CRITICAL SOLUTIONS IN THE DYEING OF COTTON TEXTILE MATERIALS


Over the decades there have been several papers on the coloration of cotton-based textiles. The number of articles dealing with the processing of cotton, including preparation, dyeing, and finishing, may be in the thousands. An investigation of the possible causes of problems occurring in the coloration of textiles revealed that a comprehensive review of case studies and scientific analysis would be a welcome addition to the already rich pool of knowledge in this area.

Please click the following link to download full article.

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Effective Colour Management for Textile Coloration- An instrumental way towards perfection


by:-   Zeeshan Khatri, G Yasin Sheikh, Khan Muhammad Brohi and Aslam A Uqaili

Textile coloration is a process of dyeing and printing textiles by use of colorants that include dyes and pigments. Meeting stringent requirement from Buyer that demands right colour on right time is not a simple task to achieve. The colour produced by the application of either dyes or pigments on textiles must be close matched with reference (standard) provided by buyer. The process of colour matching is a lengthy process and needs many trials to get close match. The colour quantification through instruments helps to cut most of the lead time, however, there is a serious need to manage colour during colour approval stage and coloration process. This paper presents a strategy towards effective colour management by using available instruments and techniques that involve in colour measurement and management systems, trips to control colour intelligently and give way to get closer match to the buyer’s reference in a shortest possible time.

To download full article please click on the following link.

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CO2 technology for water free dyeing………….


CO2 Technology

People are more familiar with the practical uses of carbon dioxide (CO2) than they realize. CO2 is the same stuff that puts the fizz in your soda at every restaurant, and dry ice is nothing more than the solid form of carbon dioxide.

CO2 can exist as a gas, solid, liquid or supercritical fluid, depending on pressure and temperature. We develop applications which use CO2 in its liquid or supercritical states.

As a solvent CO2 has gas-like viscosity and liquid-like density. It’s low surface tension and high diffusivity allow for exceptional penetration and material / particle transportation properties. It’s gentle processing is ideal for delicate or sensitive materials.

CO2 is becoming a key commercial and industrial solvent, largely due to its highly effective solvency powers and low environmental impact. Alternative solvents, such as hazardous chemicals or water, are rapidly being phased out. CO2 is the 21st century solvent.

No new CO2 is generated from our processes. We use CO2 that’s already there – it’s been recaptured from other industrial processes and recycled, so there is zero greenhouse gas effect. CO2 is non-toxic, non-hazardous, non-flammable, in-exhaustible and inexpensive. It leaves no residues or secondary wastes behind after processing and and has zero potential for soil or groundwater contamination. In most cases process costs are lower than comparable conventional processes.

Our solutions are implemented across the processing spectrum, including the food, pharmaceutical, flavors & fragrances, renewable energies and textiles sectors. Here’s a few of the most exciting successes:

Dyeing Textiles Without Water
The textile industry is believed to be one of the biggest consumers of water. In conventional textile dyeing large amounts of water are used, both in terms of intake of fresh water and disposal of waste water. On average an estimated 11 – 14 gallons of, otherwise drinkable, water is needed to process 1 lb of textile. FeyeCon, together with partners, developed a process to dye textiles with CO2. It’s a completely water-free dyeing process with considerably lower operational costs compared to conventional dyeing processes.
For more information visit www.dyecoo.com

Fast Diagnoses Without Toxic Solvents
In medicine certain diseases are detectable only through the sampling and testing of tissue. Conventional tissue processing techniques rely on the use of toxic chemicals and is an overnight process. FeyeCon developed a process that eliminates the need for hazardous chemicals and processes quick enough for same-day diagnosis. Laboratory staff are not exposed to toxic chemicals, no hazardous wastes are produced, laboratory efficiency is increased and patients get fast results – unprecedented improvements in histological processing.
For more information visit www.tispamedical.com

Clean Clothes, Clean Environment
Cleaning clothes, professional garments and other textile are traditionally water and/or chemical intense. “Eco friendly” and “organic” cleaning solutions have been marketed recently, none of them however are truly sustainable. Together with partners FeyeCon has developed the next generation of fabric and textile cleaning methods that use liquid CO2 as a cleaning solvent. Washing is gentle and requires no heat (to wash or dry), which translates into longer lasting garments. No water, no toxic chemicals – just clean clothes.
For more information visit www.co2nexus.com

COLORFASTNESS OF COTTON TEXTILES


INTRODUCTION

Today’s consumer is more sophisticated than ever. They are conscious not only of style and comfort, but also of care and durability. They demand a quality product. Market studies show that consumers make many purchase choices based on color. Therefore, a fabric’s ability to retain its original color is one of the most important properties of a textile product.

