Color control in electrocoat.
What is color? According to Webster s Dictionary, color is defined as a phenomenon of light or visual perception that enables one to differentiate otherwise identical objects." While the human eye can distinguish somewhere between one and ten million different shades of color (depending on which reference you consult), a scientific method for determining differences in colors has been established which has helped standardize color perception and quantify differences in color between objects. Colorimetry, the science of color measurement, provides the "rules" of color measurement, and is utilized widely in science and industry to express color in numerical terms.
The first system of color organization was developed by Albert Munsell, an artist who taught at the Massachusetts Normal Art School from 1881 to 1918. His system was based on the premise that color is three-dimensional and that the three dimensions form a color space, which he depicted visually using equally-spaced color samples. To assign numerical values to colors, Munsell used notations to describe color in terms of hue, value and chroma.
* Hue: The attribute of color that distinguishes red from blue from green, etc. There are five principle hues--red, yellow, green, blue and purple--and five intermediate hues--yellow-red, green-yellow, blue-green, purple-blue and red-purple.
* Value: Describes the darkness or lightness of a color. Dark colors have a low value and light colors have a high value; the range is from zero (black) to ten (pure white).
* Chroma: The difference from gray of a color. Chroma is also known as color strength, saturation or intensity, and has a range of zero (low intensity) to 20+ (high intensity).
Using Munsell color notation, 5R 6/12 describes a color in the middle of the red principle hue with a value of 6 and a chroma of 12. The Munsell Color Order System has stood the test of time and it continues to be used as the primary basis for defining color in the U.S.
In 1931, the Commission Internationale de L'Eclairage (CIE) in France made the first major recommendations regarding colorimetric standards, forming the basis of modern colorimetry. Colorimetry is based on the principle that the human eye contains receptors for the three primary colors (red, green and blue), and all colors are seen as mixtures of these primary colors. In colorimetry, these three primary colors are referred to as x (red), y (green) and z (blue) coordinates, or tristimulus values. Instruments known as spectrocolorimeters are modeled after this theory and they utilize three photocells as receptors to interpret color in a fashion very similar to the human eye. The basic components of a spectrocolorimeter include a light source, viewing optics, three photocells matched to a standard observer and an internal or external processor/computer, which is used to perform the calculations of color measurement.
While the x, y and z tristimulus values are useful for defining color, they do not allow for easy visualization of color. In 1976, the CIE created mathematical equations to convert tristimulus value into meaningful color space values known as [L.sup.*], [a.sup.*] and [b.sup.*]. These equations are referred to as CIELAB, and they are widely used for color determinations today. The CIELAB equations are as follows:
[L.sup.*] = 116 (y/yn)1/3 - 16
[a.sup.*] = 500 [(x/xn)1/3 - (y/yn)1/3]
[b.sup.*] = 200 [(y/yn)1/3 - (z/zn)1/3]
Spectrocolorimeters use CIELAB (and other) equations to convert tristimulus values into [L.sup.*], [a.sup.*] and [b.sup.*], which are visually depicted in Figure 1 (page 82).
[FIGURE 1 OMITTED]
The overall or total difference in color between a specimen and the standard, described in terms of [L.sup.*], [a.sup.*] and [b.sup.*] is known as Delta E or [DE.sup.*]. The term is derived from the German word for sensation, "empfindung." DE literally means the difference in sensation. In mathematical terms, DE (always a positive number) is expressed as:
[DE.sup.*] = sqrt [(D[L.sup.*])2 + ([Da.sup.*])2 + ([Db.sup.*])2]
In general, a DE of 1.00 or less is considered to be a production match. When calculating a color difference, the [L.sup.*], [a.sup.*] and [b.sup.*] values of the standard are subtracted from those of the trial. Thus, a negative DL value indicates a trial darker in color than the standard. Table 1 on this page summarizes how to interpret DL, Da and Db values.
While CIELAB math represents one of the most common set of equations used to determine color differences, other equations exist. Examples include FMC2 equations, BFD equations, Hunter equations and M&S equations, among others.
It is important to note that color can be affected by the light source. For example, a blue object may be a noticeably different shade of blue or a different color altogether when viewed outside in the sunlight compared to the color when viewed indoors under fluorescent light. This phenomenon of color variations under different light sources is called metamerism. In 1931, The CIE established several standard illuminants for use in color determinations; these illuminants are tables of spectral energy intended to represent "real" light sources. Figure 2 (page 82) illustrates the standard illuminants--D65 (average daylight), F2 (cool white fluorescent light) and A (incandescent or tungsten light).
