review of literature - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9068/11/11_chapter...

Review of Literature


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Heavily soiled areas of a garment are a cleaning challenge. For effective cleaning of these areas, one/more factors of cleaning have to be intensified. This is not desirable as stronger chemicals, elevated temperatures or enhanced agitation can damage the fabric. This damage is further aggravated due to repeated washings. High frequency ultrasonic waves offer an alternative for laundry process intensification. The review was intended to advance the understanding and outline directions for research; it was also done to get a comprehensive insight into process intensification through ultrasound, especially in laundry. The literature was reviewed under following sections: 2.1 Ultrasonic Waves 2.2 Application of Ultrasonic in Textile Industry 2.3 Ultrasonic Cleaning 2.4 Transport Phenomenon in Textile Material 2.5 Principle of Cleaning Apparel 2.6 Ultrasonic Washing of Textiles 2.1 ULTRASONIC WAVES Ultrasonic in general follows the principles delineated in acoustics; its development particularly in the early years is to some extent embedded in the broad developments in the scientific study of sound waves. Investigations of high frequency waves did not originate until the 19th Century. The era of modern ultrasonic started in about 1917, with Langevin’s use of high frequency acoustic waves and quartz resonators for submarine detection ( www.acousticalsociety.org). Major developments in the field of acoustics and ultrasound are summarized in Appendix II. Since that time, the field has grown enormously with applications found in science, industry and other areas (FIGURE 2.1). Broadly it is used for Ultrasonic Testing, a type of non destructive testing which uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic Inspection can be used for flaw detection where sound waves of 3000 MHz work as a microscope for evaluation, dimensional measurements and material characterizations (www.ndt.ed.org/ed.res). Ultrasonic Range Finding also called SONAR (Sound Navigation and Ranging) is used for scanning ocean depths, to guide ocean going vessels and also for clandestine activities and electronic eavesdropping (Klien, 1948).


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FIGURE 2.1: APPLICATIONS OF ULTRASONIC

Food Processing

- Microbial Inactivation

- Emulsification - Crystallization - Meat Processing - Food Analysis

Miscellaneous

- Forensics - Archeology - Aerosol Operations - Weaponry - Underwater

Communications Healthcare &

Medical - Diagnostics - Surgery - Occupational Therapy - Cancer Treatment - Doppler Ultrasound

Industrial

- Mining, Alloying - Metal recovery - Cutting, Grinding,

Welding & Drilling - Waste Water

Processing

SONAR (Sound Navigation and Ranging) - Guiding

Submarines - Electronic

Eavesdropping

Non-Destructive Testing

- Underwater Detection

- Flaw Detection in Metals

- Inspection

Applications

Of Ultrasonic


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Most Medical Applications use ultrasound in the frequency range of 1-15 MHz. Ultrasonic techniques are complementary to other physical methods used in surgery, therapy and diagnosis (Duck et al., 2001; Bushberg, 2002). In Industrial Applications, ultrasonic is used for cutting, slicing, grinding, welding, drilling, drawing and extrusion of metal, plastic and glass (Shoh, 1975; Lynnworth, 1975). It is used in fiber and optic drawing, mining, alloying, sieving, precious metal recovery, solder powder making and even waste water processing. Ultrasonic techniques are increasingly finding use in the Food Processing industry for both analysis and modification of foods. Low intensity ultrasonic is used to study composition, structure, physical state and flow rate while high intensity ultrasonic is used to alter either physically or chemically the properties of foods. Primarily it is used to generate emulsions, to inhibit enzymes, for rapid germination of seeds, acceleration of microbial fermentation, in wine clarification by removal of fluid suspensions. It is an effective means for homogenization, pasteurization, emulsification, crystallization, enhanced oxidation and dispersion of dry powder. It also finds use in preservation of food through microbial inactivation and in meat processing for tenderizing of meat (Povey, 1981; McClements, 1995). Besides these ultrasonic has been explored for various Miscellaneous uses like Forensics and Archaeology [bone cleaning, selective erosion, serial number restoration etc.]; Sonoluminescence for possibility of nuclear fusion reaction (Suslick, 1986); for remote sensing of sound (Cheeke, 2002); in weaponry, e.g. Long range acoustic device (LRAD) etc. ( www.acousticalsociety.org). 2.1.1 THEORY AND FUNDAMENTALS OF ULTRASONIC Ultrasound is high frequency sound wave inaudible to humans ranging from 18 kHz-10MHz. In practice, three ranges of frequencies are reported for distinct uses: low frequency or power ultrasound (20-100 kHz), medium frequency (300-1000 kHz) and high frequency or diagnostic ultrasound (2-10 MHz) (Ince et al., 2001). Ultrasonic vibrations travel in the form of a wave similar to the way light travels but unlike light waves which can travel in vacuum, ultrasound requires an elastic medium such as a liquid or solid with elastic properties for propagation. Ultrasound produces its physical and chemical effects through several different mechanisms but the most important is the phenomenon of cavitation. Cavitation is


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the formation, growth and collapse of gas or vapor filled micro bubbles or cavities under the influence of pressure variation in medium. Different kinds of cavitations based on the cause of pressure variation are: 1. Hydrodynamic Cavitation is caused by the pressure variation in the flow of a

liquid, due to variation in the velocity of flow. 2. Acoustic Cavitation is due to pressure variation in the liquid caused by passage

of an acoustic wave. 3. Optic Cavitation is due to high-intensity light such as laser which results in

rupture of liquid. 4. Particle Cavitation is produced by any type of elementary particle (for example

proton) rupturing the liquid as in a bubble chamber. Of these only the hydrodynamic and acoustic (or ultrasonic) cavitation have potential towards large-scale application, due to the simplicity of the method of creating them while others are basically useful for fundamental research in cavitation due to high cost of operation (Moholkar, 2002). Acoustic cavitation produced by high frequency ultrasound waves was the focus of this research. Ultrasound waves consist of expansion (rarefaction) and compression cycles. Compression cycles exert a positive pressure on the liquid and push molecules together, while expansion cycles exert a negative pressure and pull molecules apart (FIGURE 2.2). Cavities or “voids” can be generated during the expansion cycle of a sound wave, of sufficient intensity. Cavities are created in the liquid when the distance between the molecules exceeds the critical molecular distance necessary to hold the liquid together (for water molecules the critical molecular distance R is 10−8 m).

FIGURE 2.2: ACOUSTIC WAVE

(Source: Moholkar, 2002)

Molecules Compression Rarefaction

Ultrasonic wave Compression Rarefaction


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The intensity of the sound wave needed to induce cavitation depends on the type and purity of the liquid. Pure liquids have very high tensile strength and it is difficult to produce significant negative pressures to create cavities. The theoretical pressure amplitude to cause cavitation in water is approximately 1500 bar. However, in practice acoustic cavitation occurs at far lower pressure amplitude, less than 5 bar. This is because the liquids are impure and usually contain numerous small solid particles, pre-existing dissolved gases and more especially the trapped gas–vapor nuclei, the tensile strength of the liquid is reduced as a result. These solid particles and gas bubbles represent weak points in the liquid, the place where nucleation of the bubbles occurs. A cavitation bubble in irradiated liquid continually absorbs energy from alternating compression and the expansion cycles of the sound wave. This causes the bubbles to grow in expansion cycles and contract in compression cycles. FIGURE 2.3 shows an imploding cavity in a liquid irradiated with ultrasound captured in a high-speed flash photomicrograph.

FIGURE 2.3: HIGH-SPEED FLASH PHOTOMICROGRAPH OF IMPLODING CAVITY (Courtesy: National Center for Physical Acoustics, University of Mississippi)

Two forms of cavitation are well known: stable and transient. When bubbles undergo a stable oscillatory motion for several acoustic cycles it is called stable cavitation. A transient cavity has a lifetime of one, or at most, a few acoustic cycles, a bubble grows very rapidly to double its initial size and, finally, collapses violently in less than a microsecond. For frequencies less than 100 kHz, transient cavitation dominates. The implosion of cavities creates an unusual environment for chemical reactions. The vapor and gases inside the cavity are extremely compressed during cavity collapse. On collapse large increases in temperature and pressure are generated creating high local pressure up to 1000 atm. and a high transitory temperature up to 5000 K/5500°C inside the bubble and 2100°C in the liquid that surrounds the cavity.


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Such conditions are limited to a very small region and the heat produced during cavitation is dissipated very quickly (heating and cooling rates greater than 109 K/s). As a result, the surrounding liquid remains at the ambient temperature. The critical size of the bubble depends on the liquid and the frequency of the sound. At 20 kHz the size of the bubble is roughly 170 µm and at 1 MHz it is 3.3 µm. There are millions of these bubbles created and collapsing every second, at 20 kHz this happens 20,000 times per second. It is this transient cavitation which is responsible for process intensification (Suslick, 1989; Mason, 1999). This phenomenon of acoustic cavitation consists of at least three distinct and successive stages: nucleation, bubble growth (expansion), and under proper conditions implosive collapse. The first stage is a nucleated process, by which cavitational nuclei are generated from microbubbles trapped in microcrevices of suspended particles within the liquid. In the second stage, the bubbles grow and expand depending on the intensity of the applied sound wave. With high-intensity ultrasound, a small cavity grows rapidly through inertial effects, whereas at lower intensities the growth occurs through “rectified diffusion”, proceeding at a much slower rate, and lasting many more acoustic cycles before expansion. The third stage of cavitation occurs only if the intensity of the ultrasound wave exceeds that of the “acoustic cavitational threshold” (typically a few watts/cm2 for ordinary liquids exposed to 20 kHz). At this condition, the microbubbles overgrow to the extent where they can no longer efficiently absorb energy from the sound environment to sustain themselves, and implode violently, therefore, in a so called “catastrophic collapse” (Mason and Lorimer, 1990; Suslick, 1990). Sound at high density levels in gas and liquid is accompanied by stationary flows known as acoustic streaming. This streaming, which perturbs the boundary layer, is responsible for the observed effects involving the acceleration of transport process and the cleaning of contaminated surfaces (Rozenberg 1971). 2.1.2 FACTORS AFFECTING CAVITATION The prominent factors that affect the radial bubble motion are: • Frequency of the sound wave Frequency has a significant effect on cavitation as it alters the critical size of cavitation bubble (Adewuyi, 2001). In general, the energy available in a cavitation


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bubble increases with bubble size which corresponds to decreasing ultrasonic frequency (McQueen, 1986). Lower frequency produces more violent cavitation and, as a consequence, higher localized temperatures and pressures. At very high frequency, the expansion part of the sound wave is too short to permit molecules to be pulled apart sufficiently to generate a bubble. Weak cavitation or no cavitation in megahertz range is observed (Mason, 1993; Ince et al., 2001) • The Acoustic Pressure Amplitude The collapse time, the temperature and the pressure on collapse are all dependent on acoustic amplitude. At higher acoustic amplitude, cavitational bubble collapse is more violent as the bubble motion becomes transient (Adewuyi, 2001). For a given frequency, amplitude is a function of electrical power supplied and has to be above the minimum threshold value for cavitation to happen. The minimum power required for cavitation varies greatly with the colligative properties and temperature of the liquid and also with the nature and concentration of dissolved substances.

