Abstract
Softwood contains up to 40 wt% of lignin, which makes it the most abundant polyphenolic biopolymer on earth. The applications of Kraft lignin, because of its poor solubility, are not as numerous as other water-soluble lignin derivatives, such as lignosulfonates. We investigated the ultrasound-assisted carboxymethylation of two softwood LignoForce Kraft lignins in sodium hydroxide with sodium chloroacetate (SCA) as the carboxylate group donor to yield a water-soluble product. Thermogravimetric analyzer, gel permeation chromatography, and Fourier-transformed infrared spectroscopy characterized the physico-chemical properties of the carboxymethylated lignins. A photometric dispersion analyzer determined the ability of the carboxymethylated lignin to disperse kaolin clay aqueous suspensions. A SCA/lignin ratio of 12 and ultrasound at 20 W and 20 kHz yielded carboxymethylated lignins with a dispersion ability comparable to dioctyl sulfosuccinate sodium salt, a common dispersant. Ultrasound intensified the process by reducing the reaction time from 4 h (conventional process) to 30 min.
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