(590c) Fabrication and Property Testing of Carbon Foams Using 100% Kraft Lignin

Lignin is one of the major components of lignocellulosic biomass and the most important natural phenolic polymer. It is a wood by-product composed of 60-65wt% carbon with an annual production over 70 million tons worldwide from the paper and pulping industries [1]. Much research has been done on the processes involving lignin thermal decomposition to sustainable/renewable carbon-based materials like active carbons, carbon fibers, templated carbon, and graphene [2], but less work has been done on carbon foam production from lignin. Mansmann and Winter [3] developed a process to produce carbon foam from an aqueous lignin solution. However, only ligninsulfonates were used as the lignin precursors, polyethylene oxide and acrylic acid-acrylamide were added as the copolymer, and tremendous energy was consumed to vaporize water from the ligninsulfonate solution in the heating process. Spradling and Amie [4] described a process to produce carbon foam from lignosulfonate. The lignin resource was limited to lignosulfonates in both previous works [3, 4] while Kraft lignin, which comprises 95% of the lignin produced annually, was not used for carbon foam production [5]. Currently, carbon foams are mainly produced by the process of blowing the carbon precursors, which are usually first decomposed in a closed vessel at high temperature, under high pressure [6]. This accounts for the high cost of the product since high temperature and high-pressure facilities are required for production. Furthermore, the product size and properties are limited by the facility [7]. Furthermore, catalysts, foaming/blowing agents, surfactants, and/or crosslinking agents may be used for the foam preparation [8], this further increases the cost of the product.

This work reports a simple manufacturing process to produce carbon foams in an open vessel using Kraft lignin as the only carbon precursor; no solvents, catalysts, blowing agents, surfactants, and crosslinking copolymers are needed in this developed method. The process includes pressing the carbon foam precursors into a lignin block in a mold at room temperature, then the lignin block is heated to create a lignin foam block at a temperature range of 400-600 °C, and the resultant lignin carbon foam (LCF) is finally obtained by carbonization at 1100 °C. The properties obtained and structural examination showed a density of 0.18 – 0.68 g/cm³; porosity of 69.3 – 91.5%; open-cell rate of 95%-98%; pore size ranges from 10 to 1000 μm; pore wall thickness is distributed between 10 and 100 μm; compressive strength of 7.03 ± 1.25 MPa – 30.16 ± 2.41 MPa; thermal conductivities of LCF samples increased with increasing foam bulk density; the bulk electrical conductivities of LCF samples also increased with increasing bulk density. Lignin carbon foam blocks also showed high levels of both fire and termite resistance. Fire resistance testing showed no damage occurred when the LCF sample was heated with an acetylene/air flame over 1000 °C for more than 3 minutes. Termite testing showed that termites ignored the lignin carbon foam blocks, but readily consumed the pine feeder block as a food source.