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RAISING THE BAR FOR SUSTAINABLE TEXTILE DYEING S. Aishwariya and S. Thamima
Assistant Professor and PG Student, Department of Textiles and Clothing,
Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, India.
Abstract
Unprocessed raw textile has no market value. The grey cloth made through weaving and
knitting is expected to pass through stages of treatments with water and chemicals that
possibly improve their value. This processing involves synthetic and toxic materials that
are making the entire supply chain, non-sustainable. The industry, however, has tried on
replacing the conventional methods that use more energy and water resources to
techniques that result in lower carbon footprints This paper deals with the road map to
sustainable dyeing technologies and focuses on dyeing based on ultrasonic waves, critical
carbon dioxide, microwaves, and the inkjet printing technology. The process and benefits
of each are discussed in this paper.
Keywords: ultrasonic dyeing, critical CO2 dyeing, microwave-assisted dyeing, inkjet
printing technology, sustainable dyeing, eco-friendly dyeing, textile processing.
IntroductionTextile coloration is an integral part of the value addition of textile material. Dyeing
and printing increase visual beauty, aesthetic appeal, and uniqueness of the fabric. The
textile design and manufacturing are crafted with colors that help in fixing the brand image,
suit a particular season, age group, geographical positioning, and also the pricing. It is
obvious that dyed materials are more preferred and the application of chemicals to make a
fabric more appealing, costs human life and environment. Textiles is the second largest
polluting textile industry and this realization has paved the way to shift its entire supply
chain towards eco-friendly alternatives. Some of the core concepts like ultrasonic dyeing,
critical CO2 dyeing, microwave-assisted dyeing, inkjet printing technology are discussed in
this paper. Textile is one of the largest consumers of freshwater, which is used in dyeing
and cleaning. In this era of water scarcity and water pollution issues bombarding, textile
processing is aimed at waterless processing. Increased expense in sourcing water, the
complexity of the textile wastewater, strict legislation on expelling treated water into the
river bodies have been added factors for looking at alternatives.
1. Ultrasonic dyeing Sound can be defined as the vibrations that travel through the air or any medium
which on reaching human or animal ears is heard. Sound waves can be classified as
infrasound (16 kHz up to 106 kHz), audible sound (up to 16Hz), and ultrasound (20 Mhz -
500Mhz) (Figure 1). Among these, the oscillating waves with frequency bigger than the
maximum limit of human hearing, which makes them inaudible to the human ear is called
ultrasonic waves. The interesting property of these waves is its ability to reflect or refract
like light waves. They can travel through a vacuum, create vibrations in low viscosity
liquids, move with the speed of the sound, and travel with uniform velocity in a
homogenous medium. It can mold metal and plastic. The researchers have found that bats
and frogs use these waves to communicate between them, especially the bats emit high
pitch sound in low frequency, to migrate from one place to another. The ultrasonic
detectors which are employed in calculating distances use the same principle.
Figure 1. The spectrum of Sound Waves
Conventionally, ultrasonic waves a variety of applications in everyday life (Figure 2).
They are used for cleaning devices made of glass, metal, and ceramic to remove the dirt,
chips, oil, and grease. The aircraft sludge and lubricants are usually removed by the use of
ultrasonic waves. In the medical field, Sonography is a technique that helps in capturing
images of the body’s internal organs, which is done using ultrasonic waves. In textile
dyeing, the conventional methods used huge tones of water and energy which had various
allegations which made the way for sustainable solutions. Repeated dyeing to achieve the
better shade that consumes huge quantities of water and energy was a challenge.
In 1941, an alternative technology using ultrasonic waves was designed and textiles
were soaked, washed, and cleaned using these waves. There were studies conducted on
checking the effect of dyeing synthetic materials like polyester and acetate using direct
and reactive dyed with the help of ultrasonic waves. The experiment involved the use of air
bubbles in the dye bath along with the waves. It was observed that ultrasonic dyeing of
textiles helped in increasing the dye penetration on to the fibers, which resulted in better
fastness property. Besides, it does not harm the surface of the fibers, that was an added
advantage. Spectrophotometry was used in evaluating the dye penetration in the fabric.
