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The branched structure of LDPE.

Low-density polyethylene (LDPE) is one of several varieties of polyethylene (PE).[1] PE’s, the dominant synthetic polymer, are produced in many forms that differ in terms of molecular weight, branching, and the incorporation of comonomers. Often these features are captured in terms of the density of the material. All forms of PE are colorless, odorless, rather chemically inert solids produced by the polymerization of ethylene, the monomer. LLDPE is a substantially linear polymer (polyethylene), with significant numbers of short branches. In 2013 the worldwide LDPE market reached a volume of about US$33 billion.[2] A variety of LDPE’s are known, which mainly differ by the degree of linearity vs branching.[1]

LDPE can be further classified: LDPE and linear low-density PE. The former is produced by free-radical polymerization and the latter by coordination polymerization (e.g. Ziegler-Natta catalysis.)

Properties

LDPE is a thermoplastic. PE’s are often classified by their densities.[3]

Polyethylene (PE) densities and structure[1]
PE type density (g/cm3) structure
LDPE 0.924 highly branch, some cross-links
LLDPE 0.922 some short branches
HDPE 0.961 no branching

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.

Chemical resistance

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[3] and 90 °C (194 °F) for a short time. Made in translucent and opaque variations, it is quite flexible and tough.

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

Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic.[6] 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:

History

LDPE was the first grade of polyethylene. It was discovered by worker at Imperial Chemical Industries (ICI) using a high pressure process and free radical polymerization.[9] Major innovations were made in the 1950 by commercial, governmental, and university labs, focusing especially on catalysts, which allowed the production of PE without free radicals. Some of this work led to the Nobel Prize in Chemistry.[1]

Recycling

LDPE has SPI resin ID code 4

The EPA estimates 5.7% of LDPE (resin identification code 4) is recycled in the United States.[10] Despite its designation with the recycling symbol, it cannot be as commonly recycled as No. 1 (polyethylene terephthalate) or 2 plastics (high-density polyethylene).[11][12]

See also

References

  1. ^ a b c d Jeremic, Dusan (2014). “Polyethylene”. Ullmann’s Encyclopedia of Industrial Chemistry. pp. 1–42. doi:10.1002/14356007.a21_487.pub3. ISBN 978-3-527-30673-2.
  2. ^ “Market Study: Polyethylene LDPE (2nd edition)”. Ceresana.
  3. ^ a b BPF. “Polyethylene (Low Density) LDPE, LLDPE”. www.bpf.co.uk. Retrieved 2018-06-14.
  4. ^ Qiu, Xia-Wen; Pei, Jie; Wu, Chen-Chou; Song, Lin; Bao, Lian-Jun; Zeng, Eddy Y. (August 2021). “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. Bibcode:2021Chmsp.27730235Q. doi:10.1016/j.chemosphere.2021.130235. PMID 33794435. S2CID 232764239.
  5. ^ “LDPE Chemical Compatibility Chart”. CP Lab Safety.
  6. ^ a b Achilias, D.S.; Roupakias, C.; Megalokonomos, P.; Lappas, A.A.; Antonakou, Ε.V. (November 2007). “Chemical recycling of plastic wastes made from polyethylene (LDPE and HDPE) and polypropylene (PP)”. Journal of Hazardous Materials. 149 (3): 536–542. Bibcode:2007JHzM..149..536A. doi:10.1016/j.jhazmat.2007.06.076. PMID 17681427.
  7. ^ LDPE products and applications. Exxon Mobil Corporation
  8. ^ DOW LDPE 5004I. IDES – The Plastics Web
  9. ^ Dennis Malpass (2010). Introduction to Industrial Polyethylene: Properties, Catalysts, and Processes. John Wiley and Sons. pp. 1–. ISBN 978-0-470-62598-9.
  10. ^ “Municipal Solid Waste Generation, Recycling, and Disposal in the United States” (PDF). EPA.gov. 15 November 2016. Archived from the original (PDF) on May 3, 2012.
  11. ^ Brady, Angela (28 December 2018). “How to Recycle Number 4 Plastic”. San Francisco Chronicle.
  12. ^ “Exactly What Every Plastic Recycling Symbol Really Means”. Good Housekeeping. 18 February 2022.