It is known to employ lignosulfonate compounds, including sodium salts of lignosulfonates, as an additive, such as a dispersant, in textile dyestuffs and printing pigments. Such lignosulfonate compounds are generally produced as a by-product of the wood pulping industry by either the sulfite or kraft process.
Such sulfonated lignin products provide three basic functions in dyestuff compositions:
(1) They assist in reducing the dye particles to a fine size;
(2) They maintain a dispersing medium for the dyestuff; and
(3) They are used as a diluent.The advantages of employing sulfonated lignins as dispersants in dyestuff compositions are based on their unique physical properties which include good compatibility with many dye systems, outstanding dispersant characteristics at ambient and elevated temperatures, and availability. There are certain disadvantages in employing lignins, whether they are sulfite lignins or sulfonated kraft lignins, as dispersants.
Negative factors in the use of such lignins as dyestuff additives relate to problems of high inorganic salt content, i.e., electrolyte content, when lowered in pH, foaming, high pH, fiber staining, poor heat stability, and high viscosity.
These adverse properties are troublesome to dyers and many attempts have been made to overcome these and other disadvantages.Inorganic electrolyte content of lignin dispersants and dyestuff additives greatly effect their use in a specific dyestuff additive formulation. High electrolyte content of a lignin dispersant imposes unwanted side effects on hydrophobic dyestuffs. In vat dyes, high salt content of the lignin additives can cause harmful rheological effects during storage of the dyes.
The viscosity of the oxidized form in the presence of salts generally increases to a level where the dye mixture can only be removed from a storage container with considerable difficulty.
Recent use in this country of double strength dyes over powder dyes has necessitated a reduced application level of the lignin dispersants in order to accomodate the increased amount of dye, thus dictating that the dispersant be in its purest state possible.A number of technological developments have resulted in new methods and processes to modify sulfonated lignins to reduce the negative aspects of employing such materials as dye dispersants without simultaneously causing any major adverse effects upon those properties which render sulfonated lignins desirable as dyestuff dispersants.
In the paper industry, lignin is obtained as a by-product from spent pulping liquors, known as black liquor, where lignocellulosic materials, such as wood, straw, cornstalks, bagasse and the like are processed to separate the cellulosic pulp from the lignin. The lignins employed in the process of this invention may readily be obtained from the kraft wood pulping process wherein the natural lignin is present as a sodium salt. In kraft pulping, the wood is subjected to the effects of strong alkali. The lignin forms a soluble sodium salt in the alkaline region which is separated from the cellulose and dissolves in the pulping liquor. The lignin is then recovered from the black liquor by acidification.
Acidification of black liquor containing the lignin salt generally may be accomplished by the introduction of carbon dioxide which converts the phenolic hydroxide groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor and, as a result, it precipitates out. To precipitate the alkali lignins from the black liquor as water-insoluble products, black liquor initially having a pH of around 13.0 is acidified to about a pH of 10.5 at which point the lignin begins to precipitate. The lignin precipitate can be further purified by reducing the pH level to pH 2, where the lignin is coagulated and washed with water to obtain a lignin product identified as “A” lignin.