Page 24 - The Pocket Guide to Outdoor Knots

customers could afford. There are two grades, nylon 6,6 (discovered in the Du
Pont laboratories) and nylon 6 (subsequently developed by I.G. Farbenindustrie),

the numbers indicative of their respective molecular structures. The main
advantage of nylon ropes is that they stretch under load, by anything from 10–

40%, regaining their original length when the load is removed. This makes them
the obvious choice for tow-lines, mooring lines and any other use—such as

climbing—where the energy of a sudden fall or other abrupt shock loading may
have to be safely absorbed.

Polyester (sold as Terylene™ or Dacron™) came next, a British
development resulting from research at the Calico Printers Association, the sole

rights of which were taken up by Imperial Chemical Industries. It is about 75%
as strong as nylon, but equally so wet or dry. It does not stretch half as much,

and pre-stretching during manufacture can eliminate most of what latent
elasticity it does possess. Consequently it is ideal for any sort of task, such as to

replace wire stays or guy-lines, where high tensile strength must be coupled with
an absence of noticeable stretch.

Polyethylene (Polythene) appeared in the late 1950s, when the simple
hydrocarbon ethylene was incorporated into a rope making fiber. It was a

cheaper product than either nylon or Terylene, hard-wearing and durable; and,
although the ropes made from it were not as strong as the other two, it was still

superior to the traditional vegetable fiber yarns. With a density less than that of
water, polyethylene was the first synthetic fiber to float. Practical enough for use

in the fishing industry and for agricultural baling twines, it is less suitable for

general-purpose knot tying.
Polypropylene represented the major break-through in synthetic fibers,
however, when it was discovered that it could be polymerized (made into long

molecular chains) to create a fiber considerably stronger and even lighter than

polyethylene and extruded from low-cost machines. The raw material (a by-
product of the oil industry) was very cheap, so that synthetic ropes made from it

could at last compete with sisal and even manila on almost equal terms. It soon
became available in a variety of forms, ranging from filaments to split-films

which could be twisted into yarns to make a variety of cordage. Polypropylene