Understanding Pigments: The Third Mark Harber October, 2000 Step to Higher Quality and Consistency
Putting great color in your product is part of the pigments. However, they are less opaque and systems approach for resolving issues of sub- would have to be used at higher loading levels to standard properties and appearance. achieve similar whiteness and opacity.
This article on pigments is the third in a four-part Titanium Dioxide is used in the majority of the series about the interrelationship of the material products made by the cast polymer industry. Tita- components used in marble and solid surface nium Dioxide-based colors include most whites, manufacturing. These AOC-authored articles re- pastels, earth tones and off-whites such as bone, spond to the challenge that the cast polymer in- ivory, beige or biscuit. As noted in Table 1, non- dustries aspire to higher standards of quality and white synthetic oxides are combined with Titani- consistency. Because resolving cast polymer is- um Dioxide to create pastels and earth tones for sues requires a systems approach, other articles cultured marble and solid surface applications. in this series address resins, gel coats and pro- cessing. All articles begin with background infor- Phthalocyanine pigments, or "Phthalos," impart mation on the main subject matter, followed by deep colors such as the automotive "Hunter ten related issues and guidelines. Green" of a sport utility vehicle or the high strength Blue used in ballpoint pens. Because A BACKGROUND ON COLORANTS they are so deep when used by themselves, In their natural state, cast polymer resins meet a Phthalo Blue and Phthalo Green are normally variety of performance requirements but are lack- blended with other pigments, many times Titani- ing in the color that draws the customer to the um Dioxide. end product. Achieving a marketable color re- quires the addition of one or more pigments. Among black pigments, Iron Oxide Black is used Each pigment type or chemistry has an inherent to achieve a "jet" black look in cast polymer prod- property for imparting color to the final formula- ucts. Carbon blacks are used judiciously in many tion. Gray and Platinum colors, and as a shading com- ponent in many other formulations. Ultramarine Most of the primary pigments used in the cast blues, which have a deep "royal blue" color, can polymer industry are listed in Table 1. The first be blended with Titanium Dioxide to create the pigment listed, Titanium Dioxide, is often referred appearance of "Bright White" colors. to as "tee-eye-oh-two" after its chemical symbol, TiO2. Titanium dioxide is the most widely used ANALYZING COLOR AND CONSISTENCY white pigment because of its unique ability to pro- At AOC, scientists and technicians generally em- vide exceptional opacity and to lend whiteness ploy the CIE L*a*b* Color Scale for analyzing col- and brightness. Because of this opacity property, or. The Scale is a three-dimensional "map" that Titanium Dioxide is better able to "hide" the color determines the location of a specific color by lo- of the base resin than other commercially availa- cating the intersection of three different axes of ble white pigments. For example, Zinc Oxide (ZnO) color values. By using this system, a color analyst or zinc sulfide (ZnS) can be employed as white can numerically express or analyze if a product's
Understanding Pigments, continued
color is on target or if it is getting too blue or yel- 2. Dry or wet...the choice is yours. low, too green or red, too light or dark. See the For some operations, adding dry pigment to the sidebar report, "The CIE L*a*b* Scale 'Elevator,'" cast polymer batch can be cost and performance for details on this color evaluation system. effective. However, dry pigment addition is much less "forgiving" of formulation and process varia- While several different instruments are available tion. In dry form, a pigment is usually in a small for measuring color, visual assessment is still a particle size and irregular shape that causes it to very powerful and important technique. Cast poly- stick on equipment and workers. Without strict mers are not the only products where sight is attention to bin and hopper design, the handling used to make a subjective appraisal. Thus it is of solid pigments can lead to "bridging" and "rat- important the human observer agrees with and holing" problems. General maintenance and up- sophisticated color computer. Many important keep are also increased. purchasing decisions, such as homes, clothes and automobiles, are influenced entirely by visual Dry pigments take longer to blend into the cast color assessment. polymer system to ensure uniform distribution of color and to eliminate the possible appearance of GUIDELINES TO BETTER COLOR undispersed pigment specks. Furthermore, the Establishing color parameters and maintaining resin adsorption characteristics of dry pigments color consistency is a complex proposition. A list can lead to variations in gel time, higher catalyst of basic troubleshooting tips has a useful purpose demand and differences in green strength devel- but do not always address individual shop prefer- opment. ences and routines. In an alternative approach, let's look at key issues and guidelines for optimiz- Most operations prefer the convenience, simplici- ing color quality and consistency. These guide- ty and effectiveness of liquid or paste pigment lines are not presented in any particular order of dispersion. With pigment dispersions, the color is importance. suspended in a "vehicle" of monomer free polyes- ter resin. When added to the matrix resin of a Three of these guidelines - on record