Polyethylene is a type of polymer that is thermoplastic, meaning that it can be melted to a liquid and remolded as it returns to a solid state. It is chemically synthesized from ethylene, a compound that's usually made from petroleum or natural gas. Other non-official names for this compound include polythene or polyethylyne; and it is also abbreviated as PE. It is used in making other plastic compounds much often than it's used in its pure form. Though it has a wide variety of uses, it can be harmful to humans and to the environment.
Production and Uses
Of all the plastics produced for industrial and commercial products, polyethylene is the most common. As an example, 280 million metric tonnes of it was produced in 2011 alone. Over five times as much PE is manufactured each year than a closely-related compound, polypropylene (PP). The largest use for these polymers is in packaging materials, like films and foam; and for bottles and other containers that can be used in food, medical, and other consumer industries.
The characteristics of a plastic can be adjusted by combining it with various plasticizers, which are substances added to plastics to make them more durable, flexible, and transparent. Adding chromium/silica makes high-density polyethylene (HDPE), which is used to create sturdy products like garbage containers. Combining it with organic olefin compounds makes a type of low-density PE (LDPE) that is used for plastic grocery or shopping bags. Other common forms of polyethylene are ultra high molecular weight PE (UHMWPE), which is used in bullet-proof vests and knee joint replacements; and medium-density PE (MDPE), which is crack resistant for applications in gas pipe pressure fittings.
Plastics based on the PE molecule are widespread because the compound has physical characteristics that are considered safe and useful in a range of environments. These traits include the fact that it remains pliable for an extended period of time while remaining inert and impervious to damage by most liquids. Since its softness and strength level can be easily adjusted and it can be dyed many colors, it is often used in consumer products from food wrap to shampoo bottles, milk containers, toys, and grocery bags.
Depending on the compounds its bonded with, PE's level of toxicity and flammability varies considerably. There are concerns about two versions of the compound in particular, both of which are often used for medical and consumer purposes. Polyethylene-glycol (PEG), which acts as a binding agent for many drugs and is also found in products like shampoo and toothpaste, can cause allergic reactions in certain individuals. Some people experience nausea, flatulence, and diarrhea after being exposed to it, while others get a hives-like rash. The elderly seem to be particularly prone to these side effects.
In addition, harmful chemicals — including the plasticizer phthalate — may leach from polyethylene-terephthalate (PET), which has been widely used in the plastic bottling industry. Phthalate is associated with hormonal imbalances, increases in allergies, and reduced fertility. Some studies show that it may also contribute to the development of obesity and breast cancer.
While PE may help to make many useful and durable products possible, its environmental impact concerns many experts. It doesn't biodegrade easily, and can sit in a landfill for hundreds of years. About 20%-24% of all landfill space in the US alone is taken up by plastics, including polyethylene products. However, recycling may reduce this problem, since PE scrap can be melted down and reused.
Additionally, an aerobic bacteria called Sphingomonas can significantly reduce the amount of time it takes some forms of PE to break down, though it is not yet widely used. Environmental preservation efforts have also led to the development of bioplastics, with the aim of creating polyethylene from ethanol made from sugarcane.