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Dispersants, also known as super dispersants, are a special type of surfactant characterized by their molecular structure, which contains two groups with opposing solubility and polarity. One of these is the shorter polar group, called the hydrophilic group, which has a molecular structure that easily orients on the surface of a material or at the interface of two phases, thereby reducing interfacial tension and providing excellent dispersion effects in aqueous dispersion systems.

Types of Dispersants Used in Aqueous Pigment Dispersions:

1. Inorganic Dispersants, such as polyphosphate esters, silicates, etc.

2. Organic Small Molecule Dispersants, such as alkyl polyethers or anionic surfactants of the phosphate type.

3. Super Dispersants, such as sodium polyacrylate and acrylic-(methacrylic) copolymers.

Traditional dispersants face certain limitations in their molecular structures: hydrophilic groups do not strongly bond to particle surfaces with low polarities or non-polar surfaces, leading to desorption and re-flocculation of the particles after dispersion; hydrophobic groups often lack sufficient carbon chain lengths (generally not exceeding 18 carbon atoms), making it difficult to provide adequate steric hindrance in non-aqueous dispersion systems to maintain stability. To overcome these limitations, a new class of super dispersants has been developed that exhibits unique dispersion effects in non-aqueous systems. Their main features include: rapid and thorough wetting of particles; significantly increased solid particle content in grinding materials, conserving processing equipment and energy consumption; and uniform dispersion with good stability, resulting in significantly improved end-use performance of the dispersion system.

The common types of super dispersants used in aqueous pigment dispersions are polyelectrolyte dispersants and non-ionic dispersants. Their structures can include random copolymers, graft copolymers, and block copolymers. The structure of super dispersants is composed of two parts:
Anchor Groups: Frequently found groups include -R2N, -R3N+, -COOH, -COO-, -SO3H, -SO2-, -PO42-, polyamines, polyols, and polyethers. These can form multiple anchoring points on the particle surface through various energetic interactions, increasing adsorption strength and reducing desorption.
Solvated Chains: Common types include polyesters, polyethers, polyolefins, and polyacrylate. They can be categorized based on polarity: low-polarity polyolefin chains; medium-polarity polyester or polyacrylate chains; and strongly polar polyether chains. In dispersion media with matched polarities, the solvated chains exhibit good compatibility with the dispersion medium, adopting relatively extended conformations to form a sufficiently thick protective layer on solid particle surfaces.

Selection of Super Dispersants:

The selection primarily considers two factors:

1.Surface Properties of Pigment Particles: This includes surface polarity, acid-base characteristics, and functional groups.

- For inorganic pigments with strong surface polarity and some organic pigments, super dispersants that can form single-point anchoring functional groups via dipole-dipole interactions, hydrogen bonding, or ionic bonding are selected.

- For most organic pigments and some inorganic pigments with low polarity surfaces, super dispersants with multi-point anchoring functional groups are used to enhance overall adsorption strength.

- Organic pigments often require super dispersants, and care must be taken to ensure compatibility between the resin and the dispersant. Poorly compatible dispersants result in coiled extended chains, leading to thinner adsorption layers and low steric hindrance effects.

- Generally, super dispersants with amino anchor groups are effective on acidic pigments, while those with acidic groups work better on basic pigments.

2. Polarity of the Dispersion Medium and its Solubility of the Solvated Chain Segments: The dispersion efficiency for each pigment is influenced by the interactions among the pigment, resin solution, and additives. The solvent plays a significant role, particularly the dispersion medium, which influences the mobility and dispersibility of the pigment particles. To ensure that the super dispersant provides adequate spatial stability for the pigment particles in aqueous solutions, the solvated chain segments must adopt sufficiently extended conformations within the medium. Therefore, it is essential to select solvent chains that are highly compatible with the aqueous solution.

Identification of Super Dispersants:

Super dispersants exhibit better dispersing activity. At the same processing viscosity, they can substantially increase the pigment content in the slurry, thereby enhancing processing efficiency or can lower the viscosity of slurries with the same pigment content. This property alone can distinguish between high molecular weight dispersants and low molecular weight dispersants. Experiments with difficult-to-disperse carbon black can easily highlight this distinction. Low molecular dispersants often struggle to achieve effective dispersion at high carbon black concentrations due to insufficient wetting, leading to poor dispersion and high slurry viscosity. In contrast, super dispersants effectively address this issue.

Super dispersants display better storage stability. Color pastes produced with super dispersants maintain good storage stability for extended periods, whereas pastes made with low molecular weight dispersants often display poor stability, especially under thermal cycling tests, leading to easy re-flocculation or aggregation.

Since super dispersants exhibit resin-like properties, with molecular weights reaching or exceeding those of coating resins, this characteristic is an easy means of identification. A sample of the dispersant can be dried in an oven; if the residue forms a solid resin film, it is identified as a high molecular weight dispersant. It is important to note that standard super dispersants yield a light yellow or yellow resin film upon drying. If the residue forms a transparent, brittle film, it may only indicate modified acrylic resin, which, while exhibiting some dispersing effect, cannot be classified as a high molecular weight dispersant.

Application of Super Dispersants:

To achieve optimal dispersion effects, the application of super dispersants is crucial. In terms of the order of addition, for inorganic pigments in polar resins containing active functional groups, they can be added before or after the resin without significant impact since the resin plays a major role. However, if the resin lacks active functionality, it is advisable to add the pigment first, followed by the dispersant, and finally the resin.

The amount of dispersant added is typically determined based on the surface characteristics of the pigment, particularly its acid-base properties, specific surface area, and shape. The optimal value is often established to achieve a dense monomolecular adsorptive layer on the pigment particle surface. Excessive amounts can increase costs and affect product quality, while insufficient amounts may not achieve the desired dispersion effect. Each pigment has a specific optimal concentration value in a particular dispersion system, which is influenced by the pigment's specific surface area, oil absorption, slurry fineness, sand milling time, and the characteristics of the sand-milling resin; hence, the usage must be appropriate and determined through repeated trials.