Pigment - Part 1: What is Color

Pigment Part 1: What is Color Color plays an essential role in our day-to-day lives. From birth we are taught to react to colors logically or emotionally. Colors have meaning, which vary from culture to culture and continent to continent. It governs and controls traffic, triggers strong emotions, and is used to describe moods. Light, perceived by the human eye, is the product of electromagnetic waves in a small range of wavelengths. Different wavelengths are perceived as different colors. Color is therefore a perceptual phenomenon, which depends on the observer and the conditions in which the color is observed (our eye-brain is very accommodating in adjusting for varying environmental conditions). Three things are required for the presence of color: - an object, - a light source (illuminant), and - an observer. 1.1/ The Illuminant Color has been successfully used for object tracking and recognition. However, the color of an object changes if the illuminant's color changes. To see colors, energy in the form of light is required. Color sensation is produced by physical stimuli associated with the various wavelengths in the visible portion of the electromagnetic spectrum. To understand color better, we must recognize the origin of light. Light comes from a wide variety of sources and consists of electromagnetic radiation, a form of energy that spreads in a wave motion. Figure 1: Visual color spectrum All visible light is made up of a mixture of colors, which combine in different proportions to make up each distinctive light. The way we measure light is by a Spectral Power Distribution. In Figure 1, the visual color spectrum begins at 400 nm and finishes at 700 nm. Everything below 400 nm is called ultraviolet (UV) and everything above 700 nm is referred to as infrared (IR). It is not possible for the naked human eye to see ultraviolet or infrared light. Average North Sky Daylight (Illuminant D65) Spectrum of Fluorescent Light Incandescent Light (Illuminant A) Figure 2: Sources of light Note: (Vertical axes: spectral distribution) White light is composed of a select group of colors; each one characterized by a specific range of wavelengths, which it absorbs. These are the colors of the spectrum - red, orange, yellow, green, blue and violet. Incandescence and luminescence are two main ways of creating light. Incandescence is light from heat energy. Heating the source of a light bulb to a sufficiently high temperature will cause it to glow. The stars and sun glow via incandescence. Luminescence, also known as cold light, is light from other sources of energy independent of heating. It can be generated at room or even lower temperatures. Quantum physics explains luminescence as movements of electrons from their ground-state (lowest-energy level) into a state of high energy. When returning to its ground- state, the electron gives back the energy in the form of a photon of light. If the time interval between the two steps is short (few microseconds), the process is called fluorescence; if the interval is long (some hours), the process is called phosphorescence. The combination of these wavelengths in light can change according to the light source. For this reason, colors can look different when compared under the influence of daylight, fluorescent light or sodium lamps. Natural sunlight varies widely. It can be very blue, particularly around midday, looking north. Direct sunlight usually is seen as golden, but, at sunset, it can be bright red. Artificial light can be yellow, from sodium vapor, blue-green from mercury vapor, or it can be yellow, from an incandescent light bulb, or varying colors from fluorescent light. The graphs in Figure 2 show average north sky daylight (Illuminant D65), a cool white fluorescent light (Illuminant F), and an incandescent light (Illuminant A). Several phenomena can occur when light hits an object. Transmission occurs if the light passes through the object, which is the case with transparent colors. It is referred to as reflection if, for example, a blue object reflects the part of the color spectrum that represents blue and the remaining light is absorbed. The reflection curve of white will show roughly equal intensities close to 100% reflection in all wavelengths of the spectrum. Refraction or scattering is when light changes direction as it passes from one medium to another, like from the polymer to a pigment or filler particle in a plastic part. Scattering is influenced by the difference in refractive index between a particle and its surroundings, particle size, and wavelength of light. An opaque color provides a high scattering performance. A translucent color shows a combination of transmission and scattering. Absorption happens if most wavelengths of the visible spectrum are absorbed. Black surfaces absorb almost all light. 1.2/ The Object An object appears in a particular color because the light, which is reflected from its surface, is made up exactly of wavelengths, which combine to create the color observed. The object absorbs all other wavelengths. For example, a blue object reflects the blue light spectrum, but absorbs red, orange, yellow, green and violet, which are most of the other wavelengths. A red object reflects the red spectrum but absorbs most of the orange, yellow, green, blue and violet. Figure 1 : Absorption and Reflection of Light Black and white colors are different to other colors in terms of the way that they reflect and absorb light. A white object reflects almost all colors while a black object absorbs most colors completely. Other significant influences on the color of an object are shape and surface effects. For instance, an object can be spherical or square, dull or glossy, transparent, opaque or translucent. It may also appear metallic, pearlescent, fluorescent, or phosphorescent. Viewing angles also affect our perception of