ABSTRACT
High-frequency, high intensity ultrasound influences chemical reactions and transport phenomena via two principal mechanisms - acoustic streaming and cavitation. Bubble implosions associated with acoustic cavitation generate very high temperatures (1000s of Kelvin) and pressures (100s of atmospheres) which can accelerate many chemical reaction rates by several orders of magnitude. Acoustic streaming, the unidirectional high-velocity flow induced by the pressure gradient can substantially enhance heat, mass and momentum-fluxes. These aspects of acoustically-driven process intensification are investigated here for leather dyeing application: Dynamics of dye uptake by leather under varying conditions of ultrasonic frequency (58-192 kHz) and input power (maximum 500 Watts, variable between 40% to 100%).Experimental data indicate that a combination of cavitation and acoustic streaming e.g., by using dual frequency systems that combine a low frequency (e.g., 58 kHz) with a higher frequency (e.g., 192 kHz),at 60% power level proves to be optimum condition, at 100% power level 58 kHz frequency results in higher percentage exhaustion of dye.

1. INTRODUCTION & BACKGROUND
Ultrasound technology is used as a modern and very environment-friendly process in an increasing number of applications and processes of the chemical industry. Particularly noteworthy are the application options of this technology in pharmacy, chemistry, biotechnology, and environmental engineering. Ultrasound process technology is a unique method for the activation and acceleration of processes in chemistry, chemical engineering, petrochemical and biotechnology.

Ultrasound is a sound wave with a frequency above the human audible range of 16 kHz. Ultrasound may be broadly classified according to frequency range as power ultrasound (20100 kHz) and diagnostic ultrasound (110 MHz).The power ultrasound is commonly employed for enhancing the physical processes such as cleaning, emulsification, degassing, crystallization, extraction, etc., and for accelerating/performing chemical reactions.

References [6] have studied the possible effect of ultrasound on micromixing through the phenomenon of acoustic cavitation. References [10 and 11] have noticed that ultrasound has the added advantages of improving the process efficiency, reducing the process time and to improve the quality of leather. They showed that the rate of diffusion of dye through pores of leather matrix was enhanced by the use of ultrasound using ultrasonic cleaner (150W and 33 KHz) and compared with the conventional drumming conditions.

1.1 CAVITATION
Richards and Loomis reported in 1927 on the chemical effects of high power ultrasound. They described two types of chemical reaction:
1.The acceleration of conventional reactions by ultrasound, and
2. Redox processes in aqueous solutions.

In a liquid medium, the effects of low frequency (i.e., <40 KHz) ultrasound are dominated by the phenomenon called cavitation. At higher frequencies (100KHz-upto megahertz), acoustic streaming becomes a significant contributor.

An ultrasonic system basically has three components,

1.Ultrasonic generator
2.Ultrasonic transducer
3.Tank with cleaning liquid

Figures 4.8 (A & B) show how the improvement factor varies with time. Its clear that 58 KHz, and (58 192) KHz, 100% power showed high enhancement factor throughout and dual (58 192) KHz, 60% power level have better enhancement throughout the dyeing process compared to all other frequencies.

4.9 PROPERTIES OF THE DYED CRUST LEATHER
I .Color value of the dyed leather
The color measurement values for the leathers dyed in the presence of ultrasound compared with leathers dyed in absence of ultrasound with 4% acid red dye for 120 minutes was tabulated below.

Table: 6.1.9 (i) Color value of dyed leather at 4% dye concentration

Color measurement quantify that leather dyed in presence of ultrasound has greater color values compared to that of control leathers. The values L*, a* and b* are the variables in the CIELAB color space and explained as follows.

More negative value of L* denotes darker shade and more positive value of L* for more light shade of the color. More negative value of a* implies more green color and more positive value of a* for more red color. More negative value of b* means more blue color and more positive value of b* for more yellow color. Lower L* values for leathers dyed with ultrasound indicate that ultrasound helps in getting brighter shades. Also, higher a* values for leathers dyed with ultrasound indicate that ultrasound helps in getting more redness, the color of the dye used.

