Aurora Turcanu

Natural dyes in modern textile dyehouses — how to combine experiences of two centuries to meet the demands of the future?

Abstract Plant materials which are available from farming regions in the moderate Austrian climate were investigated to serve as sources for natural dyes in textile dyeing operations. The extraction of the dye components from the plant materials was performed with boiling water without addition of chemicals or solvents. Based upon a rigorous selection of possible plant sources, a selection of natural dyestuffs applicable in a one-bath dyeing step was established. A broad variation in shade and color depth can be achieved by applying mixtures of natural dyestuffs in various combinations of iron- and alum-mordants. More than 60% of tested dyeings achieved acceptable fastness properties. On the basis of the developed natural dyestuff-based dyeing procedures, a comparison was made between the effluents from processes based upon them and those based upon the current ‘state-of-the-art’ techniques utilizing synthetic dyes. The comparison revealed that a lowering of the chemical load released with waste water can be expected by shifting to the plant-based dyes.

Anthraquinones as mediators for the indirect cathodic reduction of dispersed organic dyestuffs

Abstract 9,10-Anthraquinone-2-sulfonic acid Na-salt (AQS2), 9,10-anthraquinone-1,5-disulfonic Na-salt (AQDS1,5) and 1,4-dihydroxy-9,10-anthraquinone (DHAQ1,4) were investigated as to their ability to act as mediators for indirect electrochemical reduction of dispersed organic dyestuffs. A characterisation of the reducing efficiency was obtained from CV-experiments when comparing the cathodic current maxima of the AQ solution containing the dyestuff with the cathodic peak current without reducible vat dye. Voltammograms in a flow cell show that the limiting current density is dependent mainly on the diffusion transport of the AQ, while the addition of reducible dispersed dyestuff is of minor influence. Spectroelectrochemical experiments performed with 9,10-anthraquinone-1,5-disulfonic Na-salt in presence of Vat Yellow 1 permit a photometric observation of the coupled redox reactions.

Indirect Electrochemical Reduction of Dispersed Indigo Dyestuff

Iron-triethanolamine complexes are well suited for the indirect reduction of dispersed dyestuffs, e.g., finely dispersed indigo in a dyebath. Experiments to develop basic data for the scale-up of an electrochemical cell usable for the indirect reduction of indigo, vat dyes, and sulfur dyes are described. Planar electrodes show that the current density at the cathode is limited by diffusion controlled transport phenomena in the mediator system. The characterization of the iron triethanolamine mediator system is performed by reduction experiments in a divided flow cell with parallel geometry. An optimization of this cell type was achieved with three-dimensional flow through electrodes, leading to high specific electrode areas and a good space/time yield with the investigated electrochemical system. The characterization of the different cell types was performed with electrolysis experiments using the mediator alone and in the presence of dispersed indigo dyestuff

Process balance and product quality in the production of natural indigo from Polygonum tinctorium Ait. applying low-technology methods

Indigo is the most important blue component in the class of natural dyes for cellulose and protein fibres. In the moderate European climate Polygonum tinctorium Ait. could be an interesting source for natural indigo (Vat blue 1). Following a cultivation of the plant material a simple procedure for the extraction of the indigo precursor indican was investigated with regard to crop and quality of dye obtained. The dependence of the crop on the storage conditions of the harvested plant material was investigated. The results quantify the distinct sensitivity of the fresh material to the time of storage before extraction with regard to the amount of natural indigo obtained, the photometrically determined indigo content in the product and the shade and colour depth observed in standardised dyeing experiments. A basic set of data is presented, which describes the process in terms of consumption of energy, water and chemicals and organic waste released from the extraction step.

Electrochemical Vat Dyeing Combination of an Electrolyzer with a Dyeing Apparatus

A standard dyeing apparatus for yarn on X-cones has been coupled to a multicathode eletrolyzer for indirect reduction of dispersed oxidized vat dyes. An alkaline solution containing 0.01 M iron-triethanolamine complex was used as mediator solution. In test runs current efficiencies better than 80% were obtained at cell currents up to 10 A. The cathode was built up from 10 insulated sheets of steel fabric with a total area of 0.96 m 2 . In batch experiments cones of cotton yarn with an average mass of 900 g yarn were dyed. Different dyes were applied in light to dark shades at a dyeing temperature of 60(C. During the electrochemical dyeing process the cell must carry out different functions: reduction of dissolved oxygen, indirect reduction of dispersed oxidized vat dye buildup of a certain reduction capacity against air oxygen, and control of redox potential in the dyeing apparatus by adjustment of cell current. A description of the concentrations of dissolved oxygen and reduced form of the mediator during electrolysis can be achieved by a simple mathematical model, which permits efficient optimization of important technical parameters for process design and scale-up.

