Valorization of waste biomass for the fabrication of isocyanate-free polyurethane foams†

Abstract

Polyurethane (PU) foams are key players within the large foam market, with applications such as thermal insulating materials, cushioning, protective equipment, etc. With the current regulatory constraints on the use of toxic isocyanates and the ambitious goals to increase the renewable content of plastics while valorizing waste, isocyanate-free liquid formulations containing biofillers that are able to easily self-foam are needed for more sustainable PU foams in the future. Herein, we incorporate various abundant waste stream-sourced biofillers (proteins, lignin derivatives, and polysaccharides) into isocyanate-free PU formulations composed of CO₂-based poly(cyclic carbonate)s, diamines and a catalyst. The formulations containing up to 30 wt% of biofillers are foamed at 100 °C without adding any external foaming agent. Moisture naturally present in the biofillers partially hydrolyses the cyclic carbonates, which generates the blowing agent (CO₂). The biofiller, even at a low content (1 wt%), stabilizes the growing cells, providing homogeneous foams. Although the nature of the biofiller does not significantly affect the foams’ density and morphology, their mechanical properties are strongly affected, for example from a rigid foam with 10 wt% keratin (compression modulus (E) = 21.9 MPa) to a flexible one with chitosan (E = 0.2 MPa). Preliminary studies indicate that the biofiller does not prevent the foam recycling into polymer films by hot pressing. Virtually any kind of moisture-containing biowaste can be used as a water reservoir to foam the formulations while increasing the bio-based content of the material, which reaches 97% when selecting bio-based monomers. This process valorizes abundant waste stream-sourced biofillers for producing more sustainable PU foams.