Application of Ultrasound in the Preparation of Cotton and Silk Fabric
West Bengal University of Technology
Govt. College of Engineering and Textile Technology
Berhampore, Murshidabad, India
Sound is a form of energy, just like electricity and light. Sound is made when air molecules vibrate and move in a pattern called waves, or sound waves .
Sound generated above the human hearing range (20 Hz to 20 kHz) is called ultrasound. Ultrasonic vibrations travel in the form of a wave, similar to light. However, unlike light waves, which can travel in a vacuum (empty space); ultrasound requires an elastic medium such as a gas, liquid or solid. However, the frequency range normally employed in ultrasonic non-destructive testing and thickness range is 100 kHz to 50 MHz. Although ultrasound behaves in a similar manner to audible sound, it has a much shorter wavelength .
The earliest form of an ultrasonic transducer was a whistle developed by Sir Francis Galton (1822-1911) in 1883 to investigate threshold frequency of human hearing.
The first commercial application of ultrasonic appeared around 1917 and was the first “eco-sounder” invented and developed by Paul Langévin (1872-1946). The original “echo-sounder” eventually became underwater sound navigation and ranging for submarine detection during World War-2 .
2. GENERAL PRINCIPLE
Being a sound wave, ultrasound is transmitted through any substance, solid, liquid or gas which possess elastic properties. The movement of the vibrating body is communicated to the molecules of the medium, each of which transmits the motion to an adjoining molecule before returning to approximately its original position. For liquids and gases, particle oscillation takes place in the direction of the wave and produces longitudinal waves (In a longitudinal wave the particle displacement is parallel to the direction of wave propagation) (fig.1a). Solids, however, they also possess shear elasticity, can also support tangential stresses giving rise to transverse waves (In a transverse wave the particle displacement is perpendicular to the direction of wave propagation), in which particle movement takes place perpendicular to the direction of the wave (fig. 1b) .
2.2 Bubble formation and the factors affecting cavitation threshold:
A bubble is a globule (small round particles) of one substance in another, usually gas in a liquid. Due to the marangoni effect (the mass transfer along an interface between two fluids due to surface tension gradient), bubbles may remain intact when they reach the surface of the immersive substance.
Cavitation is the formation of vapour cavities in liquid i.e. small liquid-cavitation-free zones (“bubbles” or “void”)-that are the consequencies of cavitational forces acting upon the cavitational liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low .
The effects are as follows:
2.2.1 Effect of gas and particulate matter:
The progression of a sound wave through a liquid medium caused the molecules to oscillate about their mean position. During the compression cycle, the average distance between the molecules decreased, whilst during rarefaction the distances increased. If a sufficiently large negative pressure is applied to the liquid, such that the average distance between the molecules exceeds the critical molecular distance necessary to hold the liquid intact, the liquid will break down and voids or cavities will be created i.e. cavitation bubbles will be formed (cavitation is the formation of gas bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. Cavitation is usually divided into two classes of behaviour: inertial cavitation, and non-inertial cavitation). Once produced these cavities, voids or bubbles, may grow in size until the maximum of the negative pressure has been reached.
Estimates of the acoustic pressure necessary to cause cavitation in water has led to a value of approximately 1500 atm. In practice cavitation occurs at considerably lower values and this is undoubtedly due to the presence of weak-spots in the liquid which lower the liquid’s tensile strength. There is now sufficient experimental evidence to suggest that one cause of weak-spots is the presence of gas molecules in liquid. For example, it has been observed that the degassing of liquids has led to increase in the cavitation threshold i.e. to increase in the values of the applied acoustic pressure necessary before cavitation bubbles were observed. Further, the application of external pressure which would cause any suspended gas molecules to dissolve, thereby effectively removing the gas nuclei, has also been found to lead to increases in the cavitation threshold.
To create bubble in water, provide the maximum rarefaction. Thus, there will probably be several different types of cavity in the liquid:
- The empty cavity (true cavitation),
- The vapour filled cavity,
- The gas filled cavity, unless the liquid is totally degassed, and
- A combination of vapour and gas filled cavities .
2.2.2 Effects of viscosity:
Since it is necessary for the negative pressure in the rarefaction cycle to overcome the natural cohesive forces acting in the liquid, any increase in these forces will increase the threshold of cavitation. One method of increasing these forces is to increase the viscosity of the liquid.
The effect, though not insignificant, is hardly dramatic. Taking corn and castor oils as examples, a ten-fold increase in viscosity has only led to a 30% increase in the acoustic pressure needed bring about cavitation .
2.2.3 Effects of applied frequency:
To completely rupture a liquid and hence provide a void, which may subsequently become filled with gas or vapour, requires a finite time. For sound waves with high frequencies, the time required to create the bubble may be longer than available during the rarefaction cycle. Thus, it might anticipate that as the frequency increases the production of cavitation bubbles become more difficult to achieve in the available time and the greater sound intensities will need to be employed, over these shorter periods, to ensure that the cohesive forces of the liquid are overcome.
