Ultrasonic represents a special branch of general acoustics, the science of mechanical oscillations of solids, liquids and gaseous media. With reference to the properties of human ear, high frequency inaudible oscillations are Ultrasonic or supersonic. In other words, while the normal range of human hearing is in between 16Hz and 16 kHz, Ultrasonic frequencies lie between 20 kHz and 500 MHz. Expressed in physical terms, it is the sound produced by mechanical oscillation of elastic media.
The occurrence of sound presupposes the existence of material it can present itself in solid, liquid or gaseous media. Wet processing of textiles uses large quantities of water, and electrical and thermal energy. Most of these processes involve the use of chemicals for assisting, accelerating or retarding their rates and carried out at elevated temperatures to transfer mass from processing liquid medium across the surface of the textile material in a reasonable time.
Scaling up from lab-scale trials to pilot plant trials is difficult. In order for ultrasound to provide its beneficial results during dyeing, high intensities are required. Producing high intensity, uniform ultrasound in a large vessel is difficult.
Ultrasound reduces processing time and energy consumption, maintains or improves product quality, and reduces the use of auxiliary chemicals. In essence, the use of ultrasound for dyeing will use electricity to replace expensive thermal energy and chemicals, which have to be treated in wastewater.
Ultrasound energy is sound waves with frequencies above 20,000 oscillations per second, which is above the upper limit of human hearing. In 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 liquid. When the bubbles burst, 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 Ultrasonic effects observed in solid/ liquid or liquid/liquid systems.
The figure below shows the waves produced by ultrasound
Generation of waves
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 piezo-electric and magnatostrictive transducers. The working mechanism of sirens and whistles allow an optimal transfer of the Ultrasonic sound to the ambient air.
In the case of magnatostrictive and or piezo-electric transducers of Ultrasonic waves, the generators as such will only produce low oscillation amplitudes, which are difficult to transfer to gases. The occurrence of cavities depends upon several factors such as the frequency and intensity of waves, temperature and vapor pressure of liquids.
Basic design of instrument
An Ultrasonic generator generates the high frequency energy for the process. The electric variation lies between 20 and 40 KHz. The electronic vibrations of the generator transmitted to the ultrasound head by a shielded wire and converted into mechanical vibrations by ceramic piezo ring. The ceramic ring is fit into a metal body, which enhances the vibration reaches its maximum, which lies in the range.
To put the ultrasound into effect a close contact between the sonotrode and the goods is necessary. This is done by working on a solid surface, e.g., glass or by pressing a respective counter tool to the sonotrode. Thus, the vibrations are transmitting to the material to be processed and create innerfriction, heat and possibly processing. The sequence of operations of the plant is controlled by microcomputers.
Ultrasound technology can be attributed to various mechanisms. The effect of Ultrasonic depends on various factors like radiation pressure, heat, streaming, cavitations, agitation, interface instability and friction, and diffusion and mechanical rupture.
The mechanisms involved are increasing swelling in water, reducing glass transition temperature of the fiber (dilation of amorphous regions), increasing the fiber/dye bath partition coefficient, enhancing transport of the dye to the fiber surface by reducing the boundary layer, and breaking up of micelles and high molecular weight aggregates into uniform dispersions in the dye bath.
Increasing swelling in water
The swelling of both mercerized and un mercerized cotton fibers with water alone and with ultrasound has been studied. Ultrasound causes significant fiber swelling compared to water alone. Since cotton fibers are non-uniform and hard to measure, tests were repeated several times and the results averaged.
Several methods of measurement were used to confirm the results. For un mercerized cotton, swelling with water alone ranges from 10% to 20%, but with ultrasound the range is from 25% to almost 50%. For mercerized cotton, swelling with water alone is only about 3%, but with ultrasound it is about 35%. The mercerization process causes permanent swelling of the cotton fiber.
Of interest here is that ultrasound still causes additional swelling. The fiber swelling is observed for at least an hour after the removal of the ultrasound, as long as the fiber remains in water. When removed from water and allowed to dry, the fiber returns to its original diameter.
Dye particle size
As Dr. G. Mock, R. McCall and D. Klutz carried out experiments. Five different vat dyes of known structure were examined, most of which had previously been used in ultrasound dyeing trials. A 5-gpl solution of each dye was prepared and then divided in half to give two samples for each dye. One sample of each dye was treated with 20 kHz ultrasound at 250C for 60 Min; the other sample was used as a control untreated sample. The dye samples were measured with a Honeywell Microtrac Particle Size Analyzer.
