Dyes refer to the organic substances enable the strong coloring of fiber and some other materials. There are various kinds of dyes. According to the source, they can be divided into natural dyes (such as vegetable dyes, animal dyes and mineral dyes, etc.) and synthetic dyes (or artificial dyes). According to the molecular structure, it can be divided into azo dyes, anthraquinone dyes, phthalocyanine dyes, aromatic dyes and nitro dyes. Based on the application methods, it can be divided into acid dyes, basic dyes, sulfur dyes, reactive dyes, disperse dyes, direct dyes and so on. Dyes have color, but substance having color does not necessarily belong to dyes. Dye must have a chromophore and auxochrome group. Dye contained in the ink should also have water-soluble group such as sulfonic acid group.
Natural dyes are dyes obtained from animals, plants and minerals. According to sources they can be divided into: ① plant dyes; dyes extracted from the roots, stems, leaves and fruit of some plants such as indigo extracted from the leaves of indigo (blue); curcumin extracted from turmeric (yellow); alizarin extracted from madder (red) and so on; ② animal dyes; dyes extracted from animal body such as carmine extracted from cochineal, etc; ③ mineral dyes; dyes extracted from the colored inorganic substance of mineral such as chrome yellow, ultramarine blue, brown and other manganese. Owing to its various disadvantages compared with artificial dyes such as incomplete chromatography, application inconvenience and poor fastness, most natural dyes have been eliminated except for a few still in use.
Synthetic dyes, also known as "artificial dyes." is mainly made through the chemical processing of coal tar (or oil processing) fractionation products (such as benzene, naphthalene, anthracene and carbazole, etc.), sometimes are also known as coal tar dyes. Since the earliest several synthetic dyes all took aniline as raw material, it is also known as "aniline dyes." Compared with the natural dyes, synthetic dyes have various kinds with complete chromatography and most of them being colorful, washable and being able to be produced in large-scale. Thus, almost all of the current so-called dye refers to synthetic dyes with its dyeing products being one of the common samples of Forensic evidence.
Pigment is a colored, black, white or fluorescent particulate organic or inorganic solid. It is not soluble in water, oil or other applied media, but can be uniformly dispersed with the dispersion process being unaffected by the physical and chemical effects of the medium and still retaining its unique crystal or particle structure. Based on the chemical composition, it is divided into inorganic and organic pigments; according to color, it can be divided into white, red, yellow, black and other pigments; according to the source, it is divided into natural and synthetic pigments with natural pigments including mineral (inorganic) such as ocher, cinnabar, laterite and realgar, etc., also derivatives from animals and plants (organic) such as Garcinia and cochineal red; the synthetic pigments also include organic and inorganic pigments; according to the applications, it is classified into coloring pigment, extender pigments and functional pigments. Coloring pigment is inorganic and organic pigments that can endow the media with a variety of colors; extender pigment is a non-coloring and non-opacity pigments. In early time, it is mainly used to reduce product cost and improve the performance of coatings, plastics, rubber and other products. Therefore it is also called filler. Now there are natural products and synthetic products which have been processed into ultrafine-grained type. Owing to the low impurity content, fine particles and being available for artificial synthesis and various kinds of surface modifications, the synthetic products can also be used to reduce the amount of the paint with opacity (such as titanium dioxide) and improve the product performance, thereby now it is more suitable to be called as constitution (incremental) pigment. The main varieties of the extender pigment are mostly processed natural minerals such as natural calcium carbonate, barite powder, talc, mica, diatomaceous earth, quartz powder, clay and asbestos, etc. with a small number of synthetic products such as precipitated calcium carbonate, precipitated barium sulfate and synthetic silica; functional pigments is pigments that endow the product with particular function such as temperature-indicating paints make the color of the product change with temperature; anti-fouling paint with capability of poisoning and killing water creatures; luminescent pigment which is lighting in the dark; anti-rust paint for preventing corrosion and pearlescent pigments with pearl luster.
Pigment is widely applied to paint, printing, rubber, plastics, paper, fibers, ceramics and other industries. It can endow the product with decorative, opacity, durability or special features. With advances in technology, a variety of high performance pigments will be further developed.