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Dyneema

From Academic Kids

Dyneema or Spectra is a synthetic fiber, 15 times stronger than steel and three times stronger than Kevlar. It is usually used in bulletproof vests, bow strings and sails on high-performance yachts. Dyneema was invented by DSM in 1979. It has been in commercial production since 1990 at a plant in Heerlen, the Netherlands. In the Far East, DSM has a cooperation agreement with Toyobo Co. for commercial production in Japan. In the United States, DSM has granted a license to Allied Signal. Spectra was developed independently by Allied Signal (now Honeywell) in the USA. Though the production details will undoubtedly be different, the result is comparable to Dyneema. In this article, we will refer to both Dyneema and Spectra as Dyneema

Structure of Dyneema, with n greater than 100,000
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Structure of Dyneema, with n greater than 100,000
Contents

Properties

Dyneema is a type of olefin 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. The fibre 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 or 152 degrees Celsius, and according to DSM, it is not advisable to use Dyneema at temperatures exceeding 80 to 100°C for long periods of time. It becomes brittle at temperatures below -150°C.

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, Dyneema does not absorb water readily, but it also does not wet easily, which makes bonding it to other polymers difficult. For the same reasons, skin does not interact with it strongly, making the fiber surface feel slippery. Similarly, aromatic polymers are often susceptible to aromatic solvents due to phenyl stacking, an effect aliphatic polymers like Dyneema are also immune to. Since Dyneema 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.

Most people do not like the way Dyneema feels. Fabrics made of Dyneema do not offer great dexterity, and will feel slick and quite different from cotton. It is not often used in fabric, for these reasons. Another problem, in some applications, is that Dyneema will creep, meaning it will stretch, and continue to stretch continually, when placed under tensile stress.

Production

Dyneema is synthesized from monomers of ethylene, which are bonded together to form what is called ultra high molecular weight polyethylene (or UHMWPE). These are molecules of polyethylene which are several orders of magnitude longer than familiar, high-density polyethylene due to an exotic synthesis process based on metallocene catalysts. HDPE molecules generally have between 700 and 1,800 monomer units per molecule, while UHMWPE molecules tend to have 100,000 to 250,000 monomers each. The material is also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE). The polymers are aligned randomly when they are produced. To make Dyneema, they are dissolved and drawn into fibers as the solvent evaporates, causing the polymer chains to orient in the direction of the fiber.

The production of Dyneema fibres demands relatively little energy and uses no aggressive chemicals. The product can easily be recycled, so environmental pollution from product and process is minimal.

See also

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