The second half of the twentieth century brought chemistry into rope manufacture and with it a wide variety of synthetic rope materials. The fibers are made to any length desired and are built of long chain molecules in various formulas. Most synthetic ropes are at least twice as strong as equal-sized natural fiber ropes, and many are even stronger.

Another advantage to using synthetic ropes is that rot and fungus are not as much of a problem, so the ropes do not have to be dried as thoroughly for storage, and in general they weather much better. This is why they are used almost exclusively in boating and rescue applications. But even though synthetic ropes handle and knot well, the general slipperiness of synthetic cordage makes for less security in knot holding; extra care must be taken that a given knot will hold.

There are other concerns as well. Synthetic ropes are more susceptible to degradation from sunlight than natural fiber materials and are sometimes chemically treated to mitigate this. Also, because of their chemical makeup, most synthetic ropes will melt near heat, and they can even melt from the friction of a knot tightening quickly under a load. Some people like to melt the ends of a rope with a lighter to keep them from fraying. This can also be done as a test to distinguish natural from synthetic ropes, since natural fibers burn and char, but do not melt. You may think that you can distinguish between a natural and a synthetic rope, but that's not always the case. Synthetic ropes come in a wide variety of textures and colors, many of which mimic natural fiber ropes.

Nylon Cordage

The term nylon, like most terms for synthetic materials, actually refers to more than one formula or specific material of similar properties. Nylon cordage is used in cordage of all sizes from large mooring cables to fishing line and thread. It is somewhat elastic, and hence can absorb more energy from shock loading (temporary tension when the rope comes under sudden strain) than other materials. It is one of the stronger synthetics, sinks in water, and is slightly weaker when wet.

Polyester Cordage

Polyester, also known as Terylene and Dacron, is similar to nylon because it's nearly as strong, but it has much less stretch. It has better resistance to acids and sunlight, and also sinks in water. Polyester ropes are often used in boating and other applications where little stretch and less resistance to shock loading are required.

Polypropylene and Polyethylene Cordage

Polypropylene and polyethylene both float in water, and polypropylene is a favored material for water rescue and water ski ropes. They are both weaker than nylon or polyester, but still stronger by weight than natural fiber ropes. They are both very sensitive to heat, with polyethylene being quite flammable. Both must be protected from prolonged exposure to sunlight, and both are used in inexpensive general utility ropes.

Other Options

Many other synthetic materials are in use and continue to be developed. High-performance fiber ropes, such as Kevlar and Spectra, are stronger per weight than steel. These and other varieties of high-modulus fibers, though more expensive, offer unique properties for special applications. Detailed information about these new offerings can be obtained from the manufacturers.

Many activities use a very narrow range of rope materials, and you should refer to information sources on those applications to determine what is used. All the properties of rope materials affect the suitability of ropes for given applications, and with a little knowledge you will be able to take them into account for your cordage needs.