From Corn to Water Cup to the Circular Economy

With each passing day, there are more and more plastic products in our households. Plastic bags and similar items are often used thoughtlessly and quickly discarded, which is a significant environmental problem due to the extreme durability of these plastics. That is why chemistry research increasingly focuses on new methods for producing biodegradable macromolecules from renewable raw materials. At the same time, the recycling of bioplastics is becoming ever more important, as this enables a more sustainable use of these materials. The team at the Chair of Bioinorganic Chemistry at RWTH, led by Professor Sonja Herres-Pawlis, is creating new non-toxic catalysts for bioplastics and their recycling as part of numerous third-party-funded projects.

Facing Enormous Challenges

More and more frequently, we hear about the dangers of plastic particles to people and the environment. The rapidly increasing consumption of plastics presents us with enormous challenges. More than 300 million tons of plastic are produced annually, compared to only 1.5 million tons in the 1950s. It is estimated that five to twelve million tons of plastic waste enter the world's oceans each year, mainly through rivers and coastal areas. There, sunlight, salt water, wind, and waves cause the plastics to break down into tiny particles that are ingested by marine life. Plasticizers in plastics accumulate in the fatty tissue of animals and thus enter the food chain. The effects on humans and wildlife have not been fully researched, but estimates are that up to 100,000 marine mammals and one million seabirds die annually from pollution. Eventually, we ingest the microplastic particles through our food.

To solve this problem, more research is now done on sustainable, biodegradable plastics. These plastics can fully decompose under biological conditions within twelve weeks at 60 °Celsius.

Bioplastics and Their Applications

Bioplastics offer solutions to two major problems: our dependence on fossil fuels and our culture of generating vast amounts of non-compostable plastic waste. Bioplastics can be biodegradable or made from renewable resources, sometimes both. An important group of biodegradable bioplastics are aliphatic polyesters such as polylactic acid (PLA). They can replace conventional plastics made from petroleum and can be turned into compost at home. Other bioplastics, such as polybutylene succinate (PBS), are made from a mixture of renewable and fossil raw materials and are equally compostable. Bioplastics are used in packaging, clothing, medicine, and pharmaceuticals. They could also be used for optical applications such as Plexiglas or for beverage bottles if problems such as CO₂ permeability are solved.

Polylactide (PLA) and Its Production

Polylactide (PLA) is made from lactide, which is extracted from corn, sugar beets, or sugar cane. Other than that, bioplastic production mostly uses raw materials unsuitable for food production. Recently, alternative starch sources such as corn straw are also being investigated so that even as bioplastic production increases, it will not compete with food production. The glucose in plant fibers is first fermented to lactic acid, which is then linked to short chains (“oligomerized”) and converted to dilactide. Metal catalysts can open this cyclic ester in a process known as ring-opening polymerization and connect it to form polyester chains. Industrially, this is done using toxic tin compounds, which the RWTH researchers want to replace with non-toxic zinc or iron guanidine catalysts to improve environmental compatibility and efficiency. Guanidines are basic donor ligands that can be adapted to the desired application via modular synthesis. Currently, guanidine complexes are the fastest catalysts for lactide polymerization under industrial conditions.

The polymerization reaction, i.e., the synthesis reactions that convert similar or dissimilar monomers into polymers, can be monitored closely using time-resolved Raman spectroscopy, from which kinetic models of the reaction process can be derived. This reveals whether the catalysts mediate polymerization in a controlled manner. Reaction control is also important to control the resulting plastics’ chain lengths and chain distribution, which can be used to adjust the desired properties, such as degradation rate, flexibility, and hardness.

Disposing of Bioplastics

Bioplastics can be recycled, incinerated, or degraded under industrial composting conditions. Various certification organizations monitor and regulate the bioplastics industry to ensure that products are correctly labeled and disposed of. The composting of bioplastics requires special equipment, as it requires higher temperatures and air circulation than food waste composting. The compost produced can then be used for agriculture or planting trees.

Recycling of Bioplastics

However, bioplastics are really much too valuable to be used only once. They are made from valuable raw materials and can be used several times. Mechanical recycling requires little energy but is only  worthwhile with gentle processes and large quantities of waste. Chemical recycling, however, can also work for polymer blends; our catalysts succeed in rapidly degrading them to lactate esters, which in turn are used as green solvents.

Summary

Bioplastics offer promising solutions to the plastic waste problem. They can be made from renewable raw materials, are biodegradable, and can replace conventional plastics in many applications. However, more research and development work is needed to improve their properties and efficiency and to ensure that they can be disposed of in an environmentally friendly manner.

– Author: Sonja Herres-Pawlis