CHAPTER 3

CLASSIFICATION OF DYES

The English chemist William Perkins accidentally discovered the synthetic dye, mauve, in 1856, while he was trying to synthesize quinine in aniline. Before that time, all coloring materials were extracted from barks, roots, seeds, leaves and shellfish. Most synthetic dyestuffs are made from coal-tar derivatives. Their manufacture is a highly complicated technical operation, which is constantly changing. There is variation in the manner in which different fibers respond to dyestuffs and even the same fibers do not produce a full range of colors with a particular type of dye.

Basic or Cationic Dyes

This group was the first of the synthetic dyes to be taken out of coal-tar derivatives. As textile dyes, they have been largely replaced by later developments. However, they are still used in discharge printing, and for preparing leather, paper, wood, and straw. More recently they have been successfully used with some readymade fibers, especially the acrylics. The name means that these are dyes with an organic material, which is soluble in a simple acid. Basic dyes were originally used to color wool, silk, linen, hemp, etc., without the use of a mordant, or using agent. With a mordant like tannic acid they were used on cotton and rayon. Basic dyes give brilliant colors with exceptional fastness to acrylic fibers. They can be used on basic dyeable variants of nylon and polyester.

Nowadays basic dyes are no longer used to any great extent on cotton or linen and seldom on wool. Since they are cheap, however, they are used for hemp, jute and similar fibers.

Their most important use today is on acrylics. They can also be used on basic dyeable variants of nylon and polyester.

The Direct Dyes

Historically, the direct dyes followed the basic dyes and were widely hailed because they made it unnecessary to use a mordant or binder in dyeing cotton. The colors are not as brilliant as those in the basic dyes but they have better fastness to light and washing, and such fastness can be measurably improved by after treatments (diazotized and developed.) Direct dyes can be used on cotton, linen, rayon, wool, silk and nylon. These dyes usually have azo linkage –N=N- and high molecular weight. They are water soluble because of sulfonic acid groups.

The Acid Dyes

This is a very large and important group of dyestuffs. While an acid dye is a salt the color comes from the acidic component, while in the basic dye it's from the organic base. The first acid dyes were combinations of basic dyes with sulphuric or nitric acid.

Adding metallic salts especially chrome to the dyed fabric in an after-treatment generally has increased colorfastness of acid dyes. Acid dyes cannot be used for wool tops but are used in dyeing wool piece goods, silk, nylon, and some of the other manmade fibers. If a mordant is used they will successfully dye cotton and linen, though this is seldom done today. The ordinary type of acid dye is reserved largely for apparel fabrics and for knitting and rug yarns. A great deal of it is used on nylon carpeting.

The Premetalized Dyes

This is an important group of acid dyes, which have been complexed with metallic ions to improve light fastness on wool and nylon.

The Sulphur Dyes

The sulphur dyes provide very deep shades, which have excellent resistance to washing but poor resistance to sunlight. They will dye cotton, linen, and rayon, but not brightly. A problem with sulphur dyes especially the black colors is that they make the fabric tender, or weaken its structure, so that it breaks easily. Sulphur dyed fabrics therefore usually must be treated with alkalis to neutralize the acids, which have formed.

Azoic Dyes

These dyes are used primarily for bright red shades in dyeing and printing since most other classes of fast dyes are lacking in good red dyes. Azoic dyes, called Naphthols in the industry, are actually manufactured in the fabric by applying one half of the dye. The other half is then put on and they combine to form the finished color. Unless they are carefully applied and well washed, they have poor fastness to rubbing or crocking.

The Vat Dyes

These are perhaps the best known group of dyes in use today because of their all round fastness to washing and sunlight on cotton and rayon.

The term vat comes from the old indigo method dyeing in a vat: indigo had to be reduced to light form. Vat dyes are made from indigo, anthraquinone and carbazole. They are successfully used on cotton, linen, rayon, wool, silk, and sometimes nylon. Vat dyes are also used in the continuous piece of dyeing process sometimes called the pigment application process. In this method the dyes are not reduced before application, but after they have been introduced into the fabric. This makes for a dyeing superior appearance and economy. There are no light red vat dyes.

Soluble Vats: There are no water-soluble preparations for dyes.

Indigo: The oldest known vat dyestuff, formerly made from the indigo plant, but now made synthetically.

Collective Dyes

Collectives are the latest dyestuff and because they react chemically with cotton, viscose, linen, wool and silk they are very fast to washing treatments. They can be dyed and printed by many methods and for the first time, the whole spectrum of color can be put onto cloth using just one class of dyes. Substituting a reactive group on a direct dye produces these dyes.

