Visiting adress: Skaraborgsvägen 3. Postal adress: 501 90 Borås. Website: www.hb.se/ths Thesis for the Degree of Master in Science With a major in Textile Engineering The Swedish School of Textiles 2017-09-05 Report no: 2017.14.10. Antimony diffusion from polyester textiles upon exhaust dyeing. Shah Miran Patwary
47
Embed
Antimony diffusion from polyester textiles upon exhaust ......dyeing of polyester fabrics/yarns and analyzing the dyeing waste water, to determine the amount of antimony diffusion.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Visiting adress: Skaraborgsvägen 3. Postal adress: 501 90 Borås.
Website: www.hb.se/ths
Thesis for the Degree of Master in Science With a major in Textile Engineering
The Swedish School of Textiles 2017-09-05 Report no: 2017.14.10.
Antimony diffusion from polyester textiles
upon exhaust dyeing.
Shah Miran Patwary
Abstract
In recent years, environmental authorities in Sweden are reporting about high content
of antimony in waste water that is discharging from polyester textile dyeing industries.
It is known from available scientific publications that, antimony and its compound is
harmful for both human and environment. While dyeing of polyester textiles have a
commercial importance and in regards to the environmental issues, the industries are
looking for the factors those results in high concentrations of antimony in their dyeing
waste water. Dyeing of polyester textile requires high-temperature application in
association with dyes and process aid chemicals. The waste water that is being
produced after dyeing contains a complex mixture of chemicals, where antimony is one
of that mixture.
To comply with the industries interest, this master thesis work involved the exhaust
dyeing of polyester fabrics/yarns and analyzing the dyeing waste water, to determine
the amount of antimony diffusion. According to literature studies, the antimony
compounds are widely used as catalyst for polyethylene terepthalate (PET)
polymerization and hence antimony is present in polyester textiles. The entire
experimental work intended to understand the variation of antimony concentration and
the factors that are causing high antimony diffusion from polyester textiles during
dyeing. The materials which are polyester yarn and fabrics were collected from 3
different dyeing industries of Sweden and the materials were in 9 different types. From
material analysis (before dyeing) it has been found there were variations in antimony
concentration among the materials. The process parameters that have varied during
exhaust dyeing were dyeing temperature, cycle time and process aid chemical (leveling
agent) adding options. With the variation in process parameters, the dyeing has
performed and the dyeing waste waters have analyzed through inductively coupled
plasma sector field mass spectroscopy (ICP-SFMS). The expectations from the
experiments were, under specific dyeing process and with same antimony
concentration, the materials varying in filament/fiber diameter; big diameter
filament/fiber will diffuse less antimony compared to the small diameter filaments.
Also, process-wise the antimony diffusion ratio among the materials will accordingly
follow the Fick’s diffusion model.
To face the environmental issues with sustainability, entire thesis work could provide
concentrated knowledge’s with literature evidence for the dyeing industries. As
literature study indicates, the dyeing temperature, temperature ramp set and cycle time
play major factor while comparing for the expected diffusion ratio. As a gentle process
parameter, comparatively lower temperature and cycle time results lower antimony
diffusion. The use of levelling agent could be reduced to a level with the
appropriateness while added for dyeing. Also, a strong follow-up is needed in the
supply chain, for lowering the initial antimony content in the materials. Overall, the
findings of this thesis work also keep an importance to do further research on the
polyester textile, as during the experiments most of the materials haven’t reacted
accordingly as they were expected to react with the Fick’s diffusion model.
Keywords: PET polymer, polymerization, di-antimony tri-oxide, polyester textiles,
exhaust dyeing, antimony diffusion, waste water, health & safety, environmental
issues.
