Materials used for Ropes:
High Modulus Polyethelyne HMPE, Dyneema®, Spectra®

HMPE has many names

HMPE has many names: high-performance polyethylene (HPPE) Ultra-high-molecular-weight polyethylene (UHMWPE or sometimes shortened to UHMW), also known as high-performance polyethylene (HPPE).

In the sailing and boat world it is mainly known by its trade names: Dyneema®, Spectra®, and sometimes as Novabraid®.

Chemical Structure of High Molecular Weight Polyethylene is the Key to its Outstanding Features

It has extremely long molecule chains with no groups sticking out of the molecule. This gives no easy attack point for chemical interaction and attack. The long molecule are extremely strong particularly after being aligned by stretching.

Because of its lack of interaction with other substances, it is not attracted to water, doesn't stick to anything including hands, and resists attachment by micro organisms.

Gel spinning is used to manufacture Spectra® and Deneema fibers. There are 3 main steps.

1. The UHMW-PE is transformed into a gel using a solvent.
2. The fiber is extruded pulled, or otherwise spun from the HMPE solution and some solvent is evaporated out.
3. The rest of the solvent is removed while the Polyethylene fiber is stretched many times its initial length

A very Good article from MIT on HMPE worth reading if you are interested in the structure and chemical properties.

Physical Properties of HMPE, (UHMWPE)

Manufacturers of ropes for boat use and general marine applications wax poetic and can only speak in superlatives when describing HMPE!

• It does NOT absorb water, in fact Polyethylene is Hydrofuge and repels it. This means that it never absorbs water.
• Dyneema® and Spectra® are not weakened when wet as nylon is.
• Its melting point is around 144 to 152 °C (291 to 306 °F) This is quite low compared to many ropes intended for boat use.
• HMPE feels very slippery to the hand.
• Has a density of as little as .95 and floats on Sea Water
• Polyethylene is subject to creep under continuous load.
• It is stong. It is twice as strong as hardened steel (per unit area)
• HMPE has a very low coefficient of friction. In this it is similar to Teflon which is very slippery.
• Great abrasion Resistance (better than Teflon) in some forms it is 15 times more resistant to abrasion than carbon steel
• UHMW-PE is self-lubricating
• UHMW-PE has good UV resistance.
• Low Elasticity. Unlike Nylon UHMWPE does not stretch much.

High-strength steels have comparable yield strengths, and low-carbon steels have yield strengths much lower. BUT since steel has a specific gravity of roughly 7.8, this gives strength-to-weight ratios for these materials in a range from 10 to 100 times higher than steel. Strength-to-weight ratios for Dyneema® are about 40% higher than for aramid such as Kevlar and Twaron.

Chemical Properties of HMPE, (UHMWPE)

Since UHMWPE does not contain chemical groups such as esters, amides or hydroxylic groups that are susceptible to attack from aggressive agents it is not very active chemically.

Besides making the molecule very inert to most chemical attack, (It is slightly suseptible to oxydizing acids), it repels water.

Uses for HMPE, (UHMWPE)

UHMWPE has one of the highest strength to weight ratios of any manmade fibers. It has low weight (It floats in Seawater) and high uv and chemical resistance

Excellent vibration damping, flex fatigue and internal fiber-friction are other features of High molecular weight polyethylene

It is used in high-performance sails and rigging in yachting. Having low stretch lets the sails retain an optimal shape and a brilliant white appearance plus exceptional resistance to abrasion. The sails’ useful life exceeds that of other materials—many times over!

UHMWPE is also used in Body Armour personal and vehicle armor, cut-resistant gloves and clothing, climbing equipment, fishing line, spear lines for spearguns and bow strings, suspension lines on sport parachutes and paragliders, kites, and kites lines for kites sports. Spectra® is also used as a high-end wakeboard line.

Because the line floats in Seawater it is used for Ships mooring lines and as tow lines for boats of every size. It is considerably lighter than steel of equal weight so it often replaces metal ropes.

It is used for lifting slings and cables.

Spun UHMWPE fibers excel as fishing line, as they have less stretch, are more abrasion-resistant, and are thinner than traditional monofilament line. Special knots and fastening techniques are often recommended. Fishing knots in Spectra® from Charkbait

UHMWPE is used as arip-stop reinforcement for luggage fabric, and as fishing and fish farming nets.

Dyneema® was used for the space tether in the ESA/Russian Young Engineers' Satellite 2.

Advantages of UHMWPE

• Very Strong with a high strength to weight ratio. (Strengths as high as 350,000 psi). Stength is not affected by water
• Light enoug to float. (Some mixtures do not float)
• UV Stable
• Low electrical conductivity
• Chemically inert except for strong oxydizing acids
• Smooth and slippery, self lubricating
• Resistant to fatigue, internal friction and abrasion
• Low Stretch, vibration damping
• Transparent to radar

Disadvantages of UHMWPE

• Low Melting point 144 to 152 °C (291 to 306 °F) and even lower recommended use (less than 80°C)
• Subject to Creep under tensile load. Various mixtures help address this problem in rigging applications.
• Because it is so slippery it does not hold knots very well. The triple fisherman's knot has been recommeded by climbers. All about Knots discusses knot strength.
• Rope layers can distort and twist because of the low friction.
• It is difficult to cut smoothly because it is so slippery and resistant.
• 4-5 times more expensive than polyester ropes.

The safe loading of a rope is often 1/10 to 1/12 of its breaking strength. Remember that knots weaken a rope (50-80% reduction) as does age, wear, chemical attack etc.

Dyneema® and Spectra® Characteristics

NOTE that because of different formulations and manufacturing process of raw material characteristics such as weight, strength and melting point vary. Other factors such as manufacturing of rope, core composition, sheath composition or number of strands can have large effect on the way the rope handles. Keep in mind also that modern high performance ropes are often combinations of fibers rather than single fiber composition. In this way the benefits of each fiber can be incorporated.

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.

Many of the strength figures I quote come from Wikipedia or from the actual manufacturer. I sometimes make mistakes (!!?!) in transcribing the data.