Kraft lignin carboxylation

Many of the chemical reactions used to modify lignosulfonates are also used to modify kraft lignins. These include ozonation, alkaline—air oxidation, condensation with formaldehyde and carboxylation with chloroacetic acid (100), and epoxysuccinate (101). In addition, cationic kraft lignins can be prepared by reaction with glycidjiamine (102). 

Lignins are most usually employed as various types of sodium lig-nosulfonate and are products of sulfite pulping. (Kraft lignins from the Kraft pulping process are also available.) They are used as dispersants, binders, sequestrants, and emulsifiers. Sodium lignosulfonate product variables include color, MW, and the degree of carboxylation and sulfonation on the lignin backbone. 

Alkaline peroxide treatment of an isolated residual kraft lignin resulted in small decreases in the concentration of carboxylic, phenolic and aliphatic OH groups, according to the P NMR spectrum of its trimethyl phosphite derivative [71]. Alkaline (and acidic) peroxide was used to bleach hydroxypropyl lignin [72]. 

The effect of the maleic anhydride content in the copolymerization reaction mixture on the degree of carboxylation is illustrated by the data in Table I. Carboxylation of kraft lignin and ligninsulfonates may easily reach levels of 0.7 to 0.8 maleic anhydride units per lignin-building Cg-unit, if the lignin to maleic anhydride ratio in the reactor increases to 2 1. 

Lignin is a major component of plants where it serves as a binding agent for cellulose and other materials, and the kraft process of paper production, heating with alkali and sulfide, produces polyhydroxy phenolic, carboxylic acid, and sulfide functional groups in a soluble black liquor mixture [5]. Acidification precipitates this modified lignin as a powder, hereafter referred to as limin, a 20630% by product in the manufacture of paper. 

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