Coatings Technology. Today s high-tech pigments provide value beyond aesthetics.

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1 20 D+D OCTOBER 2014 Coatings Technology The Color Durability Today s high-tech pigments provide value beyond aesthetics. By Mark M. Ryan Jr., The Shepherd Color Co. From the time of the cave dwellers, who used abalone shells and quartz rock as mortar and pestle to grind natural pigments from stone into paints to adorn the walls of their caves, human beings have expressed themselves, communicated with, and enjoyed color. Art and science have expanded the available colors from those basic natural pigments to highly engineered colorants with features and properties beyond the visible spectrum, but all too often the brightest colors have the least stability. Fading colors on buildings, even though they re just on the surface, can diminish the impression of the whole building. That can be an expensive problem for those who design, construct and own buildings. Fortunately, a special class of pigment, CICP (Complex Inorganic Color Pigment), combines vivid color with extreme durability for colorfastness that withstands the elements. The pigment also helps create dark colors that are exceptionally resistant to heating from sunlight, with all the benefits such a characteristic brings. This article examines the complex chemistry behind CICPs, from a formulation chemist s or engineer s perspective. First, let s take a look at some color and pigment basics. This splash of color shows the range of bright pigment colors available from CICP chemistry. The brighter yellows and oranges are new. All but the violets are highly durable. Photo courtesy of Shepherd Color Co. What is Color? Entire seminars, textbooks and even careers are built on understanding the nuances of color. In its most basic form, what we call color is the interaction of a light source, an object and an observer. A light source has a distribution of energy at a certain wavelength. We know sunlight, for instance, differs from an LED bulb or a fluorescent light. When sunlight hits an opaque object, the light is either absorbed or reflected. A white object reflects most of the energy. A black object absorbs most of the energy. Aesthetically pleasing colors selectively absorb and reflect different wavelengths providing different spectral curves.

2 Coatings Technology 21 The reflected wavelengths travel to the observer. The anatomy, physiology and neuroscience behind what happens between light entering our eyes and the creation of the sensation of color and form is a highly complex system. Thankfully, it proceeds without much thought on our part. That s how the light source, object and observer form the basis of color. On exteriors, the light source we work with is the sun. The sun is generally beyond human control, though we can maximize, minimize or take advantage of its effects based on how we build. The observer function depends on individual variables like genetics and age, and ors or transparencies that can be modified by adding colorants. Dyes and Pigments Colorants can be described in a number of ways, including by their chemical nature and the mechanism they use to make color. The Complex Inorganic Color Pigments (CICPs) bridge the gap between the durable, but limited hues of simple oxides and the chromatic, but less durable organic pigments. Here, a steep-slope standing-seam roof shows off a coating pigmented with Cobalt Aluminate Blue CICP. Photo courtesy of Shepherd Color Co. (Facing page) Bushman cave art, like this painting of an antelope in a shallow cave in the mountains of Drakensberg, South Africa, is as old as 2,400 years. The ancient artists mixed ground stone with fat to create their media. istockphoto/ecopic biggest general difference is whether the colorant is a dye or a pigment. A dye is soluble in a vehicle that is being colored, such as a resin or solvent, and disperses to individual molecules of the colorant. A pigment remains as discrete particles separate from the vehicle to which it is added. The pigment in house paint may seem to be soluble in the paint, but it is actually just finely divided and dispersed into particles about 1 micron in size, invisible to the human eye. Pigments are preferable for coloring high-durability materials since dyes, because of their solubility, are more apt to is also largely beyond design and engineering controls. Objects, from toys to towers, are what we can manipulate. Plastics, paints, coatings, ceramics and other materials all have base colmigrate out of a material. Pigment particles are largely fixed in the vehicle once it sets. Pigments include two main families organic and inorganic. Organic pigments Mostly based on nitrogen, carbon, hydrogen and oxygen, organic pigments produce colors based on the structure and bonds between elements and their interaction with light waves. Organic pigments are often highly chromatic with bright masstone colors and good tint strength when mixed with white. While the stability and durability of organic pigments runs a wide gamut, because of their basic organic structure, they are susceptible in varying degrees to attacks by acids, bases, ultraviolet (UV) light, elevated temperatures and other extremes. Inorganic pigments These can be either natural or synthetic. The most prevalent are the simple oxides that are made of a metal ion and oxygen in a matrix structure. The most common of these would be white titanium dioxide and red iron oxide and green chrome (III) oxide. The simple inorganic oxides provide high opacity colors that are very durable but not very chromatic. The range of colors they can be used to formulate is limited. A Special Class CICPs bridge the gap between the durable but limited hues of simple oxides and the chromatic, but less durable organic pigments. Instead of one metal in the oxide structure, the CICPs are based on two or more metals. They re most commonly created by intimately blending simple oxides like iron oxide and chrome oxide and firing them in a kiln at over 800 C (1,472 F). At these temperatures the metal ions jump back and forth. They make a new chemical that is neither, in this case, chrome nor iron oxide, but a chromium-iron oxide with different properties from either single-metal oxide.