Dyes for Manmade Fibers

Dyeing man made fibers such as acetate, the polyacids, polyesters and acrylics, etc., has proved to be a challenge to dyers. Each new fiber, as it emerges from the laboratory, must be carefully analyzed and tested for its reaction to different dyestuffs. The process has been continuous experimentation with new developments turning up constantly.

To date both basic and acid dyes have been used as well as what is known as disperse colors. A dispersed dye may be any one of a number of slightly soluble dyestuffs dispersed, or held in suspension in the dye bath. Perhaps the best known example is the dispersed dye method for coloring acetate, which cannot be dyed by any other technique. For acetate dyes, the dye substance is derived from anthraquinone and azoic dyes. It is ground in a colloid mill. When dispersed in dye bath (colloidal suspension), the particles are microscopic and cannot be detected by the naked eye.

Alizarin Dyes: These are vegetables dyes, originally derived from the madder plant and now produced synthetically. They are used on wool and sometimes on cotton. They produce a brilliant turkey red, among other colors.

Aniline Black: They are produced from the chemical aniline, and are usually associated with the color black. Aniline black is a fast black, much used on cottons, and is developed by oxidizing the aniline on the fiber. It is very fast tot light, washing, and chlorine.

Chrome Dyes: These are a special type of acid dyes and they are used to color animal fibers, especially woolens and worsteds. They will react well on a fabric with metals such as chromium. The process, however, tends to dull the color brilliance but does provide high light fastness and washfastness.

Neutral Dyes: These are metal containing acid dyes and the metal is added in manufacture.

Acetate of Disperse Dyes: Disperse dyes were originally developed for dyeing secondary cellulose acetate fibers. These dyes are relatively insoluble in water and are prepared for dyeing by being ground into relatively fine powder in the presence of dispersing agents. In the dye bath, a suspension of the dye particle dispersion produces a very dilute solution of the dyes, which are then absorbed by the fibers. This dye class is used to dye polyester, nylon, acetate and triacetate fibers.

Types of Natural Dyes

Natural dyes can be sorted into three categories: natural dyes obtained from plants (Indigo), those obtained from animals (cochineal), and those obtained from minerals (ocher). Although some fabrics such as silk and wool can be colored simply by being dipped in the dye, others such as cotton require a mordant.

A mordant is an element, which aids the chemical reaction that takes place between the dye and the fiber so that the dye is absorbed. Containers used for dying must be non-reactive (enamel, stainless steel.) brass, copper or iron pots will do their own mordanting. Not all dyes need mordants, such as lichens and walnut hulls, they are called substantive dyes. If they do need a mordant, they are called adjective dyes.

Common mordants are: ALUM, usually used with cream of tartar, which helps Evenness and brightens slightly; IRON (or copperas) which saddens or darken colors, bringing out green shades: TIN, usually used with cream of tartar, which blooms or brightens colors, especially reds, oranges and yellows; BLUE VITRIOL which saddens colors and brings out greens and TANNIC ACID used for tans and browns.

One example of natural dye obtained form plants is madder, which is obtained form the roots of the madder plant. The plants are dug up, the roots washed and dried and ground into powder. During the 19th century, the most widely available fabrics were those which had been dyed with madder - the 'turkey red' that was considered brilliant and exotic. The madder plant continued to be used for dyeing until the mid 1800s when a synthetic substitute was developed.

Another example of a natural dye obtained from plants is wood. Until the middle Ages, Europeans used wood to create a blue fabric dye. The wood was widely used in Europe. The coloring was in the leaves, which were dried and ground mixed with water, and made into a paste. This dye was supplanted by indigo an ancient shrub well known to the Egyptians and Indians. Like wood, its color lay in its leaflets and branches. The leaves were Fermented, the sediment purified, and the remaining substance was pressed into cakes.

Indigo prevailed as the preferred blue dye for a number of reasons. It is s substantive dye, needing no mordant, yet the color achieved is extremely fast to washing and to light. The manufacture of natural indigo lasted well into the early 1900s.

Natural dyes obtained from minerals Ocher is a dye obtained from an impure earthy ore of iron or a ferruginous clay, usually red (hematite) or yellow (limonite). In addition to being the principal ore of iron, hematite is a constituent of a number of abrasives and pigments.

Natural dyes obtained from animals

A good example is cochineal, which is a brilliant red dye produced from insects living on cactus plants. The properties of the cochineal bug was discovered by pre Colombian Indians who would dry the females in the sun, and then ground the dried bodies to produce a rich, red powder. When mixed with water, the powder produced a deep, vibrant red coloring. In fact, most cherries today are given their bright red appearance through the artificial color " carmine ", which comes from the cochineal insect.

Name of Dyes from plants. Major Natural :

ANNATO is from the pulpy part of the seeds of Indian plant. Bixa orellana : Fugitive orange red color.

BRAZILWOOD is from wood of the tree, Caesalpinia Echinacea: bright red color.