Following results were obtained
Ultrasound reduced the average size of the dye particles for each of the dyes tested. The effect ranged from virtually nothing in the case of Vat Black 25 to greater than 1.75 microns in the case of Vat Violet 1. The effect of ultrasound on particle size is most evident when examining the before and after graphic distributions drawn by the analyzer. Ultrasound had the greatest effect on vat dyes with a bimodal particle distribution. After these dye samples were treated with ultrasound the large particles were completely eliminated. Without ultrasound vat dyes may contain particles larger than 14 microns, but when vat dyes are treated with ultrasound the largest dye particles are smaller than 2 microns.
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. Before the dye can penetrate into the amorphous regions of synthetic fibers, 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.
Ultrasound intensity is the measure of the available energy per unit volume of the sample or material. Application of Ultrasonic can be thus be of low intensity and High intensity. In the low intensity application, input power levels are low enough that there is never any change in the state of the medium. Typical examples are the non-destructive testing of materials and measurement of elastic properties of materials. High intensity applications, wherein phase changes have more severe effect on the medium, are generally important for wet processes. In most chemical reactions, reaction rate is found to increase with intensity.
There are many industrial applications of the ultrasound, including in the fields of biology; biochemistry, engineering, geology, geography, and medicine where the application ranges from using high frequency sound to "see" unborn baby to destruction of stones and cancerous cells inside the body. The application in chemistry is mainly for physical measurements and also as method of improving reaction rates and/or product yields.
In textile applications
The effect of ultrasound on textile substrates and polymers started after the introduction of synthetic materials and their blends to the industry. These include application in mechanical processes (weaving, finishing and making up for cutting and welding woven, non-woven and knitted fabrics) and wet processes (sizing, scouring bleaching, dyeing, etc). It deals with the application of ultrasound in the mechanical processes of industrial as well as apparel textiles. Ultrasonic equipment for cutting and welding has gained increase acceptance in all sectors of the international textile industry from weaving, through finishing to the making-up operation.
A piece of textile is a non-homogeneous porous medium. A textile comprises of yarns, and the yarns are made up of fibers. A woven textile fabric often has dual porosity: inter-yarn porosity and intra-yarn porosity. 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.
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 and mass transfer from the liquid boundary layer to the bulk liquid.
The relative contribution of each step to the overall mass transfer in 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. In essence, the use of ultrasound for dyeing will use electricity to replace expensive thermal energy and chemicals, which have to be treated in waste water. The Ultrasound Consortium consists of Greenville Machinery, Branson Ultrasonics, Blackstone Ultrasonic, and Cotton Incorporated. A proposed application of Ultrasonic to textile processing consists of direct Ultrasonic energy transmission to the textile materials, through a horn and anvil, for continuous wetting or dyeing.
The apparatus was designed and built for pilot plant testing with the intent to introduce direct Ultrasonic energy to textile materials. To collect data more easily and accurately during the testing, data acquisition devices were interfaced with the Blackstone tank. The measurements desired are time, water/dye temperature, fabric yarn speed, and ultrasound power.
Approximately 25 pilot plant trials using the Blackstone cleaning tank to dye 100% cotton knit with direct dyes have been performed and analyzed. The material with and without the addition of ultrasound was evaluated for their fastness properties, exhaustion rates, and final color. The variables in the study were salt, temperature, and the addition of ultrasound. The ultrasound technique had been applied in the different areas of textile wet processing including desizing, scouring, bleaching, washing, dyeing and finishing, etc.
It was found that the use of degraded starch followed by Ultrasonic desizing could lead to considerable energy saving as compared to conventional starch sizing and desizing. Fiber degradation is also reduced, and final whiteness and wet ability of the fabric are the same as those of without Ultrasonic.
The possibility of dyeing textiles using ultrasound was started in 1941 - dyeing of cotton with direct dyes, wool with acid dyes, polyamide and acetate fiber with disperse dyes. The significant increases in the rate of dyeing with disperse dyes on polyamide and acetate were obtained. Ultrasound is more beneficial to the application of water insoluble dyes to the hydrophobic fibres. Effects dispersion and degassing are promoted by the mechanical action of cavitation, while diffusion due to medical action and heating of surface.
Ultrasound irradiation also produces a greater evenness in color. The dyeing results are affected by frequency of ultrasound used. Frequency of 50 or 100 cis produces no effect while frequency of 22 to 1 75 Kc/s have been found to be most effective.
