The plastic that floats yet outpulls steel eight times over by weight
Ultra-high-molecular-weight polyethylene is chemically ordinary polyethylene, only with chains stretched to millions of daltons. Spun into fibres like Dyneema, it becomes light enough to float on water yet strong enough for body armour, with a strength-to-weight ratio about eight times that of high-strength steel.
Ordinary high-density polyethylene has somewhere between 700 and 1,800 repeating units per molecule. The ultra-high variety packs 100,000 to 250,000, giving a molecular mass of roughly 2 to 6 million daltons. The attraction between neighbouring chains is weak atom for atom, but because the chains are so long, the overlaps are enormous and add up to a material that shrugs off shear. It has the highest impact strength of any thermoplastic; in standard notched Izod tests the samples often simply refuse to break.
Its chemistry is plain, and that is its trick. Lacking the polar groups that let most polymers bond with water, it barely absorbs moisture, resists wetting and feels slippery to the touch, with friction comparable to Teflon but better resistance to abrasion. The same plainness makes it hard to glue to anything. Weak points include heat, with melting starting at 127.7 degrees Celsius, and creep: under steady tension it keeps slowly stretching.
Ruhrchemie commercialised the polymer in the 1950s, and since the 1960s it has been the preferred bearing material in joint replacement implants. The fibres arrived later. DSM launched Dyneema in the late 1970s and Honeywell sells Spectra, both made by gel spinning, in which a dilute suspension of about 5 percent polymer is forced through a spinneret and then drawn through ovens that can exceed 200 metres to align the chains. The conventional process is thirsty: around 20 kilograms of oil for every kilogram of yarn, and up to 200 kilograms of hexane to remove it. A newer route using supercritical carbon dioxide reportedly cuts the carbon footprint by more than half.
The finished fibre turns up in armour plates, cut-resistant gloves, climbing gear, bow strings, paraglider lines and kite lines. Winch ropes made from it store far less energy than steel cable, so they barely snap back if they break, and they float, which makes recovering them from water easier. Anglers like its thinness and low stretch.
Source: Ultra-high-molecular-weight polyethylene
