It is known to employ lignosulfonate compounds as an additive for textile dyestuffs and printing pigments. Such lignin compounds are generally produced as a by-product of the wood pulping industry by either the sulfite (lignosulfonates) or kraft process (alkali lignins which subsequently may be sulfonated). Such sulfonated lignin products provide three basic functions in the dyestuff composition:
(1) They assist in reducing the dye particles to a fine size; PA1 (2) They maintain a dispersing medium for the dyestuff; and PA1 (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 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.
Electrolyte content in specific dyestuff formulations can greatly effect their performance qualities. For example, negative rheological performance has been determined when electrolyte was present in conjunction with hydrophobic dyes. High salt content in the lignin dispersant thus imposes undesirable side effects in such systems. In vat dyes, high salt content in 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. The following U.S. patents are directed to reacting and modifying lignins to make them more suitable as dye dispersants: U.S. Pat. Nos. 4,001,202 to P. Dilling et al., 4,184,845 to S. Y. Lin; 4,131,564 to P. Dilling; 3,158,520 to L. A. Baisdell; 3,094,515 to K. F. Keirstead et al; 3,726,850 to Detroit; 2,680,113 to E. Adler et al; 3,769,272 to Hintz; 3,841,887 to Falkehag et al; 4,131,564 to P. Dilling; 4,355,996 to P. Dilling et al; and 4,308,203 to Lin.
U.S. Pat. Nos. 2,525,433; 2,690,973; and 3,503,762 disclose the use of lignins as an additive in printing inks, pastes, and gels. The foregoing patent art is cited to show the state of the art and is not intended to be all inclusive of lignin modifications for use as dye additives.
More recently in my co-pending commonly assigned U.S. patent applications Ser. No. 601,047, now U.S. Pat. No. 4,521,336, and Ser. No. 601,049, now U.S. Pat. No. 4,551,151, filed Apr. 16, 1984, there is described a process for preparing sulfonated lignins particularly suited for use as dyestuff dispersants, and the products produced thereby, wherein the viscosity of sulfonated lignins can be reduced with improvement in heat stability properties by removing the low molecular weight component of the lignin and thereafter sulfonating the resultant lignin material with sodium sulfite (Na.sub.2 SO.sub.3) and an aldehyde at a pH of about 8.0 to 9.2. Also, in my commonly assigned copending U.S. patent application Ser. No. 532,677, filed September 16, 1983, now abandoned, there is disclosed a process for producing sulfonated lignins for use as dye dispersants and additives, and the products produced thereby, wherein dispersability and heat stability of the lignins are improved by post-sulfonation cross-linking of the sulfonated lignins at controlled lower pH levels, typically a pH of between about 7.0 to 8.5.
In my co-pending, commonly assigned parent U.S. patent application Ser. No. 657,973 filed Oct. 5, 1984, there is disclosed a method of producing low electrolyte sodium salt sulfometnylated lignins for use as dye and print paste additives employing the following steps: (1) lignin material from the black liquor of a kraft paper-making process is ionized and methylolated in an alkaline liquid medium by treatment with an aldehyde compound, (2) the methylolated lignin is treated with an acid, such as H.sub.2 SO.sub.4, to lower the pH to an acid level and precipitate the methylolated lignin, (3) the precipitated methylolated lignin is washed with water to remove inorganic salts and residual reactants therefrom, and (4) the washed, methylolated lignin is treated with a sulfonating composition, such as a sodium sulfite or sodium bisulfite compound, at an acid pH level to produce a sodium salt sulfomethylated lignin product having low electrolyte content. By reducing the precipitated lignin to an acid level and washing it with water, undesirable inorganic salts and electrolytes are removed from the lignin to produce the improved product suitable for use in dye and print paste compositions.
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. (other sulfate, as well as sulfite, lignins may be employed also.)
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 9.5 at which point the lignin exists in a precipitated form. 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.
Lignin obtained from the kraft, soda or other alkaline processes is not recovered as a sulfonated product, but is sulfonated by reacting the material with a bisulfite or sulfite compound. A sulfonated lignin is understood to be any lignin containing at least an effective amount of sulfonate groups to give water solubility in moderately acid and higher pH solutions.
One of the conventional processes for sulfonating lignin involves sulfomethylation of the alkali lignin by reacting the lignin with sodium sulfite and formaldehyde. Such a process is described in Adler et al U.S. Pat. No. 2,680,113. Sulfomethylation acts upon the aromatic phenolic nuclei of the lignin molecule in such a manner that --CH.sub.2 SO.sub.3 .sup.- Na.sup.+ groups are bonded to the aromatic phenolic ring. It is also possible to sulfonate the lignin side-chain of the aromatic nucleus by sodium sulfite treatment of the lignin in the absence of formaldehyde. Sulfomethylation of the alkali lignin has been carried out generally at a pH level of 9.0 or higher, in order to ensure optimum phenol ionization and solubility of the lignin for sulfomethylation.
Although prior art sulfomethylated lignins existed as relatively pure products when sold at a high alkaline pH composition concentration, their use as a dyestuff additive requires adjustment by a dyer to an acidic pH level commonly employed in most textile dyeing by the addition of acid. Since the amount of acid required to either neutralize or acidity the lignin corresponds directly to the electrolyte formation which enters into a given dyestuff formulation, acidification produces increased undesirable electrolytes in the composition. In lowering the pH of the lignosulfonates, the phenolic groups which convert from ionized form to acid form initially produce a buffering effect in the higher alkaline region, such that their initial response to acidification is minimal. This results in large amounts of acid being necessary to initially move the lignin dyestuff additive toward acidic when the starting pH of the lignin compositions is in the higher alkaline range, resulting in corresponding higher amounts of electrolytes being present in the dye composition.
Acidification of the lignin by the dyer not only produces undesirable increases of electrolytes in the dyestuff composition, but also is an added expense to the dyer in preparation of the lignin additive for use.
Difficulties have also been experienced in attempts to use lignosulfonates as additives in synthetic printing pastes. Such printing pastes are highly vulnerable to electrolytes such that, in their presence, printing gel viscosity is reduced causing dyestuff run-off problems during printing. Since lignosulfonate additive compositions typically have exhibited high electrolyte content in printing paste pH applications, they have found only limited utility in such applications.