- Ultra-high-molecular-weight polyethylene
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. It is highly resistant to corrosive chemicals with exception of oxidizing acids; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating; and is highly resistant to abrasion, in some forms being 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 polytetrafluoroethylene (Teflon), but UHMWPE has better abrasion resistance than PTFE. 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.
UHMWPE fibers, commercialized 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.
Structure and properties
UHMWPE is a type of polyolefin. It is made up of extremely long chains of polyethylene, which all align in the same direction. It derives its strength largely from the length of each individual molecule (chain). Van der Waals bonds between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is bonded to the others with so many Van der Waals bonds that the whole of the inter-molecule strength is high. In this way, large tensile loads are not limited as much by the comparative weakness of each Van der Waals bond.
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, nor 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.
When UHMWPE is annealed , the material is heated to 135 °C to 138 °C in an oven or a liquid bath of silicone oil or glycerine. The material is then cooled down at a rate of 5 °C/h to at least 65 °C. Finally, the material is wrapped in an insulating blanket for 24 hours to bring to room temperature.
UHMWPE is synthesized from monomer of ethylene, which are bonded together to form ultra-high-molecular-weight polyethylene (or UHMWPE). These are molecules of polyethylene that are several orders of magnitude longer than familiar high-density polyethylene due to a synthesis process based on metallocene catalysts. In general, HDPE molecules have between 700 and 1,800 monomer units per molecule, whereas UHMWPE molecules tend to have 100,000 to 250,000 monomers each.
UHMWPE is processed using the following methods: compression molding, ram extrusion, gel spinning, and sintering. Several European companies began compression molding UHMW in the early 1960s. Gel-spinning arrived much later and was intended for different applications.
In gel spinning, a precisely heated gel of UHMWPE is processed by an extruder through a spinneret. The extrudate is drawn through the air and then cooled in a water bath. The end-result is a fiber with a high degree of molecular orientation, and therefore exceptional tensile strength. Gel spinning depends on isolating individual chain molecules in the solvent so that intermolecular entanglements are minimal. Entanglements make chain orientation more difficult, and lower the strength of the final product.
Ballistic vests can be made of UHMWPE.
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 (350,000 psi) and specific gravity as low as 0.97 (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 specific gravity of roughly 7.8, this gives strength-to-weight ratios for these materials in a range from 10 to 100 times higher than steel. Strength-to-weight ratios for Dyneema are about 40% higher than for aramid.
UHMWPE fibers are used in armor, in particular, personal armor and on occasion as vehicle armor, cut-resistant gloves, bow strings, climbing equipment, fishing line, spear lines for spearguns, high-performance sails, suspension lines on sport parachutes and paragliders, rigging in yachting, kites, and kites lines for kites sports. Spectra is also used as a high-end wakeboard line.
For personal 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 utilize a form of Spectra or Dyneema fabric. Dyneema provides puncture resistance to protective clothing in the sport of fencing.
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 yarn 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. "Spec Lines" as they are called in the towboat community are commonly used for face wires.
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.
It is also used in lifting applications for manufacturing low weight, and heavy duty lifting slings .
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.
Medical applications in total joint replacement
UHMWPE has over 40 years of clinical history as a successful biomaterial for use in hip, knee, and (since the 1980s), for spine implants. An online repository of information and review articles related to medical grade UHMWPE, known as the UHMWPE Lexicon, was started online in 2000.
Joint replacement components have historically been made from "GUR" resins. These powder materials are produced by Ticona, typically converted into semi-forms by companies such as Quadrant and Orthoplastics, and then machined into implant components and sterilised by device manufacturers.
UHMWPE was first used clinically in 1962 by Sir John Charnley and emerged as the dominant bearing material for total hip and knee replacements in the 1970s. Details about the "discovery" of UHMWPE for orthopedic applications by Charnley and his engineering associate Harry Craven are available Throughout its history, there were unsuccessful attempts to modify UHMWPE to improve its clinical performance until the development of highly crosslinked UHMWPE in the late 1990s.
One unsuccessful attempt to modify UHMWPE was by blending the powder with carbon fibers. This reinforced UHMWPE was released clinically as "Poly Two" by Zimmer in the 1970s. The carbon fibers had poor compatibility with the UHMWPE matrix and its clinical performance was inferior to virgin UHMWPE.
