What is the migration resistance of pigments in plastics? How is it tested?

What is pigment migration resistance in plastics? How is it tested?
The dispersion of pigments is of great significance to the coloring of plastics. For pigments for coloring, the most important properties are tinting strength and hiding power. Tinting power is the measure of the coloring ability and efficiency of pigments. When two kinds of plastics make the coloring object obtain the same color, the coloring power with less dosage is higher; hiding power is the ability of the pigment to cover the background color of the coloring object. These two indicators are closely related to the dispersion of pigments.

1. Dispersion principle
Generally speaking, the better the dispersion of the pigment, the higher its tinting and hiding power. The reason is that the coloring and covering of the product by the pigment are achieved through the complex interaction between the surface and the light. The pigment is well dispersed, its average particle size is small, and the specific surface area is large. The effect on the light is strong, and the appearance of the colored product will be improved Appears uniform, delicate, less color spots, small color difference. In addition, the amount of pigment used to achieve the same coloring and hiding effect can be reduced, thereby reducing the coloring cost.
The concept of “decentralization” in engineering generally includes the following two types:
①The volume reduction and unification process of the dispersion medium;
② The uniform process of the dispersed medium in the spatial distribution.
For the pigment in the resin, the large particles are broken into small particles and finally unified in a certain scale range, and these particles are evenly distributed in the resin matrix. Generally speaking, in plastic coloring, the uniform distribution of pigments is not difficult, the key lies in how to fully refine them.
State of the pigment before dispersion
primary particles
It exists in the form of single crystal, massive, spherical or large crystal composed of microcrystal.

(Figure: Primary particle size of pigment)

Aggregate

It is formed by the adsorption of primary particle surfaces to each other.

Aggregates

It is a loose combination of primary particles or aggregates, or a mixture of the two adsorbing each other on the sides and corners.
The dispersion and refinement of pigments is the process of pulverizing and refining the aggregates.
Pigment Dispersion Process
In the dispersion of pigments, the first step is to use a wetting agent to wet the pigments, so that the cohesion between the pigments is reduced, which is convenient for the pulverization and refinement of the pigments in the second step. The pulverized pigment is further coated to reduce the surface energy of the newly formed interface, so that the pigment will not be re-agglomerated during further processing, and the pigment after the third step can be uniformly dispersed in the melt , thus, the dispersion process of pigments can be divided into three steps: wetting, refinement and stabilized mixing dispersion after refinement.
Pigment wetting
Dispersion requires wetting first, and the wetting process is to wet and coat the surface of the pigment with a wetting agent (surface treatment agent).
Pigment refinement (crushing)
The refinement of the pigment is to break the aggregate particles, which mainly depends on the free movement between the pigment particles (impact stress) and the stress (shear stress) of the pigment aggregates through the surrounding medium.
There are three basic methods of refinement: dry mixing, liquid shearing and melt shearing. In the actual production process, these methods are used in combination, which is more conducive to the advantages of each method.
Pigment particle size problem
The particle size of the pigment after dispersion is appropriate, which can be explained by the following data.
1. If the particle size of the pigment is greater than 30 μm, spots and streaks will appear on the surface of the product;
2.10～30μm, the surface of the product is dull;
3. When the particle size is less than 5μm, it can be used for general products, but for products with strict requirementsProducts will also affect their mechanical properties, electrical properties and processability;
4. For fibers (single filament diameter 20-30 μm) and ultra-thin films (less than 10 μm), the pigment particle size should be less than 1 μm.
Many researches have proved that in general, the smaller the particle size of the pigment, the higher its tinting strength. The particle size of the pigment is also related to the hiding power. This is because the particle size becomes smaller and the surface area increases after the pigment is dispersed, so the hiding power increases. In addition, when the particle diameter of azo pigment is 0.1 μm and that of phthalocyanine pigment is 0.05 μm, it has the highest tinting strength.
Effect of Pigment Dispersion on Product Appearance
The dispersion of pigments directly affects the appearance of colored products (spots, streaks, gloss, color and transparency); processability also directly affects the quality of colored products, such as strength, elongation, aging resistance and resistivity. As the pigment size increases, the impact strength and elongation of the product decrease. Well-dispersed, its average particle size is small, its specific surface area is large, and its effect on light is strong. The appearance of colored products will appear uniform, delicate, with less stains and color differences, and at the same time it can reduce costs.
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Influence of Pigment Dispersion on the Quality of Colored Products
Tinting power and decolorizing power
A measure of the coloring ability and efficiency of a pigment. When two pigments are used to obtain the same color of the object to be colored, the coloring power of the lesser amount is higher. When titanium dioxide is mixed with another pigment, it is possible to obtain the ability of the mixture to exhibit its own pigment. For titanium dioxide, after it is mixed with a dark pigment, the lighter the color of the mixture, the stronger its decolorizing power.
Influencing factors of decolorizing power
The size of the achromatic power is related to its scattering ability and refractive index;
The size, shape, structure and particle size distribution of the particles have a significant impact on the decolorizing power, and the decolorizing power increases with the decrease of the particle size and the increase of the uniformity of the particle size;
The decolorizing power of titanium dioxide is also related to the preparation conditions.
Common plastic coloring methods
1. Dry mix coloring
It can also be used for floating dyeing, powder coloring and pure pigment coloring, and can produce ordinary plastic products. It simplifies the granulation process and directly uses injection molding, extrusion and other processing methods to produce colored products, avoiding resin degradation due to granulation heating.
2. Coloring with dry pigment mixture

