High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber. HDPE is commonly recycled, and has the number "2" as its resin identification code.
In 2007, the global HDPE market reached a volume of more than 30 million tons.
HDPE is known for its large strength-to-density ratio. The density of HDPE can range from 930 to 970 kg/m3. Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength than LDPE. The difference in strength exceeds the difference in density, giving HDPE a higher specific strength. It is also harder and more opaque and can withstand somewhat higher temperatures (120 °C/248 °F for short periods). High-density polyethylene, unlike polypropylene, cannot withstand normally required autoclaving conditions. The lack of branching is ensured by an appropriate choice of catalyst (e.g., Ziegler–Natta catalysts) and reaction conditions.
The physical properties of HDPE can vary depending on the molding process that is used to manufacture a specific sample; to some degree a determining factor are the international standardized testing methods employed to identify these properties for a specific process. For example, in Rotational Molding, to identify the environmental stress crack resistance of a sample, the Notched Constant Tensile Load Test (NCTL) is put to use.
HDPE is resistant to many different solvents and has a wide variety of applications:
- 3D printer filament
- Arena board (puck board)
- Backpacking frames
- Ballistic plates
- Bottle caps
- Chemical-resistant piping
- Coax cable inner insulator
- Conduit protector for electrical or communications cables
- Corrosion protection for steel pipelines
- Electrical and plumbing boxes
- Far-IR lenses
- Folding chairs and tables
- Food storage containers
- Fuel tanks for vehicles
- Geomembrane for hydraulic applications (such as canals and bank reinforcements) and chemical containment
- Geothermal heat transfer piping systems
- Heat-resistant firework mortars
- Housewrap (Tyvek)
- Hovercraft: The material is too heavy and dense for such craft but is still used occasionally
- Lasts for shoes
- Microwave telescope windows
- Natural gas distribution pipe systems
- Piping for fluid, slurry and gas purposes
- Plastic bags
- Plastic bottles suitable both for recycling (such as milk jugs) or re-use
- Plastic lumber
- Plastic surgery (skeletal and facial reconstruction)
- Potable water mains
- Root barrier
- Sewage mains
- Snowboard rails and boxes
- Stone paper
- Storage sheds
- Swimming pool installation
- Telecom ducts
- Water pipes for domestic water supply and agricultural processes 
- Wood plastic composites (utilizing recycled polymers)
HDPE is also used for cell liners in subtitle D sanitary landfills, wherein large sheets of HDPE are either extrusion welded or wedge welded to form a homogeneous chemical-resistant barrier, with the intention of preventing the pollution of soil and groundwater by the liquid constituents of solid waste.
HDPE is preferred by the pyrotechnics trade for mortars over steel or PVC tubes, being more durable and safer: HDPE tends to rip or tear in a malfunction instead of shattering and becoming shrapnel like the other materials.
Milk bottles, jugs, and other hollow goods manufactured through blow molding are the most important application area for HDPE, accounting for one-third of worldwide production, or more than 8 million tonnes.
Above all, China, where beverage bottles made from HDPE were first imported in 2005, is a growing market for rigid HDPE packaging, as a result of its improving standard of living. In India and other highly populated, emerging nations, infrastructure expansion includes the deployment of pipes and cable insulation made from HDPE. The material has benefited from discussions about possible health and environmental problems caused by PVC and polycarbonate associated Bisphenol A, as well as its advantages over glass, metal, and cardboard.
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