Materials used for Ropes:
Polyester

Polyester is sometimes called Terylene, Dacron, or Trevira, these are trade names for the same material. Polyester is widely used for boat lines and for sail material because it combines good strength and durability, relatively low stretch and reasonable prices.

Polyester fibres are very fine and hair like, typically about 0.023 mm dia. These fibres are almost always white. It is virtually impossible to distinguish between nylon and polyester by appearance alone.

Polyester Physical Properties

Polyester has a specific gravity of 1.38 which means it does NOT float.

Polyester is very strong, it has a similar tensile strength to Nylon 6, and is slightly stronger than regular Nylon.

Unlike Nylon or Polypropylene, Polyester is not stretchy. Usually any stretch present in Polyester is minimized by pre-stretching during the manufacturing process. It also has low creep under load.

Unlike Nylon, Polyester retains its strength when wet.

Melting Point 240 degree Celcius. Varies with different composition

UV resistance is Excellent. Typically a polyester rope will only lose 10% of its breaking strength after 2 years of outdoor use. This is excluding cuts or severe abrasion. Polyester is often chosen as the outside coating for a cored rope made from a stronger but uv sensitive materials such as kevlar.

Excellent abrasion resistance.

Polyester is non conductive to electricity.

Strength and Weight of Polyester Rope

Tensile Strength vs. Working Load

Tensile strength is determined by stretching new rope, UNDER LABORATORY CONDITIONS, until it breaks. This is done several times and an average is determined. Rope manufacturers also use Tenacity as a measurement of tensile strength of the rope.

Working Load: is the ACTUAL LOAD that a rope can be expected to safely and reliably be put under. It is only a fraction of the tensile strength, often only 15% - 25% of the tensile strength, or even less, in critical applications.

Most knots reduce the TENSILE STRENGTH by 40% - 50%. The use of grommets and splices instead of knots can improve the working load.

As the rope age it's strength is reduced by Ultra Violet exposure, oxidation, salt and chlorine, oil, slippage or twisting of the core, twisting of the whole rope, and wear caused by shock loads, and abrasion.

When ropes are chosen, the required SAFETY FACTOR is a major consideration. Rope manufacturers talk of 5:1 safety factor for general use and 10:1 for more critical applications such as climbing ropes. This means that maximum working load should be 1/5 at least or 1/10 or less, of the rope's tensile strength.

If a rope is subject to shock loads then this can cause a weakening of the rope. This is not visible damage but even one strong shock, can seriously reduce the strength of a rope.

Table of Comparative strength of Polyester Rope

https://www.engineeringtoolbox.com/polyester-rope-strength-d_1514.html

This table gives typical strength values compared to the diameter of the rope. Strength will vary from brand to brand, from variations of material, and from type of rope such as twisted or braided. These figures are given as examples. If you have a critical application, check with specific manufacturers.

Units

lbf: The pound-force is equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth. In other words a weight of one pound will exert a force of 1 lbf on a rope.

kgf: The Kilogram force is the force exerted by one kilogram of mass under standard gravity.

Minimum Tensile Strength The maximum amount of tensile stress that a rope can sustain before failure, (breaking or permanent deformation). Tensile strength specifies the point when a material goes from elastic(that is, it can recover it's original shape) to plastic deformation (where the stretch is permanent and will not go back after the load is removed). It is expressed as the minimum tensile stress (force per unit area) needed to split the material apart.

The following table is for double braid polyester rope. Engineering toolbox has strength table for twisted polyester rope

Polyester Chemical Resistance

Good resistance to Alkalis and Acids at room temperature. As the temperature increases, the resistance decreases.

Polyester has excellent resistance to Petroleum based products, Bleaches and Solvents. That doesn't mean it is not affected. Don't soak your lines or sails in strong bleach for long periods, they will be weakened.

Advantages and Disadvantages of Polyester

Advantages

• Good chemical resistance at regular temperatures
• Excellent UV resistance
• Polyester is slightly stronger than Nylon. Unlike nylon it does not lose strength when wet.
• Low Stretch
• Stiffer than Nylon
• Well suited to Static Loads
• Well priced and widely available.

Disadvantages

• Polyester does not float.
• Some braids of polyester are quite stiff and don't work very well in blocks.
• Polyester is not suitable if the load is subject to jerking. It does not give as nylon does. In these applications it might only be used as the outside protective layer.

Polyester Uses

Among the conventional synthetic ropes available Polyester has the lowest stretch and best abrasion resistance, combined with good UV resistance.

It is commonly used for halyards and sheets. for example Novabraid supplies Marine lines: NOVABRAID XLE Double Braid Dacron Sheet Halyard Line 3/8" x 100' white/Blue and 1/2" X 100' Double Braid/Yacht Braid Premium Polyester Halyard Rigging Line

Polyester is the most common readily available rope and sail material. It is often referred to as Dacron or Terylene. Sailrite sells rope along with sailcloth and other sail related materials.

It also finds use for anchor rode. It is stiffer than nylon and not as stretchy.

There have been cases where nylon anchor lines have failed because high temperatures have been generated during strong weather, particularly when it's hot. This caused the nylon anchor/mooring lines to soften and fail. Polyester with it's melting point of 482 F might be a better choice than Nylon which melts at 428 F. A snubber or longer chain might be helpful in absorbing shock forces.

Polyester rope or polyester combinations are often found for winches, capstans and block and tackles where its low stretch and low creep are an advantage.

It is extensively used for cargo lifting straps, tow straps: 4.5 ton polyester tow strap with hooks. where a stretch and bounce effect could destabilize a load.

There are a number of ropes offered that combine polyester outer braid with nylon inner core to get the low stretch and resistance to wear of polyester with the cushioning effect of nylon. It helps control the stretch of nylon in some climbing roopes: Samson Arbormaster Black/Red/White 16 Strand Climbing Rope (1/2") - 150' Hank

It is also combined with aramids again to provide protection and wearability in the outside layer, while the core provides more strength. Many such ropes are marketed as survival rope: Sleeved Spectra Kevlar Cord Durable Orange 100ft 325lbs Strength

The safe loading of a rope is a 1/10 to 1/12 of its breaking strength. Remember that knots weaken a rope as does age, wear, chemical attack etc.

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I try to be accurate and check my figures, but mistakes happen. Check the suitability of any material against the technical information provided by the manufacturer.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine