We're not going to discuss the three highest volume polyethylenes, LDPE, LLDPE and HDPE because they're already described pretty well at our sister website, the Macrogalleria. Free radical synthesis of PE is described here which gives LDPE. The other two most common types are a result of using transition metal complexes like metallocenes and Ziegler-Natta catalysts.
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In fact, that's how UHMWPE is made, using such catalysts with careful control of monomer purity and polymerization conditions. The 3D image at the top of the page is of purely linear PE, which is what you need to make the high performance fibers Spectra and Dyneema. Below are images of the branched homopolymer (LDPE) next to a crosslinked version that is found in PEX (click on either to pop up a 3D version you can rotate and zoom in on).
Polyethylene Bullet-proof Vests?
Let's start with the high performance fibers, then. These are made from essentially all-linear high-density PE chains but with one important difference compared to regular HDPE: they're really, really high molecular weight, which is why they're termed UHMWPE or ultra-high molecular weight PE. In fact, their molecular weights are so high you can't do anything with them, like melt processing. Sure, they're thermoplastics, in one sense, but because the chains are so long, like into the millions of daltons, they have an enormous number of entanglements. Take a look at some of the analogies for polymers to get a better idea what these entanglements involve.
"But wait! You said these huge polymers were used to make the fibers were talking about, like those in the picture on the right. How can they not be processible?" Ah, good question, and as the Bard said, "There's the rub." Meaning, of course, that learning how to process the UHMWPE into the fibers was the crucial discovery for these ultra-performance materials. That's similar to how Kevlar was finally commercialized, not because it was a new polymer but because it was so rigid and entangled that you needed special new conditions to spin fibers of it. And only then could we get material that is so useful in so many ways.
Now, most polymer scientists and engineers would think about processing a thermoplastic polymer into fiber form using heat and some kind of extruder, a huge, expensive machine used to make fibers. Already told you that won't work and why. But think a minute: if the key is to increase molecular mobility and this high molecular weight PE is all tangled up, what trick can you play? Let me illustrate.
I have granddaughters, and every once in a while I get asked to brush their long hair. Ugh! Tangles and more tangles from playing out in the wind, playing like, well, like children. What to do? Hate to pull hard and bring on tears. Answer turns out to be simple: spray a little lubricant on the tangled hair so the individual strands can slip and slide past each other. And you can buy that stuff in a spray bottle! Works like a charm.
"I don't get it...Wait a minute! You implying we should spray lubricant on the PE chains?" Good guess, my friend, and that's almost exactly right. No need to spray it on if you can submerge the whole mass you're trying to process into some kind of solvent or lubricant. Long story short, that's what you have to do to PE spin fibers. You don't need to dissolve all the chains in a solvent, just swell them up so they can disentangle. That solvent swollen mass of UHMWPE is called a "gel," and the process is called "gel spinning," oddly enough.
The figure below shows a schematic of the process. Of course, it took a lot of tweaking and experimentation to get the conditions just right, but hey, "time is money." Or is it "money is time?" Whatever, making these fibers has resulted in a lot of uses that are worth a lot of money (or time, if you prefer). We'll get to those in a minute.
The figure above is complicated because the process is complex. That's because it's not just one thing that is important in making these wonder polymers. First, let's point out that the enormous strength of these polymers comes from a very simple concept: pulling together. I know, you've probably heard that phrase in a totally different context, maybe in a work related, team building exercise. Whatever. The key here is this: if you can line up all of those really high molecular weight chains in the same direction in a fiber, when you pull on the fiber, you're actually pulling directly on the chains themselves.
Two things to point out. Very few polymers of any kind can be lined up like that. If they could be, we'd see incredible properties for lots of other fibers. Second is the fact that, even with pretty good alignment, the chains in most fibers can just slip past each other making the fibers pretty weak. So what gives with UHMWPE? Simply that little things can add up. It's the strength of the interactions between fibers that transfer forces along the chains from one to the others. Pull on one chain and because they're all held together, you're pulling on many of them.
And what are those intermolecular forces? In Kevlar, they consist of a combination of aromatic-stacking and pi-pi interactions, plus hydrogen bonding between amide groups. Neither exist in UHMWPE. Instead, the only forces are van der Waals, which are the weakest type of intermolecular interactions that exist between molecules. So how can they add up to be so important? Simply because the chains are so long. With Kevlar, all you need are chains of less than a hundred thousand molecular weight or so. But with UHMWPE, you need chains that are millions of molecular weight, so long that the van der Waals forces can add up to be more than the strength of the carbon-carbon bonds in the chains themselves.
