Lecture 1 - Pigments: First Part PDF

Compare between dyes and pigments Both belong to the class of colorants Pigments are substances consisting of small particles that are practically insoluble in an application system. Dyes are practically completely soluble in the medium of application. Pigment particles, when applied, have...

Compare between dyes and pigments Both belong to the class of colorants Pigments are substances consisting of small particles that are practically insoluble in an application system. Dyes are practically completely soluble in the medium of application. Pigment particles, when applied, have to be attached to surfaces (substrates) by additional materials, such as binders. Dyes are applied to various substrates, such as textiles, leather, paper, or hair, using a liquid in which they are dissolved. In contrast to pigments, dyes must have an affinity to the substrates on which they are fixed. There are inorganic and organic pigments, but dyes are organic compounds. 1  Pigments are substances consisting of small particles that are practically insoluble in an application system and that are used as a colorant or because of their anticorrosive or magnetic properties. The term “pigment” descends from the Latin word “pigmentum”. It was originally used in the sense of a coloring matter The use of the word was later extended to indicate colored decoration. The term “pigment” was also used in the late Middle Ages for plant and vegetable extracts, particularly for those with coloring properties. Applications The most important application media for pigments are automotive and industrial coatings, paints, plastics, printing inks, and building materials. Other uses of pigments are in paper, rubber, glass, porcelain, and artists’ colors. Paint and coating films have to be thin, in most cases, when they are cured. Pigments with high tinting strength and good hiding power combined with suitable dispersion properties are, therefore, needed in this application. The aim is to obtain , a homogeneous suspension of pigment particles on the surface and final coating of the required characteristics. 2  Application technical behavior Pigment–binder interaction Binders are used in paintings to disperse pigments and allow adhesion to the painting surface (or substrate). Binder chemistry Pigment binders are made using polymer latexes. Latex: Colloidal dispersion of polymer particles in a liquid. -The degree of dispersion of the pigment in the application medium has a significant influence on the properties of a paint or a coating system. 3 Fillers (extenders) are powders that, like pigments, are practically insoluble in the application system. -They are typically white and are used because of specific chemical and physical properties. -Is used to improve the technical characteristics, to influence optical and coloristic properties, or to increase the volume. -Fillers are also used to lower the consumption of more expensive binder components. 4  Pigments can be classified into natural and synthetic. Pigments are can be classified according to their chemical composition and with respect to their optical and technical properties. Pigments can be classified into organic and inorganic pigments, while dyes are organic. 5  Another classification for inorganic pigments is based on the chemical composition. 6  Transparent pigments are characterized by very small particles with sizes in the range below 100 nm and large specific surface areas. Most of these pigments consist of particles which are even smaller than 30 nm. They are classified as nanomaterials. Pigmentation with these pigments leads to a transparent appearance of the application systems. Luminescent pigments show optical effects based on the ability to absorb radiation and emit it as light of longer wavelength with a time delay (phosphorescence) or without a time delay (fluorescence). Light emission often occurs in the visible spectral range. External energy is necessary to enable luminescent materials to generate light. 7  Functional pigments are not about color but about different physical properties. Functional pigments can be electrically conductive, IR-reflective, magnetic, anticorrosive, UV-absorbing. Some of the transparent pigments and the luminescent pigments can also be classified as functional pigments. 8  A pigmented system is described as a dispersion of pigment particles in a transparent binding medium, which is initially liquid, and after hardening a solid. In many cases, a thin film is formed, e.g., a paint layer, with thicknesses of a few to hundreds of micrometers. Incident light undergoes reflection, refraction, scattering, and absorption in interaction with the pigmented system that results in the visual appearance of the pigmented medium. 