Twisted composite rope is made by combining two or more fiber – or polymer – types.
In some cases two separate yarn types are used, in other cases the polymers are combined in liquid form and extruded into a yarn with the desired combination of properties.
The most popular of this type for use by specialty contractors is a polyester/polyolefin combination, often referred to as ‘poly dac’, ‘poly dacron’ or ‘poly-plus’. It is made by covering polyolefin yarns with polyester yarns to create a strand, then twisting three of these strands together to form the rope. The resulting product is less expensive than 100 percent polyester, nearly as strong, lighter in weight, and with polyester’s superior resistance to sunlight. Poly dac is used in lifelines by window washing contractors, tree trimming contractors, and others requiring fall protection, as guy lines, and in a number of hoisting operations.
It holds up better than nylon when exposed to sunlight, and is almost as strong. And it has better dielectric properties (is less likely to conduct current) than nylon. And in some diameters, because it has a good ‘hand’ and doesn’t shed as natural fiber products do, it is used in place of Manila in undulating and other exercise applications.
On the other hand, it does not absorb shock loads as well as nylon and its dielectric properties are not as good as those of polypropylene. Another popular composite rope, designed for use as an arborist climbing rope, is one made with nylon yarns inside polyester strands.
Much of the cotton rope available today is not really cotton but a cotton and spun polyester composite. Most sash cords and clotheslines are these poly-cotton blends. So if 100% cotton is required, it’s a good idea to be very specific when ordering.