Wetting, dispersing and dispersing stability of pigments

Sep. 09, 2020

1、 1. Overview

In the printing and dyeing industry of pigment, ink, pigment printing, rubber, plastic, coloring and some types of dyes, the pigment and dye powder used must have good wettability and dispersive state to the medium used. The size and distribution of pigment particles directly affect the storage stability of products, the quality of coating (or film), and the pigment properties of colorants also play an important role in the smooth printing and dyeing process. Therefore, effective dispersion process is indispensable in coating production, and this process is not a simple crushing process, but also includes changing the surface physical state of pigment particles, so that all pigment particles are evenly distributed in the use medium, and have good storage stability, so as to prevent re aggregation or precipitation before use.

The ideal dispersion system is the pigment dispersion state with "primary particles", but in the storage process, due to the high surface energy of particles, it is easy to agglomerate or flocculate. Therefore, it is assumed that in the process of pigment production, some specific surfactants or other derivatives can be added to change the surface properties between "pigment water" or "pigment non-aqueous solvent", so as to prevent the aggregation of fine particles, and to prepare the pigment particles with soft structure and easy redispersion, namely the so-called dispersible pigments. The dispersibility and degree of dispersion (particle size and distribution) of the pigment not only affect the appearance color of the pigment, but also lead to the color uniformity, such as streaks and other defects.

It can be seen that the size of pigment particles is directly related to the final effect of pigment application. In addition to the molecular structure, the smaller the particle size and the larger the specific surface area, the higher the pigment strength.

 

In order to make the organic pigment products have ideal application performance, many researchers have paid attention to the study of the wetting characteristics of pigment particles in the medium, the reasonable dispersion process and the stability of the dispersion system.


二、 Wetting of pigment particles

 

The dispersion of pigment in the medium can be divided into three stages of wetting, mechanical crushing of aggregates, agglomerates and stabilization of anti flocculation. Therefore, wetting the particles in the dispersion medium is the first step in the dispersion process. When the solid surface contacts with the liquid, the original "solid gas" interface disappears and a new "solid-liquid" interface is formed. This phenomenon is called wetting. Generally, polar solid substances are easy to be wetted by polar liquids. Most of the solids that can be wetted by water are hydrophilic polar substances, while those that cannot be wetted by water are nonpolar or general small substances.

 

The wetting angle or contact angle (0) between "liquid-solid" interfaces can be used as the scale. When 0 > 90 ° the solid surface is not wetted by the liquid, if 0 < 90 ° it can be wetted. In other words, the smaller the wetting angle is, the better the wetting is; the larger the wetting angle is, it is difficult to wet when the liquid is close to 180 degrees. When the liquid is placed on the surface of pigment particles, the force of each interface is as shown in Fig. 1, RS is solid (pigment)

 

RZ is the surface tension of liquid, and RSL is the interfacial tension between solid and liquid.


三、 Dispersion technology of pigment

 

The dispersion technology and equipment of pigments in the medium used are mainly determined by the quality of dispersion, the cost of producing dispersions and the dosage form of dispersed pigments (dry powder or paste). For example, for the pigment used in viscose fiber pulp coloring, the particle size of the pigment should be about 1U in order to pass through the diameter of the tapping hole. However, for some other applications, too high dispersion is unnecessary because the finer the pigment particles, the higher the surface energy, the easier to promote the flocculation of particles. Generally, the pigment particles are moderate, but the distribution of particle size is required Concentration.


The technology and equipment should be selected to make the pigment aggregate be crushed as quickly and economically as possible. Of course, the crushing effect and speed depend on the properties (soft and hard) of pigment paste and the efficiency of equipment. If the paste is prepared by wet filter instead of dry pigment powder, the pigment content is 15-40%, and the material is not very viscous, the vertical or horizontal ball mill suitable for low viscosity should be selected instead of multi (three) rod mill and kneader. On the contrary, the paste made from dry pigment powder is not suitable for ball mill.


