High-density polyethylene (HDPE) orpolyethylene high-density (PEHD) is athermoplasticpolymer produced from the monomerethylene. It is sometimes called "alkathene" or "polythene" when used forHDPE pipes.[1] With a high strength-to-density ratio, HDPE is used in the production ofplastic bottles, corrosion-resistant piping,geomembranes andplastic lumber. HDPE is commonly recycled, and has the number "2" as itsresin identification code.
In 2008, the global HDPE market reached a volume of more than 30 million tons.[2]
| Density | 961 kg/m3 |
|---|---|
| Melting point | 131.8 °C (269.24 °F) |
| Temperature of crystallization | 121.9 °C (251.42 °F) |
| Latent heat of fusion | 188.6 kJ/kg. |
| Thermal conductivity | 0.54 W/m.°C. at °C. |
| Specific heat capacity | 1331 to 2400 J/kg-K |
| Specific heat (solid) | 2.9 kJ/kg. °C. |
| Crystallinity | 61% |
HDPE is known for its high strength-to-density ratio.[4] The density of HDPE ranges from 930 to 970 kg/m3.[5] Although the density of HDPE is only marginally higher than that oflow-density polyethylene, HDPE has littlebranching, giving it strongerintermolecular forces andtensile strength (38 MPa versus 21 MPa) than LDPE.[6] The difference in strength exceeds the difference in density, giving HDPE a higherspecific strength.[7] It is also harder and more opaque and can withstand somewhat highertemperatures (120 °C/248 °F for short periods). High-density polyethylene, unlikepolypropylene, cannot withstand normally requiredautoclaving conditions. The lack of branching is ensured by an appropriate choice ofcatalyst (e.g.,Ziegler–Natta catalysts) andreaction conditions.
HDPE is resistant to many differentsolvents, and is exceptionally challenging to glue; joints are typically made by welding.
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 is the international standardized testing methods employed to identify these properties for a specific process. For example, inrotational molding (rotomolding), to identify the environmental stress crack resistance of a sample, thenotched constant tensile load test (NCTL) is put to use.[8]
Owing to these desirable properties, pipes constructed out of HDPE are ideally applicable fordrinking water[9] and waste water (storm and sewage).[10]
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HDPE has a wide variety of applications; for applications that fall within the properties of other polymers, the choice to use HDPE is usually economic:
HDPE is also used forcell liners in United Statessubtitle Dsanitary landfills, wherein large sheets of HDPE are eitherextrusion welded or wedge welded to form ahomogeneous chemical-resistant barrier, with the intention of preventing thepollution ofsoil andgroundwater by the liquid constituents ofsolid waste.
HDPE is preferred by thepyrotechnics trade for mortars over steel orPVC 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 throughblow 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 improvingstandard of living. In India and other highly populated, emerging nations, infrastructure expansion includes the deployment of pipes and cable insulation made from HDPE.[2] The material has benefited from discussions about possible health and environmental problems caused by PVC andpolycarbonate associatedbisphenol A (BPA), as well as its advantages over glass, metal, and cardboard.
Industrial production of HDPE from ethylene happens through either Ziegler-Natta polymerization or the Phillips slurry process. The Ziegler-Natta method uses a combination of catalysts, including titanium tetrachloride, in contact with gaseous ethylene to precipitate high-density polyethylene.[17] In a similar way, the Phillips slurry process uses silica-based catalysts in contact with a fast-moving hydrocarbon and polyethylene slurry to precipitate high density polyethylene.[18]
Processing will determine the properties of the HDPE. The method used to synthesize the HDPE is crucial because the micro structure of the HDPE will vary. The Phillips Slurry process results in HDPE with less branching and more precise molecular weights than the Ziegler process, but the Ziegler process provides greater flexibility in the type of polyethylene produced.[18]
The molecular weight of HDPE refers to the length of the polyethylene chains, and helps determine properties such as flexibility, yield strength, and melt temperature. After the precipitate is formed, the temperature, pressure, and cooling time during processing will dictate the degree of crystallinity, with a higher degree of crystallinity resulting in greater rigidity and chemical resistance.[19] Depending on the application, the method and processing steps can be adjusted for an ideal result.
Once the HDPE has been synthesized, it is ready to be used in commercial products. Industrial production methods for HDPE products include injection molding for complex shapes such as toys. Extrusion molding is used for constant-profile products such as pipes and films. Blow molding is intended for hollow products, specifically bottles and plastic bags. Rotational molding is used for large, seamless parts such as chemical drums and kayaks.[19] The method used during processing depends on the product requirements, with each having benefits for a given application.
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