Developers Produce Sustainable Polyurethane Without Toxic Isocyanate

Polyurethanes (PUR) are found in many medical products, such as medical catheter tubes, but the production of these high-demand plastics relies on toxic isocyanate. Fraunhofer researchers, however, have developed an alternative production process using harmless dicarbamate.

Chemical compounds like isocyanate are toxic and trigger allergies or asthma, but they remain indispensable for the chemical industry. They are necessary to produce PUR, plastics that are highly versatile and used in numerous products. Although the end product no longer contains isocyanates, special safety precautions are necessary during manufacturing to keep them away from humans and to avoid health hazards. For the first time, Fraunhofer researchers have succeeded in producing polyurethanes without using isocyanates in the CO2NIPU (nonisocyanate polyurethane, NIPU) project.

Dicarbamate as a Substitute for Isocyanate

To achieve this materials breakthrough, project manager Christoph Herfurth from the Fraunhofer Institute for Applied Polymer Research IAP and his team replaced isocyanate with harmless dicarbamate. The innovative process makes plastics production easier and safer. Employees no longer must undergo special training to protect themselves from the toxic substance. Moreover, the process results in lower greenhouse gas emissions because the researchers are using carbon dioxide to produce dicarbamate. They are also developing recycling methods for used PUR materials.

In addition to Fraunhofer IAP, participants in the CO2NIPU project were the Fraunhofer Institute for Chemical Technologies (ICT), the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) and the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT). “The molecular structures of polyurethanes made from dicarbamates are identical to those of conventional PU made from isocyanates. This means that existing expertise can be built upon to achieve the material properties required for the end product or application,” Herfurth noted.

Modular System for Material Properties

The researchers have further developed the process with a view to industrial feasibility. Different chemicals are mixed in specific proportions to produce the desired properties. So-called chain extenders help to cross-link the molecular groups and ensure elastic or adhesive properties. Polymer diols serve to soften the plastic, while the dicarbamate, as a substitute for isocyanate, initiates the chemical process. After mixing, these chemicals are melted and stirred at temperatures between 180 and 190 degrees Celsius. After cooling, the experts test for characteristics such as tensile strength and elasticity. 

Isocyanates are highly reactive, which is why polyurethanes often form within only a few minutes. Although using less reactive dicarbamates extends the same process out to six to eight hours, this makes it much easier to control and regulate. This reduces scrap and quality fluctuations in production.

Circular Economy for the Plastics Industry

Dicarbamates are produced using a high-pressure process at project partner Fraunhofer UMSICHT. Methanol and carbon dioxide are reacted with diamines at a pressure of 50 bar to synthesize dicarbamates. Fraunhofer ICT develops recycling processes for used polyurethanes, such as from old foam materials, which are then reprocessed to produce new PUR products. “We are thus contributing to the goal of a sustainable circular economy without greenhouse gas emissions,” Herfurth said.

As an initial application, Fraunhofer researchers are focusing on manufacturing biocompatible catheter tubes for medical applications. Fraunhofer IFAM uses the variable modular system to develop adhesives that bond cannulas to the tube.

The technology for producing isocyanate-free polyurethanes is now also working outside the laboratory. “We are now able to produce several kilograms of NIPU in our pilot plant. In the next step, it will be possible to produce several hundred kilograms of NIPU at the Fraunhofer Pilot Plant Center for Polymer Synthesis and Processing PAZ in Schkopau,” Herfurth concluded.