Report on Textile dyes and their application process

Contents

Serial	Topic	Page
1	Introduction, Coloration & Dyes	1
2	Classification of dyes	1,2
3	Classification table according to application	2,3
4	Information about Popular dyes	3,4,5,6
5	Other important dyes	6,7
6	Brief discussion about dyes selection on fibers	7,8,9
7	References	9

Introduction

Production of colored textiles is one of the basic technologies in human civilization. Textile consumption is steadily increasing worldwide, following the growth of world population and stimulated by a growing GDP in many countries, primarily in Asia .The scale and growth of the dyes industry has been complicate linked to that of the textile industry. World textile production has grown steadily to an estimated 35 ×1000000 ton in 1990 . The two most important textile fibers are cotton and polyester. Consequently, dye manufacturers tend to concentrate their efforts on producing dyes for these two fibers. The estimated world production in 1990 was 1x 1000000t. The figure is significantly smaller than that for textile fibers because a little dye goes a long way. For example, 1 t of dye is sufficient to color 42 000 suits . Overall the demand of dyeing chemicals are rapidly increasing from the beginning of industrialization of textile.

Textile Coloration

Textile coloration is a generalize process of textile where the fiber become colored by specific dyestuff with the help of dye- fiber interaction. In textile dyeing process , the use of dyestuff is mandatory.

Dyes

Dyes are the chemical which is use to make colored textile. It has a vast derivative area . Here we are giving a short discussion about dyestuff classification and their selective methodology below.

Classification of Dyes

There are several ways for classification of dyes. Those are listed below,

Dyes according to the source

A very common classification of the dyestuff  is based on the source from which it is made. According to source, dyes are generally two types. Those are as follows,

• Natural dyes

• Synthetic dyes

Natural dyes

Man has been used coloring materials over thousand years. Most of that case the sources of these coloring materials are nature. Which dyes are the common derivatives of natural resources those are called as natural dyes. Natural dyes are often negatively charged. For special case it might be positively charged. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged. The use of dyes is very ancient. Kermes (natural red 3) is identified in the bible book of  Exodus, where references are made to scarlet colored linen.

Today, many of the traditional dye sources are rarely but some of our most common dyes are still derived from natural sources. These are also termed as natural dyes.

Synthetic Dyes

Dyes derived from organic or inorganic compound are known as synthetic dyes. Examples of this class of dyes are Direct, Acid, Basic, Reactive, Mordant, Metal complex, Vat, Sulfur , Disperse dye etc.

Dyes according to the nuclear structure

Though not very popular but dyes can be categorized into types by using this method of classification:
• Cationic Dyes
• Anionic Dyes

Industrial Classification of the Dyes

As globally majority of the dyestuff is primarily consumed by the textile industry. So, at this level a classification can be done according to their performances in the dyeing processes. Worldwide around 60% of the dyestuffs are based on azo dyes that gets consumed by in the textile finishing process. Major classes of dyes in textile finishing are given here. Major Dye classes and the substrates:
• Protein Textile Dyes
• Cellulose Textile Dyes
• Synthetic Textile Dyes

Now the most common specified classification of dyes according to their application are giving on the table shown below :

Specification	Application area	Method of application
Acid	Nylon, wool, silk, paper, inks, and leather.	Usually from neutral to acidic dyebaths.
Azoic	Printing inks and pigments.	Fiber impregnated with coupling component and treated with a solution of stabilized diazonium salt.
Basic	Paper, polyacrylonitrile, modified nylon, polyester and inks.	Applied from acidic dyebaths.
Direct	Cotton, rayon, paper,leather and nylon.	Applied from neutral or slightly alkaline baths containing additional electrolyte.
Disperse	Polyester, polyamide, acetate, acrylic and plastics.	Fine aqueous dispersions often applied by high temperature/pressure or lower temperature carrier methods; dye may be padded on cloth and baked on or thermo fixed.
Reactive	Cotton, wool, silk, and nylon.	Reactive site on dye reacts with functional group on fiber to bind dye covalently under influence of heat and pH (alkaline).
Solvent	Plastics, gasoline, varnishes, lacquers, stains, inks, fats, oils, and waxes.	Dissolution in the substrate.
Sulphur	Cotton and rayon.	Aromatic substrate vatted with sodium sulfide and reoxidized to insoluble sulfur-containing products on fiber.
Vat	Cotton, rayon, and wool.	Water-insoluble dyes solubilized by reducing with sodium hydrogensulfite, then exhausted on fiber and reoxidized.
Mordant	Wool, leather, and anodized aluminium.	Applied in conjunction with Cr salts

Now a brief discussion about some of most popular dyes are listed below :

Direct dyes

The name ‘direct dye’ alludes to the fact that these dyes do not require any form of ‘fixing’. They are almost always azo dyes, with some similarities to acid dyes. They also have sulphonate functionality, but in this case, it is only to improve solubility, as the negative charges on dye and fibre will repel each other. Their flat shape and their length enable them to lie along-side cellulose fibres and maximize the Van-der-Waals, dipole and hydrogen bonds.