The colorfastness or color retention of cotton textiles is influenced by a number of variables that occur both pre-consumer and post-consumer. This report summarizes how variations in raw materials, chemicals, manufacturing processes and consumer practices all have an effect on the performance characteristics of a fabric. Manufacturers must understand how the many variables affect colorfastness to achieve the ultimate goal of consumer satisfaction.

COLORFASTNESS AND TEST METHODS

Colorfastness is defined by the American Association of Textile Chemists and Colorists as “the resistance of a material to change in any of its color characteristics, to transfer its colorant(s) to adjacent materials, or both, as a result of the exposure of the material to any environment that might be encountered during the processing, testing, storage, or use of the material.” In other words, it is a fabric’s ability to retain its color throughout its intended life cycle. There are many types of colorfastness properties that must be considered to provide the consumer with an acceptable product. The American Association of Textile Chemists and Colorists has over thirty test methods that evaluate different colorfastness properties. These include, but are not limited to wash, light, crock, dry cleaning, perspiration, abrasion and heat. The type of product being manufactured determines which types of colorfastness are important and therefore which test methods are relevant. For example, upholstery fabrics must have excellent lightfastness and crockfastness properties, whereas washfastness is important for clothing fabrics. Manufacturers must know a fabric’s intended end use in order to make processing decisions that will produce a product of acceptable performance.

TEXTILE MANUFACTURING PROCESSES AFFECTING COLORFASTNESS

1. Preparation

Many aspects in the textile manufacturing process of taking a loom state fabric to a finished product have an effect on the colorfastness properties. Preparation is the first stage of textile wet processing. Cotton fibers are approximately 95% cellulose. The non-cellulosic portion consists of natural products such as waxes, sugars, metals, and man-made products such as processing aids, grease, plastic, and rubber. To achieve optimum dyeing and finishing conditions, it is important that these impurities are thoroughly removed with minimal damage to the cotton fiber.

2.Dye Selection

Dyeing is the crucial step in determining the colorfastness performance of a fabric. The American Association of Textile Chemists and Colorists define a dye as “a colorant applied to or formed in a substrate, via the molecularly dispersed state, which exhibits some degree of permanence.” Dyeing is accomplished by immersing the textile in a dye bath, applying heat and chemicals to drive the dye onto the textile, and then rinsing the substrate to remove the surface dye. These principles are illustrated below.

Different dye classes are used for each fiber type. The table below shows which dyes can be used for which fibers.

Dye Classes Available for Different Fibers

Fiber Dyestuffs
Cotton & manmade cellulosics Direct, Vat, Sulfur, Naphthol, Reactive, Pigment
Polyester Disperse, Basic
Nylon Disperse, Acid, Premetallized
Acetate Disperse
Wool & Silk Acid, Premetallized
Acrylic Dispersed, Basic

Dye selection must be based on desired performance criteria, manufacturing restrictions and the costs a market can bear for each end product. Every dye has unique colorfastness properties. Some dyes are known for their excellent washfastness characteristics and others are known for their lightfastness properties. The structure of the dye, the amount of dye, its method of bonding to the fabric and dyeing procedures all contribute to a dye’s performance characteristics. Dye combinations in a specific formulation must also be evaluated for their effect on colorfastness. Heavy shades often have reduced fastness properties. When high concentrations of dye are required, proper rinsing and washing off procedures are essential. However, due to entrapped dye particles within the cellulose structure, some unbound dye molecules can still remain and contribute to color loss and dye transfer

DYES FOR COTTON

Dyes can be categorized based on the mechanism by which they become fixed to a fiber. Dyes used for cotton fibers can be categorized into the surface bonding, adhesion, or covalent bonding mechanisms.