3 22 D+D OCTOBER 2014 Chromium-iron oxide is a dark black powder with IR-reflective properties that helps the materials it s in to resist solar heating and deterioration roof coatings, for instance. Another CICP is formed by intimately blending black cobalt oxide with white alumina oxide. When formulators fire the resulting gray powder in a kiln at over 1,200 C (2,192 F), it forms a bright blue pigment known as cobalt aluminate, or cobalt blue, used by artists and coatings companies the world over. CICPs are inherently stable colors because they include more than one metal ion type and because of the high-temperature synthesis of the pigment at over 800 C. Simple oxides can be susceptible to degradation. Yellow iron oxide, for example, can change color at 190 C (374 F), red iron oxides can be degraded by acids. A brown CICP, on the other hand, made from iron and titanium, has much higher heat stability at over 700 C (1,292 F) and is extremely acid-stable. The color is highly stable because the mechanism for developing this color resists degradation. Instead of relying on the organic materials double bond conjugation to absorb wavelengths of light, CICP color is produced through light s interaction with electrons in the d-orbitals in the pigment crystal structure and their transitions from one energy level to another. By carefully selecting the right chemical compositions, and careful processing, formulators can produce a wide range of highly durable pigments including: Cobalt Aluminate: red-shade blue Cobalt Chrome Aluminate: green-shade blue Cobalt Chromite: dark green Cobalt Titanate: bright green Nickel Titanate: green-shade yellow Niobium Tin Pyrochlore: middle-shade yellow Chrome Titanate: red-shade yellow Rutile Tin Zinc: orange Iron Titanate: brown Magnesium Titanate: iron-free brown Copper Chromite: jet black Chromium Iron Oxide: IR-reflective black Through the careful blending of these CICPs with each other, manufacturers can create a wide range of aesthetically pleasing colors. These pigments are inherently safe because the high-temperature calcination renders them inert. The chrome in these pigments is not the Cr6+ species that is gaining increasing regulatory scrutiny. Many CICP pigment chemistries are safe enough for FDA-approved food packaging and other sensitive applications. Of course, these pigments are dry powders and must be incorporated into a binder. The final properties of the paint, plastic, coating, ceramic or other material depends largely on the durability and properties of the binder. The highly durable CICP color will not be stable if the binder breaks down. Further, once durable colored pigments go into a binder, they must be carefully formulated into usable systems such as fluoropolymers, siliconemodified polyesters and acrylics. Pushing Frontiers of Durable Color Many pigments in the yellow and orange color space meet requirements for chromaticity, opacity, heat stability, economics and durability. The common trade-off of the bright organic-based pigments versus the higher-stability inorganic pigments can be seen in Figure 1 on page 23. Organic pigments have always been more What appears to be a fine wooden deck is actually a high-end composite, colored to look like timber. Dark streaks not only give the surface depth, but as media pigmented with Chromium Iron Oxide, an infraredreflective CICP, they help keep the deck cool, possibly prolonging its service life. Photo courtesy of Shepherd Color Co. chromatic, offering redder tones than the standard inorganic pigments. But now, two CICP chemistries challenge this status quo. Rutile Tin Zinc (RTZ) orange variants have been pushed further into red-tone yellows until true orange shades have been developed. These RTZ orange pigments have higher chromaticity and redness than standard yellow titanate pigments. Being inorganic, the RTZ orange improvements show excellent stability and opacity. Niobium Tin Pyrochlore (NTP) yellow is a new chemistry that has several interesting

4 Coatings Technology 23 Fig. 1: Common pigments relationship in color space and beneficial properties. It is redder and has higher opacity when compared to other common inorganic pigments and doesn t compete directly because of its different properties. NTP Yellow has a color very similar to lead chromate (PY34) middle-chrome. Compared to similarly colored organic alternatives, NTP Yellow has higher heat stability and opacity and makes an excellent alternative to lead chromates in high-performance applications. Redder Reds The NTP Yellow and RTZ Orange also work well with other pigments to push new frontiers in durable color space. Before the use of the NTP and RTZ pigments, if a totally inorganic color match were desired for heat sta- bility, opacity or exterior durability, a common formulation would be based on blends of standard inorganic pigments. Colors represented by these blends are described in Figure 2, on the line labeled Blends of BiV- PBr24. These blends extend from the very chromatic and very green-shade yellow to redder shades with a drop off in chromaticity. With the introduction of RTZ Orange pigments, formulators can draw a new line describing blends of standard green-shade yellows and RTZ Orange. Those blends can be seen in Figure 3, page 24, with the line labeled Blends of BiV-RTZ. Once again, starting at the very chromatic but greenshade yellow and moving redder with increasing amounts of RTZ Orange, these blends have higher chromaticity for a given red shade than the previously common blends. The difference between the lines represents the increase in color space by substituting the RTZ Orange for standard titanate yellows. If the NTP Yellow is added to these pigment mixes, two separate line segments can describe the blends of standard green-shade yellow and NTP along with NTP and RTZ blends. Those blends are described in Figure 3 by Blends of BiV-NTP and Blends of NTP-RTZ. This new frontier further expands the durable color space by using all inorganic pigments. While the RTZ Orange pigment has a very pleasing and chromatic masstone, it s also an effective colorant to have in a color palette because of its ability to add redness to pigment blends without a large decrease in chromaticity. This increase in the durable color envelope means many common commercial colors and standards can be matched with all inorganic pigmentation that meets ever-tightening regulatory requirements. For example, Table 1 shows the advantages of using NTP Yellow and RTZ Orange for formulations of a common color often called Safety Yellow and known as RAL As with many colors in the yellow color space, there are several ways to match the color. One way uses organic colorants. Another way, the BiV Based, represents a formula based on a standard green-shade Table 1: RAL 1003 color matches High BiV Pigment Chemistry Performance Based Organic P.Y. 154 Benzimidazolone 67.2 P.Y. 184 Bismuth Vanadate 67.0 P.Y. 216 Rutile Tin Zinc 4.5 P.Br. 24 Chromium Titanate NTP Yellow Niobium Tin Pyrochlore 72.5 P.Y. 139 Isoindoline 8.0 P.O. 62 Benzimidazolone 10.2 P.Bk. 7 Carbon Black 0.1 P.W. 6 Titanium Dioxide 22.5 Fig. 2: Color space boundary of pigment blends Chroma (C*)

