Knots/Synthetic fibre

Synthetic fibres include polypropylene, nylon, polyesters (e.g. PET, LCP, HPE, Vectran), polyethylene (e.g. Spectra), aramids (e.g. Twaron, Technora and Kevlar) and polyaramids (eg Dralon, Tiptolon). Some ropes are constructed of mixtures of several fibres or use co-polymer fibres.

Here, we will talk about the generic kinds of synthetic cordage — if you require specific information on a particular fiber, you should consult the spec sheet available from the retailer.

Strength
Synthetic fibres are stronger than natural fibres, so they are preferred for high-performance applications such as climbing. As well, they are resistant to rot and mildew, and aren't edible so rodents will not eat the cordage.

Water
Nylon loses about 15% of its strength when wet, which is recovered again when dry. Polyester is slightly weaker than nylon, but retains its strength whether wet or dry. Polypropylene degrades when exposed to UV radiation, unless specifically treated.

Elasticity
Nylon stretches, which means it will absorb some of the energy of sudden loading — this means there is a lesser risk of breakage from suddenly overloading. This makes it suitable as a tow rope, certain climbing ropes (though not all), for mooring boats, and fishing lines. These applications require cordage which can withstand shock-loading. Nylon returns to its original length after unloaded.

Polyester is not elastic — what little elasticity it has is removed during the manufacturing process. This makes it ideal for applications where shock-loading is unlikely and stretch is undesirable. One such application is on sailboats: the shrouds, stays, and other standard vertical rigging as well as halyards, sheets and other running rigging through blocks, tackles or purchases.

Polypropylene is weaker still, but is very cheap, and floats. This makes it ideal for non-technical applications such as rescue lines and ski tow lines.

Specialty synthetics

 * Kevlar, Twaron and Technora
 * These are aramid derivatives


 * Spectra and Dyneema
 * HMPE or high modulus polyethylene (UV-stable)


 * Vectran
 * LCP or poly(p-phenylene-3,6 benzobisoxazole)

These synthetics are quite expensive, but have higher tensile strength than steel. Some have poor resistance to abrasion, low flex fatigue or are UV-unstable. However, when you buy cordage, they will normally have a polyester sheath which protects the inner cordage. These types of cordage are suitable for technical applications — check the specs for further information.