What is the migration resistance of pigments in plastics? How is it tested?
The dispersion of pigments is important for coloring plastics. The most important properties for coloring pigments are tinting power and hiding power. The coloring power is the measure of coloring ability and efficiency of pigment, when two kinds of plastics get the same color of the colored material, the coloring power of the one with less dosage is higher; the covering power is the ability of the pigment to cover the base color of the colored material. These two indexes are closely related to the dispersion of pigment.
I. Dispersion principle
Generally speaking, the better the dispersion of pigment, the higher its coloring power and covering power. The reason is that the coloring and covering of the pigment on the products are reached through the complex action of its surface and light. If the pigment is well dispersed, its average particle size is small, its specific surface area is large, its action on the light is strong, and the appearance of the coloring products will appear even and delicate, with less color spots and small color difference. In addition, to achieve the same coloring and covering effect when the amount of pigment can be reduced, so that the coloring cost reduction.
The concept of “dispersion” in engineering generally includes the following two kinds:
① The reduction and unification process of the dispersed medium in volume;
The uniformity of the spatial distribution of the dispersed medium.
For the pigment in resin, it means that the large particles are broken into small particles and finally unified in a certain scale range, and these particles are uniformly distributed in the resin matrix. Generally speaking, in plastic coloring, the uniform distribution of pigment is not difficult, the key lies in how to refine it sufficiently.
Form of pigment before dispersion
Primary particles
In the form of large crystals consisting of single crystals, lumps, spheres or microcrystals.
(Figure: Primary particle size of pigment)
Cohesion
Consists of primary particles adsorbed to each other on the surface.
Agglomerates
A mixture consisting of a loose combination of primary particles or agglomerates, or both adsorbed to each other at the edges and corners.
The dispersion of pigments is refined by the process of pulverizing and refining the agglomerates.
The process of pigment dispersion
The first step of pigment dispersion is to use wetting agent to wet the pigment, so that the cohesion between the pigments is reduced, which facilitates the crushing and refining of the pigment in the second step. After crushing and refining, the pigment is further coated to reduce the surface energy of the newly formed interface, so that the pigment will not produce re-cohesion phenomenon during further processing. The third step of refining the pigment can be uniformly dispersed in the melt, thus, the dispersion process of pigment can be divided into three steps: wetting, refining and stabilized mixing and dispersing after refining.
Wetting of pigment
Wetting is the first step of dispersion. The wetting process is to use wetting agent (surface treatment agent) to wet and cover the surface of pigment.
Refinement (crushing) of pigments
Refinement of pigment is the breaking of agglomerate particles, which mainly relies on the free movement between pigment particles (impact stress) and the stress of the pigment agglomerate through the surrounding medium (shear stress).
There are three basic methods of refinement as follows: dry mixing method, liquid shear method and melt shear method. The actual production process is the joint use of these methods, more conducive to the advantages of each method.
Pigment dispersion particle size problem
The following data can be used to illustrate how big the particle size of pigment after dispersion is suitable.
1. If the particle size of pigment is larger than 30μm, the surface of the product will produce spots and streaks;
2.10～30μm, the surface of the product has no luster;
3. When the particle size is less than 5μm, for general products, it can meet the use, but for the products with strict requirements, it will affect the mechanical properties, electrical properties and processability;
4. For fiber (single filament diameter of 20-30μm) and ultra-thin film (less than 10μm), the pigment particle size should be less than 1μm.
Many researches have proved that in general, the smaller the pigment particle size is, the higher its coloring power is. Pigment particle size is also related to covering power, which is due to the fact that the pigment is dispersed and processed, the particle size becomes smaller, the surface area increases, so the covering power increases. In addition, azo pigment particle diameter in 0.1μm, phthalocyanine pigment particle size in 0.05μm, has the highest coloring power.
The influence of pigment dispersion on the appearance of products
The dispersion of pigment directly affects the appearance of coloring products (spots, streaks, gloss, color and transparency); the processability, also directly affects the quality of coloring products, such as strength, elongation, aging resistance and resistivity. With the increase of pigment diameter particles, the impact strength and elongation of products decrease. Well dispersed, its average particle size is small, the specific surface area is large, the role of the light is strong, the appearance of the coloring products will appear uniform, delicate, less color spot, color difference is small, and at the same time can reduce costs.
Influence of pigment dispersion on the quality of coloring products
Coloring power and achromatic power
It is the measure of coloring ability and efficiency of pigment. When two kinds of pigment make the coloring object get the same color, the one with less dosage has high coloring power. When titanium dioxide is mixed with another pigment, it can get a mixture showing its own pigment’s ability.
