1.1.5. Ultrasonic offers many potential advantages in
textile wet processing:
- Energy savings by dyeing at lower temperatures and
reduced processing times
- Environmental improvements by reduced consumption of
auxiliary chemicals
- Processing enhancement by allowing real-time control
- of color shade
- Slower overall processing costs, thereby increasing
industry competitiveness.
1.2. Microwaves:
Microwaves are electromagnetic waves whose frequency ranges
from 1000MHz to10,00,000 MHz. Microwaves are so called since they are defined
in terms of their wavelength in the sense that micro refers to tiny. In other
words the wavelengths of microwaves are short at the above range of frequency,
typically from few cms to few mm. The higher frequency edge of microwave
borders on the infrared and visible light region of the spectrum.
1.2.1. Microwave dyeing:
Microwave dyeing takes into account only the dielectric and
the thermal properties. The dielectric property refers to the intrinsic
electrical properties that affect the dyeing by dipolar rotation of the dye and
influences the microwave field upon the dipoles.
The aqueous solution of dye has two components which are
polar, in the high frequency microwave field oscillating at 2450MHz. It
influences the vibrational energy in the water molecules and the dye molecules.
The heating mechanism is through ionic conduction, which is a type of
resistance heating. Depending on the acceleration of the ions through the dye
solution, it results in collision of dye molecules with the molecules of the
fiber. The mordant helps and affects the penetration of the dye and also the
depth to which the penetration takes place in the fabric. This makes microwave
superior to conventional dyeing techniques.
2. Electrochemical dyeing14:
The vat and sulphur dyes are insoluble in water; therefore
for their application it is necessary to convert them into water-soluble form
using suitable reducing agent and alkali. Different reducing agents use for vat
and shulphur dyes are briefly reviewed with emphasis on the emerging technique
of electro chemical reduction.
2.1. Reducing agent for the vat dyes:
Sodium dye thionite is the universal and mainly used reducing
agent for the vat dyes. It is also known as sodium hydrosulphite, which has
chemical formula Na2S2O4. It reduces the
entire vat dye at the temp range 300-600 C and above. Sodium dithionite
dissociates properly and liberates nascent hydrogen.
Na2S2O4 + 4H2O
2NaHSO4 + 6H+
Na2S2O4 + NaOH 2NaSO3 + 2H+
Sodium dithionite is very unstable and get decomposed (oxidative)
and thermally to several byproducts. Some are acidic in nature .the stability
of the alkaline solution of sodium dithionite decreased with increased with
temperature; increased surface exposed to the air and decreased agitation bath.
2.2. Vat dyeing by electrochemical method
Dyestar has patented an electrochemical dyeing process that it developed jointly with the textile machinery manufacturer Thies GmbH & Co.
and the institute of textile chemistry and textile physics at the university of
Innsbruck in Dornbirn Austria According to the company, the process uses an electric current instead of chemical reducing agents, giving it a number of
technical, economic and ecological benefits.Dyestar have developed a vat dye, Indanthrane
blue E-BC, specifically for this electrochemical dyeing process. The dye liquor
used in electrochemical dyeing with Indanthrane blue E-BC can be reused in an
unlimited number of times and contamination of dye house effluent is close to
zero.
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