Ultra high molecular weight polyethylene
From Wikipedia, the free encyclopedia
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, very low coefficient of friction, is self lubricating and is highly resistant to abrasion (10 times more resistant to abrasion than Carbon Steel). Its coefficient of friction is significantly lower than nylon and acetal, and is comparable to teflon, but UHMWPE has better abrasion resistance than teflon. It is odorless, tasteless, and nontoxic.
Polymerization of UHMWPE was commercialized 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, commercialized in the late 1970s by DSM, are widely used in ballistic protection, defense applications, and increasingly, in medical devices as well.
Contents |
[edit] Overview
[edit] 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 or 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.
[edit] Chemistry and properties
For details, see the properties section of Ultra high molecular weight polyethylene.
Dyneema fibers derive their strength from the extreme length of each individual molecule. 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.
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. This contrasts strongly with other high-performance fibers, which tend to be quite heat-resistant.
The fibers feel slippery, similar to polypropylene and other hydrophobic fibers. Most people do not like the way Dyneema feels; for this reason, it is not often used in fabric. The slipperiness also makes such fibers less suitable for use in fibre reinforced plastics.
Another problem, in some applications, is that Dyneema will creep, meaning it will deform when placed under any long term stress. Like other olefins, it is very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms.
[edit] 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]
[edit] 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.
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.
[edit] Usage
[edit] Fiber applications
Dyneema or Spectra is a synthetic fiber based on ultra high molecular weight polyethylene which is 15 times stronger than steel and up to 40% stronger than Aramid. It is usually used in bulletproof vests, bow strings, climbing equipment, fishing line and high performance sails, rigging in yachting,kites and kites lines for kites sports. 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. Honeywell has developed a chemically identical product on its own. The Honeywell product is sold under the brand name Spectra. Though the production details will undoubtedly be different, the resulting materials are comparable. This article refers to both materials by the name Dyneema. Dyneema is a registered trademark of Royal DSM N.V. (The Netherlands).
Other trade names for consolidated UHMWPE materials include TIVAR by Poly Hi Solidur, and Polystone-M by Rochling Engineered Plastics.
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.
Both Spectra and Dyneema excel as fishing line as they have less stretch, are more abrasion resistant, and are thinner than traditional monofilament line.
In recent years certain items of climbing equipment have started making use of Dyneema. In particular "slings", sewn loops of material that can be wrapped around sections of rock, hitched (tied) to other pieces of equipment or even tied directly to a tensioned line using a special prussik knot, have benefited from this material. It has limited applications however as items made from this material do not stretch and therefore a fall on them involves considerable shock loading of the other pieces of equipment and the climber's body. They are however much lighter and finer than the alternatives (nylon) and therefore are very popular. Usually sold in lengths of 10, 30, 60, 120, or 400 cm at either 8, 10 or 12 mm width, these slings have a breaking strength of around 22 kN. Cordelettes and prussik loops made from Spectra accessory cord are popular as well. Because Spectra is so slippery, the recommended knot for these applications is a triple fisherman's knot rather than the traditional double fisherman's knot. A good hitch to use with this material is the Palomar knot.
High-performance ropes for sailing and parasailing are made of Dyneema as well.
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.
It is also used in snowboards, often in combination with carbon fiber, reinforcing the fiberglass composite material, adding stiffness and improving its flex characteristics.
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.
The extremely low friction coefficient of UHMWPE makes it a common topsheet for boxes in terrain parks.
[edit] Medical applications
UHMWPE has been very effective since its first use by Sir John Charnley and Harry Craven in 1962. There has been much studies about the clinical consequences of wear debris and this has led to the development of newer improved forms of UHMWPE. Carbon re-inforced UHMWPE was introduced in 1970's but its had lower fatigue resistance compared with the UHMWPE. The efficiency of the material can be improved by changes in method of sterilisation, changes in method of manufacturing and the use of higher doeses of irradiation increases the level of cross-linking within the UHMWPE. In the early 1990's Hylamer7 orthopedic bearing material was introduced as a new form of UHMWPE. The morphology of the crystalline structure was changed, the crystallinity increased, yield strength, resistance to deformation and modulus increased as well. However Hylamer7 has also been reported to lead to severe eccentric wear. Furthermore, studies has revealed that the exothermic polymerisation process does not significantly increase the adjacent bone temperature to cause thermal necrosis of bone if the cement layer is within the range used in surgery. However the method of cement mixing increases the cement-bone interface temperature above the limit that might cause impaired bone regeneration in total hip replacement surgery. 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.[2] An online repository of information and review articles related to medical grade UHMWPE, known as the UHMWPE Lexicon, was started online in 2000 [2].
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 sterilized by device manufacturers.
[edit] 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.
[edit] Small Bicycle Parts
Freestyle BMX company Tree Bicycle Co. Uses UHMW in a guard used to protect a bicycles front sprocket/chainwheel during a stunt in which one grinds on their sprocket guard.
[edit] See also
- Olefin
- Aramid
- Twaron
- Kevlar
- Synthetic fiber
- Linear low density polyethylene
- Low density polyethylene
- High density polyethylene
- Body Armor
[edit] References
- ^ Hoechst: Annealing (Stress Relief) of Hostalen GUR
- ^ Kurtz SM, The UHMWPE Handbook, Academic Press, New York, 2004.
- The UHMWPE Lexicon. http://www.uhmwpe.org
[edit] External links
- DSM Homepage
- The UHMWPE Lexicon
- Ticona Homepage
- Orthoplastics Homepage
- Poly Hi Solidur (Quadrant) Homepage
- Honeywell's Spectra page
- Comparison of rope materials.
- Tote Systems Australia Dyneema Page
- Lightweight ballistic composites: Military and law-enforcement applications. Edited by A Bhatnagar, Honeywell International, USA
- Poly Hi Solidur Products