FIGURE 2.4: TYPICAL CHARACTERISTICS OF AN ULTRASONIC WAVE

• Colligative Properties of the Solvent Colligative properties of the medium include vapor pressure, surface tension, viscosity, density, as well as other properties that relate to number of atoms, ions or molecules in the medium. The bubble dynamics in the acoustic field is described by the Rayleigh-Plesset equation:

� �� 32 �� 1� � � � � � 2�� 4�� Where R is the radius (m) of cavitation bubble at any time, µ is the viscosity of the liquid medium (Ns/m2), σ is the surface tension (N/m), Pi is the pressure inside the


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bubble (N/m2) and P∞ is the pressure in the liquid far from the bubble (N/m2) (Vichare et al., 2000). The intermolecular forces in the liquid must be overcome in order to form the bubbles. Cavities are more readily formed when using a solvent with high vapor pressure (VP), low viscosity (µ) and low surface tension (σ); however the intensity of cavitation is benefitted by using solvents of opposite characteristics (i.e. low VP, high µ, σ, and density ρ) as solvents with high densities, surface tensions and viscosities have higher threshold for cavitation but on cavitation they produce harsher conditions (Adewuyi, 2001). Low VP is suitable as at high VP, more vapor enters the cavitation bubble during its formation and the bubble collapse is cushioned and less violent (Peters, 1996; Mason and Lorimer, 1990). • Properties of Gases There are several properties of gases that can affect the cavitation intensity. These are Solubility of Gas - Dissolved gases form the nuclei for cavitation but the greater the solubility of the gas is the more gas molecules penetrate the cavity (Adewuyi, 2001). Therefore, a less intense shock wave is created on bubble collapse. This is the reason why degassing of the solution is recommended to enhance the cavitational impact. Various methods of degassing are mentioned in literature. These are: increasing temperature (Adewuyi, 2001); switching on the ultrasound in the bath prior to start of experiment which causes the air bubbles to migrate towards the surface (Datar et al., 1996; Perincek et al., 2009); simultaneous application of ultrasound and heating significantly quicker degassing (Mason, 1991); stripping out all the air by saturating with CO2 and later, sodium hydroxide is added to convert all the dissolved CO2 to carbonate at pH of 9-10 (Moholkar et al., 2003); addition of chemicals like reducing agents to remove oxygen in the cleaning tank (Niemczweski, 1999); addition of surface active agent (detergent) which reduces the surface tension of water, this allows the gas to be expelled quickly (Juárez et al., 2009) and also by letting the water in a cleaning tank stand overnight at a constant temperature to achieve equilibrium of the gas content (Pohlman et al., 1972). One of the most popular means employed in industry for degassing is duty cycle control. Duty cycle is a measure of the fraction of the time that a generator’s ultrasonic


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output is turned on over a given period. At a duty cycle of 50% the ultrasonic output of a generator spends half of the time on and half of the time off. The primary effects of duty cycle are twofold. First, change in total number of cavitation events over a given period i.e. fewer implosions and second effect is it reduces the degas time of fluid. Time averaged radii of oscillating bubbles tend to increase during sonication through rectified diffusion (Crum, 1984). This means that large bubbles continue to grow to a size where buoyancy forces drive them to the surface of the fluid. The off times introduced by a duty cycle help to give these bubbles an opportunity to travel to the surface. This is an active mechanism by which a bubble population purges itself of large bubbles as well as a pumping action that tends to degas a solution. Due to this mechanism bubble nuclei and small bubbles grow until such time as they are of resonant size and undergo transient collapse. The shattered bubble fragments from this collapse are then new nuclei that further seed the bubble population in a self-sustaining cycle. Members of a bubble population that fall just above the resonant size are allowed to dissolve to a smaller radius during the off times introduced by a duty cycle. These bubbles then have a large interaction cross-section and contribute to the cleaning process (www.ctgclean.com). These are the two ways in which duty cycle, sometimes called pulse mode ultrasonics, can improve overall performance. Heat Capacity Ratio (Cp/Cv ) or polytropic ratio (γ )of the gas in the bubble affects the amount of heat released and hence the final temperature produced during cavity collapse. Higher temperatures and pressures are generated with monoatomic gases with larger ratios of specific heat (He, Ne, Ar) than diatomics (N2, O2), or polyatomic gases (CO2) with lower heat capacity ratios. Thermal Conductivity of the Gas- Gas with low thermal conductivity reduces heat dissipation from cavitation site favoring higher collapse temperature (Mason, 1991; Adewuyi, 2001). • External Pressure Literature points to increase in reaction rates with increase in pressure but with increasing external pressure, the vapor pressure of the liquid decreases and higher intensity is necessary to induce cavitation. An increase in intensity means increase in acoustic amplitude (Adewuyi, 2001).


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• External Temperature Higher external temperature reduces the intensity necessary to induce cavitation due to the increased vapor pressure of the liquid. But on the other hand as the external temperature increases more vapor diffuses into the cavity and the cavity collapse is cushioned and is less violent. Therefore, Sono-chemical reactions proceed more slowly as ambient temperatures increase (Mason, 1999; Adewuyi; 2001). A brief summary of these factors is presented in TABLE 2.1. TABLE 2.1: FUNCTIONAL DEPENDENCY OF THE CAVITATION ENERGY ON SOME STANDARD

VARIABLES Variable Symbol Dependence of Bubble Energy Frequency ω ω-2 Pressure Amplitude P P5/3 Surface Tension σ σ1/3 Density ρ ρ-1/2 Bubble Radius R R2

There is no simple relationship between the discussed parameters, but this knowledge is crucial in designing experimental conditions so that the sonochemical effects are maximized. These effects are, primarily, responsible for process enhancement during textile processing in presence of ultrasound. 2.2 APPLICATION OF ULTRASONIC IN THE TEXTILES INDUSTRY The textile industry worldwide copes with high energy costs, rapid technological developments and time limitations. Many non-traditional techniques such as use of radio frequency, microwave energy, infrared heating and ultrasound are being explored by researchers to reduce processing time and energy consumption and improve product quality. Use of Ultrasonic in textile wet processes is not a new one, on the contrary there are many reports from the 1950’s and 1960’s describing beneficial effects of ultrasound. Although most of the research has been in the area of ultrasonic dyeing, its use is being explored for other areas like eco friendly treatment of effluents from textile processing industry, apparel manufacturing and in leather industry. Following is a brief summary of its applications in various areas of textile industry.


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2.2.1 TEXTILES PROCESSING INDUSTRY 2.2.1.1 FIBER CLEANING Integrating an acoustic sensor with an optical color sensors helps to identify white or similarly colored foreign material/s, these otherwise would cause light colored lines in the final fabric produced (www.loptex.it). Hemp fibers can be separated using ultrasonic waves (Vantreese, 1998) and also remove dust from cotton fibers (www.informaworld.com). 2.2.1.2 ULTRASOUND IN TEXTILES WET PROCESSING It is proposed that the enhancing effect of ultrasound in fabric processing is due to fiber swelling, reduction in particle size and decrease in degree of crystallinity. Swelling of the fibers increases the uptake capacity of fiber and thereby facilitates easier penetration of processing chemicals where as reduction in particle size increases their diffusion coefficient. The decrease in crystalline areas and therefore a corresponding increase in amorphous regions would make the fiber more absorbent and may be responsible for better and faster penetration of processing chemicals which would lead to process enhancement. This was corroborated by several studies. Increased swelling of cotton fibers both mercerized (2.5 times) and unmercerized (33%) (Carr et al., 1996) and decrease in Vat dye particle size from 14 micron to less than 2 micron with use of ultrasound was seen (McCall et al., 1998). The fiber returned to its original diameter on drying. Similar improvements were reported by Klutz (1997); Lee and Kim (2001) and by Ignjatovic et al. (2001) where increased fiber swelling and reduced particle size lead to higher diffusivity in presence of ultrasound but no deterioration in mechanical properties was seen (40 kHz, 25oC, 90/180 minutes). X-ray diffraction study on untreated, ultrasonically and conventionally pre-treated linen fibers with sodium per borate showed 70.41, 67.51 and 64.90 per cent crystallinity respectively, corroborating reduction of crystalline areas on exposure to ultrasound (Ahmed et al., 2007). 2.2.1.2.1 FABRIC PROCESSING The main objective of using ultrasound in fabric processing is to reduce the process time and/or make process conditions less severe by conducting operations at lower temperature, or use of milder reagents at less concentration, or minimizing the use of


