High Performance Polyethylene Rope

Polyethylene is the most widely used polymer plastic. It is used for grocery bags, trash bags, drop cloths for painters, the plastic harness for beverage cans in a 6-pack, and many more products. How this "everyday" polymer is transformed into the "top-of-the-line" of high-performance industrial fibers is the story of major advances in polymer chemistry, physics, and industrial processes in the last 50 years. Polyethylene was first made in 1933 by Reginald Gibson and Eric Fawcett at Imperial Chemical Industries in England using a high-pressure reaction vessel. At 2000 atmospheres pressure and 170º C. they obtained a small amount of a white, waxy solid that they determined to be a polymer of ethylene.

Polymers are chemical compounds that consist of long, chainlike molecules made up of multiple repeating units. Polyethylene is a long chain of carbon and hydrogen atoms held together by covalent bonds. (A covalent bond holds two atoms together by sharing one or more electrons. A single bond means that only one electron is shared between two atoms, whereas a double bond, represented by a double line, means that two electrons are shared.) The chain can be very long. The diagram below, which shows the structure of a rather short polyethylene molecule, can be thought of as the top view. It shows a chain of carbon atoms with hydrogen atoms bonded to both sides. The chain appears to be straight.

However, when this structure is viewed from side, as shown in the middle drawing below, the chain of carbon atoms is corrugated, not straight. (In this view H₂ means that one hydrogen atom is coming out of the screen towards you and the other hydrogen is hidden from view behind the carbon atom.) Thus, it is easy for the molecular chain to bend, kink, and fold back on itself.

The high-pressure process was used to make polyethylene during the Second World War. The polymer was extruded to make film and coated around wires and cables for insulation. It was in high demand for submarine cables, radar, and other applications. The high-pressure process, however, was costly.

In 1953 Karl Ziegler discovered a catalyst that allowed polyethylene to be produced under conditions much closer to room temperature and pressure. The polymerization process is illustrated in the diagram above. In a reactor vessel ethylene gas is injected into a slurry contain solvent, some polyethylene, and catalyst. In the presence of the catalyst an ethylene molecule attaches itself to the end of the polyethylene chain molecule and the molecule becomes longer. The process continues by adding molecules of ethylene to the ends of the polyethylene chains, producing very long polymer chains. This process revolutionized the manufacture of polyethylene, greatly lowering production costs and permitting a wide variety of products to be manufactured. Ziegler was awarded the Nobel Prize for Chemistry in 1964 for his discovery.

The ethylene repeat unit or monomer is shown in the condensed formula in the diagram on the right above. In the commercial product high-molecular weight polyethylene, also known as high-density polyethylene (HDPE), n is on the order of 100,000. It takes approximately 10⁵ moles of ethylene gas to make 1 mole of HDPE. By carefully selecting the catalyst and precisely controlling the manufacturing conditions even longer polymers can be made. Ultra-high molecular weight polyethylene (UHMWPE) is the starting material to make high-performance polyethylene rope. The molecular weight of UHMWPE is in the range of 4 to 6 million. This means that if one took 1 mole or 1 gram molecular weight of these ultra-long molecules, it would weigh between 4 and 6 metric tonnes. By comparison 1 mole or 1 gram molecular weight of iron weighs only 55.5 grams. (There are 6.023 x 10²³ molecules in a mole, a.k.a. Avagadro's number.)

Because the molecular chains are so long and the chain structure is "corrugated," polyethylene forms small domains of folded chain. These domains are called crystallites because they give good "reflections" in an X-ray diffraction pattern. In polyethylene used as starting material for many products, these domains have random orientations.

However, when the polyethylene is formed into a sheet, ribbon, or fiber and stretched or drawn, the crystallites become aligned, i.e. they develop a preferred orientation.

Preferred orientation has a large influence on strength and other material properties of polyethylene. In order to produce high-strength, high-performance fiber, the kinks, folds, and tangles need to be taken out of very long molecular chains. How does one untangle a bowl of long spaghetti? How does one untangle a large ball of tangled fishing line?

Gel-spinning is a major advance in industrial processing. UHMWPE is purchased from a supplier in the form of "crumbs" or granules. The UHMWPE is dissolved in an organic solvent to make a 1% solution or gel. Thus, long thread-like molecules are widely separated and "swimming" in a liquid. As shown in the diagram above, the gel is forced under pressure out through many small openings of a spinnerette to form "fibers." Each "fiber" contains a number of long chain molecules–the polyethylene polymer–oriented parallel to one another. The fibers go around the large drum in the diagram. However, this drum rotates fast enough to greatly stretch the fiber. The result is that more folds and kinks are eliminated. The end product of this process is Spectra 900, a high-strength, high-performance fiber. As can be seen in the photo below, when a large load was placed on a Spectra rope wound around the bitt, it was the steel in the bitt that failed, not the rope. When installed, the bitt had a uniform diameter. The white arrows show where the diameter was reduced. (Bitts are hardware attached to the deck of a ship to secure ropes and lines.)

Plasma rope has the highest strength and lowest stretch of any commercially-available synthetic rope. The stretch at failure in tensile tests is less than 5%. Using a 10:1 safety factor, the stretch at the maximum working load is less than 0.5 %.

With its trademark purple color, Plasma 12 Strand rope is shown in the photo to the left. The low stretch of this rope is close to the stretch of the electrical conductor used in Precision Lift's electric long line.