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Abstract: Pigments are the core coloring functional components in water-based coatings, inks, and color matching systems. Their dispersion uniformity, particle size stability, and coloring strength directly determine the color saturation, coverage, glossiness, and appearance uniformity of coatings. Due to their large specific surface area and high surface free energy, pigment powders are prone to agglomeration, flocculation, settling, and coarsening in aqueous systems, which seriously reduces the color development performance and storage stability of coatings. Polymer dispersants, as key functional additives in pigment dispersion systems, can achieve efficient depolymerization and long-term stable suspension of pigment particles through the synergistic effects of molecular anchoring adsorption, interface wetting, electrostatic repulsion, and steric hindrance. This article systematically elaborates on the aggregation mechanism and dispersion process of pigments in aqueous phase, comprehensively reviews the structural characteristics and adaptability of small molecule dispersants, traditional polycarboxylate dispersants, and polymer hyperdispersants, and focuses on analyzing the dispersion differences and selection rules of inorganic pigments, organic pigments, and carbon black systems. It explores the effects of dispersant addition amount, molecular structure, and additive compatibility on the viscosity, grinding fineness, storage stability, floating color and flower development, and final performance of paint films. It summarizes the technical difficulties in current pigment dispersion systems and looks forward to the development trend of high-performance, low residue, multi-functional green dispersants, providing theoretical basis and technical reference for the optimization design of water-based pigment pastes and building coating formulations.

Keywords: polymer dispersant; Pigment dispersion; Color paste; Agglomeration mechanism; Super dispersant; Color display performance

1 Introduction

In water-based coatings, printing inks, industrial coatings, and architectural stone like coating systems, pigments undertake important functions such as coloring, covering, weather resistance, and UV shielding. According to their chemical structure, pigments can be divided into two categories: inorganic pigments and organic pigments. Inorganic pigments include titanium dioxide, iron oxide red, iron oxide yellow, talc powder, kaolin, etc., which have the characteristics of high covering power, excellent weather resistance, and low cost; Organic pigments mainly include phthalocyanine blue, phthalocyanine green, quinacridone, azo pigments, etc., which have the advantages of bright colors, high color saturation, and rich color tones. Whether it is inorganic or organic pigments, the original powder exists in the form of aggregates, and a large number of primary particles are tightly packed through van der Waals forces and hydrogen bonding. If directly put into the aqueous system, they cannot spontaneously dissociate into a uniform single particle state, and are prone to quality defects such as particle aggregation, delamination, color paste coarsening, uneven coloring, and coating discoloration.

Pigment dispersion is one of the most core and critical processes in coating production, and the dispersion effect directly determines the appearance quality and storage stability of the finished coating product. Traditional paint production relies solely on mechanical external forces such as high-speed stirring, sanding, and ball milling to break pigment aggregates. However, mechanical shearing alone cannot solve the problem of particle re agglomeration, and ultrafine pigment particles after grinding are prone to re aggregation and growth during storage. Therefore, it is necessary to introduce dispersants to quickly wet, coat, and stabilize the newly generated pigment interface, fundamentally suppressing particle agglomeration. With the development of water-based coatings towards high gloss, high weather resistance, high color strength, low VOC, and excellent storage stability, traditional small molecule wetting and dispersing agents are no longer able to meet the stability requirements of high-end pigment systems. Polymer hyperdispersants with multi-point anchoring and long-chain solvation structures have gradually become the mainstream development direction in the field of pigment dispersion.

Analysis of pigment aggregation mechanism and dispersion process

Pigment powders generally exhibit severe agglomeration in aqueous phase, which can be mainly divided into two types: soft agglomeration and hard agglomeration. Soft agglomeration is mainly caused by van der Waals forces and electrostatic attraction between particles, with a relatively loose structure that can be rapidly dissociated through mechanical shear; Hard agglomeration originates from the chemical bonding formed by the dehydration and condensation of hydroxyl groups on the surface of particles. The structure is dense and stable, and it is difficult to completely break it by mechanical grinding alone. This is the main reason for the difficulty in exploring the fineness of pigments and the long-term coarsening of pigments. The pigment particles have a small particle size and a large specific surface area, and the system interface energy is high. In order to achieve thermodynamic stability, the particles will spontaneously aggregate, reduce the specific surface area, and ultimately lead to sedimentation, layering, and decreased coloring power of the pigment paste.

