1.1.2. Dyeing:

The use of ultrasound in the dyeing of textile can be explained as: when ultrasound waves are absorbed in the liquid system the phenomenon of cavitation takes place. Cavitation can liberate entrapped gases from liquid or porous materials like textiles, dyebath etc. The influence of ultrasound on dyeing is explained to have three-way effects:

(I) Dispersion: Breaking up of micelles and high molecular weight aggregates in to uniform dispersion in the dyebath.

(ll) Degassing: Expulsion ( dissolved or entrapped gases or air molecules from fiber capillaries and interstices at the cross over points of fiber in to liquid and removed cavitations.

(Ill) Diffusion: Accelerating the rate of diffusion of dye inside the fiber by piercing the insulating layer covering the fiber and accelerating the interaction between dye and fiber.

Effects 1 and II are promoted by the mechanical action of cavitation, while effect III is due to both the mechanical action and the heating of the fiber surface. In case of water soluble dyes, ultrasound constitutes mostly an effective means of mechanical agitation, whereas in case of pigments, which are not soluble in water, ultrasound provides means of pigment dispersion and penetration, which is not provided by the conventional method. The dyeing results are affected by the frequency of the ultrasound used. Irradiation at very low frequencies of the order of 50 or 100 cps produces no effects. Frequencies in the range between 22 and 175 KHz have been found to be most effective, the latter frequency being preferable for silk, wool and nylon.

1.1.3. Diffusion mechanism:

The diffusion of dye inside the fiber is speeded up in the ultrasonic field. The speed of dye diffusion inside the fiber depends upon the size of the dye molecular and the state of the fiber i.e. the smaller the dye molecule and greater the fiber swelling the higher is the mobility of the dye molecules and the quicker they penetrate inside the fiber.

Another factor, which influences the diffusion of dye inside the fiber, is its activated state. The dye diffuses in the fiber pores, which are full of water and at the same time it is adsorbed by the adjacent macromolecules. Owing to adsorption only a small part of dye can freely move inside.

The dye molecules spent much of their time in vibrating to and fro before they are adsorbed on the surface. Because of the simultaneous adsorption and diffusion, the diffusion slows down if the rate of adsorption is slow. However, because of the intense cavitation force in the ultrasonic field the dye molecules arrive at the fiber surfaces at a much faster rate as they gain additional kinetic energy. The dye must be in the activated state to diffuse. This activated state to be brought about by ultrasonic energy, which furnishes the vibrating molecules with the critical energy they need to break their static equilibrium and thus to diffuse.

Pressure of the ultrasonic radiation on the surface of the fiber is another factor, which influences the diffusion process. There may be some loosening in the crystalline structure although most transient but of great significance in speeding up the rate of diffusion. Therefore, dyeing carried out at low temperature in the ultrasonic field showed adsorption equivalent to that in dyeing carried out without ultrasound at higher temperature.

1.1.4. Equipment for ultrasound:

Generator and converter or cleaning bath are the two main components of ultrasound equipment. Generator converts 50 to 60 Hz alternate current to electrical energy of high frequency. This electrical energy is fed to the transducer where it is transformed to mechanical vibration. The transducer system vibrates longitudinally transmitting waves into liquid medium. As these waves propagate cavitation occurs. Prototype dyeing machine was designed for continuous dyeing of yam and fabric. The system mainly consists of the tank, transport system and microprocessor, which is used to monitor the process. Ultrasonic tank is of 92 x 60 cm dimensions and capacity up to 200 liters. Temperature can be varied up to 100C by thermostatic control.