A second attempt to modify UHMWPE was by high-pressure recrystallisation. This recrystallised UHMWPE was released clinically as "Hylamer" by DePuy in the late 1980s. When gamma irradiated in air, this material exhibited susceptibility to oxidation, resulting in inferior clinical performance related to virgin UHMWPE. Today, the poor clinical history of Hylamer is largely attributed to its sterilisation method, and there has been a resurgence of interest in studying this material (at least among certain research circles). Hylamer fell out of favor in the United States in the late 1990s with the development of highly crosslinked UHMWPE materials, however negative clinical reports from Europe about Hylamer continue to surface in the literature.
Highly crosslinked UHMWPE materials were clinically introduced starting in 1998 and have rapidly become the standard of care for total hip replacements, at least in the United States. These new materials are crosslinked with gamma or electron beam radiation (50-105 kGy) and then thermally processed to improve their oxidation resistance. Five-year clinical data, from several centers, are now available demonstrating their superiority relative to conventional UHMWPE for total hip replacement (see Arthroplasty). Clinical studies are still underway to investigate the performance of highly crosslinked UHMWPE for knee replacement.
Another important medical advancement for UHMWPE in the past decade has been the increase in use of fibers for sutures. Medical-grade fibers for surgical applications are produced by DSM under the "Dyneema Purity" trade name.
UHMW is used in the manufacture of PVC (vinyl) windows and doors, as it can stand up to the heat required to soften the PVC-based materials and is used as a form/chamber filler for the various PVC shape profiles in order for those materials to be 'bent' or shaped around a template.
UHMWPE is also used in the manufacture of Hydraulic Seals and Bearings. It is best suited for Medium mechanical duties in water, Oil Hydraulics, pneumatics, and unlubricated applications. It has a good abrasion resistance but is better suited to soft mating surfaces.
- Cross-linked polyethylene (PEX)
- High-density polyethylene (HDPE)
- Linear low-density polyethylene (LLDPE)
- Low-density polyethylene (LDPE)
- Medium-density polyethylene (MDPE)
- Stretch wrap
- ^ a b c d e f g h i Steven M. Kurtz (2004). The UHMWPE handbook: ultra-high molecular weight polyethylene in total joint replacement. Academic Press. ISBN 978-0-12-429851-4. http://books.google.com/books?id=bkuFjppEdMcC. Retrieved 19 September 2011.
- ^ Hoechst: Annealing (Stress Relief) of Hostalen GUR
- ^ A.J. Pennings, R.J. van der Hooft, A.R. Postema, W. Hoogsteen, and G. ten Brinke (1986). "High-speed gel-spinning of ultra-high molecular weight polyethylene". Polymer Bulletin 16: 167-174. doi:10.1007/BF00955487. http://msc.eldoc.ub.rug.nl/FILES/root/BrinkeGten/1986/PolymBullPennings/1986PolymBullPennings.pdf.
- ^ Tensile and creep properties of UHMWPE fibres
- ^ "Dyneema". Tote Systems Australia. http://www.tote.com.au/dyneema.htm.
- ^ Lightweight ballistic composites: Military and law-enforcement applications. ed. A Bhatnagar, Honeywell International
- ^ Monty Phan, Lou Dolinar (February 27, 2003). "Outfitting the Army of One - Technology has given today's troops better vision, tougher body armor, global tracking systems - and more comfortable underwear" (Nassau and Queens edition ed.). Newsday. pp. B.06.
- ^ Tom Moyer, Paul Tusting, Chris Harmston (2000). "Comparative Testing of High Strength Cord" (PDF). http://www.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf.
- ^ "Cord testing" (PDF). http://www.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf.
- ^ AMSTEEL. samsonrope.com
- ^ a b c d e UHMWPE Lexicon
- ^ Ticona GUR page
- Southern et al., The Properties of Polyethylene Crystallized Under the Orientation and Pressure Effects of a Pressure Capillary Viscometer, Journal of Applied Polymer Science vol. 14, pp. 2305–2317 (1970).
- Kanamoto, On Ultra-High Tensile by Drawing Single Crystal Mats of High Molecular Weight Polyethylene, Polymer Journal vol. 15, No. 4, pp. 327–329 (1983).
- US Patent 5342567 Process for producing high tenacity and high modulus polyethylene fibers, issued 1994-08-30
- Polymer Gel Spinning Machine Christine A. Odero, MIT, 1994
- Patent application 20070148452 High strength polyethylene fiber, 2007-06-28
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