Polyethylene

OVERVIEW

Polyethylene (pol-ee-ETH-uh-leen) is a thermosetting white solid resistant to high temperatures, most inorganic and organic chemicals, and physical impact. It is also an electrical non-conductor. A thermosetting polymer is one that, once it is melted and formed, can not be re-melted. Polyethylene is available in a variety of forms, the most common of which are high-density (HD or HDPE), low density (LD or LDPE), linear low density (LLD or LLDPE) and cross-linked (CLPE). These forms of the compound differ with respect to the structure of the polyethylene chains and their relationship to each other. For example, if all of the polyethylene chains are straight chains without branches, they can pack together tightly forming a high density product. By contrast, low density polyethylene consists of shorter chains with many side branches on them. The side branches prevent adjacent polymer chains from getting too close to each other. In cross-linked polyethylene, adjacent polymer chains actually form chemical bonds with each other, holding them in a regular, almost crystalline pattern.

KEY FACTS

OTHER NAMES:

Ethene homopolymer

FORMULA:

-[-CH₂-CH₂-]-_n

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Organic polymer

STATE:

Solid

MOLECULAR WEIGHT:

1,500 to 100,000 g/mol

MELTING POINT:

Varies: about 85°C −110°C (185°F-230°F)

BOILING POINT:

Not applicable; decomposes above melting point

SOLUBILITY:

Insoluble in water and most organic solvents; soluble in hydrocarbons and halogenated hydrocarbons

Polyethylene was first prepared accidentally in 1889 by the German chemist Hans von Pechmann (1850–1904). Von Pechmann was heating diazomethane (H₂C=N=N) when he observed the formation of a white, waxy solid. When his colleagues identified the substance as containing repeated methylene (-CH₂) units, they called the material polymethylene. Von Pechmann did not pursue his discovery, nor did any of his colleagues until the 1930s. Then, in 1933, two chemists at the English firm of Imperial Chemicals Industries (ICI), Reginald Gibson and Eric Fawcett, accidentally re-discovered polyethylene. While attempting to pressurize a mixture of ethylene and benzaldehyde, Gibson and Fawcett observed the formation of a white waxy solid within the pressure vessel. At first, they were unable to account for this reaction. Eventually, however, they discovered that a tiny hole in the pressure vessel had allowed oxygen to seep into the tank, catalyzing the conversion of ethylene to polyethylene. Two years later, two other ICI chemists, J. C. Swallow and M. W. Perrin duplicated the Gibson-Fawcett experiment and devised an efficient commercial method for making the polymer.

Beginning in the 1950s, researchers in a number of countries began to explore the use of catalysts to increase the efficiency of reactions by which polyethylene is made. For example, Robert Banks and John Hogan at Phillips Petroleum invented a procedure using chromium trioxide (Cr₂O₃) for the preparation of high-density polyethylene and another form of the product known as crystalline polyethylene. In 1953, the German chemist Karl Ziegler (1898–1973) used titanium halides and organoaluminum compounds to make polyethylene under even lower temperatures and pressures. And in 1976, the German chemists Walter Kaminsky (1941–) and Hansjörg Sinn (1929–) invented a third method of production using metallocenes (organic compounds that contain a metal) as a catalyst.

HOW IT IS MADE

Polyethylene is made by polymerizing ethylene (ethene; CH₂=CH₂). Polymerization occurs when the double bond in ethylene breaks, allowing one molecule of ethylene to combine with a second molecule of ethylene: CH₂=CH₂ + CH₂=CH₂ → CH₃CH₂CH=CH₂. The product of that reaction also contains a double bond, allowing the reaction to be repeated: CH₃CH₂CH=CH₂ + CH₂=CH₂ → CH₃CH₂CH₂CH₂CH=CH₂. Once again, the final product contains a double bond, and the reaction can be repeated again and again and again.

Interesting Facts

Phillips Petroleum had difficulty maintaining quality control in the early years of making polyethylene. As a result, it produced large quantities of the product that could not be sold for commercial, industrial, or household use. The company faced financial ruin. Fortunately, a new toy came into existence in the mid-1950s, the hula-hoop. A hula-hoop is simply a ring of plastic that could be made with low-grade polyethylene. The first company to market hula-hoops, called Wham-O, bought up much of Phillips' defective polyethylene stock for its hula-hoops. The company sold more than twenty million hula-hoops in its first six months of existence at a cost of $1.98 each. Wham-O was a booming success, and it saved Phillips from financial ruin.

Polymerization occurs when some outside agent provides the energy to break the double bond in ethylene to get the reaction started. Heat, light, ultraviolet radiation, a beam of electrons, and gamma rays have all been used to initiate polymerization. Polymerization occurs at lower energy levels, and it has been the search for catalysts to achieve this objective that have led to the processes developed by Banks and Hogan, Ziegler, Kaminsky and Sinn, and other researchers.

COMMON USES AND POTENTIAL HAZARDS

An estimated 55 million metric tons (60 million short tons) of polyethylene are produced worldwide each year. In the United States, an estimated 7 million metric tons (8 million short tons) of high-density polyethylene will be produced in 2006 and an estimated 2.7 million metric tons (3.0 million short tons) of low-density polyethylene will be made. The greatest fraction of HDPE is used in the manufacture of molded products, film and sheeting, pipes and tubing, fibers, and gasoline and oil containers. The most important application of LDPE is in the manufacture of packaging films for foods as well as coatings that are sprayed or otherwise applied to all kinds of surfaces. It is also used widely to make liners for drums and other shipping containers, wire and cable coating, trash bags, squeeze bottles, inexpensive dinnerware, drop cloths, swimming pool covers, toys, and electric insulation.

No health hazards from polyethylene in any form have yet been identified.

Words to Know

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

POLYMER

A compound consisting of very large molecules made of one or two small repeated units called monomers.

FOR FURTHER INFORMATION

Meikle, Jeffrey L. American Plastic: A Cultural History. http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc14/icsc1488.htm (accessed on October 24, 2005).

"Polyethylene." In World of Invention. 2nd ed. Edited by Kimberley A. McGrath and Bridget Travers. Detroit, MI: Gale, 1999.

"Polyethylene Specifications." Boedeker Plastics. http://www.boedeker.com/polye_p.htm (accessed on October 24, 2005).

See AlsoPolypropylene

pol·y·eth·yl·ene / ˌpälēˈe[unvoicedth]əlēn/ • n. a tough, light, flexible synthetic resin made by polymerizing ethylene, chiefly used for plastic bags, food containers, and other packaging.