Alkoxylation Lignin

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

Step polymers can be reacted with alkoxylignin to form alko lignin esters, thermodynamic and equilibrium forces dictate that the best products from this chemistry will come from step- reaction polymers capped with isocyanate groups. New products of alkoxylated lignin covalently bonded to cellulose acetate or caprolactam are known and are available tor utilization. 

The interpretation was further complicated when we measured the alkoxyl contents of the lignins before and after hydrolysis in aqueous acid. 

Table V. Hydrolysis of Acetal Lignins (Alkoxyl Measurements)...

XS = sulfonation, M = methylolation, MO = miscellaneous oxidation, G = grafting, P = phenolation, CM = carboxymethylation, MAC = maleic anhydride copolymerization, MCR = miscellaneous carboxyla-tion reactions, E = epichlorohydrin (also in conjunction with pheno-lated lignin), A = alkoxylation (i.e., ethylene, propylene, and butylene oxides), M+EP = modification with compounds containing unsaturated end groups ( divalent hydrocarbons ) followed by epoxidation with peroxide, MA = methacrylic acid. 

The alkoxylation of lignin is possible in a solvent (dimethylformamide, N-methylpyrrolidone or in liquid PO [20]). A process using a lignin-glycerol mixture (3 1) in a polyether polyol based on lignin was developed [20]. The catalysts of this reaction are KOH, but a tertiary amine, such as dimethylaminoethanol is preferred. By alkoxylation with a PO-EO mixture (e.g., 18-25% ethylene oxide EO) a totally liquid dark-brown lignin-based polyether polyol with a viscosity in the range 4,700-8,000 mPa-s at 25 °C, with an hydroxyl number... 

During the alkoxylation reaction, PO or EO are added to all hydroxyl groups from the reaction system (phenolic and aliphatic hydroxyl groups), but firstly to the phenolic hydroxyl groups. Lignin-based polyester polyols formed by the reaction of lignin with e-caprolactone were developed [10, 12, 21, 22]. 

Even though lignins are very heterogenous in nature, they have one eommon feature they all possess alkoxylated benzene rings, which is exactly the object of the attaek by ligninases. Ligninases are oxidative, extracellular, and nonspeeific. 

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