Important Properties of a Pigment-Dispersing Agent
Important Properties of a Pigment-Dispersing Agent
SpecialChem - Apr 1, 2013
Pigments are primarily used in coatings and inks to provide color, opacity and to control gloss. Additional benefits obtained from pigments include enhanced protection from surface abrasion and UV degradation. Pigments consist of tiny insoluble solid particles, generally less than 1µm in diameter and are classified as either prime pigments, extender pigments, or specialty pigments.
Prime pigments are used to provide color and opacity. A variety of natural and synthetic prime pigments are available for use in coatings and inks to provide a complete spectrum of colors. They can either be inorganic, such as titanium dioxide, carbon black and zinc chromate, or they can be organic, such as phthalocyanine green, quinacridone red and diarylide yellow. Inorganic pigments are easy to disperse, provide excellent opacity, enhance durability, and are lower in cost as compared to organic pigments. Organic pigments are primarily used to achieve bright clean colors.
Extender pigments are used in coatings and inks to reduce gloss, enhance opacity, and lower costs. The refractive index of some extender pigments is close to the refractive index of resin binders used in coatings. This property enables extender pigments to enhance the surface abrasion resistance of clear coatings. Examples of extender pigments include silica, mica, clay and alumina.
Specialty pigments are used to provide a specific property, such as corrosion resistance, conductivity, self-healing, infrared reflection, and special visual effects including pearlescent finishes.
In order to optimize the performance of a pigment, the pigment must be completely and uniformly dispersed throughout the coating or ink.
In March, we asked our members the following question: When selecting a pigment-dispersing agent, which properties are most important?
Here are the results of the voting:
 Fig:1 Most Important Properties of a Pigment-Dispersing Agent |
The question regarding pigment dispersion received a total of 378 votes, indicating that this topic is of utmost importance to our members. Pigment dispersion is a three-step process requiring: (1) efficient pigment dispersion, (2) prevention of particle agglomeration, and (3) prevention of coagulation during storage.
All of the above received 75.9% of the total votes cast. Thus, the majority of our members are concerned about the total pigment dispersion process, not just one of the above three steps. Pigment manufacturers offer varying particle sizes of pigments that are targeted for particular end uses. However, these fine pigment particles tend to agglomerate during storage and shipping. Therefore, the coating or ink manufacturer has to separate the pigment particles back to their original individual particle size and prevent the separated pigments from reagglomerating. To optimize product performance, the formulator must implement all three of the above pigment dispersion steps.
Efficient pigment dispersion received 14.3% of the votes and came in second place as the most important property when selecting a pigment-dispersing agent. Pigment dispersion is typically carried out by a milling operation in which mechanical energy is used to separate the agglomerated pigment into individual particles. The first step in dispersing pigments is to displace air and water on the pigment surface using an effective dispersant agent. This process requires a dispersant that completely wets the pigment particles and prevents the pigment particles from reagglomerating. Dispersants are classified as either a surfactant or a polymeric additive. Formulators have a multitude of dispersing agents available, each containing different chemical and physical properties. Surfactantdispersing agents are available as cationic, anionic, and nonionic additives. These include low molecular weight fatty acids, polyesters, polyamides, phosphoric esters, and aminic associative groups. Polymeric dispersants typically contain urethane and acrylic backbones modified with polyether or polyester side chains and have molecular weights between 5,000 and 30,000 g/mol. Polymeric dispersantstypically are nonionic. The above-mentioned dispersants are replacing the traditionally used alkyl phenoxy ethoxylates (APEOs) because APEOs have been found to be toxic to aquatic life.
Prevention of particle agglomeration came in third place, receiving only 5.3% of the votes. Dispersants function by the following process: one end of the dispersant is absorbed on the surface of the pigment particles and the other end repels particles from agglomerating. The polymeric dispersants, having high molecular weights, repel pigments via steric repulsion. Low molecular weight surfactants, having ionic end groups, repel pigments through electrostatic repulsion. Individual pigments are chemically different. Some pigments are attracted to water and some are not attracted to water. Dispersants have to be selected to accommodate the chemical needs of the pigments. When changing pigmentation in a formula, the dispersant needs to be checked to see if it is stable in the formulation. Good dispersion provides the end user with better opacity and color stability.
Prevention of coagulation during storage came in a close fourth place, receiving 4.5% of the votes. Pigment coagulation during storage can be prevented by selecting a dispersing agent that is (1) compatible with the type of pigment and any surface pretreatment of the pigment being used, (2) compatible with the chemistry of coating binder, and (3) compatible with the processing conditions used to disperse the pigment. It is also important to prevent the dispersant from being displaced from the pigment when the milled pigment is letdown into the coating binder.
Advances in dispersant technology are now making it easier for formulators to optimize the color quality of pigments used in paints.