Dyneema

Ultra high molecular weight polyethylene (UHMWPE), also known as high-modulus polyethylene (HMPE) or high-performance polyethylene (HPPE), is a subset of the thermoplastic polyethylene. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million. 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 of any thermoplastic presently made. It is highly resistant to corrosive chemicals, with exception of oxidizing acids. It has extremely low moisture absorption, has a very low coefficient of friction, is self-lubricating, and is highly resistant to abrasion (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 Teflon, but UHMWPE has better abrasion resistance than Teflon. It is odorless, tasteless, and nontoxic.

Polymerisation of UHMWPE was commercialised in the 1950s by Ruhrchemie AG, which changed names over the years; today UHMWPE powder materials are produced by Ticona. UHMWPE is available commercially either as consolidated forms, such as sheets or rods, and as fibers. UHMWPE powder may also be directly molded into the final shape of a product. Because of its resistance to wear and impact, UHMWPE continues to find increasing industrial applications, including the automotive and bottling sectors, for example. Since the 1960s, UHMWPE has also been the material of choice for total joint arthroplasty in orthopedic and spine implants.

UHMWPE fibers, commercialised in the late 1970s by the Dutch chemicals company DSM, are widely used in ballistic protection, defense applications, and increasingly in medical devices as well.

Applications

Dyneema and Spectra are gel spun through a spinneret to form oriented-strand synthetic fibers of UHMWPE, which have yield strengths as high as 2.4 GPa and density as low as 0.97 kg/l (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 density approximately equal to 7.8 kg/l, this gives strength/weight ratios for these materials in a range from 10 to 100 times higher than for steel. Strength to weight ratios for Dyneema are about 40% higher than for Aramid.

UHMWPE fibers are used in ballistic vests, ballistic helmets and ballistic vehicle protection, bow strings, climbing equipment, fishing line, spear lines for spearguns, high-performance sails, suspension lines on sport parachutes, rigging in yachting, kites, and kites lines for kites sports. Spectra is also used as a high-end Wakeboard line.

For body 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 utilise a form of Spectra or Dyneema fabric.

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

In climbing, cord and webbing made of combinations of UHMWPE and nylon yarns have gained popularity for their low weight and bulk, though, unlike their nylon counterparts, they exhibit very low elasticity, making them unsuitable for limiting forces in a fall. Also, low elasticity translates to low toughness. The fiber's very high lubricity leads to poor knot-holding ability, and has led to the recommendation to use the triple fisherman's knot rather than the traditional double fisherman's knot in 6mm UHMWPE core cord to avoid a particular failure mechanism of the double fisherman's, where first the sheath fails at the knot, then the core slips through.

Owing to its low density, ships' hawsers and cables can be made from the fibre, and float on sea water.

It is used in skis and snowboards, often in combination with carbon fiber, reinforcing the fiberglass composite material, adding stiffness and improving its flex characteristics. The UHMWPE is often used as the base layer, which contacts the snow, and includes abrasives to absorb and retain wax.

High-performance ropes for sailing and parasailing are made of UHMWPE, due to their low stretch and high strength. Dyneema was used for the 30-kilometre space tether in the ESA/Russian Young Engineers' Satellite 2 of September, 2007. The extremely low friction coefficient of UHMWPE makes it a common topsheet for boxes in terrain parks.