Ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE), also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE), is a thermoplastic. 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 oxidising acids. It has extremely low moisture absorption, 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 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. 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.

Overview

Structure and properties

UHMWPE 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 degrees Celsius, and according to DSM, it is not advisable to use UHMWPE fibers 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, 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. Similarly, aromatic polymers are often susceptible to aromatic solvents due to aromatic stacking interactions, 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.

Under tensile load, UHMWPE will deform continually as long as the stress is present - an effect called creep.

Annealing

To anneal UHMWPE the material should be heated to 135 °C to 138 °C in an oven or a liquid bath of silicone oil or glycerine. The material must then be cooled down at a rate of 5 °C / hour to at least 65 °C. Finally the material should be wrapped in an insulating blanket for 24 hours to bring to room temperature.[1]

Production

UHMWPE 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 a 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.

Finished UHMWPE is produced by 4 major methods: compression molding, ram extrusion, gel spinning, and sintering. The leading manufacturers of each process UHMWPE in different ways:
  • compression molding (PPD, Hutchinson, NorthAmerican)
  • ram extrusion and fabrication (Garland Manufacturing, Artek)
  • gel spinning - armor and cordage (Dyneema)
  • sintering - Medical (Quadrant, Solus, Perplas)
Dyneema fibers are made using a DSM patented (1979) method called 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.[2]

Capacity

Production capacity of DSM Dyneema is about 5000-6000 tons/yr by 2008 as the 10th production line will be finished in 2008[3], [4]. 5 lines are located in Heerlen, The Netherlands and 5 in Greenville, North Carolina (USA).

Trade Names

Dyneema is a registered trademark of Royal DSM N.V. (The Netherlands). 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 a production facility in Greenville, North Carolina which is the largest production facility in the United States for UHMWPE fiber.[5]

Honeywell developed a chemically identical product, which is sold under the brand name Spectra. Though the production details are somewhat different, the resulting materials are comparable in properties. [6]

Other trade names for consolidated UHMWPE materials include TIVAR by Poly Hi Solidur, and Polystone-M by Rochling Engineered Plastics.

Usage

Fiber applications

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)[7]. This gives a strength/weight ratio as much as 15 times stronger than steel and up to 40% stronger than Aramid.

UHMWPE fibers are used in bulletproof vests, bow strings, climbing equipment, fishing line, spear lines for spearguns, high performance sails, suspension lines on sport parachutes, rigging in yachting, kites and kites lines for kites sports. Spectra is also used as a high-end Wakeboard line.

For body armor, the fibers are generally aligned and bonded into sheets, which are then layered at various angles to give the resulting composite material strength in all directions.[8] [9] Recently developed additions to the US Military's Interceptor body armor, designed to offer arm and leg protection, are said to utilise a form of Spectra or Dyneema fabric.

Spun UHMWPE fibers excel as fishing line as they have less stretch, are more abrasion resistant, and are thinner than traditional monofilament line.

Equipment used for climbing includes cord and webbing made of combinations of Dyneema and nylon yarns. Dyneema "Slings", multi-purpose sewn loops of webbing, 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. Dyneema'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 Dyneema 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.[10][11]

It is also used in both 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, that contacts the snow and is structured with abrasives to absorb and retain wax.

High-performance ropes for sailing and parasailing are made of Dyneema as well. Dyneema is the preferred material for sport kite lines for two main reasons. First the low stretch means that control inputs to the kite are transferred quickly and secondly the low friction allows the kite to remain controllable up to about ten twists in the line.

Dyneema was used for the 30-kilometre space tether in the failed 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 most recently (since the 1980s), for spine implants. [12] An online repository of information and review articles related to medical grade UHMWPE, known as the UHMWPE Lexicon, was started online in 2000.[13]

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 [12], and then machined into implant components and sterilised by device manufacturers.[14]

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 [13] Historically, there were unsuccessful attempts to modify UHMWPE to improve its clinical performance until the development of highly crosslinked UHMWPE in the late 1990s. [12]

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.[12] The carbon fibers had poor compatibility with the UHMWPE matrix and its clinical performance was inferior to virgin UHMWPE. [12]

A second attempt to modify UHMWPE was by high pressure recystallisation. This recrystallised UHMWPE was released clinically as "Hylamer" by DePuy in the late 1980s.[12] 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).[13] 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 has rapidly become the standard of care for total hip replacements, at least in the United States.[12] These new materials are crosslinked with gamma or electron beam radiation (50-105 kGy) and then thermally processed to improve its oxidation resistance.[12] Five year clinical data, from several centers, are now available demonstrating its superiority relative to conventional UHMWPE for total hip replacement (see Arthroplasty).[13] Clinical studies are still underway to investigate the performance of highly crosslinked UHMWPE for knee replacement.[13]

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 currently produced by DSM under the "Dyneema Purity" trade name.

Skis

The bottom of most modern skis—the surface that contacts the snow—is coated with UHMWPE. In this context, the material is known as P-tex. Because the material is a thermoplastic, gouges can easily be filled.

Small Bicycle Parts

Freestyle BMX company Tree Bicycle Co. uses UHMWPE in a guard used to protect a bicycles front sprocket/chainwheel during a stunt in which one grinds on their sprocket guard.

Inline Skating

Aggressive inline skating use UHMWPE for sole plates and frames to assist how well the skates slide when performing grinding tricks

Garland Manufacturing has sponsored both solar cars and inline skating projects with its GARDUR brand of UHMWPE. The company has limited-run capabilities for new applications in sport markets.

