Ultra-high-molecular-weight polyethylene

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Sep. 30, 2024

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Ultra-high-molecular-weight polyethylene

Very long-chain polyethylene with high impact strength

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Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a specific type of thermoplastic polyethylene with exceptionally long chains. Often referred to as high-modulus polyethylene (HMPE), its molecular mass typically ranges between 3.5 and 7.5 million amu.[1] The extended molecular chains enhance load transfer to the polymer backbone, resulting in robust intermolecular interactions. As a result, UHMWPE boasts remarkable toughness and the highest impact strength of any known thermoplastic.[2]

UHMWPE is characterized as odorless, tasteless, and nontoxic.[3] It offers achievements similar to those observed in high-density polyethylene (HDPE), while enhancing resistance against concentrated acids, alkalis, and a variety of organic solvents.[4] Not only is it extremely resistant to corrosive chemicals, except oxidizing acids, but it also exhibits minimal moisture absorption, a very low coefficient of friction, and self-lubricating properties (refer to boundary lubrication). In fact, in some cases, it is 15 times more resistant to wear and abrasion than carbon steel, and its coefficient of friction is notably lower than nylon and acetal, aligning more closely with polytetrafluoroethylene (PTFE, Teflon)—despite outperforming PTFE in abrasion resistance.[5][6]

Development

The commercialization of UHMWPE polymerization can be traced back to Ruhrchemie AG, a company that has evolved over the years.[1][7] UHMWPE powder materials are now produced by various firms such as Ticona, Braskem, Teijin (Endumax), Celanese, and Mitsui, allowing for direct molding into final product shapes. The processed forms of UHMWPE are available as fibers or consolidated materials, such as sheets or rods. Its unique properties make it increasingly sought after in industrial applications across the automotive and bottling sectors, as well as for total joint arthroplasty in orthopedic and spinal implants since the 1980s.[1]

Fibers derived from UHMWPE, marketed under names like Dyneema, were developed in the late 1980s by DSM, while Honeywell introduced Spectra. Today, both fibers are extensively utilized in ballistic protection, defense sectors, and medical applications alongside hiking gear, climbing accessories, and various other industries.

Structure and Properties

As a polyolefin, UHMWPE consists of extensively elongated polyethylene chains, all aligned uniformly. Its strength arises predominantly from the individual molecule lengths. Though van der Waals forces among the molecules are relatively weak, the significant overlaps facilitated by long chains allow for the effective transfer of shear stresses from one molecule to another. This cumulative network bolsters the overall intermolecular strength, allowing UHMWPE to withstand substantial tensile loading.

When processed into fibers, the polymer chains can achieve over 95% parallel orientation, along with crystallinity levels ranging from 39% to 75%. In comparison, Kevlar's strength stems from robust bonding among shorter molecular structures.

The molecular simplicity grants UHMWPE unique surface and chemical properties that are not commonly found in high-performance polymers. For instance, while polar groups in most polymers are prone to bonding with water, UHMWPE does not absorb or wet easily, which complicates bonding with other polymers. This quality, along with its surface being resistant to interactions with skin, gives UHMWPE fibers a notably slippery texture. Additionally, UHMWPE resists aggressive agents, UV radiation, and microbes due to its lack of chemically reactive groups like esters or amides.

When subjected to tensile load, UHMWPE can continuously deform as long as the stress persists—an observation known as creep.

In terms of processing, UHMWPE can be annealed at temperatures between 135°C and 138°C in environments like an oven or silicone oil baths. It is then gradually cooled to 65°C or lower and insulated for 24 hours to return to room temperature.[10]

Production

Ultra-high-molecular-weight polyethylene is produced from ethylene monomers that are chemically bonded to form the base polyethylene unit. The chains generated in this synthesis process are significantly longer than those found in standard high-density polyethylene (HDPE), primarily due to metallocene catalysts, yielding UHMWPE molecules with around 100,000 to 250,000 monomer units while HDPE contains only 700 to 1,800 monomers.

Processing methods for UHMWPE include compression molding, ram extrusion, gel spinning, and sintering. European manufacturers initiated compression molding UHMWPE in the early 2000s, with gel spinning emerging later for differing application requirements.

