Paint Encyclopedia: (10) Pigment Dispersion And Preparation Of Paint - Paint, Paint - Paint Industry

HC coating Network : Pigment dispersion is a key step in preparation of paint

, pigment dispersed directly affect the coating quality and production efficiency.

1, pigment dispersion process Pigment dispersion of three processes: wetting, dispersion and stability.

wetting Pigment surface water, the air Solvent (Paints) the replacement is called wetting. Solvent-based paint is not wetting problem, because solvents (paint) the surface tension is always lower than the general surface tension of the pigment. However, wetting a process, especially since the pigment is an aggregate, paint solvent into the gap needed. When the solvent viscosity is low, the wetting speed quickly. Paints and solvents should pay attention to increase the order of, first, add additional solvent paints. In Water-based paint , Due to the higher surface tension of water, the organic pigment wetting have difficulties, and needs Wetting agent To reduce the surface tension of water.

grinding and dispersion In Pigment process, the pigment particle size is controlled in accordance with specified requirements, but because the van der Waals forces between particles, pigment fine particles gathered with each other, as aggregates, which will be open to them to re-disperse This will require shearing or impact force. The abrasive coating is mainly shear forces.

Viscosity in the study have known, when the shear rate (D) fixed, the shear stress ( ) is and viscosity ( ) is proportional, that is: = D

Shows high viscosity, shear force, the ground is favorable, but the grinding equipment, the capacity of the motor system determines the maximum value of , the viscosity can not be too high.

Wetting of the pigment dispersed by impact force when the hope that the lower viscosity medium (paint), but when grinding high viscosity; to make full use of distributed equipment, hopes to paint a large batch dispersion. How to balance these three requirements? According to Mooney formula, in the system as little more pigment plus polymer, is a three worlds approach.

stability Pigment dispersion after, there are still a tendency to aggregate with each other, that flocculation tendencies, this needs to be stable particles have been dispersed them, which is protected, otherwise, because flocculation can cause opacity, color strength and so on down, and even poly Results. For pigment particles stabilized, mainly by two ways:

stability of the pigment surface electric charge, that is formed on the surface double layer, using the opposite charge repulsive force, the particles remained stable. Plus a number of surfactants or inorganic Dispersant Such as multi-phosphate and hydroxyl amine, can achieve this purpose. Protective effect of three-dimensional stereoscopic

protective effect of protective effect, also known as entropy. An adsorption layer of paint surface, when the adsorbed layer reaches a certain thickness (> 8? 9nm), their mutual repulsion can not protect the particles together. If the system only solvent because the adsorption layer is too thin, repulsive force is not enough, the particles can not be stable. If the increase in the number of solvent-based paint in the long chain surfactant, surfactant adsorbed on the polar pigment, the non-polar side toward the paint, can form a thick adsorption layer (8? 9nm), but surfactant adsorption in the paint is only one point, it is very easy to replace solvent molecules down, if the rate of some polymers, polymer adsorption on the surface of pigment particles can form adsorption layer 50nm thick, and there are a number of polymer adsorption point, and this can he last the next, not from pigments, which may well play a protective role.

2, the stabilizing role of dispersion

A stable pigment dispersion should not occur in the store when the following three phenomena:

pigment deposition occurred; excessive pigment flocculation occurred, to the detriment of the apparent rheological properties and coating;

As pigment and medium physical or chemical action between the viscosity increase caused.

pigment deposition Stokes formula = 2 2 ( 1- 2) g/9 representation of spherical particles in a liquid medium in the rate of settlement, the formula for the whereabouts of the velocity , for the particle radius, 1 for the particle density, 2 liquid density, is liquid viscosity, g is gravitational acceleration. Although the paint does not fully comply with the settlement of the requirements of this formula, but can be used as a basis for discussion. Can be seen from the formula, with the reduction of particle radius decreases with the particle and medium density difference declines, also decreased with the increase of viscosity, so as far as possible with the particle radius is small, low-density pigment and high viscosity of the medium to prevent sedimentation. ,

When the pigment adsorption of low molecular weight polymer or surfactant, the particle diameter will increase, not conducive to anti-settlement, but the density of particles dropped to prevent settlement, both compared to the previous one kind of effect can be ignored. When using high molecular weight polymer, the particle adsorption layer thicker, can speed up the settling velocity, but because of the low density of adsorption layer thick and has a good protective effect of the space, preventing flocculation, thus preventing settlement, even if there is settlement, deposition layer is very loose, not serious flocculation and aggregation of dispersed by stirring easy to restore. Proportion of pigment to prevent the settlement of a method of treatment is the use of surfactants, such as treatment with stearic acid Calcium carbonate . Another way to prevent the settlement is to increase the medium viscosity, which can make use of paint, "thixotropy" has made, that is, when the paint place, the viscosity is high, can be frozen into a jelly. In order to paint a thixotropic paint can add thixotropic agents or Thickener , Solvent-based paint in mainly hydrogenated castor oil, organic bentonite (montmorillonite), and alcohol and aluminum.

pigment flocculation Pigment particles in the medium continued to Brownian motion, that is, thermal motion, each particle has some kinetic energy. Between particles and particle collisions continue to occur, if the particle's kinetic energy can overcome the repulsive force between particles can lead to close contact with each other, resulting in flocculation. Dispersion is not stable on the flocculation rate (the number of half-life of a particle that) that can be used the following formula: t1 / 2 = 3 /4kTn0 the formula t1 / 2 half-life for the particles, that is half the required number of particles time, the medium viscosity, k the Boltzmann constant, T is the temperature, kT is the average kinetic energy of particles, n0 the initial number of particles. As can be seen from the type to raise the viscosity can be reduced flocculation. But in fact, alone is not sufficient to improve the viscosity stability of coatings dispersion, important way to prevent the particle collision in contact with each other.

Pigment particles close to each other usually has three main forces: dispersion forces, resistance to static electricity and space. Dispersion force is attractive; static electricity can also be attractive to repulsive, dispersion in the coating is mainly repulsive;

by: gaga

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