[FIGURE 2 OMITTED]
Illuminant selection usually depends upon where the painted object is intended to be located. For example, the color of exterior products such as agriculture or construction equipment is usually tested with the D65 illuminant, while the F2 illuminant would be the logical choice for interior goods such as metal office furniture or air diffusers. For products that will encounter a variety of potential light sources, it is important to rule out metamerism during pigment selection early in the formulation process.
In addition to choosing the math and the illuminant for the spectrocolorimeter settings, you must select the angle of the observer (2[degrees] or 10[degrees]), determine whether to include or exclude UV light and specular reflectance (gloss), choose a large or small aperture, and you must ensure that the unit is properly calibrated. All of these settings on your spectrocolorimeter can impact the results when testing the color of your parts.
FORMULATING A COLOR MATCH
From a formulation standpoint, there are many aspects to consider before a match to a color standard can be achieved, which is simply the color that is required on the painted part. Pigment selection is critical. For electrocoat formulations, pigment conductivity is one of the limiting factors, since conductive pigments in most electrocoat applications tend to "interfere" with the electrical charge of the rectifier, which is intended to drive the conductive resinous portion of the paint to the part. This "interference" can lead to uneven film builds and abnormal appearance, and can also exacerbate a phenomenon known as preferential pigment deposition, in which the more conductive pigments will coat out faster than the less conductive pigments. Preferential pigment deposition negatively impacts color stability in an operational electrocoat tank and makes it difficult to control color in a consistent fashion. A good rule of thumb for formulating electrocoat paints is to choose pigments with conductivity values less than 100 microSiemens.
Two other traits of pigments to consider are the oil absorption values and surface treatments. These characteristics can impact the quantity and quality of the grind vehicles and surfactants used to disperse the pigment agglomerates during the milling process. There are other nuances. For example, red pigments don't tend to hide very well (organic reds are particularly transparent), and while some pigments don't fare well in long-term exterior durability testing, others do. All of these aspects must be considered. From a color standpoint, you must determine the correct blend of pigments to obtain a color match to your standard while imparting sufficient hiding power. Finding the proper ratio of pigments is sometimes difficult, but many spectrocolorimeters contain colormatch programs which can minimize the number of trials required to match the standard. Jiffy mills and other similar devices are sufficient for preparing small, laboratory-scale pilot batches, and tints can be added for "tweaking" after a rough color match is obtained. Tints, which are highly concentrated, single-pigment paints used for adjusting color, are usually incorporated into the final formula in relatively small quantities, preferably less than two percent of the total volume. Co-grinding dry pigments is less expensive than incorporating high volumes of multiple tints into the thin down portion of the formula.
Once you find the blend of pigments that gives the color, hiding power and appearance characteristics you desire, it is important to test shear stability and heat stability to ensure the pigment does not settle abnormally or destabilize. This phenomenon, sometimes called "kickout," can cause ultrafilter and bag filter fouling, plus it can lead to rough horizontal surfaces on the painted parts. Since an electrocoat paint bath is continuously agitated and heated, it is important to evaluate stability in your pigmented electrocoat feed components. Keep in mind that heavier pigments and pigments with high particle sizes such as Ti[O.sub.2] and kaolin clay extenders are more prone to settling than lower density pigments or those with small particle sizes. And throughout the entire formulation process, you must choose the overall paint chemistry that will impart the desired properties of the final paint film, such as durability, corrosion resistance, UV resistance, et cetera. Color-controllable electrocoat pastes can be produced in any quality including anionic epoxy, anionic acrylic, cationic epoxy or cationic acrylic. However, acrylic electrocoat products are more suitable for color control as epoxies tend to turn yellow during the cure cycle.
Electrocoat pastes are manufactured using a horizontal or vertical grinding mill with suitable media (zirconox beads are common). Cowles agitator blades are useful in breaking down large pigment agglomerates prior to milling, which minimizes the dwell time when dispersing pigments on the mill. Paste formulations should be optimized by using the correct ratio of pigments, grind vehicle (binder) and solvents for best results. One device used to determine the end-point of the grinding process is the Hegman grind gauge. Most color-controlled electrocoat pastes are processed on the mill until a Hegman reading of 7.0 is obtained. However, the end-point determination needs to be evaluated in conjunction with batch history, using repeatable flow rates and dwell times on the mill to determine when to stop processing the batch. Over-grinding color controlled electrocoat pastes leads to over-developing the color, which results in adjustments to the batch and slower cycle times through the production facility.
