Application of Carbon Black in Aqueous Pigment Systems

Carbon black, with its large surface area, small particle size, and high structural complexity, tends to aggregate, forming larger particles that are challenging to stabilize and disperse within various systems. This dispersion challenge is particularly pronounced in aqueous solutions, posing limitations on the application of carbon black. As society increasingly emphasizes environmental considerations, the utilization of carbon black pigments in aqueous systems like water-based coatings and inks has gained significant attention. The key prerequisite for preparing aqueous carbon black dispersions is ensuring the hydrophilicity of carbon black, thereby achieving highly stable aqueous dispersion systems. Consequently, the dispersion stability of carbon black in aqueous systems has become a focal point of research within the carbon black industry and its downstream sectors.

This paper's first chapter provides an overview of four methods for preparing aqueous dispersion systems of carbon black. Among these methods, oxidative modification enjoys high utilization in industrial production. This approach enhances the presence of oxygen-containing functional groups on the surface of carbon black, thereby improving its dispersibility. However, due to the relatively low steric hindrance on the carbon black surface, its stability is not entirely guaranteed. Graft modification, on the other hand, introduces long-chain polymers on the surface of carbon black, providing substantial steric hindrance, and yielding stable aqueous dispersion systems. Nevertheless, graft modification demands stringent conditions and is less suitable for widespread industrial application. Surface coating, while unrelated to the variety of carbon black, involves complex steps and is not suitable for industrial production. Direct dispersion, due to its simplicity, easily controllable conditions, and excellent product reproducibility, has gained wide acceptance. However, it requires the selection of appropriate mechanical dispersion methods and dispersants to attain stable dispersions. Each of these methods has its merits and limitations. To obtain stable aqueous dispersion systems, this paper combines oxidative modification and direct dispersion. Using oxidized carbon black as the starting material, it is mixed with dispersants, wetting agents, defoamers, agents, and deionized water in specific proportions. Subsequently, the mixture is ground and dispersed using a vertical sand mill to produce aqueous pigment dispersions.

Experimental PrinciplesThis paper utilizes a combination of oxidative modification and direct dispersion methods to prepare aqueous carbon black pigment dispersions. Oxidation increases the oxygen-containing functional groups on the surface of carbon black, improving its wetting properties. Combined with mechanical dispersion methods, this process facilitates the rupture of carbon black's secondary structure into smaller particles, allowing dispersants to effectively link to the carbon black's surface, followed by re-aggregation of the dispersed carbon black particles.

Aqueous pigment dispersions are pigment-filled suspensions prepared by grinding and dispersing in a water medium with the addition of surfactants. The production of carbon black aqueous pigment dispersions involves several essential steps:

1. Wetting: Replacement of air on the carbon black surface with the liquid medium used in grinding, transitioning from a solid/gas (pigment/air) interface to a solid/liquid (pigment/grinding medium) interface.

2. Dispersion: Breaking up carbon black aggregates into primary particles through mechanical separation. The size and duration of input play crucial roles in achieving complete separation of each pigment primary particle.

3. Stability: Maintaining the separation achieved during the dispersion phase and ensuring long-term control of the dispersion. Wetting agents and dispersants assist in wetting, dispersion, and stable dispersion of the solution throughout the process.

Experimental ProcedureAqueous pigment dispersions contain carbon black and dispersants, with the addition of defoamers, rheology modifiers, anti-settling agents, preservatives, and humectants as auxiliary agents. High-quality dispersions should not precipitate, flocculate, or exhibit phenomena like floating or bleeding. This study primarily focuses on the research and development of aqueous carbon black dispersions, primarily utilizing dispersants, wetting agents, and defoamers. Other auxiliary agents will be investigated further upon product development, which is beyond the scope of this paper.

To prepare carbon black aqueous pigment dispersions, it is essential to select suitable dispersants. In this study, dispersants from three manufacturers, totaling five types, were used. The benchmark carbon black used is BH-1, with a uniform carbon black content of 30%. The grinding time, motor speed, and bead-to-solution ratio in the vertical sand mill experiments were consistent. The resulting dispersions were mixed with pure acrylic acid resin and used to create 200-micron coatings on test cards, allowing observation of coating uniformity.

4.3.3A certain amount of carbon black, dispersant HY2200, wetting agent HY1600, and deionized water, along with zirconium dioxide beads in corresponding proportions, were added to the vertical sand mill for grinding. The variables in this experiment included grinding time, carbon black variety, and content, influencing the performance of carbon black.

Experimental ParametersFill ratio: 50%Carbon black mass percentage: 30%, 35%, 40%Zirconium bead-to-solution mass ratio: 1.5:1HY2200 mass percentage (relative to carbon black): 10%, 15%, 25%, 35%HY1600 mass percentage (relative to carbon black): 2%Motor speed: 1500 rpmGrinding time: 1-8 hours

4.4 Characterization of Aqueous Pigment DispersionsBefore establishing a quantitative approach, reliable testing and analysis techniques are essential. In the field of application, testing and analysis of aqueous pigment dispersions include the following parameters: fineness, pH value, electrical conductivity, and tinting strength of the dispersion. The tinting strength of aqueous pigment dispersions, representing the coloring power of black pigment dispersions when mixed with white dispersions, directly affects the gloss and darkness of downstream coating products. Therefore, this paper utilizes tinting strength as a performance indicator for aqueous carbon black pigment dispersions. The specific testing procedure is as follows:

A latex paint containing approximately 20% titanium dioxide and an aqueous pigment dispersion with equal mass were thoroughly mixed in porcelain cups. After thorough mixing, a 200-micron coating was prepared on test cards using a coating applicator. After air drying, the tinting strength value was measured using a colorimeter.

Selection of DispersantsBased on the steric hindrance stabilization mechanism, dispersant molecules typically comprise two parts: anchoring groups, such as -NR2, -N+R3, -COOH, -COO-, -SO3H, among others, that can adsorb onto the pigment surface through various forces and van der Waals forces. The other part consists of solvating chains with long carbon chains, which form strong steric hindrance. Therefore, dispersants for carbon black must possess an affinity group for carbon black, allowing them to adsorb to the surface while extending in water, forming a robust and thick adsorption layer conducive to dispersion in aqueous systems.

Wetting agents, in conjunction with dispersants, play a crucial role in the dispersion process of carbon black. They not only reduce the surface tension of carbon black but also possess a molecular structure with a strong affinity for carbon black, reducing the viscosity of the system. Wetting agents assist dispersants in entering the interstitial spaces between particles, enabling the dispersants to separate all particles effectively.