UHMWPE: gel spinning

Ultra high molecular weight polyethylene (UHMWPE) fabricated as tethers using a technique known as gel spinning creates a material which is 40 times stronger than kevlar and 100 times greater strength-to-weight ratio than steel cables. By weaving together multiple layers fibrous layers of high strength polyethylene tensile strengths greater than steel are able to be achieved. The project is most notably known for its employment as tethering cables which hold ultra-high inflatable wind turbines which have recently been designed and tested. These tethering cables, unlike steel, meet the requirements of both the tensile strength required to hold such a weight and the lightness needed to allow the turbine to float in the atmosphere. Fabrication techniques such as these begin to break away from the traditional industrialized methods currently in place, and begin to look towards bottom-up approaches to create higher strength materials. The weaving of fibrous tissues is often found biology and it will be beneficial to find a material and building method for our architecture which mimics and recreates these systems, consuming less energy and allowing for more efficient building technologies.-JB

Ultra high molecular weight polyethylene (UHMWPE or sometimes shortened to UHMW), 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.[citation needed] 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, Braskem, and Mitsui. 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.[1]

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.

HMWPE is a type of polyolefin, and, despite relatively weak Van der Waals bonds between its molecules, derives ample strength from the length of each individual molecule. It is made up of extremely long chains of polyethylene, which all align in the same direction. Each chain is bonded to the others with so many Van der Waals bonds that the whole can support great tensile loads.

When formed to fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystallinity of up to 85%. 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 144 to 152 °C (291 to 306 °F), and, according to DSM, it is not advisable to use UHMWPE fibers at temperatures exceeding 80 to 100 °C (176 to 212 °F) for long periods of time. It becomes brittle at temperatures below −150 °C (−240 °F).

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, but it also does not get 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.

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)[6]. 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-to-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.-Wikipedia


Categories: Fabrication, Research, Technology


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