What is polyethylene?

Any polyethylene is a polymer consisting of ethylene units (C 2 H 4, molecular weight 28 Da). It was first synthesized back in 1936, at high temperature and pressure, obtaining chains weighing from 20 thousand to 500 thousand, with long branches. Such high pressure polyethylene (LDPE) is still the most common, although using special catalysts, linear LDPE with shorter side chains is also obtained.

From top to bottom: 1) low-pressure polyethylene with a weakly branched chain; 2) linear high-pressure polyethylene (HDPE) with many short branches; 3) High pressure branched high pressure polyethylene

And since 1954, the polymer has been produced under fairly moderate conditions: low-pressure polyethylene with a mass of 80 thousand to 800 thousand differs in high strength and softening temperature. Finally, using complex organometallic catalysts, they synthesize ultra-high molecular weight polyethylene (UHMWPE) weighing millions of daltons.

Ultra-high molecular weight polyethylene reinforced with chemical crosslinking is durable, has a low friction coefficient and high biocompatibility. This allows you to use it even for joint prostheses.

The UHMWPE chains are so long and tangled that in the molten state the polymer will be very viscous, remotely resembling rubber. Its processing requires special methods, but it allows to obtain products of exceptional properties. It’s like threads in a ball: short ones are easy to pull out without tearing, and long ones are tangled with each other and with themselves, it is much more difficult to pull them apart. The same is with polyethylene: the longer the molecular chains, the more there will be “links” between them. These physical cross-links make UHMWPE chemically inert and biocompatible, which allows its use in medical prostheses. The low coefficient of friction is comparable to Teflon, and the wear-resistant UHMWPE coatings can improve the performance of bearings.

Wear-resistant polyethylene is used even in dry friction units, such as plain bearings.

If you orient the UHMWPE chains in any general direction, such a polymer will become incredibly strong: a single fiber with a diameter thinner than a human hair (20-60 μm) can withstand a load of about 1 kg. In terms of unit mass, the strength of the ropes made from UHMWPE fibers will be much higher than that of steel. In this case, the density of the material will remain less than that of water: such cables do not sink and do not get wet.

The article “Plastic World” was published in the magazine Popular Mechanics (No. 12, December 2017). Do you like the article?

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