II. Scanning Electron Microscopic (SEM) Analysis
The SEM photographs indicate regular fiber structure for the leathers dyed with ultrasound as compared to that of control. The photographs also show that fiber bundles are intact and not affected due to the use of ultrasound in leather dyeing process.

III Physical properties of the dyed leathers
(i).Fastness properties
Dry-rub fastness is better for leather dyed in presence of ultrasound while comparing with that of control leathers. This implies that color of the leathers dyed in presence of ultrasound can withstand better way.

4.8 DIFFUSION COEFFICIENT

The results show that 44.44 times more improvement in the diffusion coefficient for dyeing with acid red due to the presence of ultrasound (58 KHz, 100% power level after 2 hours of run) compared to dyeing in absence of ultrasound. This can be attributed to the fact that ultrasound helps in reducing the diffusion resistance in a better manner

1: Fastness is bad, 5: fastness is excellent C: Control without ultrasound

(ii) Strength properties

SEM analysis also implies that strength properties were not affected due to the effect of ultrasound during the dyeing process. This is also proved in earlier studies [11].

4.SUMMARY AND CONCLUSION

In leather dyeing process different frequency at various power levels shows different percentage of exhaustion of dye. For 58 kHz at various power level(40-100%) shows 40-80% increase in dye exhaustion than controls. Similarly, for 132kHz it is 40-60% increase , for 192 kHz it is 45-65% increase and for dual frequency it is 45-55% increase for lower power levels and 65-85 %increase for higher power levels. The optimum operating conditions to have higher dye uptake by leather was found to be 58 KHz at 100% power level where cavitation phenomena is more predominant and (58 192) kHz dual frequency, at 60% power level where combined effect of acoustic cavitation and streaming are observed. It is also shown that enhancement diffusivity of dye into leather was found to be higher at lower frequency where cavitation is predominant .And also dual frequency enhances mass transfer rate due to combined effect of acoustic cavitation and acoustic streaming.

As power level lowers cavitation intensity decrease hence percentage dye exhaustion decreases. The percentage exhaustion of dye is 1.2 times more than the static control when leather dyeing was carried out in mechanical shaker operated at 50rpm and both maintained at room temperature. This proves that rate of diffusion of dye also depends on agitation factor.

As the amount of dye offered increases percentage exhaustion of dye decreases. At 2% and 4% concentration of offered dye, it was found to have 3 and 2 fold times increase in percentage dye exhaustion than static control and 0.68 times and 0.35 times increase in percentage dye exhaustion when compared with leathers dyed in mechanical shaker respectively.

As amount of dye offered increases, dye uptake per gram of leather also increases. Dye uptake per gram of leather at 2% offered dye concentration at 58 kHz,100% power level and (58 192) kHz 60% power level is more than 4% offered dye concentration of static control.

Enhancement intensity is not directly related to cavitation intensity but depends on frequency and power level. Cavitation intensity decreases with increase in viscosity and density of the fluid.

Diffusion coefficient for leather dyeing in the presence of ultrasound is 44.44 times more than that of control after 2 hours of run. It is 225 times increasing than control after 1 hour run at 58 kHz, 100% power level. Improvement/enhancement factor also shows that 58 kHz and dual frequency are better.

Color measurement quantify that leather dyed in presence of ultrasound has greater color values compared to that of control. SEM results indicates that regular fiber structure was observed for leathers dyed with ultrasound as compared to that of control and also strength properties were not affected due to ultrasound. Even dry rub fastness was better for leather dyed in presence of ultrasound compared to control.

REFERENCES

1.C Vanhille, C Campos-Pozuelo, A numerical formulation for non-linear ultrasonic waves propagation in fluids, Ultrasonics 42(2004) 1123-1128.