Popular Abstract
Since the last couple of decades, the environmental concerns are getting highlighted in
our everyday life. When it comes about the textile coloration industries waste water
discharge, it even becomes more critical. During coloration of textiles a lot of water,
chemicals, and energy are used; hence the waste water right after coloration looks like
a strong poison for the aquatic nature. Now, this poisonous water is getting even more
poisonous with the high rise of a known chemical that is already in that water. That
chemical is called as antimony and that leach out from the polyester textiles during
coloration. To get a polyester textile it is also necessary to use it in the beginning of
production and so it sits in the textile until it exposes for coloration. The health and
safety ensuring authorities have already declared it as harmful for human and also for
nature. So, it becomes urgent for the polyester textile coloration industries to know the
reasons that result in such high increase of leakage during coloration.
This thesis work gives the insights of the reasons and differences of the antimony
leakage among different polyester textiles. In the beginning of the research work,
different polyester materials were collected and initially tested for antimony’s
presence. Also, the collected polyester materials were different in shape thickness and
outlook; they were mostly polyester yarns and fabrics. The method for finding out the
reasons of leakage was the actual coloration that happens in the industries. Four
different processes of coloration have applied for a single type of material. The
processes were varied for the coloration temperature, coloration time and coloration
recipe. After every process, the waste water from each coloration process was
collected and tested for antimony’s presence in that water. The expectations were
thinner polyester textiles will give higher leakage compared to thicker polyester textiles
for each process.
After all coloration process, the results about the antimony’s presence in waste water
haven’t shown the same phenomena that have expected. Most of the polyester textile
has even lower leakage that was expected but still exceeding the common limits that
have to be ensured. Higher coloration temperature and time are causing higher leakage
compared to a little lower coloration temperature and time. Also without the presence
of a color levelling chemical during coloration, it is showing lower antimony leakage
for most of the polyester textiles. As, a conclusion of this research, the coloration
industries can use the knowledge’s of the coloration process variations and can utilize
them with a better planning. So, that the amount of antimony leakage could be lowered
down and the nature will get less impacted with sustainability issues.
Acknowledgements
I would like to give a big thank you to my thesis advisor Mr. Anders Persson at
Swedish School of Textiles, for his utmost supervision throughout my work. It was
really a good experience for me to work under your direction, which will help me a lot
in my future works. I am also grateful to Ms. Ulrika Noren for helping me with the
dyeing machines in our laboratory. When it comes to the external support with
materials, reagents and technical advice, I would like to especially acknowledge Mr.
Stefan Gustafson of FOV fabrics for his valuable dyeing experience contribution. Also,
I would like to thank the technical experts of Ludvig Svensson AB and Almedahls AB
who also contributed their materials to take part in this research work and strengthened
it. The professional supports from Ak lab was truly lab on analyzing the materials and
waste water during my experiments.
I am so grateful to TEKO and especially to Ms. Weronika Rhenby of TEKO, who have
offered such topic to Swedish School of Textile and thus I got the opportunity to take it
a step forward. I hope such collaboration will be continued and we, the students will
get more opportunity with the future research work.
Table of Contents Title Page Number 1. Introduction………………………………. 1 2. Literature Review………………………… 1-20 2.1 The PET Polymer and Polymerization 1-4 2.1.1 The Polycondensation Catalyst 4-5 2.1.2 Crystalline behavior in PET 5 2.2 PET Products & Textile Fibres 5-8 2.2.1 PET Bottles 5-6 2.2.2 PET Textile Fibres 6-7 2.2.2.1 Filament yarns 7 2.2.2.2 Textured Yarns 7 2.2.2.3 Partially oriented Yarns 8 2.2.2.4 Fully oriented yarns 8 2.2.2.5 Spun yarns 8 2.2.3 Di antimony tri-oxide as flame retardant 8-9 2.3 Dyeing of Polyester fabrics 9-15 2.3.1 Dyeing parameters and auxiliary chemicals 11-12 2.3.1.1 Temperature 11 2.3.1.2 pH of dye bath 11 2.3.1.3 Particle size in dispersion of dyes 11 2.3.1.4 Dispersing agent 11 2.3.1.5 Levelling agent 12 2.4 Main method of commercial polyester dyeing 12-15 2.4.1 Carrier Dyeing 12 2.4.2 High Temperature Dyeing 13-14 2.4.2.1 Adsorption Phase 13 2.4.2.2 Diffusion Phase 13-14 2.4.2.3 Clearing Phase 14 2.4.3 Thermosol Dyeing Process 14 2.4.4 Supercritical CO₂ Dyeing 15 2.5 Antimony Diffusion studies on PET 15-17 2.6 Research Problem Description 17-20 2.6 Research Questions 20
3. Experimental Works …………………………… 20-29 3.1 Materials 20-23 3.1.1 Dyes and Chemicals 22 3.1.2 Dyeing Machine 22-23 3.2 Methods 23-29 3.2.1 The diameter measurement (microscope). 23-24 3.2.2 Analyzing of antimony content in the materials. 24-25 3.2.2.1 Working procedure 24-25 3.2.3 The dyeing of the materials 25-28 3.2.4 Dyeing waste water analysis 28-29
4. Results & Discussion………………………. 29-36 4.1 Results for materials diameter and antimony content 31-33 4.2 Results for antimony diffusion after dyeing 33-36
texturized by heat setting in a twisted condition to give pleasing aesthetics as mentioned
above.
