Innovative and Emerging Technologies for Textile Dyeing and Finishing E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
With the public enhanced awareness towards eco-preservation, eco-safety and health concerns, environmentally benign, nontoxic and sustainable bioresource materials produced mainly from non-food crops have revolutionized all industrial sectors particularly textile industry. In recent years, textile industries in developed countries are getting increasing interest in global interest due to the varied and changing world market conditions in terms of price, durability and fiber mixtures as well as design, colors, weight, ease of handling and product safety. The increasing environmental and health concerns owing to the use of large quantities of water and hazardous chemicals in conventional textile finishing processes lead to the design and development of new dyeing strategies and technologies. Effluents produced from these textiles wet processing industries are very diverse in chemical composition, ranging from inorganic finishing agents, surfactants, chlorine compounds, salts, total phosphate to polymers and organic products. This aspect forced western countries to exploit their high technical skills in the advancements of textile materials for high quality technical performances, and development of cleaner production technologies for cost effective and value-added textile materials. Therefore, vast and effective research investigations have been undertaken all over the world to minimize the negative environmental impact of synthetic chemical agents through the sustainable harvest of eco-friendly bioresource materials. The book will discuss following research developments in academic and industry: * Improvement in dye extraction and its applications * Impact of textile dyeing on environment * Textile finishing by natural and ecofriendly means * Natural dyes as environmental-friendly bioresource products * Textile effluent remediation via physical, chemical and biological processes.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Eco-Friendly Stimuli and Their Impact on the Tinctorial Capacity of Textile Materials
1.1 Introduction
1.2 Characterization of Ultrasound Physical Stimuli
1.3 Microwave as Stimulus for the Textile Wet Processes
1.4 Conclusions
References
2 Skincare Finishes to Textiles
2.1 Introduction
2.2 Types of Skincare Textiles
2.3 Techniques for Skincare Compounds Embedding
2.4 Techniques for Microparticles Applying on Textile Supports
2.5 Conclusion
References
3 Recent Advances in Synthetic Dyes
3.1 Introduction
3.2 Synthetic Dyes in Textile Coloration
3.3 Synthetic Dyes in Dye-Sensitized Solar Cells
3.4 Synthetic Dyes in Liquid Crystal Display
3.5 Synthetic Dyes in Fluorescent Sensors
3.6 Synthetic Dyes in the Detection of G-Quadruplex DNA
3.7 Conclusion
References
4 Natural Dye Extraction and Dyeing of Different Fibers: A Review
4.1 Introduction
4.2 Classification of Natural Dyes
4.3 Color-Based Classification
4.4 Dyeing With Natural Color From
Calendula offcinalis
4.5 Extraction of Natural Dyes
4.6 Dyeing Methods
4.7 Conclusions
References
5 Airflow, Foam, and Supercritical Carbon Dioxide Dyeing Technologies
5.1 Introduction
5.2 Airflow Dyeing Technology
5.3 Foam Dyeing Technology
5.4 Supercritical Carbon Dioxide Dyeing Technology
5.5 Conclusion
References
6 Colored Nanofiber Production: A Literature Review and Case Study
6.1 Introduction
6.2 Electrospinning
6.3 Colored Nanofiber Production
6.4 A Case Study: Silver Cyclohexane Mono Carboxylate: β-Cyclodextrine Inclusion Complex Doped Colored Functional Poly(Vinyl Alcohol) Nanoweb Production
6.5 Conclusion
Acknowledgements
References
7 The Effect of Plasma Treatment on Dyeing of Natural Fibers
7.1 Introduction
7.2 Types of Plasma
7.3 Plasma Application on Natural Fibers
7.4 Environmental Impact
7.5 Conclusions
References
8 The Effect of Plasma Treatment on Dyeing of Synthetic Fibers
8.1 Introduction
8.2 Mechanism of Plasma Interaction With the Substrate
8.3 Achievable Functionalities by Plasma Application
8.4 Application on Synthetic Fibers
8.5 The Current Standpoint of Plasma Application
8.6 Conclusions
References
9 Ozone-Based Finishing of Textile Materials
9.1 Introduction
9.2 Application of Ozone in Textile Finishing
9.3 Conclusion with Future Expectation
References
10 Ultrasound-Based Wet Processes in Textile Industry
10.1 Introduction
10.2 Application of Ultrasound in Textile Finishing
10.3 Conclusion With Future Expectation
References
11 Synthetic and Natural UV Protective Agents for Textile Finishing
11.1 Introduction
11.2 Role of Textiles in Protective Clothing
11.3 Factors Influencing Ultraviolet Radiation
11.4 Susceptibility of Various Textiles on UV Radiations
11.5 Method of Analysis and Standard
11.6 Synthetic Organic Compounds for UV Protection of Textiles
11.7 Ultraviolet Protection of Textiles From Natural Dyes
11.8 Nanotechnological Interventions in UV Protective Textiles
11.9 Graphene as UV Blocker for Textiles
11.10 Conclusion
References
12 Hydrophobic and Oleophobic Finishes for Textiles
12.1 Introduction
12.2 Textiles With Special Wettability Properties
12.3 Liquid Repellent Treatments of Textile Materials
12.4 Characterization Methods of Repellency Degree
12.5 Properties Desired of Liquid-Repellent Coatings
12.6 Environmental Impact
12.7 Conclusions
References
13 Flame Retardant Finish for Textile Fibers
13.1 Introduction
13.2 Importance of Flame Retardant Finish
13.3 Factors Affecting the Flammability of Textiles
13.4 Mechanism of Combustion of Textile Fibers
13.5 Flame Retardants
13.6 Flame Retardant Finish for Different Polymers
13.7 Different Flame Retardant Techniques
13.8 Assessment of Flame Retardancy
13.9 Application of Flame Retardant Textiles
13.10 Environmental Issues and Sustainable Flame Retardants
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Modifications of the absorption bands for the acrylic fiber determined...
Chapter 2
Table 2.1 The main compounds of essential oils [87–89].
Chapter 4
Table 4.1 Natural yellow dyes [39].
Table 4.2 Important red natural colors [39, 40].
Table 4.3 Important natural brown color.
Chapter 5
Table 5.1 Airflow dyeing machines [10].
Table 5.2 Comparison of liquor ratios of different equipment based on rope dyein...
Table 5.3 Comparison of specific input factors for dyeing with reactive dyestuff...
Chapter 6
Table 6.1 Number and percentage of publications related to electrospinning and/o...
Table 6.2 Number of publications related to electrospinning and/or nanofiber pro...
Table 6.3 Number of publications related to colored electrospinning and/or nanof...
Table 6.4 Results obtained by Pedicini and Farris [29].
Table 6.5 Color change of PFO/MEH-PPV/PMMA blend nanofibers depending on the rat...
Table 6.6 Electrospinning parameters.
Table 6.7 Properties of the solutions used to produce PVA/Ag-CC:β-CD nanofibers.
