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Chemistry

Are shopping bags impacting global warming?

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Plastic grocery bag (Fruits in a plastic bag by Anna Shvets)

Every time we push through the supermarket door, we can immediately perceive the smell of freshly baked bread, recognising our favourite snacks. This is when a plastic bag becomes handy!

  1. ) Plastic shopping bags are regularly used by consumers to carry the goods they purchase at the supermarket due to their low prices. Plastic bags were first developed in 1959 by Sten Gustaf Thulin. Before plastic bags became prevalent, paper bags were the norm. However, these were considered harmful for the environment due to their link to deforestation; another reason for using plastic bags was that they are much stronger than their paper alternatives, especially when it rains. Some companies have already shifted towards removing single-use plastics .For example, Morrisons already replaces them with reusable paper bags.
  2. ) A study conducted at the University of Hawaii has shown that low density polyethene (LDPE) in the ocean emits methane and ethene greenhouse gases (R. Sarah, F. Sara, T. Samuel, PLOS ONE, 2018, https://doi.org/10.1371/journal.pone.0200574). when it degrades under light . But what is LDPE, and how is it made?

LDPE is a type of a polyethene. There are two main types of polyethene, LDPE and high density polyethene (HDPE). HDPE has its polymer chains lining up in an orderly fashion, as seen in figure 1, resulting in high density, crystalline structure, and strong attractive forces between the polymers.

Figure 1: Chemical structure of HDPE. (The diagram was influenced by B. George, N. Alan, P. Gordon, Britannica, 2016).

Scheme 1: Polymerisation of ethene. n represents the number of ethene monomers. This article influenced this scheme.

Production of LDPE (shown in scheme 1) can be achieved by compression of the ethene gas monomer in a tubular reactor; the polymerization then occurs, resulting in polymer chains with side branches, as seen in figure 2.

Figure 2: Chemical structure of branched LDPE. (The diagram was influenced by B. George, N. Alan, P. Gordon, Britannica, 2016).

As a result of the irregular side branches, the polymer does not line up regularly, meaning LDPE’s structure is not crystalline, LDPE is of lower density, and the attractive forces between polymer chains are weakened. Due to LDPE’s long and short-chain branches, it does not pack tightly into the crystalline form. This gives it better ductility (the ability to be plastically deformed), which is vital for shopping bags. Other applications of LDPE are laminations and films for food packaging.

3.) Since polyethene has a carbon-carbon backbone (G. Berit, P. Merle, M. Matthew, Environmental Science: Processes and Impacts, DOI https://doi.org/10.1039/C5EM00207A) it is vulnerable to photo-initiated oxidative degradation, which is the most significant abiotic (i.e. not involving living organisms) degradation pathway under outdoor conditions. This degradation pathway is limited, as this process happens only when unsaturated double bonds are present, but since small quantities of external impurities are incorporated into the PE structure, this process occurs to some extent.  Scheme 2 demonstrates this.


Scheme 2: Polymer photodegradation process. This was redrawn from (G. Berit, P. Merle, M. Matthew, Environmental Science: Processes and Impacts, DOI https://doi.org/10.1039/C5EM00207A).

In the initiation step, light breaks the main polymer chain’s chemical bonds, producing free radicals (G. Berit, P. Merle, M. Matthew, Environmental Science: Processes and Impacts, DOI https://doi.org/10.1039/C5EM00207A). Free radicals react with O2, forming a peroxy radical. In addition, further radical reactions occur and result in autoxidation. This is followed by propagation, which eventually leads to chain crosslinking. The termination step finalises the process, where the radical reaction forms the inert products. Those smaller polymer fragments are small enough to pass through microorganisms’ cell walls, where they are used for biochemical processes and thus undergo degradation. In anaerobic biodegradation (when organisms do not use oxygen), methane is produced as a result. In aerobic biodegradation, (when organisms use oxygen) H2O and CO2 are also produced (which are greenhouse gases ). We are not going to go into detail about biochemical processes and enzymes involved in PE breakdown because exact details are yet not known (S. Ghatge, Y. Yang, J. Ahn, Applied Biological Chemistry, 2020, 63, doi.org/10.1186/s13765-020-00511-3)! If this blog sparked your enthusiasm in plastic, we recommend reading this blog, which talks about natural latex production in sunflowers.

Have you learned anything new and fascinating? We would be happy to hear your opinion in the comments below!

Contributors: Maxim Semchenkov (research, writing, illustrations), Tom Silcox (proof-reading), Natalie Fey (review).

Picture credits:

Main image: by Anna Shvets, https://www.pexels.com/photo/fruits-in-a-plastic-bag-3645504/

All other figures drawn ourselves.

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