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auxiliaries, thereby improving the effluent quality and making the process more eco-friendly. Its use has been explored for following preparatory processes: SCOURING: Ultrasonic assisted scouring of wool fibers showed significant improvement in handle, while giving comparable results to conventional method (Carr, 1989). DESIZING: Industrial trials carried out on ultrasonic machine showed 10% quality improvement for de-oiling of polyamide and desizing of cotton compared to the traditional treatments (Vouters et al., 2004). Ultrasound intensified the phenomena of diffusion and improved effectiveness of processes. DEGUMMING OF SILK: Increased rate of degumming of Persian silk was observed in presence of ultrasound, due to increase in the soap dispersion and diffusion capabilities (Mahmoodi et al., 2010). BLEACHING: Accelerated bleaching and improved whiteness for cotton in various forms such as raw fiber, ring-spun yarns and knitted fabrics was reported by Safonov (1984) and later Mistik (2005) with hydrogen peroxide in presence of ultrasonic energy (Intensity- 2.5W/cm2). Use of ultrasound as an environmentally friendly heating technology for pre-treatment of linen fibers with sodium per borate gave better whiteness index at lower temperature for all studied treatment times (Ahmed et al., 2007). BIO-PREPARATION OF FABRICS: A more recent application of ultrasound in wet textile processing is in the area of enzymatic treatment of the fabrics which is increasingly becoming popular due to milder processing conditions, replacement of harsh chemicals, resulting in more environmentally friendly processes. Enzymes are large protein molecules with molecular weight ranging from 12,000 to over 1,50,000 and have low diffusion rates. They mostly attack the external cellulose fibers in cotton yarn, resulting in excessive damage of the fibers, also they are expensive. A combination of ultrasound with conventional enzymatic textile treatments offers significant advantages such as a reduction in the consumption of enzymes (thus reducing the cost of operation), shorter process times, less fiber damage and greater uniformity of the enzyme treatment (Yachmenev et al. 1999, 2004). ENZYMATIC DESIZING, SCOURING AND BLEACHING: 25-30% increase in weight loss at low 0.2% concentration of cellulase enzyme (cellusoft l) with introduction of


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ultrasound (16 and 20 kHz, 1.4 kW) was seen. Subsequent studies using enzyme pectinase for scouring of cotton gave comparable results to conventional alkaline scouring but in lesser time (Yachmenev et al., 1998a,b). Combined enzymatic desizing, using amylase and scouring with pectinase resulted in higher desizing degree and wettability value with ultrasound in short processing time of 20 minutes at low concentration. The tensile strength of cotton remained unaffected after ultrasound treatment in all the experiments (Karaboga et al., 2007). Increased whiteness effect in ultrasound assisted peroxide bleaching of cotton fabric using laccase enzyme (Carlos et al., 2006) as well as for linen fabric (26 kHz, 180 W) was reported (Okeil et al., 2010). X-ray diffraction studies showed that fabric was less crystalline and more hydrophilic with exposure to ultrasonic waves as a result this bleached fabric when subsequently dyed showed slightly higher dye uptake for both reactive and cationic dyes as well as improvement in fastness properties was noted. The improvements in the enzymatic wet textile processes observed by the application of ultrasound are attributed to several effects: 1. Desorption of the enzyme molecules from the surface of the cellulose fiber, thus

increasing the number of free sites for the enzyme reaction 2. Deagglomeration of the enzyme molecules; which could otherwise decrease the

enzyme activity 3. Faster transport of the enzyme molecules towards the textile surface by

accelerating the rate of diffusion 4. Acceleration of the overall reaction rate due to removal of the products of

enzymatic hydrolysis from the reaction zone. 5. Degassing or expulsion of dissolved or entrapped gas or air molecules from fiber

capillaries and interstices at the crossover points of fabric into liquid and removal by cavitation (Yachmenev et al., 2001).

2.2.1.2.2 DYEING Dyeing is a process of transferring the dye from solution phase to fiber phase. To promote this transfer four primarily responsible factors are agitation, temperature gradient, concentration gradient and interaction between dye molecule and reactive groups present in the fiber. Enhancing these factors to increase the mass transfer rate


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is not always possible or desirable. Operating at higher temperatures or increasing the concentration of dye/auxiliaries or even agitation can be detrimental to the fabric as well as to the environment, besides being uneconomical. Use of ultrasonic energy for process enhancement on the other hand is a promising alternative and has been explored for more than 60 years now. Ultrasonic assisted dyeing is in fact one of the most promising and well researched area of application. One of the earliest studies on the effect of audible sound (9.5 kHz) on the dyeing of cellulose by substantive dyes has been done by Sokolov and Tumansky (1941). They reported a 2-3 fold rise in the dyeing rate with application of sound waves. Subsequently, Brauer (1951) reported 25% reduction in dyeing time of cellulose with vat dyes by application of ultrasound. Rath and Merk (1952) studied effect of both audible sound and ultrasound on the dyeing of cotton, viscose, and wool, with acid and direct dyes. They reported that the effect of ultrasound frequencies was independent of the system, while the effect of audible sound was system specific. Alexander and Meek (1953) reported acceleration in the dyeing rate of cotton, wool, acetate, and nylon with direct, acid, and disperse dyes with ultrasound similar to stirring for cotton and wool, and higher than stirring for acetate and nylon. They, therefore, concluded that ultrasound has a greater effect on the dyeing of hydrophobic textile than hydrophilic textiles. Since then the work that has been carried on covering various classes of dyes including natural dyes is briefly reviewed here: DIRECT DYES: Smith et al. (1988) studied the dyeing kinetics of 10 different dyes, each yielding a different rate of dyeing. He proposed following possible reasons for the observed rate enhancement with ultrasound: (1) Disruption of the boundary layer between the textile and the dye liquor; (2) Increased segmental mobility in the amorphous regions of cellulose increasing the dye diffusion rate; (3) Decreased dye aggregation and dye solubility in the bath. Thakore (1988a,b; 1990) studied the diffusion and permeability coefficient of direct dye on cellulose and reported effects of several parameters such as fabric construction, temperature, intensity of ultrasound waves, dye bath concentration, electrolyte concentration on the dyeing kinetics. He concluded that the dyeing rate enhancement is more pronounced for thicker fabrics, which cannot be dyed evenly with conventional techniques. Also reduction in dye bath concentration, dyeing time and temperature with ultrasound was seen which


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increased with its intensity above a certain threshold. Smith and Thakore (1991) have also studied the fiber reactive dye hydrolysis and reported 50-500% acceleration in reaction rate under the influence of ultrasound. 50% reduction in salt and energy consumption for same exhaustion level as conventional method was achieved while dyeing cotton with Solophenyl Blue FGL 220 and Solophenyl Scarlet BNL 200 (Mock et al., 1995). REACTIVE DYES: Dyeing of nylon-6 fibers in presence of ultrasound was first investigated by Shimizu et al. (1989) and later by Kamel et al. (2003a). Reactive, disperse, acid and acid mordant dyes in a low ultrasound field (27 kHz and 38.5 kHz) were investigated and, in all cases, increase in dyeing rate and decrease in activation energies were observed (Shimizu et al., 1989). Ultrasound improved dye fixation and exhaustion for C.I. Reactive Red 120 by 2% and for C.I. Reactive Black 5 by 6% at 80°C for 100 min, with no undue effect on the fastness properties (Öner et al., 1995). Ultrasound assisted dyeing (38.5 kHz, 500W) with C.I. Reactive Red 55, C.I. Reactive Red 24, C.I. Reactive Blue 19 and C.I. Reactive Black 5 gave higher absorbance values than conventional method due to enhanced dye uptake in diffusion phase (Kamel et al., 2003b). Polyamide (micro fiber)/Lycra 85/15 blend knitted fabric was dyed with: C.I. Reactive Blue 4, C.I. Reactive Blue 15 and C.I. Reactive Blue 52 each containing different chromophore and reactive groups, increased dye transfer and percentage exhaustion with ultrasound at all studied times for all three dyes was observed (Merdan et al., 2004). BASIC DYES: Ultrasonic assisted dyeing of acrylic fabrics with C.I. Astrazon Basic Red 5BL enhanced K/S by about 28.5% compared to conventional heating (Kamel et al., 2010). DISPERSE DYES: Polyester fibers (PET and PBT) dyed with C.I. Disperse Orange 25 and C.I. Disperse Blue 79 in presence of low frequency ultrasound (26 kHz, 120W) showed 4-fold increase in the dye uptake of PBT fibers. A combination of ultrasound and dye carrier (Dilatin TCI) gave a further rise of 184% in the dye uptake. Although the dye uptake obtained with ultrasound technique was only 75% of that obtained with the conventional techniques, the effect of a carrier and ultrasound together was significantly larger than either individually for polyester fibers (Saligram et al., 1993). Similar results were reported while investigating low-temperature ultrasonic


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dyeing of polyester fabrics with disperse dyes in other studies (Ahmad and Lomas, 1996; Lee et al., 2001, 2003). VAT DYES: Better shade depth (Birla et al., 1996) and 17% less dye was used (McCall et al., 1998) in ultrasound assisted continuous vat dyeing while giving similar results. CATIONIC, ACID AND METAL-COMPLEX DYES: Silk filament yarn dyed using cationic, acid, basic and metal-complex dyes showed increased dye uptake for all classes of dyes at lower dyeing temperatures(45°C & 50°C) and a shorter dyeing time(15 min), with ultrasound(26 kHz/120W) as compared with conventional dyeing (85°C, 60 min.) with no apparent fiber damage. For basic and acid dyes absorbance value for ultrasonic was approximately three times that of conventional method at 50°C in half the time (Shukla and Mathur, 1995). A project to develop and commercialize open width dyeing machine incorporating the technique of ultrasonic for energy conservation was taken up by SASMIRA (Silk and Art Silk Manufacturing Industrial Research Association) sponsored by Department of Science and Technology, Government of India. The resultant conservation of time and energy has been emphasized for all substrates (cotton, silk, nylon and polyester) dyed with various classes of dyes: Low dyeing temperature of 50 - 55°C (40 % savings); 20-25% increase in exhaustion and fixation; 30 % reduction in dyeing time; Uniform dyeing; lesser selvedge to selvedge variation; lesser load to effluent (Mathur et al., 2004). ULTRASONIC ASSISTED EXTRACTION AND APPLICATION OF NATURAL DYES: Several studies report enhancement in dye extraction from various plant and animal sources. Higher dye uptake and color value due to better penetration of dye, reduction in processing time, energy saving, better environmental impact and also efficient dye bath reuse with use of ultrasound compared to conventional method was seen. Studies have been carried out on cotton, wool and silk fabrics as well as yarns. Work carried out on wool with Lac dye showed an enhanced effect of about 41% in the dye extraction (without boiling the water which helped in retaining the brightness of the extracted dye) and 47% in dye uptake compared to conventional heating (Kamel et al., 2005). However prolonged exposure to ultrasonic waves, beyond the optimum level, may lead to dye degradation. Subsequently Kamel et al. (2007a,b) extended their work on cationized cotton fabrics with, Lac where dye-uptake was about 66.5%


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more than the conventional heating. Ultrasonic efficiency ∆k% was calculated using the formula given below and was found to be 10.88%.