2.2.2.3 Partially Oriented Yarn (POY) The partially oriented yarn is produced by melt spinning. During the spinning process, the
filaments are stretched or drawn as much as from their original size to orient the polymer to
meet the desired evenness, strength, shrinkage, and elongation properties. Hence the term
partially oriented yarn refers to multi-filament that is only partially stretched. POY generally
have lower tenacity and less uniform than fully oriented yarn (FOY). POY is mainly used in
texturizing to make textured yarn and can be used in warping for weaving and warp knitting
of fabrics.
2.2.2.4 Fully Oriented Yarn (FOY) Fully oriented polyester yarn, actually manufactured by the similar process as it is for POY.
As an exception in the process, the yarn is produced at higher spinning speeds than POY. The
spinning coupled with the intermediate drawing which is integrated into the process itself.
Such process allows high stabilization through fiber orientation and crystallization. FOY is
mainly used as weft or warp in making fabrics.
2.2.2.5 Spun Yarns (Staple Yarns) They are made of staple or cut PET fibers. The staple fiber may be bright, semi dull or dull
and tenacity may be different as regular, mid or high. It may be polished to reduce crimp and
to increase luster. It may either be spun alone or blended with other staple fiber from natural
sources such as cotton, wool or rayon and then spun into yarn.
The basic difference between the PET based bottle and fiber is the crystallinity of polymer
chain orientation- Amorphous, Semi Crystalline & Crystalline and the manufacturing
method. Non-crystalline PET is highly transparent while fully crystalline PET is opaque.
Apart from the natural physical and chemical composition sometimes other type of functional
property can be imparted on the textile fibers. The following section is a example of such
functional property that can be develop into the fibers in terms of regulatory needs.
2.2.3 Di antimony trioxide as a flame retardant for PET Textiles Flame retardancy is a special property that is imparted in textiles for creating additional
performances of the textiles. This property has actually added in textiles to follow different
international standards on fire safety issues. Like other European countries and rest of the
world, In Sweden, there is regulation of fire protection. According to the Swedish Consumer
Agency, there are a number of rules regarding textiles that are used as for home furnishing
purpose (Annika Westling, 1999) -
• KOVFS 1988:2- related to the ignition of the folded furniture, need to be ensured that
they must not be ignited by a cigarette flame.
• KOVFS 1990:1- applies to mattresses and seeks an assurance that they cannot be
ignited by a cigarette.
• KOVFS 1985:5- applies to lining textiles and indicates that they can catch fire
quickly, could be within a second, but they should not burn too fast.
There is also an EU directive in such case, so called the PPE Directive, which also includes
firefighting and consists of several parts (Annika Westling, 1999)-
- The ignition- the material must not be ignited or melted by a small gas layer.
- Insulation protection, heat radiation must not be transported too fast through the material.
- Insulation protection, protection against open flame for a shorter period.