The process was fast, simple, and safe which suggests its sustainable nature. The
benefits in the processing were less quantity of dye, increased dye absorption, uniform
distribution of dyes on the material, quick processing time, less power consumption, less
pollution with dyed water. This makes it economically and ecologically the best alternative
solution. Less dye, less dyeing time, and low dyeing temperature are advantages of
ultrasonic dyeing. The effect is more in hydrophobic, fibers dyed with insoluble dyes than
hydrophilic with ionic dyes. It is also observed that the rate of dyeing, increased dye
uptake, the effect is more on coated textiles. Ultrasonic cavitation speeds up the rate of
dye diffusion and allows dye penetration more rapid, the sonication speeds the reaction
between dye and fiber. Dispersing is uniform due to the sonic waves and the uniform
dyeing was possible even at a low temperature of 30ºC, which is 230% more than color
strength compared to conventional dyeing.
Figure 2. Application of Ultrasonic waves
Methods of producing ultrasonic waves a. Formation of cavitation: Ultrasound energy is sound waves with frequencies above
20,000 oscillations per second. In liquid, these high-frequency waves cause the formation
of cavitation (microscopic bubbles) and insignificant heating of the liquid. In textile dyeing,
ultrasonic waves are absorbed in the liquid system and the cavitation/bubbles are created.
This can liberate entrapped gases from liquid or materials like textiles, dye bath.
b. Compression or rarefaction: There is a compression/ rarefaction during each cycle of
a wave. When ultrasonic waves are absorbed in the liquid system, the phenomenon of
cavitation takes place, which is an alternative wave formation, the oscillation of tiny
bubbles/ cavities. When the bubbles collapse, they generate tiny but powerful shock
waves into the liquid. In a dye bath, the vibration of waves makes compressions or
rarefactions. This creates minute vapor bubbles of 500 nanometer in size, which can
collapse and cause shock waves throughout the bath
c. Streaming: The waves push the water along with the bubbles producing a flow of water
called streaming away from the sound source. The two phenomena attributed to
ultrasound are the rapid movement of liquids caused by variation of sonic pressure which
subjects the solvent to compression and rarefaction and micro streaming. Simultaneous
formation and collapsing of tiny air bubbles result in a large increase in pressure and
temperature at the microscopic level. If the bubbles collapse in textile materials, it will
result in the formation of high-velocity micro-jet particles with high velocities directed
towards the solid surface. These microjets can give rise to intra yarn flow, increase in the
rate of the mass transfer between the intra-yarn and inter yarn pores. On the other hand,
they may be carried along with the sound waves if they do not collapse immediately. This,
in turn, pushes water along with the bubbles producing a flow of water called streaming
away from the sound source.
Textile processing using Ultrasonic wavesVarious studies have reported the use of ultrasonic waves in dyeing natural,
synthetic, and regenerated textile fibers. The studies on using ultrasonic waves for dyeing
of wool is quite interesting. They have shown positive effects on the dyeing behavior and
increased dye exhaustion rate on dyeing woolen fibers with acid dyes. It could be due to
the reduced boundary in liquor surrounding the fiber which is the characteristic feature in
the ultrasonic irradiated environment. Low dyeing temperature for wool can be more
effective using ultrasonic waves, the resultant fiber has a good wash and fastness
property. In textile finishing, the herbs are coated on textile materials for its medicinal and
novel properties. In this scenario, the herbal extraction can be done by the use of
ultrasonic waves. Cavitation in the liquid is good with the use of these ultrasonic waves
however the pH, salt concentration, time, pressure, and power of ultrasonic waves will
affect the finishing process. In another study, natural dye lac with varying percentages of
dye, temperature, and power resulted in a different amount of dyeing material. A dried and
grounded sample mixed with methanol and placed in an ultrasonic bath for 30 minutes.
