Frothed black liquor as a renewable cost effective precursor to low-density lignin and carbon foams

Abstract By whipping air into a solution containing the pulp processing by-product black liquor, crosslinker and surfactant, stable air-in-black liquor froths were produced. The air volume in the froths was controlled by tuning the viscosity of the black liquor and the whipping time. By the selectively crosslinking the renewable lignin and hemicellulose content in the black liquor froths, lignin foams possessing a porosity of up to 88% have been produced. The porosity, pore and pore throat sizes decreased with increasing viscosity of the continuous liquid phase of the black liquor froths. The resulting lignin foams were pyrolysed to produce renewable carbon foams, which retained the shape and macropore structure of the lignin foam precursor well, opening the door to structurally designable carbon foams from lignin. The work demonstrated that froth templating using an industrial by-product is a viable low-cost method to produce both stable lignin and carbon foams.