There are two primary reasons to coat an object – to protect and to beautify. The inclusion of color into a formula satisfies the latter objective. Before delving into the particulars of pigmentation, a few definitions are in order. Colorant pigments are sometimes referred to as prime or opacifying pigments. Prime pigment differentiates these materials from extender pigments, also known as extenders or fillers, which are used to lower formula costs. Extender pigments ideally impart little or no extraneous effects to the coating including color or opacity. Opacifiers by definition obscure or cover a substrate. Colorant pigment, prime pigment and opacifier are synonymous and can be used interchangeably. We’ll use the general term pigment to describe colorant pigments.
What is a Pigment?
Pigments are small, insoluble particles that provide color and opacity. Dyes also provide color; however, they are differentiated from pigments because of their solubility in polymers. Pigment particles provide color and opacity by being dispersed in a medium such as a powder coating binder. Dyes are used in some powder coatings; however, it is relatively uncommon mainly due to their typical lack of opacity and UV durability.
Both pigments and dyes provide color because they inherently absorb certain wavelengths of visible light (ranging from approximately 380 to 730 nanometers). This absorption is observed then processed by the human eye and is interpreted by what humans describe as color. The general range of wavelengths of visible 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 through the extrusion process and provides an even dispersion of the pigment particles throughout the powder binder.
The original pigments and dyes used to color paints and coatings were found in nature and consisted of metal oxides found in the earth (e.g. iron oxide) and plant-based compounds (e.g. chlorophyll). These materials varied in quality and purity and are no longer used in industrial coatings.
Organic vs. Inorganic Pigments
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. Inorganic pigments are typically based on simple or complex metal oxides and have higher specific gravities and usually lower oil absorption than their organic counterparts. Table 2 depicts some general differences between organic and inorganic pigments.
Color Index
Colorant pigments are classified by a convention established jointly by the Society of Dyers and Colourists (SDC) in the United Kingdom and Association of Textile Chemists and Colorists (AATCC) in the United States. Color Index refers to this system and represents the most commonly accepted taxonomy format in powder coating formulation. The system uses the following format: the prefix is comprised of the first letter “P” which signifies pigment and the subsequent letter(s) identify the color. This prefix is followed by a number, sequentially assigned by when the pigment was recognized by the governing groups. Table 3 shows the most common prefixes used in powder coating formulation.
Pigment Requirements
Besides providing color, pigments must meet the following criteria to be used in powder coatings.
Heat Stability
Not only must they endure the elevated temperatures of the extrusion process, they must also maintain their color and integrity at the bake conditions experienced on the applicators’ finishing line. In some cases, pigments must also maintain performance at elevated service temperatures to which a coated object is subjected (e.g. exhaust parts, grills).
Ease of Dispersion
Prime pigments are dispersed throughout the binder during the extrusion process. Pigments are typically received as agglomerates of primary particles. These agglomerates need to be fractured to provide the most color development. Some pigments are readily dispersed under standard processing conditions, while others are more difficult to disperse.
Non-toxicity
From their inception, powder coatings have been touted as being not only environmentally friendly but also safe to handle. In concert with this theme, the majority of powder formulators have historically avoided using toxic raw materials. A number of pigments exist that are relatively toxic. These include certain metal oxide compounds, such as chrome, lead, cadmium, and molybdenum. Organic pigment technology has given the formulator suitable replacements for all the toxic inorganic pigments. Consequently, toxic pigments are seldom, if ever, found in a powder coating formula.
Pigment Characteristics and Performance
Hue
This characteristic refers to the color of the pigment.
Chroma
This characteristic describes a pigment’s brightness or intensity of color.
Tint Strength
This characteristic refers to how much color a given amount of pigment provides when mixed with a white pigment such as titanium dioxide.
Metamerism (metameric failure)
This refers to the phenomenon where a color varies under different light sources. Colors may be perceived to match under one light but not another. For example, two colors may appear to match under fluorescent light but not sunlight or hal opacify. Some suppliers refer to their pigment grades as transparent or opaque. Higher concentrations give more hiding, usually with increased raw material cost, and in some cases reduced coating smoothness. Opacity can be measured by coating a specialized black and white test panel (see ASTM D6441 test method) at a prescribed film thickness, then observing the color difference between the black portion and the white portion of the panel.
