Synthesis of polymers from organic solvent liquefied biomass: A review

Biomass liquefaction with organic solvents is a unique thermochemical conversion process for biomass utilizations. It combines the useful functional groups from both biomass and organic solvents used in liquefaction, thus obtaining a large variety of polymers. This review is focused on the resin products synthesized from organic solvent liquefied biomass, including phenolic, polyurethane, epoxy, polyesters, etc. Many biomass species, such as wood, corn stover, waste paper, and wood bark, have been investigated as the feedstocks for liquefied biomass-based resin products. Phenol liquefied biomass was studied the most mainly based on expectation to utilize the aromatic structures of lignin in biomass as the substitute for phenol in phenolic resin synthesis. Further condensation reaction of phenol liquefied biomass with formaldehyde to synthesize novolac or resol phenolic resins was proven to be an efficient route to convert the un-reacted phenol from liquefaction to resins and improve the physical mechanical properties of the resulting resins. Except for phenolic resins, most other polymers from liquefied biomass were synthesized based on the utilization of hydroxyl groups in liquefied biomass. Polyhydric alcohols liquefied biomass was used as polyols to synthesize polyurethanes, epoxy, and polyesters. To achieve comparable mechanical strength of resulting resin products, the common liquefaction solvent to biomass ratio was 3/1, which indicates a biomass substitution around 25% to petroleum raw materials. A few cases could reach 50% substitution with biomass pretreatment before liquefaction. Three main concerns with the liquefied biomass-based resins were their relatively higher viscosities, highly hydrophilic characters, and relatively lower cross-linking density in cured resins.