9
To ensure that a textile product have such high flame retardancy, different flame retardants
are often added, otherwise the risk of fire may be very high at given temperatures of ordinary
textiles. Flame retardants can be applied in several different ways (Annika Westling, 1999) -
1) Surface chemical treatment (Example- padding).
2) Impregnation with a functional finish which either reacts with the fiber
molecules or is polymerized in the fiber cavities. (Example- Trevira CS)
Di antimony trioxide, Sb2O3, usually used as a so-called brand inhibitor and the category
synergist inhibitors. The synergist inhibitor means, they are used in combination with other
compounds such as, halogen compounds (Chlorine). The purpose is to prevent ignition, but
also to reduce the spread of the flame.
As a summary of the above sections, it could be seen how PET and di antimony trioxide were
related in terms of PET material composition. With regards to the research problem, it is also
important to know how the polyester textiles get dyed and the conditions of dyeing. The
following section represents all necessary knowledge about polyester textiles dyeing.
2.3 Dyeing of Polyester textiles For the dyeing of polyester textiles, it always requires a well precision of temperature and
pressure control as the morphology of PET requires. It is difficult to dye the homogeneous
polyester fiber within its glass transition temperature which is around 67- 81 °C (Aspland,
1997). The glass transition temperature of polyester is close to the boiling point of water
under normal pressure. To have a uniform dyeing and level of dyeing it is often required to
dye the polyester fiber above their Tg.
Figure-3: Dye molecules in amorphous and crystalline region of polyester fibre during
dyeing.
The normally adopted temperature in polyester textiles dyeing industries is in between100-
135°C and the cycle time for dyeing depends on the expected color depth. So, higher the dye
diffusion in the fiber higher the depth of color will be. If it is also looked over the polymeric
structure it will be found that the PET polymers contain both crystalline and amorphous
region in its polymer chain (figure 3). Without selecting a high temperature and pressure it is
tough to dye, as in PET the amorphous part of the fiber get dyed only. It is also said that the
Polymeric chains of polyesters are closely packed and held together by strong forces, so for
10
penetration of dye molecules, this forces must need to be overcome. Dye molecule can
penetrate only when it finds space (void) in compact polymeric structure. Such voids are
formed due to the thermal mobility of polymeric chains upon application of heat. So, higher
the temperature, greater chance is for the mobility and void formation. For dyeing polyester
fibers, in practical terms disperse dyes are selected as suitable. For having hydrophobic
properties, these dyes are capable of penetrating into the similarly hydrophobic polyester
fiber. According to Rouette, 2000; disperse dyes has poor solubility in water. For this reason,
the dispersing agent is often added to the dyebath to maintain the dispersion stability,
especially in the case of high-temperature dyeing (Burkinshaw, 1995).
According to Murray&Mortimer, 1971 the four stages of the processing mechanism for
disperse dyeing are as follows and can be represent through the figure number 4.
i. Some of the dye dissolve (dispersion) in the water of the dyebath.
ii. Molecules of dye (micelles) are transferred from solution to the surface of the fibre.
iii.The solution in the dyebath replenished by the dissolution of more solid material from the
dispersion.
iv. The adsorbed dye diffuses in a monomolecular state to the fibre.
Figure-4: Schematic diagram of dye dispersion and dye diffusion as described by A.
Johnson, 1989.
The transfer process from the aqueous solution to the fiber is comparable with the extraction
of a solute from one solvent by a second. Here immiscible solvent and similar laws of
partition are applicable. The rates of the first and second stages of the processing mechanism
are governed by the solubility. It is well established that dyeing with disperse dyes is the
transfer of dye molecules from a molecular dispersion into the fibre and because of the
linearity of the isotherms obtained, the amount of dye adsorbed [D] 𝒶𝒹 relative to the
concentration in the bath [D]s can be expressed by a partition coefficient K (A.
Johnson,1989), i.e.