Pigments are sometimes used to color cotton fabrics, however they are not considered dyes. They are completely insoluble in water and have no affinity for cotton fibers. Some type of resin, adhesive, or bonding agent must be used to fix them to the cotton fiber. Typically, they exhibit good colorfastness to light and poor colorfastness to washing.

Direct dyes are water soluble and categorized into the surface bonding type dye because they are absorbed by the cellulose. There is no chemical reaction, but rather a chemical attraction. The affinity is a result of hydrogen bonding of the dye molecule to the hydroxyl groups in the cellulose. After the dyestuff is dissolved in the water, a salt is added to control the absorption rate of the dye into the fiber. Direct dyes are fairly inexpensive and available in a wide range of shades. Typically, they exhibit good lightfastness and poor washfastness. However, by applying a fixing agent after dyeing the washfastness can be improved dramatically.

Vat, sulfur, and naphthol dyes are fine suspensions of water insoluble pigments, which adhere to the cotton fiber by undergoing an intermediate chemical state in which they become water-soluble and have an affinity for the fiber. Typically, vat dyes exhibit very good colorfastness properties. Sulfur dyes are used to achieve a low cost deep black. They exhibit fair colorfastness properties, although the lighter shades tend to have poor lightfastness. Naphthol dyes are available in brilliant colors at low cost, but application requirements limit their use. They exhibit good lightfastness and washfastness, but poor crockfastness.

Reactive dyes attach to the cellulose fiber by forming a strong covalent (molecular) chemical bond. These dyes were developed in the 1950’s as an economical process for achieving acceptable colorfastness in cellulosic fibers. Bright shades and excellent washfastness properties are the trademark of reactive dyes. One concern regarding reactive dyes is their susceptibility to damage from chlorine. Another is that lighter shades tend to have reduced lightfastness properties.

The following table summarizes the fastness properties of the dye categories or classes available for dyeing cotton fabrics. Keep in mind that these are generalizations. Every dye is unique and some dyes within a particular class may behave differently.

Finishing

Finishing is the final stage of textile wet processing. Different types of finishes can be utilized depending on the desired performance characteristics of the end product. Resin and enzyme treatments are common finishing techniques that can influence the colorfastness of textile fabrics. Crosslinking resins are used to improve the durable press or wrinkle resistance of a fabric. Generally, resin treated fabrics demonstrate improved color retention to laundering. However, this increase in color retention comes at the expense of reduced physical properties of the fabric. Silicone softeners incorporated into the resin finish bath may further improve color retention for some fabrics. Softeners and resins play a key role in reducing surface abrasion and therefore improved overall wash performance. Cellulase enzymes are used to remove surface fibers that can create a fuzzy appearance on the surface of a fabric. Generally, enzyme treated fabrics show improved ability to maintain their original color and appearance after multiple home launderings. The degree of improvement from any of these finishing techniques is highly dependent on the individual dyes used in a particular formulation to achieve a given shade

CONSUMER PRACTICES

Manufacturers can follow every recommendation and precaution to produce a fabric with optimum performance characteristics. However, colorfastness properties are also influenced by consumer practices. These include laundry detergent selection and wash procedures. Therefore, when evaluating colorfastness properties of a product it is important to use the appropriate test method that accurately reflects the consumer laundry practices. Due to higher energy costs consumers are laundering clothes at lower temperatures. For this reason detergent with “color safe” or activated peroxy bleaching agents, which improve cleaning efficacy at lower wash temperatures, are one of the fastest growing segments of the home laundry market. Some fabrics may fade a little when home laundered with standard detergent, but fabrics laundered with detergents containing activated bleach can show significant losses in color strength as determined by the sensitivity of the dye to those detergents. Another type of detergent available to consumers is those containing enzymes, which remove surface cellulosic fibers from the fabric. Many times the loss or apparent loss of color can be attributed to surface changes in the fabric caused by abrasion during laundering. Detergents containing enzymes generally reduce the color change associated with home laundering by decreasing the fuzziness of a fabric’s surface. Wash procedures also influence a fabric’s ability to retain its color. Consumer practices such as washing clothes inverted, reducing the wash load size, adding softener to the final rinse and reducing the tumble dry time minimize color loss.