The temperature was increased from 20- 60 °C after an hour. The process was repeated a
few times to get the required amount of extract. This process is based on producing colors
by addition and subtractions of textile materials by ultrasonic waves. Unlike gases and
liquid, in solids both longitudinal and transverse waves are transmitted. The effects of
ultrasonic arise when sound is circulated through the medium. In liquids, longitudinal
vibrations of molecules generate compressions and rarefactions, i.e., areas of high and
low local pressure. The latter results in the formation of cavities, i.e., very small vapor
bubbles of 500nm in size, which can collapse and cause shock waves throughout the
medium.
Figure 3. Ultrasonic waves in washing textiles
Ultrasonic waves are used in the washing of fabrics using ultrasonic vibration,
cavitations, and transducer working on the surface of the material (Figure 3). The
advantages are deep and effective cleaning, low energy requirement, less water, less
detergent, fewer accessories and does not harm the fabric. The merits of ultrasonic dyeing
are its ability to save energy by dyeing a low temperature which enables in power
consumption. The reduction in consumption of dyes and synthetics contributes less load to
the effluents. Reduced processing time, reduced usage of auxiliary chemicals,
enhancement of color shade, reduced processing cost, improvement in quality, and easier
process control. The demerits of ultrasonic dyeing are the difficulty in producing high-
intensity ultrasound waves in the chamber. To conclude, the major idea of using the
ultrasonic dyeing is to improve dye productivity, washing fastness, reducing both energy
cost and water usage.
2. Supercritical Carbon dioxide Dyeing (SC-CO2 Dyeing)Carbon dioxide is available in abundance, ecologically harmless, non -toxic, and
non-explosive. A supercritical fluid has both the gaseous property of being able to
penetrate anything and the liquid property of being able to dissolve materials into their
components. Supercritical fluid (SCF) denotes highly compressed gases that combine
properties of gases and liquids intriguingly. It is a substance that can either be liquid or
gas, used in a state above the critical temperature and pressure where gases and liquids
can coexist. This shows unique properties that are different from those of either gases or
liquids under standard conditions (Figure 4). Supercritical fluid carbon dioxide (SCF) is
obtained by heating carbon dioxide above 31º C (88° F) and pressurizing it. In this stage, it
becomes supercritical which is an expanded liquid or a heavily compressed gas. Carbon
dioxide is the most commonly used supercritical solvent, which is made in excess by
industry with high purity. It is inexpensive, non-toxic, non-flammable and has a near
ambient critical temperature (31°C). The high density and good diffusivity make
supercritical CO2 a very good and popular solvent. High performance, economic,
sustainable, ecofriendly is its added merits. The SC-CO2 dyeing is mainly used for
synthetic dyes but also proved effective for natural dyes.
Figure 4. Properties of Supercritical fluid
The process eliminates water consumption, effluent generation, reduced energy
consumption, fewer air pollutants, faster dyeing, no need for surfactants and chemicals
with the ultimatum, which is 95 percent of the CO2 used in the process can be recycled for
repeated use. The demerits are high cost of installation and process is done as batches.
The dye solubility requires high temperature and pressure. The entire process is now
focused on synthetic textiles however it is found to be working best on polyester.
Figure 5. Advertisement of DyeCoo promoting Waterless campaign
Supercritical carbon dioxide is used in various industries like Food and
Nutraceuticals, Perfumes and Cosmetics, Pharmaceuticals, Electronics, Aerogels,
Ceramic and innovative Materials, Oil and gas, Waste treatment, and Waste valorization.
The decaffeination of coffee, extraction of natural flavors, isolation of compounds for
fragrances is some examples. In textiles, a Netherlands based DyeCoo Textile Systems,
was the first in the world to have made a commercial success in treating synthetic textiles
with CO2 and without water (Figure 5). At the end of research for more than eight years,
Nike has adopted to use recycled carbon dioxide to color synthetic textiles that have
proved to reduce effluent to a great extent. Energy consumption is found to be reduced by
40% and color consistency is 98%. Statistics quote by 2015, 39 million tons of polyester
will be dyed, whereas processing one kilogram of textile consumes 100-150 liters of water.