The complete pigment dispersion process mainly includes three stages: wetting, depolymerization, and stabilization, which are interconnected and indispensable. The first stage is the wetting stage, where dispersant molecules quickly replace the air and water vapor adsorbed on the surface of pigment particles, reducing the solid-liquid interfacial tension and allowing the aqueous medium to fully penetrate into the internal pores of pigment aggregates, weakening the binding force between particles; The second stage is mechanical depolymerization, in which large-sized pigment aggregates are broken into ultrafine primary particles under the high-speed shear and impact of the sand mill; The third stage is the interface stability stage, where dispersant molecules quickly adsorb onto the interface of the newly formed pigment, forming a uniform and dense coating layer that prevents particles from colliding and aggregating again, achieving long-term stability of the pigment system.

3 Types and structural performance characteristics of pigment dispersants

Based on differences in molecular structure, molecular weight, and stability mechanism, water-based dispersants for pigments are mainly divided into four categories: small molecule wetting dispersants, traditional acrylic acid homopolymer dispersants, acrylic acid maleic anhydride copolymer dispersants, and comb shaped polymer super dispersants. The molecular structure of different types of dispersants determines their suitability for pigment systems and stability effects.

3.1 Small molecule wetting and dispersing agent

Small molecule dispersants mainly include anionic phosphate esters, sodium dodecyl sulfate, alkyl sulfates, etc. They have low molecular weight and fast wetting speed, which can quickly reduce the interfacial tension of the system and assist in pigment wetting and crushing. But its molecular structure is simple, the adsorption group is single, the adsorption force on the pigment surface is weak, the coating is not dense, and it is prone to desorption under system shear, temperature changes, and long-term storage conditions, leading to secondary flocculation of the pigment. At the same time, small molecule dispersants remaining inside the paint film will increase the hydrophilicity of the paint film, reduce the water resistance, scrub resistance, and weather resistance of the coating. Currently, it is only used for low-end latex paint basic dispersion systems and is difficult to adapt to high gloss and high stability organic pigments and carbon black systems.

3.2 Traditional polyacrylate dispersants

Sodium polyacrylate is the most commonly used inorganic pigment dispersant in architectural coatings, with molecular chains rich in high-density carboxyl polar groups. After ionization in water, it carries a large amount of negative charge and can firmly adsorb on the surface of highly polar inorganic pigments such as titanium dioxide and iron oxide, relying on electrostatic repulsion to achieve particle stability. This type of dispersant has high cost-effectiveness, significant viscosity reduction effect, excellent grinding efficiency, and can effectively improve the fluidity and storage stability of inorganic pigment pigments. However, polyacrylic acid salts are linear homopolymers that lack long-chain solvation structures, have weak steric hindrance ability, and have poor adsorption stability for organic pigments and carbon black with low surface polarity and few adsorption sites. They are prone to floating, flocculation, and coarsening defects.

3.3 Acrylic acid maleic anhydride copolymer dispersant

The acrylic acid maleic anhydride binary copolymer significantly improves the molecular carboxyl density and structural uniformity by introducing maleic anhydride monomer. Compared with ordinary sodium polyacrylate, it has stronger electrostatic adsorption ability and moderate steric hindrance effect. This type of dispersant has stronger adaptability to inorganic pigments, which can further improve the uniformity of pigment dispersion, suppress pigment settling and agglomeration, and also has a certain ability to resist electrolyte interference, making it suitable for building coating systems with high filler content. But it is still a linear molecular structure with limited anchoring and adsorption ability for low surface energy organic pigments, which cannot meet the dispersion requirements of high-end color pastes.