When using pigments with very different dispersibility, and when plastic processing manufacturers find it difficult to match colors, they can use powdered color matching pigment mixtures provided by pigment suppliers.
3. Paste mixture coloring
It is a pigment paste that uses mixing and kneading equipment such as three-roll mills and kneaders, and uses liquid carriers such as plasticizers and polyols to grind pigments into paste.
4. Pigment mixture
It is a concentrate of pigment, and it is finely dispersed into resin or dispersant.
Main equipment for pigment dispersion
Kneader: mainly used for mixing and dispersing of high-viscosity substances;
Branetali mixer: mainly used for mixing and separating emulsions of various viscosities;
High-speed dispersing machine: it is the simplest structure in the pigment dispersing equipment;
Three-roll mill: mainly used for the processing of paint with low volatility;
Ball mill: mainly used to produce volatile paints and various inks, liquid colorants and color pastes, etc.;
Sand mill: mainly used for the processing of volatile paints and various inks and the production of liquid colorants and latex paints;
Colloid mill: It is a medium-free dispersion equipment, consisting of a rotor and a stator;
High-speed mixer: mainly used for mixing and dispersing dry blended powder resin;
Continuous mixer: It is a rotor mixer, which is mainly used for batching some substances and mixing various fibers in plastics.
How to improve the dispersibility of pigment?
Methods to achieve high-quality dispersion in liquid media (choice of carrier)
It has good wettability to both the pigment and the grinding surface;
“Viscous” matching method, that is, low viscosity is selected in the processing method that relies on particle collision with each other, and high viscosity is selected in the processing method that relies on the shear force through the liquid layer;
Chemically compatible with pigments under processing conditions. Choose the right amount of pigments and additives for proper flow in the machine.
Method to achieve high quality dispersion by melt shearing (resin selection)
Good wettability to both pigments and abrasive surfaces;
High adhesive strength;
Viscosity is little affected by temperature and is chemically compatible with pigments under processing conditions. Compatible with the fluidity of the base resin (especially when preparing color masterbatches) to reduce the difficulty of dilution caused by different hot melt flow rates;
Choose the right amount of pigment so that high shear forces can be generated during operation;
Premix the dry ingredients;
Use shear to flow resin instead of possible external heat (eg Banbury).