When the goal of the company is to conserve water, energy, and chemical consumption
such techniques are handy.
Figure 6. Steps involved in Supercritical Carbon dioxide Dyeing
Textile Dyeing using SCF-CO2The textiles to be dyed are wrapped on a dyeing beam to achieve an equal dyeing
result. The dyeing beam is placed in the dyeing autoclave and the dyestuff is filled into the
receiver. When the pressure vessels are closed, carbon dioxide is fed in several stages
which are explained in Figures 6 & 7.
a. Pretreatment: In the first step the textiles are cleaned from pollutants and sticking
auxiliary materials from the production because materials like wax, oils, and other
hydrophobic substances can disturb the dyeing process. With the pressurization pump
liquid CO2 from the buffer tank is compressed to supercritical pressure and heated up in
the heat exchanger to supercritical temperature. The supercritical CO2 flows through the
textiles in the dyeing autoclave and, besides, solves carefully all sticking pollutions from
the fibers. The loaded CO2 flows via an expansion valve and becomes by the pressure
decrease gaseous. Thereby the solution power is reduced and the extracted pollutions
precipitate and are collected in the separator. The purified CO2 is liquefied in the
condenser and is led via the buffer vessel back into the circulation.
Figure 7. The schematic diagram for Dyeing using Supercritical Carbon dioxide
b. Dyeing: After the pre-treatment the actual dyeing process begins by switching off the
dyestuff receiver into the CO2 circulation. The supercritical CO2 solves the dyestuff in the
dyestuff receiver and flows through the dyeing autoclave. The CO2 loaded with dyestuff is
delivered through the textiles and the dyestuff is adsorbed in the fibers. After the dyeing
autoclave the CO2 flows through a filter to the circulating pump and afterward is fortified in
the dyestuff receiver with fresh dyestuff and is led as long as in the circulation, until the
desired dyeing intensity of the textiles is achieved.
c. After Treatment: After finishing the dyeing step the CO2 circuit and dyed Material are
cleaned from excess dyestuff. Therefore, the dyestuff receiver is taken out of the CO2
circuit and the loaded CO2 is expanded via the expansion valve into the separator. The
excess dyestuff precipitates fall out and are collected in the separator. The CO2 is
circulated as long as the plant and the textiles are cleaned from the excess dyestuff
leftovers. After finishing the complete dyeing process the CO2 circulation is stopped and
the dyeing autoclave is depressurized to atmospheric conditions. The dyed textiles are
taken out of the autoclave.
In a nutshell:
The test begins in a given condition, the gas-like diffusion of supercritical CO2
happens where the dye is evenly dispersed into the pores and crevices of the fiber.
The dyestuff is fed in the autoclave, and the dyeing instrument is flushed with liquid
CO2.
The preheated liquefied CO2 absorbs the dye and performs dyeing operation (like
solvent dyeing). With the increase of pressure CO2 becomes gaseous and loses its
dissolving capacity. The residues of dye are then separated after liquefication.
Carbon dioxide which is free from dye goes back into the collecting tank after the
dyeing process. The circulation of CO2 is stopped and the dyeing autoclave is
depressurized.
The unused dye powder is seen to have deposited at the bottom of the machine.
The technique does not produce any drainage and is considered a sustainable
solution for conventional dyeing.
3. Microwave-Assisted DyeingMicrowaves are of 3 and 300 GHz frequency with a corresponding electrical
wavelength between λ = c/f = 10 cm and λ = 1 mm, respectively. They were popular during
the Second World war as an important part of radar systems. The use of radiofrequency
heating using microwaves was discovered in 1946 and today more than 60 million homes
have a microwave oven in their houses. Microwave radiation spreads inside a matter
similar to the light waves. They are reflected by metals, absorbed by a dielectric material,
and implemented through other materials without any loss of energy. The organic solvent
can absorb microwave radiation, while ceramic, quartz and most thermoplastic material
absorb microwave irradiation to moderate levels.
