ABSTRACT
Conventional cotton textiles dyed with direct dyes suffer from the problems of poor exhaustion and moderate to poor washing fastness due to the weak dye-fibre binding forces of hydrogen bonding and Vander Waals attractions. In addition, the dye effluent contains large amounts of dissolved and undissolved electrolytes which are harmful to the aquatic environment. In this work an attempt is made to change the dye-fibre binding force to that of ionic or electrostatic bonding (stronger bond) through cationisation (i.e. imparting positive charge) on the cotton. Cationisation involved the use of CHIPTAC which was converted to its epoxide form called EPTAC which reacted with the fibre to form complexes having two positive heads (dye sites) for formation of ionic bonds with the dye. Cotton cellulose (100%) sliver was purified and cationised prior to dyeing with a direct dye Cupro brilliant blue 2BL (CI Direct Blue158:1). All the dyed samples (cationised and uncationised) were characterised (FTIR), subjected to performance tests (wash and light fastness) as well as kinetics and thermodynamics parameters were studied. Results indicated an improvement in dye exhaustion ranging from 31 to 65% for uncatinised samples (control) to 60 to 85% cationised samples i.e. 51.04% increase. Improvement in wash and light fastness were seen, control samples gave an average rating of 2-3 (colour change) and 3 (staining), cationised samples have 4 (colour change) while 3 to 4 (staining). Light fastness showed percentage increase of 50 to 112.5%. Cationisation resulted in improvement of affinity for the cationised cotton cellulose by the dye ranging from -549.743 to -5253.67 as compared to treated control which showed very low affinity. Activation energy, heat of dyeing, entropy of cationised samples gave more favourable values that enhance dyeing condition than that given by uncatinised samples (control).
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TABLE OF CONTENTS
TITLE PAGE I DECLARATION II
CERTIFICATION III
ACKNOWLEDMENT IV
ABSTACT V
TABLE OF CONTENT VI
LIST OF FIGURES XI
LIST OF TABLES XII
LIST OF PLATES XIII
LIST OF APPENDICES XIV
LIST OF SYMBOLS AND ABBREVIATIONS XV
1.0 INTRODUCTION 1
1.1: COTTON 1
1.2: Chemistry of Cotton 4
1.2.1: Behaviour of Cotton in water 4
1.3: Direct dyes and their Application to Cotton 5
1.3. 1: General 5
1.3.1.1: Dyes 6
1.3.1.2: Colour 7
1.3.2: Fastness Properties of Direct dyes 7
8
1.3.2.1: Wash fastness 7
1.3.2.2: Light fastness 8
1.4 Modification of Cotton for Better dyeability 8
1.4.1: Cotton Modification using Quaternary Ammonium Salt (QUATS). 11
1.4.2: Cationisation of Cotton Cellulose. 11
1.4.3: Conversion of CHIPTAC to EPTAC. 12
1.4.4: Reaction of EPTAC with Cellulose. 13
1.4.5: Hydrolysis of EPTAC 13
1.5: Kinetics and Thermodynamics of Dyeing 14
1.5.1: Kinetics of Dyeing. 16
1.5.1.1: Dyeing Temperature 17
1.5.1.2: Agitation (Dye liquor circulation) 18
1.5.1.3: Fibre structure 18
1.5.1.4: Diffusion of Dye into Fibre. 18
1.5.1.5: Measurement of Diffusion Coefficient 19
1.5.2: Adsorption Isotherms 21
1.5.2.1: Langmuir Isotherm 22
1.5.2.2: Freundlich Isotherm. 24
1.5.2.2.3: Nernst Isotherm 25
1.5.3: Affinity of Dye for Fibre 26
1.5.4: Heat of Dyeing. 27
1.5.5: Entropy of Dyeing. 29
1.5.6: Kinetics of Dyeing. 30
1.5.6.1: Activation Energy. 30
1.5.7: Exhaustion Characteristics. 32
9
1.6: Statement of the Problem 34
1.7: Aims and Objectives of the Research. 34
1.8: Scope of the Research 34 CHAPTER TWO
2.0: LITERATURE REVIEW 35
CHAPTER THREE
3.0: MATERIALS AND METHODS 37
3.1: Materials and Chemicals. 37
3.2: Equipment and Apparatus. 37
3.3: Scouring of Cotton Sliver. 37
3.3.1: Procedure. 38
3.4. Preparation of EPTAC and cationisation of cotton 38
3.4.1: Preparation of EPTAC. 38
3.4.2: Cationization of Scoured Cotton Sliver using EPTAC. 38
3.5: FTIR CHARACTERISATION. 39
3.6: Dyeing Scoured Cotton Sliver with Direct dye. 39
3.6.1: Infinite Dyeing of Cationized Cotton to Determine Kinetic Parameters. 39
3.6.2: Dyeing Cationized Cotton to Determine Thermodynamic Parameters. 40
3.7: FASTNESS PROPERTIES OF THE DYED SAMPLES. 41
3.7.1: Wash Fastness. 41
3.7.1.1: Methodology of wash fastness test. 41
3.7.2: Light Fastness. 42
3.8: MEASUREMENT OF ABSORBANCE USING COLORIMETER. 43
3.8.1: Procedure. 43
3.8.2: Determination of concentrations of dye in solution [D]S1 and fabric [D]f 44
10
3.9: Construction of Calibration curve for the Direct dye (C.I Direct Blue 158:1). 44
3.9.1: Usage of calibration curve. 45
CHAPTER FOUR
4.0: RESULTS AND DISCUSSION 46
4.1: Cationisation of the cotton cellulose 46
4.1.1: The results of (FTIR) Spectroscopy in Table 4:1 46
4.2: MEASUREMENTS OF FASTNESS PROPERTIES 47
4.2.1: Wash Fastness. 47
4.2.2: Light Fastness 48
4.3 THERMODYNAMIC PARAMETERS STUDIES. 50
4.3.1: Calibration curve for the direct dye. 50
4.3.2: Available Sites for Dye attachment (Saturation limit). 51
4.3.3: Affinities of control and cationised samples. 52
4.3.4 Entropy of dyeing. 53
4.3.5: Heat of Dyeing (ΔH) 54
4.3.6 Kinetic Studies 55
4.3.6.1 Diffusion coefficient 55
