Chinese researchers decouple curing and foaming in recyclable NIPU foam

The team from Guizhou University and Guizhou Minzu University said conventional production of non-isocyanate polyurethane (NIPU) foam is complicated by the interaction between crosslinking and foaming. Because the two reactions occur simultaneously, changing one can adversely affect the other.

The researchers instead produced stable cyclocarbonate-functionalized prepolymers that can be stored as processable intermediates for at least 30 days.

They then used rheological measurements to identify what they describe as the viscosity explosion point: a reproducible stage in the crosslinking process at which foaming can be initiated. This replaces the empirical timing generally used in one-pot NIPU foam production.

The resulting closed-cell foams had densities ranging from 0.302–0.498 g/cm³. Cell sizes could be adjusted between 0.18 mm and 0.63 mm, compared with a fixed cell size of approximately 0.38 mm using the conventional one-pot process cited by the researchers.

Compressive strength reached 3.03 MPa, compared with approximately 0.26 MPa for the one-pot reference process.

The researchers incorporated 1,4-phenylenediboronic acid into the formulation to create dynamic boronic ester bonds. These form a covalent adaptable network, allowing the crosslinked material to rearrange without permanently breaking down.

Foams containing the dynamic network achieved self-healing efficiencies above 83% and could be recycled in a closed loop without losing mechanical integrity, according to the paper.

The prepolymer could also be shaped by compression molding before foaming, offering another route to controlling the dimensions and thermomechanical properties of the finished material.

The researchers said the work provides a framework for separating competing reaction kinetics in sustainable polymer-foam production. Further development would be required to establish whether the process can be transferred from laboratory preparation to commercial manufacturing.

Photo by Hans Reniers on Unsplash

The study was published in ACS Applied Polymer Materials