5 24 D+D OCTOBER 2014 Spectral Irradiance (W/m 2 /nm 1 ) UV Visible n-ir Wavelength (nm) Fig. 3: Solar spectrum reaching Earth s surface yellow, but shows that an organic pigment still has to be used to meet this color. In the high-performance formulation, an all-inorganic formula can be used because of the color properties of the NTP and RTZ pigments. This formula has advantages in heat stability, opacity and exterior durability versus the other two organic pigment formulations. While NTP Yellow and RTZ Orange are individually interesting pigments, together they provide high-performance coloring options in the critical middle/red-shade yellow color space with a blend of properties not seen in other pigments. These options are especially relevant, given increasing regulatory restrictions and the desire to reduce the use of pigments based on deprecated chemicals. Beyond Color CICP pigments have properties beyond coloristic. Among the most important are the near-infrared properties. As can be seen in Figure 3, the sun s energy is not limited to the nanometer (nm) range that our eyes are sensitive to and perceive as color. The damaging UV energy represents a few percent of the sun s energy while the rest is roughly divided between the visible and the invisible near-infrared. Since all these wavelengths contribute to how the sun heats objects, but only some of them cause the visual sensation of color, optimally, pigments would absorb in the visible for aesthetically pleasing color but reflect in the near-ir to remain cooler. This property of reflectance is quantified with the test method ASTM C1549 Standard Test Method for Determination of Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer. It s reported as TSR (Total Solar Reflectance) as either a percent (0-100 percent) or as a fraction (0.00 to 1.00). To remain cool, the larger the TSR measurement the better. White, usually pigmented with titanium dioxide, is the coolest color because it reflects most of the sun s energy and has TSR values around Reflective Is the New Black However, designers and owners sometimes want color choices beyond white. The properties of chromatic color and IR-reflectivity are not present in all pigments, especially in darker colors approaching jet black. Carbon black is the most common black colorant because of its high absorption in the visible spectrum for jet black colors. Unfortunately, this high absorbance continues from the visible into the near-ir. Carbon black has a TSR around The optimum IR pigment will have the absorbance of carbon black in the visible spectrum for dark colors, but the reflectivity of titanium dioxide in the invisible near-ir spectrum. Some CICP chemistries approach this model of absorbance and reflectivity. In the nm range, the IR Black has good absorbance for dark colors (L* about 27), but once past 700 nm the absorbance gives way to reflectivity. This gives advanced versions of IR Black TSRs of around 0.28 versus a TSR of carbon black of around The increase in TSR leads to a reduction in heat build-up from direct solar energy. Other CICP pigments exhibit these hyperspectral properties of chromatic color with IRreflectivity as well. Most organic colorants also have high absorbance in the visible spectrum for chromatic colors, but have different near-ir properties. Organic pigments are often near-ir transmitting instead of near-ir reflecting. Organic pigment-based color formulas depend on reflective substrates to reflect the near-ir energy that passes through the colored material. If the substrate isn t reflective, the TSR will likely be lower. IR-reflective CICP blacks offer the ability to match aesthetically pleasing colors that have lower solar-induced heat build-up than standard pigmentation. This can be a critical technology in some climates, as heat build-up can induce warping and degradation, especially in PVC. The technology can also be used to meet building requirements and code specifications, and potentially, to save energy. CICPs provide high-performance tools to color formulators, engineers and chemists in the building envelope product market. Products containing CICP colors give designers, architects and building owners more options when it comes to expanding the investmentgrade color envelope. Beyond bright colors, they add properties of durability, functionality, product differentiation and regulatory acceptance. About the Author Mark Ryan is the marketing manager for the Shepherd Color Co. Mark has been with Shepherd for 15 years and has more than 20 years of industry experience. He has presented around the world on topics such as IR reflective pigments, highdurability color and environmentally friendly color options. He has a chemistry degree from Saint Louis University and an MBA from the University of Missouri at St. Louis. His favorite color is blue. D+D

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