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Materials used for Ropes; Polypropylene

Polypropylene is one of the three most common rope materials besides nylon and polyester. It has some stellar qualities that make it a good choice.

Polypropylene (PP) Physical Properties

Polypropylene is a thermoplastic polymer (turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently). it is rugged and unusually resistant to solvents, bases and acids.

It has a melting point of 170°Centigrade. Its usual appearance is as a coarse bristle, or tape like coloured fiber, often yellow, black, or orange. It is sometimes also offered as fine white fiber.

Polypropylene is normally tough and flexible. Polypropylene is reasonably economical, and can be made translucent (although usually not for rope) when uncolored but it is often opaque or colored using pigments. Polypropylene has good resistance to fatigue and is often found in live hinges on plastic containers with built in lids.

Polypropylene has a specific gravity of .91 (water is 1) and so it is lighter than water and FLOATS.

Because it floats polypropylene is the rope of choice when used in the water. It is used for water-ski ropes and other small ropes used for utilitarian purposes. It is used extensively in the fishing industry. Because it floats it is not likely to get tangled in a motor prop.

Polypropylene is not one unique material but rather a range of plastics with a range of characteristics depending on the crystal form and the exact chemical composition.

The melting point and flow rate depends on the molecular weight. Better flowing polypropylene might be desirable for moulding but at the cost of impact strength.

Polypropylene Rope Strength

Diameter of Rope (Minimum) Breaking Strength Safe Load
(Safety Factor=12)
Weight of Rope
(in) (mm) (lbf) (kN) (lbf) (kN) (lbm/ft) (kg/m)
1/4 6 1125 5.00 93.8 0.42 0.01 0.02
5/16 8 1710 7.61 143 0.63 0.02 0.03
3/8 10 2430 10.8 203 0.90 0.03 0.04
7/16 11 3150 14.0 263 1.17 0.04 0.05
1/2 12 3780 16.8 315 1.40 0.05 0.07
9/16 14 4590 20.4 383 1.70 0.06 0.09
5/8 16 5580 24.8 465 2.07 0.07 0.11
3/4 18 7650 34.0 638 2.84 0.10 0.15
7/8 22 10350 46.0 863 3.84 0.14 0.21
1 24 12825 57.0 1070 4.76 0.18 0.27

Polypropylene Degradation

Polypropylene is liable to chain degradation from exposure to heat and UV radiation such as that present in sunlight. Degradation shows up as a network of fine cracks and crazes that become deeper and more severe with time of exposure.

Rope becomes chalky looking and as strands on the outside break, it becomes fuzzier and discouloured.

For external applications, UV-absorbing additives are be used. Dyes and Carbon black provides protection from UV damage. Polypropylene can also be oxidized at high temperatures, a common problem during moulding operations. Anti-oxidants are normally added to prevent polymer degradation.

Polypropylene Rope Styles

Polypropylene rope is made either from continuous monofilament similar to but slightly thicker than polyester and nylon. It is also chopped into shorter strands which get twisted like natural fibers.

It is sometimes a thicker monofilament, resembling straw or bristles, typically 0.1 to 0.15 mm dia. In this form it may either be a continuous fibre, or it may be cut into short lengths and then processed like natural fibres to form staple yarn. Canada Cordage sells twisted polypropylene rope in this form. It can be used where the look of natural rope is desirable but the advantages of synthetic are useful.

Another form of polypropylene resembles a thin tape, typically 0.06 to 0.1 mm thick. The tape is sometimes twisted so it appears to be a circular fibre. This tape may be split so it appears to be a collection of small flat fibres which cling to each other.

Monofilament polypropylene ropes are often black, yellow or orange. Colouring helps prevent UV degradation. It is also possible to get white monofilament polypropylene that has some other form of UV protection in white. Polypropylene nylon and polyester fibres are almost impossible to separate by appearance but polypropylene is usually slightly thicker and stiffer.

Advantages and Disadvantages of Polypropylene Rope

Advantages

Disadvantages

Polypropylene is sometimes added to ropes in combination with other fibers to make a rope that floats, but has better strength and UV resistance.


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