Colorants for inks include pigments and dyes. Pigments are divided into organic pigments and inorganic pigments. The former has bright hue, strong coloring power and short drying time, so it is widely used in inks, such as azo and phthalocyanine pigments; the latter Light resistance, heat resistance, solvent resistance, and concealment are all good, such as titanium white, cadmium red, chrome green, ultramarine blue, etc.
Pigments are colored in the form of particles and do not dissolve. They are the most commonly used colorants in inks. While the dye is formulated into a solution when in use, it is colored in a molecular state, and the coloring effect is not as good as that of the pigment. The colorant can give the ink different colors and color density, and make the ink have a certain viscosity and dryness.
The primary conditions for selecting colorants when manufacturing inks are bright, bright, saturated, shiny, and stable.
The color strength depends on the colorant's own structure, preparation method and dispersion performance. The coloring power of the coloring agent directly affects the performance of the ink. The coloring power is low, the amount of coloring agent is large, and the solid content of the ink increases, which reduces the adjustable margin of ink performance and is not conducive to the adjustment of ink properties.
Transparency refers to the degree to which the ink film transmits light after the colorant is formulated into ink and printed on paper or other materials to form a film. When the refractive index of the colorant in the color ink is approximately equal to that of the binder, it is a transparent ink.
Hiding power is the ability of the ink film containing the colorant on the printed product to cover the bottom layer. The greater the difference between the refractive index of the colorant in the color ink and the refractive index of the binder, the smaller the transparency of the ink and the stronger the hiding power.
The printed products not only require colorful, but also good gloss. The colorant of the same structure, the different manufacturing conditions and processing technology, the gloss of the ink after the modulation is quite different.
Oil absorption refers to the ability of colorant to blend with oil, and is generally expressed by two oil absorptions. The first oil absorption is the amount of oil needed when the colorant is made into a paste; the second oil absorption is the amount of oil needed when a certain amount of the colorant is made into ink. The oil absorption is closely related to the particle size, dispersion, and wetting ability of the colorant, and also related to the water content of the colorant and the acidity of the oil. It is also affected by the lipophilicity, wettability, particle shape, and surface electrostatic properties of the colorant. And so on.
The colorant has a large oil absorption and the ink concentration is not easy to increase, which is not conducive to the adjustment of ink properties.
The dispersibility is generally related to the wetting ability of the colorant surface, the size of the surface area, the size of the crystal form, and the state of particle aggregation, which can reflect its dispersion in the bonding material. Dispersibility is related to the performance of the colorant.
Organic pigments in inks generally have some solubility. It is not a good thing that pigments can be dissolved by solvents, especially plastic inks, where the pigments are easily dissolved in the plasticizer and unreacted monomers in the plastic compounding agent. Lithographic inks will come into contact with water during printing, and soluble dyes cannot be used. Therefore, when making this type of ink, the pigment must be carefully selected.
The pigment used in the ink must be very fine particles, mostly 0.02-0.05μm, with a specific surface area of 40-100㎡/g and large surface energy.