4.3.7 Activation Energy 56
4.3.8: Dye Exhaustion 57
CHAPTER FIVE
5.0: SUMMARY, CONCLUSION AND RECOMMENDATION 58
5.1 SUMMARY 58
5.2 CONCLUSION 59
5.3 RECOMMENDATION 60
REFERENCES 61
11
APPENDIX I 65
APPENDIX II 85
PLATES
CHAPTER ONE
1.0: INTRODUCTION
1.1 Cotton
Cotton fibres grow around seed of cottonplants; it is hydrophilic in nature and dominantly the most abundant source of clothing the world over. Length of cotton fibre ranges from 10-35mm; depending on quality and cotton variety, it has length to width ratio of 3×104μm by 15μm. Generally colour of raw cotton varies from creamy white to grey depending on climatic conditions and the environment it was grown. Native cottonconsists of aggregated fine micro fibrils which are flexible and are attracted to each other by hydrogen bonds. These fibrils crystallize into lamellae (sheets) Adam. (2000), Chavan et al.,( 1998).
In its raw form cotton contains impurities in the order of 4 to 12%, mainly waxes, protein matter, pectin, vegetable matter, ash etc. These impurities are gotten rid of through processes such as scouring, bleaching etc giving pure cellulose Goli,(2008).Practically pure cotton cellulose(90 to 99%)consist of linear condensation polymer known as poly(1,4-β-D-)anhydroglucopyranose whose repeat unit is a monomer called cellobiose. Cellobiose consist of two glucose units with six carbon atoms joined by glucosidiclinkage. One molecule of glucose contains nearly 6000 to 10000 units of cellobiose. Saowaneeet al.,(2005). The cross sectional area of cotton fibre strand showing primary wall, lumen, reversal and impurities (pectin, fats, and waxes).
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Fig.1.1 Morphological structure of cotton. Adapted fromSundaresan et al.,( 2006).
Fig. 1.2 Molecular structure of cotton. Adapted from Sundaresan et al.,(2006. Saowanee et al.,( 2005).
Structurally cotton cellulose consists of one primary and twosecondary hydroxyl groups in each glucose unit. These hydroxyl groups form the reacting sites for substitution reactions such as etherification, esterification and modification such as cationization. Goli , (2008).
Cotton cellulose fibre is not electrically inert, it carries small amount of negative chargesof11mv which is mainly due to presence of carboxylic acid groups formed as a result of oxidation at primary hydroxyl site(-CH2OH).At pH 8, more hydroxyl groups at hydroxyl-methyl side chains may also be ionized thereby increasing the amount of negative charges significantly. Saowanee et al., (2005).
Negative charges on the surfaceof cottoncelluloserepel anionic dyes within dye bath;as a result of this problem efficiency of dye exhaustion and fixation are drastically reduced. To
20
avert this unwanted situation, many studies were carried out on cotton dyeing, aimed at improving dye uptake as well as fastness properties (Saowanee et al., 2005). Majority of the researches in this area centered on introduction of cationic sites into the cotton for interaction with anionic dyes Subramanian et al., (2006).
Cotton is suitable to all weather conditions; however it is most suitable to the tropics owing to its high moisture regain of 18%. Furthermore, crystalline structure of cotton cellulose ranges from 65 to 70% with 30 to 35% being amorphous. Bird et al., (1975).
In addition 20-41% of space in cotton is not occupied due to lumen and spaces between fibrils. Presence of spacesenhances modification of cotton by giving accessibility tochemical reagents. Subramanian et al., (2006).Common dyes used in dyeing cotton fabrics include direct, vat, azoic and reactive dyes. Nkeonye, (1989).
Generally in cotton cellulose structure there are no specific sites for dye attachment; dyes are retainedby means of H-bond, mechanical retention, or physical forces such asVander Waals forcesand dipole-dipole moment.
This short coming explains the reasons for the difficulty in exhaustion and retainingofthe dye molecules in the fibres. Bird et al.,(1975. Gile, (1963).
These reasons mentioned above are causesfor low dye exhaustion and poor wash fastness properties associated with most dyes used in cotton dyeing. However, improvements were recorded by making use of salts and other after treatments. Birdet al.,(1975), Gileset al., (1963), Haruna,(2011).
1.2: Chemistry of Cotton Cellulose
1.2.1:Behaviour of Cotton in Water
Dyeing of cotton fabric, yarn or loose fibres takes place in solution of dye in water either in cold or hot condition. Water molecules do form H-bonds with each other:
21
Thisbehaviour explains the high boiling point of water. Bird et al.,(1975), Gileset al., (1963).It is of paramount importance to first and foremost analyze how cotton cellulose materials behave when in contact with water.As water with a formula H20 dissociates into hydroxyl ions (OH-) and hydrogen ions (H+),with cotton cellulose having numerous number of hydroxyl groups distributed all over are in contact, repulsive forces are thus developed which in earnest aggravate the difficulty of dyeing cotton cellulose. Bird et al.,(1975).
