Polyurethane is one of the most important synthetic polymers, and it is synthesized through a polyaddition reaction between a polyisocyanate (a polymeric molecule with two or more isocyanate groups, such as toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI)) and a polyol (a polymer with two or more reactive hydroxyl groups, such as polyethylene adipate and poly(tetramethylene ether)glycol). Both the polyisocyanates and the polyols are currently derived from petroleum oil. Polyurethane has varied applications in different areas from liquid coatings and paints, tough elastomers, rigid foams for packing and insulation, to flexible foam in mattress and car seats .
Lignin is one of the three major components in plant cell walls and the most abundant aromatic polymer in the nature . Structurally, lignin is a 3-D networked polymer biosynthesized in plants from three monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, through radical coupling processes . Lignin plays a vital function in the plant’s defense system against degrading enzymes and diseases. The lignin also binds fibers together to form a strong and tough matrix of plants and provides mechanical support to the plant vessels for the transportation of water and nutrients . However, the physical and chemical nature and functions of lignin make it troublesome in the utilization and conversion of lignocellulosic biomass. For example, lignin has to be removed (dissolved) during chemical pulping of wood to release/produce intact, strong, and bleachable fibers (pulp) for making paper. In bioconversion of lignocellulosic biomass to fuel ethanol, lignin is one of the major recalcitrance sources of the cellulosic substrates to cellulases. Furthermore, the lignin isolated from either chemical pulping or biorefining has not been utilized in a value-added way, and the most common lignin utilization is still steam and power production through combustion.
Extensive efforts have been made to explore high-value applications of lignin, in particular in polymeric materials, such phenolic and epoxy resins . Considering the fact that lignin is a polymer with a fair amount of hydroxyl (phenolic and aliphatic) and carboxylic groups that own reactive hydrogen, lignin has the potential to replace polyols in polyurethane production. For example, polyurethane film was prepared from organosolv lignin with polyethylene glycol as co-polyol and soft segments  with or without catalyst . Polyurethane foam was prepared from kraft lignin using polyethylene glycol as solvent . Water-soluble lignosulfonate from sulfite pulping was used to prepare rigid polyurethane foams in glycols . Lignin from straw steam explosion was also investigated for polyurethane preparation . A polyurethane elastomer (film) was prepared from flax soda lignin with polyethylene adipate and ethylene glycol as co-polyol and soft segment, but the resultant polyurethane film was heterogeneous and did not have adequate mechanical strength for any application when lignin content was over 10% (wt.) . Because of the solid state and less accessible hydroxyl groups of lignin, chemical modification such as oxypropylation with alkylene oxide was proposed to improve the accessibility of the hydroxyl groups, which could convert lignin into liquid polyol with extended chain and exposed hydroxyl groups [5, 12]. As a follow-up, recently, liquid polyol from oxypropylated pine kraft lignin was used to prepare rigid polyurethane foam . The same group also investigated the reinforcement of rigid polyurethane foam from oxypropylated ethanol organosolv lignin with cellulose nanowhiskers .
Organosolv ethanol process uses aqueous ethanol to extract lignin from lignocelluloses in the presence of small amount of inorganic acid as catalyst. It was developed in 1970s and commercialized in 1980s at pilot scale for producing pulp from hardwood for papermaking [15–17]. Recently, we reevaluated the organosolv process as a pretreatment method of woody biomass for cellulose ethanol production. It was found that the organosolv process was an effective pretreatment for both hardwood and softwood and the resultant cellulosic substrates had a ready digestibility with cellulases [18–21]. The isolated organosolv lignin during the pretreatment had attractive properties such as high purity, low molecular weight and narrow distribution, and more functional groups and the lignin was expected to have great potential in developing high-value lignin products [18, 22]. However, the products and market of organosolv lignin have not been sufficiently developed. It is believed that the successful commercialization of organosolv pretreatment is greatly dependent on whether the organosolv lignin can be utilized efficiently and in value-added ways, which is expected to offset the high cost of the organosolv process.
In the present research, hardwood ethanol organosolv lignin (HEL) was evaluated to replace synthesized polyol to prepare rigid polyurethane foam and compared with hardwood kraft lignin (HKL). The effect of lignin addition on foam preparation (viscosity of polyols) and foam properties (density, compressive strength, and cellular structure) was investigated. Chain extenders (glycerol and butanediol) were examined for improving the properties of the lignin-based polyurethane foams.