[D]𝒶𝒹 / [D]s = K ……. (Equation-1)
11
As more dye is introduced into the system a point of equilibrium will be reached, at which
the amount of dye in the dye bath exceeds the solubility. In the ideal case further additions of
dye will not bring further change in the concentration of dye in solution, and hence no change
will take place in the concentration of dye on the fiber. Dye molecules that have been
adsorbed on the fiber surface and then diffused into fiber interior follow a simple mechanism,
Fick’s equation (Patterson & Sheldon, 1959). So, at any point of the fibre surface, the rate of
dye diffusion through the unit area is directly proportional to the concentration gradient of the
dye.
2.3.1 Dyeing parameters & auxiliary chemicals For each textile materials the dyeing process is dependent on numerous factors, such as water
absorbency of the materials. Typically, synthetic textile materials, for example: polyester is
hydrophobic in nature and thus requires different mechanism for dyeing, while comparing to
natural textile materials. Also among synthetic materials the dyeing process parameters varies
according to the polymeric nature of the material. As studied by Waters, 1950; polyester
fibers dye very slowly at temperatures much below 100°C. Several factors affect the dyeing
of polyester fiber with disperse dye such as the temperature of dyeing, pH of the dyebath, the
particle size of the dye, dispersing agents and leveling agents.
2.3.1.1 Temperature The adsorption and diffusion of disperse dyes on polyester fiber are greatly influenced by
temperature selection during dyeing. With an increase in temperature the mobility of the
polymer chains increases in the amorphous regions of the fiber. It was found that polyester
fiber that has been dyed at low temperature is 700-1000 times slower than those measured for
the same dyes on secondary cellulose acetate and nylon (Nunn, 1979). For dye-hydrophobic
fiber system, the affinity of the disperse dye decreases with the increase of dyeing
temperature (Bird et al, 1959 and White, 1960) while by increasing the temperature, the
saturation value of the dye into fiber can be increased.
2.3.1.2 pH of Dyebath
Generally, commercial dyeing of polyester with disperse dyes, is carried out between the pH
range 5.5 and 6.5. For maintaining this pH, generally, acetic acid is used. At this pH, dye
exhaustion is satisfactory. During color development, correct pH needs to maintain for
superior color fastness and for stable color. S. Shakra at el. (1978), have studied that behavior
of the dye and the magnitude of dye uptake greatly affected at different pH.
2.3.1.3 Particle size in dispersion of the dyes According to Kenneth & Skelly,1973; the aqueous solubility of disperse dye particles in a
dispersion increase with decreasing particle size. Thus an increase in the severity of milling
accompanies a reduction in the particle size of the dye that enhances the solubility and
adsorption of disperse dye. But, here a factor involved in terms of fastness while selecting the
small particle sized dyes. With smaller particle size dye, it becomes easy for the dye to get
penetrate into the fiber while dyeing, but after dyeing process, the fiber exhibits poor color
fastness property, compared to big particle sized dyes.
2.3.1.4 Dispersing agent Disperse dyes are slightly soluble in water and often crystalline while varying in particle size.
These characteristics can cause uneven dyeing. In order to achieve the required particle size
and distribution (Heimanns, 1981), the disperse dye is milled in the presence of a dispersing
agent. The dispersing agents are anionic and chemical mixtures of lignin sulphonates or
polycondensates of arylsulphonic acids combined with formaldehyde to facilitate milling by
preventing agglomeration of the dye particles.
12
2.3.1.5 Leveling Agent Leveling agent used in the dyeing process as an aid for even dyeing process and hence an
additional agent used during dyeing. The fundamental mechanisms that contribute to level
dyeing are (S.K. Laga et al. 2014):
• Controlling the exhaustion of dye for even take up.
• Migration of dye for fiber sorption.
Leveling agents can be either non-ionic or anionic surfactants, which can increase the
solubility of the dye, thereby lowering the initial strike and overall rate of the dye uptake. The
slower dye uptake can also cause a restraining effect resulting in a loss of color yield.