Long story short: you have to have UHMWPE for the fibers to be really, really strong. BUT, and it's a really big but, they have to be perfectly lined up. And that's where the complex process in the figure above comes in. First you have to spin from a gel state, that allows the chains to slip past each other, get pulled out of the tangled mess and partially lined up. Notice I said "partially." That's because you're playing a game with the solvent that makes the gel in the first place: it's still there after this first step and has to be removed in a non-solvent or evaporation process. And THAT leaves partially non-aligned segments in the more-or-less microscopically aligned fibers.
So the schematic in part "C" of the figure above is what you get from gel spinning in part "A", but it's the almost perfectly aligned chains in "D" that come from the process in part "B" that give the fibers their ultimate strength.
The real question you might be asking is, "So what is this ultimate strength you're talking about? It's not what you call it that matters but what it can do, right?" You bet, and what it can do is incredible. The graph above shows one property, probably the most useful one for this material. It's tenacity which is tensile strength divided by denier or linear density. And as the graph shows, UHMWPE fibers (Spectra on the graph) is stronger than ANY of the other ones, including Kevlar and even steel. "WOW!" I say. That's why these fibers are finding so many uses. One example is in cables for tying up barges, ships and rigging on oil wells where the strength is important, yes, but so is the density and weight. These cables are much less dense which means lighter. They're also more flexible and that means they're a whole lot easier to manhandle and better yet, they float in water!
"And what about bullet proof vests you alluded to? Isn't that what this is all about?" Well, yes and no. Yes, these fibers work great in bullet resistant applications (like body armor), and they're lighter and stronger than alternatives. So yes, there are several companies making body armor with them. But the "no" is because that's not the only thing they're used for. And all those other uses add up to much bigger markets than body armor.
And PEX? Crosslinked Polyethylene?
Turns out there's more than one way to make PE stronger. Sure, you can go through the complicated process used for ultra-oriented PE fibers described above, or you could do something simple and very effective: crosslink the polymer chains. Does it work? You bet. Is it easy, cost effective and commercially useful? Darn tootin' it is! And that's PEX we're talking about now.
The diagram below shows several different ways to accomplish this crosslinking, fulfilling the old adage that there's more than one way to skin a cat (yuch). Not sure which process is used for which applications but doesn't really matter. Key is the property enhancement, which entails mechanical strength, resistance to deformation at elevated temperatures, and property maintenance under repeated abrasion.
By Minihaa - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=But in fact, it's not just a simple matter of adding some crosslinking to the polyethylene (in whatever form it's in). If you want the best properties, you have to first (or later) impart as much crystallinity as is feasible and that normally means using high density polyethylene or HDPE. As shown in the diagram below, those crystalline domains also act as crosslinks as long as the use temperature is below Tm. But don't forget that Tg, the glass transition temperature, is also important. And if the use temperature goes higher than Tg? That's where the chemical crosslinks come into play. Even though the chains in the amorphous regions above their Tg would really like to slither and slide every which way (meaning deform under any kind of pressure or force), they can't. The crosslinks keep them all in the shape they're supposed to be in. Neat, huh?
Two of the important applications of PEX are in the areas of plumbing and artificial joints. PEX piping for home and commercial construction has virtually taken over from traditional metal piping. Not only is it much faster, cheaper and easier to install, but it helps conserve copper, one of the metals we need for other applications like electrical wiring. And artificial joints? The smooth, abrasion resistant surfaces of molded HDPE that is then crosslinked with radiation means longer functionality in this increasingly important area of medicine. As we all age, our joints wear out and don't work well anymore. Solution: take out the old and put in the new. Fascinating area worth further investigation, just not here. So come back next month for another edition of "Polymer of the Month."
And just to complicate things here, it's actually possible to have a polyethylene copolymer that behaves very much like PEX under most conditions. The comonomer is an acrylic or methacrylic acid and after synthesis, the copolymer acid groups are neutralized with basic metal salts such as NaOH. Zn salts are also used. These neutralized copolymers are called "ionomers," and have the unique property that the carboxylate salt moieties tend to group together and act just like the chemical crosslinks of PEX. Except they're not, either chemical crosslinks or exactly like those in PEX. That means that such ionomers can be thermally processed to make things like golf ball covers- flexible, smooth and resistant to tearing. You just can't leave them out in the sun and rain for too long or they degrade. Which is actually why PE ionomers are not used in plastic plumbing pipe: hot water would make them leak.