CONCLUSIONS

The colorfastness of cotton textiles can be a complicated subject. Fiber quality, yarn formation, fabric construction, textile wet processes and consumer practices can all have an influence on the performance characteristics of a fabric. Of these variables, the choices made during textile wet processing have the most significant effect on the colorfastness properties. Dye selection is of the utmost importance. Consumer practices such as detergent selection and laundering techniques also play a major role in the color retention of a fabric. Customer satisfaction should improve as manufacturers gain experience and knowledge in understanding and controlling the many aspects that influence colorfastness.

REf. TECHNICAL BULLETIN( www.cottoninc.com)

Chemical forces responsible for dyeing


The internal surface of fibers and its importance

The nature fibres, i.e. the cellulosic and protein fibres have exceedingly large internal surfaces, which are the walls of the channels between the bundles of long-chain molecules composing the fibres. The number of such channels is immense, of the order of ten million in the cross-section of, e.g. cotton or a wool fiber, and the total surface of their walls is of the order of 100 m2 or five acres per lb. This is about one thousand times as the outer surface of the fiber.

Whets the fiber is wetted, water rapidly penetrates and swells a large proportion of these channels, and Dyes in solution are then able to diffuse into the channels or pores. They can however enter only a relatively s proportion of the total internal space, because the remainder is in pores too small to admit a dye molecule. Many of the synthetic polymer fibres have much less internal surface than the natural fibres, but the dye used with such fibres are able to penetrate between the fiber molecules even though water cannot always do so.

Dyes are surface-active substances, that is, when dissolved in water their molecules tend to concentrate more closely together at a surface than in the body of the solution .the surface (or interface) can be that between the solution and either air or a fibre. The first action in any dyeing operation is the concentration of dye molecules at as much of the
internal surface of the fibre as they can reach. The concentration so produced is not usually sufficient to give a useful deep coloration to the fibre, and for such coloration other factors must be brought into play. These are the chemical forces, which can operate between a dye molecule, and fibre molecule, which are classified below, and also those between the dye molecules themselves. Which can cause their association into larger units?

The main physical and chemical effects between fibres and dyes

Broadly, four main chemical effects subsequently responsible for the substantively of the dye for the fibre are list below:

· Hydrogen bonds
· Non-polar or van der Walls’ forces
· Electrostatic or ionic forces –
· Covalent bonds.

These seldom act in isolation; usually at least two operate in any dyeing process.

· The hydrogen bond

This is the ‘secondary valency’ by which a hydrogen atom in e.g. a hydrogen group can form a weak association with another atom. Most fibres and dyes contain groups that can take part in this form of combination. There is evidence for the importance of hydrogen bonds in dyeing some man-made fibres. e.g. cellulose acetates and possible cellulosic protein fibres.

· Non-polar forces

This is a manifestation of the universal tendency, of atoms and molecules to attract one another. They seem to be particularly effective in attracting a dye to a fibre when the two have certain special characteristics, e.g. either when they both have long and fairly flat molecules, as with cellulose and direct or vat dyes and also with cellulose acetate and disperse dyes, or when they both contain a considerable proportion of purely hydrocarbon groups (aliphatic or aromatic) as with some dyes applied to wool and most dyes applied to polyester. In the latter cases the presence of the water of the dye bath assists the dye-fibre attraction because hydrocarbon groups tend to escape from water and associate together. This effect is known as ‘hydrophobic bonding’.

· Ionic forces

The third form of attraction between dye and fibre is due to difference of electric charge between them. In water, fibres become negatively charged and. Since most water soluble
dyes are anionic, their coloured ion carrying a negative charged, adsorption does not occur readily. It is then necessary to reduce or even reverse the charge on the fibre before the dye ion can approach closely enough, for the non-polar forces to become effective. (This does not apply with the u of cationic, i.e. positively charged dyeing of acrylic fibres.)
Adding salt to the bath can have the required effect with cellulose fibre, a suitable with protein fibres and nylon. In the latter case, the reaction for wool dyeing in presence of acid can be illustrated by a series of simple chemical equations.

· Covalent bonds

Only reactive dyes are attached to the fibre by a covalent bond, which is much stronger than the previously mentioned forces and difficult to break down. Some degree of breakdown, shown by bleeding of the dye from the fibre, can with some types of reactive dye be produced to small extent by drastic treatment with acid or alkali, and almost completely by a treatment For 3 h in boiling 49% aqueous hydrazine solution.