Table 6.8 Colors of Ag-CC:β-CD doped colored PVA nanofibers at various pH values...
Chapter 9
Table 9.1 A brief of some oxidants/species with relative oxidation powers by ref...
Table 9.2 The general physical features of ozone.
Chapter 11
Table 11.1 UV radiation exposure categories and sun-protective measures.
Table 11.2 UPF classifications AS/NZS 4399:2017.
Table 11.3 International standards related to UV protection of textiles.
Chapter 12
Table 12.1 Liquid repellency terminology.
Table 12.2 Summary of hydrophobic or oleophobic coatings for textile materials b...
Table 12.3 Summary of commercial fluorine-based coatings available on the market...
Table 12.4 Fluorine ≤C6 functionalization treatments for textile materials.
Table 12.5 Summary of commercial fluorine-free based coatings available on the m...
Table 12.6 Sustainable superhydrophobic treatments for textile materials
Table 12.7 Comparison of the environmental impact of liquid-repellent coatings.
Table 12.8 Comparison of liquid-repellent coatings.
Chapter 13
Table 13.1 Outlines of important test methods for different textiles [5, 35].
Table 13.2 Limiting oxygen index (loi) values of different types of fibers [35].
List of Illustrations
Chapter 1
Figure 1.1 Transversal cross-section and longitudinal aspect of Romanian acrylic...
Figure 1.2 FTIR spectra for Romanian acrylic fiber, before (1) and after 15 min ...
Figure 1.3 The influence of ultrasonication on the diameter of acrylic fiber Mel...
Figure 1.4 Influence of the ultrasound treatment on the dye affinity (a), dyeing...
Figure 1.5 The effects of microwaves on the eco-friendly processes (extraction c...
Figure 1.6 Spectra for Melana before and after microwave irradiation (MW).
Figure 1.7 Spectra for Dralon L before and after microwave irradiation (MW).
Figure 1.8 Diffractograms of Melana before and after microwave treatments.
Figure 1.9 Diffractograms of Dralon L before and after microwave treatments.
Figure 1.10 Fluorescence spectra for Melana (a) and Dralon L (b) samples dyed wi...
Figure 1.11 Color and fluorescence intensities of acrylic samples dyed with 10% ...
Chapter 2
Figure 2.1 Types of skincare textiles.
Figure 2.2 The advantages and disadvantages of exposure to UV radiation.
Figure 2.3 The main anti-aging compounds.
Figure 2.4 The types of moisturizers.
Figure 2.5 Action mode of essential oils.
Figure 2.6 Applications of fragranced textiles.
Figure 2.7 The most used essential oils with relaxing and refreshing effects.
Figure 2.8 Microencapsulation methods.
Figure 2.9 The morphology of the capsule.
Figure 2.10 The methods of applying active compounds on textile supports.
Figure 2.11 Exhaustion technique on jigger.
Figure 2.12 Microcapsule applying by padding method.
Figure 2.13 Microcapsule applying by the pad-batch method.
Figure 2.14 Microcapsule applying by the pad-steam method.
Figure 2.15 Microcapsule applying by spraying method.
Figure 2.16 Microcapsule applying by the printing method.
Figure 2.17 Nanosols applying on textiles.
Chapter 3
Figure 3.1 Azo disperse dyes based on coumarin and bridged with thiophene. Repri...
Figure 3.2 Chemical structures of CNU-1 to CNU-10. Reprinted from permission [17...
Scheme 3.1 Synthetic route of reactive disperse dyes. Reprinted from permission ...
Scheme 3.2 Synthesis of anthraquinone based disperse dyes.
Figure 3.3 Ring opening upon deprotonation of phenolphthalein halochromic dye (l...
Scheme 3.3 One-pot synthesis of EDOT based dyes.
Figure 3.4 Molecular structure of few D–π–A dyes.
Figure 3.5 Chemical structure of BODIPY dyes with thiophene spacer for conjugati...
Figure 3.6 Chemical structure of red dyes based on diketo pyrrolo-pyrrole chrmop...
Scheme 3.4 Synthetic route for the fabrication of fluorescent sensors based on t...
Chapter 4
Figure 4.1 Indigo.
Figure 4.2 Dibromoindirubin.
Figure 4.3 Berberine.
Figure 4.4 Benzoquinone.
Figure 4.5 Naphthoquinone.
Figure 4.6 Anthraquinone.
Figure 4.7 Flavone.
Figure 4.8 Flavonol.
Figure 4.9 Isoflavonoid.
Figure 4.10 Anthocyanidin.
Figure 4.11 Brazilin.
Figure 4.12 Brazilein.
Figure 4.13 Lycopene.
Figure 4.14 β-Carotene.
Figure 4.15 Forming the linkage between dye and fiber through the agency of mord...
Chapter 5
Figure 5.1 Illustration of an airflow dyeing machine with an indication of air c...
Figure 5.2 The THEN Airflow Synergy dyeing machine. (1) blower, (2) aerodynamic ...
Figure 5.3 Foam sequence of processing (adapted from [20]).
Figure 5.4 Foam dyeing process stages (adapted from [25]).
Figure 5.5 Foam dyeing application using a padding mangle principle. Typically, ...
Figure 5.6 The phase diagram of a pure CO
2
.
Figure 5.7 A simplified scheme of a typical scCO
2
dyeing plant for textiles; (1)...
Figure 5.8 DyeCoo’s scCO
2
dyeing machine (DyeOx) with three dyeing vessels [50].
Figure 5.9 Schematic of the dyeing mechanism of polyester fiber in scCO
2
.
Chapter 6
Figure 6.1 SEM images of PVA nanofibers and Ag-CC:β-CD doped PVA nanofibers.
Figure 6.2 FTIR Spectra of Ag-CC, PVA nanofibers and Ag-CC:β-CD doped PVA.
Figure 6.3 TGA thermograms of PVA nanofibers and Ag-CC:β-CD doped PVA nanofibers...
Figure 6.4 XRD peaks of PVA nanofibers and Ag-CC:β-CD doped PVA nanofibers.
Chapter 7
Figure 7.1 A typical diagram of plasma application on textile substrate.
Figure 7.2 Effect of plasma treatment on cotton fabric (drawn by the data report...
Figure 7.3 Scanning microscope image of cotton (a) before and (b) after oxygen p...
Figure 7.4 Comparative wettability and tensile property of conventional wet trea...
Figure 7.5 Effect of plasma treatment (a) on surface energy and color strength (...
Figure 7.6 Color strength of plasma-assisted printed cotton (drawn from data rep...
Figure 7.7 Scanning electron microscopic images of wool fiber (a) before and (b)...
Figure 7.8 Effect of plasma treatment on (a) wettability (drawn from the data re...
Figure 7.9 Influence of plasma pre-treatment on (a) half dyeing time and (b) equ...
Figure 7.10 Influence of plasma on the equilibrium adsorption of lac dye per uni...
Chapter 8
Figure 8.1 Schematic diagram of solid, liquid, gas and plasma state of a materia...
