Detailed explanation on the problem of the dispersion method of carbon black pigment

Dispersing carbon black must first understand the dispersion mechanism of carbon black pigment.

Carbon black is well known as one of the hard-to-disperse pigments, so carbon black needs to be dispersed in the carrier more than any other pigment. Although almost all users of carbon black have more or less mastery of techniques for effectively dispersing carbon black, the understanding of the dispersion process and mechanism of carbon black is relatively weak, so it is necessary to do the dispersion process and mechanism of carbon black. A brief description.

Dispersion process and mechanism of carbon black pigment:

The dispersion process of ink black is a physical process, and many independent sub-processes are included in this overall process. The dispersion of carbon black in another medium actually means that the agglomerates in the carbon black powder are destroyed into smaller particles and simultaneously wetted by the medium. The dispersion process of carbon black is also accompanied by a random and uniform distribution process of carbon black particles. If the viscosity of the medium is low, the dispersed carbon black particles are easily aggregated due to the attraction between the particles, that is, the so-called pigment particles reagglomerate. effect. This action lowers the coloring strength of the carbon black, and in order to prevent the occurrence of re-agglomeration, it is necessary to stabilize the carbon black dispersion state.

The dispersion process of carbon black can be divided into the following stages:

1. Crushing: Since the usual fineness (45μm) of commercial grade carbon black pigments is far from the requirements of the downstream field, it is necessary to crush and disperse the carbon black when using carbon black, and only the carbon black is broken and dispersed to a certain extent. In the fineness range, the coloring performance of carbon black or other required properties can be fully exerted. For example, the operation occurring in an extruder, a kneader, a three-roll mill, an internal mixer, a high agitator, a ball mill, a sand mill, etc., is a crushing of the pigment. As a characteristic of the pigment, the degree of breakage of the carbon black is also related to the "fastness" of the self particles.

2. Wetting: The interaction of the surface of free carbon black particles with the surrounding medium is an important factor in the wetting of carbon black. The energy relationship of carbon black wetting can be explained by the laws of thermodynamics, in which the surface tension and surface energy of carbon black are important auxiliary parameters for the heat of wetting, and the wetting process of carbon black powder and agglomerates can also be described from the kinetic point of view. The wetting time is not only related to the pores and geometry of the carbon black powder, but also increases with the viscosity of the medium and decreases with increasing wetting tension. Under practical conditions, increasing the temperature accelerates the wetting process, but an increase in temperature (reduced viscosity) leads to a decrease in shear (crushing). The wetting volume can be used as a measure of the pore volume or packing density of the carbon black, which is closely related to the relative tinting strength of the carbon black and thus indirectly related to the quality of the dispersion of the carbon black in the final product. Therefore, in order to improve the dispersibility of carbon black in practice, low molecular weight additives (such as an enhancer or a viscosity promoter) are often incorporated into the medium.

3. Distribution: By increasing the processing temperature and reducing the viscosity of the system, the system is under turbulent conditions; and prolonging the residence time of the carbon black in the processing equipment; the wet carbon black particles can be better distributed. However, in practice, extended residence time will be limited by production requirements. Therefore, for the plastics industry, torpedo heads, perforated plates, shearing parts, and kneading sections are usually installed in injection molding machines and extruders, and other design measures are added to improve the crushing action and prolong the residence time.