For quality control purposes, specifications for color-controlled electrocoat pastes should be maintained at +/- 0.3 or less for DL, Da and Db. This will keep the overall color difference between the batch and the standard at a DE of less than 1.0, which is generally considered to be a production match. Since the color of electrocoat paint in an operating electrocoat tank usually tends to drift due to a variety of factors (preferential pigment deposition, color shifts caused by shear, settling due to poor agitation, etc.), the actual final color of a produced batch may vary greatly from the color standard. For example, it is typical for the color in anionic electrocoat paint baths to drift dark, therefore production batches of color controlled anionic pastes are often sent out light to compensate for this drift. The same logic applies if the final color in an operating electrocoat tank drifts in any direction; the color of the paste is adjusted to compensate for the drift. So if the color on line is two units yellow compared to the standard, the paste is adjusted to be two units bluer; if the color on line is one unit red compared to the standard, the paste is adjusted to be shipped out one unit greener. If you refer to the [L.sup.*], [a.sup.*] and [b.sup.*] diagram in Figure 1, the preceding examples demonstrate that production batches and electrocoat paste formulas are adjusted to be the opposite color of the direction the color is drifting. Ultimately, these types of adjustments lead to minimal tinting on-line and a closer match to the color standard for the parts being coated.
In an operational electrocoat tank, many factors exist that can have an impact on the final color of the coated part. Among them:
* Oven conditions. Overbaking and/or poor oven exhaust will have a negative impact on color. The color will typically be dark and yellow if either of these conditions is present. It is important to ensure the proper bake schedule is being utilized and that the oven has sufficient exhaust to minimize accumulation of fumes. Fumes that emit from the part during the curing process can be especially detrimental to lighter colored coatings, such as off-whites, beiges and light grays.
* Film build variation. Color shifts can occur with as little as 0.1 mils difference in dry film thickness (DFT). Light colors are affected by film build variations to a greater extent than dark colors.
* Rack loading. If large and small parts go through the cure oven simultaneously, the large parts can act as a heat sink, which can adversely affect the cure and color control of the lighter parts.
* Contamination. High levels of iron and ionic contamination, or problems with bacteria can all lead to yellowing of the paint film. Directing permeate into your waste stream can minimize ionic contaminants, but actions must be taken to completely eliminate any contamination source from the electrocoat bath.
* Bath parameters. It is important to keep the solids, P/B, conductivity, pH and solvent levels within the recommended operating ranges to ensure proper color control. Maintaining the correct chemical balance in the electrocoat bath is critical to keep pigments and paint solids in suspension. Tanks that operate out of specification can experience poor solubility and other problems, which can lead to abnormal settling and "kick-out" or instability of paint solids.
* Substrate condition. Properly cleaned and evenly pretreated substrates lend themselves well to color-controllable electrocoat paint. Additionally, the type of substrate can impact color results, so if multiple substrates are coated out of the same electrocoat bath, some color differences may be seen between them.
The following examples demonstrate how some of these factors can impact color. Note that these examples apply only for the specific colors and products that were evaluated, but some general information about certain color trends can be gleaned.
In Table 2, the data show that as the film build increases, the color shifts light, red and yellow. For this particular light colored anionic acrylic product, DL (lightness/darkness) and Db (yellow/blue) change significantly as the film build increases. Light products tend to hide the substrate better at higher film builds, so the color results at lower DFT are affected by poor hiding over the pretreated substrate.
In Table 3, the data show that as the cure temperature increases, the color shifts dark, green and yellow. The tendency of epoxy paints to turn yellow during the cure cycle is exacerbated as the bake temperature increases and is even more pronounced if oven exhaust fumes are present during curing. This is known as "fume yellowing."
Table 4 shows that as the dwell time in the cure oven increases, the color shifts dark and significantly yellow. As in Table 3, cure effects are amplified for epoxy paints.
In Table 5, the color shifts light and blue as the bath solids increase.
In Table 6, the color shifts dark and red as the P/B ratio increases. Note that P/B effects on color are more pronounced in baths that operate out of spec low in solids, and that low P/B baths do not tend to hide the substrate well, especially for light colors.
The importance of maintaining an in-spec, contamination-free electrocoat bath cannot be overstated. Additionally, the substrate type, pretreatment qualit, and especially cure conditions can all impact color of the final paint film.