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.[1] 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.[2]
UHMWPE is odorless, tasteless, and nontoxic.[3] 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.[4] It is highly resistant to corrosive chemicals except oxidizing acids; 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 acetal and is comparable to that of polytetrafluoroethylene (PTFE, Teflon), but UHMWPE has better abrasion resistance than PTFE.[5][6]
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Polymerization of UHMWPE was commercialized in the s by Ruhrchemie AG,[1][7] which has changed names over the years. Today UHMWPE powder materials, which may be directly molded into a product's final shape, are produced by Ticona, Braskem, Teijin (Endumax), Celanese, and Mitsui. Processed UHMWPE is available commercially either as fibers or in consolidated form, such as sheets or rods. Because of its resistance to wear and impact, UHMWPE continues to find increasing industrial applications, including the automotive and bottling sectors. Since the s, UHMWPE has also been the material of choice for total joint arthroplasty in orthopedic and spine implants.[1]
UHMWPE fibers branded as Dyneema, commercialized in the late s by the Dutch chemical company DSM, and as Spectra, commercialized by Honeywell (then AlliedSignal), are widely used in ballistic protection, defense applications, and increasingly in medical devices, sailing, hiking equipment, climbing, and many other industries.
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Structure of UHMWPE, with n greater than 100,000UHMWPE is a type of polyolefin. It is made up of extremely long chains of polyethylene, which all align in the same direction. It derives its strength largely from the length of each individual molecule (chain). Van der Waals forces between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is attracted to the others with so many van der Waals forces that the whole of the inter-molecular strength is high. In this way, large tensile loads are not limited as much by the comparative weakness of each localized van der Waals force.
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 130 to 136 °C (266 to 277 °F),[8] and, according to DSM, it is not advisable to use UHMWPE fibres at temperatures exceeding 80 to 100 °C (176 to 212 °F) for long periods of time. It becomes brittle at temperatures below &#;150 °C (&#;240 °F).[9]
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, nor 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 UHMWPE are immune to. Since UHMWPE 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.
When UHMWPE is annealed, the material is heated to between 135 °C (275 °F) and 138 °C (280 °F) 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 (2.5 °F/ks) to 65 °C (149 °F) or less. Finally, the material is wrapped in an insulating blanket for 24 hours to bring to room temperature.[10]
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Ultra-high-molecular-weight polyethylene (UHMWPE) is synthesized from its monomer ethylene, which is bonded together to form the base polyethylene product. These molecules are several orders of magnitude longer than those of familiar high-density polyethylene (HDPE) due to a synthesis process based on metallocene catalysts, resulting in UHMWPE molecules typically having 100,000 to 250,000 monomer units per molecule each compared to HDPE's 700 to 1,800 monomers.
UHMWPE is processed variously by compression moulding, ram extrusion, gel spinning, and sintering. Several European companies began compression molding UHMWPE in the early s. Gel-spinning arrived much later and was intended for different applications.
In gel spinning a precisely heated gel (of a low concentration of UHMWPE in an oil) is extruded through a spinneret. The extrudate is drawn through the air, the oil extracted with a solvent which does not affect the UHMWPE, and then dried removing the solvent. 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.[11]
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For more information, please visit Bullet-Proof Helmet.
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LIROS Dyneema hollowDyneema and Spectra are brands of lightweight high-strength oriented-strand gels spun through a spinneret. They have yield strengths as high as 2.4 GPa (350,000 psi) and density as low as 0.97 g/cm (0.087 oz/in) (for Dyneema SK75).[12] High-strength steels have comparable yield strengths, and low-carbon steels have yield strengths much lower (around 0.5 GPa (73,000 psi)). Since steel has a specific gravity of roughly 7.8, these materials have a strength-to-weight ratios eight times that of high-strength steels. Strength-to-weight ratios for UHMWPE are about 40% higher than for aramid. The high qualities of UHMWPE filament were discovered by Albert Pennings in , but commercially viable products were made available by DSM in and Southern Ropes soon after.[13]
Derivatives of UHMWPE yarn are used in composite plates in armor, in particular, personal armor and on occasion as vehicle armor. Civil applications containing UHMWPE fibers are cut-resistant gloves, tear-resistant hosiery, bow strings, climbing equipment, automotive winching, fishing line, spear lines for spearguns, high-performance sails, suspension lines on sport parachutes and paragliders, rigging in yachting, kites, and kite 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.[14][15] 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 UHMWPE fabric.[16] A multitude of UHMWPE woven fabrics are available in the market and are used as shoe liners, pantyhose,[17] fencing clothing, stab-resistant vests, and composite liners for vehicles.[18]
The use of UHMWPE rope for automotive winching offers several advantages over the more common steel wire rope. The key reason for changing to UHMWPE rope is improved safety. The lower mass of UHMWPE rope, coupled with significantly lower elongation at breaking, carries far less energy than steel or nylon, which leads to almost no snap-back. UHMWPE rope does not develop kinks that can cause weak spots, and any frayed areas that may develop along the surface of the rope cannot pierce the skin like broken steel wire strands can. UHMWPE rope is less dense than water, making water recoveries easier as the recovery cable is easier to locate than wire rope. The bright colours available also aid with visibility should the rope become submerged or dirty. Another advantage in automotive applications is the reduced weight of UHMWPE rope over steel cables. A typical 11 mm (0.43 in) UHMWPE rope of 30 m (98 ft) can weigh around 2 kg (4.4 lb), the equivalent steel wire rope would weigh around 13 kg (29 lb). One notable drawback of UHMWPE rope is its susceptibility to UV damage, so many users will fit winch covers in order to protect the cable when not in use. It is also vulnerable to heat damage from contact with hot components.
