The paper presents strategies on how to improve recyclability through the smarter design of polymers—and how biobased polymers could shift to carbon neutral alternatives.
The production of plastic is constantly increasing: global plastic production now stands at over 300 million tonnes per year.
Meanwhile, the recycling of plastic has room for improvement. Of the 14 percent that is collected globally for recycling, 8 percent is made into plastic of inferior quality, while 4 percent is lost in the process and only 2 percent is recycled into plastic of the same or equivalent quality.
A considerable amount of the packaging produced is not made to be recycled at all. In fact, 95 percent of plastics’ value as packaging is lost after a very brief first use.
Plastic is an umbrella term for a large group of materials with different properties and potential applications. It can be everything from hard and strong to soft and pliable, and can also be adapted to withstand extreme heat or cold.
Plastic consists mainly of one or more polymers that have been blended with additives. A polymer consists of one or more types of small building-block molecules.
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Most of the plastic manufactured is made from fossil oil, but less than 1% of total plastic is also produced from biobased material such as sugar cane (sugar), maize (starch), and plant-based oil.
“We must start to focus more on polymer design than we have done previously. It’s not enough to focus only on the polymers’ material properties, or on the design of the product, we must also develop polymers that can contribute to an increased rate of recycling,” says Professor Rajni Hatti Kaul, at Lund University.
She and her colleagues suggest that there are several parameters that are particularly important in the design of bio-based polymers for improving their application in plastic products (e.g. hardness, softness, durability, flexibility, stretchability, etc.) as well as their tolerance to recycling.
At present, plastic is mainly recycled using mechanical recycling, in which it is sorted, ground, washed and extruded.
As mechanical recycling involves high temperatures, there is a need to develop bio-based polymers that can withstand the high melt temperature or glass transition temperature (the temperature at which the polymer changes from a hard, glassy state to a rubbery state during a temperature rise).
“The higher the temperature the polymer withstands without losing its properties, the higher the quality it has as recycled material. Besides the challenges of the mixed plastic waste, the deteriorated material properties after recycling is currently a major problem in creating demand for recycled plastic,” says Rajni Hatti Kaul.
Polymer design that enables selective degrading of polymers to building-block molecules again, through so-called chemical recycling, is also an interesting area for research.
“Selective degrading of bio-based polymers is an excellent solution for separating and recycling polymers from different types of plastic blends. Then we gain more possibilities to use it again,” says Rajni Hatti Kaul.
Progress in the design of bio-based recyclable polymers also requires more research and development. Above all there is a need to develop a portfolio of different carbon-neutral building block molecules that can provide the desired characteristics for polymers.
To this end, microorganisms and enzymes are important tools both for the production and recycling of bio-based polymers.
There is also a need for research on the potential to develop better biocatalysts (enzymes) for polymer synthesis (polymerization is a process in which monomers—small molecules—bind to create one, usually long, polymer chain) and for selective degrading of polymers.
There is much to be gained from the better design of biobased polymers. In addition to contributing to increased plastic recycling and functionality, more optimal processes could make the production of polymers more energy- and resource-efficient.
Developing bio-based plastic alternatives, in which the polymer is made from agriculture or forestry residues instead of fossil oil, or waste gases like carbon dioxide, is also essential if plastic production is to become sustainable over time without any impact on land use.
“There are, of course, major obstacles to carbon-neutral alternatives, not least regarding the price, as fossil oil is still the cheaper alternative. However, a greater focus on polymer design is an important step in the right direction for circular plastics economy,” concludes Rajni Hatti Kaul.
Published on phys.org