Figure 8. Microwaves in the electromagnetic spectrum
The use of microwaves, in dye extraction and application, is termed as microwave-
assisted textile dyeing. This is done using the dielectric and thermal properties of the
solution. The dielectric property refers to the intrinsic electrical properties that affect the
dyeing by dipolar rotation of the dye and influences the microwave field upon the dipoles.
The aqueous solution of dye has two polar components, in the high-frequency microwave
field oscillating at 2450MHz. It influences the vibrational energy in the water molecules and
the dye molecules. The thermal (heating) mechanism is through ionic conduction, which is
a type of resistance heating. Depending on the acceleration of the ions through the dye
solution, the collision of dye molecules occurs within the molecules of the fiber. The
mordant helps in the penetration of the dye resulting in superior dyeing compared to
traditional methods. This is an effective method of dyeing small amounts of fiber or fabric
in the microwave using reactive dyes.
Figure 9. Comparing the conventional and microwave heating method
The dry yarn or fiber to be dyed is weighed and thoroughly soaked the fiber/yarn
into the dyebath (overnight soaking or one hour in hot water). The dye powder is weighed
and mixed ¾ of a cup of hot water and stirred thoroughly. A wide shallow microwave-safe
container such as pyrex is taken and the fiber is distributed to cover the base in one-inch
thickness, a spoon can be used for even distribution of fibers. The content is covered
loosely and cooked for 6 minutes, after which it is allowed to cool. All the dye should be
absorbed into the fiber leaving just cloudy water. Rinsing in cold water and drying in shade
is done. Uniform dyeing is obtained however in some cases of yarn dyeing white streaks
are seen, which will be a defect in dyeing. Uneven depth of color, more amount of dye in
the water are other drawbacks.
Both mordant and dye extraction can be done using microwaves, which is a clean
source of energy ad easily made available. In a particular study, the stock solution of
0.5%, 1%, and 2% will be prepared by boiling 0.5g, 1g, and 2 g of mordant in 100 ml of
water at 90 degrees Celcius and above, checking the best temperature to extract mordant.
It is then subject to microwave at 900 W power supply. The extract is filtered and used as
a mordant. The maximum adsorption wavelength and optical density of both dye solution
and mordant were checked using a single and double beam spectrophotometer. The
dyeing was done with the liquor and material ratio to be 30:1. The fabric in the mordant
solution was heated to 90 degrees for one hour. After pre-mordanting, excess mordant
was squeezed and dyed in the same Rota Dyer machine for one hour at 90 degrees. The
dyed fabric is washed in cold water followed by testing of wash and light fastness property
to characterize the material. The technique demonstrated the quick extraction of dye
however increase in shade was not obtained.
Microwaves will be able to start chemical reactions through selective heating, which
results in new materials to be formed, which is not evident in conventional processing
techniques. These waves are used in making composites, ceramics, polymers, minerals,
and powders. Microwaves use the selective heating technique by which the energy wasted
can be minimized. Homogenized heating, power consumption, and reduced operational
time are the merits of using microwaves in textile processing. The high cost of the
equipment, limited applications, variation in dielectric properties with temperature, and
ingrained inefficiency of electric power are some of the demerits of the technique. This is
however less popular at industrial scale and usually done in the lab, or for sample dyeing.
4. Ink Jet or Digital Printing
Figure 10. Inkjet printing machine
The domestic printer helps in printing any image from the computer to a paper. This
is of different types like a laser, inkjet, etc. The same technology is also in printing designs
on textiles and termed as digital printing/ inkjet printing. It is with the advent of digital color
printing various design possibilities have opened up and the technique seems to perform
better than conventional screen printing. Inkjet is a technology wherein there is no printing
master and hence only the ink drops make contact with the substrate. It is therefore
classified as a non-impact printing method. Digital printing includes pre-treatment of the
fabric before the printing process. Pre-treatment of textiles in preparation for ink-jet printing
is carried out because the inclusion of auxiliary chemicals and thickeners into the low
viscosity ink has proved troublesome. Thus, the methodology is akin to `two-phase'
conventional printing as opposed to the `all-in' approach. In the latter case all the dyes,
chemicals, and thickeners required are included in the print paste, whereas in the former
some of the ingredients, particularly chemicals, are applied before or after printing.