2.Jianyong Wu, Xiuchum Ge, Oxidative burst jasmonic acid biosynthesis and taxol production induce by low-energy ultrasound in Taxus chinensis cell suspension cultures. Biotechnology and bioengineering vol 85,Issue 7,(2004) 714-721

3. J.P.Xie, J.F.Ding, G.E.Attenburrow, T.J.Mason, Influence of power ultrasound on leather processing. Part I: Dyeing, J.Am.LeatherChem.Assoc. 94 (1999)146-157.

4.S.Sushlick, Ultrasound-Its Chemical, Physical and Biological effects, VCH publishers, New York, 1988.

5.M.Chouvellon, A Largillier, T Fournel, P Boldo, Y Gonthier, Velocity study in an ultrasonic reactor, Ultrasonic Sonochemistry 7(2000) 207-211.

6.Monnier H, Wilhelm AM, Delmas H, Influence of ultrasound on mixing on the molecular scale for water and viscous liquid, Ultrasonic Sonochemistry March 6 (1-2) (1999) 67-74.

7.Nilesh P Vichase, Parag R Gogate, Vishwas Y Dindore, Aniruddha B Pandit, mixing time analysis of a sonochemical reactor, Ultrasonic Sonochemistry 8 (2001) 23-33.
8.Scherba, G., Weigel, R.M., OBrien, W.D., Quantitative assessment of the germicidal efficacy of ultrasonic energy. Applied and Environmental Microbiology 57 (7) (1991) 2079 2084.

9.Sigve Jjotta, One some non-linear effects in ultrasonic fields, Ultrasonics 38 (2000) 278-283.

10.Sivakumar, P.G Rao, Application of power ultrasound in leather processing: an eco-friendly approach, J.Cleaner Prod.9 (1) (2001) 25-33.

11.V. Sivakumar, P.G Rao, Studies on the use of power ultrasound in leather dyeing, Ultrasonic Sonochemistry 10(2003) 85-94.

12.Vijayanand S Moholkar, Sander Rekveld, Marijn M C G Warmoes Kerken, Modelling of the acoustic pressure fields and the distribution of the cavitation phenomena in a dual frequency sonic processor, Ultrasonics 38 (2000) 666-670

13.Yasumasa Ito, Koiji Nagata and Satoru Komori, The effects of high frequency ultrasound on turbulent liquid mixing with a rapid chemical reaction, Physics of fluids Vol 14, Issue 12, Dec (2002) 4362-4371.

The ultrasonic generator transforms the line voltage with 50 or 60 Hz to a frequency corresponding to the operative frequency of the transducer. Usually this frequency is in the range of 20-200 kHz. The resulting electric oscillations are supplied to the transducer by a cable. The transducer transforms these electric oscillations into mechanical sound waves. So, the liquid is set in motion. Each wave leads to alternating phases of high and low pressure according to the transducer expanding and contracting.

On contracting, this means the phase of low pressure, innumerable small vacuum bubbles form in the liquid due to its restricted tractability as in figure1. These small cavities collapse during the following expanding of the transducer which means the phase of high pressure they implode. This phenomenon is called cavitation.

Around the cavitation bubble, local high pressures develop considerable turbulences as well as fluid motions due to the sudden implosion. These events set up forces to remove dirt particles from the surface of the cleaning goods. Cavitation bubbles mainly arise at the interface between the liquid and the object to be cleaned. That is exactly where they are desired.

1.2. STRUCTURE OF LEATHER

Animal skin/hide is tanned to get resistance against physical, chemical and biological effects. Tanned skin/hide is called leather. Collagen is a fibrous protein present in skin involved in leather making. Non-collagenous materials are removed during leather processing. The leather cross-section can be classified as three layers namely, corium minor (grain layer), corium major I (middle layer) and II (flesh layer). The collagen molecules have a triple helical structure and are arranged in a quarter staggered five-stranded pentagonal fashion to form a micro fibril. Numbers of micro fibrils constitute a fibril with about 7000 collagen molecules and diameter of about 10−5 cm. Numbers of collagen fibrils constitute a fiber. The skin/hide matrix is composed of a three-dimensional weave of collagen fiber-bundles. The diameter of the collagen fiber-bundles varies from 0.01 to 0.05 cm. Collagen fiber-bundles present in the corium minor are more compactly woven and smaller in diameter than those present in corium major. Therefore, leather can be considered as a membrane capable of physical as well as chemical adsorption having non-uniform pores.

