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DyneemaDyneema or Spectra fiber is expensive and extremely high-performance. Fifteen times stronger than steel, it finds use in such applications as bulletproof vests, bow strings, and the sails of high-performance yachts. Made by a solvent-assisted drawing process from extremely high molecular weight polyethylene, its polymer chains are several orders of magnitude longer (that is, they have more repeat units) than familiar high-density polyethylene due to an exotic synthesis process based on metallocene catalysts. The material is also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE), and is of a class of chemical known as olefins. Olefin vs. aramid Its strength is comparable to that of kevlar, but its density and resistance to high temperature are both noticeably lower. This suggests that it will eventually replace Kevlar and other aramids in high-strength applications, but not in high-temperature environments. It also lacks kevlar's sensitivity to alkaline environments (and so might be used in concrete, etc.), but has much lower surface energy and may be harder to wet in composite applications. Aramid bonding These contrasts can all be explained by comparing the directionality of bonding in the two materials. Aramid fibers have relatively short chains, but rely on strong bonds (π-bonding between aromatic rings & polar bonds between amide groups) between molecules to link aligned chains into a single fiber. These bonds tend to lock the molecules into a crystalline structure and make melting impossible, but the presence of certain ions (notably calcium) weakens them. Strong unsatisfied bonds at the surface make the fiber feel "sticky". Olefin bonding By contrast, each molecule in Spectra contains so many monomers that even weak Van der Waals bonding, when averaged over such a long chain length, can be enough to hold the fiber together. However, thermal excitations are local, and can disrupt the crystalline order of a given chain piece-by-piece, giving it a melting point of only 165 degrees Celsius. The weak bonding at the sides of the chain means that the edges of a crystal form and break easily, making the fiber surface feel "slippery". Reference: #Comparison of rope materials.
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