∆�% � ��

� 100

Where �� and ��are the rate constants of dyeing with ultrasonic and conventional heating, respectively. Similar work was carried out using berberine, a natural basic dye (Kamel et al., 2007a) and cochineal (Kamel et al., 2009). Factors affecting dye extraction such as ultrasound power, particle size, extraction temperature and time were studied. The results indicated that for each dye there is an optimum ultrasonic power level for maximizing extraction (Atul, 1996). In a study, Eclipta leaf extract, Acacia catechu and Tectona grandis were used on pre mordanted cotton fabric for dyeing using both conventional and sonicator methods. They reported 7-9% higher efficiency with ultrasound (Vankar et al., 2007, 2008c). They also used Nerium Oleander flowers for ultrasonic dyeing of pre-mordanted cotton and silk fabrics. Dye uptake was 87% in presence of ultrasonic and 43% in its absence (Vnkaer et al., 2008a). Similarly higher dye uptake was obtained when cotton, silk fabrics & wool yarns were sonicator dyed using Mahonia napaulensis DC, a natural dye traditionally used by Apatani tribe of Arunachal Pradesh (Vankar, 2008b). The enhancements observed in ultrasound-assisted dyeing processes in above reviewed studies are generally attributed to cavitation phenomena and, as a consequence, other mechanical effects are produced such as: • Dispersion (breaking up of aggregates with high relative molecular mass) • Diffusion (accelerating the rate of diffusion of dye inside the fiber by piercing the

insulating layer covering the fiber and accelerating the interaction or chemical reaction)

• Degassing (expulsion of dissolved or entrapped air from fiber capillaries into liquid and removal by cavitation, thus facilitating dye–fiber contact) (Chuz, 1962).

• Intense agitation of the liquid ENHANCED EFFECT OF ULTRASONIC ASSISTED PRE/POST TREATMENTS ON DYE UPTAKE: Studies also show positive impact of use of ultrasonic on dye uptake and fastness even when the fabric was either pretreated (Okeil et al., 2010) or post


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treated; like washing etc. (Akalin et al., 2004) in presence of ultrasonic energy. Ultrasound-assisted, clearing of poly (lactic acid) fibers dyed with disperse dyes gave comparable results to traditional reduction clearing in terms of fastness and color. It was suggested that this alternative would reduce risk of hydrolytic damage to the fiber, reduce chemical consumption and also generate a more environmentally acceptable effluent than reduction clearing (Burkinshaw and Jeong, 2008 a,b,c). In most industrial processes vat dyes, used to dye cellulosic fibers, are chemically reduced by sodium dithionite. This process produces large amounts of sodium sulphate and sulphite as byproducts which increase the costs of waste water treatment in the textile plants. In a study Roessler et al. (2002) focused their investigations on the replacement of sodium dithionite by the use of ultrasound to accelerate the vatting procedure and increase the conversion. Sun et al. (2009) reported improved dye exhaustion and fixation with ultrasounic pre- treatment of dye solution, no change in the fastness properties of dyed fabrics was observed. It is believed that of all wet textile processes ultrasound-assisted dyeing is the most promising area. Enhancement effect of ultrasonic in terms of time and energy saving, process enhancement by real time control of color shade, environmental improvements by reduced consumption of dyes and other auxiliary chemicals and increased color yields compared to conventional dyeing were observed for mostly all kinds of fibers with natural as well as synthetic dyes. Less dye and other auxiliaries were needed and less effluent was produced. Firstly, when dyeing with direct dyes which require a large electrolyte addition to achieve exhaustion, secondly with disperse dyes, which exhaust only with a carrier or under high temperature, and finally with reactive dyes which require large amounts of electrolyte for fixation. Ultrasound and its beneficial effects were seen not only during dyeing but even when fabric or dye solution was exposed to it either before or after dyeing. 2.2.1.2.3 FINISHING In finishing, use of ultrasound has been explored for anti- shrinkage treatments, washing after dyeing or other pretreatments as well as for extraction of trace elements from the fabrics to make the fabric non-toxic. RELAXATION/ANTI-SHRINKAGE TREATMENT: In wet chlorination of wool, micrographs showed that the amount of descaling achieved with 0.1g/l active chlorine in presence


25

of ultrasound was same as with 0.2g/l in absence of ultrasound. The decline of mechanical properties in ultrasonic bath was less than in conventional bath (Azari et al., 2007). Compared to conventional relaxation methods for cotton knitted fabrics, with use of ultrasonic waves, the area geometry constant value achieved was higher with improved uniformity of loops (Jeddi et al., 2007). WASHING AFTER PROCESSING: Enhancement in washing process efficiency was observed with ultrasound in scouring of printed fabrics (Kovaleva et al., 1985). In open-width washing of mercerized cotton, washing efficiency at 50°C in presence of ultrasound was found equal to that at 87°C without ultrasound, which offers prospects for energy saving. The efficiency decreased with increasing fabric speed and was higher for the dual frequency processor than single frequency (Rathi et al., 1997). Ultrasonic wet cleaning as an alternative to the dry cleaning using conventional dry cleaning solvents to remove hydrophobic soils was also proposed (Haskelll et al., 1995). ULTRASONIC EXTRACTION: Presence of trace elements causes problems during fiber processing, fabric production, dyeing, bleaching and finishing. Ultrasonic method was optimized for extraction of selected 23 elements from cotton in both static as well as dynamic conditions to make the fabric non-toxic and suitable for apparel purposes (Rezic, 2009). In dynamic condition analytes were removed as soon as they were transferred from solid matrix to the solvent. The ultrasonic method of extraction was recommended for its efficiency, rapidity, low cost and environmental acceptability. Ultrasound assisted extraction was also used on the historical 19th Century fragile silk banner as the most appropriate sample preparation step for the identification of the resinous binder (Rezic et al., 2008). DRYING: Ultrasonic is being favoured as a drying technology of the future, as it offers all the benefits of conventional drying systems, along with additional benefits of being eco-friendly and low energy consumtion (Majumdar, 1996). A process of atomization and pumping of liquid contained in the material towards the outside is produced. This process facilitates the subsequent drying (Juarez et al., 2009) ADAPTING ULTRASONIC TECHNOLOGY FOR INDUSTRIAL FINISHING EQUIPMENT: An attempt was made to adapt ultrasonic technology to jigger machine for textile finishing. Industrial trials carried out on this machine showed 10% increase in quality


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and confirmed the robustness of applicators, supporting up to 90°C temperatures during the 4 hour period. With use of ultrasound following benefits were envisaged: Reduction in the consumption of dyes and chemicals resulting in a 20–30% reduction in the amount of effluents; 20% savings in water; energy saving of 52% for same quality or 40 % for better quality. Since the applicators did not constitute a technological breaking for textile industries, the proposed additional cost was only about 10% (Vouters et al., 2004). A study (Perincek et al., 2009) investigated the spacing and alignment of transducers so that homogeneous acoustic distribution in the treatment bath is achieved. Results of the study indicated that since different fabrics have different degree of hydrophility, the treatment bath parameters like temperature, bath volume etc. have to be accordingly tailored, further work is underway to develop an optimum design for ultrasound treatment bath. 2.2.1.2.4 EFFLUENT TREATMENT Recently, there has been a great deal of research into resolving environmental problems caused by textile processing specifically dyeing and finishing. Direct discharge of effluents from the processing house can cause the formation of toxic aromatic amines under anaerobic conditions in waters, and contaminate the soil and groundwater. Various novel processes are being introduced and ultrasound is also being studied as an alternative solution for environmental problems. This process works on the principle of generating free radicals and their subsequent attack on the contaminant molecules with the aim of either, completely mineralizing the contaminants or converting it into less harmful or lower chain compounds which can then be treated biologically. During ultrasonic irradiation there is formation of highly concentrated oxidizing species such as hydroxyl radicals (OH), hydrogen radicals (H), hydroperoxyl radicals (HO2) and Hydrogen peroxide (H2O2), and localized high temperatures and pressures. The term “mineralization” means the final products of the degradation process are carbon dioxide, short-chain organic acid, oxalate, formate and inorganic ions like sulphate and nitrate. A comprehensive review of studies involving the use of ultrasound/ sonochemical processes to treat a variety of chemical contaminants such as aromatic compounds, organic dyes, organic and inorganic gaseous pollutants etc. has been compiled by Adewuyi, 2001 and Ince et al., 2001.


