Aramid Fibers, trade names Kevlar®, Twaron®, Nomex®, Technora®
Aramid fibers are another group of super-heros of the fiber world. Kevlar and other polyamides, all conjure up images of ultra strong materials that are elbowing out more conventional construction materials such as steel. Although not used in boats as ropes very much it is sometimes used in high end composite for sailboats and kayaks.
What is Aramid (Kevlar®, Twaron®)
"Aramid" is formed from "aromatic polyamide". The US Federal Trade Commission defines Aramid Fibers as:
"A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings."
Aramid fiber is produced by spinning a solid fiber from a liquid chemical blend. This causes the polymer chains to orientate in the direction of the fibre increasing strength.
Kevlar is expensive and dangerous to manufacture partly because it is dissolved in concentrated sulfuric acid. This is necessary to keep the highly insoluble polymer in solution during synthesis and spinning.
Unlike HDPE high molecular weight polyethylene, the kevlar® molecule is polar. This allows other substances including water to attach themselves to the Aramid. This allows it to be more active chemically than UHMWPE (Dyneema®, Spectra®) It also means it can be bonded for example to epoxy, and it is wettable.
Aramid (such as Kevlar®, Twaron®, Technora® and more) Properties are Impressive!
- Para-aramid fibers such as Kevlar® and Twaron®, which are slightly different, have outstanding strength-to-weight properties, and have high tenacity which makes it difficult to cut or fray.
- High Rigidity Young's modulus (stiffness): 130-179 GPa compared to carbon Fiber 300 GPa and glass 81 GPa, low elongation to Break (does not stretch much).
- Resistant to organic solvents, but sensitive to Chlorine, Some Acids and Bases.
- Good resistance to abrasion and cutting
- Tensile Strength is Slightly less than E glass fiber
- No melting point. Resistant to thermal degradation. Low flammability
- Good fabric integrity at elevated temperatures
- Nonconductive under regular conditions, but can absorb water and salt water.
- Sensitive to degradation from ultraviolet radiation
- Sensitive to shock load. Can fail unexpectedly if subject to shock.
- Relatively low compression strength compared to Carbon fiber.
1-Aramid fibers have High Strength to Weight Ratio
Force per unit area at failure / Density = is the Strength to Weight ratio of this material.
Kevlar® is very strong and is slightly stronger than Carbon Fiber per unit weight.
|MATERIAL||Strength to weight|
|Ultimate Strength MPa||Density g/cm3|
|Spectra® fiber (UHMWPE)||3619||2300-3500||.97|
|E Glass Fibre||1307||3450||2.57|
|E Glass laminate||775||1500||1.97|
|S Glass Fibre||1906||4710||2.47|
|Balsa axial load||521||83||.16|
|Steel alloy ASTM A36||254||400||7.8|
Note: strength and rigidity are different properties, strength is resistance to breaking, rigidity is resistance to bending or stretching
Note also: All the values on this page are for comparison. There are variations between different manufacturing processes, alloys, spiders and wood samples.
Aramid fibres have remarkable strength to weight ratio when compared to other commercial fibres. Aramid fibre exhibits similar tensile strength to glass fibre, but can be twice as stiff. Aramid is very tough showing significant energy absorption but compared to carbon fiber, it is lower in compressive strength and has poorer adhesion to the matrix. It is also susceptible to moisture absorption.
Kevlar® (and other aramids) is subject to stress rupture. This is the sudden failure in a material held for long periods of time under loads which are sometimes considerably less than the tensile strength of the material.
Although Aramid is very strong, a sudden impact can seriously damage it. Climbers who tried Kevlar® ropes learned that a rope that had been subject to a serious shock needed to be replaced since it could not be relied upon to retain its strength. It is not subject to creep as UHMWPE does.
2-Kevlar® (ARAMID) is Much Stiffer than Glass
but Not as Stiff as Carbon Fiber.
The Rigidity of a material is measured by its Youngs' Modulus.
Kevlar® is quite stiff and has low stretching at break.
|Rubber (small strain)||0.01-0.1|
|Low density polyethylene||0.2|
|UHMWPE (such as Dyneena or Spectra)||.7|
|Pine wood (along grain)||8.963|
|Oak wood (along grain)||11|
|Aramid (such as Kevlar and Twaron)||70.5-112.4|
|Brass and bronze||100-125|
|Glass-reinforced plastic (70/30 by weight fibre/matrix, unidirectional, along grain)||40-45|
|Carbon fiber (depends on direction and type)||300-400|
|Carbon fiber reinforced plastic (70/30 fibre/matrix, unidirectional, along grain)||181|
|Single-walled carbon nanotube||1,000+|
3-Aramid Fibers are Chemically quite Resistant.
Aramids are not sensitive to organic solvents or oil.
Because they are resistant to corrosion they can be used in marine environments as stays or antenna guy-wires and dont's corrode in Sea Water. They need to be protected from the sun.
Kevlar®, Twaron® and other Aramids are sensitive to strong acids, bases, and certain oxidizers, like chlorine bleach* (sodium hypochlorite). Exposure to these cause degradation of the fiber. Regular chlorine bleach (e.g. Clorox®) cannot be used with Kevlar®, oxygen bleaches such as sodium perborate (e.g. OxiClean®) can be used without damaging the Aramid fiber. Hydrogen Peroxide cannot be used to whiten Aramid though.