Choice of Dyes


REUTLINGEN, GERMANY - NOVEMBER 17:  A student ...

Image by Getty Images via @daylife

One reason for the existence of the great number of commercial dyes that any textile material may be have to withstand one or more of a wide variety of processes of manufacture and later be subjected to a variety of different types of wear and tear in use. The correct of choice of dye for any given circumstance, in fact, requires considerable knowledge and experience, and nothing more than a bare outline of the underlying principles can be given here. A few typical examples, selected at random, of some of the matters to he considered in making the choice are given below under four main headings:

1 Nature of wear and Tear in Use

Many textiles must withstand severe exposure to sunlight or to repeated washing. Thus curtains and fabrics for outer garments must have good fastness to light. And fabrics for awnings and deck chair must withstand sunlight and also rain: knitted wool material should be fast to washing; shirting and handkerchiefs must withstand boiling in soap solution; and so on.

2 Nature of Manufacturing Processes

Cotton fabric having colored threads on a white ground may have to be subjected to boiling, with alkali under pressure (kier boiling) and bleaching after weaving. The first kind of dye chose for bland fabrics should be withstood the dyeing conditions of the second kind of dye.

3 Nature of Dyeing Process

Apart from the above treatments to which the already dyed materials are subjected, the nature of the dyeing process is important in determining the choice of dye, Thus in the dyeing of fabric only the most level-dyeing dyes can be used, because the slightest inequality in colour in different areas of the cloth would spoil the appearance. If loose fiber is being dyed, however, levelness is of less importance, because any portions of uneven appearance in the mass will be evenly distributed when the fiber is subsequently manufactured into yarn. Again, in using package dyeing machine, which the dye-liquor is pumped through a container packed tight with loose fiber, or through a cake or thick reel
of yarn. it is important for the dye to be either in true solution or present as extremely fine particles.

4 Dyeing Costs

The prices of different dyes are quite different. It is better to choose the economic dyes in Practice if all above mentioned requirements are conformed.

Classification of Dyes


There are several ways for classification of dyes. It should be noted that each class of dye has a very unique chemistry, structure and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification is given hereunder.
• Classification based on the source of materials
Chemical classification of the Dyes- Based on the nature of their respective chromophores.
• Dyes according to the nuclear structure
• Industrial Classification of the Dyes

Classification based on the source of materials

A very common classification of the dyestuff is based on the source from which it is made. Accordingly the classification could be:
Natural Dyes
Synthetic Dyes

Natural dyes

Colouring materials have been used for many thousands of years by man. Leather, cloth, food, pottery and housing have all been modified in this way. The two old ways were to cover with a pigment (painting), or to colour the whole mass (dyeing). Pigments for painting were usually made from ground up coloured rocks and minerals, and the dyes were obtained from animals and plants. Today, many of the traditional dye sources are rarely, if ever, used (onion skins, for instance). However, some of our most common dyes are still derived from natural sources. These are termed natural dyes. The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern:

–Natural + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and which may still be derived from animals or plants. Note that this is a classification based on the dye’s source and colour. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It gives no information about the mechanism by which staining occurs.
Natural dyes are often negatively charged. Positively charged natural dyes do exist, but are not common. In other words, the coloured part of the molecule is usually the anion. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged. Many, but by no means all, natural dyes require the use of a mordant.
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 coloured linen. Similar dyes are carmine (natural red 4) and lac (natural red 25). These three dyes are close chemical relatives, obtained from insects of the genus Coccus. All require a mordant. The most commonly used natural dye is undoubtedly hematein (natural black 1), obtained from the heartwood of a tree. This dye also requires a mordant.

Saffron (natural yellow 6), is obtained from the stigmata of Crocus sativus, and is used without a mordant, staining as an acid dye. Although its use is very ancient, it is more common now as a colouring and spice for food than for dyeing, due to its expense.