However presence of intra molecular attractive forces of polarity and H-bond enhance easy wetting of cotton cellulose despite the repulsion. Gileset al., (1963), Bird et al., (1975), Goli, (2008).
All the mentioned phenomena above result in lowering of surface tension on the water surface which thus ease water absorption and the cotton cellulose gets swollen; this provides more surface area of contact with dye molecules. Chavan et al.,(1998).
1.3: Direct Dyes and their Application to Cotton
1.3.1: General
Cotton cellulose in loose fibres, yarn or fabricsis usually dyed with direct, reactive or vat dyes owing to absence of specific sites for dye attachment. Presence of minute negative charges on the surface of cotton cellulose makes its dyeing difficultGileset al., (1963).
Even with direct or reactive dyes, dyeing of cotton exhibits low exhaustion. To improve the situation, large amount of electrolyte has to be employed which could only result in 60-65% dye utilization. Shaukat et al.,(2009).
Among the popular dyes for cotton cellulose, reactive dyes are given credit by having better fastness properties than the others due to their ability to form covalent bond with cotton cellulose. Bird et al., (1975). Apart from wasting 30-35% of dyes in cotton cellulose dyeing,
22
large amount of water is employed, this plus large quantity of electrolyte constitutes serious threat to our environment as far as pollution is concerned. Adam, (2000).
In Nigeria, textile effluent is one among the disturbing sources of environmental pollutants. Hazards associated with this waste are contamination of our little available drinking water, killing of aquatic organisms and epidemics such as:
Cholera, typhoid fever and the like. To dye one (1) kilogram of cotton material, an average of 10 kilograms of effluent will be discharged into the environment. Averagely 740grams solute, heat and gases are also discharged into the environment. World statistics has shown that textile industries contribute nearly twenty millions kilograms waste water annually into the sea. Burcu et al.,(2011),Haruna, (2011).
Direct dyes are most common known dyes and are the easiest to use, requiring no addition of any chemicals for their application. One of the disadvantages associated with direct dyes is their poor fastness properties to agencies such as washing, light, perspiration etc. However other avenues are currently being exploited in improving these short comings, though at the expense of products production cost. Gileset al.,(1963).
1.3.1.1:Dyes
Dyes are organic compounds which have ability of imparting colouring effect on textile substrates, leather, foods etc. In their chemical structure, a dye consist of mainly two parts namely: Chromophore and Auxochrome join together by a linking group. Birdet al., (1975)
. Dyes are usually applied in aqueous solution either in molecular or colloidal state of dispersion. Dyes selectively retain some of the wavelength of light that fall upon it, this reason for it natural colour. Oguntona, (1986). Furthermore dyes are classified into various groups based on their chemical composition, mode of application etc. Bird et al.,(1975).
1.3.1.2: Colours.
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Colour constitute complex phenomena that its theory remain a question of perception and area of view, colour theoreticians, artists, physicians and philosophers view light on different angles, while some view it as pigment, to others its simply ray of light while to some it is mainly waves. Oguntona, (1989).
Colours are classified into primary, secondary and tertiary, it can added, subtracted or mixed to give certain desiring properties at the same time colour have three important dimensions or properties namely: Hue, Value and Intensity. Oguntona, (1989).
1.3.2: Fastness Properties of Direct Dyes.
1.3.2.1: Wash Fastness.
Washfastness of direct dyes on cotton material imply ability of the dye molecules to withstand treatment in soap solution at cold or elevated temperature. This action merely means solubility as well as rate of movement outward of the dye molecules from the fibre in the presence of soap (synthetic detergent). Bird et al.,(1975).
These two phenomena rested on fibre-dye attachment. For direct dyes, wash fastness is usually poor in ideal cases, as forces involved are weak and non-ionic linkages. In this work however cationic sites for dye attachment are provided via cationisation to enhance absorption and retention of the anionic direct dye (Haruna (2011).
1.3.2.2: Light Fastness.
When cotton materials dyed with direct dye are exposed to light, fading will result.
The fading process is a complex problem; to explain the phenomena, it is necessary to talk on the basis of absorption of radiant energy in form of photons.
24
These photons provide excitation energy to the dye molecules which escape resulting in disruption of bonds and rupture of chromophores and auxochromes of dye components and hence colour fading. Nkeonye, (2010).
Escape.
D (normal) D* Faded colour.
Figure 1.3: Action of photons on dye molecules.
1.4: Modification of Cotton for Better Dyeability.
Environmental research is paving way toward strenuous attention on industries such as cosmetics, leather, plastics, food, paper, textiles and the likes, owing to their high usage of dyes and colorants. Burcuet al., (2011). World production of colorants stands at one million metric tons annually; out of this 50% are textile dyes. Though dyes contaminate the environment and affect our health, yet dyes are important to us in clothing, carpets, furnishing, curtains etc. Presently more attention is concentrated on cotton dyeing industries which use nearly 70% of all dyes produced. Invariably most dyes in use are non-toxic yet there are still few that are carcinogenic. Azlan et al.,(2008).
Whatever the nature of dyes, its presence in water isundesirable, most of them being recalcitrant organic molecules that are resistant to aerobic activities. Burcu et al., (2011),Hauser, (2000).