This is important for anyone cleaning boat ropes or sails containing Kevlar. Polyester (Dacron) is less sensitive to bleach but Kevlar's strength will be destroyed! (Dacron will be weakened by prolonged bleach exposure so use only in diluted form and for short periods). Nylon is also attacked by bleach.
4-Aramid Fibers are Abrasion and Cutting Resistant
Kevlar® is often used as protection against cutting. It is used worldwide for bullet proof and armor material. Kevlar gloves are often used to protect against sharp materials in construction industry.
The high abrasion resistance of kevlar helps prevent the whiskers that steel cables can produce when a strand breaks.DuPont kevlar® cut testing PDF
5-Tensile Strength of Kevlar is lower than E Glass
This table is offered as a comparison only since there are a great number of variables.
|Carbon steel 1090||650|
|High density polyethylene (HDPE)||37|
|Stainless steel AISI 302||860|
|Aluminium alloy 2014-T6||483|
|Aluminium alloy 6063-T6||248|
|E-Glass in a laminate||1500|
|Carbon fiber alone||4127|
|Carbon fiber in a laminate||1600|
|Pine wood (parallel to grain)||40|
NOTE: A reader wrote to point out an error in the specs of UHMWPE, which I corrected. She also made a valid point that I was comparing unoriented polymers with oriented fibers. ... Polymers tend to be vastly stronger in their oriented axis. Thank you.
6-Aramid is a Star when it comes to Heat Resistance
Aramid does not melt and is highly resistant to burning and shows Thermal Degradation at 400°F but is not burnt till 500°C. NOTE the different units Degradation of Kelar at 400°Farenheit but but burning at 500° Centigrade, welcome to Canada!
This makes it a very successful material for protective clothing and any fabric that is used near fire. Welding and foundry protective clothing, firefighting equipment and racing suits are a few examples.Modern Apparel makes protective equipment and sells Kevlar® gloves and firefighting equipment.
This makes it a very successful material for protective clothing and any fabric that is used near fire.Guardian Protective Clothing from Calgary Canada makes protective clothing and sells Aramid fiber protective Clothing used for firefighting, racing, oil patch, welding.
Under normal conditions it will not burn or sustain a flame. It is used as an Asbestos substitute
7-Good fabric integrity at elevated temperatures
Because it is so stable in high heat it does not distort at high temperature.
Kevlar® is nonconductive but because it can absorb water, precautions need to be taken. Kevlar® cable covered with waterproof coating supplied by Barry Cordage is used as guy lines for transmission towers and other energized electrical environment
Another advantage of using non conductive guy-wiring, is that unlike steel cabling, there is no electrical or radio interference from Kevlar® and other aramids.
9-Sensitive to degradation from Ultraviolet Radiation
Aramid fibers will degrade in Sunlight and in high UV environment. Protective coatings are applied or it is enclosed in a layer of protective fiber as in some mixed fiber ropes.
Major Uses of Aramid Fibers
- ropes and cables although its severe weakening under impact limits its use on boats and climbing. It is more satisfactory in static load situations. Cables used as guy-wires for hydro tower erection for Hydro Quebec is one application of aramid cable.
- sail cloth (not necessarily racing boat sails)
- flame-resistant clothing
- protective clothing and helmets
- body armor, although it is being replaced by stronger polyethylene products such as Dyneema®.
- as a composite materials it is often combined with carbon fiber.
- asbestos replacement
- hot air filtration fabrics
- tyre and mechanical rubber goods reinforcement
- sporting goods
- speaker woofers
- Its use as a bow string has pretty much been eliminated because it tends to fail fairly quickly and catastrophically.
In Conclusion Kevlar's® and other aramids properties:
Aramid fibres have the highest strength to weight ratio compared to other commercially available fibres. Kevlar® manufactured by DuPont is one familiar brand name. Aramid fibre exhibits similar tensile strength to glass fibre, but can have modulus at least two times as great. Aramid is very tough allowing significant energy absorption but, compared to carbon, it is lower in compressive strength and has poorer adhesion to the matrix. It is also susceptible to moisture absorption. Aramid fibre properties depend on the manufacturing process and can vary quite a lot depending on the intended end use.
Aramid lines for boats
Because of the relatively poor compression strength of aramids, lines other than stays and standing rigging or guy-wires, are rarely used. Special attachment are required because standard grommets weaken the line. When aramid ropes are used, the lines are not stiff and flatten easily, so that when they go through a block the compression is minimized. When a rope goes through a block one side is under tension the other is compressed. This is more marked if the rope is stiff. Larger blocks also help minimise this.
Because of UV degradation aramid lines are usually enclosed in a protective covering.
On a personal note, we once had a kevlar canoe and found that if it hit a rock the kevlar tended to crack.
I try to be accurate and check my figures, but mistakes happen. Check the suitability of any material against the technical information provided by the manufacturer.
Test results can vary considerably from sample to sample, material manufacturing, or environmental conditions. The tables I provide are for comparison, not for planning critical builds. I believe the figures are as accurate as I can find but I did not do the tests myself.
Many of the strength figures I quote come from Wikipedia or from the actual manufacturer. I sometimes make mistakes (!!?!) in transcribing the data.
There are several different varieties of Kevlar®, Twaron® and Nomex®. Although they are all in the general class of Aramids their properties vary within a range. Aramids are also being produced in China under other names.email me if you find mistakes, I'll fix them and we'll all benefit: Christine