Front1 Most high-level short track speed skaters use a cut resistant suit made of Dyneema or a comparable textile like Kevlar.

Skateboarding

Downhill and trick slide skateboarders and use UHMWPE for the plastic pucks used on sliding gloves.

Hockey

Ice hockey and field hockey sticks against US Patent 5333857 use Dyneema as a material to reinforce the body of the stick.[15]

See also

References

1. ^ Hoechst: Annealing (Stress Relief) of Hostalen GUR
2. ^ A.J. Pennings*, R.J. van der Hooft, A.R. Postema, W. Hoogsteen, and G. ten Brinke, High-speed gel-spinning of ultra-high molecular weight polyethylene, Polymer Bulletin 16, 167-174 (1986)
3. ^ Dyneema capacity 400 or 600 tons/yr per production line
4. ^ 2008 expansion to ten lines
5. ^ DSM Dyneema Homepage
6. ^ Honeywell Spectra
7. ^ Tensile and creep properties of UHMWPE fibres
8. ^ Tote Systems Australia Dyneema Page
9. ^ Lightweight ballistic composites: Military and law-enforcement applications. ed. A Bhatnagar, Honeywell International, USA
10. ^ Tom Moyer, Paul Tusting, Chris Harmston,(2000) Comparative Testing of High Strength Cord
11. ^ Cord testing
12. ^ Kurtz SM, The UHMWPE Handbook, Academic Press, New York, 2004.
13. ^ UHMWPE Lexicon (medical applications)
14. ^ Ticona GUR page
15. ^ US Patent 5333857 Dyneema reinforced hockey stick

External links

Further reading

  • Southern et al., The Properties of Polyethylene Crystallized Under the Orientation and Pressure Effects of a Pressure Capillary Viscometer (1990), John Wiley & Sons, Inc. 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)) .
"Polythene" redirects here. For the Feeder album, see Polythene (album).


Polyethylene (IUPAC name polyethene) is a thermoplastic commodity heavily used in consumer products.
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A thermoplastic is a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently.
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molecular mass (abbreviated Mr) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12).
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The coefficient of friction is a dimensionless quantity used to calculate the force of friction (static or kinetic).

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Abrasion may mean:
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Carbon steel, also called plain carbon steel, is a metal alloy, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are manganese (1.
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Nylon is a generic designation for a family of synthetic polymers first produced on February 28, 1935 by Wallace Carothers at DuPont. Nylon is one of the most common polymers used as a fiber.
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Polyoxymethylene (POM), also known as polyacetal, acetal resin, polytrioxane, polyformaldehyde, and paraformaldehyde, is an engineering plastic used to make gears (for vehicles) bushings and other mechanical parts (cf.
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Teflon is a trademark of DuPont and is commonly used for the chemical compound polytetrafluoroethylene.

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Ticona is an international manufacturer of engineering polymers. The company's head office has been located at the Ticona/Celanese-Plant in the German town of Kelsterbach near Frankfurt since 2002.
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Arthroplasty (literally "formation of joint") is an operative procedure of orthopaedic surgery performed, in which the arthritic or dysfunctional joint surface is replaced with something better or by remodeling or realigning the joint by osteotomy or some other procedure.
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Orthopedic surgery or orthopedics (also spelled orthopaedics, see below) is the branch of surgery concerned with acute, chronic, traumatic, and overuse injuries and other disorders of the musculoskeletal system.
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vertebral column (backbone or spine) is a column of 34 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs. It houses the spinal cord in its spinal canal.
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Royal DSM N.V.

Public (Euronext: DSM )
Founded 1902
Headquarters Heerlen, the Netherlands

Key people Feike Sijbesma, chairman of the managing board
Industry Chemicals
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A polyolefin is a polymer produced from a simple olefin (also called an alkene) as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene.
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van der Waals force is sometimes used as a synonym for the totality of non-covalent forces (also known as intermolecular forces). These forces, which act between stable molecules, are weak compared to those appearing in chemical bonding.
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"Polythene" redirects here. For the Feeder album, see Polythene (album).


Polyethylene (IUPAC name polyethene) is a thermoplastic commodity heavily used in consumer products.
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Tensile stress (or tension) is the stress state leading to expansion; that is, the tensile stress may be increased until the reach of tensile strength, namely the limit state of stress.
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The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
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Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale
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brittle if it is liable to fracture when subjected to stress. That is, it has little tendency to deform (or strain) before fracture. This fracture absorbs relatively little energy, even in materials of high strength, and usually makes a snapping sound.
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Chemical polarity, also known as bond polarity or simply polarity, is a concept in chemistry which describes how equally bonding electrons are shared between atoms.
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Wetting is the contact between a fluid and a surface, when the two are brought into contact. When a liquid has a high surface tension (strong internal bonds), it will form a droplet, whereas a liquid with low surface tension will spread out over a greater area (bonding to the
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Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone.
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Stacking in supramolecular chemistry refers to a stacked arrangement of aromatic molecules, which interact through aromatic interactions. The most popular example of a stacked system is found for consecutive base pairs in DNA.
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In chemistry, aliphatic compounds are organic compounds in which carbon atoms are joined together in straight or branched chains or in rings, that can be either saturated or unsaturated, but not aromatic.[1] The simplest aliphatic compound is methane (CH4).
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Esters are a class of chemical compounds and functional groups. Esters consist of an inorganic or organic acid in which at least one -OH (hydroxy) group is replaced by an -O-alkyl (alkoxy) group.
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amide is one of two kinds of compounds:
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