High-density polyethylene

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HDPE has SPI resin ID code 2 Symbol Resin Code 2 HDPE.svg
HDPE has SPI resin ID code 2

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. [1] 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.

Contents

In 2008, the global HDPE market reached a volume of more than 30 million tons. [2]

Properties

Thermophysical properties of high density polyethylene (HDPE) [3]
Density961 kg/m3
Melting point131.8 °C (269.24 °F)
Temperature of crystallization121.9 °C (251.42 °F)
Latent heat of fusion188.6 kJ/kg.
Thermal conductivity 0.54 W/m.°C. at °C.
Specific heat capacity1331 to 2400 J/kg-K
Specific heat (solid)2.9 kJ/kg. °C.
Crystallinity61%

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 of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength (38 MPa versus 21 MPa) than LDPE. [6] The difference in strength exceeds the difference in density, giving HDPE a higher specific strength. [7] 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.

HDPE is resistant to many different solvents, 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, in rotational molding, to identify the environmental stress crack resistance of a sample, the notched constant tensile load test (NCTL) is put to use. [8]

Owing to these desirable properties, pipes constructed out of HDPE are ideally applicable for drinking water [9] and waste water (storm and sewage). [10]

Applications

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 sheet which has been extrusion welded T Joint cross section.jpg
HDPE sheet which has been extrusion welded

HDPE is also used for cell liners in United States 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. [2] The material has benefited from discussions about possible health and environmental problems caused by PVC and polycarbonate associated bisphenol A (BPA), as well as its advantages over glass, metal, and cardboard.

Production

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.

See also

Related Research Articles

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Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, cups, jars, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

<span class="mw-page-title-main">Thermoplastic</span> Plastic that softens with heat and hardens on cooling

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<span class="mw-page-title-main">Polypropylene</span> Thermoplastic polymer

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<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

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<span class="mw-page-title-main">Low-density polyethylene</span> Chemical compound

Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Dr John C. Swallow and M.W Perrin who were working for Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE is recycled in the United States. Despite competition from more modern polymers, LDPE continues to be an important plastic grade. In 2013 the worldwide LDPE market reached a volume of about US$33 billion.

<span class="mw-page-title-main">Linear low-density polyethylene</span> Polymer

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<span class="mw-page-title-main">Plastic bottle</span> Narrow-necked container

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<span class="mw-page-title-main">Plastic pipework</span> Tubular section or hollow cylinder made of plastic

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<span class="mw-page-title-main">Twinwall plastic</span>

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<span class="mw-page-title-main">Expanded polyethylene</span>

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HDPE pipe is a type of flexible plastic pipe used to transfer fluids and gases. It is often employed for replacing aging concrete or steel main pipelines. Constructed from the thermoplastic HDPE, it has low permeability and robust molecular bonding, making it suitable for high-pressure pipelines. HDPE pipe is often used for water mains, gas mains, sewer mains, slurry transfer lines, rural irrigation, fire-suppression system supply lines, electrical and communication conduits, and stormwater and drainage pipes.

References

  1. ↑ Pipe materials. level.org.nz
  2. 1 2 "Market Study: Polyethylene HDPE". Ceresana Research.
  3. ↑ Araújo, J. R.; Waldman, W. R.; De Paoli, M. A. (2008-10-01). "Thermal properties of high density polyethylene composites with natural fibres: Coupling agent effect". Polymer Degradation and Stability. 93 (10): 1770–1775. doi:10.1016/j.polymdegradstab.2008.07.021. ISSN   0141-3910.
  4. ↑ Thermoforming HDPE Archived 2012-02-05 at the Wayback Machine . Dermnet.org.nz
  5. ↑ Typical Properties of Polyethylene (PE). Ides.com. Retrieved on 2011-12-30.
  6. ↑ Askeland, Donald R. (2016). The science and engineering of materials. Wendelin J. Wright (7 ed.). Boston, MA. p. 594. ISBN   978-1-305-07676-1. OCLC   903959750.{{cite book}}: CS1 maint: location missing publisher (link)
  7. ↑ Compare Materials: HDPE and LDPE. Makeitfrom.com. Retrieved on 2011-12-30.
  8. ↑ www.rotomolding.org. Retrieved 2016-4-20.
  9. 1 2 3 "Acu-Water | HDPE Blueline Water Pipe". Acu-Tech Piping Systems.
  10. 1 2 "Acu-Sewer Pressure Pipe for Sewer Mains". Acu-Tech Piping Systems.
  11. ↑ "Puck Board (HDPE Sheets)". Professional Plastics. Retrieved 24 December 2018.
  12. ↑ AstroRad. European Space Agency . 25 January 2019.
  13. ↑ Gaza, Razvan (14 July 2018). "International Science Aboard Orion EM-1: The Matroshka AstroRad Radiation Experiment (MARE) Payload" (PDF). nasa.gov. Retrieved 27 August 2019.
  14. ↑ "Acu-Gas Yellow High Pressure HDPE Pipe". Acu-Tech Piping Systems.
  15. ↑ Dermnet.org.nz Archived 2013-04-12 at the Wayback Machine . Dermnet.org.nz (2011-07-01). Retrieved on 2011-12-30.
  16. ↑ "Acu-Comms White Communications Conduit". Acu-Tech Piping Systems.
  17. ↑ "Ziegler-Natta catalyst | Polymerization, Olefins, Alkylaluminums | Britannica". www.britannica.com. Retrieved 2023-11-16.
  18. 1 2 Dunn, A. S. (1990). "Principles of polymer systems, 3rd edn ferdinand rodriguez, taylor & francis, new york, 1989. pp. xiv + 640, £35.00. isbn 0-89116-176-7". British Polymer Journal. 23 (4): 361. doi:10.1002/pi.1990.4980230411. ISSN   0007-1641.
  19. 1 2 Gasson, Peter C. (June 2011). "Materials Sciences and Engineering – Eighth edition. W. D. Callister and D. G. Rethwisch John Wiley and Sons, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK. 2010. 968pp. Illustrated. £47.99. ISBN 978-0-470-50586-1". The Aeronautical Journal. 115 (1168): 388–389. doi:10.1017/s0001924000005947. ISSN   0001-9240.
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