Discussion on the Stability of Carbon Black Paste-Blog

--- Examining Paste Preparation and Storage Issues from the Perspective of Carbon Black Characteristics

Black pastes are widely used in coating, ink, and plastic systems, serving as an important component for achieving color performance and hiding power in products. However, issues such as dispersion difficulties, sedimentation, skinning, or viscosity changes often occur during the preparation and storage of pastes, affecting their final application performance.

The root causes of these phenomena are closely related to the physical and chemical characteristics of carbon black. This article will analyze the factors affecting paste stability and optimization directions, starting from the structural characteristics of carbon black.

I. Characteristics of Carbon Black and Key Challenges in Paste Preparation

Carbon black is an inorganic pigment with a large specific surface area and high surface energy, possessing excellent light absorption capacity and coloring performance, but also exhibiting a strong tendency to agglomerate and high adsorption capacity.

During the paste preparation stage, the following aspects are key factors affecting dispersion quality:

1.Strong Agglomeration Tendency

The primary particle size of carbon black is typically 8–100 nm, making it very prone to form agglomerates via van der Waals forces. If the grinding energy is insufficient or the dispersion system is mismatched, agglomerates cannot be effectively broken down, leading to insufficient jetness of the paste, a brownish tint, and reduced gloss.

2.High Oil Absorption

High-structure carbon black has numerous pores and branched structures, resulting in strong adsorption capacity.

During grinding, it tends to adsorb resins and solvents, reducing system fluidity and causing phenomena like "dry-up" and "difficult grinding."

3.Poor Wettability

The carbon black surface is hydrophobic. When the system is highly polar or water-based, wetting difficulties, floating, and flooding are common. Selecting appropriate dispersants to reduce interfacial tension and improve wettability is particularly important for enhancing dispersion efficiency.

4.Insufficient Dispersant Compatibility

Different types of carbon black have significant differences in their surface chemical properties. Solvent-based systems are suitable for polyamide or polyester-type dispersants, while water-based systems are more suited for block copolymers or phosphate ester structures. If the dispersant is selected improperly, while initial dispersion might be achieved, issues like re-coarsening or flocculation can easily occur during storage.

5.Control of Grinding Energy

 Insufficient grinding leads to residual coarse particles, while over-grinding may disrupt the adsorption equilibrium, causing instability phenomena such as thickening or gelling in the system. Appropriate grinding energy ranges should be set according to the structure and specific surface area of different carbon blacks.

II. Typical Problems During Storage and Mechanism Analysis

Even if good results are achieved in the dispersion stage, paste stability can still decline during storage. Common problems include:

1.Sedimentation and Stratification

When the system viscosity is low or the particle surface adsorption layer is insufficient, carbon black particles gradually settle under gravity. Low-structure or large particle size carbon blacks, due to their higher density and smaller specific surface area, are more prone to sedimentation and stratification, resulting in an uneven state with a thin upper layer and a thick lower layer.

2.Skinning Phenomenon

If the paste is in direct contact with air during storage, the resin on the surface may form a film due to oxidation or solvent evaporation. High specific surface area carbon black can accelerate solvent evaporation, thereby exacerbating the skinning problem. Skin fragments mixed into the system can easily cause spraying defects or film particles.

3.Abnormal Viscosity Changes

Both an increase or decrease in paste viscosity during storage are abnormal phenomena. Viscosity increase is often due to the continuous adsorption of dispersant or resin by the carbon black, while viscosity decrease might be caused by pigment agglomeration, solvent loss, or system exudation. Both affect application performance and printability.

4.Decrease in Jetness and Hiding Power

Under prolonged exposure to light or high temperatures, the carbon black surface is prone to slight oxidation, reducing its light absorption capacity. Simultaneously, migration or aging of the dispersant can lead to a relaxation of the system structure, resulting in phenomena like graying or yellowing.

III. Strategies for Improving Paste Stability

To enhance the preparation and storage stability of carbon black pastes, the following aspects can be addressed:

1.Selecting the Appropriate Carbon Black Type

For systems requiring high jetness, fine particle, high specific surface area carbon black can be chosen, but the dispersion system design must be strengthened. For systems requiring high fluidity and storage stability, low-structure carbon black is preferable.

2.Optimizing the Dispersion System

The polarity and molecular structure of the dispersant should match the surface characteristics of the carbon black and the resin system. Adding appropriate amounts of wetting agents, leveling agents, or defoamers can improve system stability, but excess should be avoided to prevent adhesion loss.

3.Controlling the Storage Environment

It is recommended to store pastes in a cool, dark place at 5–30 °C and 40–60% relative humidity, away from heat sources and direct sunlight. After opening, containers should be sealed promptly to prevent air ingress and solvent evaporation.

4.Improving Packaging Form

Choose containers with good solvent resistance and sealing performance, such as metal drums or high-density polyethylene drums. For frequently opened packages, nitrogen purging or anti-skinning inner membrane designs can be used to reduce oxidative effects.

5.Adding Functional Additives

Rational use of anti-skinning agents can delay oxidation reactions; appropriate adjustment with thickeners or diluents helps maintain the system's rheological balance. However, compatibility must be noted to prevent adverse effects on color shade and tinting strength.

IV. A Stability Mindset Based on Carbon Black Characteristics

The long-term stability of a paste does not rely on the accumulation of additives, but rather on the understanding and control of the carbon black itself. The particle size, structure, specific surface area, volatile content, and surface chemical properties of carbon black collectively determine its dispersion performance and storage behavior. Through rational carbon black selection and system design, it is possible to achieve high jetness while balancing dispersibility and storage stability. Through precise adjustment of the structure and surface chemical properties of specialty carbon blacks, Anhui Black Cat enables carbon blacks to maintain excellent jetness while possessing better wetting and stability performance, providing more reliable coloring solutions for the coatings, inks, and plastics industries.

V.Conclusion

The stability issues of carbon black pastes are essentially a manifestation of the interaction between carbon black characteristics and the system. Only by deeply understanding the structure and interfacial behavior of carbon black can one truly achieve "deep blackness, effective separation, and stable storage" in the paste preparation and storage processes.