By Susan Bates
Technical Specialists, Color Technology
M.A. Hanna Color and Additives
EDITED BY Jean M. Hoffman
Color, on a scientific level, is merely how the eye and brain interpret electromagnetic energy. But it is also one of the most powerful marketing forces in industry. As consumers continue to demand a vibrant look on everything from cell phones to automobiles, color may be the deciding factor between a new product's success or failure.
Color once consisted of simply slapping a thin coat of paint on a product's surface. But thanks to the widespread adoption of engineered thermoplastics, color is now integral to design. In much the same way that engineers turn to databases and tables to weigh the right material physical properties for their applications, they are now examining color. A wealth of information and expanding color palette help pave the way for more design freedom and new levels of product aesthetics.
Unfortunately, color is not a constant. For example, what are thought to be identical colors can look markedly different as lighting and resin systems change. Automotive designers, for instance, often deal with half a dozen or more resins in a car's interior. Only through careful collaboration with a colorant supplier and proper selection of pigments and dyes will a unified color scheme be possible. In addition, not only must the colors match on the showroom floor, industry standards dictate that they maintain the same gloss, grain, and color for 10 years. The match must remain consistent even under harsh UV exposure and the ravages of summer heat inside a locked car.
The good news is that as with many engineering challenges, color is well on the way to better control. Breakthroughs in the knowledge about human color perception is one reason. A second is that instrumentation and methodologies have progressed to the point that colorists now readily create acceptable color matches in a wider variety of resins and viewing conditions.
HOW PLASTICS TAKE COLOR
Pigments and dyes in plastic resins are derived from a number of organic and inorganic compounds. Each is often tailored with a set of specific characteristics suited to a particular resin or application. A decade ago, standard practice involved custom batches of precolored resins. Today, metering and mixing color during molding eliminates the need to handle numerous lots of colored resin for different products.
Color concentrates take many forms. These include encapsulated colorants and additives in a polymer matrix, super concentrates, wax-based flakes, and liquid or dry nonencapsulated additives.
Inorganic pigments include metal oxides and sulfides. Titanium dioxide, for example, is universally used for white, while titanium-dioxide-coated mica yields a pearles-cent effect. Iron oxides give a wide assortment of reds, oranges, blacks, or yellows. Organic pigments include carbon black, phthalocyanines, azo derivatives, quinacridones, and more. Organics are taking the place of heavy-metal oxides, such as cadmium sulfide. Many pigment grades are now FDA compliant.
Dyes almost never pose dispersion problems, but many show poor-to-fair light fastness. PET soda bottles, with a variety of clear colors, best illustrate dyes at work. Various organic formulations are used for dyes, including anthraquinone for reds and azo for a wide range of colors. Glow-in-the-daylight colors are generally derived from dyes.
The cost of color varies widely, based on both the relative cost of colorant raw materials and the relative strength of the colorant. The latter affects the color letdown ratio the percent by weight of colorant to base resin. A high-strength, low-cost colorant will clearly cost much less than a low-strength, high-cost colorant.
In theory, color matching should merely consist of measuring an original sample with a good color meter, then creating a color batch whose parameters match the sample. In actuality, there is quite a difference among measuring devices and there remains a strong element of art to the business of color matching.
The instruments for determining color generally measure sample reflectance. They range from relatively simple colorimeters to highly complex spectrophotometers. At the low end, marginally accurate colorimeters use three or four filters in the light path to a photodetector that's been calibrated to a standard sample.
At the high end, full-range spectrophotometers effectively detect each nm of light in the visible range 380 to 780 nm. Though few spectrophotometers actually carry the full 400 photo-sensitive detectors needed for full-range detection, they generally exhibit far better repeatability, reproducibility, and accuracy than typical colorimeters.
The majority of instruments define color using the Commission Internationale de l'Éclairage (CIE) or International Commission on Illumination L*a*b* Model. This model imposes uniformly spaced scales on the decidedly nonlinear way humans perceive colors, and uses an X, Y, Z-coordinate system for color descriptions.
L* in the CIE L*a*b* colorspace is the whiteness or lightness axis, from white to black. The "a*" axis represents the redness-greenness coordinate. If "a*" is positive, there is more redness than greenness; if it's negative there's more green. It is normally used with "b*," the yellowness-blueness coordinate, as part of the chromaticity or chromaticity color difference. Chroma (C*) indicates the degree of departure of the color from a gray of the same lightness. Hue (H*) describes perceived colors such as red, yellow, green, or blue. Pure white, black, and grays possess no hue. CIE L*a*b* color-space gives repeatable and precise color descriptions and helps determine a color sample's specific spectrophotometric values. Precise spectrophotometric values alone, however, cannot guarantee a good match. Because color is a human perception, experts need to massage the data using the empirical evidence and their personal experience with pigments and dyes.
THE PERILS OF METAMERISM
The same object can look different to two different people or two spectrophotometers, referred to as observer metamerism. Or they can differ in two distinct light sources object metamerism.
