Pigment Selection Criteria

Pigment Selection Criteria When developing and selecting a pigment, manufacturers and users apply different tests suited to their purposes. This article will focus particularly on the criteria that producers implement and emphasize. While users also perform many of these tests, what they entail will be the subject of a separate article. Principal tests applied by manufacturers: • Surface area – BET and oil absorption, • Dispersion, • Rheology, • Color tone and strength, • Chemical resistance, • Durability – indoor and outdoor, • Overpaint adhesion, • Pigment stability – flocculation, floating, sedimentation, • Gloss. Looking at these in somewhat more detail: For pigment surface area, the easiest test to apply is the "Oil Absorption Test." This is determined by how much linseed oil 100 grams of pigment absorbs. This test is defined as the "Rub Out" test, and the lower this amount, the better the flow characteristics. • Pigment surface area is also determined by BET value (Brunauer Emmett Teller). Specific surface area is assessed by nitrogen absorbance. This value is used particularly to predict how much additive will be needed. • Another important test is the determination of particle size distribution. Pigment particles in a batch vary in size from very fine particles to coarse particles. When their distribution is examined, a bell curve is observed, and the peak of this bell curve provides information about the average particle size. However, this bell curve can be broad or narrower. A narrower curve indicates that a large proportion of particles are at the average particle size. • The next important test is examination of the pigment's dispersion. After previous tests have established the surface area, oil absorption values, and average particle size of the pigment, once the amount of additive needed for dispersion is estimated through the BET value, dispersion is then examined. During dispersion, the paint's color strength shows an increase to a certain value (preferably 100%). At this point, each pigment particle is separated from the others to the maximum possible degree. After reaching this maximum point, color strength remains constant for a certain period. However, once the dispersion time reaches a certain critical point, color strength begins to decrease and shows a declining tendency, with color strength becoming weaker. This test will provide insight into how quickly maximum color strength can be achieved and from when it should not be continued. Maintaining dispersion time at an optimum level is important for time savings and energy conservation. • After optimum dispersion time is determined, the next stage involves determining whether the pigment provides the desired rheology. Rheology is the study of flow and deformation of materials. The pigment added to a paint system irreversibly changes the rheological properties of the medium. The commonly used term in this area is viscosity. Viscosity is the resistance, drag, or friction on many objects moving within a liquid, or the resistance that impedes the flow of a liquid (due to chemical attraction and physical shape of molecules). • The test applied in the next stage is that of color tone and strength. This test can be performed in the simplest way by eye, or scientifically through a spectrophotometer. Depending on the application, the pigment is tested in full tone with and without TiO2 in certain proportions to determine color tone and strength. • After the color measurement stage is completed, the next test determines the pigment's chemical resistance. This test is applied in both acidic and alkaline environments. Based on the result of this test, the suitability of the pigment for its intended use is determined. • Another test equally important as chemical resistance is solvent resistance. For paint produced in liquid form or after it has become a film, it is important to predict the pigment's movement within the dry film. The pigment is primarily expected not to dissolve in the solvent, since the system in which it is used will mostly contain a solvent. Furthermore, after paint is applied, pigments within the paint film layer, although the system appears solid, may have the capability to move, which is called bleeding and is an undesired property. If this property is weak, the pigment, if another paint film is applied over that film, can move into that film and stain the upper film layer. • As the final stage approaches, one of the most important properties is the pigment's ability to maintain its color tone and strength in the medium in which it is applied. Common errors are generally made in determining this property. Color is created through the absorption or reflection of visible light wavelengths as sunlight and other light sources strike the paint film. The visible light energy that is absorbed is primarily what destroys the pigment. For this reason, the fastest test method for this test is the WOM device, which most closely simulates sunlight. However, UV-CON devices, which are more commonly used and cheaper and emit UV light, are generally employed. However, these devices do not test pigment durability but rather test the polymeric material that makes up the paint film, which is damaged by UV. The paint film created is tested in the WOM device for a period ranging from 250 hours to 4000 hours depending on application. Based on comparative studies with natural environment results and the amounts of visible light energy absorbed in outdoor conditions, approximately 1200 hours of WOM testing is estimated to correspond to 1 year outdoors. However, studies conducted in different regions of the world still show that there is not 100% equivalence. Although this equivalence is not clear-cut, this test is the most reliable test regarding pigment durability. Assoc. Prof. Gürses Öner Regional Sales Manager – Turkey SUDARSHAN Chemical Industries Ltd.