INTRODUCTIONPigments and dyes both come under the classification of word colorant meaning A substance used for coloring. Further a colorant can be elaborated as any chemical that is colored or can become colored in a particular environment/under certain condition and can be applied/used to impart the property of color to an item. Pigments are colorant composed of particles that are in soluble in the application medium. They have no substantivity for the material on which they are applied. Since the particles are too large to penetrate in to the substrate, the pigments are there fore easily removed unless fixed with adhesive.
Pigment dyeing like pigment printing is a phenomenon in which insoluble Coloring matter called pigment, having no affinity with the substrate is applied on to fabric. The color is being fixed are being attached to the fabric by virtue of binding agents. The role binders hence becomes extremely important since the pigment Is some what sandwiched between the substrate and final layer of binder. This Explanation may be too simple, but certainly explain basic idea of pigment application.
Historical Background of PigmentThe natural instinct of humans for beautifying and improving their own bodies, clothes and surroundings has been responsible for extensive use of colors since the times immemorial. The sources of natural colored substances then were vegetable, animal and minerals. The pigments were obtained from colored minerals that were ground to fine powders, mixed with a suitable binding agent like boiled oils, albumen, vegetable gums or waxes, and then fixed by heating or air-drying for producing the color effects. The earliest known examples are the well known cave paintings in France and Spain that have been carbon-dated to be of about 5,000 years old. Use of the natural mineral pigments is still continuing in the artistic canvas-paintings, colouring of dwellings and also in some wood paints. However with slow and painstaking scientific developments, the natural mineral pigments were gradually replaced by inorganic pigments that were made from lead, chromium, iron and other metal compounds. An interesting use of such pigments that lasted till the 20th century was printing of tents and awnings. The mineral-khaki pigment, used extensively for the military uniforms all over the World, was produced by depositing chromium and iron hydroxides inside the fibers and this pigment survived till the end of the Second World-War. The first organic pigment dispersions were commercially made in 1920s in Germany and marketed under the trade name of Hansa Yellows. However their full acceptability in textile dyeing and printing had to wait for another 10 years when the giant German chemical manufacturing firms mastered the technology of emulsion copolymerisation of olefinic compounds like vinyl esters, acrylic acid esters, butadiene, acrylonitrile etc that served as excellent binding agents for the organic pigments to the textile materials. Pigment dyeing was introduced in Pakistan in early 1980s, where it first replaced ground printing by the rotary screen printing machine. The entire ground of fabric was dyed on padding mangle rather than printed followed by drying and high temperature thermo fixation (curing). The process starts on thermosol dyeing machine and stenter frame but mainly done in preferred on stenter frames because the final chemical finish was added to pigment dyeing2 |Page
recipe (subjected to compatibility) and process on stenter thus saving a process. At that time only the light shades up to 2g/l were dyed successfully on stenter while a slight higher depth up to 3-4 g/l was achieved on thermosol dyeing machine a depth more than this produced inferior quality and cost ineffective product hence pigment dyeing was limited to light shades only .with the development of better pigments and binder, fabrics with high pigment depth can be successfully dyed, thus extending the limitation of pigment dyeing process
CLASSIFICATION OF PIGMENTSPigments have been chemically classified into the following classes.
CONVERTED INTO PIGMENTS DYES: Dyes converted by precipitation on substratesWith the introduction of synthetic dyes, attempts were made prepare pigments from them by method based on those that had been used with natural dyes. Many soluble compounds can be made insoluble by precipitating them as salt of heavy metals in the presence of alumina hydrated. Use of alumina hydrate as a basic substrate for making pigments from dyes has lost its commercial importance now.
Dye converted by precipitation with complex acids:The precipitation of pigments using the mordants that were employed when the same substances were being used as dyes began in initial years of twenty century, but these pigments possessed very poor light fastness. It was noted that some amines could be precipitated by treatment with acid in the presence of sodium phosphate and sodium tungstate or molybdate. When these precipitants were tried on basic dyes, pigments were produced with higher light fastness then that of parent dyes. After the Second World War, copper toners were made by precipitating basic dyes with copper (ii) hexacynoferrate resulting in pigments that were less brilliant, lower in strength and light fastness with their predecessors.