Structure of a typical direct dye is given below,

Note that the sulphonate groups are spread evenly along the molecule on the opposite side to the hydrogen bonding -OH groups, to minimize any repulsive effects.

                                                    Fig : Structre of  Direct dyes

Vat dyes

Vat dyes are a good example of the cross-over between dyes and pigments. Large, planar and often containing multi-ring systems, vat dyes come exclusively from the carbonyl class of dyes (for example, indigo). The ring systems of the vat dyes help to strengthen the Van-der-Waals forces between dye and fibre.

                                                            Fig : Structure of  Vat dyes

Basic dyes

Basic dyes possess cationic functional groups such as -NR3+ or =NR2+. The name ‘basic dye’ refers to when these dyes were still used to dye wool in an alkaline bath. Protein in basic conditions develops a negative charge as the -COOH groups are deprotonated to give -COO-Basic dyes perform poorly on natural fibres, but work very well on acrylics. A general structure of an acrylic type polymer is shown below. It is simplified, and doesn’t show any anionic groups which are often present.

                                                          Fig : Structure of  Basic dyes

Reactive Dyes

A reactive dye will form a covalent bond with the appropriate textile functionality. This is of great interest, since, once attached, they are very difficult to remove. The first reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially availablreactive dyes for protein and polyamide fibres. In theory, reactive dyes have been developed for other fibres, but these are not yet practical commercially. Although reactive dyes have been a goal for quite some time, the breakthrough came fairly late, in 1954. Prior to then, attempts to react the dye and fibres involved harsh conditions that often resulted in degradation of the textile.

Basic interaction of reactive dyes are shown below,

Disperse dyes

Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. The general structure of disperse dyes is small, planar and non-ionic, with attached polar functional groups like -NO2 and -CN. The shape makes it easier for the dye to slide between the tightly-packed polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures.

The dye is generally applied under pressure, at temperatures of about 130°C. At this temperature, thermal agitation causes the polymer’s structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-der-Waals and dipole forces.

A common structure of disperse dyes are given to another page ,

                                                   Fig : Structure of   Disperse dyes

Mordant dyes

Mordant is a Latin word meaning ‘to bite’. Mordants act as ‘fixing agents’ to improve the colour fastness of some acid dyes, which have the ability to form complexes with metal ions. Mordants are usually metal salts; alum was commonly used for ancient dyes, but there is a large range of other metallic salt mordants available. Each one gives a different colour with any particular dye, by forming an insoluble complex with the dye molecules.

Fig : Structure of Mordant dyes

Other important dyes

A number of other classes have also been established, based among others on application that includes the following:

·         Leather Dyes – Used for leather.

·         Oxidation Dyes – Used mainly for hair.

·         Optical Brighteners – Used primarily for textile fibres and paper.

·         Solvent Dyes – For application in wood staining and production of coloured lacquers, solvent inks, waxes and colouring oils etc.

·         Fluorescent Dyes – A very innovative dye. Used for application in sports good etc.

·         Fuel Dyes – As the name suggests it is used in fuels.

·         Smoke Dyes – Used in military activities.

·         Sublimation Dyes – For application in textile printing.

·         Inkjet Dyes – Writing industry including the inkjet printers.

·         Leuco Dyes – Has a wide variety of applications including electronic industries and papers.

Among this large specified area of dyes, the specific fiber needs the specific one. So the dye selection for fiber is very important issue in textile dyeing process.

Dye selection

The selection of a specific dye on specific fiber for dyeing application  are termed as dye selection process.

Here some common reason of dyestuff selection factors for re-known textile  materials are given.