Instrumentation can be used to detect observer metamerism, which facilitates proper formulation to minimize the phenomenon. Object metamerism, on the other hand, is tougher to solve. This is because colorants respond or reflect differently across the spectrum under various lighting conditions.
The color of a car, for example, changes under nighttime parking-lot lights when compared to a bright sunny day. Part of this has to do with subtle shifts in light levels that change the response of the human eye. However, a much greater impact on the color change comes instead from how pigments in the car's paint respond to the excitation from the different light sources.
Multicomponent assemblies made from different materials also create thorny color-matching problems. When two objects such as a paint chip and a plastic part are compared they will often match under one light and not another. They are said to be a conditional match. And are referred to as a "metameric pair."
Colorant suppliers minimize metamerism by specifying standards and tolerances for all possible lighting conditions. Special viewing rooms are also helpful in replicating potential light sources and pointing out metameric effects. And experts often recom-mend using samples made from the actual resins, rather than from stock color chips or printed swatches.
Likewise, color concentrate manufacturers may use different pigments and dye systems to match the same color, so designers must exercise caution when switching suppliers. Substitute concentrates may produce a match under one type of light but cause a dramatic color shift under another.
There are three components that are required for color.
Color: Object + Visual System + Light Source
These three variables are referred to as the color perception triad. In order to specify and maintain color in a material, repeatability in all three areas is required.
The light source: Color perception varies with the light source. A given color will appear different under daylight, an incandescent bulb, or a fluorescent light. Daylight favors blues and greens. Fluorescent light bulbs, on the other hand, use chemicals that emit light at particular wavelengths. This results in nonsun peaks. Cool-white fluorescent lights, for example, peak in dark blue and again in green-yellow.
Depending on the angle of the sun in the sky and the surrounding atmosphere, sunlight itself varies. Likewise, the glow from an incandescent bulb changes with age or wattage. For this reason, computerized color-science technology uses mathematical models, or "illuminants" to describe light sources. For instance, Illuminant A represents incandescent light; Illuminant F depicts one particular kind of fluorescent bulb; and Illuminant D65 denotes daylight.
The sensor or visual system: The second variable, the visual system, consists of the eyes and brain. Two kinds of photosensitive cells are found in the human retina, rods and cones. Rods function at night. Cones respond to bright light and are responsible for color vision. Both connect to the optic nerve and transmit their signal to the brain.
The brain assembles several million bits of red, green, or blue data to compute observed colors. Most color measurement or color reproduction depends on this red-green-blue (RGB) triad and uses a computerized version of the brain's internal algorithm to compute color.
The eye's peak sensitivity is around 560 to 630 nm or yellow. The least sensitive spectral range is blue at 400 to 480 nm. The result is that a "pure" blue will appear darker than a "pure" green or red, which are clustered around the yellow.
Because the brain translates the eye's input, humans often perceive colors the way they want to see them. For example, a seacoast village observed early in the morning may, colorwise, look the same as it did the previous afternoon. But if a color photograph is taken, the film's interaction with the early morning throws a strong orange-red cast over the seascape. The human eye and brain simply tune that color cast out. The mind's ability to tune color out makes human color perception both inconsistent and inaccurate.
The design world demands consistency. Therefore, a great deal of experimentation and study goes into the science of color. Ongoing enhancements in instrumentation and color theory have made it possible to virtually standardize the contribution of the visual system to the color perception triad. Such standardization lets colorists discern, specify, and recreate very narrow bands of color and color effects.
The object: The third variable is the object. Object or material-created modifications of light include transmission, absorption, scattering, fluorescence, and reflectance.
Absorption is key to color. In designed materials, pigments and dyes selectively absorb and scatter light at particular wavelengths. Dyes dissolve in the medium they are coloring, while pigments don't. Pigments disperse throughout the material and tend to increase a part's opacity.
Each colored object has a characteristic spectrum of reflected light, the shape and intensity of which are determined by the object's natural or synthetic colorants. Spectral reflectance curves plot the level of reflection from an object at each point across the spectrum against relative reflected energy, expressed as a percentage of the original light energy.
Pigments and dyes absorb or subtract certain wavelengths of light. The resulting reflectance spectrum is the light that is not absorbed. Mixing or blending pigments and dyes results in shades that combine the absorption of each colorant and is called subtractive mixing. The subtractive primary colors are yellow, cyan, and magenta.
Mixing all three of the subtractive primaries takes away all wavelengths and yields black. Computer programs model subtractive mixing via mathematical rules and formulate color matches from them.
The final three modifications, scattering, fluorescence, and reflectance also affect color, but fall more into the realm of special effects. A surface that effectively scatters light gives a ground-glass effect. When subsurface scattering and limited transmittance combine, the result is an opalescent effect. Fluorescent pigments create glow-in-the-daylight color effects. Reflectance or mirrored surfaces are also important because color matching instruments bounce calibrated light sources off objects to their sensors.