Figure 8.2 The interconnections among the mechanisms and properties induced by p...
Figure 8.3 Schematic representation of (a) before, (b) during, and (c) after pla...
Figure 8.4 Chemical structure of polyethylene terephthalate (PET) (a) and the pr...
Figure 8.5 Effect of oxygen plasma treatment on color strength of PET fabrics dy...
Figure 8.6 Effect of dielectric barrier discharge (DBD) plasma treatment on ligh...
Figure 8.7 Surface atomic composition of untreated and dielectric barrier discha...
Chapter 9
Figure 9.1 A general approach for the water–chemical–heat consumptions during fi...
Figure 9.2 Formation and deformation of ozone in atmosphere.
Figure 9.3 Denim fabrics and the warp-wef yarns.
Figure 9.4 A general washing processes for denim garments.
Chapter 10
Figure 10.1 A general flow chart in textile production.
Figure 10.2 Some advantages and disadvantages of irradiation methods in textile ...
Figure 10.3 Possible usages of ultrasound in wet processes of textiles.
Figure 10.4 A brief for possible applications of ultrasound in textile coloratio...
Chapter 11
Figure 11.1 The protective action of sunscreen, fabric, fabric with UV protectin...
Chapter 12
Figure 12.1 Contact angle measurement.
Figure 12.2 Effect of surface roughness on liquid droplet: (a) Wenzel state, (b)...
Figure 12.3 Repellency coatings for the textile materials.
Figure 12.4 Desirable properties of repellent coatings.
Chapter 13
Figure 13.1 Mechanism of combustion cycle of fibers [5].
Figure 13.2 Formation of halogen radicals.
Figure 13.3 Recombnination of hydrogen atoms by phosphorus species.
Figure 13.4 Phosphate acrylate monomer.
Figure 13.5 Diethyl-2-(methacryloyloxyethyl) phosphate.
Figure 13.6 Bis (diphenyl phosphate) bridged with aromatic ring.
Figure 13.7 Dialkylphosphinates salts.
Figure 13.8 Thiourea crosslinked polyamide chain.
Figure 13.9 Bifunctional P-Si containing flame retardant.
Figure 13.10 Nano-sized inorganic silicates and silsesquioxane derivatives.
Figure 13.11 Pyrolysis of cellulose.
Figure 13.12 Crosslinking of phosphoric acid.
Figure 13.13 Dehydration of cellulose by strong acid.
Figure 13.14 Thermal decomposition of ammonium salts.
Figure 13.15 Flame retardancy of ammonium polyphosphate.
Figure 13.16 Tetrakis (hydroxymethyl) phosphonium chloride.
Figure 13.17 Reaction of THPC urea ammonia.
Figure 13.18 Reaction of cellulose with N-methylol dimethylphosphonopropionamide...
Figure 13.19 Tetrabromophthalic anhydride (TBPA).
Figure 13.20 Tris (2,3-dibromopropyl) phosphate.
Figure 13.21 Cyclic phosphate/phosphonate flame retardant.
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Scrivener Publishing
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Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Innovative and Emerging Technologies for Textile Dyeing and Finishing
Edited by
Luqman Jameel Rather
Aminoddin Haji
Mohd Shabbir
This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
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Preface
In recent years, the textile industrial sectors of developed countries have been attracting increasing attention due to the diversification and transformation of world market conditions in terms of price, durability, design, color, ease of handling, and product safety. With enhanced public awareness of eco preservation and eco safety in conjunction with increasing environmental and health concerns, scientists are trying to exploit their high technical skills for the advancement of high-quality technical performance of textile materials and development of cleaner production technologies for cost-effective and value-added textile materials. Therefore, vast amounts of effective research have been undertaken all over the world to minimize the negative environmental impact of synthetic chemical agents through the sustainable harvesting of eco-friendly bioresource materials. Because the present world scenario demands more sustainability and greenness, the emphasis in this research has been on eco-friendly processes and the recovery of a safe environment without compromising the comfort properties of textile products. Therefore, this book presents an accumulation of current ideas for more sophisticated ways of improving textile dyeing and the comfort properties of textiles for the betterment of the textile industry.
Innovative and Emerging Technologies for Textile Dyeing and Finishing consists of 13 chapters on various research areas dealing with the modification of textile materials. The first two chapters deal with the eco-friendly stimuli (ultrasound and microwaves) and easy-care finishes of textile materials. The next two chapters deal with the advancements in synthetic and natural dye applications; and the fifth and sixth chapters deal with the use of advanced dying technologies (supercritical CO2 and foam dyeing) and the production of colored nanofibers respectively. Chapters seven through ten give a detailed account of the surface modification of textiles using new methods and strategies; and the remaining chapters deal with the functional finishing aspects of textile materials.
This book contains informative chapters from authors in the specialized fields of textile materials relating to dyeing, surface modification, functional finishing and nanofiber production. Thus, the editors hope that students, researchers and academicians of various fields, such as textile dyeing, chemical engineering, environmental science, materials science, etc., will find this book of great interest and useful in their curriculum. We expect this book will definitely be helpful in generating new ideas in textiles research, leading to interdisciplinary research collaborations.
In conclusion, we would like to thank those who supported this book in any way. We acknowledge the great efforts of the eminent authors without whom this book would have been unimaginable. We also appreciate the support of the publisher in showing interest in compiling a reference book on this timely subject.
Luqman Jameel RatherAminoddin HajiMohd ShabbirDecember 2020
1Eco-Friendly Stimuli and Their Impact on the Tinctorial Capacity of Textile Materials
Vasilica Popescu1* and Luminita Ciobanu2
1“Gheorghe Asachi” Technical University, Department of Chemical Engineering in Textiles and Leather, Iasi, Romania
2“Gheorghe Asachi” Technical University, Department of Knitting and Garment Engineering, Iasi, Romania
Abstract
Ultrasound and microwaves can be considered eco-friendly physical stimuli for the process of dyeing textile materials with natural or synthetic dyes. The application of these stimuli can be carried out before (pre-treatments) or during the dyeing process. The pre-treatments can be applied either to the matrix containing the natural dyes for their extraction/release, or on the surface of textile materials, in order to modify the surface morphological aspect and to change their reactivity. The use of ultrasound and microwaves causes chemical/physical changes that can be detected through Fourier-Transform Infrared Spectroscopy (FTIR) or Scanning Electron Microscopy (SEM). The physical changes can significantly influence the dyeing capacity of textile materials due to the modified amorphous-crystalline ratio, a fact confirmed by the X-ray diffraction (XRD). Compared to classic dyeing, ultrasound/microwave-assisted dyeing processes have certain advantages: improved dyeing capacity of textile materials, shorter process, improved color uniformity and good fastness properties.
Keywords: Eco-Friendly, ultrasound, microwaves, pre-treatments, spectroscopic analyses, textile dyeing
1.1 Introduction
Ultrasound and microwaves are physical stimuli with physical and chemical effects on the entire irradiated system [1–3].
