Materials used for Ropes:
Aramid Fibers, such as: Kevlar®, Twaron®, Technora®
Aramid fibers are astonishing
With a startling combination of high strength, high modulus (stiffness), toughness and thermal stability aramid fibrer is (almost) a dream come true for fiber lovers!
Its use on boats and for climbing is limited however by Kevlar's low resistance to shock. Shocking as it might seem, once an Aramid rope has been subject to sharp impact or shock load, it can be seriously compromised without showing any exterior damage. Its use on boats should be limited to static loads.
For a more information see my Kevlar characteristics page
Chemical Structure of Kevlar is the Key to its Outstanding Features
Like UHMWPE the manufacturing process used for Kevlar® aligns the crystal structure in the fiber resulting in exceptional strength.
The long molecule chains contribute to the strength of Kevlar
Unlike UHMWPE, Kevlar (Aramid) is a polar molecule. Being polar means that it will be easier for some substances to bond to it. This makes it more susceptible to chemical attack than UHMWPE but also allows it to be bonded to epoxy. It also means that Kevlar is attracted to water and will wet easily. This also gives it a less slippery feel.
Physical Properties of KEVLAR® (Aramid)
Fibers of KEVLAR® consist of long molecular chains produced from poly-paraphenylene terephthalamide. The chains are highly oriented with strong interchain bonding which result in a unique combination of properties.
- High Cut resistance makes it great for protective clothing.
- Kevlar® has high tensile strength 2920 MPa
- Critical Temperature 400°F, decomposes at 800°F Flame Resistant, Self-Extinguishing
- High Chemical Resistance but is very sensitive to Chlorine
- Low Thermal Shrinkage and it does not become brittle at very low temperatures.
- Has a specific gravity of 1.44
- Subject to UV degradation
- Will absorb moisture
- Excellent Dimensional Stability
- Elongation at break is Low 1.5-4.5%
- No Electrical Conductivity
- High Toughness (Work-To-Break)
- Can be damaged by shock loads (limits its use on boats.)
Tensile Strength of Aramid (Kevlar) ropes.
(lbs. per 100ft.)
|1/8"||3/8"||1,500 lbs.||0.65 lbs.|
Note that reported strength values vary from different manufacturers, Technora is reported to be stronger that Kevlar. I have a page that compares the strength of kevlar and technora ropes
Chemical Properties of Kevlar, Twaron, Technora
Resistant to organic solvents but will degrade in strong acids and bases.
Degrades quickly in Chlorine bleach. Kevlar ropes or sails reinforced with Aramid should never be cleaned with Bleach. Oxy Clean is safe however. Kevlar is also sensitive to Hydrogen Peroxide and it should not be used as a bleach.
Uses for Aramid Rope
Kevlar rope is often substituted for steel cable where its chemical resistance to rusting and lower weight is a real advantage.
Because it degrades in the Sun it is often encased in other protective material. Either with a chemical coating, or a rope is made where the core is aramid and the outside is a UV resistant material.
In Ropes a casing of Polyester is often used. Because Aramid does not resist live loading well, and can fail without warning if it has previously been damaged, it's use is restricted for boat lines. In climbing ropes, it was found that kevlar could be seriously compromised after stopping a fall, which induces a shock load. No sign of damage can be seen but the strength of the rope could be seriously compromized.
Knotting Kevlar can reduce strength by 50% or more so kevlar cables are often terminated with custom fittings to reduce strength loss. Custom fittings are sometimes made of epoxy laminate.
Kevlar ropes are not recommended for use over pulleys or flexing applications. It should not be subject to shock loads.
If an aramid line is to be used in a block or pulley, the diameter of the sheeve must be as large as possible. Ropes which will be asked to go through blocks are braided to be more resistant, either by making them in a very loose braid which flattens when it goes through a pulley, or by special flattened braids.
Kevlar lines often replace steel in winches. Unline dyneema UHMW it is not subject to heat degradation until well over 500 degrees F while dyneema will melt at 300 degrees F and deform way before this. There are arguments both ways because Kevlar aramids don't like shock loads and degrade in sunlight.
Kevlar has been used in a suspension bridge in Scotland and as guy wires for hydro towers.
Because Aramids such as Kevlar and Technora do not conduct electricity, they can sometimes replace steel cables in applications where Electromagnetic interference (EMI) is a factor.
Because of its high resistance to heat, aramid has replaced asbestos in many applications.
Advantages of Aramid
- Very high tensile strength with a high strength to weight ratio. Strength is not affected by water but it can absorb moisture being a polar molecule.
- Great resistance to heat, burning, or thermal degradation
- Not electrically conductive at normal condition, if wet then it can conduct electricity.
- Chemically stable except for strong acids and bases and Chlorine
- Low Stretch
- Tough and well known as difficult to cut or abrade.
Disadvantages of Aramids, Kevlar Twaron
- Does not float
- Quite expensive
- Compressive properties are relatively poor
- Difficult to cut without fraying
- Absorbs moisture
- Kevlar tends to fuzz.
- Kevlar can be invisibly damaged by shock load. It was tried for archery bows and found that it often failed catastrophically after about 1000 uses.
- Kevlar and Technora (and other aramids) require special terminators and attachment to maintain strength.
The safe loading of a rope is often 1/10 to 1/12 of its breaking strength. Remember that knots weaken a rope (50-80% reduction) as does age, wear, chemical attack and UV degradation etc.
NOTE that there are different formulations, manufacturing processes and coatings applied to Aramids. This will change properties somewhat. Waterproofing is commonly applied to kevlar guywires to prevent water absorption and possible electrical conduction of lightning.
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.
Many of the strength figures I quote come from Wikipedia or from the actual manufacturer. I sometimes make mistakes (!!?!) in transcribing the data.email me if you find mistakes, I'll fix them and we'll all benefit: Christine