1.3. LEATHER DYEING
Dyeing is a process of imparting color to the leather. In dyeing, initially dye penetrates throughout the cross-section of the leather and is then fixed in the leather fibers by electrostatic attraction. Acid/anionic dyes are used for the dyeing of chrome leather. Since chrome tanned leather is cationic in nature, the leather is neutralized to the pH of 5.26.5 to make the leather more anionic to facilitate the penetration of dyes.

1.4. USE OF POWER ULTRASOUND IN LEATHER DYEING
Leather processing involves diffusion of various chemicals through the pores of hide/skin with pore size ranging from 310−8 to 1.510−2 cm. The diffusion in conventional processing is achieved by drumming/paddle action. For a given leather to be dyed, the rate of diffusion of dye through the leather matrix depends on various factors such as concentration of dye in the dye bath, neutralization pH of leather, temperature of the dye bath, particle size of the dye, mechanical agitation. The diffusion rate can be improved by offering higher concentration of dye, maintaining higher temperature (60 C) and by increasing the mechanical agitation. If higher concentration of dye than what is actually required is offered, it will lead to poor exhaustion of dye and concomitant pollution problems. For maintaining higher temperature of the dye bath, hot water circulation is essential. If the mechanical agitation is increased, there is a possibility of damage caused to the leather.

The use of power ultrasound may accelerate the diffusion rate of chemicals diffusing through the hide/skin matrix even in static and ordinary room temperature conditions and without using higher concentration of dye. Therefore, the optimum amount of dye that is actually required can be employed, minimizing the amount of unspent dye in the waste liquor for a given processing time. Thereby, pollution load can be reduced. The applications of power ultrasound in leather processing were studied earlier for potential benefits in some of the process stages such as soaking and liming, chrome tanning, vegetable tanning, dyeing and in fat liquoring. The application of power ultrasound in textile dyeing was also studied earlier for its potential benefits reference [11].

In this paper, the results obtained for experiments carried out in the presence of ultrasound in stationary conditions and compared with control experiments carried out in the absence of ultrasound in stationary conditions as well as in mechanically agitated conditions. The effect of variation in parameters such as percentage dye given (24%), temperature (30, 50 and 60 C) and time (02 h) were studied. Experiments were carried out with acid red dye. Parameters such as percentage exhaustion of dye, dye fastness properties, mechanical strength properties and color values of leathers were compared.

3. EXPERIMENTAL DETAILS

Before dyeing of the leather samples, it is soaked in water with 1% ammonia solution for about 24 hours; since chrome tanned leathers is more cationic in nature, it is neutralized and acid red dyes are used for dyeing process. Dyeing experiments were carried out in a glass beaker with a flat bottom, clamped inside the ultrasonic tank containing water. The leathers were weighed, and correspondingly 1000% water and 4% dye concentration solution were added to the glass beaker in which sample leathers are placed for dyeing. The process beaker is positioned exactly at the centre of the tank above the transducer. The distance of the beaker from the bottom of the tank and its side face are adjusted for repeatability of the experiments. Temperature of the water bath in the tank is monitored using temperature controller.

A piezoelectric transducer mounted at the bottom of the tank generates ultrasound of particular power at the selected frequency and at the selected power level. The diffuse sound field may be operative in the heterogeneous system comprising of dye solution and leather. The spent dye liquor was taken at each 15 minutes interval and analyzed using UV-visible spectrophotometer. The experiments were conducted for continuous 2 hours and the volume of the solution left was measured. The dyed leather samples are dried for further analysis at room temperature. The control experiments were conducted similarly in a glass beaker at room temperature (static condition), in a mechanical shaker (at 50 rpm, room temperature) and in a water bath at (50o C and 60o C).