According to the chemical structure leveling agent can be made from the following:
• FattyAcid (Ethylene Urea)
• Fattyalcohol (Sulphates)
• Sulphated (Fattyamine)
• Alkylaryl (Sulphonates)
• Fattyalcoholethylene (oxidecondensate)
Leveling agent also tends to slow down the dye uptake of the fibers that also helps to produce
more uniform color in the textile fiber (S.K. Laga et al. 2014). That’s why they are also
termed as retarding agents or retarders. The use of retarders is essential in the situation in
which dye tends to rush on to the fiber and results unevenly colored textile material.
2.4 Main methods of commercial polyester dyeing There are different dyeing methods available in terms of commercial importance. According
to Waters 1950; the rate of dyeing may be raised to the level of commercial acceptability,
either by raising the working temperature to the region of 130°C, or by dyeing at the boil in
the presence of an accelerating agent. Not only from aqueous solution disperse dyes can also
be transferred to polyester fibers under dry conditions. The dry processing includes
impregnating the fibers with a suitable dispersion, then drying at a ratio and then baking at
temperatures range 190-220°C (Ingamells,1993). Most widely used commercial dyeing
method of polyester textiles have discussed in following sections.
2.4.1 Carrier dyeing The term ‘carrier’ comes as the idea of the compound that could ‘carry’ the dye into the fiber,
and will cause rapid dyeing. It is now known that the carrier is absorbed by the fiber by
modifying the structure of the amorphous regions (A. Johnson, 1989). The Carrier is an
organic compound that can dissolve or emulsify the dye in the dye bath. For the dyeing of
polyester commonly used carriers are butyl benzoate, methylnaphthalene, dichlorobenzene,
diphenyl and o-phenylphenol. It has been widely debated to understand the actual mechanism
by which a carrier accelerates the dyeing and the probable mechanism could depend on the
chemical type of the carriers. As a mechanism, when a carrier is added to a dye bath within
the dyeing recipe the fibers absorb the carrier and results in swelling. The swelling can delay
liquor flow in the yarn/ fabric packages and that can cause irregularities in dyed shade depth
levelness. Lowering the polymers glass transition temperature (Tg) can promote the overall
effect to polymer chain for movements and results in creating a free volume. According to
the dyeing industries, the commonly used temperature for dyeing with carriers is between 90-
100°C. Previously, with carrier method the dyeing machine were only capable to work with
the temperature 100°C. However, carrier dyeing has steadily declined in present days with
the development of high-temperature dyeing machines for the dyeing of polyester. In high-
temperature dyeing method, previously used carrier have replaced with high pressure at the
13
temperature 130°C. Another reason for declining the carrier method is health and
environment concerns for using the carriers. 2.4.2 High-temperature dyeing Joonseok Koh, 2011, described high-temperature dyeing as the most widespread method of
batch coloration. The temperatures (ca. 130°C) require pressurized equipment and impart
increased diffusion of the dyestuff (and therefore increase the rate of dyeing) by reducing
cohesion between polymer chains and increasing the kinetic energy of the dye molecules.
The high-temperature dyeing machines actually represent the exhaust dyeing method.
Figure 5: Phases in exhaust (HT) dyeing of polyester, as illustrated by Joonseok Koh
(2011).
A typical exhaust dyeing application sequence for polyester is showing in figure-5, the three
main phases of the process including the heating or adsorption phase, the high temperature or
diffusion phase, and the clearing phase.
2.4.2.1 Adsorption phase According to Joonseok Koh, 2011; the heating or adsorption phase is the most critical in
determining the levelness of the dyed fiber and it is essential therefore that the heating rate is
appropriate to allow controlled adsorption of the dye. Although in the dyeing of polyester,
leveling can occur through migration at top dyeing temperature, in rapid dyeing cycles the
time at top temperature is minimal and it is even more critical to ensure dye is applied in a
uniform manner during the adsorption phase. The adsorption behavior during dyeing is
strongly influenced by a number of factors. The most important factors that considered are as
the concentration of dye, temperature gradient, fiber type and auxiliary chemicals adding
options. The rate of exhaustion of a disperse dye by polyester is controlled by the rate at
which the temperature is raised. At some temperature between 80°C and 120°C the dyeing
rate for that dye reaches a maximum. The temperature range over which the dyeing rate is at
its maximum is known as the ‘critical dyeing temperature’ (CDT). High CDT belongs to the
slow-diffusing and high-energy dyes, whereas more rapidly-diffusing dyes have a lower
CDT. Specific values of CDT depend on the rate of temperature rise, dye concentration,
liquor flow rate, liquor ratio and the substrate to be dyed.
