Low-density polyethylene

Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization.[1] Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE (recycling number 4) is recycled in the United States.[2] 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.[3]

LDPE has SPI resin ID code 4
Schematic of LDPE branching structure

Properties

LDPE is defined by a density range of 0.917–0.930 g/cm3.[4] It is not reactive at room temperatures, except by strong oxidizing agents, and some solvents cause swelling. It can withstand temperatures of 80 °C continuously and 90 °C (194 °F) for a short time. Made in translucent or opaque variations, it is quite flexible and tough.

LDPE has more branching (on about 2% of the carbon atoms) than HDPE, so its intermolecular forces (instantaneous-dipole induced-dipole attraction) are weaker, its tensile strength is lower, and its resilience is higher. Also, because its molecules are less tightly packed and less crystalline due to the side branches, its density is lower.

When exposed to ambient solar radiation the plastic produces two greenhouse gases, methane and ethylene. Due to its low density properties (branching) it breaks down more easily over time, leading to higher surface areas. The production of these trace gases from virgin LDPE increase with surface area/time, with rates at the end of a 212-day incubation of 5.8 nmol g-1 d-1 of methane, 14.5 nmol g-1 d-1 of ethylene, 3.9 nmol g-1 d-1 of ethane and 9.7 nmol g-1 d-1 of propylene. When incubated in air, LDPE emits gases at rates ~2 times and ~76 times higher in comparison to water for methane and ethylene, respectively.[5]

Chemical resistance

Applications
A GEECO bowl, c.1950, still used in 2014.
A piece of packaging foam made from LDPE
A Ziploc bag made from LDPE
Facial wash gel bottle made of LDPE

LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and various molded laboratory equipment. Its most common use is in plastic bags. Other products made from it include:

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See also

References
  1. Dennis Malpass (2010). Introduction to Industrial Polyethylene: Properties, Catalysts, and Processes. John Wiley and Sons. pp. 1–. ISBN 978-0-470-62598-9.
  2. "Municipal Solid Waste Generation, Recycling, and Disposal in the United States" (PDF). EPA.gov.
  3. "Market Study: Polyethylene LDPE (2nd edition)". Ceresana.
  4. BPF. "Polyethylene (Low Density) LDPE, LLDPE". www.bpf.co.uk. Retrieved 2018-06-14.
  5. Karl, David M.; Wilson, Samuel T.; Ferrón, Sara; Royer, Sarah-Jeanne (1 August 2018). "Production of methane and ethylene from plastic in the environment". PLOS ONE. 13 (8): e0200574. doi:10.1371/journal.pone.0200574. ISSN 1932-6203. PMC 6070199. PMID 30067755. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
  6. "LDPE Chemical Compatibility Chart". CP Lab Safety.
  7. LDPE products and applications. Exxon Mobil Corporation
  8. DOW LDPE 5004I. IDES – The Plastics Web
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