Ball milling has been used in coating and oil industry for many years, especially for low viscosity materials, such as solvent based ink. It is an effective method to obtain high quality water-based pigment dispersion. The ball milling process is to disperse the pigment particles by means of the rolling of the grinding medium and the strong shear force. Under the condition of free sliding and rolling of lapping medium, the ratio of grinding medium to pigment, grinding time and dispersion should be determined according to the properties of the crushed materials and the required dispersion when the material viscosity is too high. The ball mill has the advantages of low cost and convenient experiment, but its disadvantage is that it is difficult to trickle and wash the equipment in batches.

 

There are many other types of groove equipment suitable for grinding low viscosity, such as Kady mill. It is a kind of high-speed vibration mill with special rotor and stator, which can exert high shear force on the crushed pigment position. It is also recommended to use high-pressure jet pump equipment, which is composed of three piston pumps. The low viscosity pigment slurry is sprayed on the metal surface of the dispersion valve made of special high hardness at a very high pressure. At this time, the pigment aggregate is broken by the strong shear force between the surface of the dispersion valve. Of course, for the dispersion of pigment paste with low viscosity, colloid mill, cone Zhan and "high speed mixer" can also be used.


4、 Stability of pigment dispersions

 

In the pigment and coating industry, it is usually necessary to disperse hydrophobic pigments into water or hydrophilic pigment particles into oily medium, and try to prevent floating color, condensation and precipitation during storage and application. Therefore, the stability of pigment dispersion is of great significance. The stability of pigment dispersion system is related to many factors, such as the properties of the dispersions, the surface characteristics of particles, the wetting and distribution state of the particles. In order to make the dispersions have ideal stability, the following ways can be adopted

 

I. the surfactants with appropriate concentration and ionic type are added and selectively adsorbed on the surface of pigment particles·

 

(II) the surface treatment of pigment with polymer surfactant is based on the adsorption on the surface of the particles, and the dispersion tends to be stable according to the steric barrier effect.

 

(III) increase the viscosity of the dispersion medium to control the Brownian motion and natural gravity settlement of pigment particles.

5、 Application of coating activator in pigment dispersion

 

The addition of surfactants plays an important role in the wetting, dispersing process, or the stability in the process of storage and use in the dispersion medium. Surface active molecules have lipophilic groups (hydrophobic groups) and hydrophilic groups, which disperse in molecular state when the concentration is very low in the aqueous solution. When the concentration reaches a certain concentration (i.e., critical concentration), tens or hundreds of molecules exist in the form of micelles, and they are arranged in an orderly and directional manner on the interface, thus reducing the surface tension. The surface tension reaching the critical wave intensity (usually 10 ~ ~ 10 -'mol / L) is almost a certain value.

 

The dispersibility of pigment in the medium and the stability of dispersion have an important influence on its application performance, coloring effect and product appearance. The wettability of pigment particles in the medium is one of the key steps to ensure the smooth dispersion of particles.

 

It is necessary to select surfactants and polymer additives with good compatibility according to the properties of pigments (hydrophilic and lipophilic) and the properties of medium. According to the dispersion requirements of the dispersion equipment, the dispersion effect of the products is achieved according to the dispersion equipment.

 

For pigment dispersions with good stability, we should not only consider the electrostatic repulsion, but also pay attention to the steric (spatial) barrier of the additive and the solvation of the medium. With the expansion of the use of organic pigments and the increase of dosage forms, the wetting, dispersing and dispersion stability of organic pigments are very important research fields.


Hydroxyl-Modified Vinyl Chloride/Vinyl Acetate Terpolymers

Gravure composite ink: pigment dispersion can completely replace solbin a, and the ink has high color development, especially for black, blue and other hard to disperse pigments; it is suitable for making high-performance ink, high-speed shallow screen printing is not easy to start the cutting line; improve the ink composite strength; high ester solubility transparency; and it is suitable for high-performance ink;

Reflective ink: good dispersion of pigment and excellent adhesion with pet;

Wood paint: good repainting property (more than three times, high transparency), good alkyd compatibility and fineness, good leveling performance;

Plastic paint: excellent directional arrangement of silver powder, strong sense of metal, good adhesion of substrate, ester soluble and no burning bottom;