film is rarely used in standard sailcloth styles because it shrinks more rapidly than Mylar®, is less resistant to abuse, and reduces the working life of the sail.