2.2.4 Effect of temperature:
The final factor to be considered here, and known to affect the cavitation threshold, is the temperature. In general, the threshold limit has been found to increase with decrease in temperature. This may in part be due to increases in either the surface tension or viscosity of the liquid as the temperature decreases, or it may be due to the decreases in liquid vapour pressure .
Ultrasound is used in many applications, such as homogenizing, disintegration, sonochemistry, cleaning etc. Even this is used for medical diagnosis and therapy and also as a surgical tool. Now discuss about these in the following.
3.1 Motion sensors and flow measurement:
Ultrasonic sound can be used as motion sensor, i.e. automatic door opener, where an ultrasonic sensor detects a person’s approach and opens the door.
Ultrasonic sensor can control a wide area from a single point. The flow in pipes or open channels can be measured by ultrasonic flow meters, which measure the average velocity of following liquid .
3.2 Non-destructive testing:
Ultrasonic testing is a type of non-destructive testing commonly used to find flaws in materials and to measure the thickness of objects. Frequencies of 2 to 10 MHz are common. Lower frequency ultrasound (50–500 kHz) can also be used to inspect less dense materials such as wood, concrete and cement .
3.3 Biomedical applications:
Ultrasound also has therapeutic applications, which can be highly beneficial when used with dosage precautions. Relatively high-power ultrasound can break up stony deposits or tissue, accelerate the effect of drugs in a targeted area, assist in the measurement of the elastic properties of tissue, and can be used to sort cells or small particles for research .
3.4 Ultrasonic range finding:
A common use of ultrasound is in underwater range finding; this use is also called sound navigation and ranging. An ultrasonic pulse is generated in a particular direction. If there is an object in the path of this pulse, part or all of the pulse will be reflected back to the transmitter as an echo and can be detected through the receiver path. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine the distance .
3.5 Ultrasonic Cleaner:
Ultrasonic cleaners, sometimes mistakenly called supersonic cleaners, are used at frequencies from 20 to 40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments, diving regulators and industrial parts. An ultrasonic cleaner works mostly by energy released from the collapse of millions of microscopic cavitations near the dirty surface. The bubbles made by cavitation collapse forming tiny jets directed at the surface .
Ultrasound in the 20–100 kHz range is used in chemistry. The ultrasound does not interact directly with molecules to induce the chemical change, as its typical wavelength is too long compared to the molecules. Instead, the energy causes cavitation which generates extremes of temperature and pressure in the liquid where the reaction happens. Ultrasound also breaks up solids and removes passivating layers of inert material to give a larger surface area for the reaction to occur over. Both of these effects make the reaction faster. In 2008, Atul Kumar reported synthesis of Hantzsch esters and polyhydroquinoline derivatives via multi-component reaction protocol in aqueous micelles using ultrasound .
3.7 Ultrasonic welding:
In ultrasonic welding of plastics, high frequency (15kHz to 40kHz) low amplitude vibration is used to create heat by way of friction between the materials to be joined. The interface of the two parts is specially designed to concentrate the energy for the maximum weld strength .
3.8 Ultrasound identification (USID):
The ultrasound identification data is used by IBM’s Real-Time Location Service software to not only track and visualize equipment with location accuracy to zone, room or sub-room levels, but to generate alerts and automate responses .
3.9 Acoustic microscopy:
The Acoustic Microscopy method uses a high frequency ultrasound transducer to emit sound waves that are either echoed by or transmitted through a material. The resulting acoustic signature or waveform may be interpreted to determine variations of acoustic impedance within a sample.
3.10 Sonogram of a foetus:
An ultrasound exam is a procedure that uses high-frequency sound waves to scan a woman’s abdomen and pelvic cavity, creating a picture (sonogram) of the baby and placenta. Although the terms ultrasound and sonogram are technically different, they are used interchangeably and reference the same exam.
Ultrasounds may be performed at any point during pregnancy, and the results are seen immediately on a monitor during the procedure. Transvaginal scans may be used early in pregnancy to diagnose potential ectopic or molar pregnancies .
3.11 Veterinary medicine:
Various types of treatment can be done using ultrasonic sound. Ultrasound competes with other forms of medical imaging, such as X-ray techniques and magnetic resonance imaging, it has certain desirable features—for example, Doppler motion study—that the other techniques cannot provide. In addition, among the various modern techniques for the imaging of internal organs, ultrasonic devices are by far the least expensive. Ultrasound is also used for treating joint pains and for treating certain types of tumours for which it is desirable to produce localized heating. A very effective use of ultrasound deriving from its nature as a mechanical vibration is the elimination of kidney and bladder stones .
4. APPLICATION OF ULTRASONIC SOUND IN TEXTILE PROCESSING
The effect of ultrasound on textile substrates and polymers has started after the introduction of the synthetic materials and their blends to the industry. Application of ultrasound in the preparation of fabric specially in mechanical processes (weaving, finishing, knitted & non-woven fabric etc.) and wet process (desizing, scouring, bleaching, dyeing etc.). It deals with application of ultrasound in the mechanical processes of industry and apparel textiles.
In a study on desizing of textiles with starch, size removal was affected by means of ultrasonically accelerated techniques. It was found that the use of degraded starch followed by ultrasonic desizing could lead to considerable energy savings when compared to conventional starch sizing and desizing.