Ultrasound energy of 20 KHz frequencies is suitable for inducing cavitations. The micro bubbles that are unstable solely grow in the process of oscillation .the implode violently thereby generating momentary localized high temperature and pressure. This active stage causes chemical reaction between fiber and dye, this result in better dye uptake.
The main conclusions of the study on direct dyes on cotton were:
- At lower temperatures ultrasound leads to higher final exhaustion
- As temperature increases, ultrasound causes smaller increases in final exhaustion, eventually reducing exhaustion levels below those of non-ultrasound dyed fabrics
- Ultrasound lowers wash fastness, dry crock fastness, and wet crock fastness of dyed samples
- The ultrasound power levels may be too low to provide beneficial results. Since low intensity ultrasound does not seem to be beneficial for direct dyeing of cotton on a pilot plant level, the future experiments will focus on acid dyes on nylon and disperse dyes on polyester.
Among textile fibers, polyester is a structurally compact fiber with a high level of crystallinity and without recognized dye sites. Polyester fiber is dyed using the only class of dye disperse dyes. From aqueous dispersions with a carrier at 1000C; from aqueous dispersions at 120-1400C under pressure, or a thermo fixation dyeing at 175-2100C.
There are however, several alternative methods of dying at the exploratory level- super critical carbon dioxide assisted dyeing and Ultrasonic techniques.
In the case of polyester dyeing with disperse dyes, the use of Ultrasonic waves is still more relevant due to advantages like Ultrasonic waves helping in breaking up the aggregates and thereby stabilizing the dispersion, and Ultrasonic waves accelerating the rate of diffusion of the dye inside the fiber.
The benefits include 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; and increased color yields, in certain cases.
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.
Ultrasound is an effective alternative as compared to conventional textile processing as a lot of experiments have been carried out on cellulose material like cotton with direct dyes, nylon with acid dyes, basic dyes, reactive dyes; disperse dyes & leather processing, dry cleaning of textiles using perchloroethylene. All the experiments have shown positive results by achieving reduced processing time and energy consumption in wet processing. However a lot of work had been done in this area. Still majority of applications remain untouched. They are like cellulose blends, wool blends processing, effect of pre-treatments right from desizing to bleaching, finishing using biotechnology, and washing of textiles.
This technique is a boon for textile industry. Even a one per cent saving in chemicals can save a lot. The possibility of dyeing of textiles using ultrasound was initiated in 1941 by Sokolov and Tumensky and subsequently Brauer evaluated this method for vat dyeing of celluloses in order to reduce the time of vat dyeing. The work in this line was extended to dyeing of cotton, viscose and wool using direct and acid dyes respectively.
In the same study, adsorption of disperse dyes on cellulose acetate was also investigated, which indicated that the adsorption of disperse dyes on cellulose acetate was considerably influenced by the Ultrasonic waves as against the marginal influence of dyeing rates of the direct dyes. The work was further extended for dyeing of cotton with direct dyes, wool with acid dyes polyamide and acetate with disperse dyes.
There have been several other studies on similar lines. The observations of the preliminary studies revealed that are significant increase in the rate of dyeing was obtained with disperse dyes on polyamide and acetate, and the ultrasound is more beneficial to the application of water insoluble dyes.
The work done by Saligram and his co-workers showed that optimum cavitation effect occurs at a temperature of 45-50C with the useful cavitations frequencies are in the range of 5-50 kHz. In another study an ultrasound dyeing method for polyester at low temperature has been explored, in which the use of appropriate pre-swelling of the substrate had been reported to give acceptable shade depths at 50C.
The other areas of application: Paper producers have had problems in the past with meeting environmental. Standards and they have been moving toward enzymatic treatments. Ultrasound would make these treatments more cost-effective.
There are many industrial applications of ultrasound in the fields of biology/ biochemistry engineering, geology/ geography. Non-destructive testing of various joints, assemblies and machine parts examination of people in the field of medicine using low energy Ultrasonic waves, i.e., low amplitude waves.
The use of Ultrasonic energy in physico-chemical processes offers advantages from the point of view of conservation of energy, time and chemicals. The Ultrasonic processes are summarized as low temperature processes; uniform results in case of dyeing, and other processes; increased exhaustion and fixation depending on molecular structure of dyestuffs; and lesser load to the effluent.
Thus, the technique of ultrasound can be effectively implemented for textiles, in wet processing units for conservation of energy and time with improved dyeing techniques.