These stimuli are results of the transformation of electric current, as follows:
a)
for ultrasound:
the transducer converts the electric energy into mechanical oscillation energy that spreads around as elastic sound and ultrasound waves; more concise, the electric energy is converted into acoustic energy [1–3];
b)
for microwaves:
the magnetron (the main component of the microwave oven) generates a fluctuating electromagnetic field and the water molecules are forced to change their rotation direction at a rate of 2.4 billion times per second, determining inevitable hits/displacements/friction between neighboring molecules generating the thermal energy (heat) associated to kinetic energy. This way, the electromagnetic energy is converted into thermal energy [4].
Ultrasound and microwaves have different frequency ranges:
a) the frequency range of microwaves depends on the type of ovens used [4]:
– 915 MHz for ovens used in industrial applications, characterized by 32.8 cm wavelength;
– 2.45 GHz generating a 12.2 cm wavelength used in the microwave domestic ovens.
b) ultrasound waves have a large frequency range, from 16 kHz to 1 GHz. As mentioned before, ultrasound waves are elastic acoustic waves. The elastic oscillations specific to ultrasound appear when a body vibrates and transmits part of its vibrational energy to the surrounding medium, the propagation generating waves characterized by compressions and rarefactions regions [1–3].
1.2 Characterization of Ultrasound Physical Stimuli
Ultrasound waves produce physical and chemical effects that can be valorized in different applications, including the medical field/therapy, food industry, metal ultrasonic cleaning, industrial applications, non-destructive material analysis methods, polymer chemistry, environmental protection, electrochemistry, as well as textile finishing treatments [1–3, 5].
Elastic waves are classified according to their frequency [1, 2]:
a) 0.5–15 Hz infrasonic waves;
b) 16 Hz–18 kHz audible sonic waves;
c) 20 kHz–1 GHz ultrasonic waves. They are low frequency (20–100 kHz), medium frequency (100 kHz–10 MHz) and high frequency (10 MHz–1 GHz).
The ultrasonic waves with frequencies in the 20–40 kHz range are known as conventional power ultrasound;
d) 1 GHz–10 THz hypersonic waves.
Compared to the audible sonic waves, the much higher frequency ultrasound waves can transport more energy, as well as more mass, the latter determined by the increased absorption and diffusion processes and the higher acceleration of the particles in the medium. Furthermore, ultrasound can be amplified, focused and oriented toward a pre-set direction [2, 3].
Ultrasound-assisted chemical treatments determine sonochemical reactions [6–9]. Based on the ultrasound propagation manner and the effects on the medium, these reactions refer to:
– homogeneous sonochemistry of liquid–liquid systems that generate sonoluminescence;
– heterogeneous sonochemistry of liquid–liquid and solid–liquid systems;
– sonocatalysis (increased rate of chemical reactions by means of ultrasound as catalyst).
1.2.1 Ultrasonic Equipment
An ultrasonic equipment contains [10]:
a)
the primary source of energy
—the mechanical or electric ultrasonic generator.
Usually, the frequency of the generator is determined based on the application of the ultrasonic device. For example, for the mechanical processing of solid materials, the ultrasound waves are used in the 400–800 kHz range. The ultrasound generator contains: the power source, the pilot oscillator, the pre-amplifier, the power amplifier.
b)
ultrasonic transducer
—this is the most important component that converts the electric energy into acoustic energy; it is the interface between the electric or magnetic circuit and the mechanical component.
c)
acoustic amplifier
—that focuses and amplifies the acoustic energy generated by the transducer.
1.2.2 Effects of Ultrasound in Liquid Mediums
The main effects of the ultrasound concern the increased transport of mass and heat by means of longitudinal waves that propagate easily through liquid or gas mediums, displacing the medium particles in the same direction.
When a liquid system is irradiated with ultrasound, an acoustic cavity is generated by the formation, growth and collapse of air/gas bubbles in that liquid [10, 11].
The ultrasound waves have rarefaction/expansion and compression cycles that move through the liquid medium, developing negative and positive pressures.
The rarefaction/expansion cycle causes the development of a high acoustic negative pressure which increases the intramolecular space, leading to the formation of vapor cavities (bubbles or void spaces) bigger than the critical molecular distances.
The compression cycle creates positive acoustic pressure; this pressure forces the bubbles/cavities together and compresses them which lead to their violent implosion. In the so-called “hot-spots”, this implosion/collapse produces high temperature (1,000 to 10,000 °K), as well as high pressure (100 to 1,000 bar), while in the liquid neighboring the cavity the temperature rises to 3812 °F.
This phenomenon is extremely rapid (the heating-cooling rate is approx. 10 °K/s) and is caused by a rapid adiabatic collapse that generates supercritical states such as these hot-spots [8, 9, 11]. These conditions of high temperatures and pressures determine the heterolytic dissociation of water in H and OH free radicals.
The acoustic pressure needed to generate the cavities depends on the nature/purity of the liquid, its viscosity, the volume of gas in the liquid, the level of hydrostatic pressure and temperature of the liquid [10, 11].
The phenomena produced during the acoustic cavitation generate [1–3, 5–9] the following effects:
chemical effects
like oxidation/reduction reactions (redox), syntheses, polymerization, depolymerization; the ultrasound influence concerns:
a) primary sonochemistry: reactions inside bubbles involving radicals and excited species;
b) secondary sonochemistry: diffusion of the radicals into bulk medium and then the chemical reaction with other reactants to form the reaction products.
Primary and secondary sonochemistry reactions are encountered inside the hot points due to their specific characteristics (extremely high temperatures and pressures).
sonoluminescence effects
: short lived light flashes (under 10
−8
s) produced during the cavity bubble implosions. The sonoluminescence can be explained based on different theories: theory of electric micro discharge, chemical or thermic/heat theory [10];
mechanical effects
of asymmetric cavitation appeared in liquid–solid systems: shear forces, shock waves and bubble jetting/microjets from collapsing lead to pitting, erosion, streaming and turbulent mixing. The shear waves create a microscopic turbulence in the interfacial layers around the neighbouring solid particles. The asymmetric cavitation leads to the formation of liquid micro-jets that collide with extreme force (due to their high speed, close to 400 km/h) with the solid surface of the polymer, producing surface corrosion and erosion and increasing the reactivity at surface level. Close to surface, these phenomena determine increased mass transfer rate, as well as increased reaction rate.
These determine changes in particles/surface distribution, morphology, chemical composition and reactivity. The level of the mechanical effects depends on the speed, acoustic energy parameters, nature and temperature of the cavitation liquid [1, 5, 10].
The cavitation phenomenon, as well as the implosion phenomenon, is influenced by different internal factors (properties of the solvent/liquid, the gas formed during cavitation and ultrasound characteristics) and external factors (pressure and temperature) [10, 11]. The cavitation phenomenon is positively influenced [10, 11] by the following parameters:
– low frequency, power of the ultrasound;
– greater intensity of the ultrasound;
– high vapor pressure of the liquid;
– low viscosity and surface tension of the liquid;
– high gas solubility formed during cavitation;
– ambiental factors: low pressure and high temperature.