Valu et al, in their investigation of the use of ultrasonics in the desizing of woven cotton fabric, achieved a savings in chemicals and energy as well as reduced fibre degradation. The final whiteness and wet ability of the fabrics were the same as those obtained without ultrasonics. These investigations were carried out on an industrial jig in which ultrasonic transducers were mounted on the walls. Power varied from 2kW to 4kW for different transducer position .
4.2 Scouring and Bleaching:
The scouring of wool in an ultrasonic bath in neutral and very light alkaline bath reduces the fabric damage and enhance rate of processing. The bleaching rate of cotton fabric was increased by using 20 KHz frequency for peroxide bleaching. The degree of whiteness also increased as compared to that of conventionally bleached sample.
The function of ultrasonic wave can make use of acoustic cavitation effect to produce impact on sericin, improving the removal efficiency of sericin at crossing points & achieving uniform silk fiber degumming. The function of ultrasonic wave can cut down the time process and avoid the damage of fibroin through controlling the temperature and loss of chemical agent .
The possibility of dyeing textile using ultrasonic sound was started in 1941. This can be used for dyeing of cotton using direct dyes, wool with acid dye, polyamide and acetate fibre with disperse dyes. Significant increase rate of dyeing with disperse dyes on polyamide and acetate were obtained. Ultrasound is more beneficial for the application of water insoluble dyes to the hydrophobic fibres. Ultrasound irradiation also produces a greater evenness in colour. The dyeing results are affected by frequency of used ultrasonic sound .
Finishing of textiles involving ultrasonics has been studied by several researches. Applied ultrasound at 8 and 18 kHz frequency to formaldehyde resin treatment of cotton fabric and measures the change in physical properties before and after 60 washing cycles. The crease recovery angle, even after 60 washings, is much higher than without ultrasound but resulted in a small decrease in tensile strength. In the studies of Simkovich and Yastrebinski, cotton fabric was treated with urea-formaldehyde resin in an ultrasonic field of 8 kHz frequency. The isolated fabric improved crease recovery properties.
A U.S. patent describes the method and apparatus for treating military fabrics with a liquid repellent fluoro chemical finish in the presence of high frequency ultrasonic waves. This method produced an increase in finish add-on. Another patent by Carpenter for applying a fluid treatment to textiles describes a material that is passed through a chamber in which a dispersion of fluid treating medium is ultrasonically generated. The results are claimed to be fully comparable to padding and drying and are said to be capable of producing greater levels of treatment because this method can effectively handle treating media of higher solid contents .
According to the laboratory test reports washing time of wool after different wet processing can be reduced effectively for an equivalent whiteness. According to the results of a statistically planned experimental, the washing of flax can be improved by ultrasonic vibrations. This removes non-cellulosic material more effectively than mechanical agitation and improves whiteness of the flax fibre 
5. PLANNING OF FUTURE WORK
A preliminary trial has been carried out by carrying out scouring in an ultrasonic bath under the conditions, as given in Table 1.
|TRO||A few drops||A few drops|
|Time (min)||60 minutes||90 minutes|
|Surfactant concentration (g/L)||15 g/l||15 g/l|
|Alkali concentration (g/L)||0|
Experimental conditions of ultrasonic cotton scouring trial
*At first room temperature was 29°C and after de-aeration for 3 minutes the temperature was 31°C.
From the above-mentioned trial, it was observed that ultrasonic scouring has some promises. Hence, it is proposed to carry out some in-depth studies regarding this as per conditions given in Table 2.
|TRO||A few drops|
|Temperature (°C)||Room temperature||60, 70|
|Time (min)||60, 90||60, 90|
|Surfactant concentration (g/L)||15|
|Alkali concentration (g/L)||20||30|
Table. 2: Conditions for proposed ultrasonic cotton scouring
Similarly, works carried out by Mahata et. al. on ultrasonic assisted degumming operation with some encouraging results. In continuation of this study Table gives the conditions of this experimentation.
Another is degumming of silk in room temperature and higher temperature (<=80°C) for 60 minutes and 90 minutes (tentatively) with chemicals and without chemical.
|Temperature (°C)||Room temperature||45,55|
|Surfactant concentration (g/L)||20|
Table. 3: Conditions for proposed ultrasonic cotton degumming
We also wish to do scouring and degumming in conventional method and compare the results with scoured and degummed sample which was done by ultrasound process .
- Mason T. J. & Lorimer J. P., Introduction to Applied Ultrasonics, Applied Sonochemistry: Uses of Power Ultrasound in Chemistry and Processing, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002
- Smith C. B., Clapp T. G., Thakore K., Cato M. J., Hite D., Application of Ultrasound in Textile Wet Processing Phase 1, Compliments of EPRI TEXTILE OFFICE College of Textiles North Carolina State University Raleigh, 1992
- Mahata C., Das A., Chatterjee D., Roy F., Paul P, Application of ultrasonic sound in textile processing, a B.Tech minor project dissertation submitted at G.C.E.T.T.B, 2013
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