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2.2.2 TEXTILE CONSERVATION Ultrasonic assisted cleaning treatment can have some usefulness for archaeological textiles and is an area worthy of further study. Ultrasonic was used to break up and disperse hard, oxidized grease deposits on a Central Asian pile mat. These deposits had not yielded to variety of solvents at an acceptable pH and temperature even with prolonged treatment. Microscopic examination after ultrasonic treatment showed no fibrillation (Weik and Barton, 2006). It was also used to remove ink signatures on Indonesian textiles in conjunction with a vacuum table. No damage was observed after cleaning (Anonymous, 2008). 2.2.3 LEATHER PROCESSING INDUSTRY In leather industry ultrasound improves the process efficiency by reducing the process time and improving the quality of the leather produced (SivaKumar and Rao, 2001). More than four times improvement in the diffusion process through porous skin/leather matrix with use of ultrasound was observed (Sivakumar et al., 2004). A reversible pore-size change in the leather matrix, during the ultrasound propagation was one of the possible reasons suggested. Structural and morphological studies indicated no significant change in the fiber structure and no adverse effect on the grain surface due to the use of ultrasound. Processing Ultrasound is a more eco-friendly method for leather processing as it functions as a physical activator and reduces the use of conventional chemicals in stages like soaking for cleaning the raw skin/hide, liming for loosening hair and degreasing for removing natural fat (Herfeld, 1978; Alpa, 1995; Sivakumar et al., 2009a). Enhancement of fat liquoring process and better emulsification of fat liquor based on vegetable oil due to ultrasound was observed (Xie et al., 2000; Sivakumar et al., 2005b, 2008). Tanning Reduction in process time in conventional chrome tanning with use of power ultrasound was reported (Ernst et al., 1950; Mantysälo et al, 1997). It has also been explored for vegetable tanning of leather. It was used for extraction of tannins from myrobalan nuts and compared with the conventional process. Ultrasonic method


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showed 90% extraction efficiency with no external heating while the conventional method showed 77% efficiency at 70°C for 4 hours (Sivakumar et al., 2007). Dyeing The application of power ultrasound to leather dyeing offers same advantages of better dye exhaustion, reduced dyeing time etc. as for man-made and natural fibers. Dyeing experiments in presence of ultrasound (33 kHz, 150W) gave 40% increase in dye exhaustion and 45% decrease in the dyeing time (SivaKumar and Rao, 2001). They demonstrated ultrasound helps to improve the dyeing properties of leathers such as penetration and fastness, diffusion rate of dye, efficiency of the soaking process and reduces the unspent dye in the spent liquor for a given process time, thereby reducing pollution load (Sivakumar and Rao, 2003a, 2003b, 2004). Scanning Electron Microscopy (SEM) analysis of leather revealed that fiber structure and morphology are not affected by ultrasound (Sivakumar et al., 2005a). Ultrasound assisted enhancement in natural dye extraction from beetroot and dyeing of leather was observed (Sivakumar et al., 2009b). Also dry cleaning of finished leather in presence of ultrasonic waves (22 and 44 kHz) successfully removed the oily dirt adhering to the leather without affecting its properties (Herfeld, 1978 a). 2.2.4 APPAREL MANUFACTURING

As traditional clothing is being transformed into intelligent garments by integrating smart materials, the technology used in this sector is also becoming advanced. One of these advanced technologies is ultrasonic for cutting, slitting, welding, etc. and is increasingly being used in various sectors of the textile industry from weaving to finishing, to the making-up operation. 2.2.4.1 ULTRASONIC CUTTING AND SLITTING Ultrasonic slitters are designed for continuous slitting and edge sealing of thermoplastic films; knitted, woven and non-woven fabrics made from nylon, polyester, polypropylene, modified acrylics and synthetic blends. It prevents the unraveling of knitted or woven material by sealing the slit edge using vibratory energy. Depending on the material, slitting rates may be 30-400 linear feet per minute. Nearly all textile materials (woven, knitted, and non-woven) and all raw materials (natural and synthetic


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fibers) as well as glass, aramid and carbon fibers can be cut with ultrasound (Rupp, 1995). There is no discoloration after cutting and the process is environmentally harmless, the instrument is heated only to approximately 50°C degrees generating no smoke or odors, and eliminating the risk of burns. The selvedge is cleanly cut, and neither warp nor weft ends are displaced or dissolved out. The sealed edge is tapered, without a bead (www.deloup.com). 2.2.4.2 ULTRASONIC WELDING AND BONDING Sewing, which joins individual panels together with another textile element, provides adequate strength, elasticity and aesthetic properties, but produces discontinuous joints and perforated seams. Thermal and ultrasonic bonding methods are used to overcome limitations of conventional sewing methods. Ultrasonic bonding assembles two or more layers of material by passing them between a vibrating horn and a rotary drum, frictional heat created by the vibratory energy melts and fuses the layers together in a strong, permanent bond, producing continuous and impermeable seams. Synthetics, synthetic blends with up to 50% non-synthetic fiber content, films and coated papers can also be processed in this manner. Various sealing patterns like standard single stitch, dot, right or left slant, zigzag, custom designed etc. are available (www.rincoultrasonics.com). Typical applications for ultrasonic welding include protective garments, disposable hospital gowns, shoe covers, face masks, infants’ nursery garments, filters, bags, curtains, sails and web splicing. Sealed edges and seams with no stitch holes help prevent the penetration of chemicals, liquids, blood-borne pathogens and particulates. They are also used in packaging industry (www.hermannultraschall.de). 2.3 ULTRASONIC CLEANING The cleaning action of ultrasonic energy is mainly due to transient cavitation. The implosion of small gas or vapor bubbles inside the cleaning liquid and near the surface to be cleaned imposes such stresses on the surface that it erodes the contaminant coat and removes the impurities (Crawford, 1963). In liquids, the collapsing bubbles remain spherical because the ultrasonic waves are uniform. However, if a transient acoustic bubble collapses near a solid boundary, the bubble will implode asymmetrically generating jets of liquid directed towards the surface of the solid boundary. The micro-


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jets resulting from collapsing bubbles at a solid boundary are responsible for the cleaning effect of ultrasonic waves (Suslick, 1988). An ultrasonic cleaning system consists of four fundamental components of transducer, generator, tank and cleaning solution. Performance and reliability of the system depends upon the design and construction of the transducers and generators. The overall effectiveness of the cleaning is dependent upon the cleaning liquid. The size of the tank is dependent upon the size of the parts being cleaned. The number of transducers and generators is determined by the tank size. The choice of the cleaning solution depends on parts being cleaned and contaminants to be removed. Transducers A transducer converts high frequency electric energy into mechanical motion. When excited by an electric pulse it physically changes shape and causes the tank bottom or side to move. This creates a compression wave in the liquid of the tank. By using an electrical generator that puts out a high frequency signal (20 to 250 kHz) the transducer rapidly induces compression and rarefaction waves in the liquid. Physical mass and shape of the transducer determines its resonant point (frequency at which it will change shape). According to Newton’s second law [F=ma] force is equal to inertial mass times acceleration. Therefore a heavier faster accelerating transducer will produce more cleaning force than a lighter, slower accelerating transducer (Richardson, 1981). It is for this reason; manufacturers add resonant mass to the transducer assembly. A resonant mass is a precisely machined steel or stainless metal block, perfectly sized to resonate with the transducer output. The acceleration of the transducer makes up the other half of the law, the faster the transducer accelerates the greater amount of force it will exert. (Lower frequency have higher mass but since it has to produce fewer pulses/sec. acceleration is less and vice versa.) This law applies to all ultrasonic cleaning systems regardless of frequency (Anonymous, 1997). Ultrasonic Generator Generator energizes the transducers. It converts low frequency line power at 50-60 hertz to high frequency power at 20-80 kHz or higher, so as to match the resonant frequency of the transducer (FIGURE 2.5). The generator produces an electrical signal of high voltage and sends it to the transducer when the transducer receives the signal they respond by changing shape for as long as the signal is applied. The signal from


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the generator must be close to the response range of the transducer (Gooberman, 1988). Most generators are designed in modules that operate a specific amount of transducers. The most common are 250, 500, 750, and 1000Watt sizes, adding additional generator modules to the system can operate transducer stacks of any size ([email protected]).

FIGURE 2.5: ENERGY TRANSFORMATIONS In general, ultrasonic is used in precision cleaning as well as heavy-duty cleaning applications of complex manufactured metallic parts. It has the ability to clean in narrow crevices and small holes that are not accessible by a spray washer or other methods of cleaning. Also it is effective in removing difficult soils, such as buffing compounds and baked-on carbon. In other words, ultrasonic cleaning is especially suitable for complex part configuration and stubborn contaminants. Presently, ultrasonic cleaning finds application in areas of Healthcare and Medical which includes dental and surgical instruments, Laboratory and Pharmaceutical, Automotive and Aerospace, Industrial and Manufacturing and Light Industrial and Hobbyist such as jewelry, clocks, watches and firearms etc (www.ultrawave.co.uk). In analogy with its successful application in cleaning hard surfaces, its use for washing textile material has been proposed by various authors. This is supported by its application in various wet textile processes (Section 2.2.1.2). However the mechanism responsible for process enhancement in textile is different than in other media.

ELECTRICAL

ACOUSTICAL MECHANICAL

CAVITATIONAL

ELECTRICAL MECHANICAL ACOUSTICAL CAVITATIONAL


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Cavitational collapse affects chemical processes, the possible effects of this collapse however varies in different systems. In order to scientifically understand the acoustic principles involved in a textile/water system, in particular with regard to their interactions with the fundamentals of the soil loosening and soil transport processes it was important to first achieve a better knowledge of the mass transport phenomenon in textile materials. It is this improvement in mass transfer rate which is responsible for process enhancement and process intensification. The following section describes the basic transport phenomenon in textiles and its enhancement with use of ultrasound. 2.4 TRANSPORT PHENOMENON IN TEXTILE MATERIAL The subject of mass transport in textiles has been investigated both theoretically and experimentally for over 50 years. The literature on the mass transfer in textile treats two types of processes: the mass transfer in textile finishing process, like dyeing and the mass transfer in laundry processes in washing machine. 2.4.1 MASS TRANSFER THROUGH TEXTILE MATERIAL A piece of textile is a non-homogeneous porous medium comprising of yarns that are made up of fibers. A woven textile fabric often has dual porosity, inter-yarn porosity and intra-yarn porosity (FIGURE 2.6). Diffusion and convection in the inter-yarn and intra-yarn pores of the fabric are the dominant mechanisms of mass transfer in wet textile processes.

FIGURE 2.6: SCHEMATIC DIAGRAM INDICATING THE GENERAL STRUCTURE AND DUAL

POROSITY OF THE TEXTILES (Source: Moholkar, 2002)


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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 (Moholkar et al.,

2003). FIGURE 2.7 shows the three stages of mass transfer in textile materials along with the typical concentration profiles of the diffusing substance.