Dyeing Behavior of Indigo Reduced by Indirect Electrolysis

Dispersed indigo dyestuff can be reduced by indirect electrolysis using an iron (II)-triethanolamine complex. The iron (III) form of the complex can be transformed to the iron (II) form by cathodic reduction, thus leading to a regenerable reducing agent. Electrochemically reduced indigo is tested in laboratory scale dyeing experiments, and the results of different reduction conditions in the dyebath are discussed. The influence of the concentration of the complex-system on the build-up of color depth and shade with increasing number of dips is discussed and compared with samples of the standard dyeing procedure using sodium dithionite as reducing agent. The results of the latter conventional process show that the dyestuff in the dyebath behaves in a manner similar to that when a regenerable Fe(II)-complex is used as the reducing agent.

Multi-cathode cell with flow-through electrodes for the production of iron(II)-triethanolamine complexes

Nonregenerable reducing agents used in dye houses for the application of indigo, vat dyes and sulfur dyes can be substituted by indirect cathodic reduction techniques. An electrochemical cell for the indirect cathodic reduction of dispersed indigo dyestuff using an alkaline solution of the Fe(iii/ii)-triethanolamine complex as redox mediator was constructed and tested. The cell is built up as divided cell (cathode area 5–10m2, catholyte volume 12L, anolyte volume 1.5L) with several three dimensional cathode units (up to 10) in the same cathode compartment. The cathodes were connected to a common anode and to separately adjustable power supplies. The catholyte was circulated through the porous cathode units parallel to the current direction. Two different electrode materials (copper and stainless steel) and cathode constructions were tested, resulting in an optimized cell construction. The electrochemical cell was characterized by a series of batch electrolysis experiments. Results are given dealing with the cell voltage drop in the cathode, the product yield and the current efficiency at different current densities and cell current. After an optimization step the current efficiency reaches 70–80% at 2 A m–2 current density and 7.8×10–3moldm−3 Fe(ii)-complex. The cell current is 10A.

Direct cathodic reduction of Leuco Sulfur Black 1 and Sulfur Black 1

The electrochemical activity of Leuco Sulfur Black 1 in alkaline solution is shown by the means of voltammograms of the dyestuff recorded in a flow cell. The voltammograms indicate the possibility for further dyestuff reduction by cathodic electron transfer. In batch electrolysis experiments successful reduction of Leuco Sulfur Black 1 was achieved using a multi cathode cell. Applying similar experimental conditions cathodic reduction of the insoluble oxidized dyestuff Sulfur Black 1 was achieved. In a first step the dispersed Sulfur Black 1 is reduced to give an alkaline soluble form of the dyestuff that can be reduced further to yield Leuco Sulfur Black 1. Analogous to the electrochemical behaviour of technically produced Leuco Sulfur Black 1, which contains reducing agents, the solution of Leuco Sulfur Black 1 formed by electrolysis is able to undergo further cathodic electron transfer. The cathodic dyestuff reduction is of importance for the development of sulfur dyeing processes which do not require addition of reducing chemicals.

Cathodic Decolourisation of Dyes in Concentrates from Nanofiltration and Printing Pastes

Direct cathodic reduction of dyes which contain an azo-goup in the chromophore was successfully used for decolourisation of intensively coloured concentrates from Nanofiltration treatment of textile effluents. Based on laboratory scale experiments, a technical multi-cathode electrolyser was applied for full scale decolourisation experiments at cell currents from 40 to 80 A. The absorbance of the treated wastes decreased from 60 to 80% of the initial value at an energy consumption of 2–8 kWh m−3. Experiments with addition of redox mediator indicate a significant increase in decolourisation rate; however chemical consumption is increased for 0.5–1.5 kg m−3 of waste. The decolourisation of reactive dye containing printing pastes was also achieved at the laboratory scale, where decolourisation of 60–80% was achieved.

Ca2+–Fe3+–D-gluconate-complexes in alkaline solution. Complex stabilities and electrochemical properties

The complexation of Fe3+ and Ca2+ ions by D-gluconate (DGL) in alkaline solutions was studied by potentiometric titration. Stability constants for the complexes were determined by mathematical treatment of the data with the computer program BEST. Depending on the concentration of ligand, complexes of Fe3+ ∶ DGL stoichiometry 1 ∶ 1, 1 ∶ 2 and 2 ∶ 2 were identified. In the presence of Ca2+ ions mixed binuclear complexes are formed with the Ca2+ ∶ Fe3+ ∶ DGL stoichiometries 1 ∶ 1 ∶ 1 and 1 ∶ 1 ∶ 2. The complex system was studied with cyclic voltammetry and with constant current electrolysis experiments. The uptake of Ca2+ into the FeIIIDGL complexes results in a distinct change of the electrochemical properties of the iron(II/III)-redox couple. A shift of the formal redox potential towards more negative values is observed. The Ca2+ ∶ Fe3+ ∶ DGL complexes can be regarded as models for an iron(II/III)-redox system whose electrochemical behaviour is determined by the concentration of an electrochemically inactive ion.