By Molly Sung, Secretary for the GCI
Plastic is one of the most ubiquitous materials on the planet. Everything from our toothbrushes, to pens, take-out containers, or parts used in the automotive or aeronautic industries are made from plastic. What started off as a convenient and cheap alternative to traditional materials has become a global reliance – and it’s taking its toll.
Traditional plastics are petroleum-based – and as we know, petroleum is a non-renewable resource and its extraction, processing, and use contributes to environmental pollution and climate change. When plastic bags were first gaining popularity in the 1950s and 60s, one of the selling points of using plastic bags was that they were more durable and long-lasting than paper,1 but that’s also exactly the problem. Plastic doesn’t degrade easily like paper does, so it starts to accumulate. This accumulation in landfills and, unfortunately, our waters has spurred research in the development of plastics that can break down over time.
An example of a biodegradable plastic is polylactic acid (PLA). The starting material, lactic acid, can be obtained through fermentation of crops such as sugarcane or corn, which can undergo condensation to form short chains (oligomers). Next, these oligomers undergo depolymerization to form lactide, a cyclic ester, which is then polymerized with the help of a catalyst to give PLA, shown in Figure 1.2
PLA performs comparably to the popular commercial plastic polyethylene terephthalate (PET, labelled with the “1” inside the recycling symbol). It is currently used in food packaging (such as disposable cups), as medical implants,2 and has also found renewed popularity as a common filament for 3D printing, but it’s not without its problems. The monomer, lactide, can have varying stereochemistry which influences the final polymer product and the mechanical properties of the plastic. Significant strides have been made in this area of research, but possibly the biggest barrier to using PLA is the competition with the food industry for the starting material. This is incidentally the same problem many first-generation biofuels ran into. But what if we could take food waste and turn it into usable plastics?
While there are some technologies being developed to use non-food materials like cellulose as a bioplastic, many of these methods require fairly harsh reactions. A gentler, water-based approach to make a cellulose-based plastic was recently reported by a research team from the Italian Institute of Technology and the University of Milano-Bicocca in the journal Green Chemistry.3This new technique uses waste from the food-industry, including carrot, cauliflower, radicchio, or parsley waste. The vegetable matter must first be dried and ground into a micronized powder, but otherwise no further processing or purification is required to make the veggie waste usable in this process. To make the plastic films, the researchers simply mixed the vegetable powder with a weakly acidic solution (5 % HCl w/w) at 40 °C, then removed any residual acid through dialysis and let the suspension dry in a petri dish for 48 hours. This process has a 90 % conversion of the vegetable waste into bioplastic (by weight) and the product has very promising mechanical properties (Figure 2).
In particular, in measuring the elasticity and tensile strength of the bioplastic films, it was found that the carrot film had comparable properties to polypropylene (commonly used for rigid plastic containers – otherwise referred to as number “5” plastics).
The researchers also tested important factors for plastics being considered for food storage applications. First, they studied whether the films would interact with water. The parsley film was found to absorb water fairly readily. Conversely, the carrot filmed exhibited hydrophobic behaviour – an uncommon characteristic for vegetable-derived plastics. This hydrophobic behaviour means that the moisture from food is unlikely to soak through the plastic film or structurally damage it.
One very interesting property of the radicchio waste is that it is rich in anthocyanins. Anthocyanin is what gives radicchio, red cabbage, and beets their vibrant red colour. More importantly, anthocyanins are known anti-oxidants and materials rich in these anti-oxidants are currently being investigated as food-packaging materials that extend the shelf-life of food.4 Unfortunately, these vegetable films tested to be fairly permeable to oxygen, which would offset any benefit from the antioxidant-rich radicchio film. However, the researchers showed that if the vegetable waste was blended with polyvinyl alcohol (PVA), the oxygen permeability can be reduced significantly and was even an improvement on the pure PVA.
Lastly, and very importantly, the researchers tested for the biodegradability of the films. To test the rate of biodegradation, the researchers submerged the carrot film in seawater to measure the rate of oxygen consumption by the seawater organisms responsible for the biodegradation of the film. They found that the film decomposed fairly quickly in 15 days.
These scientists have now demonstrated a very mild process in the synthesis of bioplastics that have mechanical properties similar to one of the most common commercial plastics. They have also made a plastic that, because of the presence of anthocyanins, may have applications in food storage that can help reduce food-waste.
What is especially promising about these bioplastics is how little purification of the vegetable waste is required to make them; however, there are improvements to be made. A major obstacle these materials will face is their performance in wet or humid environments as well as scaling up to an industrial process. It is clear that we need more sustainable materials and these vegetable waste plastics present an exciting new avenue towards biodegradable bioplastics.
- Laskow. How the Plastic Bag Became So Popular. The Atlantic [Online] 2014. https://www.theatlantic.com/technology/archive/2014/10/how-the-plastic-bag-became-so-popular/381065
- Gupta et al., J. Prog. Polym. Sci. 2007, 32, 4, 455-482. DOI: 10.1016/j.progpolymsci.2007.01.005
- Perotto et al., Green Chemistry, 2018, 20, 804-902. DOI: 10.1039/C7GC03368K
- N. Tran, et al., Food Chemistry, 2017, 216, 324-333. DOI: 10.1016/j.foodchem.2016.08.055
Figure from Perotto et al. 2018 reproduced with the permission of the Royal Society of Chemistry.