color 1.3/ The Observer The human eye is the defining observer of color. An observer nearly always bases acceptance of a color on visual judgment. For this reason, a color match can become highly subjective, as color vision varies widely from person to person. Characteristics such as age, gender, inherited traits, and even mood, can affect color vision. Figure 1 : Human Eye Part 2: Basics on Color Measurement People who believe that the eye is the most important observer of color, argue that judgments of color can be made purely by reference to color cards by visual matching. The clear criticism of this technique of judging color is that everyone's perception of color differs. Besides genetic abnormalities, color vision changes with age due to the build up of yellow macular pigmentation in the eye. For this reason, it is argued that all judgments of color must be based on physical measurements. However, these measurements and their interpretation must be related closely to the responses of visual observers. Therefore, color control is split into two segments: visual and instrumental. In the following pages, color measurement methods will be described: - the equipment that can be used like colorimeters and spectrophotometers - the CIELAB method of measurement - and the Munsell color measurement system 2.1./ Equipment Although the human eye can control color, there is a need for instruments in order to provide objective color measuring and evaluation as well as help in matching colors. There are two basic methods for measuring surfaces' color: • The first is to imitate the analysis made by the eye in terms of responses to three stimuli. This technique, known as "tristimulus colorimetry", sets out to measure X, Y, and Z directly. • The second method is to determine reflectance (R) for each wavelength band across the range of the spectrum to which the eye is sensitive, and then to calculate the visual responses by summing products of R and the standard values for distribution of the sensitivity of the three-color responses (2. J, and Z). The tristimulus method has theoretical advantages where the materials to be measured are fluorescent, but there are serious practical problems in assuming that a tristimulus colorimeter exactly matches human vision, that is, in eliminating color blindness from the instrument. Two commonly used types of color measurement equipments are a colorimeter and a spectrophotometer. Colorimeters A tristimulus colorimeter has three main components: • a source of illumination (usually a lamp functioning at a constant voltage); • a combination of filters used to modify the energy distribution of the incident / reflected light; • a photoelectric detector that converts the reflected light into an electrical output. Each color has a fingerprint reflectance pattern in the spectrum. The colorimeter measures color through three wide-band filters corresponding to the spectral sensitivity curves. Measurements made on a tristimulus colorimeter are comparative, the instrument being standardized on glass or ceramic standards. To achieve the most accurate measurements it is necessary to use calibrated standards of similar colors to the measured materials. This "hitching post" technique enables reasonably accurate tristimulus values to be obtained even when the colorimeter is demonstrably colorblind. Tristimulus colorimeters are most useful for quick comparison of near-matching colors. They are not very accurate. Large differences are evident between the various instrument manufacturers. However, colorimeters are less expensive than spectrophotometers. Spectrophotometers To get a precise measurement of color, it is advisable to use a spectrophotometer. A spectrophotometer measures the reflectance for each wavelength, and allows to calculate tristimulus values. The advantage over tristimulus colorimetry is that adequate information is obtained to calculate color values for any illuminant and that metamerism is automatically detected. The negative is that high quality spectrophotometers are very expensive and measurements take longer (although this disadvantage has been greatly reduced by instrument development). In a spectrophotometer, the light is usually split into a spectrum by a prism or a diffraction grating before each wavelength band is selected for measurement. Instruments have also been developed in which narrow bands are selected by interference filters. The spectral resolution of the instrument depends on the narrowness of the bands used for each successive measurement. In theory, a spectrophotometer could be set up to compare reflected light directly with incident light, but it is more usual to calibrate it against an opal glass standard itself previously calibrated by an internationally reknown laboratory. Checks must also be made on the optical zero (e.g. by measurements with a black light trap) because dust or other problems can give rise to stray light in an instrument giving then false readings. Today's spectrophotometers contain monochromators and photodiodes that measure the reflectance curve of a product's color every 10 nm or less. The analysis generates typically 30 or more data-points, with which a precise color composition can be calculated. 2.2./ CIELAB Method An organization called CIE (Commission Internationale de l'Eclairage) determined standard values that are used worldwide to measure color. The values used by CIE are called L*, a* and b* and the color measurement method is called CIELAB. L* represents the difference between light (where L*=100) and dark (where L*=0). A* represents the difference between green (-a*) and red (+a*), and b* represents the difference between yellow (+b*) and blue (-b*). Using this system any color corresponds to a place on the graph shown in Figure 3. Variables of L*, a*, b* or E* are represented as delta L*, delta a*, delta b* or delta E*, where delta E* = delta (delta L* 2 +delta a* 2 +delta b* 2 ). It represents the magnitude of the difference in color, but does not indicate the direction of the color difference. 