The cavity collapse or implosion process [10, 11] benefits from:
– low ultrasound frequency (in kHz region);
– low liquid vapor pressure;
– low gas solubility;
– low gas thermal conductivity;
– monoatomic gases;
– higher ratio of specific heat of gas;
– high external pressure and low external temperature.
1.2.3 Types of Ultrasound Cavitation
Based on the type of bubbles, there are the following types of ultrasound cavitation:
transient or inertial cavitation
: the bubbles are either voids or vapors and are produced at intensities higher than 10 W/cm
2
. They last for one or more acoustic cycles and expand to at least double their initial size before collapsing suddenly at compression and often disintegrating into smaller bubbles. This step is too short to allow any mass flow due to gas diffusion inside or out of the bubbles [1, 3, 5, 10].
stable or non-inertial cavitation:
stable bubbles consist mainly of gas and some vapor, and can be produced at low intensities (1–3 W/cm
2
). The bubbles present a non-linear oscillation for several acoustic cycles and spread within the mass of gas, determining the thermal diffusion, respectively the evaporation and condensation phenomena. The differences between the mass transfer rates at the gas–liquid interface can increase the size of the bubble. In the expansion phase, the gas diffuses from liquid into bubbles, while the reverse phenomenon happens during compression and the gas passes into liquid again. The inward diffusion is bigger than the outward one and this determines the general growth of the bubble. As the bubble grows, the acoustic and environmental conditions change and the bubble becomes a transient one and collapses; its implosion is less sudden than in the case of a transient bubble containing vapors and the collapse is attenuated by the gas [1, 3, 5, 10].
1.2.4 The Action of Ultrasound on Polymers
In the case of solids, the ultrasonic waves propagate only as longitudinal waves and the mass flow of the particles in the solid medium is perpendicular to the wave direction [10].
When polymers are irradiated with ultrasound there are usually destructive processes of depolymerization, when the macromolecular chains get broken from place to place.
There are cases when the ultrasound waves have a positive effect, generating polymerization processes through free radicals or high temperatures. It is preferable to use a heterogeneous liquid–solid/polymer system for the irradiation of polymers. In these situations, the ultrasonic waves propagate as longitudinal waves through the liquid and as transversal waves through the solid/polymer medium [1–3, 10].
The ultrasound irradiation of a polymer immersed in a liquid generates:
– destructive effects (depolymerization) for high frequencies, in the 1 to 2 MHz range;
– (occasionally) non-destructive effects (polymerization) for lower frequencies, in the 15 to 500 kHz range.
The destructive effects are caused by the cavitation phenomenon and the friction and shear forces developed in the liquid due to the ultrasonic waves. More precisely, the friction forces appear at the interface between the polymer and the liquid mass and increase with the molecular mass of the polymer until they can destroy the bonds between the macro/molecules, generating the depolymerization process. If the polymer is characterized by a low degree of polymerization and molecular mass, then the depolymerization does not occur in the presence of ultrasound because the shear/friction forces are not enough to break the chemical bonds [1–3, 5–12].
The degradation of the fibrous polymer due to depolymerization may generate new species/formations (broken macromolecules, blocking copolymers, portions of polymeric chain, macroradicals, free radicals) that may have positive effects of polymerization in certain conditions: the presence of gases, the presence and the amount of oxygen freed through cavitation, secondary reactions taking place at the same time. The radicals are a consequence of the cavitation phenomenon and the high energy levels produced by the high temperature and pressure from the bubbles collapse. They can determine the degradation of bonds from water, gases or activate different parts of the polymer. These radicals and active species appear around “hot-spots” and the neighboring liquid on an area of approximately 200 nm and last for about 2 ms [11]. They react with each other to form new molecules, radicals, macroradicals or diffuse through the liquid as oxidants [10, 11]. At least one of the active radicals/new species can start the initiation processes for new reactions (grafting or polymerization) on the degraded polymer, as well as reactions accelerating the formation process of the new polymer. The initiation–acceleration–polymerization stages depend on the activity of radicals and macroradicals, the chemical nature of the polymer, parameters of the acoustic field (the intensity of the acoustic field must be at least 0.03 W/cm2) and temperature [10, 11].
In general, the efficiency of the ultrasound irradiation of a heterogenous liquid–solid/polymer medium depends on [10]:
characteristics of the acoustic medium: intensity, frequency and duration of the ultrasonic waves;
characteristics of the ultrasound irradiated liquid: nature and purity of the solvent, concentration, nature of dissolved gases;
characteristics of the polymer: chemical structure, molecular mass, degree of polymerization, strength of bonds within the macromolecules;
parameters of the cavitation process (temperature and hydrostatic pressure) that determine the depolymerization of the solid.
1.2.5 Applications of Ultrasound in Textile Finishing
Ultrasonic waves have a large range of applications: cavitation, acceleration, cleaning, degreasing, bleaching, catalysis, extraction, erosion, curing, deagglomeration, homogenization, mixing, drying, food dehydration, liquids processing, separation, sonoluminescence, streaming, surgical, biological cell disruption, surface processing, solubilization [1–3, 5–12].
With regard to textile production, ultrasound can be used for the cleaning processes, like washing and rinsing materials, as well as other wet processes (desizing, scouring, bleaching and dyeing). In any textile finishing process, the presence of ultrasound generates strong agitation of the solution, dispersion of chemical auxiliaries in the dyebath, intensification of the diffusion process and especially a strong deairing/degassing effect due to the elimination of the air/gas in capillaries [10, 11].
In the case of ultrasound-assisted wet processes, the most studied ones are:
– washing of textile materials [12];
– pre-treatments (scouring, bleaching) before dyeing [13, 14];
– extraction of natural dyes and subsequent dyeing [15–17];
– dyeing of textile materials with synthetic dyes [18–26];
– after treatments following dyeing [27–29];
– wastewater treatments [30, 31].
1.2.5.1 Ultrasound-Assisted Washing of Textile Materials
Textile materials are made of a natural or synthetic polymer which makes them (bi)porous viscoelastic materials. The presence of air in textile supports (in fiber capillaries, in yarn pores and in the free spaces characterizing woven and knitted fabrics) diminishes the mass transfer process through diffusion and convection [11, 12].
Still, Moholkar et al. [12] showed that the air between fibers and yarns plays an important role in the ultrasound-assisted washing process. Depending on the fabric’s resistance to flow and the physical properties of the standing waves, even the position of the textile materials in reference to the propagation direction of the ultrasound can alter the performance of the ultrasonic system, affecting the washing efficiency.
In the system formed by the washing liquid and the textile material, the use of ultrasound determines the formation of standing waves, increases the system’s energy consumption, as well as the generating regions with cavitation/non-uniform cavitation activity. This study [12] offers a simple method for the optimization of the total gas content of the system in order to optimize the intensity of the cavitation process.