Figure 11. Digital printed textile as upholstery (Sofa cover and cushions)
When printing cotton the choice has generally been between reactive dyes and
pigments. The pigment printing process is simpler, as it involves three main stages (print,
dry, bake/cure), whereas reactive printing has two extra processes (print, dry, steam,
wash-off, dry). Pigment printing is therefore a more economical procedure but we avoid
the use in inkjet printing because pigments produced much duller shades than could be
achieved with dyes, and there was a tendency for nozzles to block, in other words, the
`run-ability' was poor. Reactive printing by the `all-in' method is the normal approach for
screen printing, but for jet printing it has certain dangers. As a result, the jet printing of
cotton, wool, and silk has generally been carried out by the `two-phase' method, the ink
containing only purified dyes, the thickener, and chemicals being applied to the substrate
in a pre-treatment. Although the quality of the resulting prints is excellent, the extra
expense of pre-treating the fabric by a pad/dry process makes the process uneconomical
for anything but short runs.
Figure 12. Screen Printing
The main reasons for separating the dye ink from thickeners and other chemicals and
applying them separately to the fabric. 'All-in' inks are less stable and have lower storage
stability, e.g. reactive dyes are more likely to hydrolyze when alkali is present in the ink.
Chemicals in the ink cause corrosion of jet nozzles; the deleterious effect of sodium
chloride on steel surfaces is well known, for instance; inks for use in `charged drop'
continuous printers should have low electrical conductivity. Thickeners in the ink often do
not have the desired rheological properties. Some chemicals can be utilized in pre-treated
fabric but would cause stability problems in the ink; e.g. sodium carbonate as alkali for
reactive dye fixation is acceptable on the fabric but not in the ink. The presence of large
amounts of salts in aqueous inks reduces the solubility of the dyes; concentrated inks are
required in jet printing due to the small droplet size. The advantage of applying thickeners
and chemicals separately from the dyes is that it allows the wettability and penetration
properties of the fabric to be adjusted. The conventional printing adopted in the industry
was screen printing (Figure 12), which was not sustainable due to water consumption and
toxic chemicals involved in processing. The Table 1 compares the conventional method to
the latest inkjet/ digital printing
Table 1. Comparison between Conventional Screen Printing and Digital Printing
Factors Screen printing Digital printing
Tools and technique
The process involves making a stencil using a drawn/digitized image or a photograph, attaching to a screen, placing it over the desired canvas, and spreading the ink over the image.
A computer and a printer with ink cartridges are the requisites
Efforts Takes a lot of time-consuming effort, because the screens need to be made and the process is slow
Easy to operate and gives results at the touch of a key. It is relatively quicker
Quality It offers better quality imaging as the ink gets deeply absorbed and lasts longer. Screen printing also gives clearer edges to the image printing, because of the precision that is carefully done in preparing the stencil blocks
The ink does not spread because the image is directly printed on the fabric, but tends to fade quicker. To print a colorful image, all the colors are present in the single image, and the laborer does not need a separate screen for the same.
Cost Costs escalate with the numbers of screens. If a person wants a more complex image with many colors, then individual slides for every color are created. Since this technique requires skill, the training of labor will be essential. The method is most apt if a person wanted a large quantity of fabric
The computer and printers are one time investments and digital printing is cheaper compared to screen printing as the charge is an offer for per imprinted image.
To sum up, Inkjet or Digital printing has revolutionized the way businesses create
their printed materials. It is fast, effective, and provides an alternative to the more
traditional method of textile printing. The various merits of inkjet printing are discussed
below;
▪ Quality: When it comes to quality, nothing surpasses digital printing. Images are
essentially flawless, alignment and registration issues are non-existent, and the
color is vibrant. Digital printers can also use the entire length of a printable item.
