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

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 (resin identification code 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]

Properties

LDPE is defined by a density range of 917–930 kg/m3.[4] At room temperature it is not reactive, except to strong oxidizers; some solvents cause it to swell. It can withstand temperatures of 65 °C (149 °F) continuously[4] and 90 °C (194 °F) for a short time. Made in translucent and 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. The side branches mean that its molecules are less tightly packed and less crystalline, and therefore its density is lower.

When exposed to consistent sunlight, the plastic produces significant amounts of two greenhouse gases: methane and ethylene. Because of its lower density (high branching), it breaks down more easily than other plastics; as this happens, the surface area increases. Production of these trace gases from virgin plastics increases with surface area and with time, so that LDPE emits greenhouse gases at a more unsustainable rate than other plastics. In a test at the end of 212 days' incubation, emissions recorded were 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 methane and ethylene at rates about 2 times and about 76 times, respectively, more than in water.[5]

The standard method to test plastic density is ISO 1183 part 2 (gradient columns), alternatively ISO 1183 part 1 (MVS2PRO density analyzer).[6]

Chemical resistance

Applications

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

Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic.[9] 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:

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,n word and Disposal in the United States" (PDF). EPA.gov.
  3. ^ "Market Study: Polyethylene LDPE (2nd edition)". Ceresana.
  4. ^ a b 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.
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    Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
  6. ^ "PLASTIC DENSITY IN 2 MINUTES". www.plastic-density.com. Retrieved 2021-06-18.((cite web)): CS1 maint: url-status (link)
  7. ^ Qiu, Xia-Wen; Pei, Jie; Wu, Chen-Chou; Song, Lin; Bao, Lian-Jun; Zeng, Eddy Y. (2021-08-01). "Determination of low-density polyethylene–water partition coefficients for novel halogenated flame retardants with the large volume model and co-solvent model". Chemosphere. 277: 130235. doi:10.1016/j.chemosphere.2021.130235. ISSN 0045-6535.
  8. ^ "LDPE Chemical Compatibility Chart". CP Lab Safety.
  9. ^ a b Achilias, D. S.; Roupakias, C.; Megalokonomos, P.; Lappas, A. A.; Antonakou, Ε. V. (2007-11-19). "Chemical recycling of plastic wastes made from polyethylene (LDPE and HDPE) and polypropylene (PP)". Journal of Hazardous Materials. Pollution Prevention and Restoration of the Environment. 149 (3): 536–542. doi:10.1016/j.jhazmat.2007.06.076. ISSN 0304-3894.
  10. ^ LDPE products and applications. Exxon Mobil Corporation
  11. ^ DOW LDPE 5004I. IDES – The Plastics Web