In gel spinning, a precisely heated gel consisting of a low UHMWPE concentration in oil is extruded through a spinneret. This extrudate is then drawn through the air, where the oil is removed using solvents that do not affect the UHMWPE, followed by drying the final product. This yields fibers with a high degree of molecular orientation and exceptional tensile strength, with minimal intermolecular entanglements enhancing chain orientation and product strength.[11]

Applications

Fiber

Dyneema and Spectra represent lightweight, high-strength fibers spun from oriented-strand gels. These fibers exhibit yield strengths of up to 2.4 GPa (350,000 psi) and a density as low as 0.97 g/cm³ (0.087 oz/in) (for Dyneema SK75).[12] Such characteristics match the yield strengths of high-strength steel, while the weight-to-strength ratio can be up to eight times better than high-strength steel products. Additionally, UHMWPE offers a strength-to-weight ratio approximately 40% higher than aramid fibers. Albert Pennings first identified these high-quality UHMWPE characteristics in the 1960s, with commercial products surfacing by DSM in 1979 and Southern Ropes shortly thereafter.[13]

UHMWPE yarn derivatives are utilized in armor composites, including personal protective gear and occasionally vehicle armor. In civilian contexts, UHMWPE fibers enhance cut-resistant gloves, tear-resistant garments, bowstrings, climbing gear, automotive winches, fishing lines, speargun spear lines, high-performance sails, suspension lines in sport parachuting, yachting rigging, and kite strings.

In protective gear, fibers are aligned and bonded into sheets layered at various angles, providing multidirectional strength to composite materials. Such applications have recently been included in advancements within US Military Interceptor body armor, designed to afford protection for arms and legs using UHMWPE fabric.[16] Additionally, various UHMWPE woven fabrics are commercially available for use in shoe liners, pantyhose,[17] tactical apparel, stab-resistant vests, and composite vehicle linings.[18]

The switch to UHMWPE rope in automotive winching projects numerous advantages over traditional steel wire ropes. The reduced mass of UHMWPE rope minimizes energy accumulation during operations due to lower elongation, resulting in virtually no snap-back risks. Furthermore, UHMWPE rope does not kink easily, eliminating weak points commonly associated with steel cabling. Its buoyant properties allow for easier water recoveries, complemented by visible color options for better identification underwater or amidst dirt. Importantly, the significantly lower weight of a typical 11 mm (0.43 in) UHMWPE rope, at 2 kg (4.4 lb) for 30m (98 ft), starkly contrasts to steel wire ropes that weigh around 13 kg (29 lb). A couple of limitations of UHMWPE rope include susceptibility to UV damage, necessitating winch covers for protection, and vulnerability to heat damage upon contact with hot components.

UHMWPE fibers are also preferred for fishing lines due to their reduced stretch, improved wear resistance, and thinner profile compared to monofilament alternatives.

Climbing equipment often incorporates UHMWPE along with nylon yarns for fabricating cord and webbing due to their lightweight and compactness. These materials maintain lower elasticity when compared to nylon, translating to reduced toughness. However, the incredibly high lubricity of UHMWPE fibers potentially hampers knot-holding ability; hence, using sewn 'slings' (pre-made loops) is recommended for applications rather than relying on knot tying, with the triple fisherman's knot advised when necessary.[19][20]

Additionally, UHMWPE fibers excel in marine applications, with cables and hawsers constructed from these fibers (0.97 specific gravity) designated to float in seawater. Widely known as "Spectra wires" within the towing community, these products serve as lightweight alternatives to conventional steel cables.[21]

Moreover, implementation of UHMWPE can also be found in the production of skis and snowboards, commonly used in combination with carbon fiber to enhance structural rigidity and improve flex characteristics.[clarification needed] UHMWPE often serves as the base layer in contact with snow and incorporates abrasives that enable wax absorption and retention.[clarification needed]

Due to remarkable abrasion resistance, UHMWPE finds utilization in heavy-duty lifting applications, and it is preferred as corner protection for synthetic slings.

In high-performance sailing and parasailing lines, UHMWPE is favored owing to its low stretch, impressive strength, and lightweight attributes. Furthermore, for winch-launching gliders, its superior abrasion resistance leads to reduced wear and extended lifespans compared with steel cables. Achieving lighter winch launches become feasible due to the decreased weight in the mile-long cables used.

In an innovative application, UHMWPE comprised the tether for the ESA/Russian Young Engineers' Satellite 2 in 2018, spanning 30 km (19 miles) and measuring 0.6 mm (0.024 in) in thickness.[23]

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Dyneema Composite Fabric (DCF) is a laminated construction containing a grid of Dyneema filaments sandwiched between two transparent polyester membranes. Compared to other fabrics, DCF exhibits great strength relative to weight and was initially designed for racing yacht sails under the name 'Cuben Fiber'. In recent times, it has discovered new applications, specifically in lightweight camping and backpacking gear like tents, backpacks, and bear-proof food bags.

In archery, the usage of UHMWPE for bowstrings has significantly increased, thanks to its minimal creep and stretch compared to alternatives like Dacron (PET).[citation needed] Often, pure UHMWPE fibers are blended with other materials to optimize performance, such as Vectran.