▪ Speed: Digital printing’s ability to switch over to a new label almost instantly is
another perk of using digital printing. Because there’s no lost time setting up plates
and printing machinery, your order is likely to reach its intended destination days, if
not weeks earlier.
▪ Short-run printing advantage: Digital textile printing efficiently produces
designs at run lengths as low as one yard of fabric without the need for screen
changes.
▪ Lower water and power consumption: Digital textile printing eliminate the
substantial amount of water and electrical energy one requires for rotary screen
preparation, printing, and cleanup. Even greater water and power savings can be
achieved with disperse/sublimation and pigment digital textile inks, which only
require a heat-fixation step for post-treatment.
▪ Less chemical waste: Digital textile printing results in significantly less ink usage
and waste relative to screen-printing. Taking into account the additional chemistry
and chemical waste from screen production, printing digitally offers a greener
advantage for printing.
▪ Large repeat sizes: Digital textile printers can print large designs (e.g. cartoon
characters on sheets and blankets) on roll fabric without the usual rotary screen-
printing limitation in pattern repeat size.
▪ Reduced production space requirements: By not having to prepare and store
customer screens for future use, the production footprint for digital printing is a
fraction of the size one requires for a rotary screen print facility.
▪ Less printed inventory needed: Digital textile printing permits the option to print a
design at will. This means that manufacturers with an integrated digital printing
system in their production chain can keep a stock of unprinted textiles on hand to
print as required. This reduces the need for a pre-printed inventory of fabric that
may or may not is used.
▪ Sampling and production were done on the same printer: By being able to print
samples (strike-offs) on the same printer one uses for production, digital textile print
shops can present their customers with proof samples of designs that will exactly
match the final printed material.
▪ Print flexibility: Printing houses utilizing both digital and screen technologies can
choose to print a small number of designs with different color combinations
(colorways) first with their digital textile printing solutions to test the market. They
can later opt to print higher volumes of the most desired color designs using rotary
screen technology.
▪ Variety of creative design choices for printing: Digital textile printing provides
the option to print photographic/continuous tone images, spot color pattern designs,
or a combination of both. This expands the creative printing alternatives for fashion
and interior designers.
▪ Low capital investment: The relatively low capital investment to set up a digital
textile print shop, especially compared to rotary screen-printing production, makes it
possible to start small and expand as business grows.
The demerits of Inkjet Printing are as follows;
▪ Limitation of particle size: Metallic colors cannot be printed by these machines
due to large particle size.
▪ Large Volumes are expensive: Without getting too technical, digital printing
presses run at a maximum of about 50 feet per minute. While this speed is sufficient
for low volume (10,000 – 15,000 item) projects, larger volume work will benefit from
using traditional presses that can run at speeds between 300 and 500 feet per
minute. Although traditional presses are more expensive to configure and operate,
they will be economical.
▪ Ink limitations: While digital printing certainly handles color and ink well, digital
inks tend to fade more quickly than offset inks when exposed to direct sunlight.
Also, the opacity of digital ink is not quite up to par with offset ink, because digital
ink is naturally thinner (though the difference between the two is only noticeable
when dealing with clear or metallic media). There are types of laminations available
to help prevent this problem from occurring.
6. Other sustainable technologiesCotton Pretreatment
Cotton requires more water than other textiles for dyeing. About 200 liters of water
are required to produce 1kg of fabric. Dow has developed a pretreatment process called
ECO FAST Pure that is applied before the dyeing process to produce cationic cotton. The
pretreated cotton acquires a permanent positive charge, enabling it to have a higher
affinity for negatively charged molecules such as dyes. This patented technology
decreases the use of dye and water by 50 percent for cotton dyeing. ColorZen has
innovated technology for pretreatment of raw cotton fibers using a solution comprising a
wetting agent, caustic soda, and an ammonium salt. This pretreated cotton exhibits
increased ability to retain the dye without the need for fixation chemicals, thus reducing the
usage of toxic chemicals by 95 percent and water wastage by 90 percent.