Weatherability
Some pigments can degrade when exposed to environmental elements, whereas others are quite robust. Suppliers qualify their pigments by lightfastness, which is an indicator of resistance to UV light. However, other environmental factors, such as acid rain resistance and corrosion resistance, may need to be evaluated.
Specific Gravity
Powder coatings are sold by weight but used by volume. Higher specific gravity raw materials increase the specific gravity of the finished powder. Higher specific gravity powders in turn provide less coverage per pound (or kilogram) for the applicator. It is important to take this into account when formulating a powder coating. Most organic pigments are relatively low in specific gravity (< 1.8) whereas the inorganic pigments tend to be higher in specific gravity (1.8 to 5.7).
Oil Absorption
This can be defined as how many grams of oil (similar in nature to resin) can be absorbed by 100 grams of a given compound. The oil used in testing has been standardized. Most methods use n-dibutyl phthalate. Details can be found in ASTM D285 or ASTM D1483 test methods. Oil absorption is an important property when formulating a powder coating, as higher oil absorptive materials restrict the melt flow of a powder coating, which can increase the texture of the finished coating.
Cost
In this competitive world it is paramount to provide your customer with a good performing product at a reasonable price. It is therefore requisite to establish an optimum price-performance balance.
Formulating with Colorant Pigments
In nearly all cases, prime pigments are more expensive than the binder system of a powder coating. Therefore it is very important to control the concentration used in a formula. A formulator strives to provide acceptable hiding at a reasonable coating thickness, which requires a certain minimum concentration. Levels beyond this concentration unnecessarily increase formula raw material cost and can reduce the overall profitability of the powder. Moreover, the smoothness of the coating is usually reduced with higher-than-needed levels of prime pigments. This is because most prime pigments possess a relatively high oil absorption rate.
A reasonable film thickness is roughly 1.5 to 2.5 mils (38 to 55 microns). Difficult hiding colors such as bright yellows and oranges sometimes require thicker films (2.5 to 3.0 mils – 55 to 75 microns) to provide acceptable hiding and appearance.
The ranges noted in Table 4 to the right are merely guidelines. Most formulas require a mixture of prime pigments to achieve the desired color. These mixtures may involve the use of both inorganic and organic pigments of varying strength. It is also very common to incorporate some titanium dioxide and black pigment as an undertone. It is always a good idea to ascertain the hiding of a formula before submitting it to a customer.
Organic pigments tend to produce the brightest, cleanest colors. They also are usually more expensive than inorganic pigments. It is therefore wise to try to achieve a color using lower cost inorganic pigments where possible. For instance, when shading a pastel color such as a beige, choose a mixture of lower cost yellow and red oxides rather than the more expensive organic red and yellow.
Tinting versus Solid Colors
Choice of colorant pigment is influenced by a formulator’s goal in developing that color. When producing vibrant solid colors, the first choice is usually a strong, clean, organic pigment. Whereas when tinting a pastel color, it is usually more practical to incorporate a weaker tint strength material such as an inorganic pigment into titanium dioxide. When tinting, it is very important to adequately disperse and distribute the tint pigments throughout the powder. Strong pigments provide intense color at low concentrations. It is more difficult to ensure that low concentrations are evenly distributed throughout your powder coating. Weaker pigments by their nature require higher concentrations that are easier to distribute throughout the coating during manufacture.
Some formulators choose to preblend tinting pigments to ensure more even distribution in their powders. Pre-blending or master-batching involves mixing prime pigment with either a white pigment (usually titanium dioxide) or an extender pigment. Concentration of the prime pigment in the master-batch can practically range between 1.0 and 10.0%.
Satisfying a color requirement is a balancing act involving both artistic ability and knowledge of science. Blending a few pigments to match a color takes a good eye for color in concert with a strong technical appreciation of the pigments selected. Formulating a good color match achieves the beauty aspect of a coating.