What is High-Density Polyethylene?

Karyn Maier
Karyn Maier
Karyn Maier
Karyn Maier
HDPE is often used to make laundry detergent bottles.
HDPE is often used to make laundry detergent bottles.

High-density polyethylene (HDPE) is a type of plastic made from petroleum. Since this material can be remolded by subsequent melting and shaping, it is classified as a polyethylene thermoplastic. It can also be joined in segments when welded or machined. However, it does not accept adhesives very well. Also known as polyethylene high-density (PEHD), products made of high-density polyethylene are marked by the imprint of the number “2” surrounded by the Möbius strip recognized as the universal recycling symbol.

Products made with high-density polyethylene are typically marked with a recycling symbol as a reminder to dispose of the product appropriately.
Products made with high-density polyethylene are typically marked with a recycling symbol as a reminder to dispose of the product appropriately.

As the name implies, HDPE is denser than most other polymer plastics, namely low-density polyethylene. This is due to its crystallization structure occurring in a linear fashion rather than branching out to form long chains of polyethylene. Instead, the lack of branching results in its carbon molecules bonding with more hydrogen molecules. This allows the final product to possess greater tensile strength, even though it is lighter than water. It also makes high-density polyethylene highly resistant to acids and solvents.

HDPE containers are equipped with the numeric symbol of 2 surrounded by the universal recycling symbol.
HDPE containers are equipped with the numeric symbol of 2 surrounded by the universal recycling symbol.

The production of high-density polyethylene does not happen by accident or natural event, however. In fact, the lack of branching during the polymerization process is deliberately induced by the addition of a type of reagent known as a Ziegler-Natta catalyst. Usually, these catalysts are derived from titanium compounds.

Since HDPE is so durable and chemically non-reactive, it has numerous applications in various industries. It is used in many different types of packaging containers, such as milk and laundry detergent bottles, as well as plastic grocery bags. It is also found in storage systems designed to store chemicals and fuels. In fact, high-density polyethylene is used to produce materials to act as chemical barriers, such as liners that are placed under landfills to help prevent soil and groundwater contamination. One of the most common uses of this material is in the manufacturing of wood plastic composites to make furniture, flooring, fencing, and landscaping materials.

Some people are concerned about the use of HDPE in teething rings.
Some people are concerned about the use of HDPE in teething rings.

In terms of environmental impact, products made of high-density polyethylene do not readily biodegrade in landfills. Such products can be recycled, though, albeit at the risk of losing some of its original tensile strength. Since this material is constructed of hydrogen and carbon, being subjected to high heat merely results in the release of water and carbon dioxide. However, additives, such as fire retardants, UV-stabilizers, and dyes, can produce other toxins. In addition, some environmental groups express concern over the potential hazard from the leeching of phthalates used in producing some children’s toys made from high-density polyethylene, such as teething rings.

Karyn Maier
Karyn Maier

Contributing articles to wiseGEEK is just one of Karyn’s many professional endeavors. She is also a magazine writer and columnist, mainly for health-related publications, as well as the author of four books. Karyn lives in New York’s Catskill Mountain region and specializes in topics about green living and botanical medicine.

Karyn Maier
Karyn Maier

Contributing articles to wiseGEEK is just one of Karyn’s many professional endeavors. She is also a magazine writer and columnist, mainly for health-related publications, as well as the author of four books. Karyn lives in New York’s Catskill Mountain region and specializes in topics about green living and botanical medicine.

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Discussion Comments

anon165874

what type of plastic is high density polythene and what is it used for?

anon148783

Is HDPE noncombustible?

anon119311

What is the main source of polythene high density? thanks

anon79116

All polyethylenes, both low density & high density are 100 percent recyclable and impervious to chemicals. (When battery acid came in boxes, the bags inside were low density polyethylene!) All are degraded back into their hydrocarbons by UV. Bury them and they degrade faster than glass. Leave them in sunlight and they degrade. (Cover your outside wood pile with a trash bag to experience this.)

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    • HDPE is often used to make laundry detergent bottles.
      HDPE is often used to make laundry detergent bottles.
    • Products made with high-density polyethylene are typically marked with a recycling symbol as a reminder to dispose of the product appropriately.
      Products made with high-density polyethylene are typically marked with a recycling symbol as a reminder to dispose of the product appropriately.
    • HDPE containers are equipped with the numeric symbol of 2 surrounded by the universal recycling symbol.
      HDPE containers are equipped with the numeric symbol of 2 surrounded by the universal recycling symbol.
    • Some people are concerned about the use of HDPE in teething rings.
      Some people are concerned about the use of HDPE in teething rings.