1.2.5.2 Ultrasound-Assisted Pre-Treatments (Scouring, Bleaching)
Ultrasound-assisted pre-treatments of bast fabrics (jute and linen) lead to improved scouring and bleaching effects in comparison to the classic methods, with positive influence on the dyeing process with reactive and basic dyes [13, 14].
The use of ultrasound in the case of alkali scouring and bleaching of jute fabrics [13] determines a higher amount of lignin to be removed (confirmed by the Fourier-Transform Infrared Spectroscopy—FTIR, as well as a smaller fabric mass), increased hydrophilicity, higher whiteness index, lower yellowness index, but also decreased tensile strength. Compared to classic dyeing processes, the pre-treatments assisted by ultrasound have a positive influence on the behavior of jute during dyeing with reactive and basic dyes which is confirmed by the values recorded for color strength. Ultrasonic treatments facilitate the dye exhaustion and increase color strength (due to a higher diffusion of dyes). The study authors [13] remarqued that color fastness properties (to light, washing, and rubbing) were not influenced by the use of ultrasound in the pre-treatments applied.
In the case of woven linen fabrics, the influence factors and effects of bio-scouring (in the presence of laccase) combined with ultrasound-assisted hydrogen peroxide bleaching were compared to the results of the same process not assisted by ultrasound and also to classic bleaching [14]. The improved results of the ultrasound-assisted process (power up to 180 W and frequency 26 kHz) were explained through the cavitation phenomenon that generates hot-spots, high velocity microjets directed towards the fabric surface, and the transport of bulky enzyme macromolecules on the surface and inside the linen fabric. All these phenomena facilitated better bleaching, with lower ratios of enzymes and H2O2, while the joining of bio-scouring and bleaching in a single stage allows significant cutbacks in energy, time and water consumption. Furthermore, the X-ray diffraction analysis (XRD) indicated the modification of the crystallin/amorphous ratio in the samples subjected to ultrasound, the increase of the amorphous areas leading to increased hydrophilia. The dyeing was evaluated based on the comparison of the kinetic indices (half dyeing time, specific dyeing rate constant K and dye uptake) and the dyeing efficiency (calculated using the dyeing rate constant K, specific to the processes with/without ultrasound), as well as the values for color strength and fastness properties obtained for the samples pre-treated with laccase and H2O2 in the presence/absence of ultrasound. Better results were obtained for both reactive and basic dyes when the process involved the use of ultrasound [14].
Ultrasound-assisted pre-treatments decrease the fiber degradation while the qualities of textile support remain the same.
1.2.5.3 Ultrasound as Physical Stimulus for Textile Dyeing
In a wet process such as dyeing, ultrasound may act as a physical stimulus, with positive effects.
The effects ultrasound has on dyeing [18–25] can be evaluated by comparison with the results from the conventional method (heating method in the same conditions, without ultrasound), considering:
– color strength (determined using a spectrophotometer);
– exhaustion (calculated based on the absorptions, concentrations in the fiber and solution determined using UV–VIS spectrophotometer);
– unlevelness of dyeing (determined based on the statistical analysis of the color uniformity, determined for several points on the same dyed sample);
– fiber swelling (diameter after sonication, measured with a microscope);
– dyeing kinetics: dyeing rate, diffusion coefficient, dye-up-take per unit time, time of half dyeing;
– dyeing thermodynamics: affinity;
– surface morphology or fiber aspect in transversal cross-section (highlighted by Scanning Electron Microscopy, SEM analysis);
– fastness properties (resistances to washing and rubbing);
– internal organization/physical structure, based on crystallinity, d-spacing, crystallite size, lattice strain (highlighted by XRD analysis).
1.2.5.4 Ultrasound-Assisted Extraction and Dyeing Using Natural Dyes
The irradiation with ultrasound was applied for the extraction of natural dyes [13–15], as well as the dyeing of cotton [15, 16], leather and paper [17].
Different methods for the extraction of natural dye from Cochineal insects were considered—conventional extraction, without ultrasound, respectively methods using ultrasound with different sonic power (100–500 W), at different temperatures (50–80 °C) and for different durations (15–120 min) [15, 16]. The results took into consideration the particle size of partially soluble Cochineal dye extracted from the raw source and the color strength obtained after dyeing the initially cationized cotton material, with different concentrations (0–150 g/L) of Solfix E [15] or Quat 188 [16]. These cationization agents are actually polyaminochloro-hydrin quaternary ammonium polymer with epoxide functionality (Solfix E), respectively 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (Quat 188) that were applied to cotton using the pad–dry–cure method. In both cases, the ultrasound power level of 300 W ensured the best extraction of Cochineal dye, after dyeing resulting in a color strength value of 7.50 for cotton cationized with Solfix E [15] and 6.28 for cotton cationized with Quat 188 [16]. The color strength values obtained for the dyeing processes were compared considering the presence/absence of ultrasound, power, pH, salt concentration, dyeing time and temperature. In all cases included in the comparison, the dyeing processes with ultrasound present the best color strength values, while the fastness properties varied, from fair to good, similar to the conventional process [15, 16]. The authors stated that XRD and SEM analyses can show a set of effects produced by ultrasound, increasing the dyeing affinity for natural dyes. The X-ray irradiation of the cationized cotton indicated modifications/redistributions within the cotton yarn, highlighted by the decrease of crystallinity, crystallite sizes and increasing of d-spacing between crystallites. Following ultrasonication, the crystallinity of the samples cationized with Solfix E decreased with approx. 7%, while for the samples cationized with Quat 188 the decrease is 6%. The ultrasonication slightly modifies the morphology of the cationized fiber surfaces, causing roughness, small cavitated pores that can facilitate the binding of the dye to the fibers.
In a different study mentioned in the literature [17], another natural dye, betalain, was extracted from red beetroot using ultrasonication varying the following parameters: ultrasonic output power (40–100 W), time (0–210 min), pulse mode (with–without pulse), effect of solvent system (ethanol:water ratio from 1:1 to 3:2) and amount of beetroot (1–8 g).
The results of the extraction were then compared to those obtained using control static/magnetic stirring processes, at 45 °C. When compared to the methods that did not use ultrasound, the ultrasound-assisted extraction improved the extraction efficiency by 8% (% yield of dye). The conditions for this improvement were: ultrasonication at 80 W, with pulse at 2 s, process time 3 h in the presence of ethanol as solvent (the ethanol:water ration is 1:1). The ultrasound determines the destruction of the walls of vegetal cells, freeing and immediately transporting the betalains toward the exterior due to the cavitation produced in the heterogeneous medium (liquid-beetroot) and its secondary effects: erosion, micro-jets streaming and turbulent mixing.
The extracts containing 8% dye, obtained in the presence of ultrasound, respectively static/magnetic stirring were used to dye leather and paper, for 7.5 h.