Skydiving has also seen UHMWPE take precedence over traditional Dacron for suspension lines due to its reduced bulk and lighter properties.[citation needed] While UHMWPE yields exceptional strength and wear-resistance, its susceptibility to heat exposure leads to uneven shrinkage variability during canopy deployment, consequently requiring regular line replacements. Its near-total inelasticity can result in severe opening shock, motivating the use of Dacron lines in novice and some tandem configurations where this added bulk is less critical. Conversely, high-performance parachutes designed for swooping now often employ Vectran and HMA (high-modulus aramid) in place of UHMWPE for their thinner, dimensionally stable qualities, albeit with heightened maintenance needs to mitigate the risk of catastrophic failure. Moreover, UHMWPE fibers are utilized for reserve parachute closing loops with automatic activation devices due to their remarkably low friction coefficient, pivotal for ensuring appropriate cutter activation operation.

Medical

Having a substantial clinical history, UHMWPE is utilized as a biomaterial in hip, knee, and (since the 1990s) spine implants.[1] In 2015, an online knowledge repository for medical-grade UHMWPE, called the UHMWPE Lexicon, was initiated to consolidate relevant information and reviews.[24]

Historically, joint replacement components have been manufactured using "GUR" resins supplied by Ticona, which are converted into semi-finished products by firms like Quadrant and Orthoplastics,[1] followed by machining and sterilization by device manufacturers.[25]

Initially employed clinically in 1962 by Sir John Charnley, UHMWPE evolved to dominate as the primary bearing material in hip and knee replacements during the 1990s.[24] Despite earlier attempts at modifying UHMWPE for better clinical performance, it was not until the advent of highly cross-linked UHMWPE in the late 1990s that significant advancements occurred.[1]

Past efforts included blending UHMWPE with carbon fibers to create reinforced composites branded as "Poly Two" by Zimmer in the 1970s. Unfortunately, this compound demonstrated poor compatibility, resulting in subpar clinical performance compared to virgin UHMWPE.[1]

A similar attempt involved high-pressure recrystallization, leading to the introduction of "Hylamer" by DePuy in the late 1980s. While this material appeared promising, it was found to be vulnerable to oxidation during gamma irradiation in air, leading to inferior clinical outcomes compared to conventional UHMWPE. Although Hylamer lost favor during the 1990s as new materials emerged, ongoing research continues to explore its viability, especially among certain studied communities.[24]

By the year 2000, highly cross-linked UHMWPE received clinical approval and swiftly became the standard of care in total hip replacements across the United States.[1] Incorporating innovative cross-linking methods using gamma or electron beam radiation (50-105 kGy) and subsequent thermal treatment improved its resistance to oxidation.[1] Presently, five-year clinical data from numerous centers indicate the superiority of this refined material over traditional UHMWPE in hip replacement procedures (see arthroplasty).[24] Ongoing investigations are still underway to assess the performance of more advanced UHMWPE innovations for knee replacements.[24]

In 2012, anti-oxidation measures began inclusion in UHMWPE for hip and knee arthroplasty components. Vitamin E (α-tocopherol) emerged as the predominant antioxidant employed in radiation-cross-linked UHMWPE, effectively neutralizing harmful free radicals generated during irradiation, thus augmenting oxidation resistance without necessitating thermal processes.[26] Since 2012, various companies have offered antioxidant-stabilized technologies for joint replacements utilizing both synthetic vitamin E and hindered phenol-based materials.[27]

Over the past decade, the demand for UHMWPE fibers for sutures in surgical applications has also increased significantly. Medical-grade fibers produced by DSM under the "Dyneema Purity" trade name now serve this market.[28]

Manufacturing

In manufacturing, UHMWPE proves advantageous for producing PVC (vinyl) windows and doors owing to its ability to withstand molding heat. It acts as a chamber or form filler across various PVC profile shapes to facilitate bending or shaping around templates.

Additionally, UHMWPE finds relevance in hydraulic seal and bearing production, ideal for medium mechanical duties in applications involving water, oil hydraulics, pneumatics, and situations where lubrication is absent. Its excellent abrasion resistance is noteworthy, although its performance is superior when paired with soft mating surfaces.

Wire and Cable

Typically constructed with dual insulation, Fluoropolymer/HMWPE cathodic protection cables feature a primary layer of a fluoropolymer, such as ECTFE, ensuring chemical resistance against chlorine, sulfuric acid, and hydrochloric acid. The HMWPE insulation layer follows this, imparting pliability and resilience against installation-induced abuse, further augmented by HMWPE's mechanical protection properties.[29]

Marine Infrastructure

Lastly, UHMWPE is extensively used in marine applications for mooring floating structures and vessels. Serving as the contact surface between mobile and fixed entities, UHMWPE enjoys favor in fender systems due to its unmatched characteristics:[30]

Exceptional wear resistance: superior among plastics, even better than steel
Impressive impact resilience: comparable to that of steel
Low friction in both wet and dry conditions: a self-lubricating material

Ultra-high-molecular-weight polyethylene