FIGURE 2.7: THE THREE STAGES OF MASS TRANSFER IN TEXTILE MATERIALS

(Source: Moholkar and Warmoeskerken, 2004) 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. Van den Brekel (1989) and later Gooijer (1998) studied and reported that most of the liquid flow passes the yarns through the inter-yarn pores without penetrating into the intra-yarn pores. This is because the flow resistance in the relatively small intra-yarn pores is much higher than the resistance in the relatively large inter-yarn pores; inter-yarn permeability is typically larger by a factor of 200 to 2000 than the intra-yarn permeability. ��

1200 �� 12000


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The process of mass transfer through the yarns is therefore driven by diffusion, while the mass transfer between the yarns is mainly driven by convection. Since the diffusion process is much slower than convection, the rate-determining step in the overall mass transfer will be the diffusion from intra-yarn pores to inter-yarn pores. 2.4.2 FLUID FLOW THROUGH THE FABRIC The nature of fluid flow through fabric is an important aspect of transport in wet textile processes like washing. Warmoeskerken and Boom (1999) introduced a stagnant-core and a convective-shell model to describe the flows and mass transfer through the yarns. The stagnant core in the yarn is the area in which there is no flow at all; the convective shell is the outer area of the yarn in which the liquid penetrates to some extent. The model is given schematically in FIGURE 2.8. It shows top view of the yarn held in the fluid flow perpendicular to it. The dots represent the fibers in the yarn and shows that most of the flow takes place through the larger inter-yarn pores, which is the least-resistant path for the flow. The flow in the inter-yarn region may penetrate to a small extent inside the yarn, thus giving rise to a convective shell near its periphery. The central core of the yarns however remains a stagnant zone where the mass transfer occurs by diffusion alone.

FIGURE 2.8: LIQUID FLOW AROUND AND THROUGH A TEXTILE YARN (TOP VIEW)

(Source: Warmoeskerken and Boom, 1999)


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The transfer processes in the stagnant core are based on molecular diffusion, while the transport processes in the outer convective shell are driven by convective diffusion. Since convective diffusion is much faster than molecular diffusion, the rate of reaction will be determined by the size of the stagnant core. If this core is smaller the processes will be faster. Ganguli and Eendenburg (1980) also considered two distinct regions-diffusion and convective within the textile to show the effects of hydrodynamics, the size of each region depended on the magnitude of agitation. Van der Donck et al. (1998) experiments on salt release also support the view that reduction in stagnant core size results in acceleration of mass transfer. This means that the role of mechanical energy in a wash, dyeing or rinsing process is to make the stagnant cores in the yarns as small as possible, or the conversion of the slow diffusion process to convection in the yarns, especially in the stagnant core of the yarn. 2.4.3 MASS TRANSFER INTENSIFICATION THROUGH ULTRASOUND The discussion in the previous section concludes that if mass transfer and fluid flow through textile material is accelerated, it can result in improvement in process efficiency. Several methods for this are described in literature and all of them focus on enhanced agitation of liquid; by stirring, or raising temperature of liquid etc., in some form (Matsui et al., 1978; Ganguli and Eendenburg, 1980). The need for improved methods of introducing mechanical energy into the system arises from various drawbacks of current methods such as requirement of large quantities of water and energy, long process times and resultant damage to textile in these conditions. Use of ultrasound to supply this mechanical energy is a promising alternative. In a study (Warmoeskerken et al., 2002) comparison of various conventional methods with ultrasonication to reduce stagnant core size in textile materials as a means of accelerating mass transfer was done and ultrasound was found to be most effective and efficient method. It was noted that rate of mass transport in textiles increased by a factor of 6, and size of the stagnant core decreased by a factor of 2.5 by application of ultrasound. Intense micro convection due to transient bubble motion driven by ultrasound, accelerated fluid flow and thus mass transfer in inter yarn and intra yarn pores of textile which is responsible for process enhancement (Moholkar et al., 2003)


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2.5 PRINCIPLES OF CLEANING APPAREL Four factors in proper combination are necessary in any cleaning process namely time, temperature, agitation and chemistry of the detergent. Two main methods of cleaning clothes are aqueous cleaning and dry cleaning. Aqueous cleaning is a method of removing contaminants from the textiles using water. In the dry cleaning process, the solvents are substituted for the water while the remaining factors of cleaning remain the same. Aqueous cleaning is environment friendly compared to chlorinated and other ozone depleting chemicals used in solvent cleaning. It offers advantages like reduced solid wastes and air emissions and is therefore witnessing a revival of interest from consumers, researchers as well as industry. Manufacture and marketing of household washing machines and detergents is a multibillion dollar industry worldwide (www.strategy.com). That is why substantial investment in R&D has been undertaken by manufacturers to maintain or improve their competitive position. The research in last six decades has focused on gaining a fundamental understanding of soiling and detergency mechanism. As a consequence there are numerous publications as well as text books which deal comprehensively with theory and technology of detergency (Bishop, 1995). The following brief discussion is undertaken for the purpose of integrating the concept of ultrasonic energy as a substitute for agitation (mechanical energy) in the aqueous cleaning process. 2.5.1 AQUEOUS CLEANING PROCESS Chemistry of the cleaning agent, time, temperature and agitation/mechanical energy are the keys to the aqueous cleaning process. These factors have to perform a separation process in which soil is removed from a textile substrate. In that process, two important steps can be distinguished: the soil loosening step and the soil transfer step (FIGURE 2.9). In the soil loosening step, the physical binding forces between the soil and the substrate are broken up. In the subsequent soil transfer step, the loosened soil is transported from the substrate to the wash liquor (Moholkar, 2002.) A detergent is a material intended to assist in cleaning. ‘Detergency’ is a measure of cleaning power (Bhattacharaya, 2009). Detergency is defined as “The removal of unwanted substances, called soils from a substrate immersed in some medium, generally


37

through the application of a mechanical force, in the presence of a chemical substance which may lower the adhesion of the soil to the substrate” (Florescu et al., 2002).

FIGURE 2.9: TRANSPORT OF LOOSENED PARTICLE SOIL FROM THE YARN TO BULK DUE

TO CONVECTION Detergents were developed in response to shortage of animal and vegetable fats used to make soap in World War I and II. General structure of detergent is R-SO4Na+. Detergent is structurally similar to soap but differs in water soluble portion -SO4-Na+) instead of (-COO-Na+). Surface active agent present in detergent belongs to a chemical class of highly polar, high molecular weight molecules, when added to water, these molecules form a cluster called micelles, in which polar ends of the molecules are on outside of the cluster and non polar ends of the molecules are in the middle (Bhattacharya, 2009). These molecules can be anionic, cationic or nonionic in aqueous solution. The surfactant molecules congregate at the oil-water interfaces so that an oily droplet rolls up from fabric surface and is disengaged from fabric with the aid of mechanical action in laundering process (Webb and Obendorf, 1987a,b). In the washing process, a complex mixture of soils must be removed, consisting of compounds physically adsorbed and/or covalently bound on textile substrate. The removal efficiency of surfactants depends on nature of surfactant and correlates with their adsorption on textile substrate (Fort et al., 1966b). Modern detergents include various components besides the surfactant called auxiliaries and are incorporated in small amounts to boost performance. These are builders, bleaches, enzymes, optical brightening agents, soil anti-redeposition agents, foam regulators, corrosion inhibitors, perfumes, color and fillers. These components have synergistic effects on one another and boost the overall cleaning performance. Detergents remove soil by sequestration, dissolution, wetting, emulsification, deflocculation, dispersion and/or saponification.


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Sequestration – sequestering agents are generally powders or liquids which combine with calcium, magnesium and other heavy metals in hard water. They form molecules in which these ions are held securely or sequestered, so that they cannot form insoluble soaps with fatty acids Dissolution- is a process whereby water soluble salts are dissolved in the alkaline solution and then flushed away in the rinse. Wetting – surfactants enable wetting by lowering surface tension and interfacial tension so that the cleaning solution can penetrate even the small spaces and get under the soil and lift it from the substrate. Water is not attracted to oily surface (hydrophobic), but with surfactant, wetting occurs as molecules from its hydrophilic group absorb at the oil-water interface making it spreadable on to the substrate surface. Small oil droplets become negatively charged, repel each other and remain suspended in washing water, which during rinsing is flushed away. Emulsification- is the primary means of removing oils when using alkaline or aqueous cleaning. The oils are broken up into tiny droplets that are suspended in a solution and are flushed away in the rinse. Deflocculation/ Dispersion/ Solubilization – is used in removing solid oils that have aggregated. It works by breaking the attractive forces holding the particles together, thus breaking up the solids into small fine particles that are dispersed throughout the cleaning media. The soil/liquid matrix is maintained as a dispersion or colloidal suspension, preventing agglomeration. Saponification – alkaline hydrolysis or breaking up of insoluble fats into water soluble soaps and glycerin, which help in emulsification. This is used for solvent free defluxing and degreasing (Lomax, 1996; Smulders, 2000). 2.5.1.1 PHYSICO-CHEMICAL ASPECTS OF SOILING AND SOIL RELEASE Soiling and soil release are complex phenomena involving the interrelationships of the nature of the fiber surface, fiber-fabric structure, nature of soil, chemical finish and detergent. 2.5.1.1.1 NATURE OF SOIL Soil on a textile fabric comes from two sources, body of the wearer and from the environment. Analytical studies have shown that the soil deposited on apparel and furnishing is a mixture of a fluid component usually oil or grease and a solid component


39

made up of small particles, the composition of which varies according to the source (Venkatesh et al., 1974). Lubricating oils and greases from automobiles and machinery, secretions from the human skin, oils and fats from food and cosmetics are a common source for the fluid component. A substantial part of soil present on domestic laundry is sebum secreted by sebaceous glands in the skin and is found at levels of 0.5-1% on clothing worn next to skin and bed linen. Sebum contains some hydrocarbon such as squalene but primarily it has triglycerides and fatty acids (Bishop, 1995). The major particulate constituents of soil are clay minerals of varying particle size (Kissa, 1973). Another source of soil is the wash liquor itself, soil redeposition or wet soiling, takes place during laundering due to pick up by the fabric of soil suspended in the wash liquor (Fort et al, 1966a). Many analytical studies have been undertaken to identify the constituents of natural soil present on dirty laundry so that it can be replicated for detergency studies. Analyses of natural soils have shown that both fatty and inorganic constituents are present, the exact proportions of which depend on the source of the dirt (Fort et al., 1966a). Brown (1947) determined the composition of the oily constituent of dirt from a number of domestically soiled cotton fabrics, with the following results: Free long chain fatty acids (31.4%), Long chain neutral fat (triglycerides 29.2%), Short chain fatty materials (3.3%), Acetylizable material (fatty alcohols, cholesterol, etc. 15.3%) and hydrocarbons (21%). The fatty and the inorganic constituents present in the irremovable residue left on soiled cotton fabrics after laundering were analyzed and it was found that the average composition of the oily constituents was: Free Fatty Acids (3%), Esterified Fatty Acids (50%), Lime Soaps (23%) and Unsaponifiables (24%). The major particulate constituents were clay minerals 0.2-1.00 microns in diameter. 2.5.1.1.2 MECHANISM OF SOILING Fabric is soiled either by direct contact with another soiled surface or by contact with air-borne or liquid-borne substances. During direct contact mechanical forces transfer oily and particulate soil directly from a soiled surface to the fabric surface. Soil particles floating in the atmosphere settle down on the fabric surface because of gravity or are intercepted by the fabric and the finer particles may even diffuse into the fabric structure. Liquids coming in contact with fabric evaporate leaving behind dissolved or suspended particles (Venkatesh et al., 1974). Also, small particles adhere


