Ultra high molecular weight polyethylene

Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material, with the highest impact strength (HDPE) with the added traits of being resistant to concentrated acids and alkalis, as well as numerous organic solvents. It is highly resistant to corrosive chemicals except oxidizing acids; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating (see boundary lubrication); and is highly resistant to abrasion, in some forms being 15 times more resistant to abrasion than carbon steel. Its coefficient of friction is significantly lower than that of nylon and acetal and is comparable to that of polytetrafluoroethylene (PTFE, Teflon), but UHMWPE has better abrasion resistance than PTFE.

Development

Polymerization of UHMWPE was commercialized in the 1950s by Ruhrchemie AG, which has changed names over the years. Today UHMWPE powder materials, which may be directly molded into a product's final shape, are produced by, Ticona, Braskem, Teijin (Endumax), Celanese, and Mitsui. Processed UHMWPE is available commercially either as fibers or in consolidated form, such as sheets or rods. Because of its resistance to wear and impact, UHMWPE continues to find increasing industrial applications, including the automotive and bottling sectors. Since the 1960s, UHMWPE has also been the material of choice for total joint arthroplasty in orthopedic and spine implants.

UHMWPE fibers branded as Dyneema, commercialized in the late 1970s by the Dutch chemical company DSM, and Spectra, commercialized by Honeywell (then AlliedSignal) are widely used in ballistic protection, defense applications, and increasingly in medical devices, sailing, hiking equipment, climbing, and many other industries.

Structure and properties

UHMWPE is a type of polyolefin. It is made up of extremely long chains of polyethylene, which all align in the same direction. It derives its strength largely from the length of each individual molecule (chain). Van der Waals forces between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is attracted to the others with so many van der Waals forces that the whole of the inter-molecular strength is high. In this way, large tensile loads are not limited as much by the comparative weakness of each localized van der Waals force.

When formed into fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystallinity from 39% to 75%. In contrast, Kevlar derives its strength from strong bonding between relatively short molecules.

The weak bonding between olefin molecules allows local thermal excitations to disrupt the crystalline order of a given chain piece-by-piece, giving it much poorer heat resistance than other high-strength fibers. Its melting point is around , and, according to DSM, it is not advisable to use UHMWPE fibres at temperatures exceeding for long periods of time. It becomes brittle at temperatures below .

The simple structure of the molecule also gives rise to surface and chemical properties that are rare in high-performance polymers. For example, the polar groups in most polymers easily bond to water. Because olefins have no such groups, UHMWPE does not absorb water readily, nor wet easily, which makes bonding it to other polymers difficult. For the same reasons, skin does not interact with it strongly, making the UHMWPE fiber surface feel slippery. In a similar manner, aromatic polymers are often susceptible to aromatic solvents due to aromatic stacking interactions, an effect aliphatic polymers like UHMWPE are immune to. Since UHMWPE does not contain chemical groups (such as esters, amides or hydroxylic groups) that are susceptible to attack from aggressive agents, it is very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms.

Under tensile load, UHMWPE will deform continually as long as the stress is present—an effect called ''creep''.

When UHMWPE is annealed, the material is heated to between 135 °C and 138 °C in an oven or a liquid bath of silicone oil or glycerine. The material is then cooled down at a rate of 5 °C/h to 65 °C or less. Finally, the material is wrapped in an insulating blanket for 24 hours to bring to room temperature.

Production

''Ultra-high-molecular-weight polyethylene'' (UHMWPE) is synthesized from its monomer ethylene, which is bonded together to form the base polyethylene product. These molecules are several orders of magnitude longer than those of familiar high-density polyethylene (HDPE) due to a synthesis process based on metallocene catalysts, resulting in UHMWPE molecules typically having 100,000 to 250,000 monomer units per molecule each compared to HDPE's 700 to 1,800 monomers.

UHMWPE is processed variously by compression moulding, ram extrusion, gel spinning, and sintering. Several European companies began compression molding UHMWPE in the early 1960s. Gel-spinning arrived much later and was intended for different applications.

In gel spinning a precisely heated gel of UHMWPE is extruded through a spinneret. The extrudate is drawn through the air and then cooled in a water bath. The end-result is a fiber with a high degree of molecular orientation, and therefore exceptional tensile strength. Gel spinning depends on isolating individual chain molecules in the solvent so that intermolecular entanglements are minimal. Entanglements make chain orientation more difficult, and lower the strength of the final product.

Applications

Fiber

Dyneema and Spectra are brands of lightweight high-strength oriented-strand gels spun through a spinneret. They have yield strengths as high as 2.4 GPa (2.4 kN/mm² or 350,000 psi) and density as low as 0.97 g/cm³ (for Dyneema SK75). High-strength steels have comparable yield strengths, and low-carbon steels have yield strengths much lower (around 0.5 GPa). Since steel has a specific gravity of roughly 7.8, these materials have a strength-to-weight ratios eight times that of high-strength steels. Strength-to-weight ratios for UHMWPE are about 40% higher than for aramid. The high qualities of UHMWPE filament were discovered by Albert Pennings in 1968 but commercially viable products were made available by DSM in 1990 an
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Derivatives of UHMWPE yarn are used in composite plates in armor, in particular, personal armor and on occasion as vehicle armor. Civil applications containing UHMWPE fibers are cut-resistant gloves, tear-resistant hosiery, bow strings, climbing equipment, automotive winching, fishing line, spear lines for spearguns, high-performance sails, suspension lines on sport parachutes and paragliders, rigging in yachting, kites, and kite lines for kites sports.

For personal armor, the fibers are, in general, aligned and bonded into sheets, which are then layered at various angles to give the resulting composite material strength in all directions. Recently developed additions to the US Military's Interceptor body armor, designed to offer arm and leg protection, are said to utilize a form of UHMWPE fabric. A multitude of UHMWPE woven fabrics are available in the market and are used as shoe liners, pantyhose,
fencing clothing, stab resistant vests and as composite liners for vehicles.

The use of UHMWPE rope for automotive winching offers several advantages over the more common steel wire. The key reason for changing to UHMWPE rope is improved safety. The lower mass of UHMWPE rope, coupled with significantly lower elongation at breaking, carries far less energy than steel or nylon, which leads to almost no snap-back. UHMWPE rope does not develop kinks that can cause weak spots, and any frayed areas that may develop along the surface of the rope cannot pierce the skin like broken wire strands can. UHMWPE rope is less dense than water, making water recoveries easier as the recovery cable is easier to locate than wire. The bright colours available also aid with visibility should the rope become submerged or dirty. Another advantage in automotive applications is the reduced weight of UHMWPE rope over steel cables. A typical 11 mm UHMWPE rope of 30 metres can weigh around 2 kg, the equivalent steel wire rope would weigh around 13 kg. One notable drawback of UHMWPE rope is its susceptibility to UV damage, so many users will fit winch covers in order to protect the cable when not in use. It is also vulnerable to heat damage from contact with hot components.

Spun UHMWPE fibers excel as fishing line, as they have less stretch, are more abrasion-resistant, and are thinner than the equivalent monofilament line.

In climbing, cord and webbing made of combinations of UHMWPE and nylon yarn have gained popularity for their low weight and bulk. They exhibit very low elasticity compared to their nylon counterparts, which translates to low toughness

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