2.4.2.2 Diffusion phase The dyeing of polyester is often called as a diffusion-controlled process. This is because of
the diffusion phases are shown in Fig. 5, including a convective transfer through the liquor
14
adsorption and molecular diffusion into the fiber, is the rate-determining step (Dawson
&Todd, 1979). The time needed for the adsorption phase is largely influenced by the machine
dye bath conditions. In the diffusion phase, the time required at top temperature is directly
related to the diffusion characteristics. The diffusion characteristics are related to individual
dyes and dyeing depth. In most cases, standard time is 10-20 minutes for the dyeing of pale
shades, 20-30 minutes for medium shades and 30-35 minutes or even more for deep shades.
In the high-temperature dyeing process, another important property of dyes can be observed
which is dye-fiber migration, or their tendency to level out. This phenomenon is very
important, in particular to the dyes that have been adsorbed in a non-uniform manner,
perhaps due to inadequate liquor circulation or too rapid a heating rate. While the migration
properties of disperse dyes may become a key factor if dyes are applied unevenly during the
adsorption phase, the key parameter in the diffusion phase is the diffusion rate of the disperse
dye.
2.4.2.3 Clearing phase Because disperse dyes have such limited solubility in water, some particulate disperse dye
may still be occluded on fiber surfaces after the dyeing phase is complete (Aspland, 1997).If
not removed, this surface contamination can undermine the brightness of shade as well as the
wash, sublimation, and crock fastness results. Commonly, the dyed polyester is cleared of
surface-deposited dye as well as auxiliaries (e.g. carriers, surfactants) by means of treatment
with detergent or reductive or oxidative treatments, in order to secure optimum fastness of the
dyeing and also to improve the brightness of shade (Burkinshaw,1995).The usual treatment
carried out, especially in heavy depth, is reduction- clearing, where the dyed fiber is treated
in a strong reducing bath, usually made up of sodium dithionite and caustic soda. A treatment
for 20 minutes at approximately 70-80°C, is often sufficient to clear the fiber surface, but the
ease of removal varies from chromophore to chromophore groups and dye to dye. This
clearing treatment destroys loose disperse dye through chemical reduction and provides a
good wash fastness property to the fibers when used as for fabrics.
2.4.3 Thermosol dyeing process The thermosol dyeing process is an important continuous process for dyeing polyester and
polyester/cellulose fiber mixture with disperse dyes, which is used mainly for woven and
knitted fabrics (H. K. Rouette, 2000).A dispersion of the disperse dye is padded onto the
polyester fabric. The padded fabric is then dried through a hot flue air dryer or by infrared
radiation, the latter usually giving much less migration of the dye. Final drying of the padded
fabric takes place using a heated cylinder and the fabric is then heated in air, or by contact
with a hot metal surface. The temperature range for drying is between 190-220°C and the
time for exposure is about 1-2min. The drying temperature and time are dependent on the
fabric construction. In the hot air, as the fabric approaches the maximum temperature, the
disperse dyes begin to sublime and the polyester fibers absorb their vapors (A. D. Broadbent,
2001).Dyes of lower molar mass tend to sublime more readily, but they also suffer from low
fastness and poor resistance to heat treatments. Dyes of higher molar mass have better
fastness properties but are more difficult to apply. After thermal fixation a reduction-clearing
treatment is necessary to remove loose dyes that are remaining on the fiber surfaces (R. M
Christie et al, 2000). PET seatbelt webbing is typically dyed with disperse dyes and by using
thermosol dyeing processes in which the webbing is dipped continuously into a dye solution
and passed through a hot (IR) chamber (220°C) for approximately 2–3 min. The dyestuffs
can penetrate to the molecular chains of the fibres during their exposure conditions. This
method has some advantages that could avoid a batch wise process (a conventional dyeing
method), which is regarded as time-consuming and tedious, and would speed up the dyeing
process.