The dyeing effects were quantified using the exhaustion values, CIELAB measurements and fastness properties. For dyed leather, the exhaustion was 62% when the process was assisted by ultrasound and 49% when magnetic stirring was used. The red colors (with a* >0) of the dyed materials using ultrasound had lightness values of L* = 11.1 for leather and 21.3 for paper, while dyed materials using magnetic stirring presented values of 16.0, respectively 22.3.
Still, all dyed materials presented similar fastness properties.
Both the extraction and dyeing of different materials can be carried out in a green manner using ultrasound to boost the process. Other advantages of the ultrasound include:
– eliminating environmentally unfriendly chemicals and solvents;
– ultrasound creates conditions to increase the efficiency of wet processes, as it acts directly on the liquid and “dye raw source” from the heterogeneous water–plant material system;
– it minimizes the energy consumption;
– it is an efficient and rapid alternative for the extraction of natural dyes.
1.2.5.5 Ultrasound-Assisted Dyeing Using Synthetic Dyes
The ultrasound-assisted dyeing using synthetic dyes was studied for different textile supports: wool [18–20], cotton [21], lyocell [22], cellulose acetate [23] and polyacrylonitrile fibers [10, 24, 25].
Based on the color strength values, these studies [10, 18–25] confirm that the use of ultrasound as a physical stimulus leads to better results, explained through the sum of phenomena/effects that take place in a dyebath “stimulated” by ultrasound.
In general, the improvements produced by the irradiation with ultrasound are considered to be produced by the cavitation phenomenon and other physical effects, such as:
– dye dispersion due to breaking and prevention of the formation of dye aggregates with relatively high molecular mass;
– degassing, meaning the expulsion of the gases dissolved in the dyebath, as well as the air trapped between fiber capillaries;
– fiber swelling that leads to a faster diffusion rate for the dye inside the fiber.
– strong stirring of the liquid that determines the reduction in thickness of the fiber–liquid interface facilitating the coming together of dye molecules and their adsorption on the fiber surface.
1.2.5.5.1 Ultrasound-Assisted Dyeing of Wool
The studies carried out on wool had different objectives:
– the testing of the effects of some pre-treatments assisted by ultrasound on the color strength obtained after the classic dyeing with reactive dyes (Lanasol class) or acid dyes of high, respectively low levelling (Tectilon, respectively Sandolan class) [18];
– identification of the lowest dyeing temperature that, under ultrasound stimulus, will give color strength close to the one obtained for the classic dye at 98 °C [19, 20].
The authors [18] suggested that the ultrasound-assisted pre-treatments (35–39 kHz, 10 min, 40 °C) led to good effects for degreasing and scouring, as well as they had a positive influence on the behavior at bleaching. Still, part of the thioester linkages (-S-S-) in the wool broke during ultrasonication, transforming into sulphydryl groups (-SH).
Ultrasonication determined an improvement of the cleaning effect for the wool, but had no influence on the color obtained after dyeing, especially for the acid dyes that were studied; in their case, the specific values were similar to the ones recorded for the dyed samples not subjected to ultrasound.
This study indicated that the ultrasound stimulation of certain wet processes will generate less consumption of chemicals and energy with a positive impact on the industry and the environment. Cut-backs in energy (thermal) can also be the result of an ultrasound-assisted dyeing process that requires lower temperatures (40–80 °C) than the classic process (98 °C) [19, 20]. Based on these comparisons, the authors of the study [20] concluded that by dyeing wool at low temperatures (40–70 °C) the energy savings will be 69% when the temperature is 40 °C and 28% when the temperature reaches 70 °C.
The effects of the use of ultrasound in wool dyeing were theoretically, as well as experimentally evaluated as follows:
– through kinetic and thermodynamic indices/characteristics [19], respectively through the “maximum implosion pressure in a bubble” that can be correlated with cavitation intensity [20];
– through dyeing with acid leveling dyes at different low temperatures (60–80 °C [19], respectively 40–70 °C [20]) and color evaluation with color strength, measuring DE (color variation) in comparison to the samples without ultrasound treatment, Re% (reflectance percentage) and color fastness.
The results presented in these studies confirm the synergetic effect between sonication and kinetics of wool dyeing [19], indicating that the temperature of the dyebath influences the level of the cavitation intensity [20]. In the two studies [19, 20] the optimum temperature for the ultrasound-assisted wool dyeing was found out to be 60 °C. The wool samples were dyed with high color strength, uniformity and good results for dyeing fastness to domestic laundering, even if the dyeing was ultrasound stimulated in two different manners:
a) ultrasound at 37 kHz frequency and 150 W power level, time 110 min, no leveling agent in the dyebath [19];
b) ultrasound at 25 kHz frequency, 600 W power level, using a leveling agent [20].
Furthermore, the SEM analysis indicates a non-destructive ultrasound effect on wool at 60 °C and just a slight destructive influence when the temperature reaches 80 °C [19]. The ultrasound dyeing efficiency is the result of the micro-bubbles generated by the cavitation phenomenon that depends on temperature, system hydrodynamics, placement of the sample from the ultrasonic transducer.
The selection of the optimum temperature for ultrasound-assisted dyeing must take into consideration the influence of temperature on the cavitation intensity and kinetic of dye diffusion in the fiber, as follows:
– the intensity of cavitation decreases with the increase in temperature;
– the kinetics of dye diffusion in fibers is favored by system temperature [20].
1.2.5.5.2 Ultrasound-Assisted Dyeing of Cellulosic Materials
The results concerning the ultrasound-assisted dyeing of cellulosic materials refer to: cotton [21] and 2 semi-synthetic fibers, Lyocell [22] and cellulose acetate [23].
The efficiency of ultrasound treatment (40 kHz frequency and 120 W power) for cotton dyeing was studied [21] considering its application in 3 stages:
pre-treatments
:
a) cleaning through irradiation of the cotton woven sample in water, continuous ultrasonication (0–30 min) followed by classic dyeing;
b) irradiation of the dyebath (10 min, without the presence of the textile material in the dyebath) followed by dyeing;
dyeing
for 30 min using 2 reactive dyes, at 40 and 50 °C, using continuous and intermittent sonication;
fixation period:
maintaining all dyeing parameters constant for 40 min.
For comparison purposes, a control classic dyeing (no ultrasound) was done for each dye, modifying only the process time: the selected dyeing period was 40 min, while the fixation was carried out in 50 min [21].
The acceleration efficiency of ultrasound was explained by the authors considering the dye transfer during the dyeing process. This way, the mechanism that leads to an acceleration efficiency of the ultrasound in a continuously irradiated dyebath was explained based on the degassing, deairing and dye mass transfer/transport phenomena:
1) degassing involves the expulsion of the dissolved gases in a solution and/or of the air trapped in fibers;
2) dispersion of the dye aggregates that allows a uniform dyeing, with no agglomeration of dye molecules;
3) the dye mass transfer/transport inside the cellulosic fibers, through diffusion;
4) intense agitation of the liquid.