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, Sulphure , Disperse dye etc. However using general dye chemistry as the basis for classification, textile dyestuffs are grouped into 14 categories or classes:

Group Application
Direct Cotton, Cellulosic and Blends
Vat dyes Cotton, Cellulosic and Blends
Sulphur Cotton, Cellulosic fibers
Organic pigments Cotton, Cellulosic, Blended Fabrics, paper
Reactive cellulosic fibers and fabric
Dispersed dyes Synthetic fibers
Acid Dyes Wool, Silk, Synthetic fibers, leather
Azoic Printing inks and pigments
Basic silk, wool,cotton
Oxidation dyes Hair
Developed Dyes Cellulosic fibers and Fabric
Mordant dyes Cellulosic fibers and Fabric, Silk, Wool
Optical/Fluorescent Brighteners synthetic fibers, leather, cotton, sports goods
Solvent dyes Wood Staining, solvent inks, waxes, colouring oils

Chemical classification of the Dyes

According to a system of chemical classification, dyes can be divided according to the nature of their Chromophore:

Chromophoric Group Textiles, leather
Acridine dyes, derivatives of acridine >C=N-and >C=C Texties
Anthraquinone dyes, derivatives of anthraquinone >C=O and >C=C
Arylmethane dyes; Diarylmethane dyes, based on diphenyl methane, Triarylmethane dyes, based on triphenyl methane
Azo dyes, based on a -N=N- azo structure
Cyanine dyes, derivatives of phthalocyanine
Diazonium dyes, based on diazonium salts
Nitro dyes, based on the –NO2 nitro functional group
Nitroso dyes, are based on a –N=O nitroso functional
Phthalocyanine dyes, derivatives of phthalocyanine >C=N Paper
Quinone-imine dyes, derivatives of quinine Wool and paper
Azin dyes; -Eurhodin dyes, -Safranin dyes, derivatives of safranin -C-N=C- -C-N-C Leather and textile
Xanthene dyes, derived from xanthene -O-C6H4-0 Cotton, Silk and Wool
Indophenol dyes, derivatives of indophenol >C=N-and >C=O Colour photography
Indophenol dyes, derivatives of indophenol >C=N-and >C=O Colour photography
Oxazin dyes, derivatives of oxazin -C-N=C =C-O-C= Calico printing
Oxazone dyes, derivatives of oxazone
Thiazin dyes, derivatives of thiazin
Thiazole dyes, derivatives of thiazole >C=N- and -S-0= Intermediate
Fluorene dyes, derivatives of fluorine Intermediate
Rhodamine dyes, derivatives of rhodamine Pyronin dyes

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

Cellulose Textile Dyes

  • 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 maximise the Van-der-Waals, dipole and hydrogen bonds. Below is a diagram of a typical direct dye. Note that the sulphonate groups are spread evenly along the molecule on the opposite side to the hydrogen bonding -OH groups, to minimise any repulsive effects.

image
The main problem with direct dyes is their lack of fastness during washing. However, they are cheap, so are popular for items which are less likely to require fastness during washing. Wash fastness may be improved, though, by the application of direct and developed dyes, which contain -NH2 functional groups as well as sulphonate groups. In this process, the dyed fabric is treated with sodium nitrite, which causes the dye to be converted to a diazo salt. It is then treated with a coupling compound such as 2-napthol. The resultant larger azo molecule now has more affinity for the fibres, and is less soluble.

  • 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.

Vat dyes are insoluble in water, but may become solublised by alkali reduction, for example sodium dithionite (a reducing agent) in the presence of sodium hydroxide. For this reason, they tend not to contain many other functional groups which may be vulnerable to oxidation or reduction. The leuco form produced by alkali reduction is absorbed by the cellulose and, once there, can be oxidised back to its insoluble form. Oxidation is usually performed using hydrogen peroxide, but occasionally with atmospheric oxygen under the correct conditions. Treating the dyed textile with a soap completes the process, since the soap molecules encourage the dye molecules to clump together and become crystalline.

image

The other types of dyes, for example the azo class, undergo a non-reversible change on reduction.

  • 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.

image

The most common anionic group attached to acrylic polymers is the sulphonate group, -SO3-, closely followed by the carboxylate group, -CO2-. These are either introduced as a result of co-polymerisation, or as the residues of anionic polymerisation inhibitors. It is this anionic property which makes acrylics suitable for dyeing with cationic dyes, since there will be a strong ionic interaction between dye and polymer (in effect, the opposite of the acid dye-protein fibre interaction). An example of a basic dye is shown below:

image

  • Fibre-Reactive Dyes

A fibre-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.
Early fibre-reactive dyes; The first fibre-reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially. Although fibre-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.