3.4 Comb type polymer super dispersant

Comb type super dispersant is currently the core additive of high-end pigment dispersion systems, with a molecular structure consisting of two parts: anchored main chain and hydrophilic solvation side chain. The main chain is rich in multi-point anchoring groups such as carboxyl, sulfonic acid, and amide groups, which can be firmly adsorbed on the surface of pigments through multiple interactions such as ionic bonds, hydrogen bonds, and van der Waals forces, and are not easily detached; Side linked branched long-chain polyoxyethylene ether hydrophilic groups fully extend in the aqueous phase, forming a thick and dense hydration space barrier. This type of dispersant has a dual mechanism of electrostatic stability and steric hindrance stability, and has excellent dispersion and stability effects on inorganic pigments, organic pigments, and high specific surface area carbon black. It can significantly improve the coloring strength of pigments, the gloss and color uniformity of paint films, and inhibit the floating and blooming of multi-color systems. It is the preferred dispersant for high weather resistant water-based coatings and high-end color pastes.

The dispersion and application rules of 4 different types of pigments

The surface polarity, specific surface area, and surface energy of inorganic pigments, organic pigments, and carbon black vary greatly, and there are significant differences in the selection and use of dispersants. Accurately matching the dispersant structure is the key to achieving efficient dispersion.

4.1 Application of Inorganic Pigment System Dispersion

Inorganic pigments (titanium dioxide, iron oxide, zinc oxide, talc powder) have high surface polarity and are rich in hydroxyl groups. They can form strong hydrogen bonds with carboxyl groups for adsorption, making them suitable for anionic polyacrylic acid salts and acrylic maleic anhydride copolymer dispersants. Dispersants coat pigment particles with high-density negative charges, greatly enhancing the electrostatic repulsion of particles, effectively solving the problem of high density and easy settling of inorganic pigments, significantly reducing the viscosity of pigments, improving grinding efficiency and coating coverage. In high filler building coatings such as real stone paint and sand in water, an appropriate amount of dispersant can improve the suspension of quartz sand, colored sand, and powder fillers, reduce storage stratification and aggregate settling, and improve the uniformity of coating construction.

4.2 Organic pigment system dispersion application

Organic pigments have weak surface polarity, smooth surface, and few adsorption sites. Traditional anionic dispersants have low adsorption strength and are prone to desorption, making it difficult to achieve stable dispersion. They are prone to defects such as pigment flocculation, color spots, uneven coloring, and floating colors. Comb type and block type polymer hyperdispersants, with their multi-point anchoring structure, can form a stable adsorption layer on the surface of low polarity organic pigments. Combined with long-chain steric hindrance, they effectively inhibit particle aggregation, improve the fineness and color saturation of color pastes, and significantly improve the gloss and color uniformity of paint films. They are specialized dispersion systems for color pigments such as phthalocyanine blue, organic red, and organic yellow.

4.3 Dispersion Application of Carbon Black System

Carbon black has a very large specific surface area, extremely high surface energy, and strong agglomeration ability, making it the most difficult system in the field of pigment dispersion. Carbon black powder is prone to ultrafine agglomeration, and conventional dispersants cannot completely dissociate and stabilize it. During storage, it quickly thickens, thickens, and flocculates. Specialized polymer hyperdispersants can tightly encapsulate carbon black nanoparticles through multi-point anchoring, and use ultra long hydrophilic side chains to construct a strong steric hindrance barrier, effectively preventing nanoparticle aggregation, reducing the viscosity of black paste, improving blackness and coloring strength, and ensuring long-term storage of black paste without coarsening or flocculation.