2.3./ Munsell Color System The Munsell Color System, developed in 1898 by American artist A. Munsell, is another commonly used color measurement system. Munsell aimed to create a "rational way to describe color" that would use clear decimal notation rather than color names. In 1905 he published a color notation, which has been reprinted several times and is still a standard for colorimetry. Munsell modeled his system as an orb around whose equator runs a band of colors. The axis of the orb is a scale of neutral gray values with white as the North Pole and black as the South Pole. Extending horizontally from the axis at each gray value is a gradation of color progressing from neutral gray to full saturation. With these three defining aspects, any of thousands of colors could be fully described. Munsell named these aspects, or qualities: hue, value, and chroma. Hue Munsell defined hue as the quality by which we distinguish one color from another. He selected five principle colors: red, yellow, green, blue, and purple; and five intermediate colors: yellow-red, green-yellow, blue-green, purple-blue, and red- purple. He arranged these in a wheel measured off in 100 compass points. The colors were identified as R for red, YR for red-yellow, Y for yellow etc. Each primary and intermediate color was allotted ten degrees around the compass and then further identified by its place in the segment. Value Munsell defined value as the quality by which we determine light colors from dark ones. Value is a neutral axis that refers to the gray level of the color, ranging from white to black. Chroma Chroma is the quality that distinguishes a pure hue from a gray shade. The chroma axis extends from the value axis at a right angle and the amount of chroma is noted after the value designation. Therefore, 7.5YR 7/12 indicates a yellow-red hue tending toward yellow with a value of 7 and a chroma of 12. However, chroma is not uniform for every hue at every value. Mussel saw that full chroma for individual hues might be achieved at very different places in the color sphere. In the Munsell System, reds, blues, and purples tend to be stronger hues that average higher chroma values at full saturation, while yellows and greens are weaker hues that average fullest chroma saturation relatively close to the neutral axis. In the "Munsell Book of Color", you will find the complete system in 40 pages. Each page has a different hue running around the spectrum to red and on through purple back to violet (PB in the Munsell notation). The colors on each page are arranged in rows of equal Value and in columns of equal Chroma. Each color has three references corresponding to hue, value, and chroma (ex: 5YR/5/10 is a saturated orange). Figure 1 : The Munsell System Part 3: Differences of Pigment versus Dyes Pigments Pigments are organic or inorganic, colored, white or black materials that are practically insoluble in the medium in which they are dispersed. They are distinct particles, which gives the medium their color and opacity. Figure 1 : Pigments Dispersion The smallest units are called primary particles. The structure and shape of these particles depends on the cristallinity of the pigment. During the pigment production process, primary particles generally aggregate and generate agglomerates. During the dispersion of the pigment into the polymer, high shear is generally needed to break up these agglomerates (improved tinting strength). Pigments are thus required to resist dissolving in solvents that they may contact during application, otherwise problems such as "bleeding" and migration may occur. In addition, depending on the demands of the particular application, pigments are required to be resistant to light, weathering, heat and chemicals such as acids and alkalis. Polymer Soluble Dyes Polymer Soluble Dyes are soluble in the medium in which they are dispersed. This means that there are no visible particles and the transparency of the medium is unchanged. Figure 2 : Dyes Dispersion Dye is a substance that is applied in order to impart color with some degree of permanence. Part 4: Pigment Performances Organic pigments are mainly used for applications needing high tinting strength and brilliant shades while inorganic pigments are mainly useful where high opacity is needed. Pigment performances and properties mainly depend on its chemical structures, surface properties, crystallinity, particle size and size distribution. Click on the image above to access more details on the required performance for your coating. 4.1./ Color The color of a pigment is mainly dependent on its chemical structure, which is determined by the selective absorption and reflection of various wavelengths of light at the surface of the pigment. Colored pigments absorb part of all the wavelengths of light. For example, a blue pigment reflects the blue wavelengths of the incident white light and absorbs all of the other wavelengths. Hence, a blue car in orange sodium light looks black, because sodium light contains virtually no blue component. Black pigments absorb almost all the light, whereas white pigments reflect virtually all the visible light falling on their surfaces. Fluorescent pigments have an interesting characteristic. As well as having high reflection in specific areas of the visible spectrum, they also absorb light in areas outside the visible spectrum (ultra-violets that human eye can not detect), splitting the energy up, and re-emitting it in the visible spectrum. Hence, they appear to emit more light than actually falls upon them, producing their brilliant color. Pigment Color Titanium Dioxide Excellent Iron Oxide Fair Prussian blue Excellent Lead chromate Excellent Carbon black Excellent Monoazo Excellent