Isocyanate-free polyurethane
Polyurethane is a true all-rounder: it is found in mattresses and car seats, in insulation boards, paints and catheter tubing. However, there is a catch to its production – it requires isocyanates, which are toxic and can trigger allergies or asthma.
In the CO2NIPU project, four Fraunhofer Institutes have found a way to produce polyurethane entirely without isocyanates. One of the raw materials: CO₂. What the process can do, where it reaches its limits and what the next steps are.
CO₂ explained briefly
What is CO₂? Carbon dioxide is a chemical compound consisting of one carbon atom and two oxygen atoms. At room temperature, it is a colourless, odourless and non-flammable gas. It makes up around 0.04 per cent of the Earth’s atmosphere.
How is CO₂ produced? It occurs naturally during cellular respiration in humans, animals and plants, in decomposition processes and through volcanic activity. Human activity produces it primarily through the burning of coal, oil and gas – in power stations, transport and industry.
Why is CO₂ a problem? CO₂ is the most significant man-made greenhouse gas. If its concentration in the atmosphere rises, the greenhouse effect intensifies, thereby accelerating global warming.
How can CO₂ become a raw material? CO₂ contains carbon, the basic building block of plastics. If it is captured, for example at power stations, it can partially replace fossil raw materials. In the CO2NIPU project , CO₂ reacts with methanol and diamines to form dicarbamates, from which polyurethane is produced.
Why isocyanates are a problem in polyurethane production
Traditionally, polyurethane is produced by reacting polyols with isocyanates. The reaction is rapid, well-understood and deeply established in industry – for almost 90 years.
The weak point here is the isocyanates themselves. Common examples such as MDI and TDI irritate the skin, eyes and respiratory tract and can trigger allergies and asthma. The manufacturing and processing stages are particularly critical. The EU has now responded: From August 2023, products containing more than 0.1 per cent monomeric diisocyanate may only be processed in a professional context by trained users. For businesses, this means additional effort – and a good reason to look for alternatives.
Polyurethane, isocyanates and NIPU explained briefly
What is polyurethane? Polyurethane (PU or PUR) is a versatile plastic developed by Otto Bayer in 1937 in Leverkusen. It is traditionally produced through the reaction of polyols with isocyanates. Depending on the formulation, the spectrum ranges from soft foams and elastomers to hard thermosets, as well as adhesives and coatings.
Where is polyurethane used? According to Fraunhofer IAP, PU is found in, amongst other things, mattresses, packaging, sports equipment, seals, paints, adhesives, construction foam and medical devices.
Is cured polyurethane dangerous? In the cured material, the isocyanates have undergone a chemical reaction. The risk mainly concerns the manufacture and processing of, for example, fresh PU foam or liquid resins.
What is NIPU? NIPU stands for Non-Isocyanate Polyurethanes, i.e. polyurethanes produced without isocyanates. In the CO2NIPU project, dicarbamate takes on the role of the isocyanate. According to Fraunhofer, the result has the same molecular structure as conventional PU.
How does isocyanate-free polyurethane (NIPU) made from CO₂ work?
Isocyanate-free polyurethanes – NIPU for short, standing for Non-Isocyanate Polyurethanes – have been the subject of research for some time. According to Fraunhofer, the CO2NIPU project has now succeeded for the first time in producing polyurethanes entirely without isocyanates. The project is led by the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam, with Dr Christoph Herfurth as project manager. The Fraunhofer Institutes ICT, IFAM and UMSICHT are also involved.
The crux of the approach? Instead of isocyanate, dicarbamate is used. According to Fraunhofer, it is considered safe – meaning that special training for staff would not be necessary. The dicarbamate is produced at Fraunhofer UMSICHT using a high-pressure process: at 50 bar, methanol and CO₂ react with diamines. According to the IAP annual report, the amines are also to be sourced from the chemical recycling of old polyurethanes. In this way, a greenhouse gas becomes a building block for a plastic – and waste material is set to provide another one straight away. According to the IAP, the CO₂ is to be captured in as concentrated a form as possible directly at source, for example in power stations.
The process works like a building block system comprising three components: the dicarbamate initiates the chemical reaction, polymer diols soften the plastic, and chain extenders cross-link the molecular groups, imparting elastic or adhesive properties as required. The components are mixed, melted at 180 to 190 degrees Celsius and stirred. The team then tests the tensile strength and elasticity.
Polyurethanes made from dicarbamates have identical molecular structures to conventional PURs made from isocyanates.
And because the end result is the same molecular structure, existing know-how from the conventional PU sector can be applied. According to the IAP, the processing properties are also similar to those of conventional thermoplastic polyurethanes (TPU), which facilitates integration into existing processes. The speed, however, differs significantly. Conventional PU is often produced in a matter of minutes, whereas the dicarbamate process takes six to eight hours. What sounds like a test of patience is seen by the researchers as an advantage: the slower reaction is easier to control and reduces waste and quality fluctuations. Furthermore, according to the IAP, the synthesis can be carried out in a manner analogous to conventional polycondensation processes, such as those used for PET – the researchers therefore consider large-scale production to be a likely prospect.
Limitations and initial applications
The first specific application is biocompatible catheter tubing. It is no coincidence that medical technology was chosen for this application. According to the IAP’s , NIPUs exhibit fewer cross-links, do not form gel particles and achieve a high degree of batch-to-batch consistency – properties that are particularly important for medical devices. Furthermore, the materials can be certified as biocompatible. The partner institutes complete the picture: Fraunhofer IFAM is using the same modular system to develop adhesives for attaching cannulas to tubes.
Incidentally, the idea of incorporating CO₂ into polyurethane is not entirely new. Covestro commissioned a plant in Dormagen back in 2016 with a capacity of 5,000 tonnes per year (chemie.de). According to the company, in this process CO₂ replaces around 20 per cent of the polyol. Among other things, this produces flexible foams for mattresses and upholstered furniture. The difference is that, in this approach, the CO₂-polyol still reacts with isocyanate. CO₂NIPU targets the other component – and renders the isocyanate superfluous. As promising as the approach is, it is by no means a sure-fire success. Some limitations apply to CO₂-based plastics in general, whilst others are specific to CO₂NIPU:
- CO₂ is chemically inert. Chemically speaking, the molecule is ‘exhausted’ and can only be made to react with the application of energy, pressure or catalysts. In the case of CO₂NIPU, this requires 50 bar for the dicarbamate synthesis, as well as temperatures of 180 to 190 degrees Celsius over several hours.
- Not a permanent CO₂ sink. The CO₂ remains bound for the product’s entire lifespan. If the plastic is incinerated at the end of its life, it is released back into the atmosphere.
- Foams, coatings and adhesives are the next hurdle. Thermoplastic NIPUs are already in use. For reactive formulations such as foams, coatings and adhesives, however, specific reactivity profiles are required – the IAP itself identifies this as a key challenge.
The next K 2028 will take place in Düsseldorf from 18 to 25 October 2028. By then, it should become clear whether the leap from the technical centre to pilot scale will be successful – and whether isocyanate-free polyurethanes will gain acceptance beyond the medical technology sector. Alternative raw materials and the circular economy are already among the industry’s key issues today.
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Author: Ann-Kathrin Ullrich | K-Mag
Ann-Kathrin Ullrich enjoys explaining how new ideas from research, technology and industry are changing everyday life. Alongside her work for EuroShop-Mag, she focuses on plastics and rubber in K-Mag: from new materials and processing methods through to testing technology and issues relating to the circular economy.