15
2.4.4 Supercritical CO2 dyeing process Supercritical CO2 dyeing is a unique method to dye the polyester, which is currently used in
few commercial industries. The dyeing method and the appropriate machine with it, is under
commercialization by Dyecoo (www.dyecoo.com). In this dyeing method, instead of water
CO₂ used as the dyeing medium in a closed loop process. As a method of dyeing, when
carbon dioxide is heated above 31°C within a pressure of 74bar, it goes under a supercritical
condition. The term supercritical is the state that can be seen in between a liquid and a
heavily compressed gas. In this supercritical point CO₂ has a liquid-like density that helps to
dissolve the hydrophobic dyes (example-disperse). Also gas-like low viscosities of CO₂ in
this supercritical point, helps to dye-fibre diffusion, which leads to shorter dyeing times. By
changing the temperature and pressure conditions in dyeing, it is possible to extract the
spinning oil and the removal of excess dye from the fabric in one plant. In CO₂ dyeing,
drying of the fabrics is not required as in the end of the process CO₂ released in gaseous state.
Also after dyeing, the used CO₂ can be recycled up to 90% after collecting in a separator, and
this can be done through the closed loop system. The CO₂ dyeing technology requires 100%
pure dyes and no process chemicals needed in this process. Also with this process, it is
possible to achieve a dye uptake of more than 98%. During the supercritical condition of
CO₂, the dyestuff penetrates deep into the fibres which can create colors with excellent shade
depth and can provide excellent quality. Since there is no need of process chemicals and
water in this process hence no need of waste water treatment. Efficient color absorption, short
cycle time, and no waste water treatment make this process as energy saving and low
operating cost tag for polyester fabric dyeing.
The sections in above, tell about the general information’s about PET (polyester) textiles and
its available dyeing process. It was also noticeable from the beginning of the literature review
(section 2.1.1) that antimony becomes an integral part of the PET polymer for being used a
polymerization catalyst. Also, during dyeing of the polyester materials, several mechanisms
take place to diffuse the dye molecules into the fiber. So, in regards to the research problem,
it was anticipated that the dyeing has an influence in terms of diffusion. But, only to review
the diffusion mechanism it was necessary to have further information’s from similar type of
diffusion studies. The following section 2.5 gives an indication on how antimony diffuses
from the PET trays into the food stimulants under particular time and temperature conditions. 2.5 Antimony diffusion studies on PET In recent days the antimony diffusion has been studied by so many PET packaging industries
to understand the release of some extent within an exposure condition and duration. The most
important parameters for the prediction of the migration of a chemical compound are the
concentration of the migrant in the material, the diffusion coefficient of the chemical species
in the polymer as well as the partition coefficient between the polymer and the contact media.
The partition coefficient, however, plays in the case of PET a minor role because the
equilibrium is not reached assuming typical a shelf life of beverages. This is due to the low
diffusivity of the extracted PET polymer (Franz and Welle 2008), which means that for the
prediction of the chemical compounds migration the diffusion coefficients are the most
important factors. A study has conducted by Swiss federal office of public health, for
determining the antimony diffusion from PET trays. In that study, the coefficients for
antimony diffusion were determined within the exposure (time & temperature) durations. The
results of that study indicate that the migration of antimony follows a behavior as it shows on
Ficks 2nd Law of diffusion. The Fick’s 2nd law states that, the change in concentration with
time in a particular region is proportional to the change in the concentration gradient at that
point in the system. So, the equation for such diffusion ratio can be written as: