Plastic-Free Science: Spotlight on Oxo-biodegradable plastics

I am a firm believer that revolutions are formed by numbers but won with knowledge. For this reason, I have been inspired to use my background as a research engineer, and my passion to protect our beautiful planet to write a series of articles focusing on the science and engineering aspects of different alternative plastics. The articles will put a ‘spotlight on...’ different products to arm our plastic-free revolution with the knowledge required to recommend and avoid products. The series is an on-going project so any suggestions of products will be welcomed - Charlotte

 

Oxo-biodegradable plastics are currently being developed as an alternative product to plastic; the aim of the research is to replace single use plastic products like carrier bags and cling film

 

What is an oxo-biodegradable plastic?

Oxo-biodegradable plastics are plastics that degrade when placed under certain conditions. The breakdown process is called oxo-degradation and it takes place over two-stages in which the plastic undergoes peroxidation followed by bio-assimilation. Peroxidation is the process of breaking down the long chain hydrocarbons, in oxo-biodegradable plastics this stage is possible due to the reaction between the additives with exposure to heat and UV light. The second stage of the oxo-degradation process is bio-assimilation, in this process bacteria, enzymes and/or fungi digest the broken-down material resulting in the plastic degrading into CO2 and humus. Humus is a type of biomass that increases the fertilisation of soil.

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How are oxo-biodegradable plastics made?

The simplest plastics are made by connecting long chains of molecules containing only hydrogen and carbon (hydrocarbons), using heat. The liquid plastic is then moulded into the required shape and left to cool. Additives can be introduced while heating to give more desirable characteristics like rigidity, or the ability to resist higher temperatures. Oxo-biodegradable plastics are made with additives susceptible to light, heat and moisture. When exposed to these conditions, they react and the bonds holding the plastic together weaken. This weakening causes the plastic to breakdown into smaller and smaller pieces. The additives for oxo-biodegradable plastics are usually metallic such as cobalt, magnesium, manganese, zinc, iron or nickel.

 

What gets left behind?

There is a clear difference in opinion on whether the chemical additives used to promote oxo-biodegradation are harmful to the environment or not. This difference in opinion usually correlates to whether the research is conducted commercially or independently. Most independent studies find that the additives used can hinder the ability to recycle or reuse the products. They also find that the additives used to prolong the plastics shelf life can slow the biodegradation process so much, that they no longer fulfil international standards.

A study was conducted into how oxo-biodegradable plastics breakdown in sea water. It was found that, over 40 weeks the surface area of the original plastic had only decreased by 2%. The plastic had also lost the ability to transmit UV light by 90%; blocking of UV light will stop the oxo-biodegradation process. This means the plastic will remain whole, it will also block light from reaching marine life below that use it for photosynthesis.

Finally, as oxo-biodegradable plastics remain a carbon-based product it still requires oil drilling for manufacture. Even though the end state of the plastic is more promising, the carbon emissions and use of finite resources remains. Further to this, there are now additional process emissions being added from metal mining to source the metallic additives required; these include acid leaching into water streams and carbon emissions.

 The change is surface area with time for standard polyethylene and two oxo-biodegradable products (TDPA 1 + 2) (O’Brine 2010)

The change is surface area with time for standard polyethylene and two oxo-biodegradable products (TDPA 1 + 2) (O’Brine 2010)

What can it be used to replace?

Some oxo-biodegradable products are already commercially used within the agricultural industry for; thin films used to wrap waste, twine for binding hay and controlled release fertilizer. As these plastics are essentially traditional plastics with an extra additive, it is very easy to see why they are considered so promising. The manufacturers are likely to support simply adding something extra to the recipe as opposed to a complete overhaul of the chemical process, and the consumer still gets the properties of the plastics they already use. In theory oxo-degradable plastics could fully replace traditional plastics with some fine tuning of the additive recipe.

 

Conclusion

Oxo-biodegradable plastics are a great product, in theory. As the production does not require a big change to current production steps, manufacturers do not have to invest substantial time and money to adopt a more ‘green’ product and are therefore more likely to favour the change. However, the issue remains that the process still releases large volumes of carbon emissions and utilises finite resources. The main concern surrounding oxo-biodegradable plastics is how they influence existing waste management processes. They are inherently single use due to the time frame in which they are required to biodegrade. To combat this issue additives can be used to prolong shelf life however, this slows the process to a point where they are no longer of a high enough standard to be considered biodegradable. It has been reported that oxo-biodegradable plastics negatively affect the overall recycling process as they cannot be easily distinguished from conventional, recyclable plastics and the additives reduce the effectiveness of chemical processes required for plastics recycling.

It is for the above reasons that oxo-biodegradable plastics are not supported by the ‘New plastics economy’ who are pushing for a ban of oxo-degradable plastics and are recommending further research is completed. The ‘New plastics economy’ are collaboration of companies (Inc. Mars, Unilever, PepsiCo and The Coca Cola Company), policymakers, academics and NGO’s working towards creating a common direction for new plastics research.

 

References

Chapiro. A. 1975. Mechanism of Peroxidation of solid polymers as derived from the kinetics of the induced graft co-polymerizaton. Journal of polymer science. 50(1).

Kyrikou I. D Briassoullis. 2007. Biodegradarion of agricultural Plastic Films: A Critical Review. Journal of Polymers and the Environment.  15(2)

Mjs packaging. Unknown. What are oxo-biodegradable additives. [online]. [accessed on 07/05/18]. Available from: http://www.mjspackaging.com/blog/what-are-oxo-biodegradable-additives/

New Plastics Economy. Unknown. Oxo-degradable plastic packaging is not a solution to plastic pollution, and does not fit in a circular economy. [online]. [accessed on 07/05/18]. Available from: https://newplasticseconomy.org/assets/doc/Oxo-paper-13.03.18.pdf

O’Brine T. R C Thompson. 2010. Degradation of plastic carrier bags in the marine environment. Marine Pollution Bulletin. 60(12).

Scott G. D M Wiles. 2001. Programmed-Life Plastics from Polyolefins: A new look at sustainability. Biomacromolecules. 2(3).

Scott. 2000. Green Polymers. Polymer Degradation and Stability. 68(1).

Siracusa V. P Rocculi. S Romani and M D Rosa. 2008. Biodegradable polymers for food packaging: a review. Trends in Food Science and Technology. 19(12).