Creating high-performance plastic with water only

Materials World magazine
3 Jun 2020

Two additional types of organic high-performance polymers may now be produced with hot water. Idha Valeur reports.

‘An inherently environmentally benign and straightforward synthetic strategy for generating so-called organic high-performance polymers’, is possible, says Miriam Unterlass, Assistant Professor at TU Wien in Vienna, Austria. The researchers have used hot water instead of solvents such as dimethylformamide, N-methyl-2-pyrrolidone, or polyphosphoric acid to produce polybenzimidazoles and pyrone polymers. 

Unterlass explains how organic high-performance polymers are used in every microelectronic device, such as in mobile phones and laptops, and the importance of removing solvents from the synthesis process as ‘many are problematic for environment and health’. 

Unterlass, who works in the Institute of Materials Chemistry and Institute of Applied Synthetic Chemistry at TU Wien, explains that for a material to be classified as an organic high-performance polymer, it ‘must exhibit an excellent thermal stability of >500°C under nitrogen atmosphere. However, typically, organic high-performance polymers also show many other outstanding properties, such as high resistance against aggressive chemicals and mechanical stress, which makes them suitable for a multitude of extremely demanding applications, e.g. in the aeronautics, aerospace, automotive, or sports equipment industry’.

Unterlass and her team have proved that these polymers can be created hydrothermally. Their previous research synthesised organic dyes, also known as polyimides, and now they have synthesised two additional polymers – polybenzimidazoles and pyrone polymers. 

The simplicity of the process is one of its main benefits, according to Unterlass. ‘For our hydrothermal reactions, we work in closed pressure vessels, so-called autoclaves, which are only filled with our starting materials and water…Pressure is built up upon heating the closed autoclave …If a certain maximum pressure was exceeded, steam would be released through an emergency release valve.’ 

The typical temperatures the team are heating to is between 180 and 250°C. These temperatures roughly equate to pressures between 10 and 40 bar. She highlights that, in certain cases, there might be a need to work under more extreme conditions where temperatures would reach up to 350°C, which generates a pressure of about 165 bar. 

‘Under these high-pressure and high-temperature conditions, the properties of water change drastically. Solubility of aromatic organic compounds increases. Certain types of reactions are significantly facilitated,’ says Unterlass.  

‘This is the reason why we can prepare various types of organic high-performance polymers in the first place. Under regular conditions, this would be simply impossible. After a sufficiently long reaction time, we let the pressure vessel cool down to room temperature, the pressure inside decreases and we can then isolate the desired product as a powder.’

When asked why this has not been trialled before, Unterlass believes it is because the process is ‘rather counterintuitive’. 
She adds, ‘The types of reactions we perform are all condensations which liberate water as reaction by-product. At first glance, carrying out these reactions under hydrothermal conditions shouldn’t be possible. It looks like violating Le Chatelier’s principle – of course it does not.’ 

With the process now able to create three high-performance polymers – dyes and also inorganic-organic hybrid materials – Unterlass believes there is scope for more. ‘I am absolutely convinced that it is possible to generate more types of polymers than the three we have reported so far hydrothermally…There are plenty of other high-performance polymers or engineering plastics the hydrothermal synthesis of which could, and should, be attempted.’

Unterlass and her team have found a spin-off company, UGP Materials GmbH, which is currently working to industrialise and upscale the manufacturing method.