Many researchesAdam, (2000), Azlan et al., (2008), Burcu et al.,(2011), Christie, (2006), Haruna, (2011) have been carried out involving modification of cotton to improve its dyeability as well as getting better fastness properties and optimum dye exhaustion without using salts or electrolytes, thereby minimizing the amount of waste water effluent discharges into our environment. Hauser, (2000).
25
In the past, cotton modification centered on treatment with polyacrylamide or chitosan resulting into conversion of hydroxyl groups in cellulose to amide groups thus making the fibres behave as either wool or nylon. Saowanee et al.,(2007); Subramanian et al., (2006).Invariably absence of specific sites for dye attachment in cellulose structure is main hindrance for cotton dyeability with anionic dyes; in addition, whenAever cotton material is immersed in water it develops zeta negative potential which repels anionic dyes. These problems led to the low exhaustion as well as poor fastness properties associated with cotton dyeing with either direct or reactive dyes Goli et al.,(2008); Haruna, (2011), Karnik, (2002).
Cotton modification in recent times involves anchoring the fibre with positive charges in its structure thereby facilitating reactivity with anionic dyes. One of the popular methods in cotton modification is the use of 1,4 bis[2 chloro-4(1-amino-2-hydroxy-3-trimethyl ammonium propane chloride)-S-triazine-6-amino]-benzene dichloride.This treatment confers to cotton two dye reactive sites on its structure Jong, (2007), Karnik, (2002).
CH3 . CH2Cl-
N+
CH3 NH NH CH3
N+ OH NH NH OH
Cl-H2C CH3
Cl
Figure1.4 Structure of 1,4bis[2-chloro-4(1-amino-2-hydroxy-3-trimethyl ammonium propane chloride)-S-triazine-6-amino] benzene dichloride.Adapted fromAdam,T.(2000), Chavan et al., (2000), Karnik,(2002).
Previously, 1,2-dichloroethan e was popular in cotton modification Hauser,(2000). In this treatment amide groups are imparted on the cotton structure thereby making it behave as a polyamide fibre.Chitin, a naturally abundant mucus poly saccharide and supporting material of crustaceans insect is another alternative; it is to be chemically converted to chitosan which
26
is a natural polymer containing large amounts of amino groups. Commonly found in shells of crabs and shrimps, in its chemical structure, chitin consists of 2-acetamido-2-deoxy-β-D-glucose the main functional groups are hydroxyl and amino whichserve for the above treatment.Azlan et al., (2009).
CH2OH CH2OH
HH O H O
OHH OOH H
H N C CH2 H N C CH2
H O H O n
Figure 1.5 Molecular structure of Chitin (Azlan et al., 2008, Saowanee et al., 2005)
1.4.1: Cotton Modification using Quaternary Ammonium Salt(QUATS).
Quaternary ammonium salt popularly known as QUATS is used to impart permanent positive charges on cellulose structure of cotton; presence of these charges make the cotton fibre dyeable with anionic dyes, with better exhaustion and good fastness properties. Haruna, (2011).
Many researchersAzlanet al., (2008), Saowaneeet al., (2005), Konaghattaet al.,(2010) aminated epoxy derivatives to modify cotton cellulose for better dyeability. Others modified cotton by using epoxy dimethyl-3-propane followed by an ethyl iodide quaternization which yielded ions exchanger and acid dyes were successfully applied to the modified cotton cellulose. Goli,(2008).
R2
|
R1 — N+ — R3X-
R4
27
Fig 1.6 General formula for quaternary ammonium salt (QUATS)Haruna,( 2011).
Where R1, R2, R3 and R4 are alkyl groups andX- is a halide ion (Cl, or Br).
1.4.2: Cationisation of Cotton Cellulose.
The 3-chloro-2-hydroxyl ammonium chloride contains chlorohydroxyl groups which form 2,3- epoxy propyl trimethyl ammonium chloride under alkalineconditions.
This epoxide has two positive heads, one head for dye attachment and the other for permanent bonding with the fibre. The chemical 3-chloro-2hydroxyl propyl trimethyl ammonium chloride is popularly known as CHIPTAC while the epoxide is called EPTAC Subramanian et al.,(2006), Karnik, (2002).
The general structure of EPTAC is illustrated in Fig.2.6 below
R1
H2C CH CH2+N R2 X-
O R3
Fig 1.7 shows general chemical structure of EPTAC. Adapted from Adam,(2000), Karnik, (2002), Goli,(2008).
Where R1, R2, R3 and R4 are alkyl groups and X- is halide ion (Cl, or Br).
1.4.3: Conversion of CHIPTAC to EPTAC.
A solution of CHIPTAC in the right container is treated with a solution of caustic
soda prepared from a standard stock solution. The caustic soda solution is added in
drops over a period of 20 minutes. The mixture is then allowed to stand forfurther
28
30minutes to effect completereaction. Within the time span, CHIPTAC will be
converted to the epoxide form(EPTAC). The loose cotton cellulosic fibres are then
treated with EPTAC to achieve cationisation.Shaukat et al., (2009, Adam, (2000),
Chavan et al. , (2000). Karnik, (2002).
CHIPTAC EPTAC
1.4.4: REACTION OF EPTAC WITH CELLULOSE.
By treating cotton in alkalinecondition with EPTAC cationization of cellulose is
achieved, thus the fibre acquires positive charges which will now serve as sites for anionic
dyes attachment sites. It is worth noting however that there might be a side reaction which
will yield unwanted product named 2,3-dihydroxypropyltrimethyl ammoniumchloride, which
does not react with cotton cellulose at all ( Goli et al.,(2008); Haruna,(2011).
