Chemistry of Paper (RSC Paperbacks)
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J C Roberts. Walmart Tell us if something is incorrect. Add to Cart. Arrives by Friday, Sep Free pickup Tue, Oct 1. Ships to San Leandro, Davis St. The rods provide, essentially, a monochromatic view of the world, allowing perception only of lightness and darkness. The sensitivity of rods to light depends on the presence of a photosensitive pigment known as rhodopsin, which consists chemically of the carotenoid retinal bonded to the protein opsin.
Rhodopsin is continuously generated in the eye and is also destroyed by bleaching on exposure to light. At high levels of illumination, however, the rate of bleaching is high so that only a small amount of rhodopsin is present and the rods consequently have low sensitivity to light. At these higher levels of illumination, it is only the cone cells that are sensitive. The cones The Physical and Chemical Basis of Colour 17 provide us with full colour vision as well as the ability to perceive lightness and darkness. The sensitivity of cones to light depends on the presence of the photosensitive pigment iodopsin, which is retained up to high levels of illumination.
Thus, in normal daylight when the rods are inactive, vision is provided virtually entirely by the response of the cone cells. Under ideal conditions, a normal observer can distinguish about 10 million separate colours. Short cones are most sensitive to blue light, the maximum response being at a wavelength of about nm. Medium cones are most sensitive to green light, the maximum response being at about wavelength of about nm. Long cones are most sensitive to red light, the maximum response being at about nm. This book is focused on the industrially important organic dyes and pigments and, to a certain extent, inorganic pigments and thus deals almost exclusively with colour generated by the mechanisms described by group c.
Absorption is the process by which radiant energy is utilised to raise molecules in the object to higher energy states. In general, if only absorption is involved when light interacts with an object, then the object will appear transparent as the light that is not absorbed is transmitted through the object.
A dye in solution owes its colour to the selective absorption by dye molecules of certain wavelengths of visible light. The remaining wavelengths of light are transmitted, thus giving rise to the observed colour. The absorption of visible light energy by the molecule promotes electrons in the molecule from a low energy state, or ground state, to a higher energy state, or excited state. The dye molecule is therefore said to undergo an electronic transition during this excitation process.
The Figure 2. As a consequence, for example, a yellow dye, which absorbs short wavelength blue light, requires a higher excitation energy than, say, a red dye which absorbs longer wavelength bluish-green light Table 2. A shift of the absorption band towards longer wavelengths i. The third attribute, brightness, may be described in various other ways, for example as brilliance or vividness.
This characteristic of the colour depends on the absence of wavelengths of transmitted light other than those of the hue concerned. Dyes which exhibit bright colours show narrow absorption bands, whereas broad absorption bands are characteristic of dull colours, such as browns, navy blues and olive greens.
In many ways, this technique may be considered as complementary to the use of visible absorption spectroscopy for the measurement of transparent dye solutions. These three attributes may be described using the concept of colour space, which shows the relationships of colours to one another and which illustrates the three-dimensional nature of colour, as portrayed in Figure Figure 2. The hue of a particular colour is represented in a colour circle.
The three additive primaries, red, green and blue are equally spaced around the colour circle. The three subtractive primaries, yellow, magenta and cyan are located between the pairs of additive primaries from which they are obtained by mixing. The second attribute, chroma, increases with distance from the centre of the circle.kick-cocoa.info/components/buhenowo/zafyv-come-recuperare-i.php
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The third attribute, lightness, requires a third dimension that is at right angles to the plane of the colour circle. The achromatic colours, white and black, are located at either extreme of the lightness scale. Colour measurement, and its mathematical basis, generally referred to as colour physics, is a welldeveloped and well-documented science and is not considered in further detail here. Fluorescence and Phosphorescence Most dyes and pigments owe their colour to the selective absorption of incident light. These compounds are referred to as luminescent.
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During the time the molecule spends in the excited state, energy is dissipated from the higher vibrational levels, and the lowest vibrational level is attained. Fluorescence occurs if the molecule then emits light as it reverts from this level to various vibrational levels in the ground state. Non-radiative processes, the most important of which is generally collisional deactivation, also gives rise to dissipation of energy from the excited state.
Another process which may occur is intersystem crossing to a triplet state. This gives the dye its particularly visual brilliance.
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When incorporated into a white substrate, such as a textile fabric or a plastic article, FBAs provide a particularly appealing bluish cast. One of the most important uses of FBAs is in washing powders to impart a bluish whiteness to washed fabrics. DYES AND PIGMENTS Colour may be introduced into manufactured articles, for example textiles and plastics, or into a range of other application media, for example paints and printing inks, for a variety of reasons, but most commonly the purpose is to enhance the appearance and attractiveness of a product and improve its market appeal.
The desired colour is generally achieved by the incorporation into the product of coloured compounds referred to as dyes and pigments. The term colorant is frequently used to encompass both types of colouring materials.
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Dyes and pigments are both commonly supplied by the manufacturers as coloured powders. Indeed, as the discussion of their molecular structures contained in subsequent chapters of this book will illustrate, the two groups of colouring materials may often be quite similar chemically. Dyes and pigments are distinguished on the basis of their solubility characteristics: essentially, dyes are soluble, pigments are insoluble. The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water.
Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic Chapter 2 24 industries see Chapter In contrast, pigments are colouring materials that are required to be completely insoluble in the medium into which they are incorporated.
The principal traditional applications of pigments are in paints, printing inks and plastics, although they are also used more widely, for example, in the coloration of building materials, such as concrete and cement, and in ceramics and glass.