Capturing CO2: polymers have an appetite

Cooling the planet is therefore possible. Although certain techniques are based on the albedo effect, they are not sufficient to significantly reduce the average temperature. This is why capturing CO2, whether it is already present in the atmosphere or still being emitted - in factory chimneys for example - is the other avenue being explored.

Plastics go green

Green roofs are undoubtedly gaining in popularity. On paper, they seem to be a panacea. And for good reason: firstly, they provide particularly effective thermal insulation for the buildings that have them and, contrary to what one might think, they make them more waterproof. They also respond to the political desire to compensate with vegetation for each m2 of new concrete or tarmac surface built.

A green roof must not affect the structure of a building. The soil is therefore perfectly insulated by a layer of polymers.

Above all, green roofs help to reduce CO2 emissions because the plants absorb the gas and give off oxygen. Their only drawback is that they cannot be installed on all buildings. The slope must not be too steep and the structure must be solid, because a green roof weighs a lot! To green a roof, it is not enough to throw down a few shovelfuls of earth and wait for nature to do its work.

Before the vegetation can be planted, the structure must be properly drained and insulated and, above all, roots must be prevented from damaging it. It is therefore a real technical sandwich of polymers that is stacked on the supporting element, generally a concrete slab. The main reason why plastics are preferred to metals is that they are much lighter, insensitive to oxidation and humidity and are considered to be the best insulators available.
The first step involves installing a moisture barrier. This is a very strong membrane, which in most cases is made of polypropylene and a polyolefin veil that prevents water vapour from diffusing into the structure. Next comes an insulator. Depending on the designer of the green roof, the insulator can be expanded or extruded polystyrene or polyisocyanurate, a polyurethane foam enriched with cyanurate. The latter’s advantage is its extreme fire resistance and rigidity. This material is used to insulate the tanks of LNG carriers, for example. A polyethylene film is then placed on top, the function of which is to act as a barrier to the toughest and most invasive roots. On top of this is a film of ethylene-propylene-diene monomer (EPDM), an elastomer that ensures a perfect seal. Finally, there are plastic trays which are usually made of polymers. Their function is to receive the substrate (light topsoil) and to stabilise it to prevent it from running off in the event of heavy rain.

The technique for installing green roofs has little in common with that used for indoor or outdoor green walls. The constraints are not at all the same since their functions are different. A green wall is first and foremost a CO2 trap; it has no other function than to be pleasing to the eye. Here again, polymers are important because the substrate is directly fixed to a PVC sheet, a waterproof material that will protect the wall.

Although less complex than a roof, the green wall still needs to be insulated from the façade. This is why designers usually use PVC sheets..

As for the substrate, it is a felt cloth continuously soaked with nutrients on which the roots will graft and develop. This is the same technique used in hydroponics, which is becoming increasingly popular in cities, where many sheds are now being recycled into real urban farms. In addition to the ability to capture the CO2 that is widely present in urban areas, these crops promote short circuits.

A breath of CO2 for polymers

By focusing on the CO2 already emitted, all these techniques obviously have a positive impact, but it remains insufficient. Things are continuing to evolve rapidly, particularly in the field of transport (see our last article on cars Tomorrow’s vehicles: polymers lending their weight to the future ), and many start-ups are trying to develop machines capable of sucking up the CO2. Among the most advanced is the Swiss company Climeworks which, since 2009, has been trying to capture CO2 from the air by filtering it. It has set itself the goal of extracting 1% of the world's carbon dioxide emissions by 2025. That's very soon indeed!
Their “machines” consist of modular CO₂ collectors that can be stacked according to the desired suction power and are powered solely by green energy sources like solar power.
From a theoretical point of view, the process is quite simple. The collectors capture carbon dioxide by trapping it in a membrane made of a highly selective filter material. We don’t know much about the exact nature of this filter except that it is a polymeric innovation. Once the filter reaches saturation, the collector is closed and the whole assembly is heated to between 80 and 100°C to release the carbon dioxide. The pure, highly concentrated carbon dioxide is then collected. Climeworks currently has 17 plants in operation or under construction throughout Europe, the most emblematic being those in Switzerland and Iceland. Once the CO2 has been extracted, it can be injected deep into the ground where it will mineralise over time.
Better still, it can be sold as a raw material to industrial stakeholders who need it. An important agreement has been concluded in this respect between Climeworks and Swiss water brand Valser, which now uses this highly purified CO2 to gasify its sparkling water.
Re-using the collected CO2 is also a very topical issue and a source of interesting initiatives such as that of the German company Covestro, which captures this gas and reintroduces it into polyurethane foam, for example.
Climeworks, for its part, wants to go even further and is currently working on finding solutions aimed at completely eliminating the carbon dioxide after recovery. The company says it is close to achieving this and hopes to be able to eliminate several thousand tonnes this year.

Capturing CO2 directly in the atmosphere is the technique developed by a few startups around the world, including the Swiss company Climeworks. The technology is based on polymer membranes.

Finally, on the basis that the best way to combat excess CO2 is not to emit it at all, other laboratories are looking at ways of capturing it during the post-combustion phase of factory fumes, i.e. at the exit of the chimneys. This method is called sequestration. In fact, the fumes pass through very high efficiency filters made up of a porous membrane itself composed of polymers capable of specifically selecting the CO2 in gas mixtures. In addition, the heat released by the fumes amplifies the chemical processes at work during sequestration. Although still in the development phase, this technique could also be a future solution for all vehicles with combustion engines. Unless, by then, they are all electric.

To find out more about developments in this technology, see our EPFL interview