1, the introduction
Coating is widely used in our daily life, and plays a protective, decorative and special role in the painted parts. The coating is mainly composed of film-forming substances (resin), fillers, additives and solvents (or water). With the development of industry and the improvement of living standards, pigments are more and more used in the paint and ink industry to give a better appearance. Unlike dyes, pigments are generally insoluble in the medium used and, in most cases, exist as aggregates. In order to obtain good coloring power, hiding power and chromaticity, the pigment aggregates must be opened and kept stable. If the pigment is not dispersed well, many defects may occur, such as flocculation, dimming, color shift, float/bloom, precipitation, etc. This paper introduces the structure of pigments and dispersants, and the role of dispersants in dispersing pigments.
2. Pigment morphology and particle size
In paints, pigments are mostly used as suspensions.
The size range of pigment is very wide, generally ranging from 0.05 m to 1 mm. However, when the particle size of pigment is between 0.05 ~ 0.50 m, it has the best coloring power, gloss, hiding power and weather resistance. Figure 1 shows the contrast of different pigment particle sizes.
The smallest particles formed in the pigment manufacturing process are called primitive particles, which exist as single crystals or groups of crystals with very small particle sizes. And between the original particles with the surface and the surface of the formation of the mass, called aggregates, aggregates are relatively close, the general dispersion equipment is difficult to disperse it into the original particles. The original particles and aggregates are combined by van der Waals force to form a larger pigment granular mass, which is called agglomerate. The particles of the agglomerate come into contact with each other at points, edges and angles. The interaction between the particles is small, which can be dispersed into the original particles or aggregates by mechanical force, as shown in figure 2.We want the pigment to disperse into a suspension of the original particles (ideally), or at least a suspension of aggregates and the original particles. However, the tendency of the system is to recover from the high energy state to the low energy state. Structurally, the flocs are very similar to the agglomerates, but the color between the flocs is resin solution rather than air. The presence of dispersants can prevent or slow down flocculation.
3.Basic structure and type of dispersant
Dispersant is also called the wetting dispersant, it besides has wet function, its reactive end can adsorption on the surface of the paint that crushed into fine particles, the other end of the solvation have enter into an adsorption layer (adsorption base, the more the longer the chain link, the adsorption layer thickness), electrical charge repulsion repulsion (coating) or entropy (solvent-based coating), make the pigment particles dispersed suspension in have been for a long time, avoid again flocculation, thus to ensure that the paint system made of storage stability.
There are many kinds of dispersants, and according to preliminary estimates, there are more than 1,000 substances in the world that are dispersants. Distinguish by its structure now, can divide into the following 7 kinds.
Anionic wetting dispersant
Most of them are composed of non-polar hydrocarbon chains with negative charge and polar hydrophilic groups. The two groups are at both ends of the molecule, forming an asymmetric hydrophilic and hydrophilic molecular structure. Its varieties are: sodium oleate C17H33COONa, carboxylate, sulfate (r-o-so3na), sulfonate (r-so3na), etc. Anion dispersants have good compatibility and are widely used in waterborne coatings and inks. Polycarboxylic acid polymers can also be used in solvent based coatings and widely used as controlled flocculation dispersants.
Cationic wetting dispersant
Non-polar compound with positive charge, mainly amine salt, quaternary amine salt, pyridinium salt, etc. Cationic surfactant has a strong adsorption force and a good dispersion effect on carbon black, various iron oxides and organic pigments. However, it should be noted that it can react with the carboxyl group in the base material and should not be used together with anionic dispersant.
Nonionic wetting dispersant
It does not ionize in water, does not have electric charge, adsorbs weakly in the pigment surface, mainly USES in the water system paint. It is mainly divided into glycol type and polyol type to reduce surface tension and improve wettability. Used together with anionic dispersant as wetting agent or emulsifier, it is widely used in waterborne color paste, waterborne paint and ink.
Amphiphilic wetting dispersant
It’s a compound of anions and cations. The typical application is a phosphate ester type polymer. Such polymers have higher acid values and may affect interlaminar adhesion.
Opinions on dispersants with different structures are shown in FIG. 7.
Electrically neutral wetting dispersant
The size of anion and cationic organic groups in the molecule is basically equal, and the whole molecule is neutral, but has polarity. For example, oil amino oleate C18H35NH3OOCC17H33 belongs to this type and is widely used in coatings.
Macromolecular super-dispersant
Macromolecular dispersants are the most commonly used and have the best stability. Polymeric dispersant also divided into many yourself in more ester polyols – ethylene imine type block copolymer dispersant, acrylate polymer dispersant, polyurethane or polyester polymer dispersant etc., because of their anchor groups in a winding with resin adsorption, on the other side and pigment particles pack again, thus the storage stability is good.
Controlled free radical hyperdispersant
With the latest controlled free radical polymerization technology (CFPP), the dispersant structure can be more regular. Commonly used methods are: GTP, ATRP (atomic transfer free radical polymerization), RAFT (reversible addition breaking chain transfer controllable free radical polymerization, including c-raft and s-raft, etc.), NMP, SFRP (stable free radical polymerization), TEMPO, etc. By using controlled free radical polymerization technology, the relative molecular mass distribution of the dispersant can be more concentrated, and the anchoring group can be more concentrated and the efficiency can be higher.
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