Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Reexamination Certificate
2000-03-06
2001-11-06
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
C205S554000
Reexamination Certificate
active
06312583
ABSTRACT:
Among the sulphur dyes group, there are grouped together dyes with the same principles of manufacture and the same manner of dyeing. Sulphur dyes are produced by reacting suitable organic substances with sulphur, alkali sulphides or alkali polysulphides. The products produced contain recurring organic structural elements which are bonded together by bisulphide groups. The chemical constitution is not reliably known in the majority of cases.
For the purpose of dyeing, the sulphur dyes are reduced using different reduction processes. With this, a portion of the disulfide bridges are split reductively (equation 1), the products produced have lower molar masses, are soluble in aqueous alkaline solution, and can be used for the purpose of dyeing as they also have an affinity to fibres, for example, to cellulose fibres.
R-S-S-R+2e
−
←→R-S
−
+R-S
−
(R=organic residue) (1)
As, in the dyeing process, the dye bath containing the reduced dye has to be protected from undesired oxidation of the dye by air, reduction chemicals are always also added to the dye bath. If both the reduction of the dye and the prevention of oxidation by the air are to be obtained by the addition of reduction chemicals, very large quantities of reduction chemicals are needed, which give rise to corresponding expenses in both the dyeing process and the disposal of the waste water from the dyeing which is used. Some improvement is obtained with the process according to DE-OS 1 906 083, which recommends dye reduction with the aid of cathodic reduction in aqueous solution at current densities between 5 mA.cm
−2
and 50 mA.cm
−2
.
Probably because of the high current density used, the known process must be interrupted because of the development of hydrogen when the concentration of reduction equivalents in the solution is somewhere in the region of 70-150 Ah.kg
−1
. Although the solution would be useable, it would be partially oxidised by the oxygen occurring in the dyeing process. A chemical reduction agent is therefore added to the electrochemically reduced dye, which agent in total makes up half of the entire reduction equivalent. While with the process according to DE-OS 1 906 083 approximately half of the chemical reduction agent can consequently be dispensed with, this process cannot be used with the commercially available pre-reduced sulphur dyes, which are normally treated in the dye solution with further stabilising reduction agents. With the content of reduction equivalents obtained through pre-reduction, immediate development of hydrogen would occur.
While the prior publication described describes process times lasting several hours, and therefore refers to possibly increasing the current strengths used, in order to reduce this time, the invention is based on the unexpected recognition that by means of a significant reduction in the current density on the cathodes, the electrochemical reduction can be continued far enough that even after oxidation by the oxygen occurring during the dyeing process, the dye is still sufficiently reduced.
The invention thus relates to a process for reduction of sulphur dyes to an analytically determinable concentration of at least 150 Ah.kg
−1
of reduction equivalents in the solution, in relation to the solid dye, wherein at least 50 Ah.kg
−1
can be introduced by cathodic reduction. The process has a current density of between 0.5 mA cm
−2
and 5 mA.cm-2, preferably between 0.8 mA.cm
−2
and 2 mA.cm
−2
with a dyestuff concentration of 100 g.l
−1
. At least 70% of the reduction equivalents, preferably substantially all of the reduction equivalents are introduced electrochemically.
The known process is improved in that with a concentration of reduction equivalents over 150 Ah.kg
−1
, cathodic reduction takes place. The determination of the equivalent quantity of reduction agent can take place by means of potentiometric titration of the electrolyte with iodine solution and calculation using Faraday's law. The quantity of reduction agent present after electrolysis in the catholyte can also be determined by measurement of the charge which has flowed.
With sulphur black (Diresul 4G-EV fl., Sulfur black 1, Colour Index 53185), a typical instance of the class of sulphur dyes, this value is where there is at least 180 Ah.kg
−1
of solid dye. If the dyes are already present in a reduced form, which can be obtained, for example, by chemical additives during manufacture of the product, the charge which has to be applied additionally through cathodic reduction reduces. Any skilled person is capable of determining the degree of reduction already present by analytical testing of the condition of the initial dye, and of calculating from this the reduction equivalent still to be produced by cathodic reduction, such that overall reduction equivalents of at least 180 Ah.kg
−1
of solid dye are present. In the calculation, the concentration of dye in the technical product must also be taken into consideration, as numerous technical commercial products contain only a specific amount of mass of solid dye because of regulating agents (for example, salts, dispersing agents) or in the case of the manufacturing of liquid products, because of their preparation.
REFERENCES:
patent: 3640803 (1972-02-01), Frind et al.
patent: 3953307 (1976-04-01), Supanekar et al.
patent: 5244549 (1993-09-01), Bechtold
patent: 1906083 A (1970-08-01), None
patent: 9015182 A (1990-12-01), None
Bechtold Thomas
Burtscher Eduard
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Thao
Wong Edna
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