While the software used by many spectrocolorimeters contains helpful color-matching programs that can assist with color adjustments, most of these programs require inputting copious amounts of information about the paste formulations and the specific kinds of tints that exist for your particular product. Much of this information can be obtained only by conducting numerous lab trials to determine average tint strength and the effects each tint has on various colors of products. Described below is a comparatively simple technique that can be used to determine the amounts and types of tints required to adjust color in an off-color electrocoat tank. Since this is a small-scale activity, conducting this work requires a laboratory equipped to electrocoat panels from a small container. Additional equipment includes a spectrocolorimeter, a film build gauge and a small weighing scale. First, here are some general guidelines about tint additions:
If positive, add black tint.
If negative, add white tint.
If positive, add green tint or blue tint.
If negative, add red tint.
If positive, add blue tint or black tint.
If negative, add yellow tint.
* It is difficult to make a red redder, a blue bluer, a green greener, etc. Higher tint quantities will be required.
* Light colors can be significantly affected by relatively small amounts of tints, and nearly all tint additions (except white tint and sometimes yellow tint) will drive the color darker quickly.
* It is difficult to lighten the color in most electrocoat baths, so expect to add a lot of white tint if the color of your electrocoat paint tank is dark.
* Adding tints may impact other color aspects as well as the one of interest. For instance, if the color of a part is too yellow (positive Db), the addition of blue tint will drive Db negative, but it will also drive DL and Da negative as well.
* Correct film thickness is critical for color measurements.
* Exercise caution if your labwork shows that a large amount of tinting is required (>3% by volume); tints typically do not contain catalyst, so there is a risk of under-curing the parts for certain paint technologies. Additionally, tints are rarely formulated at the same pigment to binder ratio as electrocoat pastes, so adding too much tint can potentially affect the hiding power and the gloss of the coating.
With these things in mind, the following method can be used to determine the appropriate amount of tints to add to an off-color electrocoat bath:
1. Begin by using your spectrocolorimeter to evaluate the color of a part that is representative of the ware being coated. Make sure the unit is properly calibrated and that the appropriate settings are in place (math, illuminant, aperture, UV, gloss, etc.). Record DE, DL, Da and Db. For example, let's say the product is an off-white cationic acrylic with the following color results: DE/DL/Da/Db = 1.98/1.16/0.53/1.52 (light, red, yellow) at 1.0 mils DFT.
2. Weigh out 1,800 grams of your electrocoat paint bath into an appropriate sized container and coat panels at the correct film build using the same substrate and pretreatment as the parts on line, then bake several panels and evaluate the color. If the color of your lab coatout is reasonably close to the color of the actual production part (within DE of 0.5 or less), then you are ready to start adding tints to your small paint bath. If the color is different than the part, then you should conduct trouble shooting activities on line to determine if the source of the discrepancy. The common culprit is the cure oven, as fume yellowing and/or over-baking are fairly common. If you are not able to resolve the color discrepancy, it may be necessary to utilize a correction factor to compensate for the amount of tints that need to be added.
3. When you are ready to add tints to your lab bath, start with small amounts so you can determine the net shift in color. In this case, the color of the part is light, red and yellow, so the tint of choice is blue since it will drive the color dark, green and blue. Remember, the goal is to end up (numerically) in the center of Figure 1 to get the DE as close to zero as possible, so the tints to select are ones that will drive the color in the opposite direction of the DL, Da and Db results. Let's say that after a one gram addition into your 1,800 gram bath the color results are DE/DL/Da/Db = 0.37/0.24/-0.28/0.06 (light, green, yellow) at 1.0 mils DFT. The net shift in color obtained from one gram of blue is 0.92 units dark (1.16 - 0.24), 0.81 units green (0.53 - (-0.28) (the color shifted green from 0.53 to zero, then beyond zero to - 0.28, so you add the total shift)) and 1.46 units blue (1.52 - 0.06). This is a relatively simple example; some color adjustments require adding several different tints, each of which can potentially affect the total color shift in a different way. Add tints one at a time, then coat panels and check the color after each addition. Recording the net shift in color after each tint addition will help you determine which tints to add to your electrocoat tank to adjust the color.