3.1 DYEING
Full chrome crust leathers, retanned with 12% syntan and fat liquored with 10% fat liquor were used for the dyeing experiments. Circular samples of diameter 9 cm were cut parallel to the backbone at corresponding portions from both sides of the leather and taken for comparison of the experiments with ultrasound and control experiments without ultrasound. In the case of dyeing with ultrasound, the leather was placed in the process beaker in such a way that its flesh side was kept at the bottom and facing the transducer.

All the percentages of chemicals used are based on the crust (dried) weight of leather. The leathers were wetback for 24 hours with 1% ammonia and 1000% water. Then the dyeing was carried out with X % (4% dye and 2% ) acid red dye The dye used for the experiments was acid red dye (Bordeax IV , CI Acid red-119); other chemicals used were LR grade and distilled water . The percentage dye exhaustion was calculated using the formula,

Improvement factor in % dye exhaustion =
(%exhaustion of dye with ultrasound / %exhaustion of dye without ultrasound)

3.2 ANALYSIS OF DYE IN THE SPENT DYE LIQUOR

Using a UV-Visible spectrophotometer, the collected samples at each interval was analyzed for unspent dye by measuring the absorbance value at the wavelength max (523 nm) of the acid red dye used, after suitably diluting the spent dye liquor. Then the amount of dye present in the spent liquor was calculated from the calibration graph drawn for the dye.

3.3 COLOR VALUE OF THE DYED LEATHER

Quantification of color of the dyed leather was made according to the Commission International de lEclairage (CIE) system of color measurement with 100 standard observer data. L*, a*and b*values for both the grain as well as the flesh side of the dyed leathers were obtained using Reflected spectrophotometer (Greta Macbeth Spetrolino, Switzerland).

3.4 SCANNING ELECTRON MICROSCOPIC (SEM) ANALYSIS

Leather fiber structure, another important property for final leather and influence of ultrasound on the same, has been studied using SEM analysis. Dyed leather samples (58 KHz at 100% power level, 58 192 KHz at 60% and 100% power level and control leather without ultrasound) were cut into uniform sizes and then gold coated using Edwards sputtering device. Analysis was performed using Leica Cambridge Stereoscan 440 Scanning Electron Microscope. SEM analysis of the leather dyed with ultrasound and without ultrasound has been made.

3.5 PHYSICAL PROPERTIES OF THE DYED LEATHERS

Fastness properties of the dyed crust leathers such as dry-rub fastness was measured using a Satra fastness tester adopting standard Satra test procedures (PM 8/Satra).Strength properties of the leather with and without ultrasound was also tested. Leather samples for the physical testing were taken parallel to backbone from the circular dyed leather samples following the IUP/1 procedure for sampling and testing as published [23].The circular samples taken for ultrasonic experiment and for control were within 18-cm distance from each other at the sampling portion to avoid the possible differences due to the variation in area.

4. RESULTS AND DISCUSSION
4.1 EFFECT OF TIME

Dye uptake was studied during the course of the dyeing process for a total dyeing time of 2 h with and without ultrasound for acid red dye. Improvements in the dye exhaustion throughout the dyeing process for acid red at different frequency and at different power levels were observed. Similar graphs were obtained for other frequencies (132,192 and 58 192 kHz).

For 58 KHz

Figures 6.1.1-A (i & ii) show that exhaustion of dye during the course of the dyeing process is better throughout the dyeing process due to the presence of ultrasound when compared with control. About (40-80 %) exhaustion of dye can be achieved in 2 h dyeing time using ultrasound while compared to only (20-30%) in absence of ultrasound in stationary condition.

For 58 KHz 192 KHz

Figure 6.1.1-B (i & ii) show that exhaustion of dye during the course of the dyeing process is better throughout the dyeing process due to the presence of ultrasound when compared with control. About ( 45-55%) exhaustion of dye can be achieved in 2 h dyeing time at low power levels (i.e. 40 and 50% power levels)using ultrasound where as for higher power level it is (65-85%) exhaustion of dye while compared to only (20-30% ) in absence of ultrasound in stationary condition. This observed increase in higher power level is due to the fact that leather dyeing is better for combined effect of acoustic cavitation and acoustic streaming.