Disazo Excellent Phthalocyanine Excellent 4.2./ Color Strength As well as color, color strength (or tinctorial strength) must be considered when choosing a pigment. Color strength is the facility with which a colored pigment maintains its characteristic color when mixed with another pigment. The higher the color strength, the less pigment is required to achieve a standard depth of shade. Chemical structure is one of the factors that influence the color strength of a pigment. • In organic pigments, color strength depends on the ability to absorb certain wavelengths of light. Highly conjugated molecules and highly aromatic ones show increased color strength. • Inorganic pigments that are colored due to having metals in two valency states, show high color strength. In contrast, those that have a cation trapped in a crystal lattice are weakly colored. Particle size also influences the color strength of a pigment. Higher color strength is obtained with smaller particles. Manufacturing conditions are the main factor that influences the particle size of pigment crystals. Pigment manufacturers play a crucial role: • They can reduce the size of the particles by preventing the growth of crystals during synthesis, • and they can increase color strength by efficient dispersion. Pigment dispersion will also take a major role in the color strengh of the paint. Indeed, it will impart colloidal stability to the finer particles, avoiding their flocculation and using their full intrinsic color strength. [...]... resting of pigment/ binder premixes prior to their dissolving or grinding helps to accomplish the wetting stage and always eases and accelerates dispersing processes 3 Distribution demands the pigment to be equally dispersed throughout the binder system A lower viscosity tends to lead to a more even pigment distribution 4 Stabilization prevents the pigments from re-agglomerating The pigment dispersion is stabilized... optimal pigment particle size and long-term stabilization of the dispersed particle in the formulation Most organic pigments show better transparency as dispersion improves, while in the case of the larger particle size inorganic pigments, opacity is improved by good dispersion The dispersion process consists of the permanent breaking down of agglomerates into, as far as possible, primary particles... pigments Red pigments Violet pigments Blue pigments Green pigments Special effect pigments Extender pigments Corrosion-inhibiting pigments 7.1./ White pigments All white pigments are inorganic The more used white pigment is Titanium Dioxide It became the dominant white pigment after the Second World War Pigment Discovery Date White Lead 4th century before Christ Zinc Oxide middle of 18th century Lithopone... only moderate solvent resistance Uses The only use for this pigment is in camouflage paints, as its infrared spectra satisfies various standards 7.3./ Brown pigments Iron oxide is the most important brown pigment, but a few organic pigments are used for speciality applications: • • • • Iron oxides Metal complex Benzimidazolone (brown-red pigment) Azo condensation (brown-red pigment) 7.3.1/Iron Oxide... currently being developed by pigment manufacturers 5.1.2/ Particale shape The chemical structure, the crystalline structure or the synthesis of a pigment determine the shape of particles The primary particles of a pigment may be nodular, spherical, prismatic, acicular or lamellar Figure 1 : Particle shapes Primary particles are composed of single particles The smaller these particles, the greater their... equally-charged local sites on the pigment surface come into contact with one another Two particles having the same charges give a repelling effect The resulting Coulomb-repulsion of the charged particles allows the system to remain stable 6 Steric stabilization A pigment is said to be sterically stabilized when the surface of the solid particles are completely covered by polymers, making particle-to-particle... (flocculation) 7 Part 7: Main Families of Pigments Colored pigments are categorised as either organic or inorganic Each have distinct characteristics which, in the past, were used to distinguish one from the other For example, organic pigments are traditionally transparent However, modern manufacturing techniques are capable of imparting properties not previously associated with the chemical type: it is now... width or height Particle size is an average diameter of primary particles Typical ranges are: • • • • carbon black - 0.01 to 0.08 µm; titanium dioxide - 0.22 to 0.24 µm organics - 0.01 to 1.00 µm; inorganics - 0.10 to 5.00 µm; Extender pigments can be among the coarsest pigment particles, up to 50 µm, but other types can be exceptionally fine (e.g the precipitated silicas) The pigment' s particle size... pages 5.1./ Pigments Characteristics 5.1.1.1/ pigment structure Pigments can be crystalline or non-crystalline (amorphous) In crystalline pigments the atoms within each molecule are arranged in a well structured pattern, however, in amorphorous pigments the atoms are randomly arranged It is also possible for materials to have several different crystalline forms - known as polymorphism Color is dependent... possible This is achieved by surrounding the particles as soon as they are formed with a coating, which prevents the growth of crystals The most common products used for this coating are rosin or rosin derivatives This is particularly useful for printing ink pigments that are required to have high transparency and it has the added advantage that such pigments are more easily dispersed Iron oxide pigments . Pigment Part 1: What is Color Color plays an essential role in our day-to-day lives. From birth we are taught to react to colors logically. choosing a pigment. Color strength is the facility with which a colored pigment maintains its characteristic color when mixed with another pigment. The