Sound is transmitted through a medium by inducing vibrational motion of the molecules through which it is traveling. This vibrational motion represents the sound frequency. Ultrasound is sound of a frequency that is above the threshold of human hearing. The lowest audible frequency for humans is about 18Hz and the highest is normally around 18-20 kHz for adults, above which it becomes inaudible and is defined as ultrasound. In recent decades the use of ultrasound technology has established an important place in different industrial processes such as the medical field, and has started to revolutionize environmental protection.
The idea of using ultrasound in textile wet processes is not a new one. On the contrary there are many reports from the 1950s and 1960s describing the beneficial effects of ultrasound in textile wet processes. In spite of encouraging results from laboratory-scale studies, the ultrasound-assisted wet textile processes ha snow been implemented on an industrial scale by Day & Me Stockholm.
In practice, three ranges of frequencies (Fig.1) are reported for three distinct uses of ultrasound: low frequency or conventional power ultrasound (20-100 kHz), medium frequency ultrasound and diagnostic or high frequency ultrasound (2-10 MHz).
Power ultrasound can enhance a wide variety of chemical and physical processes, mainly due to the phenomenon known as cavitation in a liquid medium that is the growth and explosive collapse of microscopic bubbles. Sudden and explosive collapse of these bubbles can generate ‘‘hot spots’’, i.e., localized high temperature, high pressure, shock waves and severe shear force capable of breaking chemical bonds.
High temperature and pressures resulting from the collapse of the transient cavitation bubbles are responsible for all the observed effects of ultrasound. Parameters which affect cavitation and bubble collapse are:
Properties of the solvent: The solvent used to perform sample treatment with ultrasonication must be carefully chosen. As a general rule, most applications are performed in water. However, other less polar liquids, such as some organics, can be also used, depending on the intended purpose. Cavities are more readily formed when using a solvent with high vapor pressure, low viscosity and low surface tension. But at high vapor pressure more vapor enters the cavitation bubble during its formation and the bubble collapse is cushioned and less violent.
Properties of gases: Soluble gases should result in the formation of a larger number of cavitation nuclei, but the greater the solubility of the gas is the more gas molecules should penetrate the cavity. Therefore, a less violent and intense shock wave is created on bubble collapse.
External pressure: With increasing external pressure, the vapor pressure of the liquid decreases and higher intensity is necessary to induce cavitation. In addition, there is an increment in the intensity of the cavitational bubble collapse and, consequently, an enhancement in sonochemical effects is obtained. For a specific frequency there is a particular external pressure that will provide an optimum sonochemical reaction.
External temperature: Higher external temperature reduces the intensity necessary to induce cavitation due to the increased vapor pressure of the liquid. At higher external temperatures more vapor diffuses into the cavity, and the cavity collapse is cushioned and less violent .
Frequency of the sound wave: At high sonic frequencies, on the order of the MHz, the production of cavitation bubbles becomes more difficult than at low sonic frequencies, of the order of the kHz. To achieve cavitation, as the sonic frequency increases, so the intensity of the applied sound must be increased, to ensure that the cohesive forces of the liquid media are overcome and voids are created. 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.
Cavitation induced by ultrasound will allow accelerating processes and obtaining the same results as existing techniques but with a lower temperature and low dye and chemical concentrations. For this reason textile wet processes assisted by ultrasound are of high interest for the textile industry.
Some of the benefits of using of ultrasonics in dyeing can be listed as below;
energy savings by dyeing at lower temperatures and reduced processing times,
environmental improvements by reduced consumption of auxiliary chemicals,
lower overall processing costs (due to less energy and chemical consumption), thereby increasing industry competitiveness.
Improvements observed in ultrasound-assisted dyeing processes are generally attributed to three main phenomenons,
Dispersion: breaking up of micelles and high molecular weight aggregates into uniform dispersions in the dye bath,
Degassing: expulsion of dissolved or entrapped gas or air molecules from fiber into liquid and removal by cavitation, thus facilitating dye-fiber contact, and
Diffusion: accelerating the rate of dye diffusion inside the fiber by piercing the insulating layer covering the fiber and accelerating the interaction or chemical reaction, if any, between dye and fiber.
A good textile dyeing process must provide a satisfactory uptake of dye bath and an adequate penetration of dye into the fiber, with the practical advantages of good wet fastness and uniform coloration. The conventional methods for wool dyeing are based on long times at temperature close to the boiling point, in order to ensure good results of dye penetration and leveling. These conditions can damage the fibers, with negative effects on the characteristics of the finished material. Such damage can be minimized by reducing the operation time or, better yet, by reducing the dyeing temperature.