Ultrasonic Assisted Wet Processing

Last Updated on 04/05/2021

Aravin Prince Periyasamy
Asst Professor, Dept of Textile Technology,
DKTE Textile Engineering Institute, Kolhapur, India
E-mail: aravinprince@gmail.com

Ultrasonic-Assisted Wet Processing
The first commercial application of ultrasonic’s appeared around 1917 and was the first “echo-sounder” invented and developed by Paul Langevin (1872–1946). Ultrasonic frequencies lie between 20 kHz and 500 MHz (see Figure-1). The use of ultrasound in textile wet processing offers many potential advantages including energy savings, process enhancement and reduced processing times, enumerates. Ultrasound energy is sound waves with frequencies above 20,000 oscillations per second, which is above the upper limit of human hearing. In a liquid, these high-frequency waves cause the formation of microscopic bubbles or cavitations. They also cause insignificant heating of the liquid. Ultrasound causes cavitational bubbles to form in the liquid. When the bubbles collapse, they generate tiny but powerful shock waves. In a solid, both longitudinal and transverse waves can be transmitted whereas in gas and liquids only longitudinal waves can be transmitted. In liquids, longitudinal vibrations of molecules generate compression and refractions, i.e., areas of high pressure and low local pressure. The latter gives rise to cavities or bubbles, which expand and finally during the compression phase, collapse violently generating shock waves. The phenomena of bubble formation and collapse (known as cavitations) are generally responsible for most of the ultrasonic effects observed in solid/liquid or liquid/liquid systems. Figure-2 shows the waves produced by ultrasound.

Figure-1: Classification of sound according to the frequency

Figure-2: Representation of some typical characteristics of an ultrasonic wave

Ultrasonic waves can be generated by a variety of ways. Most generally known are the different configurations of whistles, hooters, and sirens as well as piezoelectric and magnetostrictive transducers. The working mechanism of sirens and whistles allow an optimal transfer of the Ultrasonic sound to the ambient air. In the case of magnetostrictive and/or piezoelectric transducers of ultrasonic waves, the generators as such will only produce low oscillation amplitudes, which are difficult to transfer to gases (see Figure-3). The occurrence of cavities depends upon several factors such as the frequency and intensity of waves, temperature, and vapor pressure of liquids.

Glass Transition Temperature
One proposed mechanism responsible for the effects of ultrasound in textile wet processing was the possible dilation of amorphous regions, i.e., decreasing the effective glass transition temperature in synthetic fibers, particularly for polyester, it is a necessary one making it possible to dye at a lower temperature. Before dyeing, the dyes can penetrate the amorphous regions of synthetic fibers, and the fiber must be heated above its effective glass transition temperature. In commercial dyeing, plasticizers are often added to lower glass transition temperatures. Since ultrasound allows fibers to be dyed at lower temperatures, it was thought that ultrasound might lower the glass transition temperature. As mentioned earlier, diffusion and convection in the inter-yarn and intra-yarn pores of the fabric form the dominant mechanisms of mass transfer in wet textile processes (see Figure-4).

Figure-4: Liquid flow around and through a yarn

The major steps in mass transfer in textile materials are:

Mass transfer from intra-yarn pores to inter-yarn pores
Mass transfer from the inter-yarn pores to the liquid boundary layer between the textile and the bulk liquid
Mass transfer from the liquid boundary layer to the bulk liquid

The relative contribution of each of these steps to the overall mass transfer in the textile materials can be determined by the hydrodynamics of the flow through the textile material. Ultrasonic may be employed to reduce processing time and energy consumption, maintain or improve product quality, and reduce the use of auxiliary chemicals. The use of ultrasound for dyeing will use electricity to replace expensive thermal energy and chemicals, which should be treated in wastewater.

Advantages of Ultrasonic Assisted Dyeing:
The advantages of using ultrasound in dyeing are:

It saves energy.
It makes it possible to dye PET fibers at a lower temperature.
It reduces the pollution load.
It improves the processing efficiency (with better color depth).
It causes little damage (wear and tear) to materials.
The processing cost is low.

Ultrasound Energy:
Ultrasound energy is sound waves with frequencies above 20,000 oscillations per second, which is above the upper limit of human hearing.

Figure-5: Generation of ultrasound energy

The ultrasonic waves can be generated by variety of ways. Mostly it is produced by piezo-electric and magnatostrictive transducers.

Mechanisms of Ultrasound Energy:

Increasing swelling of fiber in water.
Reducing glass transition (Tg) temperature of the fiber.
Reduce the size of the dye particles. It helps to enhance the transport of the dye to the fiber.

Applications of Ultrasound Energy:

It degraded starch followed by ultrasonic desizing could lead to considerably energy saving as compared to conventional starch sizing and desizing.
The scouring of wool in neutral and very light alkaline bath reduces the fiber damage and enhance rate of processing.
It is more beneficial to the application of water insoluble dyes to the hydrophobic fibers.
Among the textile fibers, polyester is structurally compact fiber with a high level of crystallinity and without recognized dye sites.
Ultrasonic waves accelerate the rate of diffusion of the disperse dye inside the polyester fiber.

Benefits of Ultrasound Energy:

Energy savings by dyeing at lower temperatures and reduced processing times.
Environmental improvements by reduced consumption of auxiliary chemicals.
Increased color yields.
Enzymatic treatments supplemented with ultrasonic energy resulted in shorter processing times, less consumption of expensive enzymes, less fiber damage, and better uniformity treatment to the fabric.

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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.