keeping, casting formulation, the dispersion gives the fin- manufacturing repeatability, and communicating ished part molded-in color. In pigment disper- with your suppliers - may look familiar. These uni- sions, potential clumps of solid color concentrate versal principles are important enough to be in- are broken down by high-shear premixing. cluded in all four articles of this series. The wet dispersion is easily added and blended 1. Judging the color that meets the eye. into the overall formulation for easier mainte- The way a color is judged can vary by the lighting nance of color consistency from run to run. environment. The same sample will take on a different appearance under a conventional incan- This is especially important for "mix and match" descent light bulb, outdoor daylight, and fluores- components, such as a back splash that is pro- cent office lighting. There are even more variables duced at a different time and with a different to consider for a company that has more than one batch than a vanity. visual inspector and when color-critical parts are manufactured and shipped from several loca- 3. Veining without or with monomer. tions. Shop workers take pride in developing techniques that create just the right effect with veining The appearance of some parts may change simp- pastes. When veining pastes are supplied without ly by changing the angle of illuminating light. Nor- styrene monomer, the resulting veins tend to be mal wear, scuffing, chalking or surface erosion on more subdued. Veining pastes supplied with mon- a part can also significantly alter the way overall omer tend to be bolder and brighter. However, the color and appearance is judged. Surface gloss monomer-containing paste calls for closer atten- plays a huge role in the perceiving color and ap- tion to reaction and cure to ensure consistency pearance of finished parts. and quality.
Understanding Pigments, continued
4. Master the art and science of blending. es, changing the vehicle chemistry may be the Quality is improved when there is a greater under- most effective way to minimize the impact anoth- standing of the individual and total contributions er change would have on existing shop prefer- of resin, catalyst, filler and pigment. To ensure ences and practices. formulation consistency, it is best to maintain consistent parameters for the mixing procedure. 8. Put it in writing. This includes: the order of ingredient addition, the Good record keeping starts by saving the manu- time of addition, the length of each ingredient facturing and quality control records that your pig- blending cycle, and the temperature during addi- ment supplier provides. Combine that information tion. Part quality is also enhanced when ingredi- with a record of any variables that may affect ents are blended with clean and well maintained profitability and customer-acceptance. Keep a equipment. record of how changes and variations in the for- mulation and process affect the original optimized 5. Optimize your formulation for color. formulation. Test the pigment you intend to use at differing levels to achieve the best balance of cost and ap- 9. Seek repeatability, and repeat it. pearance. A general rule of thumb is to make and The key to color consistency is establishing con- compare test specimens at 0.25% increments sistency in your formulation and shop conditions. (0.25%, 0.50%, 0.75%, 1.00%, etc.). While higher In a never-ending quest for improvement, you can pigment loading will help reduce color variability, make methodical and incremental changes to pigment loadings are generally held to less than determine which factors increase quality, con- 2%. If any changes are made to a formulation, sistency and productivity. test the proposed formulation against the current formulation. 10. Talk to your material suppliers. Take advantage of outside technical expertise. A 6. Not all pigments, resins, catalysts and fillers professional technical service representative can are created equal. help you avoid or minimize "trial and error." Mak- Every ingredient in a cast polymer formulation ing good parts sooner saves time and money. Be contributes to the color of the final part. It is ex- specific about the make-up of your formulation tremely rare to find a "drop-in" replacement for an and details about your process and shop. The en- existing ingredient that will result in the identical terprising supplier wants to join you on a path of finished part color. For titanium dioxide alone, our mutual profitability and success. studies have shown that different grades can vary by almost 10% on the lightness (L*) scale. The ABOUT THE AUTHOR most widely used fillers cast polymers are calcium Mark S. Harber joined AOC (The Alpha Corpora- carbonate and aluminum tri-hydrate. If a change tion) in 1982, and he has served as Technical in filler is made, a color shift is likely because of Service Manager for AOC’s Chroma Tek® color- differences in color, particle size distribution and/ ants since 1986. He received a B.S. degree in or resin adsorption characteristics. Also expect Mechanical Engineering from the University of changes in color if the amount or type of catalyst Memphis, Memphis, Tennessee. is changed.
7. The solution to your problem may be in the ve- hicle. If color shifts because of a formulation ingredient change, share that information with all your for- mulation ingredient vendors. They may be able to provide information on how the formulation change affects their particular ingredient. It can be especially beneficial to work with a pigment dispersion supplier who offers the option of multi- ple vehicles for carrying the pigment. In some cas-