Ultra-high-molecular-weight polyethylene ( UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, ( HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. 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 force of any thermoplastic presently made.Stein, H. L. (1998). Ultrahigh molecular weight polyethylenes (uhmwpe). Engineered Materials Handbook, 2, 167–171.
UHMWPE is odorless, tasteless, and nontoxic.Wong, D. W. S.; Camirand, W. M.; Pavlath, A. E.; Krochta, J. M.; Baldwin, E. A. and Nisperos-Carriedo, M. O. (eds.) (1994) "Development of edible coatings for minimally processed fruits and vegetables" pp. 65–88 in Edible coatings and films to improve food quality, Technomic Publishing Company, Lancaster, PA. . It embodies all the characteristics of high-density polyethylene (HDPE) with the added traits of being resistant to concentrated acids and alkalis, as well as numerous organic solvents. It is highly resistant to corrosive chemicals except ; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating (see boundary lubrication); 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 polyoxymethylene and is comparable to that of polytetrafluoroethylene (PTFE, Teflon), but UHMWPE has better abrasion resistance than PTFE.
UHMWPE fibers, commercialized in the late 1970s by the Dutch chemical company DSM, are widely used in ballistic protection, defense applications, and increasingly in medical devices.
When formed into fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystallinity from 39% to 75%. 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 , ultra high molecular weight polyethylene; UHMWPE. chemyq.com and, according to DSM, it is not advisable to use UHMWPE fibres at temperatures exceeding for long periods of time. It becomes brittle at temperatures below .
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 molecule groups in most polymers easily bond to water. Because olefins have no such groups, UHMWPE does not absorb water readily, nor wetting 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 , or 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 65 °C or less. Finally, the material is wrapped in an insulating blanket for 24 hours to bring to room temperature.Hoechst: Annealing (Stress Relief) of Hostalen GUR
UHMWPE is processed variously by compression moulding, 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 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 Reptation are minimal. Entanglements make chain orientation more difficult, and lower the strength of the final product.
UHMWPE are used as composite plates in armor, in particular, personal armor and on occasion as vehicle armor. Civil applications containing UHMWPE fibers are cut-resistant gloves, bow strings, climbing equipment, fishing line, spear lines for spearfishing, high-performance , suspension lines on sport and , rigging in yachting, kites, and kites lines for kites sports.
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.Bhatnagar, A. (ed.) (2006) Lightweight Ballistic Composites: Military and Law-Enforcement Applications. Honeywell International. 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 yarns have gained popularity for their low weight and bulk. They exhibit very low elasticity compared to their nylon counterparts. Also, low elasticity translates to low toughness. The fiber's very high lubricity leads to poor knot-holding ability, and is mostly used in pre-sewn 'slings' (loops of webbing)- relying on knots to join sections of UHMWPE is generally not recommended, and if necessary it is recommended to use the triple fisherman's knot rather than the traditional double fisherman's knot.
Ships' and rope made from the fiber (0.97 specific gravity) float on sea water. "Spectra Wires" as they are called in the towing boat community are commonly used for face wires as a lighter alternative to steel 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 . Due to its extreme abrasion resistance it is also used as an excellent corner protection for synthetic lifting slings.
Dyneema was used for the 30-kilometre space tether in the ESA/Russian Young Engineers' Satellite 2 of September, 2007.
Dyneema Composite Fabric (DCF) is a laminated material consisting of a grid of Dyneema threads sandwiched between two thin transparent polyester membranes. This material is very strong for its weight, and was originally developed for use in racing yacht sails under the name 'Cuben Fiber'. More recently it has found new applications, most notably in the manufacture of lightweight and ultralight camping and backpacking equipment such as tents and backpacks.
In archery, UHMWPE is widely used as a material for bowstrings because of its low creep and stretch compared to, for example, Dacron (PET). Besides pure UHMWPE fibers, most manufacturers use blends to further reduce the creep and stretch of the material. In these blends, the UHMWPE fibers are blended with, for example, Vectran.
In skydiving, Spectra is one of the most common materials used for suspension lines, largely supplanting the earlier-used Dacron, being lighter and less bulky. Spectra has excellent strength and wear-resistance, but is not dimensionally stable (i.e. shrinks) when exposed to heat, which leads to gradual and uneven shrinkage of different lines as they are subject to differing amounts of friction during canopy deployment, necessitating periodic line replacement. It is also almost completely inelastic, which can exacerbate the opening shock. For that reason, Dacron (PET) lines continue to be used in student and some tandem systems, where the added bulk is less of a concern than the potential for an injurious opening. In turn, in high performance parachutes used for Canopy piloting, Spectra is replaced with Vectran and HMA (high-modulus aramid), which are even thinner and dimensionally stable, but exhibit greater wear and require much more frequent maintenance to prevent catastrophic failure. Spectra / Dyneema are also used for reserve parachute closing loops when used with automatic activation devices, where their extremely low coefficient of friction is critical for proper operation in the event of cutter activation.
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. GHR HMW-PE and VHMW-PE. ticona.com
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. Throughout its history, there were unsuccessful attempts to modify UHMWPE to improve its clinical performance until the development of highly 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 recrystallization. This recrystallized 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 relative to virgin UHMWPE. Today, the poor clinical history of Hylamer is largely attributed to its sterilization 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 cross-linked UHMWPE materials, however negative clinical reports from Europe about Hylamer continue to surface in the literature.
Highly cross-linked UHMWPE materials were clinically introduced in 1998 and have rapidly become the standard of care for total hip replacements, at least in the United States. These new materials are cross-linked 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 cross-linked UHMWPE for knee replacement.
In 2007, manufacturers started incorporating anti-oxidants into UHMWPE for hip and knee arthroplasty bearing surfaces. Vitamin E (a-tocopherol) is the most common anti-oxidant used in radiation-cross-linked UHMWPE for medical applications. The anti-oxidant helps quench free radicals that are introduced during the irradiation process, imparting improved oxidation resistance to the UHMWPE without the need for thermal treatment.Wannomae, K. K., Micheli, B. R.; Lozynsky, A. J. and Muratoglu, O. K. (2010) "A new method of stabilising irradiated UHMWPE using Vitamin E and mechanical annealing". 56th Annual Meeting of the Orthopedic Research Society, 2290. Several companies have been selling antioxidant-stabilized joint replacement technologies since 2007, using both synthetic vitamin E as well as hindered phenol-based antioxidants.Spiegelberg, S.H. (2012) "UHMWPE for total joint arthroplasty: Past, present, and future". Bonezone.
Another important medical advancement for UHMWPE in the past decade has been the increase in use of fibers for Surgical suture. Medical-grade fibers for surgical applications are produced by DSM under the "Dyneema Purity" trade name.
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.