b. Natural or Engineered MicroorganismsColorifix employs a synthetic biological approach by using bacteria to color the
textiles, which can reduce the use of water by up to 10 times. The innovative steps in this
process are to fix the dye-producing bacteria directly onto the fabric using a carbon source
solution, followed by deposition and fixation of the dye onto fabrics with a single heating
cycle by the lysis of the microorganisms. This technology does not require a dye extraction
process, which uses organic solvents, or fixing and reducing agents containing organic
compounds. University of California researchers are developing denim dyes using
genetically modified E.coli bacteria to produce indican, which can then be turned into
indigo by enzymatic treatment. This new process removes the need for harsh chemical
reducing agents for indigo dye solubilization, replacing it with an enzyme. However, the
process still needs optimization in the recovery of indican for its sustainability.
c. Innovative Dyes and AuxiliariesHuntsman Textile Effects introduced Avitera, a line of polyreactive dyes for cotton
that readily bonds to fiber, in contrast to the conventional reactive dyes. Avitera dyes use
tri-functional chemical reactivity that provides a high reaction and fixation rate with
cellulosic fiber, leaving very little unfixed dye to be removed. This dramatically reduces
water and energy usage by up to 50 percent and uses up to 20 percent less salt. And
Huntsman Corporation recently developed the diffusion accelerant Univadine E3-3D, a
dyeing auxiliary that enhances the diffusion of a dye into the polyester. This diffusion
accelerant is said to achieve high-performance dyeing of polyester microfibers and is free
of hazardous chemicals, thus complying with current and anticipated industry sustainability
standards
d. Powder Dyes from Textile FibersOfficina+39, an Italy based company, developed the sustainable dye range
Recycrom using recycled clothing, fiber material, and textile scraps. It developed a
sophisticated eight-step system (patent pending) in which all the fabric fibers are
crystallized into an extremely fine powder that can be used as a pigment dye for fabrics
and garments made of cotton, wool, nylon, or any natural fiber. Recycrom can be applied
to the fabrics using various methods such as exhaustion dyeing, dipping, spraying, screen
printing, and coating. Recycrom is applied as a suspension while most dyes are used as a
chemical solution and hence can be easily filtered from the water, thus reducing the
environmental impact.
e. Hybrid PigmentsEco foot has developed hybrid pigments composed of a dye chemically linked to a
polymer particle that reacts with cellulose fibers at temperatures as low as 25ºC. This
technology does not require the use of salt, which otherwise is crucial to driving the dye
into the fabric. This technology can be applied for dyeing cotton garments at low
temperatures and also to wool in a more ecological process. Eco foot-Indigo, a hybrid
pigment used in dyeing denim, avoids using toxic reducing agents that are traditionally
used in converting indigo pigment to a water-soluble form. Common reducing agents are
considered environmentally unfavorable, as the sulfite and sulfate generated in the
dyebath can cause various problems when discharged into the wastewater. Eco foot also
developed auxiliaries to prevent hydrolysis of the dye in the dyeing process, which typically
requires harsh washing-off procedures to remove the hydrolyzed dye. Together with hybrid
pigments and auxiliaries, more than 50 percent of water in the intermediate and final rinses
can be saved in the total process of preparation and dyeing.
Conclusion The next biggest global challenge is forecasted to be the Greywater Footprint,
which is the hazard caused by textile dyed wastewater that is polluting the river bodies.
Huntsman Dyes launched the water and energy-efficient technology for reactive dyeing of
natural fiber. In 2010, Levi Strauss & Co., launched its denim made without water,
polyester, and synthetics using air dyeing technique. This has a lot of options to create
diversified designs in different prints and colors on opposite sides of the fabric. Many such
technologies are being developed to create sustainable alternatives to cease pollution.
Water conservation, energy alternatives should be the themes to be discussed for living in
cohesion with nature.
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_vis-a-vis_dyeings
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