The first fibre-reactive dyes contained the 1,3-5-triazinyl group, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fibre degradation occurred. ICI launched a range of dyes based on this chemistry, called the Procion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colours. Procion dyes could also be applied in batches, or continuously.
The general structure of a fibre-reactive dye is shown below:

image

A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilise as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O- groups. These can then attack electron-poor regions of the fibre-reactive group, and perform either aromatic nucleophilic substitution to aromatics or nucleophilic addition to alkenes.

Nucleophilic substitution; Aromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favourable -electron system. However, nucleophilic subsitutions are not very common on aromatics, given their already high electron density. To encourage nucloephilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack. For example:

image

But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added.

image

However, this will only work if there is a good leaving group, such as -Cl or -N2. The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye:

image

Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilised as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens:

image

One problem is that instead of reacting with the -OH grous on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolysed. The two reactions compete, and this unfavourable because the hydrolysed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water.

Nucleophilic addition;Alkenes are quite reactive due to the electron-rich -bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favoured generally, because of the repulsion between the Nu- and the electron-rich bond. However, they will occur if there are sufficient electron withdrawing groups are attached to the alkene, much as before, with aromatic substitution. In this case, the process is known as Michael addition or Conjugate addition.

For this reaction type, the most important dye class is the Remazol reactive dye. This dye type reacts in the presence of a base such as HO-. The mechanism for the reaction of one of these dyes is shown below:

image

As before, the intermediate is resonance stabilised, but this has not been shown.

Protein Textile Dyes

Acid dyes
Acidic dyes are highly water soluble, and have better light fastness than basic dyes. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.

Acid-leveling dyes: These planar dyes tend to be small or medium sized, and show moderate inter-molecular attractions for wool fibres. This means that the dye molecules can move fairly easily through the fibres and achieve an even colour. This is somewhat similar to the process that occurs during chromatography- the molecules with the strongest affinity for the substrate move the least distance from the point of origin whereas molecules with less affinity move much further. However, the low affinity means that these dyes are not always very resistant to washing.

Acid-milling dyes: Acid-milling dyes are larger than acid-leveling dyes, and show a much stronger affinity for wool fibres. Because of this, the resultant colour may be less even (see explanation above), but they are much more resistant to washing. As well as intermolecular interactions, intramolecular interactions play an important part in the properties of the dye. Compare the two molecules shown below. They are isomers, but the one on the right (with hydrogen bonding) shows a much greater resistance to washing in alkali, and much increased light fastness.

Acid dye colours: Usually, yellow, orange and red acid dyes are azo compounds, with blues and greens often come from the carbonyl class, particularly anthraquinones (see the example below).An example of an acid dye is Alizarine Pure Blue B. It is a sulphonated aminoanthraquinone

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  • 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. Chromium salts such as sodium or potassium dichromate are commonly used now for synthetic mordant dyes. The diagrams below show C.I. Mordant Black 1 with and without a chromium (III) ion. Chromium (III) forms 6-coordinate complexes, so two Mordant Black molecules would attach to one ion. Only one is shown below for clarity.

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Mordants do not have to be metal salts. Organic molecules such as tannic acid and tartaric acid can be used as well.

Synthetic Textile Dyes

  • 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.

The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer.

Classes of disperse dye: The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl
class, and a few from the nitro and polymethine classes.

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  • Solvent dyes

Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. One class of dyes is an exception to this. These colours are applied by dissolving in the target, which is invariably a lipid or non-polar solvent.

The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern: – solvent + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and whose primary mechanism of staining is by dissolving in the target. Note that this is a functional and colour classification. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible, but are rare.

As a general principle, solvent dyes do not ionise. Many are azo dyes which have undergone some molecular rearrangement and lost the ability to ionise. In the process they gained the ability to dissolve in non-polar materials such as triglycerides. They are commonly used to stain such materials in sections. They are frequently called lysochrome dyes. The prefix lyso means dissolve, and chrome means colour. Sudan III (solvent red 23), sudan IV (solvent red 24) and oil red O (solvent red 27) are commonly used for demonstrating fat in sections. Sudan black B (solvent black 3) is also very effective, but can also stain ionically under some circumstances.

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
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