Aramid Fibers, trade names Kevlar, Twaron, Nomex

Aramid fibers are another group of super-heros of the fiber world. Kevlar and Twaron, carbon fiber, Dyneema and Spectra 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 disolved 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 themselve 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 (Kevlar, Twaron, Technora) Properties are Impressive!

1. Para-aramid fibers such as Kevlar and Twaron, which are slightly different, have outstanding strength-to-weight properties, and have high tenacity
2. High Rigidity Young's modulus (stiffness): 130-179 GPa compared to carbon Fiber 300 GPa and glass 81 GPa
3. Resistant to organic solvents, but sensitive to Chlorine, Some Acids and Bases.
4. Good resistance to abrasion and cutting
5. Tensile Strength is Slightly less than E glass fiber
6. No melting point. Resistant to thermal degradation. Low flammability
7. Good fabric integrity at elevated temperatures
8. Nonconductive under regular conditions, but can absorb water and salt water.
9. Sensitive to degradation from ultraviolet radiation

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.

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 have be twice as stiff. Aramid is very tough showing significant energy absorption but when, 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 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.

Material	Young's Modulus GPa
PTFE (Teflon)	0.5
Rubber (small strain)	0.01–0.1
PTFE (Teflon)	0.5
Low density polyethylene	0.2
UHMWPE (such as Dyneena or Spectra)	.7
Polypropylene	1.5-2
Nylon	2–4
Pine wood (along grain)	8.963
Oak wood (along grain)	11
Aluminium	69
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
Steel	200
Single-walled carbon nanotube	1,000+

3-Aramid Fibers are Chemically quite Resistant.

Aramids are not sensitive to organic solvents or oil.

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 not particularly sensitive to bleach but Kevlar's strength will be destroyed! Nylon is also attacked by bleach.

Here are laundry instruction for Kevlar protective gloves.

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.

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.

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.

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 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 and sells Nomex 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.

8-Non Conductive

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

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 guywires for hydro tower erection for Hydro Quebec is one application of Kevlar 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
• tire and mechanical rubber goods reinforcement
• sporting goods
• drumheads
• speaker woofers
• Its use as a bow string has pretty much been eliminated because it tends to fail fairly quickly and catastrophically.

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.

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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 comarison, 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