EPTAC COTTON EPTAC-COTTON COMPLEX
FIG. 1.9Cationisation of cellulose. Adapted from Karnik, (2002, Haruna, (2011), Goli,
(2008).
1.4.5: Hydrolysis of EPTAC.
Fig 1.8 Conversion of CHIPTAC to EPTAC. Adapted from Goli, (2008), Karnik, (2002)
N
+ CH3
H3C
Cl
HO
CH3
N
+
CH3
CH3
O
CH3
Cl NaOH
–
Cl
–
Cell OH
N
+ CH3
CH3
H3C
O
+ Cell HO
– Cl
–
N
+
CH3
CH3
O
CH3 Cl
–
29
High solubility of EPTAC in water makes it easy for it to be hydrolyzed forming the unreactive 2, 3-dihydroxypropyltrimethylammoniumchloride which does not
react with cotton cellulose, for this EPTAC has to be applied to thecotton cellulose before dyeing; however, alkalis are used duringtheapplicationasfixing agent.
Potassium hydroxide is given preference because it improves wash fastness of the dyeing; though sodium hydroxide or its carbonate can also be used, but affects brightness as well as colour yield. Goli, (2008).
O CH3 CH3
H2O
N+CH2- Cl- OH N+CH3Cl- + NaCl
CH3OH- OH CH3
Fig 1.10 Hydrolysis of EPTAC in water. Adapted from Goli, (2008)
1.5: Kinetics and Thermodynamics of Dyeing
Generally all activities encountered during any dyeing process have direct relevance to the dye itself as well as the material (substrate) plus other variables. Montra et al (2006).In this case, which is dyeing cationised cotton with anionic dyes involving no application of salt or electrolyte, there are bound to be a lot of expectations to emanate there from. This unique case in which positive charges are imparted on the cotton cellulose chemical structure by the action of EPTAC poses another challenge. It is therefore important to make good study of the action of anionic dye on uncationised cotton celluloseand compare all the kinetics and thermodynamics parameters of the dyeing, Adam, (2000), Goli, (2008).Whenever cotton cellulosic fibre is immersed in dye solution under suitable condition, the fibre becomes coloured; thus the colour in the solution continuously decreases as long as the fibre is not saturated. In this process, there is diffusion ofthe dye molecules from dye solution to the
30
interior part of the fibre. Dye affinity for the fibre is the driving force; it determines the rate of the reaction, Goli,(2008).Furthermore, rate or speed of any simple process is a function of temperature. As dyeing involves adsorption, sorption and desorption, temperature plays an important role. From equation for heat of dyeing below it is evidently clear that there is a relationship between temperature and rate of dyeing, Giles et al,(1963), Saowanee et al.,(2007).
-ΔH =RΔlnKT
Δln K = -ΔH———————————- (1)
Δ( 1/T)R
.
Where:
ΔH is heat of dyeing, T is temperature, K is affinity and R is gas constant
Activation energy is given by the equation:
dln(k) = -ΔE* ——————————–(2)
d(1/T) R
Where:
k is the rate constant and ΔE* is activation energy.
The two equations above paved way for the fact that there is a strong inter- relationship between all the parameters quoted.
1.5.1: Kinetics of Dyeing.
31
Kinetics is a branch of physical chemistry. It is of prime importance to the practical dyer as valuable information regarding the rate or speed at which dye is absorbed by a given textile material from a dye bath can be obtained.
In talking kinetics of dyeing, it is necessary to explain adsorption curves which are plots of amount of dye adsorbedon- y – axis against time on- x – axis.
Equilibrium point
Time (min)
Fig: 1.11 Dye adsorption curve. Adapted from, Bird et al (1975).
Rate of dye adsorption is dependent on many factors apart from the nature of both dye and fibre such as:
(I) Temperature
(II) Agitation (circulation of dye liquor)
(III) Fibre structure
(IV) Dye bath volume (liquor ratio)
(V) pH of the dye bath (acidic, neutral or alkaline)
(VI) Electrolyte content of the dye bath
(VII) Dye affinity
(VIII) Concentration of the dye in the bath
Other parameters are migration, diffusion and dye anchoring (attachment to the fibre), Bird et al, (1975).
1
2
3
Dye
Adsorbed
32
1.5.1.1: Dyeing Temperature
Although dyeing can occur at cool or hot condition depending on the nature of dye, temperature change is known to significantly affect the rate of dyeing since the molecules in the bath are in constant random motion, having kinetic energy. Supplying additional heat by increasing temperature means imparting additional kinetic energy which can cause increase in the rate of reaction in endothermic situation or a decrease for exothermic condition, Burcu et al, (2011). Effect of temperature on dyeing is investigated by plotting temperature against time of dyeing(Burcu et al, (2011), Montra et al, (2006, Saowanee et al,(2005).
% Exhaustion
600C 400C
Time (min)
Fig: 1.12 Curve showing effect of temperature on time of dyeing. Adapted fromGiles et al, (1963).
1.5.1.2: Agitation (Dye liquor circulation)
Stirring dye solution is noted to have direct bearing on the rate of dyeing, for in many cases this influences exhaustion rate and reduces the time of dyeing as well as impart an improvement in levelness, Gile et al, (1963).
33
1.5.1.3: Fibre structure
Substantivity of dye for given substrate is purely dependent on chemical structure of the dye and that of fibre; however, modification of the fibre structure can alter this either positively or otherwise, sometimes this can make a dye which is not substantive to a material to become substantive.Other parameters such as liquor ratio, pH,electrolyte, dye affinity and dye concentration all influence dyeing in their own ways, Giles et al, (1963).