Spun UHMWPE fibers excel as fishing line, as they have less stretch, are more abrasion-resistant, and are thinner than the equivalent monofilament line.
In climbing, cord and webbing made of combinations of UHMWPE and nylon yarn have gained popularity for their low weight and bulk. They exhibit very low elasticity compared to their nylon counterparts, which translates to low toughness. The fiber's very high lubricity causes poor knot-holding ability, and it 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.[19][20]
Ships' hawsers and cables 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 [21] 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.[clarification needed] The UHMWPE is often used as the base layer, which contacts the snow, and includes abrasives to absorb and retain wax.[clarification needed]
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.
High-performance lines (such as backstays) for sailing and parasailing are made of UHMWPE, due to their low stretch, high strength, and low weight.[22] Similarly, UHMWPE is often used for winch-launching gliders from the ground, as, in comparison with steel cable, its superior abrasion resistance results in less wear when running along the ground and into the winch, increasing the time between failures. The lower weight on the mile-long cables used also results in higher winch launches.
UHMWPE was used for the 30 km (19 mi) long, 0.6 mm (0.024 in) thick space tether in the ESA/Russian Young Engineers' Satellite 2 of September, .[23]
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, backpacks, and bear-proof food bags.
In archery, UHMWPE is widely used as a material for bowstrings because of its low creep and stretch compared to, for example, Dacron (PET).[citation needed] 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, UHMWPE is one of the most common materials used for suspension lines, largely supplanting the earlier-used Dacron, being lighter and less bulky.[citation needed] UHMWPE 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 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 swooping, UHMWPE 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. UHMWPE 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.
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UHMWPE has a clinical history as a biomaterial for use in hip, knee, and (since the s), for spine implants.[1] An online repository of information and review articles related to medical grade UHMWPE, known as the UHMWPE Lexicon, was started online in .[24]
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,[1] and then machined into implant components and sterilized by device manufacturers.[25]
UHMWPE was first used clinically in by Sir John Charnley and emerged as the dominant bearing material for total hip and knee replacements in the s.[24] Throughout its history, there were unsuccessful attempts to modify UHMWPE to improve its clinical performance until the development of highly cross-linked UHMWPE in the late s.[1]
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 s.[1] The carbon fibers had poor compatibility with the UHMWPE matrix and its clinical performance was inferior to virgin UHMWPE.[1]
A second attempt to modify UHMWPE was by high-pressure recrystallization. This recrystallized UHMWPE was released clinically as "Hylamer" by DePuy in the late s.[1] 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).[24] Hylamer fell out of favor in the United States in the late s 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 and have rapidly become the standard of care for total hip replacements, at least in the United States.[1] These new materials are cross-linked with gamma or electron beam radiation (50&#;105 kGy) and then thermally processed to improve their oxidation resistance.[1] Five-year clinical data, from several centers, are now available demonstrating their superiority relative to conventional UHMWPE for total hip replacement (see arthroplasty).[24] Clinical studies are still underway to investigate the performance of highly cross-linked UHMWPE for knee replacement.[24]
In , manufacturers started incorporating anti-oxidants into UHMWPE for hip and knee arthroplasty bearing surfaces.[1] 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.[26] Several companies have been selling antioxidant-stabilized joint replacement technologies since , using both synthetic vitamin E as well as hindered phenol-based antioxidants.[27]
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 produced by DSM under the "Dyneema Purity" trade name.[28]
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UHMWPE is used in the manufacture of PVC (vinyl) windows and doors, as it can endure the heat required to soften the PVC-based materials and is used as a form/chamber filler for the various PVC shape profiles in order for those materials to be 'bent' or shaped around a template.
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.
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Fluoropolymer / HMWPE insulation cathodic protection cable is typically made with dual insulation. It features a primary layer of a fluoropolymer such as ECTFE which is chemically resistant to chlorine, sulfuric acid, and hydrochloric acid. Following the primary layer is an HMWPE insulation layer, which provides pliable strength and allows considerable abuse during installation. The HMWPE jacketing provides mechanical protection as well.[29]
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UHMWPE is used in marine structures for the mooring of ships and floating structures in general. The UHMWPE forms the contact surface between the floating structure and the fixed one. Timber was and is used for this application also. UHMWPE is chosen as facing of fender systems for berthing structures because of the following characteristics:[30]
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