Bleaching and dyeing; fluid treatment and chemical modification – Bleaching – Color stripping or subduing
Reexamination Certificate
1999-07-09
2001-04-17
Einsmann, Margaret (Department: 1754)
Bleaching and dyeing; fluid treatment and chemical modification
Bleaching
Color stripping or subduing
C008S107000, C008S109000, C008S110000, C008S115690
Reexamination Certificate
active
06217621
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the removal of organic dye molecules from fiber matrices. More particularly, it relates to the combination of a borohydride and a bisulfite and their reaction instead of sodium dithionite, as conventionally used, in the process of stripping dyed textiles.
BACKGROUND OF THE INVENTION
The stripping of organic dye molecules from textiles (e.g., woven, non-woven, and knitted cloth, and yarn) is performed when, for example, a first dyeing is unlevel and the dye house operator wishes to remove it preparatory to a second dyeing. The dye does not merely cling to the surfaces of the textile fibers but is held in the amorphous regions of fiber matrices by ionic and covalent bonds as well as by van der Waal forces and hydrogen bonding. Sodium dithionite and thiourea dioxide (also known as form amidine sulfonic acid or FAS) are the leading dye stripping agents in the textile industry. Sodium dithionite (also known as sodium hydrosulfite or “hydro”) is the most popular one and is most often used as a powder containing 70% by weight of the active material along with sodium carbonate as a fire retardant, a chelating agent such as EDTA or a sequestering agent such as STPP. The textile industry still suffers from many spontaneous fires and the concomitant emission of sulfur dioxide when the dithionite powder comes into contact with moisture. Although use of an aqueous solution of the dithionite is growing because of the health, flammability, storage, and handling problems involved with the use of the powder, the solution must be stored under an inert atmosphere in dedicated insulated tanks that must be refrigerated for long term storage. Despite those problems and the recognition that the dithionite powder is inefficient and unreliable, it has retained its standing as the preferred stripping and reducing agent.
In conventional practice, the textile is placed in an aqueous bath, the pH of the bath is adjusted according to the requirements of the particular kind of textile to be stripped (i.e., below 7.5 for acetate and from a minimum of 10.2 to about 12.3 for cotton), and the bath is then heated to about 90-160° F. before the dithionite powder is added. The stripping bath is then heated to 212° F. (“the boil”) or as high as about 230° F. when a pressurized vessel is used. The stripping is continued at temperature for about 30 minutes or more, the bath is dropped, and the stripped textile is rinsed twice. Oftentimes, the procedure must be repeated to achieve the necessary dye stripping and color removal.
It is clear, therefore, that the industry needs a more efficient and more reliable stripping system that gives satisfactory color removal even when the temperature is below the boil and the stripping time is much less than the conventional 30 minutes.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a dye stripping system that is consistently better than one in which dithionite is a raw material at temperatures above and below the boil.
It is a related object of this invention to provide a dye stripping system that provides a reclaimed textile whose acceptance of a new dye is superior in comparison with the prior art.
It is a related object of this invention to provide a dye stripping system that results in shorter production cycle times, energy savings and less substrate degradation.
It is a related object of this invention to provide a dye stripping system that eliminates the need for high pressure stripping.
It is a related object of this invention to provide a dye stripping system that eliminates the need for the chelating agents, sequestering agents, fire retardants, odor scavengers commonly used as part of the dithionite system.
These and other objects of this invention which will apparent from the following description are achieved by a dye stripping and decolorizing system comprising an aqueous bath maintained at a temperature of from about 180° to about 230° F., a bisulfite having the formula MHSO
3
dissolved in said bath, a textile immersed in said bath, said textile being made from matrix-dyed fibers, and a reducing agent produced by the reaction at a pH of from about 4 to about 7.4 of said bisulfite with from about 0.09% to about 0.35%, by weight of the textile, of a borohydride having the formula ZBH
4
; wherein M is sodium, potassium, or ammonium, and Z is sodium or potassium. The borohydride may be added to an aqueous bath already containing the bisulfite and the textile or the borohydride and bisulfite may be mixed in an add tank and the mixture added to the aqueous bath.
DETAILED DESCRIPTION OF THE INVENTION
For the purposes of this invention, textile means spun fibers, filamentary fibers, yarn, woven, non-woven, or knitted goods made from said fibers and selected from the group consisting of cellulose acetate, cellulose triacetate, silk, wool and other protein fibers, flax and other bast fibers, nylon and other polyamide fibers, acrylic, polyethylene terephthalate, and cotton and rayon fibers.
Dyes that may be removed from matrix-dyed textile fibers by the stripping system of this invention include acid dyes, basic dyes, direct dyes, mordant dyes, sulfur dyes, vat dyes, reactive dyes, and disperse dyes.
Fresh dithionite may be produced by the reaction between sodium borohydride and sodium bisulfite according to the equation:
The yield is somewhat less than 100% because of the competing reaction of the borohydride with water and other reactions but it is most often about 90% or better. The radical anion, .SO
2
—, is a possible intermediate in the reaction, as has been suggested by M. M. Cook in
Environmental Chemistry of Dyes and Pigments
, pp 33-41; A. Reife and H. Freeman, ed.; (J. Wiley, 1996). It, also, is a very strong reducing agent. Although the exact mechanism of the reaction has not been fully characterized and this invention is in no way limited by any particular mechanism, said radical anion may act as a reducing agent in this invention along with the dithionite, some portion of the borohydride, and some portion of the bisulfite.
Preferably, the amount of borohydride used in this invention is from about 0.1 to about 0.3%, more preferably from about 0.1 to about 0.25%, still more preferably from about 0.1 to about 0.15%, of the textile weight or, as they say in the dye art, on the weight of the goods (owg).
An alkaline aqueous solution containing, by weight, 12% sodium borohydride and 40% sodium hydroxide is the preferred reagent for the purposes of this invention because of its availability and stability. Such an aqueous solution is available under the trademark BOROL from Morton International, Inc. The amount of such an aqueous solution for the purposes of this invention is from about 10% to about 30%, preferably no more than about 25%, by weight, of the amount of anhydrous dithionite recommended by suppliers for the conventional dye stripping procedures in the textile industry, which vary for each particular textile and dye. For example, the recommended amounts of the 70% active powder are: about 2% on weight of goods (owg) for acetate fibers; about 1.5% owg for flax; about 10% owg for cotton; and about 8-10% owg for rayon chenille. On a molar basis, the amount of dithionite produced from the borohydride bisulfite reaction is equal to or less than the recommended amount. A proportionate amount of potassium borohydride, also commercially available, may be used.
As indicated by the above equation, 8 moles of the bisulfite are required for each mole of the borohydride but it is preferred to use a stoichiometric excess of the bisulfite. It is particularly preferred to use up to about 10% excess bisulfite on a molar basis. Moreover, when the BOROL solution is used, the sodium hydroxide present therein (40% by weight) requires an additional 3.2 moles of the bisulfite for each mole of borohydride used. Thus, in that case, it is preferable to use up to a total of about 12 moles per mole of borohydride.
In a preferred embodiment of this invention, the borohydride and the bisulfite are mixed
Ko Jeremy
Modebelu Peter
Schoots Harrie
Crimaldi Kenneth
Einsmann Margaret
Morton International Inc.
Will Joanne P.
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