Lignin-derived polyhydroxyurethanes point to NIPU pathways

The work, reported by researchers at the FAMU–FSU College of Engineering, converts lignin—an aromatic biopolymer found in plant cell walls—into functional precursors that react with carbon dioxide–derived cyclic carbonates to form PHUs. This pathway avoids conventional diisocyanates entirely.

Polyhydroxyurethane chemistry

Unlike conventional polyurethane formation (isocyanate + polyol), PHUs are synthesised via the ring-opening reaction of cyclic carbonates with amines, producing urethane linkages alongside pendant hydroxyl groups.

In this case, lignin is chemically modified to introduce reactive carbonate or hydroxyl functionalities, enabling its incorporation into the polymer backbone. Reaction with diamines yields a crosslinked or linear PHU network, depending on functionality and stoichiometry.

The presence of secondary hydroxyl groups along the backbone is a defining feature of PHUs, influencing hydrogen bonding, thermal behaviour and mechanical properties.
Material properties and processing

The resulting lignin-based PHUs are reported to exhibit:

• High aromatic content, contributing to thermal stability and rigidity
  
• Extensive hydrogen bonding, enhancing mechanical strength
  
• Improved adhesion characteristics, due to polar hydroxyl functionality
  

Compared with conventional PU systems, PHUs typically show:

• Slower reaction kinetics, due to lower reactivity of cyclic carbonates versus isocyanates
  
• Higher glass transition temperatures (Tg) in aromatic systems
  
• Potential for enhanced recyclability or reprocessability, linked to reversible hydrogen bonding networks
  

The researchers also highlight a simplified synthesis route, with fewer hazardous intermediates and lower energy requirements.

Implications for polyurethane applications

Lignin-derived PHUs are of particular interest for coatings, adhesives, sealants and elastomers (CASE), where adhesion, chemical resistance and thermal stability are critical.

Potential advantages for the PU sector include:

• Elimination of diisocyanates, addressing regulatory pressure (e.g. EU restrictions on MDI/TDI handling)
  
• Increased use of renewable aromatic feedstocks
  
• Integration of CO₂ as a chemical building block via cyclic carbonate intermediates
  

However, technical barriers remain. PHUs generally suffer from slower cure speeds and processing limitations, which may restrict their use in high-throughput applications such as flexible foams.

Illustration: ACS Sustainable Chemistry & Engineering

ACS Sustainable Chemistry & Engineering