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Technical Insights on Coatings Science
Color Formulation HOW?
Color is one of the most challenging areas of formulation. The pigments must be dispersed, stabilized and prevented from segregation or flocculation. The color must match whatever the customer wants. The type of paint dictates the technology needed to get the color correct. Automotive paints are a separate world from architectural coatings. This segment will discuss architectural coatings with a future issue targeting automotive type coatings.
Architectural paints are usually formulated at three levels of base pigmentation. The first usually contains very little or no titanium dioxide. This base is designed to be heavily tinted for dark colors. The second contains an intermediate level of titanium dioxide and is intended for many medium tone pastels. The last base is a white base which can be used as is or lightly tinted for very light colors, both with excellent hide. In all cases, the extender pigment is kept at a level to insure film build and gloss are correct. Since most paints are tinted at the store, the formulation must be compatible with the pigments, dispersants and other components contained in the tints. The paint manufacturer will often sign a secrecy agreement with the tint manufacturer and they will work together to insure compatibility. The number of pigments needed to match most end use colors is typically between 10 and 15 colorants. These can include carbon black, red iron oxide, yellow iron oxide, umber (an iron oxide), titanium dioxide, phthalo green, phthalo blue, a hansa yellow, an organic red, magenta, chrome green, and a few others.
Factors which must be controlled to obtain consistent color formulations are; paint solids content, paint titanium dioxide levels, the color of the base itself, and tint strength including hue and chroma. The lot-to-lot variance for the base and tints must be very small or color matching will not be possible. The next factor is the database for color formulation. Databases must be prepared with attention to detail. The pigments must be weighed to a high level of accuracy and mixed with a specific weight of base. These components are then mixed properly and a drawdown is made which is also highly controlled. The drawdown must be uniform and all drawdowns must be the same thickness. Variation in film thickness will result in a flawed data base. The database must be set up with proper consideration of the software from the spectrophotometer manufacturer. Coating transparency must be considered if the paint will not give full hide. The amount of base and tint must be large enough to insure accuracy. Pint samples are ideal since they will represent about 500 grams of base and 0.5 to 60 grams of tint. If the base is weighed to 0.01 gram accuracy and the tint to 0.0001 gram accuracy, the weighing errors will be insignificant.
Care must be taken to insure that a sufficient number of levels of tint have been used to establish the database. When correctly established, a good database can match a color within 0.5 delta E units or less. A poor database will result in mistints and will cost the store profits. All colorants must be evaluated for stability with the bases. One of the pigments could cause flocculation or other problems. We have noted that some pigments can alter the rheological curve of a paint. The surfactants in the tint can interact with the associative thickener to drop the viscosity. Black pigments usually have more surfactant and thus will be more prone to this problem.
The tinting of a paint also requires adequate mixing of the tint into the paint. If the paint is not shaken well before evaluation, the tint may not be uniform and will result in tinting errors. Another problem is with the tint itself and its maintenance. The tint can must be shaken before it is added to the tint dispenser. The tint dispenser must be kept clean and the tints properly stirred. Old tint dispensers may have less accuracy than modern units. All tint dispensers should be routinely calibrated by weight, not volume. This is accomplished by dispensing a set amount of each pigment into a tarred cup. If the amount is improper, the unit must be recalibrated for that pigment.
Today the customer expects more from color. Everyone wants the color to match, the paint to be uniform without settling, and every other property to be stable for years.
The End User
How should stains be evaluated?
There are two typical modes of stain evaluation. The first is a direct application of the staining component to the paint. The second is an evaluation of a stain on the substrate which may bleed through an overcoat. Classically, the stain was evaluated visually and rated. Today, with the advent of the color computer or visible spectrophotometer, the measurement of the stain can be quantitative rather than simply a subjective judgment which varied from one investigator to the next and from one laboratory to the next. There is still no absolute rating system to staining, even with a spectrometer. Most laboratories will report the stain as a delta E value. If the stain is being removed, the color of the unstained paint is measured and the color of the area which was stained and washed is measured. The difference is the delta E. Usually if the delta E is less than 0.5 the coating is judged stain resistant. For stain bleed-through, the delta E is determined analogously. On wood, tannin bleed can also be evaluated analogously. The color of the paint on a non-staining substrate such as a standard test panel is measured, then the coating on the wood substrate is measured. If the tannin bleeds through, it will cause a color change which will be detected and will yield a larger delta E.
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|March 14-18/05 Basic Composition of Coatings This course provides an overview of the components of paint and how they work. Participants are also introduced to methods for testing and manufacture of paint.|
|May 16-20/05 Introduction to Paint Formulation This course provides techniques used in|
formulating paint from raw materials. It involves formulating and making paint in the laboratory, "Hands on!"
|Coatings for Engineers available on-line anytime This course is designed to educate engineers in coatings science. Coatings systems will be covered from cleaning and surface prep to pretreatment, priming and topcoats. Specification and testing sections will aid all engineers who are charged with these tasks.|
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