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to any surface with which they come into contact because of Vander Waal forces. The contact area between soil and fiber, which depends on the force of impact, affects both the amount of soil deposited on fibers during soiling and the amount of soil remaining on the washed fabric (Kissa, 1979). In the case of hydrophobic fibers, a large surface charge density is built up on the fiber surface by friction during use or laundering and soil particles, whether charged or not, are attracted from the atmosphere or the wash liquor (Venkatesh et al., 1974). Soil is retained in the fabric by mechanical and electrostatic forces or by oil bonding. The fluid component picks up and retains particulate soil which is termed as oil bonding (Snell et al, 1950). Mechanical entrapment or occlusion of particles also takes place (1) In the inter-fiber and inter yarn spaces (macro-occlusion); (2) in the irregularities of the fiber surface (this is the mechanism responsible for entrapment of fine soil particles in surface textured fabrics like cotton, silk, wool and textured synthetic fibers); and (3) within the crevices and pores (micro-occlusion) (Compton and Hart, 1953a,b; 1954). In the case of oily soils, capillary forces influence the advance of the fluid into the yarn. The capillary pressure � is given by

� 2� �� Where � is the surface tension of the fluid on the fabric surface and � is a parameter characterizing the air column separating the fibers in a yarn, and hence depends on yarn construction. Therefore in a dry state the fluid will generally be held by the strongest forces in the surface irregularities of single fibers and at the fiber junctions or cross-over points, because in these areas a given volume of fluid will present the least interfacial area with air and the capillary forces will have maximum effect (Smith and Sherman, 1969). This was later confirmed by Webb and Obendorf (1987b) while studying the distribution of natural soils and they reported oily soil on the surfaces of polyester and cotton fibers and in the interfiber spaces of the yarn bundle. Oil was also found in the crenulation, secondary walls, and lumen of cotton fibers. Presence of waxes creates hydrophobic surfaces, which contribute to increased oil retention at fiber surface and lumen (Obendorf et al., 1982). Oil was located in the interior of some worn (cracked) polyester fibers, but no oil appeared in polyester fibers that were not damaged. Composite (oily plus particulate) soil was detected in


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the same location on many fiber surfaces. It was also seen that nonpolar soils had greater affinity for hydrophobic fibers like polyester (Moris and Prato, 1982). The nature, particle size, and distribution of soil, fabric construction (both yarn and fabric structure), fiber morphology, and its chemical nature have great influence on the soils’ retentive characteristics (Smith and Sherman, 1969). Different experimental techniques which give useful information about the physical, chemical and mechanical aspect of soiling and soil release have been attempted and reported by several authors. These are: Microscopic Techniques - Optical and electron microscopic techniques have been employed to study the nature, location and distribution of soil retained on the fabric surface as well as the manner and efficiency of soil removal under various conditions of soiling and laundering and also to assess the influence of fabric and yarn construction, fiber morphology, and surface characteristics in promoting entrapment and retention of soil (Fort et al., 1966a). Surface Energy - The surface energy of the fiber-soil interface is an important parameter but it cannot be measured directly on the fabric. Contact angles of a number of liquids of different surface tensions on the fabric surface under investigation are determined. A plot of the cosine of the contact angle against the surface tension of the liquid is a straight line which when extrapolated to cosine 1 (θ = 0°), gives the critical surface energy of the surface. Since the fabric surface is not flat, the wettability of the surface is influenced by the fabric geometry (yarn and fabric structure) and the capillarity of the fabric in addition to the surface energy. Hence the contact angle of the liquid of the lowest surface energy that does not wet the surface and the contact angle of the liquid of the highest surface energy that does wet the surface gives the critical surface energy of the fabric surface. The values of the oil-air interfacial energy of the fluids obtained in air are useful as a guide to the resistance of the fabric to fluid soiling; and the values of the critical surface tension for wetting in water, as a guide to the phenomenon of wet soiling and ease of soil removal. Work of Adhesion - Useful information can be obtained by determining the work of adhesion W using some suitable oil as reference material.

W= υ (1+ Cosθ)


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where υ is the surface tension of the oil in air or the interfacial tension of the oil against water, and θ is the contact angle. It is, however, necessary to use an oil with a contact angle greater than 90° in air, because if it is less, it will wet the fabric surface. As the soiling increases, the work of adhesion in air decreases, indicating difficulty in wetting the fabric surface with water. A decrease in the work of adhesion of oil in water, on the other hand, indicates good detergency, since soil becomes easily removable in the detergent solution (Venkatesh et al., 1974). Wettability Test - In order to get good soil removal, complete wetting of the fabric is very essential for the effective interaction with the wash liquor. A simple test [IS: 5785 (Part V)-1970] can be carried out by noting the time taken by the test fabric to become wet with distilled water. Increased wettability, i.e., a decrease in the time taken to wet the fabric surface, indicates better detergency. Zeta Potential - Under the conditions encountered in practice, most soil particles are charged, as a result there is an interaction between the electrical double layers of the fabric and soil particles in water and this interaction has an important effect on detergency. Since the electrical potential of soil particles is essentially constant, the relative interaction potential between fabric and soil is obtained from the zeta potential of the soiled fabric. 1g of fabric is plumed between the electrodes of the cell, the electrolyte is streamed through this plug, and the streaming potential is measured with an electrometer; the zeta potential is calculated. A decrease in the negative zeta potential indicates good detergency action (Katsumi and Tsuji, 1969). To assess the importance of the hydrophilicity of fibers, properties like: surface charge density, dielectric constant, conductivity, moisture regain, and permeability of the fabric to air and water have also been studied. 2.5.1.1.3 MECHANISM OF SOIL RELEASE Several mechanisms are operable during washing. Both the loosening and the rinsing consist of a number of physical processes, which depend on the type of soil, detergents, pH, agitation, etc. (Ganguli and Eendenburg, 1980). During loosening, the soil loses all the contacts with the fiber and is free to move anywhere in the three-dimensional structure of the textile. Several mechanisms of loosening have been suggested in the literature, such as breaking of fluctuating soil fiber bonds (Schott, 1975) or rolling up of the liquid layer (Kissa, 1971).


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The rinsing of the loosened soil is governed by the hydrodynamics, as it governs the diffusion of water and detergent to the soil-fiber interface (Kissa, 1975). As discussed earlier, mass transfer in the textile is a combination of convection (between larger inter-yarn pores) and diffusion (between smaller intra-yarn pores); overall mass transfer in the textiles is mainly determined by the thickness of the boundary layer between the textile and the bulk. At high Reynolds number (fast agitation of the liquid), the boundary layer vanishes, and the transport is mainly determined by convection. This effect is even more pronounced for the textiles with a higher porosity, besides this, the rigidity and the swelling characteristics of a textile in water also influence diffusion (Ganguli and Eendenburg, 1980). Soils of practical concern are liquid oily soils, solid soils, and mixed liquid-solid soils. The detergency mechanisms for oily (liquid) soils and particulate soils are different. Only liquid soils can separate spontaneously from fibers immersed in water. Solid soils require mechanical action for their dislodgement (Kissa, 1981). Oily-soil detergency has three consecutive steps, (a) An induction period, during which water and detergent diffuse into the soiled substrate, but soil removal is slow or insignificant. The length of the induction period depends upon the rate of agitation, the nature of the soil, the detergency, and the substrate. (b) During the soil-removal period, oily soil separates mainly by the roll-up mechanism other mechanisms, such as solubilization, emulsification, and soil penetration are less important. (c) In the final period, soil removal is very slow or negligible. The roll-up of oil is caused by interfacial tensions of oil on fiber, water on fiber, and between oil and water. Roll-up can be enhanced by increasing the interfacial tension between oil and the fiber surface and decreasing the interfacial tension between water and the fiber surface. In other words, the fiber surface has to be made more hydrophilic (Kissa, 1975). In the detergency process, an increase in soil removal generally occurs with increased wash water temperature (Morris and Prato, 1982, Obendorf et al., 1983). The effect of wash water temperature on soil removal depends on the textile substrate and the nature of the soil (Breen et al., 1984). The concentration of oil markedly reduced in the crenulations of the cotton fibers and the small interfiber spaces between closely spaced polyester or cotton fibers at higher temperatures (Bubl, 1970).


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Particulate soil is generally embedded in a sheath of oil spread over the fabric surface and its removal involves breaking the adhesive bond and wetting the separated soil and fiber surfaces. The particle is then transported into the wash liquor (Kissa, 1978). Earlier studies have shown that fiber topography controls micro-occlusion, while the yarn and fabric construction determine the macro-occlusion of particulate soil. Naturally-occurring soils usually contain both particulate matter and fatty or oily soil. The detergency of such mixed soils depends on the removal of the fatty film on fibers and benefit, therefore, from a hydrophilic fiber surface (Kissa, 1981). Also polar soils are easier to remove by aqueous detergent systems than non-polar soils (Chi and Obendorf, 2001). Thus soiling is a process of wetting or adsorption of fluid soil over the fabric surface, and soil release is a desorption process that involves the displacement of one interface and the formation of two new interfaces (oil-water and fabric-water). Any modification or treatment which increases the energy of the fabric-water interface and reduces the energy of the fabric-oil interface would adversely affect the soil release characteristics of the fiber. 2.5.2 EFFECT OF REPEATED WASHING Repeated washings have a cumulative deteriorating effect on textile goods. In fact washing causes more damage than use and wear. In a wash and wear trial of polyester/cotton hospital uniforms, 90% of the damage was the result of washing (Bishop, 1995). Anand et al. (2002) demonstrated that changes occurring after laundering were largely caused due to the agitation during tumble drying (FIGURE 2.10).