1.5.1.4: Diffusion of Dye into Fibre.
Diffusion is important in any dyeing process. It is the movement of dye molecules from regions of high concentration to those of low concentration, Bird et al, (1975).
Fick’s law states that:
Rate of diffusion =ds = -Ddc
dt dx
Where
ds = Amount of dye diffusing across unit area per unit time interval
dt = The time interval
dc = change in concentration
dx = change in distance
dc/ dx = concentration gradient
-D = Diffusion coefficient.
To vividly understand diffusion, it is necessary to consider the fibre to be across section of a circular rod made of concentric circles dipped in a dye bath with high concentration of dye molecules initially. Bird et al, (1975).At first a thin layer separating dye molecules in the bath and fibre surface is established; gradually concentration of dye molecules continues to build up. This development results in concentration gradient which then facilitates diffusion of dye molecules into the fibre. Gradually this process continues up to the time when there is an
34
even distribution of dye molecules all over the entire surface and interior of the fibre. At this point saturation point is reached, hence equilibrium is attained and diffusion will cease. Bird et al, (1975), Giles et al, (1963).
FIG 1.13: Illustration of ring dyeing effect
1.5.1.5: Measurement of Diffusion Coefficient
Dyeing is carried out for an equal time interval say 15, 30, 45, minutes, 1 hour etc. Adsorption of dye is determined by either stripping the dye adsorbed by each piece
for the given time, and then determining the concentration by colorimetric method or simply determining the concentration of dye in solution after dyeing:
Dye adsorbed= initial concentration – concentration in solution after dyeing.
The dye absorbed is plotted against √t (square root of time of dyeing). The slope of graph obtained gives the diffusion coefficient. Bird et al, (1975).
(√t)
Thin film
Fibre core
Dye bath
Fibre surface
Fibre interior
a
b
Dye
adsorbed
Slope = a/b = D
Diffusion coefficient
35
Fig: 1.14 Dye absorbed versus square root of dyeing time (√t). Adapted from (Giles et al, (1963), Bird et al, (1975).
Another indirect way of determining diffusion coefficient is the time of half dyeing.
In this approach, dyeing is carried out at a particular temperature, exhaustion at different times is measured till equilibrium is reached, i.e. time at which the exhaustion remains constant. By plotting % exhaustion against time gives the graph below.
Another alternative way is using infinite dye bath which is a dye bath with constant dye concentration;
in this case dyeing is carried out for a given period oftime andnegligible amount of dye solution is pipetted out over a range of constant time intervals.Absorbencies of all withdrawn dye solutions within the time intervalsare measured. Plot of absorbance against time interval can also be used to obtain diffusion coefficientMontra et al, (2006).
% Exhaustion
50 —-
t½ Time (min)
Fig 1:15 exhaustion versus time of dyeing. Bird et al, (1975).
From the y-axis (% exhaustion), point where 50% exhaustion is occurs, a line is extended to cut the curve at the point shown.By means of dotted line a vertical is drawn to cut the x- axis. Time value at this point is the time of half dyeing (t½).
100
36
It is worth noting that value of t½ characterizes the dyestuff into fast or slow dyeing. Any dyestuff that exhibited high value of t½ is slow dyeing, while lower value of t½ means fast dyeing. Giles et al, (1963).
1.5.2: Adsorption Isotherms
Isotherms simply imply similar or same temperature. In kinetics of dyeing adsorption isotherms are curves depicting dye adsorbed by the textile material in relation to the amount of dye remaining in solution as functions of the original (initial) dye concentration in the dye bath before dyeing began.
Isotherms are graphical plots, that give information or idea as to which dye can be absorbed by which textile materials. There exists three popular isotherms, with each one named after its founder, Konaghatta et al, (2010),Saowanee et al, (2005), Montra et al, (2006).
(I) Langmuir isotherm
(II) Freundlich isotherm
(III) Nernst isotherm
All these three isotherms are applicable to particular dye-fibre systems. It’s therefore important to make choice of the correct isotherm in order to obtain correct results, Bird et al, (1975).
1.5.2.1: Langmuir Isotherm
The fact that cationised cotton acquired positive charges thus giving sites for dye attachment means that this isotherm is applicable.
Langmuirisotherm is established based on the equation:
[D]f = k[D]s( [S] ƒ – [D]ƒ)
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Where
[D]ƒ = concentration of dye in fibre
[D]s = concentration of dye in solution after dyeing
[s]ƒ = maximum amount of dye the fibre can accommodate at equilibrium.
k = a constant
` [D]S
Fig: 1:16Langmuir Isotherm.
Plotting a more appropriate reciprocal relationship of these parameters in Langmuir equation:
1/[D]f = 1/k[S]f [D]s + 1/[S]f
By plotting 1/[D]fagainst 1/[D]S, a straight line curve is obtained which cut the y- axis at a point (intercept) representing 1/[S]f and the slope is 1/k[S]f.
Slope =1/k[S]f
1/[D]f
1/[S]f
[S]ƒ
[D]ƒ
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1/[D]s
Fig. 1.17: Langmuir Reciprocal plot.
1.5.2.2: Freundlich Isotherm.
This Isotherm governs absorption of direct dyes by cotton cellulose material. In this case there are no specific sites for dye attachment. Dye molecules are retained by weak forces such as: H-bond, Van der Waals forces etc. Bird et al, (1975).