4. After you have adjusted your lab bath to an acceptable color match, calculate the amount of each tint to add to your electrocoat tank as follows:
Pounds of tint to add to the electrocoat tank = (total grams of each tint added to the lab bath / weight of lab bath) x weight per gallon of your electrocoat bath x total gallons of paint in your electrocoat tank. Going back to our example, if you have a 5,000 gallon electrocoat tank with a weight per gallon of 8.52 lbs/gal:
Pounds of blue tint to add to tank = (1.0g / 1,800g) x 8.52 lbs/gal x 5,000 gal = 23.67 pounds of blue tint
To convert the tint addition to gallons, divide the total pounds of each tint by the weight per gallon of the tint:
23.67 lbs / 8.65 lbs/gal = 2.74 gal of blue tint
When it comes to actually adding tints to your electrocoat tank, it is a good practice to initially add 1/2 of the amount you think will be needed to adjust the color. This will enable you to "creep up" on the color without the risk of over-adding tints. Remember, you can always add more tints into your tank, but they cannot be removed once incorporated. For proper color control, it is recommended that you conduct daily color checks on your production parts, and keep a running log of tint additions along with the corresponding color shifts displayed in Table 7.
Color control in electrocoat paint can be achieved through a methodical approach of incorporating the appropriate amounts of various tints into the paint bath. This should be done in conjunction with controlling the color of the product as supplied, as many factors can cause the color to drift in an operational electrocoat tank.
BY WILLIAM C. LEMONS
Table 1. Color Symbols Positive Negative Symbol Value Value DL Light Dark Da Red Green Db Yellow Blue Table 2. Effect of Dry Film Thickness on Color, White Anionic Acrylic DFT (mils) DE DL Da Db 0.8 1.40 -0.64 -0.45 -1.16 0.9 0.92 -0.07 -0.35 -0.85 1.0 0.67 0.29 -0.24 -0.55 1.1 0.74 0.72 -0.08 -0.13 1.2 0.94 0.93 0.02 0.14 1.3 1.32 1.20 0.16 0.53 Table 3. Effect of Cure Temperature on Color, Gray Cationic Epoxy Bake Schedule DE DL Da Db 20' @ 300[degrees]F 0.61 0.53 0.14 -0.27 20' @ 350[degrees]F 0.51 0.26 0.07 0.43 20' @ 400[degrees]F 1.23 -0.5 -0.15 1.12 20' @ 450[degrees]F 3.09 -1.08 -0.23 2.89 Table 4. Effect of Oven Dwell Time on Color, Gray Cationic Epoxy Bake Schedule DE DL Da Db 20'@ 350[degrees]F 0.72 -0.023 -0.1 0.68 25'@ 350[degrees]F 1.26 -0.20 -0.19 1.23 30'@ 350[degrees]F 2.16 -0.56 -0.21 2.08 35'@ 350[degrees]F 3.48 -1.05 -0.22 3.31 Table 5. Effect of Bath Solids on Color, Green Cationic Acrylic Bath Solids DE DL Da Db 14% 1.04 -0.68 -0.6 -0.51 15% 0.83 0.07 -0.79 -0.24 16% 0.71 0.16 -0.68 -0.11 17% 0.84 0.56 -0.63 -0.04 Table 6. Effect of Bath Pigment to Binder Ration on Color, Beige Anionic Acrylic PB Ratio DE DL Da Db 0.35 1.27 1.13 -0.12 0.57 0.40 1.14 1.01 -0.14 0.52 0.45 0.93 0.52 0.47 0.61 0.50 0.77 0.13 0.45 0.59 Table 7. Daily Log of Color Results and Tint Activity Date DE DL Da Db DFT Gloss 10/1 0.36 -0.18 0.07 0.30 0.94 88.1 10/2 0.57 -0.20 -0.20 0.52 0.97 89.9 10/3 0.54 0.00 -0.30 0.44 1.03 88.7 10/4 0.74 0.05 -0.27 0.69 0.95 90.0 10/5 0.80 0.19 -0.52 0.58 1.00 88.7 10/6 0.87 -0.29 0.34 0.75 0.97 88.8 10/7 0.57 -0.30 0.27 0.40 1.05 88.4 10/8 0.60 -0.55 0.18 0.17 1.02 89.8 10/9 0.53 -0.29 0.20 0.40 0.98 89.7 Date Blue Red White Black Yellow 10/1 10/2 10/3 10/4 10/5 2 qts 10/6 2 qts 10/7 10/8 1 gal 10/9
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|Author:||Lemons, William C.|
|Date:||Oct 1, 2006|
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