FIGURE 2.10: MEAN PERCENTAGE CHANGES AFTER FIVE LAUNDERING CYCLES

(Anand et al., 2002)


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The fabric changes that occur during washing, such as loss in strength, shrinkage, distortion, fiber damage, color fading etc. are all dependent on fiber type, fabric construction, dye class and finishing process applied; as well as on the wash process and product. These changes are: Physical effects of water, temperature and agitation and chemical effects of ingredients of washing product. Physical effects depend on the yarn and fabric structure, the built in stresses during spinning and weaving and nature of finishes. The extent to which fibers imbibe water and swell is highly correlated with their wet Tg (Glass Transition Temperature). Recommended wash temperatures therefore are substantially lower than the fiber Tg to control damage. The nature of mechanical action depends on hand wash method and/or type of washing machine which includes drying methods, whether tumble dried or line dried. Rate of damage however would vary depending on the mechanical severity of wash method for a given fiber type and fabric construction. These changes are a result of movement of yarns relative to one another; migration of fibers with in yarns and damage to individual fibers. These lead to changes in garment shape, size and thickness; loss in mass, fuzzing, pilling, felting of shrink resistant wool and fiber shedding; fiber splitting, fibrillation and breakage (Bishop, 1995; MacKay et al., 1996; Higgins et al., 2003a; Fijan et al., 2007; Kan et al., 2009 and Sülar et al., 2009). Various systematic studies have been carried out to compare and highlight the detrimental effect of repeated washings on different fibers, fabric constructions: knitted, woven etc. and fabric finishes while employing different methods of washing and drying. Objectively measured changes; increased stiffness parameters (B, FR, G, 2HG5), reduced tensile extensibility (EMT, WT) and recovery (RT) correlated well with subjective perception of reduced flexibility, stretchiness, softness and smoothness for cotton, acrylics and wool knits (MacKay et al., 1999). Acrylics stretch and acquire a harsh handle (MacKay et al., 1996) where as cotton became more pliable and drapable with repeated washing (Orzada et al., 2009). Serious fiber fibrillation damage in cellulosic fibers after repeated washings which can affect mechanical and sensory properties was reported (Bishop, 1995). On comparison, plain woven structures were found more balanced than knit structures, as the yarns are more closely packed and woven lattice places more restraint on yarn movement. Knits achieved full relaxation in five cycles, maximum


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being in first cycle itself while woven cotton takes up to 10 cycles to relax fully (Anand et al., 2002; Higgins et al., 2003b). For acrylic knits, line drying did more damage causing progressive expansion over 50 cycles than tumble drying (MacKay et al., 1999) besides changing the hand from crisp to limp (Brown, 1970). However Higgins et al. (2003a) reported larger changes in area density and increased loss in mass with tumble drying than line drying for woven cotton. In cellulose, structural changes in fibers during hydrolysis affect its physico-mechanical properties. Decrease in the fiber tenacity in the wet-treatment cycles is the result of the degradation of the cellulose and the decrease in its dP where as the increase in the wet elastic modulus of the fiber is attributable to the “after crystallization”. Repeated washings, especially in the presence of an oxidizing agent present in detergent, result in the destruction of the molecule chains which break in the amorphous regions and can freely crystallize so that the size of the crystallites increases. Degradation and crystallization processes accelerate each other. Crystallization sets up stresses in the chains so that the energy of rupture activation is reduced and the chains break more rapidly. The breaking of the chains, on the other hand, facilitates and accelerates the crystallization process. This is responsible for high wet modulus of cotton (Serkov et al., 1979). Notably the effect of same parameters would be different on different fibers, even for same fiber type method of construction; nature of finish etc. will influence the level of damage. Chemical effects of fabric washing products are: decrease in strength (Fijan et al., 2007); structural changes in fibers (Serkov et al., 1979); deposition of insoluble calcium and magnesium salts on textile fibers which cause graying, yellowing, stiffening and development of rancid, fatty odors on the fabric (Bishop, 1995; MacKay et al., 1996). Cellulose because of its chemical structure with hydroxyl sites fixes calcium by ion exchange. Successive washings in presence of detergents with bleaching agents, depolymerize cellulose, hydrolysis of anhydroglucose rings of cellulose give the surface carboxyl groups(COOH) which is a better ion exchanger than original hydroxyls (Fijan et al., 2007). Accumulation of insoluble phosphates on fabric changes its drape as well as hand leading to stiffness (MacKay et al., 1996). A review of studies on effect of various components of laundering on mechanical properties of fabric which results in physical and sensory changes, visual and tactile,


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has revealed that even under milder conditions, the damage to the textile is substantial as the effect is cumulative. It affects both life and serviceability of textile item. The factors responsible for cleaning are the ones responsible for the damage. Reduction in intensity of these factors time, temperature, agitation or strength of washing formulation would result in corresponding decline in cleaning efficiency. There is therefore a need to develop alternate, sustainable textile washing systems that are environment and fabric friendly. 2.6 ULTRASONIC WASHING OF TEXTILES It has since long been speculated that using ultrasound waves could be an effective way of supplying mechanical energy for washing, in analogy with its numerous successful applications in the area of hard surface cleaning. The mechanical energy created by either rotation of drum in a washing machine or scrubbing, brushing etc. by hand, delivers the kinetic energy for soil loosening and soil transfer. The task of the mechanical energy in wash process is to make the stagnant core of the yarn as small as possible, so as to enhance rate of soil removal. However, since textiles have a very complex porous structure of fibers and yarns, the translation of mechanical energy into a flow within the textile pores is complicated and has been a subject of various studies. Several methods of accelerating mass transfer have been proposed and models enumerating the same have been presented. Compression of the textiles (Van Der Donck, 1997), yarn stretching (Van Der Donck et al., 1998) and pulsating flow (Van Der Donck, 1999), deforming the porous structure in textiles by bending, twist, stretch and pressure (Ganguli and Eendenburg, 1980) reportedly enhanced the soil removal considerably by influencing flow through textile layers. The findings from these studies support that the main task of the mechanical energy in a wash process is creating deformation of the textiles, thereby enhancing mass transfer in textile materials which translates into reduction of the diffusional stagnant core of textile yarns, rather than producing at random macroscale hydrodynamic phenomena (Moholkar, 2002). Mechanical energy or Agitation therefore is a critical factor, but has to be judiciously applied otherwise it can cause fabric damage, more so with repeated washing. Application of power ultrasonic has been shown in the previous section as the most


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effective and efficient method to accelerate mass transfer in textile materials by reducing the size of stagnant core, which can affect the process efficiency positively. Mechanism of Intensification In a system comprising liquid medium (water) with the textile, the phenomenon of cavitation can occur at three possible locations: (1) in the bulk liquid; (2) in the boundary layer between the textile and the bulk liquid; (3) inside the yarn of the textile (possibly due to the nucleation provided by tiny gas pockets trapped in the intra-yarn pores). Cavitation can contribute to the mass transfer enhancement in textiles and, hence, to the intensification of the textile treatment in several different ways: • The oscillatory motion of the cavitation bubble creates a spherical velocity field that

decays with the square of the distance from the bubble center. During instances of rapid bubble wall movement, where the bubble wall velocity can reach or exceed the speed of sound in the medium, this spherical velocity field can induce strong convection in the region in the close vicinity of the bubble (Moholkar, 2002).

• If the bubble is located in the close vicinity of a solid boundary, it can undergo deformation during transient collapse, which results in the formation of a high-velocity micro-jet with velocities as high as 100-150 m/s directed towards the solid surface. This micro-jet can give rise to intra-yarn flow, thus increasing the rate of the mass transfer between the intra-yarn and inter-yarn pores (Blake et al., 1986).

These mechanisms are responsible for the creation of some kind of convection in the medium; which contributes to the enhancement of the mass transfer in the textile material and, hence, will contribute to the intensification of laundry process. Although reports of attempts to adapt this technology for continuous washing of textiles for semi-industrial usage (Juárez et al., 2009), washing of medical surgery gowns to remove blood stains effectively in less time (Canoglu et al., 2004) and ultrasonic washing of wool fabric which resulted in reduced felting shrinkage (Hurren et al., 2008 ) are there. Patent literature also revealed a number of US and Japanese patents awarded for ultrasonic washing machines or a washing machine with provision for introduction of acoustic waves, but none of these innovations have reached the stage of commercialization. This is primarily due to difficulties in scaling


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up to industry level (Perincek et al., 2009) and also reticulate nature of textiles makes it difficult to generate uniform cavitation effect in the bath (Juarez et al., 2009). To clean heavily soiled areas of garment laundry process intensification is necessary. Traditional methods of increment result in enhanced fabric and environmental damage. Use of ultrasonic energy offers an alternative means of enhanced mechanical agitation that is gentle to the fabric and innocuous to the environment. In laundering, the mechanical energy created by either rotation of drum in a washing machine or scrubbing, brushing etc. by hand delivers the kinetic energy for soil loosening and soil transfer. In ultrasonic assisted washing process, this kinetic energy would be delivered by sound waves. Also, in ultrasonic agitation, mechanical and thermal energy are of very high order but at microscopic level which result in deeper but gentle cleaning in lesser time, which may lead to further conservation of energy and other resources. Despite several patents and promising research results of ultrasonic cleaning, there is no practical development in the area of textile/ apparel. This is primarily due to difficulties in scaling up to industry level (Perincek et al., 2009) and also reticulate nature of textiles makes it difficult to generate uniform cavitation effect in the bath (Juarez et al., 2009). The need, therefore, is to integrate knowledge regarding washing process parameters with principles of ultrasonic cleaning. The cleaning effectiveness of the ultrasonic assisted laundry process is related to the cavitation intensity in the cleaning medium, this in turn is a function of several acoustic and machine parameters. These need to be identified and optimized for development of an ultrasonic system for textile washing. Also application versatility of ultrasonic washing on various textile media and authentication of its gentler nature vis-à-vis conventional method needs to be done.

review of literature - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9068/11/11_chapter...

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