Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 181

7.3.2.2 Lignosulfonates

A hydrosoluble form of lignin is prepared from the by-products of the sulfite pulping
process. During the pulping which involves the use of solutions containing sulphur diox-
ide and hydrogen sulfite ions at elevated temperatures, the lignin becomes sulfonated
and as such is soluble in water and under a range of aqueous solution conditions. The
weight-averaged molecular weight of the lignosulfonate polymers is clearly above kraft
lignin and typically in the range of 5-400 kDa, and the polydispersity is at least as
high (Buchholz et al . 1992, Fredheim et al . 2002). The most widely used industrial
process is the Howard process, in which a yield of 90-95% of precipitated calcium
lignosulfonates can be obtained by adding an excess of calcium hydroxide. Ultrafiltra-
tion and treatment with a long-chained alkyl amine to form a water insoluble complex
extractable with organic solvents can also be used to separate lignosulfonates from the
spent pulping liquid.
Lignosulfonate, the far most extensively used technical lignin polymer, is utilized in
many applications including as a binder in ceramics and in animal feed or as a dispersant
through the steric stabilisation of particulates such as clays, dyes and pigments. As
the lignosulfonate macromolecule is water soluble, this class of polymer shows great
promises in future nanotechnological and surface chemistry applications beyond those
where it is already finding use.

7.4 Solution Properties of Kraft Lignin

From studies of the reaction pattern in the three different process phases of the kraft
cook, four famous principles for selective kraft pulping was established at the Swedish
Pulp and Paper Institute (STFI) and the Royal Institute of Technology (KTH) in the
mid-1970s-mid-1980s (Hartler 1978, Johansson et al . 1984).

1. The alkali profile should be levelled out, which means that a high hydroxide concen-
tration in the beginning of the cook should be avoided and that the hydroxide ion
concentration during the final part of the pulping should be increased.
2. The concentration of hydrogen sulfide ions should be held as high as possible at the
beginning of the pulping.
3. The concentration of sodium ions and dissolved lignin should be low throughout the
pulping.
4. The temperature should be relatively low.

One principle (II) is directly attributed to the nucleophilic cleavage of aryl-ether bonds in
the lignin structure that is an organochemical matter. The other three principles describe
the influence of parameters, which usually are of physicochemical importance in many
macromolecular systems. The analogies between the impact of these principles on the
kraft cook and the kraft lignin (KL) solution behaviour at various solution conditions
are found very interesting.
A polymer dissolves spontaneously in a solvent if the free energy of mixing ( G mix )
is negative. The phase transition is mainly due to increased configurational entropy of
the polymer chain or, for charged polymers, an increase in counterion entropy, due to