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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.
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.
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.
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.