What is the migration resistance of pigments in plastics? How is it tested?
The dispersion of pigments is of great significance to the coloring of plastics. For pigments used for coloring, the most important properties are tinting power and hiding power. Tinting power is a measure of the coloring ability and efficiency of pigments. When two plastics make the coloring object obtain the same color, the tinting power of the smaller amount 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 pigment is dispersed, the higher its tinting power and hiding power. The reason is that the coloring and covering of the product by the pigment is achieved through the complex interaction between its surface and light. If the pigment is well dispersed, its average particle size is small, and its specific surface area is large, its effect on light will be strong, and the appearance of the colored product will be It looks even and delicate, with fewer spots and small color differences. In addition, the amount of pigment used can be reduced to achieve the same coloring and hiding effect, thereby reducing coloring costs.
The concept of “dispersion” in engineering generally includes the following two types:
①The process of volume reduction and unification of the dispersion medium;
②The uniform process of spatial distribution of the dispersed medium.
For pigments in resin, large particles are broken into small particles and finally unified within a certain scale, and at the same time, 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 it.
The form of pigment before dispersion
Native particles
Exists in the form of large crystals composed of single crystals, massive, spherical or microcrystals.
(Picture: Primary particle size of pigment)
Condensate
It is formed by adsorbing each other on the surface of primary particles.
Aggregation
It is a loose combination of primary particles or aggregates, or a mixture of the two adsorbing each other on the edges and corners.
The dispersion and refinement of pigments is the process of crushing and refining agglomerates.
Pigment dispersion process
For the dispersion of pigments, the first step is to use a wetting agent to moisten the pigments to reduce the cohesion between the pigments, which facilitates the crushing and refinement of the pigments in the second step. The crushed and refined pigments are further coated to reduce the newly formed interface surface energy so that the pigments will not re-agglomerate during further processing. The refined pigments in the third step can be evenly dispersed in the melt. , thus, the pigment dispersion process can be divided into three steps: wetting, refinement, and stabilization mixing and dispersion after refinement.
Wetting of Pigments
Dispersion must first be moistened. The wetting process is to use a wetting agent (surface treatment agent) to wet and coat the pigment surface.
Refining (crushing) of pigments
The refinement of pigments is to break the agglomerate particles, which mainly relies on the free movement between pigment particles (impact stress) and the stress of the pigment agglomerates passing through the surrounding medium (shear stress).
There are three basic methods of refinement: dry mixing, liquid shear and melt shear. In the actual production process, these methods are used together, which is more conducive to bringing out the advantages of each method.
Pigment particle size problem
What is the appropriate particle size after the pigment is dispersed? You can use the following:�The data is explained.
1. The pigment particle size is greater than 30μm, causing spots and streaks on the surface of the product;
2.10～30μm, the surface of the product is matte;
3. When the particle size is less than 5μm, it can be used for general products, but for products with strict requirements, it 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 research works have proven that, in general, the smaller the particle size of a pigment, the higher its tinting power. The particle size of the pigment is also related to the hiding power. This is because after the pigment is dispersed, the particle size becomes smaller and the surface area increases, so the hiding power increases. In addition, when the azo pigment particle diameter is 0.1 μm and the phthalocyanine pigment particle diameter is 0.05 μm, it has the highest tinting power.
The impact of pigment dispersion on product appearance
The dispersion of pigments directly affects the appearance of colored products (spots, streaks, gloss, color and transparency); the processability also directly affects the quality of colored products, such as strength, elongation, aging resistance and resistivity. As the pigment particle size increases, the impact strength and elongation of the product decrease. If it is well dispersed, its average particle size is small, its specific surface area is large, and its effect on light is strong. The appearance of the colored products will appear uniform and delicate, with fewer stains and small color differences, and it can also reduce costs.
The influence of pigment dispersion on the quality of colored products
Tinting power and achromatic power
A measure of the tinting power and efficiency of a pigment. When two pigments cause the coloring object to obtain the same color, the one with the smaller amount has the highest tinting power. Mixing titanium dioxide with another pigment gives the mixture the ability 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 achromatic power.
Factors affecting achromatic power
The size of the achromatic force is related to its scattering ability and refractive index;
The size, shape, structure and particle size distribution of particles have a significant impact on the achromatic force. The achromatic force increases with the decrease of particle size and the increase of particle size uniformity;
The achromatic 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, powdery coloring and pure pigment coloring to produce ordinary plastic products. It simplifies the granulation process and directly uses injection molding, extrusion and other processing methods to produce colored products, avoiding degradation of the resin due to granulation heating.
2. Coloring with dry pigment mixture
When using pigments with widely different dispersities, and when plastic processing manufacturers have difficulty in color matching, you can use powdered color matching pigment mixtures provided by pigment suppliers.
3. Coloring of paste mixture
It is a pigment paste that uses mixing and kneading equipment such as a three-roller mill and a kneader, and uses liquid carriers such as plasticizers and polyols to grind the pigment into a paste.
4. Pigment mixture
It is a concentrate of pigments that has been refined and fully dispersed into resin or dispersant.
Main equipment for pigment dispersion
Kneader: mainly used for mixing and dispersing high-viscosity substances;
Branetali mixer: mainly used for mixing and separating emulsions of various viscosities;
High-speed dispersing machine: It is the simplest structure among pigment dispersing equipment;
Three-roller grinder: mainly used for processing low-volatility coatings;
Ball mill: mainly used to produce volatile coatings and various inks, liquid colorants and color pastes;
Sand mill: mainly used for the processing of volatile coatings and various inks and the production of liquid colorants, latex paints, etc.;
Colloid mill: It is a medium-free dispersing 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, mainly used for batching some substances and mixing various fibers into plastics.
How to improve pigment dispersion?
Methods to achieve high-quality dispersion in liquid media (selection of carrier)
It has good wettability to both pigments and grinding surfaces;
“Viscous” mixing methods, which select low viscosity for processing methods that rely on particle collisions with each other, and select high viscosity for processing methods that rely on shear forces 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 using melt shearing method (resin selection)
Good wettability to both pigments and grinding surfaces;
High adhesive strength;
The viscosity is slightly affected by temperature and is chemically compatible with the pigments under processing conditions. Compatible with base resin fluidity (especially when preparing masterbatch) to reduce dilution difficulties caused by different hot melt flow rates;
Choose the right amount of pigment to produce high shear during operation;
Pre-mixed dry ingredients;
Use shear to flow the resin rather than possible external heat (eg Banbury).
��, chemically compatible with the pigment under the processing conditions. Compatible with base resin fluidity (especially when preparing masterbatch) to reduce dilution difficulties caused by different hot melt flow rates;
Choose the right amount of pigment to produce high shear during operation;
Pre-mixed dry ingredients;
Use shear to flow the resin rather than possible external heat (eg Banbury).