Materials under dyeing condition are assumed to be porous solid with large area for absorption of dyes. At equilibrium the concentration of dye in the fibre is proportional to concentration in the dye solution raised to power of ―x‖ (Giles et al, 1963).Equation governing this condition is as shown below:
[D]f = K [D]SX
Where:
[D]f is dye concentration inside the fibre.
[D]S is dye concentration inside the solution.
x and K are constants with x usually equals to 0.5.
When [D]f is plotted against [D]S, the graph obtained is as shown below:
[D]f
[D]S
Fig 1:18 Freundlich isotherm.
By plotting log [D]f against log [D]S a straight line is obtained which gives better and more useful curve. log [D]ƒ = log [D]s + log k
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Slope = a/b = x
Log k
Fig: 1:19 Freundlich isotherm logarithmic plot.
1.5.2.2.3: Nernst Isotherm
This isotherm is strictly based on solution model, dye is assumed to be fully dissolving inside the fibre due to its high affinity as such dyeing is viewed as distribution of solute between two immiscible liquids (solvents) (Bird et al, 1975,Giles et al, 1963).
At equilibrium, this system obeys the equation below:
[D] ]f = K[D]S
Where
[D] f = concentration of dye in the fibre.
[D] S =concentration of dye in the solution.
K = partition co-efficient (constant)
By plotting [D]ƒ against [D]s,a straight line plot is obtained as shown below:
a
b
[D]S
Fig: 1.20 Nernst isotherm
1.5.3: Affinity of Dye for Fibre
[D]ƒ
Slope = a/b = K
Log (D)ƒ
b
a
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Affinity is a measure of how dye molecules have desire to leave dye bath and move into fibre matrix and be retained there in. Affinity is clearly understood when it is explained on the basis of exhaustion which is a ratio between concentrations of dye in material to that in liquor or is related to number of dye molecules distribution between them. When the material has more number of molecules than the dye bath, the dye is thus termed to have higher substantivity for the material.However, affinity is not determinable from single exhaustion data, but on basis of difference in chemical potential of dye molecules in the fibre matrix to that in dye solution.
Any dye that exhibits higher value of chemical potential difference between fibre and dye bath is said to have affinity for the fibre in question (Bird et al, 1975).
Beside the fact that affinity is a quantitative measure of substantivity, it also has direct bearing on the wet fastness property of the dye on the fibre. Higher-affinity simply means good wet fastness properties (Giles et al, 1963).
. Affinity is expressed as:
-ΔUo = RT ln K.
Where:
K is partition co-efficient.
R is gas constant and
T is absolute temperature.
1.5.4: Heat of Dyeing.
From thermodynamics point of view, affinity ΔUO, heat of dyeing (ΔH0) and entropy (ΔS0) have direct relationship as shown by the equation below:
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ΔU0 = Δ H0 – TΔS0
The heat of dyeing (ΔH0) when viewed from sorption and desorption of dye molecules, means difference in heat content between that needed to free one mole of dye from the solvent and that required to free same quantity from the surface or interior of the substrate by thermal agitation (Azlan et al, 2006, Bird et al, 1975, Giles et al, 1963).
High negative value of ΔHo implies high potential affinity which means the dye is substantive. Plotting affinity values at different temperatures against temperature, a straight line graph will be obtained. The straight line will cut y-axis at a point and value of the intercept gives value of ΔH0 while slope will gives value of ΔS0.
Slope = ΔS0.
ΔUO
Intercept = ΔHO
Temperature (OC)
Fig 1:21Heat of dyeing curve (a).
Furthermore, heat of dyeing can also be obtained from the equation below:
ΔHO = σ (ΔUO/T)
σ (1/T)
ΔUO and Δ HO are in standard form.
Integrating the equation it becomes:
∫ ΔHO = ∫ σ (ΔUO/T)
σ (1/T)
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so the relation becomes:
ΔHO/T = ΔUO/T +C.
By plotting ΔUo/T against 1/T, value of heat of dyeing can be determined using slope calculated therefrom. Intercept on y- axis represents the constant C which is known as molar entropy.
C
ΔUoSlope = ΔHO
1/T
Fig 1.22 Heat of dyeing curve (b)
In situations with only two values of temperatures T1 and T2 with corresponding values of affinities ΔUo1 and ΔUo2, value of heat of dyeing(ΔHo) can be calculated from the equation below (Bird et al, 1975):
ΔHo = T1ΔUo1 – T2ΔUo2
T1 – T2
In many cases of dyeing systems, value of ΔHo is negative which imply exothermic process.
1.5.5: Entropy of Dyeing.
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In solution of dye, both molecules of dye and that of solvent are in constant random motion; this situation usually results in a state of high level of disorder or randomness. This level of disorder is what is known as entropy.
The entropy governs the equilibrium state of chemical reaction at high temperature provided that heat of reaction is constant and the system is independent of temperature. At higher temperature, thedye molecules acquire high kinetic energy as such the entropy will be high. The entropy controls the equilibrium state of the system which thus facilitates movement of the dye into the fibre matrix (Burcu et al 2011, Saowanee et al ,2005, Tan, et al 2010),.
Entropy of a given dyeing system denoted by ΔSO can be obtained from graph of affinity against temperature as could be seen from the equation below:
ΔUo = ΔHo – T ΔSo
Where: ΔUo is affinity, ΔHo is heat of dyeing.
T is temperature and ΔSo is entropy.
1.5.6: Kinetics of Dyeing.
