What Is It?
Polyethylene ()n, first synthesized in 1933, looks like the simplest of molecules, but the number of ways in which the -- units can be linked is large. It is the first of the polyolefins, the bulk thermoplastic polymers that account for a dominant fraction of all polymer consumption. Polyethylene is inert, and extremely resistant to fresh and salt water, food, and most water-based solutions. Because of this it is widely used in household products and food containers.
PE is commercially produced as film, sheet, rod, foam, and fiber. Drawn PE fiber has exceptional mechanical stiffness and strength, exploited in geo-textile and structural uses. Polyethylene is cheap, and particularly easy to mold and fabricate. It accepts a wide range of colors, can be transparent, translucent, or opaque, has a pleasant, slightly waxy feel, can be textured or metal coated, but is difficult to print on. PE is a good electrical insulator with low dielectric loss, so suitable for containers for microwave cooking.
Oil containers, street bollards, milk bottles, toys, beer crates, food packaging, shrink wrap, squeeze tubes, disposable clothing, plastic bags, Tupperware, chopping boards, paper coatings, cable insulation, artificial joints, and as fibers – low cost ropes and packing tape reinforcement.
PE is FDA compliant – indeed it is so non-toxic that it can be embedded in the human body (heart valves, hip-joint cups, artificial artery). PE, PP, and PVC are made by processes that are relatively energy-efficient, making them the least energy intensive of commodity polymers. PE can be produced from renewable resources – from alcohol derived from the fermentation of sugar or starch, for instance. Its utility per kilogram far exceeds that of gasoline or fuel-oil, so that production from oil will not disadvantage it in the near future. Polyethylene is readily recyclable if it has not been coated with other materials, and – if contaminated – it can be incinerated to recover the energy it contains.
Low density polyethylene (LDPE), used for film and packaging, has branched chains which do not pack well, making it less dense than water. Medium (MDPE) and high (HDPE) density polyethylenes have longer, less branched chains, making them stiffer and stronger; they are used for containers and pipes. Linear low density polyethylene (LLDPE) is less resistant to organic solvents, but even this can be overcome by converting its surface to a fluoro-polymer by exposing it to fluorine gas. Treated in this way (when it is known as “Super PE”) it can be used for petrol tanks in cars and copes with oil, cleaning fluid, cosmetics, and that most corrosive of substances: cola concentrate. Very low density polyethylene (VLDPE) is similar to EVA and plasticized PVC.