CUDBEAR comes from the lichen, Lecanora tartarea : lilac color dye.

CUTCH is obtained from boiling the wood of Acacia catechu, native to India: rich brown color.

FUSTIC. OLD: Obtained from wood of tropical American tree. Chlorophora tinctoria: gold to yellow in color and still popular on wool.

FUSTIC. YOUNG OR ZANTE: Comes from the powered wood of the Rhus cotinus, a shrub size tree of the cashew family: yellow to dark olive in color.

INDIGO is obtained from the plant, Indigofera tinctoria: blue color.

KERMES is extracted from bodies of tiny insect, Coccus arborum ; red dye.

LAC is obtained by boiling tree incrustation produced by tiny lac insect. Tachardia lacca ; bright red color.

LOG WOOD comes from a Central American tree, Haematoxylon campechium; gives purple on wool, blue and black in silk.

MADDER comes from the roots of the plant, Rubos tinctorum ; red color. Produces alizarin.

QUERCITRON comes chiefly from then inner bark of the black oak, Quercus nigra; brown to yellow colors.

Names of Dyes from plants, minor natural :

BENGAL KINO TREE, Butea monosperma.

BABUL TREE, Acacia scorpioides.

BAEL TREE, Aegle mermelos.

INDIAN MADDER, Rubia cordifolia.

HENNA PLANT, Lawsonia inermis.

MONKEY FACE TREE, Malotus phillippinensis.

POMEGRANATEA. Punica granatum.

RED SANDLEWOOD, Pterocarpus santalinus.

SYRIAN RUE, Peganum harmala.

TAMARIND TREE, Tamarindus indica.

TULIP TREE, Thespesia populnec.

Aniline Dyes

3.171 Aniline is a chemical that is a by-product of burning coal. The first synthetic dye (mauve, Developed in 1856) was a coal tar product and contained the chemical aniline but the term "aniline" has been used to include other chemical type dyes. The synthetic dyes do not contain the chemical aniline. Take note that the class of dyes known as aniline are chemicals and as such should be treated with respect. Protective gloves and eyewear is a must and breathing the powered dyes is prohibited.

Water based Vs. Alcohol based Dyes.

Most of the colors can be found in either water based or alcohol based dyes. The alcohol-based dyes can also be reduced with lacquer thinner. The dyes may show slight differences in color depending on the reducer (alcohol or lacquer thinner) but it has been observed that lacquer thinner works just as well as denatured alcohol.

The water based dyes are reputed to be more color fast than the alcohol reduced dyes as have found the color fastness of the alcohol types to be a non issue.

Water based dyes can only be applied to unfinished and unsealed wood whereas alcohol Based dyes can be either wiped onto unfinished wood or sprayed over filled and sealed wood. Water based dyes cannot be successfully sprayed over sealed wood.

Dye selection

Water based dyes are simple to apply but on some woods may cause problems further along in the finishing process. Alcohol based dyes if wiped on are as easily applied as the water based. If you choose to wipe onto bare wood the water-based dyes have a slight advantage. Water will evaporate much slower than alcohol and as such allows the dye solution a longer working time. The longer working time can lessen the possibility of streaking. Some colors are not available in both water and alcohol based. If your chosen color is not available in a water-based dye don't let the possibility if streaking cause you to abandon your preferred color choice. In normal temperature alcohol will evaporate slowly enough to allow you adequate application time.

Food Dyes

Artificial Colors

Nearly all of the dyes found in modern foods, medicine, toothpaste, beverages, vitamins, cosmetics, etc., are synthetic. They were originally manufactured from coal tar oil, but today they are made from petroleum. A dye, which is listed as "FD&C", is permitted by the Food & Drug Administration to be added to foods, drugs and cosmetics. "D&C" means the dye may be used only in drugs and cosmetics.

Artificial Flavours :

Thousands of different chemicals, both natural and synthetic, are used as low cost substitutes for natural flavorings.

Vanillin (imitation vanilla), for example, is widely used in chocolates. Many people who believe they are reacting to chocolate are actually sensitive to the synthetic chemical flavoring.

A single artificial flavoring can be made up of anywhere from a few to hundreds of separate chemicals, many of which are derived from petroleum. For example: the formula for a synthetic raspberry flavoring is : Vanillin, Ethylvanillin, Alphaionone, Maltol, 1-(p-hydroxypheny)-3-butanone, Dimethyl sulphide, 2c,5-Dimethyl-N-(-2-pyrazinyl) pyrrole. There are no raspberries in this raspberry flavoring.

Preservatives :

Antioxidant preservatives are used primarily to prevent fats from becoming rancid. They allow foods to stay on the shelf a long time.

Most preservatives are not believed to a health hazard, but three petroleum based antioxidants have been found to trigger behavior and health problems.