Technology Interchange—Color Matching Powder Coatings
Posted on Thursday, September 24, 2020
By Kevin Biller
In a broad sense, coatings provide two distinct properties: the protection of an article and the aesthetics that conform to a desired color, gloss, and surface characteristic. This issue’s Technology Interchange will introduce you to the color matching process of creating a powder coating. Let’s start with pigments.
Definition and the Nature of Pigments
Pigments are small, insoluble particles that provide color and opacity by being dispersed in a medium, in our case a powder coating binder. Pigments provide color because they inherently absorb certain wavelengths of visible light (ranging from approximately 380 to 730 nanometers). This absorption is observed, processed by the human eye, and interpreted by what humans describe as color. The general range of wavelengths of color is depicted in Table 1.
Because pigments are comprised of relatively small particles, they tend to form agglomerates in their natural state. The incorporation of pigments into powder coating binders therefore requires deagglomeration to achieve optimal color development and opacity. This deagglomeration occurs in the extrusion process and provides an even dispersion of the pigment particles throughout the powder binder, resulting in the richest hue and highest opacity.
The colorant pigments used in powder coatings are synthetically produced and are either organic or inorganic in nature. A number of distinctions exist between these two classes of materials. Organic pigments tend to provide bright, intense color, whereas inorganic pigments are typically duller and less vibrant. Most inorganic pigments are based on simple or complex metal oxides and have higher specific gravities and lower oil absorption than their organic counterparts. Table 2 depicts some general differences between organic and inorganic pigments.
Colorant pigments are most commonly categorized by Color Index. As illustrated in Table 3, this system uses the following format: the prefix is comprised of the first letter “P,” which signifies pigment, and the subsequent
letter(s), which identify the color. This prefix is followed by a number, sequentially assigned by when the pigment was recognized by the governing groups (the Society of Dyers and Colourists [SDC] in the UK and Association of Textile Chemists and Colorists [ATCC] in the US). For example, “PR” refers to “pigment red” and “PG” refers to “pigment green” and PR-170 refers to Pigment Red 170 which is based on napthol chemistry.
Practical Color Matching Considerations
Achieving a specified color entails implementing the fundamental formulating principles detailed below. Paramount to this approach is simplicity.
Start with a Good Pigment Set
Avoid embarking upon a color match with the wrong pigments. Your color computer may recommend pigmentation from an existing formula that may have a history of its own. Many errors of judgment occur with this approach. The pigments may lack the required durability or may have been recklessly “adjusted” to respond to a color issue experienced in production.
It is best to start with a clean slate using a pigment set that has the highest probability of achieving the color target while providing the appropriate durability. By choosing the best starting point, there is less need for adjusting with additional pigments. Often a formulator starts with a less than ideal set of pigments, then attempts to adjust color with additional pigment(s). When he or she finds it impossible to match with these pigments, they have to be abandoned and a new set used.
Keep Things Simple
As a general rule, there should be two (or less), but no more than three, colorant pigments in addition to black and white pigments (as needed). Numerous issues can arise from formulating with an overly complicated pigmentation scheme including metamerism (color differences with different light sources), difficulty in scale-up, inconsistency in production, and variable color at the powder applicator.
Determine the Necessary Pigment Concentration to Achieve Opacity
When color matching, it is important to determine the pigment concentration required to achieve acceptable opacity at the specified film thickness. This is vitally important for two reasons – one, it’s a practical requirement, as insufficient opacity will create inconsistent color by the influence of the color of the underlying substrate. A lack
of opacity will make a coating appear darker, bluer, and greener when applied to a steel substrate and lighter and bluer if applied over an aluminum substrate.
Another reason to approximate the correct pigment concentration is cost. Colorant pigments typically cost more than binder components and extenders and should therefore be incorporated at levels sufficient to provide opacity but not higher than necessary.
Formulation and Performance Criteria
The following issues must be addressed when selecting a set of pigments to match a color:
A powder coating formula is exposed to heat in at least three distinct instances. First, the formula components are heated during the extrusion phase of manufacture. Extrusion temperatures typically range from around 90 to 150 degrees Celsius (194 to 302 degrees Fahrenheit) for a relatively short duration of 30 to 60 seconds. Second, the powder coating will experience a cure cycle with temperatures that can range from 120 to 200 degrees Celsius (248 to 392 degrees Fahrenheit) for typically 10 to 30 minutes. And third, a cured powder coating may experience elevated temperatures in a service environment of the coated item. Hence, it is requisite to carefully select pigments that are stable at the highest temperatures that the coating will experience throughout its product life.
