Pigments are most important to cover and assign to the coating to color, but its role is not only there, but also:
① Increase Strength
Like the role of carbon black in rubber. The active surface of pigments can be combined with macromolecular chains to form cross-linked structures. When one strand is subjected to stress, the stress is dispersed through the cross-linking point.
The interaction between pigments and macromolecules is generally a secondary valence force, Pigment which can be strengthened by chemical treatment. The size and shape of the pigment particles have an effect on the strength, the finer the particles, the better the effect.
② Increase adhesion
Coating in curing often accompanied by volume contraction, resulting in internal stress, Pigment affecting the adhesion of coatings, adding pigments can reduce shrinkage, improve adhesion.
③ improves rheological properties
Pigments can increase the viscosity of the coating, but also give the coating a good rheological properties, for example, by adding (such as gas-phase SiO2) to give Thixotropic properties.
④ Improvement of Weatherability
Carbon Black is both a black pigment and an ultraviolet absorbent.
⑤ Functional function
such as Anti-Corrosion, in the anti-corrosion pigments have a passivation effect of pigments, such as Red Dan (Pb3O4), also has a shielding effect of pigments, such as aluminum powder, mica and glass scales, Pigment as well as similar sacrificial anode zinc powder.
⑥ Reduce Luster
Adding pigments to the paint can damage the smoothness of the surface of the film, thus reducing the gloss, which is often used in varnish to eliminate light.
⑦ Cost reduction
Many pigments that do not cover and color (such as Caco3sio2, talc, etc.) are inexpensive to add to the coating without affecting the properties of the coatings, Pigment but can increase the volume and greatly reduce the cost. They are called volume pigments.
In order to select pigments to play these roles, it is necessary to understand the following properties of pigments:
The hiding force and coloring force of ⑴ pigments
The masking power of a pigment is the ability of a pigment to cover the surface of a coating so that it cannot be exposed through the film. The pigment's covering force and refractive index, crystallization type, Pigment size of particle diameter and so on, in the known pigments rutile TiO2 The refraction exponent is biggest, it and the polymer has the biggest refractive rate difference, therefore is the best white pigment. Some pigments such as silica, big white powder, refractive index and polymer similar, no contribution to the cover, known as volume pigments.
If the paint contains air, because the air refractive index is the smallest, it and polymers and pigments have a poor refractive index, so there is a good cover effect. On the blackboard with chalk to write, calcium carbonate (chalk) on the blackboard has a good cover, is because of the air, but if the chalk word wet, you can not see white, because at this time the water replaces the air, water refractive index and calcium carbonate close. Pigment Now there is a capsule-shaped pigment, that is, the small hollow ball of polymer, is the use of the cheapest air paint, if further, in the hollow ball into the titanium dioxide particles, the cover efficiency is higher. Because light passes through the polymer membrane into the air when there is scattering, Pigment from the air to the titanium dioxide particles when the scattering. Carbon Black has a good ability to absorb light, it can also be very well covered. Using the human eye's weakness, adding a little carbon black to the white pigment can reduce the dosage of titanium dioxide.
The coloring power of color pigments is the ability of the color of the whole mixture to sound in its own color, the greater the coloring power, the less the pigment dosage and the lower cost. The coloring force is related to the characteristics of pigment itself, and it has relation with its size, in general, the smaller the particle size, the greater the coloring force. General organic pigments have higher tinting strength than inorganic pigments. The dispersion of pigments has great effect on coloring, which can cause abnormal hue.
The coloring force of the pigment is independent of the covering power, and the more transparent pigment can also have a high coloring force.
⑵ resistance to light fastness
Pigments can only give the paint a good original color is not enough, Pigment the color of the coating must be durable, it is best to keep the film itself destroyed. Many pigments fade in light, darken or hue.
⑶ Permeation Color Property
Not all pigments are completely insoluble in all solvents. Sometimes painted on the bottom of the red paint, white paint pink, which indicates that the solvent in white paint dissolved a part of the pigment in the primer, this phenomenon is called "infiltration color." Red Organic pigments are particularly susceptible to infiltration.
⑷ particle size and shape
The best particle size of the pigment should generally be half of the wavelength of the light in the air, that is 0.2-0.4μm, if less than this value, the pigment loses the ability to scatter light, and greater than this value decreases the total area, which reduces the total scattering capacity of the pigment to the light. In fact, the diameter of the pigment is roughly 0.01μm (e.g. carbon black) to 50μm (e.g., some volume pigments), pigments are usually mixtures of different particle sizes. Particle size is also directly related to the amount of oil absorption, Pigment because the smaller the particle, the larger the surface area. The amount of oil absorbed will be discussed later.
Particle shape is different, its stacking and arrangement is different, so it will affect the cover of the pigment, paint rheological properties and so on. For example, rod-shaped pigments have a better effect, but also tend to poke out the surface, reduce the smoothness of the surface, which will reduce gloss, Pigment but help the adhesion of the next coating. Flaky pigments have a fence effect, which slows down the water.