Producing plastic bags uses less energy than other options but we must have responsible plans in place for their reuse and eventual disposal. This was one of the main points made by Materials Scientist Christian Müller of the Department of Chemistry and Chemical Engineering at Chalmers University of Technology, Sweden.
“We need to understand the full lifecycle from cradle to cradle,” he said.
Müller took STIAS fellows through the basics of materials science, to his group’s work in various areas to create materials that help to meet the Sustainable Development Goals (SDGs), with a few hands-on experiments along the way.
“One of the first questions I got when I arrived at STIAS was: What is materials science? Well, as a materials scientist I ‘make stuff’ and the stuff I’m particularly excited about are plastics,” he said. “I will try to convince you that in some cases we must continue to use plastics since they have functions that no other material can match. For example, plastics are outstanding insulators (meaning they do not conduct electricity), which we can use to transport renewable energy over thousands of kilometres and there are also plastics that do conduct electricity extremely well, which we can use to make cost-effective, flexible and colourful solar cells.”
Müller explained that materials science is an interdisciplinary field that uses analytical thinking from chemistry, physics and engineering. “We look at the structure, processing, properties and performance of materials like wood, stone, metals and plastics. It was recognised as a distinct field from the 1940s.”
“It falls somewhere between applied science which is about solving problems and basic science which is about answering fundamental questions,” he continued. “My team consists of materials scientists, chemists and electrical engineers. Our focus is the physical chemistry of semiconductors, polymer blends and composites for wearable electronics and energy technologies.”
“We work from single molecules to things that have an impact on the whole planet.”
Polymers to plastics
Using strings of pasta to demonstrate, Müller said: “A polymer – the basic building block of plastics – is a long string of atoms, usually carbon, that slithers like a snake. We call this motion reptation. Like snakes, polymer chains can be intertwined. To entangle they must be long enough.”
The basic science questions around polymers would include why do specific polymers have specific properties, and are there more benign ways to make polymers? While the applied engineering questions would include: in which cases should we continue to use polymers, how can we reuse and recycle polymers, and which societal challenges can polymers help to address?
Mixing polymers with other substances allows us to create different types of materials that we call plastics.
And there is plastic and plastic. Müller gave details on polyethylene – a plastic with good chemical resistance, strength, elasticity and flexibility, which has excellent insulation capabilities and is used for containers, packaging materials and even automotive components. While polypropylene has a higher melting point and is known for its durability and ability to resist heat, which it ideal for use in items such as food containers and certain types of fabrics.
“Polypropylene and polyethylene don’t mix well,” said Müller, “the molecules move away from each other so the mixture is brittle and the materials fracture. It’s also difficult to separate them economically.”
How they are processed determines their use – shopping bags are made from polyethylene which is stretchable. “The more you heat the more the molecules can align making the material stronger. Similar polyethylene, albeit with longer polymer chains, can be used to make bulletproof vests but obviously the processing is much more expensive. By weight the material can be stronger than steel.”
Müller focused in detail on some of the research questions his group is tackling and how they align with the SDGs. These include: How can we reuse and recycle plastics? – which aims to address SDG 12 which is about responsible consumption and production, as well as SDG 14 – Life below water; How can we ensure a stable supply of renewable energy? – which attempts to address SDG 7 which calls for affordable and clean energy; and, can we design more sustainable electronics? – which addresses SDG 12 calling for responsible consumption and production, and SDG 9 focusing on industry, innovation and infrastructure.
Plastics and plastics
“In materials development we need to break out of the traditional linear way of working and embrace a circular economy,” he said. “This means incorporating loops in which each stage of the process has a reuse, recycling component. Industry has moved quite far in this regard in some countries but there is still far to go. And there is some urgency – we need to turn the tide.”
He pointed to the legal basis in Europe with a 2019 European Union Directive focused on reducing single use in plastics. “16 million tonnes per year of plastic is produced in Europe. Thus far not all countries are meeting the recycling targets.”
“The film-blowing process to make plastic shopping bags is extremely efficient,” he said, “There’s no cheaper option. Paper bags would need to be used many times so that they favourably compare to plastic bags.”
Recycling is not without challenges. “Every time you use and reuse polymer chains they lose or change their properties making materials worse as they go through the cycle until they eventually break down. However, it is possible to upcycle polymers to make them act again like the original. We have to try to deepen the basic understanding and speed up the R&D processes simultaneously.”
Shuttling renewable energy
Turning to renewable energy, he said: “On a sunny, windy day energy-harvesting technologies like solar and wind are not far from meeting demand. The problem is the fluctuations and the areas with no sun in winter like Sweden – Cape Town has 10 times more sun in winter than Sweden.”
We therefore need to be able to shuttle energy between areas with and without sun and with and without wind, and this requires power cables that can carry huge amounts of electrical power.
Taking the audience on a brief historical detour, Müller explained the so-called war of the currents between Thomas Edison and George Westinghouse in the late 19th century over whether direct or alternating current (DC or AC) was superior. “Originally it was easier to change the voltage of AC, which therefore won the battle but now these technological challenges have been met making DC a strong contender for long-distance transmission. Today, DC transmission is associated with lower losses over long distances, which allows to connect wind farms and solar plants that are located far from the cities.”
“Power cables with polyethylene-based insulation can now carry up to 640 000 volts. Materials with an even lower DC conductivity through the use of polyolefin blends and co-polymers may result in new solutions.”
Bioelectronics
Müller explained that an electric current is the directed motion of an electron or an ion. Traditional electronic devices rely on electronic currents, while biological systems use ionic currents.
He explained that the technology to use mixed conductors that can sustain both electronic and ionic currents is developing rapidly and may allow to seamlessly interface traditional electronics and biology.
Overall, Müller pointed to the need for much more collaboration and discussion in all these areas. “For example, there is not always enough understanding in the packaging industry of every component and additive. But there are also good things happening – science on the factory floor and investing in new knowledge in-house. There are lots of industrial/academic collaborations but it all needs much more discussion with policy makers and the social sciences.”
Michelle Galloway: Part-time media officer at STIAS
Photograph: SCPS Photography