Vat dyes converted into pigments:Although vat dyes may be considered pigments due to their water insolubility, but there were first used as dyes. When prepare for dyeing there particles size is4 |Page
an important technical factor that influences the rate of reduction and is guarded as an industrial secret. Particles size was also found important when vat dyes were used to make pigments When methods were devised to reduce the particles size further, the only it gained commercial success to produced bright and tinctorially strong pigment.
The chemicals classes of vat colors from which suitable pigment have been developed are anthraquinones, perinones, perylenes and thioindigos. Many vat dyes have been tested and only few have met the stringent fastness standers. Seven of those found suitable in all respects all shown in following table
Common NameFlavanthrone Antharone Orange Indathrone Blue Isoviolanthrone Violet Perinone Orange Perylene Maroon Anthrapyrimidine yellow
CI VatYellowYellow 1 Orange 3 Blue 4 Violet 1 Orange 7 Red 23 Yellow 20
CI PigmentYellow Red 168 Blue 60 violet 31 Orange 43 Red 179 Yellow 108
AZO PIGMENTS:Azo pigments comprise by far the largest chemical class of compound from which pigments are made, reflecting the wide range of aromatic amines that can be diazotized and the numerous compound s to which the resulting diazo compound can be coupled, not all of course yielding pigments. The azoic pigments provide a wide range of hue from red to yellow, and orange to blue, in descending order of number5 |Page
of each hue. The classification of azo pigments is shown in the following figure;Azo pigmints
Metal - Free
Metal - Containing
PHTHALOCYANINE PIGMENTS:Phthalocynine was the first chromogenic type to be introduced I to the field of color, chemistry of organic compound concern with pigments. Copper phthalocynnine (CI Pigment Blue 15) is an example of the phthalocyanine pigments.
QUINACRIDONE PIGMENTS:In 1935 the production of liar trans-quinacrodone was reported by Liebermann and was curiously looked at as a red vat colorant but not developed commercially. Their chemical structures are based on CI Pigment Violet 19
ISOINDOLINE PIGMENTS:Any new type of pigments, called irgazin was produced in 1964 by Giegy Company. The pigments obtained were yellow pigment (CI Pigment Yellow110) and the greenish yellow pigments (CI Pigment Yellow 109).
DIOXAZINE PIGMENTS:Triphenodioxazine compound have been used to make brilliant blue direct and reactive dyes for cellulosic fiber, but only one pigment in this class CI Pigment Violet 23 is widely used.
FLUORESCENT PIGMENTS:Fluorescent pigments are based on fluorescent dyes which are soluble In certain resins. A resin colored in this way is ground to a powder of small particle size and this fluorescent pigment is dispersed into various media in the same way as other pigments. The overall result is that the paints, printing inks and plastics into which fluorescent pigments have been incorporated have vivid bright colors which attract the eye. This property is very valuable for safety clothing worn by police, rescue workers and others working in hazardous situations. Their vehicles also become much more visible when parts of them are painted with special finishes containing these colorants. Other uses include advertising and plastic toys. The resins used to make fluorescent pigments are usually toluenesulphonamide- Melamine-formaldehyde matrices. The dyes used for this purpose include CI Disperse Yellow 11, Rhodamine 6G (CI Basic Red 1) and Rhodamine B (CI Basic Violet 10. Unfortunately, the fluorescent effect is not directly proportional to the concentration of colorant present, since there is considerable quenching7 |Page
if quite low concentrations are exceeded. The light fastness of the fluorescent pigments is also less than that of many other organic pigments, but can be improved by using over layers containing ultraviolet absorbers.
INORGANIC PIGMENTS:Inorganic pigments have a very special place in the piment chemistry for several reasons; one is that there is no white inorganic pigment. White pigments are essential to provide opacity to the paints and printing inks used on metals, paper, wood, textile fabric and plastic films.
White organic pigmentsThe primary requirement for a compound to be of a white appear