The Influence of Dispersants on Coating Performance and Compatibility Issues

5.1 Performance balance law of dispersant dosage

There is a strict optimal range for the amount of dispersant added, and both insufficient and excessive amounts can cause system defects. When the dispersant is not added enough, the surface coating of the pigment is incomplete, the particle wetting is not sufficient, the grinding fineness is difficult to meet the standard, and there are a large number of unstable agglomerated particles in the system. The storage process quickly coagulates and settles, and the coloring and covering power of the coating are greatly reduced. The coating is prone to discoloration, color difference, and particle defects. When the dispersant is excessive, a large amount of free dispersant remains in the aqueous system, which not only fails to improve the dispersion effect, but also causes a series of negative problems: free polymers will increase the foam tendency of the system, increasing the difficulty of defoaming; Residual small molecule groups can enhance the hydrophilicity of the paint film, reduce the water resistance, scrub resistance, salt spray resistance, and weather resistance of the coating; At the same time, excessive anionic dispersants can interfere with the rheological control mechanism of HEUR polyurethane thickeners, causing problems such as coating viscosity attenuation, storage thinning, and construction sagging.

5.2 Conflicts in the compatibility of additives

The pigment dispersion system is a multi auxiliary compound system, and the dispersant has compatibility problems with lotion, thickener, wetting agent, bactericide and cationic auxiliary. Anionic polycarboxylate dispersants can undergo charge neutralization reactions with cationic fungicides and cationic softeners, leading to system flocculation, turbidity, and emulsion failure; Some polymer dispersants will compete to adsorb lotion interface, affecting the continuity of lotion film formation; In the water in sand colorful system, excessive dispersants can damage the protective rubber coating structure, causing the colored particles to become soft, fragmented, and mixed, directly affecting the forming quality of the stone like coating.

5.3 Effects on the Appearance and Durability of Paint Films

A suitable dispersant system can evenly disperse pigment particles, refine particle size, achieve dense and smooth film formation, high glossiness, uniform color, and no floating color or flower; The poorly dispersed system results in severe pigment aggregation, rough paint film surface, high porosity, and severe light scattering. Not only does it have poor gloss and obvious color difference, but external moisture, salt, and ultraviolet rays are also more likely to penetrate into the interior of the coating, accelerating paint film aging, powdering, loss of gloss, and peeling, greatly shortening the service life of exterior wall coatings.

6 Existing Problems and Development Trends

At present, there are still many technical shortcomings in the application of pigment dispersants in coatings: traditional dispersants have a single adaptability to pigments and cannot meet the dispersion needs of multiple systems including inorganic, organic, and carbon black; Some dispersants have strong residual properties, which negatively affect the water and weather resistance of the paint film; Small molecule dispersants have poor stability and are prone to detachment and coarsening; The high cost of high-end hyperdispersants restricts their large-scale engineering applications. With the development of green and high-performance water-based coatings, pigment dispersants will show a clear development trend: firstly, the development of bio based, low VOC, APEO free green dispersants to reduce additive residues and environmental hazards; The second is to precisely regulate the molecular structure through controllable polymerization, and prepare comb shaped and block multifunctional hyperdispersants with low addition, high efficiency, and high stability; The third is to develop specialized dispersants that are resistant to floating, hydrolysis, and low water sensitivity, balancing dispersion efficiency and paint film durability; The fourth is to adapt to high solid content and low viscosity color paste systems, reduce grinding energy consumption, and meet the low-carbon production needs of coatings.

7 Conclusion

Polymer dispersants are essential core additives in the preparation of pigment pigments and water-based coating formulations. Through the synergistic effects of wetting, electrostatic repulsion, and steric hindrance, they completely solve the problems of pigment aggregation, flocculation, settling, and coarsening. The adaptability of different structural dispersants to inorganic pigments, organic pigments, and carbon black varies significantly. Linear polycarboxylates are suitable for inorganic filler systems, while comb type hyperdispersants are suitable for high-end organic pigment and carbon black systems. The selection, dosage, and compatibility of dispersants directly determine the grinding efficiency, storage stability, color development effect, and durability of the paint film. In the future, with the continuous advancement of molecular polymerization technology and green coating technology, multifunctional, highly stable, low residue, environmentally friendly polymer dispersants will continue to be upgraded, providing solid support for the high-quality development of high-performance water-based pigment pastes and architectural coatings.

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