It is essential to carefully review the heat stability data provided by the pigment supplier prior to considering the material for use in a formula. Most pigment suppliers furnish 200 degree Celsius (392 degree Fahrenheit) data when defining a colorant’s heat stability. This should be used as a criterion for most powder formulas.
Powders formulated for resistance to extreme heat experienced in the service life of a coated object require special formulation considerations. Applications requiring this type of performance include coatings for cooking utensils/appliances/grills and exhaust components. To meet these stringent requirements, mixed metal oxides are most commonly used.
All materials naturally possess an inherent degree of UV durability based on their chemical composition. UV durability is further influenced by pigment concentration (tint vs. masstone), environmental effects such as moisture and other airborne chemicals, temperature, and surface treatments that may be present on the pigment particle.
The best guide for UV durability comes from the pigment supplier. Be careful to understand the difference between lightfastness and weather-fastness.
Lightfast refers to resistance to UV light and can be measured with accelerated weathering methods that may or may not introduce other simulated environmental factors such as moisture and temperature. Weather- fastness is defined as durability in real world environments and encompasses all the elements that a surface coating encounters in service, including UV light, moisture, temperature, acidic and alkaline airborne compounds, and dirt.
In general, mixed metal oxides provide the greatest weathering resistance. These pigments are comprised of very stable crystalline structures that resist UV, chemical, and heat exposure. In addition, a number of automotive grade organic pigments also possess good to excellent outdoor durability.
The majority of pigments used to formulate powder coatings are relatively non-toxic. Regardless, before selecting any pigment it is always wise to consult the pigment supplier, and more specifically the pigment’s Safety Data Sheet, to ensure the product is safe for its intended end use.
The United States Food and Drug Administration (FDA) lists the pigments suitable for incorporation into polymeric materials that may come into contact with food. This information can be found in the Code of Federal Regulations (CFR) Title 21 Section 178.3297.
Opacity (sometimes referred to as “hiding”) refers to the ability of a coating to eliminate the visible transmission of color from the substrate underneath the coating. Coatings are typically specified to provide a measurable amount of opacity at a specified film thickness. The formulator must incorporate enough pigment to optically cover the substrate.
Opacity of a powder can be determined by applying the coating to black and white metal panels specifically made for oven cured coatings. These standardized test panels are coated at a specified film thickness over the black and white surfaces. Percent opacity is calculated by comparing the color (reflectance) of the coating over the white surface to that of the coating over the black surface per ASTM D-6441: Measuring the Hiding Power of Powder Coatings.
Opacity can be specified as a percent at a given film thickness (e.g., 99.0 percent opacity at 2.5 mils) or may be expressed as the minimum film thickness required for achieving a specified opacity (e.g., 3.2 mils to achieve 99.0 percent opacity). Ninety-eight point five percent opacity is considered a minimum standard; however, it is wise to have at least 99.0 percent to achieve color consistency.
Specific gravity is also described as density and refers to the mass per unit volume of a material and is generally reported as grams per milliliter (cubic centimeter). The specific gravity of colorant pigments for powder coatings ranges from approximately 1.2 to 6.0. Organic pigments generally have lower specific gravity than their inorganic counterparts.
Specific gravity affects the weight of pigment incorporated in a formula. Higher specific gravity pigments occupy less volume in a powder coating formula and usually require higher weight percentages to provide color and opacity. This in turn creates a higher specific gravity powder coating. Higher specific gravity powder coatings provide less coverage per unit weight of a substrate than lower specific gravity powders. This impacts coating economics.
Cost is an important criterion to be considered with any powder coating formulation. Raw material cost is an obvious factor, but tint strength, specific gravity, and processability impact the final cost of a powder coating as well. A pigment possessing poor opacity and weak tinting strength requires higher levels in a formula. This may outweigh a lower cost per pound of a pigment. In addition, pigments requiring more intensive or elaborate processing increase the final cost of a product. This can occur with pigments that disperse poorly or inconsistently.
Color matching powder coatings is a complex challenge that requires a sound technical foundation as well as a keen eye for color and color differences. In most cases the exercise can be relatively painless if you follow the fundamentals of simplicity, durability, opacity, and cost.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.