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 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. 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.

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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. In a similar manner, 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 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.

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 (Quadrant,PPD, Hutchinson, NorthAmerican)
• ram extrusion and fabrication (Quadrant,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]

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.^[3]

Honeywell developed a product identical in chemical structure, which is sold under the brand name Spectra. Though the production details are somewhat different, the resulting materials are comparable in properties. ^[4]

Other trade names for consolidated UHMWPE materials include TIVAR by Quadrant EPP Inc., and Polystone-M by Röchling Engineering 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)^[5]. This gives a strength/weight ratio as much as 15 times stronger than that of steel^{[citation needed]} 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, in general, aligned and bonded into sheets, which are then layered at various angles to give the resulting composite material strength in all directions.^{[6] [7]} 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.^{[citation needed]}

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 yarns 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.^[8][9]

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, which 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.^{[citation needed]} 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, second, the low friction allows the kite to remain controllable up to about ten twists in the line.^{[citation needed]}

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.^{[citation needed]}

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. ^[10] An online repository of information and review articles related to medical grade UHMWPE, known as the UHMWPE Lexicon, was started online in 2000.^[11]

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 ^[10], and then machined into implant components and sterilised by device manufacturers.^[12]

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 ^[11] 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. ^[10]

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.^[10] The carbon fibers had poor compatibility with the UHMWPE matrix and its clinical performance was inferior to virgin UHMWPE. ^[10]

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.^[10] 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).^[11] 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.^[10] These new materials are crosslinked with gamma or electron beam radiation (50-105 kGy) and then thermally processed to improve its oxidation resistance.^[10] Five-year clinical data, from several centers, are now available demonstrating its superiority relative to conventional UHMWPE for total hip replacement (see Arthroplasty).^[11] Clinical studies are still underway to investigate the performance of highly crosslinked UHMWPE for knee replacement.^[11]

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.^{[citation needed]}

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.^{[citation needed]} 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.^{[citation needed]}

Front1 Most high-level short track speed skaters use a cut-resistant suit made of Dyneema or a comparable textile like Kevlar.^{[citation needed]}

Skateboarding

Downhill and trick slide skateboarders and use UHMWPE for the plastic pucks used on sliding gloves.^{[citation needed]}

Hockey

Ice hockey and field hockey sticks against US Patent 5333857 use Dyneema as a material to reinforce the body of the stick.^[13]

See also

• Olefin
• Twaron
• Zylon
• Synthetic fiber
• Linear low-density polyethylene
• Low-density polyethylene
• High-density polyethylene

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. ^ DSM Dyneema Homepage
4. ^ Honeywell Spectra
5. ^ Tensile and creep properties of UHMWPE fibres
6. ^ Tote Systems Australia Dyneema Page
7. ^ Lightweight ballistic composites: Military and law-enforcement applications. ed. A Bhatnagar, Honeywell International, USA
8. ^ Tom Moyer, Paul Tusting, Chris Harmston,(2000) Comparative Testing of High Strength Cord
9. ^ Cord testing
10. ^ ^{a b c d e f g h} Kurtz SM, The UHMWPE Handbook, Academic Press, New York, (2004) isbn:0124298516.
11. ^ ^{a b c d e} UHMWPE Lexicon
12. ^ Ticona GUR page
13. ^ US Patent 5333857 Dyneema reinforced hockey stick

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)) .