chemistry

Ultra-high-molecular-weight polyethylene

Ultra-high-molecular-weight polyethylene is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ultra-high-molecular-weight polyethylene rather than just read about it. In short: 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 typically between 2 and 6 million daltons.

Key takeaways

Ultra-high-molecular-weight polyethylene belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ultra-high-molecular-weight polyethylene to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ultra-high-molecular-weight polyethylene from memory before moving on to harder problems.

Reference excerpt

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 typically between 2 and 6 million daltons. 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. UHMWPE is odorless, tasteless, and nontoxic. It embodies all the characteristics of high-density polyethylene (HDPE), i.e., being resistant to acids, alkalis, and many corrosive chemicals except some oxidizing acids. It has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating (see boundary lubrication); and is highly resistant to abrasion. 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.

Development Polymerization of UHMWPE was commercialized in the 1950s by Ruhrchemie AG, 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 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 1960s, UHMWPE has also been the material of choice for total joint arthroplasty in orthopedic and spine implants. UHMWPE fibers branded as Dyneema, commercialized in the late 1970s 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, protective motorcycling gear, sailing, hiking equipment, climbing, and many other industries.

Structure and properties

UHMWPE is a type of polyethylene. 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. 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, aramid derives its strength from strong bonding between relatively short molecules. Its heat resistance is poorer than other high-strength fibers. Its melting point onset is 127.7 °C (261.9 °F), and, according to DSM, it is not advisable to use UHMWPE fibres at temperatures exceeding 90 °C (194 °F) for long periods of time. It becomes brittle at temperatures below −150 °C (−238 °F). 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 does it 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 and 138 °C (275 and 280 °F) in an oven or a liquid bath of silicone oil or glycerine. The material is then cooled down to 65 °C (149 °F) at a rate of 5 °C/h (9 °F/h) or less. Finally, the material is wrapped in an insulating blanket for 24 hours to bring to room temperature. UHMWPE is considered a relatively low-modulus material, with tensile strength and hardness values lower than those of many other high-performance polymers. Typical tensile yield strength values range from 19.3 to 23 MPa (2,800 to 3,340 psi). The Shore D hardness is approximately 60 to 65, which is slightly lower than that of high-density polyethylene (HDPE). Despite its lower modulus, the extremely long polymer chains and high molecular weight give UHMWPE exceptional toughness and impact resistance. UHMWPE exhibits the highest impact strength of any thermoplastic, and in standard notched Izod tests specimens often do not break (values > 1,070 J/m (240 ft⋅lb/ft)). Its elongation at break can reach 250%–450%, and its lower chain-packing efficiency gives a density of about 0.93–0.94 g/cm3 (0.54–0.54 oz/cu in), a little lower than that of HDPE.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Ultra-high-molecular-weight polyethylene

Start with the simplest possible case. Write down what Ultra-high-molecular-weight polyethylene claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ultra-high-molecular-weight polyethylene before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ultra-high-molecular-weight polyethylene ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ultra-high-molecular-weight polyethylene

In research: Ultra-high-molecular-weight polyethylene appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ultra-high-molecular-weight polyethylene in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ultra-high-molecular-weight polyethylene is common in secondary-school and first-year university syllabi. It links to neighbouring topics Body armor, Brand name materials, Bulletproofing, so understanding it makes those chapters shorter.
In everyday life: Look for Ultra-high-molecular-weight polyethylene outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ultra-high-molecular-weight polyethylene in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ultra-high-molecular-weight polyethylene means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ultra-high-molecular-weight polyethylene out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ultra-high-molecular-weight polyethylene in simple terms?

Why does Ultra-high-molecular-weight polyethylene matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ultra-high-molecular-weight polyethylene?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ultra-high-molecular-weight polyethylene.