Increasingly, plastics are being advertised as ‘made from plants’, with the implication that it is better for the planet. But is this really true? Chen et al. used LCA to compare the environmental impacts from resource extraction to manufacture (cradle to factory gate – not comparing end-of-life options) for types of PET (#1) plastic with varying proportions of biomass or fossil fuel feedstock.[1]The study considered what the plastic was made from (fossil fuels, corn or woody waste from forestry), how far it was shipped, and its properties (i.e. does it take more material to do the same job).

Comparison between PET made from different combinations of fossil fuels or plant matter modelled on the USA data for energy mix, accepted forestry practices, and travel distance from cradle to factory.

The analysis identified that there is not a clear answer – the type of material with the best and worst impacts changes depending on which impact is measured. It is important to use studies like this one to guide decisions, based on the impacts that are prioritised as being most important. The reason the results differ by type include:

  • Converting a plant to a plastic sequesters carbon in a solid form
  • Agricultural and forestry machinery is fossil-fuelled, which impacts on climate change, ozone depletion and smog
  • Power in the US (where the study is from) is mostly fossil-fuel-based, impacting climate change, ozone depletion and smog creation (note that because Aotearoa New Zealand has high rates of renewable electricity this would be much less of an issue here)
  • Current processes to convert lignocellulose (wood) to PET (#1) precursors are energy and chemical intensive, with negative impacts on most environmental categories
  • Fertilisers and pesticides for crops require energy to make, and have negative impacts on acidification, soil nutrients and ecotoxicity.

In the future, bio-based plastics may have a reduced environmental impact thanks to an increase in use of renewable electricity; industrialisation of microbial processes to convert biomass which reduces the energy and chemicals required to do this; and development of better ways to collect and repurpose a variety of waste streams that could minimise the need to use dedicated crops.

References
[1] Chen et al., “Comparative Life Cycle Assessment of Fossil and Bio-Based Polyethylene Terephthalate (Pet) Bottles,” Journal of Cleaner Production 137 (2016).

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