9 Comparison between inorganic and organic pigments Generally, inorganic pigments are more stable against light, weather, temperature, and chemicals than organic pigments. Another advantage of inorganic pigments is their lower manufacturing costs. Organic pigments need mostly multistage syntheses and more expensive raw materials for their production, which leads finally to higher prices for them in the market. Organic pigments are, however, in various cases more color intensive and, therefore, more attractive than inorganic colored pigments. A disadvantage of organic pigments is their lower stability against the factors mentioned above. Degradation of organic colorants – pigments as well as dyes – by exposure to UV light is an issue. The use of UV absorbers together with the organic colorants is very helpful to ensure a long shelf life also for these materials in their application. 10  Uses The choice of a pigment for a specific application includes several criteria to achieve an optimal application result: – color properties: color, tinting strength, lightening power, hiding power; – general chemical and physical properties: chemical composition, particle size, particle size distribution, density, moisture and salt content, content of water-soluble and acid-soluble matter, hardness; – stability properties: resistance against light (especially UV light), heat, humidity, and chemicals, retention of gloss, corrosion resistance; – behavior in binders and other application systems: dispersion properties, interaction and compatibility with binder components, solidifying properties. 11  General chemical and physical properties Inorganic pigments consist of single particles of mostly uniform chemical composition and crystal structure. Inorganic pigments can be described with respect to their optical properties, as shown in Table 1.4. The interaction of pigment particles with visible light is determines the appearance of pigments in their pure powdered form and in the application medium. 12 Jablonski diagram 13 i) Absorption of the UV-VIS radiation is accompanied by migration to an excited level in S1. ii) Excited molecules are characterized by high kinetic energy , on collision they will lose heat to reach the minimum energy level in the excited state S1 (vibrational relaxation). iii) Excited molecules will undergo further collision and return to the ground state (So) by radiationless loss of energy in the form of heat (≥ 90 % of known compounds) (internal conversion). iv) Relatively stable molecules in S1 with low kinetic energy (rigid and planar molecules) return to the ground state by fluorescence emission(less than 10% of known organic compounds can emit luminescence). v) Intersystem crossing from the singlet excited state S1 to the corresponding metastable triplet state (T1). vi) Return from T1 to So by phosphorescence emission. 14 15  Fluorescence and phosphorescence The energy difference between (S1-So) is higher than the energy difference between (T1-So), then fluorescence emission is of higher energy than for the phosphorescence emission , so phosphorescence comes at longer wavelengths than fluorescence. Lifetime of phosphorescence (10-4 -100 sec) > Lifetime of fluorescence (10-9-10-6 sec), Phosphorescent lamp can still be lightening although the light source (source of excitation) is switched off. 16 -Absorption Absorption of light and the molecule transfers to higher energy level. -Emission Transition of the molecule to lower energy state by emission of radiation. -Scattering The phenomenon by which a beam of light is redirected in many different directions upon striking a particle of matter like dust, gas molecules, or water vapors. -Transmission: light which is not absorbed -Reflection : there are 2 types ; specular and diffuse reflectance -Refraction : when light passes from one medium to another of different refractive index 17 18 19  Refraction For light, refraction follows Snell's law, which states that, for a given pair of media, the ratio of the sines of the angle of incidence and angle of refraction is equal to the ratio of the refractive indices of the two media. 20  If a substance absorbs strongly in the visible region (λmax =470 nm). What is the color of this substance? Complementary colors Colors directly opposite each other on the color wheel are said to be complementary colors. Blue and orange are complementary colors; red and green are complementary; and so are yellow and violet. Mixing together two complementary colors of light will give you white light. Substance absorbing at 470 nm (absorbs blue color), will appear orange (the complementary color for blue). -Geometric parameters such as particle size (particle diameter), particle size distribution, and particle shape are some of the most important characteristics of pigments. –Particles are definable, individual units of a pigment. The structure, size, and shape of particles can vary widely. Typical particle units in pigments are primary particles, agglomerates, and aggregates (Figure 1.7). – Agglomerates: assemblies of primary particles and/or aggregates typically adhering to one another at corners and edges; the total surface area does not differ significantly from the sum of the surface areas of the individual particles. – Aggregates: assemblies of primary particles typically adhering to one another side by side; the total surface area is smaller than the sum of the surface areas of the primary particles involved. 