4.2 EFFECT OF TEMPERATURE
As the temperature of the dye bath increases, the percentage exhaustion and dye uptake per gram of leather increases as shown in earlier studies [16]. The studies show that exhaustion of dye in a room temperature using ultrasound was better when compared to dyeing at 50C and 60 C without ultrasound in a stationary condition and comparable with dyeing carried out in static room temperature.

4.3 EFFECT OF ULTRASONIC FREQUENCIES AND POWER LEVEL

Figures (6.1.3 A and 6.1.3-B) shows that at 100% power level 58 KHz shows higher percentage of exhaustion followed by 58 192 dual frequency at 60% power level.

Figure (6.1.3 - A & B) shows that 58 KHz at 100% power level have higher diffusivity enhancement for an hour run (ED,1h) followed by 80% power level since acoustic cavitation is predominant at lower frequency with higher power level. At 60% and 50% power level dual frequency have higher diffusivity enhancement; it is due to the combined effect of acoustic cavitation and acoustic streaming. Enhancement diffusivity (ED,1h) is much higher than (ED,2h) which is due to the difference in the amount of dye available in the spent liquor for diffusion more surface available for diffusion and less mass transfer resistance.

4.4 EFFECT OF ULTRASONIC POWER LEVELS

The below figure (4.4- A) shows at 100% power level 58 KHz, the dye uptake by leather is higher since acoustic cavitation is more predominant phenomena at higher power level at lower frequency hence more mass transfer enhancement is observed, it is then followed by (58 192 KHz) dual frequency, when compared to leather dyed without ultrasound (control).
It is clear from the above figure (4.4 B) combined effect of acoustic cavitation and acoustic streaming proves to be more effective at 60 % power level. It is also observed as the power level lowers the cavitation intensity decreases and hence the % exhaustion of dye also decreases.

Enhancement diffusivity is a function of frequency and power. For the experimental data, it fits the polynomial function of order four.

4.5 EFFECT OF AGITATION

From the graphs 4.5 A and B, the percentage exhaustion of dye by leather when mechanically agitated during dyeing, operated at 50 rpm at room temperature is almost 1.2 times more than static control at room temperature. This shows that rate of diffusion also depends on the agitation factor.

4.6 EFFECT OF AMOUNT OF DYE OFFERED

There is 3 fold times increase in percentage exhaustion of dye when ultrasound was used when compared to the control process in a stationary condition and 0.68 times increase in %exhaustion at 2% dye concentration was obtained when compared with leather dyed in mechanical shaker as shown in figure 4.6-A

There is 2 fold times increase in percentage exhaustion of dye when ultrasound was used when compared to the control process in a stationary condition and 0.35 times increase in %exhaustion at 4% dye concentration was obtained when compared with leather dyed in mechanical shaker as shown in figure 4.6-B.

As the amount of dye offer increases, the percentage exhaustion of dye decreases is shown in figure 4.6-C ,it is clear by comparing the percentage exhaustion of dye by leather at 2% and 4% dye concentration operated at 58 KHz, 100% power level and 58 192 KHz (dual frequency) operated at 60% power level

The amount of dye uptake per gram of leather was more when ultrasound was employed and increased with the amount of dye given for the same dye bath volume and dye uptake per gram of leather of 2% dye concentration at 58 kHz, 100% power and (58 192) kHz, 60% power is more than 4% dye concentration of control as shown in figure 4.6-D.

4.7 CAVITATION INTENSITY MEASUREMENT
The both figures (6.1.7- A & 6.1.7-B) clearly show that there is no direct co-relation between diffusivity enhancement and cavitation intensity at the initial and at steady state. Hence ED,1h and ED,2h does not depend on cavitation intensity but by the way it is obtained.