If the first two phenomena facilitate a more rapid adsorption, the dye mass transfer/transport determines a slower diffusion process. Diffusion is caused by the breaking of the protective/contact layer of liquid covering the fibers and the penetration of dye molecules inside the fibers. This way, the bonding between the reactive dyes and fibers is stronger, leading to color uniformity [21].
In continuous sonication, the maximum accelerative efficiency of ultrasound was obtained during dyeing, while the minimum value was recorded for the fixing stage.
Pre-treatments produced the best values for exhaustion and color strength when they lasted only 10 min.
The dyeing in intermittent ultrasound field (1 min time interval) showed the best values for exhaustion and accelerative efficiency. The authors suggested that, after a short time, the intermittent action triggers a more rapid implosion of the micro-bubbles, therefore a more efficient and more accelerated action of ultrasound [21].
Ultrasonic waves were also used as stimulus for dyeing processes of a biodegradable regenerated fiber, Lyocell, with reactive dyes [22]. This fiber is used as a substitute for cotton; as it contains regenerated cellulose, Lyocell can be considered a semi-synthetic fiber. Using ultrasound at 180 W power, 53 kHz frequency and up to 0.8 W/cm2 intensity in the dyebath, the effects of ultrasound and dyeing parameters (temperature, time, concentration of chemical auxiliaries in the dyebath) on color strength values, exhaustion, dye fixing on the Lyocell fibers, color fastness and surface morphology (using SEM) were studied in reference to the classic dyeing method. The most significant results obtained with the ultrasound-assisted dyeing of Lyocell [22] were as follows:
– optimum values for color strength of 15.83 respectively 17.12 for Lyocell dyed with the 2 dyes (CI Reactive Red 195 and CI Reactive Blue 250), higher than the corresponding values when the heating method was used;
– improvement in color yield (superior to 40%);
– higher dyeing performance: approx. 7% higher than the conventional method;
– dye fixation values of 86.95% respectively 86.34% for the 2 dyes, superior to the values obtained using the heating method;
– improvement in dye fixation (>17%);
– higher fixation performance: approx. 5% higher compared to the conventionally dyed samples;
– energy savings, due to the dyeing temperature, reduced with 10 °C compared to the classic method, while the time for process equilibrium was 45 min, much shorter than for the conventional dyeing (75 min);
– cut-backs in the consumption of chemical agents in the dyebath due to the intensification of agitation, deairing and mass transport;
– maintained integrity of the surface morphology, confirmed by the SEM analysis;
– in the case of indices evaluating the pollution caused by the effluents, the ultrasound-assisted dyeing is characterized by lower values in reference to the classic process, resulting a considerable reduction of the environmental impact: 15–18% COD (Chemical Oxygen Demand) and 32–36% TDS (Total Dissolved Solids)
– good color fastness properties.
These results justify the use of ultrasound for Lyocell dyeing as a better, more efficient and more environmentally friendly option in comparison to classic dyeing [22].
It is known that the cellulose acetate is actually acetate/ester of cellulose, being considered an artificial/semi-synthetic fiber due to its semblance to polyester in requiring the use of dispersion dyes and several chemicals (dispersion agent, leveling agent, acid medium) for the dyeing process.
The study [23] demonstrated a synergistic effect of sonication on the dyeing kinetics (half dyeing times and absorption rate constants K); using the ultrasound irradiation of the dyebath (37 kHz frequency and 150 W ultrasound power), the exhaustion obtained was 80% when dyeing for 90 min, at 80 °C with the disperse dye Disperse Red 50, without any pollutant chemical agents. The same values are obtained after 120 min using the heating dyeing. Therefore, the results indicate that ultrasound can improve dye dispersion, mass transfer and dye diffusion without significantly modifying the surface structure of the fiber.
1.2.5.5.3 Ultrasound-Assisted Dyeing of Polyacrylonitrile Fibers with Synthetic Dyes
In the polyacrylonitrile (PAN) fiber–cationic dyes system, the two components have opposed ionic charges, fact that determines high dyeing rates. The high attraction between the anionic groups in PAN and the cationic groups in the dyes increases the risk for non-uniform dyeing/stains. This problem can be eliminated either by the strict control of the dyebath or the addition of equalizing anionic agents [10]. The ultrasound stimulation of the PAN dyeing is required only when the focus is on reducing the dyeing time/temperature [24] or reducing/eliminating dyeing non-uniformity (color unlevelness) [10, 25].
In these two situations ultrasound was used as a stimulus for pretreatments/cleaning [10, 25] or dyeing [24], as follows:
Popescu [10, 25] pre-treated PAN fibers of Romanian origin with ultrasound (19.5 kHz frequency and 500 W power) for 5, 15 and 30 min, in 100 mL distilled water, after which a conventional dyeing with C.I. Basic Yellow 21, respectively C.I. Basic Blue 86 was applied; the reason for this association of stages (pre-treatment with ultrasound, followed by conventional dyeing) was to use ultrasound to modify the physical structure of PAN in the first stage, so that in the next stage the dyeing will be more level, even without chemical auxiliaries or retarder added to the dyebath;
Kamel
et al.
[24] irradiated the dyebath with ultrasound (38.5 kHz and power ranging from 100 to 500 W) in order to study the influence of the dyeing parameters (ultrasonic power, dye concentration, pH, dyeing time and temperature) on the color strength values for the Egyptian PAN samples dyed with C.I. Astrazon Basic Red 5BL 200%.
The behavior of PAN under the action of ultrasound was evaluated based on the analysis of the following dependencies:
– for the Romanian Melana PAN fibre: the influence of the ultrasound treatment time on the fiber diameter after sonication, affinity, dyeing rate, diffusion coefficient, exhaustion, unlevelness and color strength values [10, 25];
– for the Egyptian PAN fiber: the influence of the ultrasonic power on the color strength and dyeing kinetics (dye-uptake per unit time, time of half dyeing t
1/2
, affinity) [24].
Furthermore, analyses such as SEM and FTIR were used in Refs. [10, 25], while Ref. [24] considered SEM and X-ray diffraction (XRD).
The ultrasonication of the acrylic fibers before dyeing [10, 25] was proved to determine internal modifications of the physical structure due to the attempt of the air trapped inside the fiber to get out, stimulated by the cavitation phenomenon; this leads to an increase of the fiber diameter with the ultrasonication time, without exceeding 15 min (Figure 1.1). Therefore, the 15 min time represents the optimum sonication duration, as the SEM analysis showed when comparing the transversal cross-sections of the sonicated PAN fibers. In the first 15 min of ultrasound treatment, the air-filled voids within the fiber united and moved from the fiber center (after t = 5 min) toward its exterior (t >15 min). After another 15 min (t = 30 min) there was a decrease of the fiber diameter due to the deairing process of the acrylic polymer, but small cracks appeared at the fiber surface, creating ways for the dye to penetrate the fibers during dyeing.
Figure 1.1 Transversal cross-section and longitudinal aspect of Romanian acrylic fiber Melana, before and after 15–30 min ultrasound treatment.