22 Color properties of inorganic pigments Fundamental aspects When light strikes the surface of a pigmented system, e.g., a pigmented film, three different events may occur: – light absorption by the pigment particles (in the case of a colored or black pigment); – diffuse reflection of light at the pigment particles (scattering); – the light passes the pigmented medium without further interaction (no striking of pigment particles, the binder is assumed to be transparent and nonabsorbent). 23 -Reflection spectra of a white pigment (TiO2), of a black pigment (Fe3O4), and of some colored pigments in a pigmented film are demonstrated in Figure 1.8. -Theoretically, the spectral curve for a pure white pigment in the visible range (400 to 700 nm) should be parallel to the baseline at a spectral reflectance of 100% and for a pure black pigment also parallel to the baseline but at a spectral reflectance of 0%. Colored pigments have very different spectra dependent on their absorption. 24  Colorimetry Colorimetry is concerned with the quantification and physical description of human color perception. 1) The CIELAB color space, also referred to as L*a*b*, is a color space defined by the International Commission on Illumination (abbreviated CIE) in 1976. It expresses color as three values: L* for the lightness and a* and b* for the four unique colors of human vision: red, green, blue and yellow. 25  a given numerical change corresponds to a similar change in color. This formula converts the standard color values X, Y, and Z into the L*, a*, b *values; L * means lightness, a* with + a* and –a* is the red–green axis, and b* with + b* and –b* is the yellow–blue axis. The three coordinates of CIELAB represent the lightness of the color (L* = 0 yields black and L* = 100 indicates white), its position between red and green (a*, where negative values indicate green and positive values indicate red) and its position between yellow and blue (b*, where negative values indicate blue and positive values indicate yellow). 26 2) Color also can be expressed by RGB parameters RGB ; Red- green- blue primary colors A color in the RGB color model is described by indicating how much of each of the red, green, and blue is included. The color is expressed by RGB parameters, each component of which can vary from zero to a defined maximum value. If all the components are at zero the result is black; if all are at maximum, the result is the brightest representable white. The image can be analyzed by a software to extract the RGB parameters. 27 -Tinting strength, lightening power, scattering power, hiding power, and transparency are the most important parameters for the efficiency of pigments in paints, coatings, and other application systems. -Tinting strength, also called coloring power, is defined as the measure of the ability of a pigment to convey its color to the surrounding medium. -In practice, only the relative tinting strength is used, i.e., for a colorant to be tested, relative to an agreed standard. The relative tinting strength is determined by the weight ratio of a reference pigment and a test pigment if both have the same coloristic appearance for an observer. -The tinting strength of a pigment is its colorant power in relation to its mass. As tinting strength goes up, the quantity (weight) of pigment needed to produce a required color intensity goes down. Tinting strength also indicates how much a pigment will dominate the color of a mixture with other pigment. 28 -Lightening power is defined as the ability of a pigment to increase the lightness of a colored, gray, or black medium. -It can also be considered as the tinting strength of a white pigment. Another definition is that the lightening power is a measure of the ability of a white pigment to increase the reflectance of an absorbing (colored, gray, or black) medium. - Scattering power is the ability of an object to re-emit incident radiation, typically light. In practice, the relative scattering power S is often used, which is the ratio of the scattering power ST of pigment to be tested to the scattering power of a reference pigment SR. 29 -Transparency is the physical property of allowing light to pass through the material without appreciable scattering of light. -The transparency of a pigmented system describes its ability to scatter light as little as possible. -Some materials, such as glass and clean water, transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent. -Materials that do not transmit light are called opaque. 30  The hiding power is an ability of a paint to hide the surface that the paint was applied to. The term “hiding power” refers to the ability of a paint or a coating to cover the contrast between a black and a white background. 31 The pigment volume concentration (PVC) σ is defined as the fractional volume of a pigment in the volume of total solids of a dry paint film: 32