Kinetics of dyeing implies study of rate or speed at which chemical reaction proceeds as well as its mechanism. Kinetics is concerned with various species of molecules type taking part in the reaction and their individual concentrations. Viewing dyeing under this concept, dyeing has numerous kinetic properties such as: exhaustion, rate of dyeing, activation energy etc. (Burcu et al 2011, Konaghatta et al 2010, Montra et al 2006, Tan et al 2010).
1.5.6.1:Activation Energy.
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Whenever reactants are brought together in a reacting medium, product will be formed. In other words, the reacting molecules acquire excess kinetic energy which helps in making the reaction to move forward and this condition favours formation of product. The excess energy is what is known as Activation Energy.
Ideally dyeing is considered as mainly diffusion process involving reacting molecules which are in constant random motion from higher concentration point to that of lower concentration (Goli,2008).
Molecules diffusing from solution into fibre matrix are function of total number of molecules present as well as the relation; e–E/RT where R is gas constant,
T is the thermodynamic temperature of reaction and E is the minimum energy value required for the diffusion to occur (Konaghatta et al 2010).
It is a known fact that any rise in temperature will cause a rise in diffusion rate for endothermic reaction only (Haruna 2011). The temperature rise brings about an increase in the activation energy of the diffusing molecules ―E‖ as could be seen from the equation below:
DT = DO –E/RT ( D T is temperature dependent).
Where:
Do = constant.
R = gas constant.
T = absolute temperature
By taking natural logarithm on both sides, the equation becomes:
ln DT = ln DO –E/RT.
2.303logDT = 2.303logDO –E/RT.
Log DT = log DO – E/2.303RT.
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By plotting a graph of log DT against 1/T a straight line curve will be obtained; its slope will be –E/2.303R, hence value of E can be calculated.
Intercept on the graph gives log DO.
Log DO
Log DT Slope = -E/2.303R
1/T
Fig 1:23 Activation energy curve.
When the value of activation energy is high, diffusion rate will be very slow as such dyeing will take longer time and from this it can easily be deduced that affinity of the dye for the fibre is also low. However low activation energy entails faster rate of dyeing and stronger dye-fibre affinity (Subramanian et al, 2006).
1.5.7: Exhaustion Characteristics.
In general all information on characteristics regarding exhaustion of dye in a textile substrate is obtainable from the exhaustion curve. In plotting this curve, percentage exhaustion which is a ratio of concentration of dye inside the fibre to that inside the original dye bath at specified condition of temperature, dye composition and concentration as well as liquor ratio are considered (Bird et al 1975).
% Dye exhaustion =mg of dye in the fibre x 100
mg of dye in original dye bath.
Exhaustion curves are constructed by making up enough number of dye baths (10-15) with the same conditions (dye assistant, temperature, liquor ratio etc.) and dye concentration.
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All the dye baths are placed in the water bath at selected temperatures and are conditioned to the temperature of the water bath. This is followed by dipping of equal mass of material inside each dye bath. These baths are then removed after reaching selected span of time. Dyed substrates are removed and rinsed thoroughly to rid them of any unfixed dye.Residual water used for rinsing should be added to residual dye baths.
Absorbencies for initial dye bath and its corresponding residual bath are measured and recorded using spectrophotometer. From values of each pair of absorbencies, % exhaustion is obtained:
% Exhaustion= Absorbance of original dye bath – absorbance of residual bath x100
Absorbance of original dye bath
By plotting % exhaustion against time, exhaustion curve is obtained:
% Exhaustion
Time (min)
Fig 1:24 Curve of % exhaustion versus time.
It can be seen that dye molecules are rapidly absorbed at first, but gradually reduce up to the time when equilibrium is attained so absorption ceased
This curve can be used to determine time it takes a system to reach 50% exhaustion which is termed as time for half dyeing (t1/2).Time of half dyeing (t1/2) is obtainable when exhaustions at different temperatures are plotted against time of dyeing. Dotted line is drawn from point of equilibrium to cut y-axis at a point, 50% of this value is extended to cut the curve, this point is extended downward to touch time axis (x-axis), and the value of the time at that point is the t1/2.
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It is worth noting that dye stuff which exhibits high value of t1/2 is slow dyeing, while that with low value of t1/2 is fast dyeing type. In addition, t1/2 can also shed more light on the substantivity of dye for a given fibre (Bird et al 1975, Giles et al 1963).
1.6: Statement of the Problem
Conventional dyeing of cotton is associated with problems of very low dye exhaustion and poor performance properties(i.e. wash and light fastness), which are attributed to lack of dye attachment sites at the same time weaker dye-fibre bonds. These problems call for after treatments processes which thus increase cost. In addition large amount of water, electrolytes, (salt) and electric energy are also usefor comparable fewthousands meters of dyed cotton fabrics.Apart from these problems, another issue is that of excessive discharge of effluents containing dissolved and undissolved salts, which when discharged, affect the natural environment and species in the ecosystem (Haruna, 2011).
1.7: Aims and Objectives ofthe Research.
The aims and objectives of this research are as follows:
i) To cationise 100% cotton cellulose in order to make it susceptible to anionic dyes
ii) To study the kinetics and thermodynamics of dyeing of cationised and uncationised cotton cellulose with anionic dyes (i.e. direct dye).
iii) To reduce environmental pollution.
iv) Reduce cost of dyeing.
1.8: Scope of the Research
This research is limited to studies on cotton sliver using a single commercial direct dye, Cuprophenyl Brilliant Blue
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