Pigments play a critical role in paint and coatings. Insufficient or excess quantities of pigments can adversely impact the appearance and performance, so it is important to be able to reliably measure the percent of pigment in the formulated product. Furthermore, due to differences in paint types, there are various methods for quantifying the percent pigment in a paint/coating. This article provides an overview of the function of pigments in paints and the different methods for measuring the percent pigment and some of the other tests that use the same pigment extraction procedures.
While the formulation of most paints and coatings is quite complex, they are made of three primary ingredients: binders, solvents, and pigments, with each serving a different function. Pigments by themselves are solids and insoluble but can be dispersed in vehicles such as various solvents, binders, or resins. Pigments (including the concentration and fineness of grind) provide color, hiding, gloss, and other properties, while binders or resins are the “glue” that hold the pigments together to create a film. Solvents are the liquids that suspend the pigments and binders and allow the paint to properly atomize and flow on the surface to which the it is applied. Although some of these materials are somewhat translucent once mixed into the resin, all these materials will be referred to as pigments in this article.
Pigment Contributions to Paint Properties
Pigments are generally, but not in all cases, the heavier solid portion of the coating and even after machining and processing retain a crystalline or particulate structure. These different structures are part of what allows pigments to give paints certain characteristics when distributed uniformly throughout a medium. Pigments can contribute to many properties of a coating such as opacity, color, level of gloss, corrosion resistance, wet paint properties (viscosity, film leveling, and settling) and even weathering and moisture resistance. Opacity, or hiding power, is the ability of a coating to obscure the substrate and alter the appearance by selective adsorption and/or scattering of light, which protects the organic binder from deterioration caused by solar radiation (ultraviolet light from the sun). A large variety of organic and inorganic pigments are also used to impart color. Titanium dioxide (TiO2) is one of the most common and important pigments in coating. It is a powerful white pigment and has a very high refractive index, which is a measure of the degree to which a substance can scatter light. This property means that TiO2 can be used to give even a thin coating substantial hiding power.
TiO2 has two different structures called rutile and anatase. Although both can be used, the rutile form has a higher refractive index and therefore provides better hiding. Rutile TiO2 also provides better chalking resistance. Ultraviolet radiation can break down molecules or binder over time, making them more water soluble and able to be washed away. This exposes more pigment, leading to a chalky appearance and texture. Rutile TiO2 absorbs more ultraviolet radiation than the anatase form, which helps to protect the binder from this degradation.
Another important property that pigments contribute to paint is the gloss level. Gloss is directly related to the pigment to binder ratio or the pigment volume concentration (PVC) of a coating. For example, having a lower PVC and a higher binder content will result in a higher gloss value of the dried paint film. As the amount of pigment in a coating increases relative to the amount of binder, the PVC can approach the value of critical pigment volume concentration or CPVC. If the CPVC is exceeded, there is insufficient binder to wet the pigment particles fully and bind them to the substrate or underling coating. In some cases, coatings formulated at or slightly below the CPVC usually have a lower permeability but better blistering resistance. “Extender pigment” is a term used for any number of carbonates, silicates, and other compounds added to paints. These pigments are often thought of as “fillers” used to reduce costs. But in other cases, they may also add valuable properties to paint like a lower gloss film or increased hardness and mechanical durability. As the PVC of a coating increases, parts of these pigment particles will begin to break the surface of the film. This roughens the surface and lowers the gloss of the dry paint film.
Other Functions of Pigments
There are plenty of other compounds used as pigments, usually to give a coating some specific function or property. Zinc metal dust is used in galvanic (sacrificial) primers to slow corrosion. Inhibitive pigments (used in primers) are also used to slow the onset of corrosion. Copper oxide is used in anti-fouling coatings to prevent growth of marine organisms on ship hulls. Mica has a thin, plate-like structure. When formulated into a coating these “plates” will overlap and help make a coating less permeable. A complete list of examples could be very long.
Although pigments are an essential part of coatings, they are also one of the most expensive ingredients; therefore, coating formulators strive to achieve optimum performance with the smallest amount of pigment possible. As described earlier in this article, too much or too little pigment can be a problem and will cause a coating to perform differently than intended. Since pigment concentration is critical, it is frequently analyzed as part of batch/lot verification testing and compared to baseline (i.e., values obtained during qualification testing).
Testing for Pigment Content in Paints
The pigment content of a paint can be tested by various methods. There are four different ASTM methods commonly used to determine the amount of pigment in a coating product:
- ASTM D2371, “Standard Test Method for Pigment Content of Solvent-Reducible Paints;”
- ASTM D2698, “Standard Test Method for Determination of the Pigment Content of Solvent-Reducible Paints by High-Speed Centrifuging;”
- ASTM D3723, “Standard Test Method for Pigment Content of Water-Emulsion Paints by Low-Temperature Ashing;” and
- ASTM D4451 “Standard Test Method for Pigment Content of Paints by Low-Temperature Ashing.”
Test method ASTM D2371 can be used to determine pigment content of paints with solvent-reducible vehicles. It is applicable to most solvent-reducible paints; a full list is provided in the standard. Briefly, the method involves using a mixture of solvents to reduce and dissolve the various components of the vehicle, then centrifuging to force the heavier insoluble pigments out of suspension. The remaining liquid containing solvents and resin/binder is then removed, and the process is repeated until all that remains is the pigments. This remaining pigment can then be weighed to determine the pigment concentration in the original coating.
ASTM D2698 is another test method that can be used for most solvent-reducible paints. Although it is commonly used to test paints with lighter pigments that may not settle during the centrifuge process used in ASTM D2371, it should not be used for paints with pigments that tend to bleed severely, or pigments that are very difficult to centrifuge (like carbon black). This method uses a high-speed centrifuge or ultracentrifuge to separate the pigment from the vehicle. The amount of each component is used to determine the original pigment concentration
Method ASTM D3723 is used to test the percent pigment in water-borne coatings with pigments that do not decompose or lose weight at temperatures below 500°C. Some examples of these types of pigments are most metal oxides, silicates, and most anhydrous inorganic salts. For this method, a known amount of paint is dispersed into a small dish with water, allowed to dry, and heated in a muffle furnace at about 450°C. This heating decomposes and removes all components of the paint except the pigment. The downside of this method is that some pigments and organic colorants can lose water of hydration or decompose at this temperature. If the remaining ash shows signs of color change or texture, this could indicate that the pigment was altered and lead to erroneous results.
ASTM D4451 is similar to test method D3723 described above, however it is used to determine the pigment content of non-water-borne paints, as well as several types of traffic marking materials. Instead of using water, the paint sample is dispersed with toluene, dried, and heated in a muffle furnace. Similar to limitations of D3723, some pigments or water contained in pigment could be lost using this method and lead to erroneous results.
After pigment is extracted from the vehicle, it can be used for additional tests. For example, the amount of certain extender pigments may need to be determined, or the pigment can be checked for the presence of heavy metals. It may be necessary to determine if the TiO2 in the coating is anatase or rutile. The amount of TiO2 can be determined using ASTM D1394. This standard uses benchtop chemical analysis methods to determine the amount of titanium dioxide and other compounds present in the pigment. Faster and easier methods include ASTM D4563 (that employs atomic absorption spectroscopy) and ASTM D4764 (that employs X-ray fluorescence spectroscopy), although these may give less reproducible results.
The role of a pigment package in a paint or coating goes well beyond color. Pigment concentrations and types are key to long term performance of a protective coating system. Different methods are used to determine the percent pigment in different paint types. Selection of the correct method and conducting the test as prescribed by the selected method will provide consistent and reliable test results.