Reactive Dyes on Cellulose and Other Fibers

Reactive dyes are the newest class of dyes for cellulose fibers.ICI introduced the first group of reactive dyes for cellulose fibers in 1956. In the dye molecule, a chromophore  is combined with one or more functional groups, the so-called anchors, that can react with cellulose. Under suitable dyeing conditions, covalent bonds are form  between dye and fiber. One-third of the dyes used for cellulose fibers today are reactive dyes. The range of available reactive dyes is wide and enables a large number of dyeing techniques to be used. Shades ranging from brilliant to muted can be obtained. They have better wet fastness properties than the less expensive direct dyes. Chlorine fastness is slightly poorer than that of vat dyes, as is light fastness under severe conditions.

In addition to cellulose, many other fibers can be dyed with reactive dyes, providedthey have chemical groups capable of forming a chemical bond with the reactive dye, e.g. wool or polyamide fibers.

Reactive Dyes on Wool and Silk

Reactive dyes produce brilliant shades on wool with good fastness. They differ from reactive dyes for cellulose fibers because the reactivity of the chemo active groups in wool is considerably higher than that of the hydroxyl groups in cellulose. To achieve level dyeing on wool, dyes with reduced reactivity must be used, and an auxiliary agent added.

Direct Dyes on Cellulosic Fibers

In contrast to some naturally occurring dyes like indigo or kermes, which must be vatted or mordanted to be applied in textile dyeing but direct or substantive dyescan be used on cellulosic fibers directly. Their use is widespread because of their easy handling.

Direct dye can also easily handle the printing on cellulosic fiber.

Sulfur Dyes on Cellulosic Fibers

The dyeing of cellulose and its blends with synthetic fibers, is the main field of application of sulfur dyes. They are also used to a limited extent to dye polyamide fibers, silk, leather, paper, and wood. These dyes are used for deeper, muted shades, such as black, dark blue, olive, brown, and green, where their favorable price has its full effect. With respect to fastness, sulfur dyes are close to vat dyes but not equal to them. They have varying light fastness; the wash fastness at 60°C is good, and their fastness to boiling water is moderate. Wet fastness can be improved by after treatment of the dyeing with quaternary (poly) ammonium compounds or formaldehyde condensation products. Most sulfur dyeing are not fast to chlorine. Therefore, faulty dyeing can be stripped by treatment with sodium hypochlorite.

Acid Dyes on Wool

The presence of aliphatic groups in the acid dye molecule contributes to a substantial increase in binding to wool, converting leveling dyes to types that are fast to fulling. The sulfonic acid groups determine not only the number of possible ionic bonds to the fiber, but also hydration, which counteracts binding.

Disperse Dyes on Polyester

The Polyester fiber is quantitatively the most important synthetic fiber. Their inexpensive production from petrochemical raw materials and excellent textile properties alone and in combination with natural fibers guarantee PES fibers universal applicability.

PES fibers are hydrophobic, water-soluble dyes do not attach. In contrast, PES fibers can be dyed easily with water-insoluble, small molecular dyes originally developed for dyeing cellulose acetate. Since the preferred dyeing mediumis an aqueous liquor, the poorly water-soluble dyes must be dispersed before application.

Vat Dyes on Cellulosic Fibers

Vat dyes are water-insoluble, organic pigments that are used to dye cotton andother cellulose fibers. The principle of vat dyeing is based on chemical reductionof these dyes to the leuco compounds, which are soluble in aqueous alkali and exhibit fiber affinity, followed by re-oxidation within the fiber to the water-insoluble starting dye.

Azo (Naphtol AS) Dyes on Cellulosic Fibers

The dyeing process is carried out in steps: First, a cotton fabric is impregnated with a solution of sodium naphtholate and, after drying, is passed through a solution of a diazotized aniline derivative to give an attractive deep dyeing. Besides bases that need to be diazotized before use in dyeing, diazotized diazonium compounds in stabilized form are also being marketed as fast color salts. Cellulosic fibers in all processing states can be dyed with Naphtol AS combinations. Handling is safe and relatively simple.

Based on these above discussion we can include that all the derivatives of dyestuff and their proper application on suitable textile materials are very important for textile coloration process.

References

‘‘Industrial Dyes’’ Edited By : K. Hunger

‘‘Industrial Color Testing Fundamentals and Techniques’’ by _ ‘Voltz  H.G’

‘‘ Textinfo.blogspot.com’’

Special Thanks to Dr. Saiful Islam Sir for his precious assistance to make this project successfully.

 

Md. Bakhtiar Rana

Department of Textile Engineering

City University Bangladesh