UV pearlescent ink
UV pearlescent ink is a special type of ink with a pearlescent effect, produced by adding mica pearlescent pigments to UV ink. Pearlescent pigments are inorganic pigments composed of mica crystals coated with metal oxides with a high refractive index, such as titanium dioxide and iron oxide. The reflective or glittering effect of the mica crystals gives the pigment surface a pearlescent color. Mica is a natural silicate. Most mica deposits are not suitable for use as pearlescent pigments. Only muscovite KAl3O10Si3(OH)2 with a density of 2.7–3.1 g/cm3 and a hardness of 2.0–3.0 in the monoclinic system is suitable for use in mica pearlescent pigments. After the transparent muscovite crystals are processed into flaky particles, metal oxides such as titanium dioxide are coated on the surface of the muscovite flakes by chemical means, which not only improves the light and weather resistance of the muscovite surface, but also allows various interference colors to be obtained by adjusting the thickness of the color film.
In application, the pearlescent pigment is evenly dispersed in the coating and forms a multi-layered distribution parallel to the surface of the material. As in a pearl, the incident light will pass through multiple reflections and interference to achieve the pearlescent effect. This pearlescent effect is different from ordinary “absorbing” pigments and “metallic” pigments. The color it displays is rich and ever-changing. The human eye perceives different glosses when looking at the same point from different angles, and it also perceives different glosses when looking at different points from the same angle. Overall, pearlescent pigments give the impression that the light is coming from within or beneath the object.
Pearlescent pigments exhibit different effects depending on their particle size. In general, the larger the particles, the higher the degree of shimmer; the smaller the particles, the stronger the covering power of the base color and the lower the degree of shimmer.
Varying the thickness or type of metal oxide coated on the mica core can also bring about different color changes. Applying pearlescent powder to the packaging field will give you more unexpected and gorgeous enjoyment.
In addition, the advantages of pearlescent pigments lie in their good physical and chemical properties: they are resistant to water, acids, alkalis, organic solvents and heat, remain unchanged at 300°C, are non-conductive, have excellent lightfastness, are non-toxic, non-irritating to skin and mucous membranes and do not cause allergic reactions.
The use of metal ions can also produce colored pearlescent pigments. Different metals give the pearlescent pigments different colors. For example, products containing compounds such as Bi, Sb, As, Cd, Zn, Mn, and Pb have stable colors. In addition, the deposition of oxides of Au, Ag, Cr, In, Sn, Ni, Cu, Ge, Co, Fe or Al on the surface of mica titanium can enhance the reflection of light by the pigment and improve the pearlescent effect. See Table 3-23.
Table 3-24 describes the gloss of mica titanium at different particle sizes, Table 3-25 describes the relationship between the hue and film thickness and coverage rate of mica titanium, Table 3-26 describes the relationship between the geometric thickness and hue of mica titanium, and Table 3-27 describes Merck’s pearlescent pigments.
UV pearlescent inks can be printed on almost all materials, such as paper, plastic, metal, glass, ceramics, fabrics, etc., especially on paper and knitted fabrics.
Pearlescent pigments are inorganic pigments with relatively large particles. Although they are transparent, they also reflect ultraviolet light the most strongly. Therefore, the amount of pigment added must be adjusted according to the effect of the pearlescent coating. If too much is added, it will not only affect the ink viscosity, but also the light curing speed of the ink.
Pearlescent pigments have a lamellar structure and are very sensitive to shearing forces. Large shearing forces can destroy the pearlescent effect. Therefore, when making the ink, the pigments cannot be dispersed using conventional three-roller mills, ball mills or sand mills. They can only be dispersed using a high-speed mixer, and the mixing must be done at a low speed to avoid destroying the lamellar structure of the mica. Pearlescent pigments can be mixed with almost all natural and synthetic resins, and they have good wettability and dispersibility, especially in polyester resins and hydroxyl acrylic resins.
Pearlescent pigments should be wetted before being added to the ink vehicle. The wetting solvent should be compatible with the ink system. Good wetting allows the pearlescent pigment to be evenly dispersed in the ink vehicle, which is the basis for obtaining high-quality pearlescent printing results. Wetting also prevents the pearlescent pigment from dusting during dispersion. Due to their good dispersibility, pearlescent pigments can generally be dispersed well in low-viscosity systems using low-speed mixing.
The pearlescent pigment particles added to UV pearlescent inks all absorb, reflect or scatter ultraviolet light, making it difficult for ultraviolet light to reach the bottom of the ink layer and affecting the curing of UV pearlescent inks, especially at the bottom. Therefore, when preparing UV pearlescent inks, it is necessary to select suitable photoinitiators with high photoinitiation efficiency and deep curing, such as ITX, TPO and 819, etc., and sometimes use sensitizers such as EDAB in combination.
Pearlescent pigments can provide a brand new, personalized tone effect. They can be used alone or in combination with transparent conventional pigments. In addition, the color of the underlying paint, when layered with them, will produce even more surprising colors. The richness of the decorative effect can be extended almost infinitely. The interference color series of pearlescent pigments can be used alone or in combination with other conventional pigments. Interference colors can produce a variety of effects depending on the viewing angle.
①“Pearl white” effect: The silver white series of pearlescent pigments can be used alone or together with other traditional pigments. It should be noted that the pigments should be transparent and the concentration added should not exceed 3%. Due to the high transparency of this pearlescent pigment, the metallic luster effect is very strong.
The “pearlescent fantasy” effect: The interference color series of pearlescent pigments can be used alone or together with other traditional colors. The interference color will produce a variety of phantom colors as the viewing angle changes. Only one mixing method is not advisable, that is, mixing different interference color pigments, because the result is a dull gray.
③The “pearlescent gold and metallic” effect. This type of pearlescent pigment is different from traditional copper or aluminum powder pigments. In addition to the usual gold color, it also provides a more attractive variety of luster. The main colors are gold luster, purple copper, bronze, and red gold. If a small amount of carbon (0.001% to 0.05%) is added, a unique gold or copper effect can be produced. Silver gray can be produced by applying pearlescent silver white pigment to a black base or mixing it with a small amount of carbon black in the ink.
Unlike common ink pigments, the following characteristics of pearlescent pigments are very important for the printing result and therefore must not be overlooked when preparing and using the ink.
(1) Fragility of the pigment
Pearlescent pigments are made of titanium dioxide (or other metal oxides) coated on mica, and have a fragile, thin flaky structure that is easily damaged. When preparing pearlescent inks, do not use dispersing devices with high shear forces or grinding functions.
(2) Particle size of the pigment
The particle size of common organic pigments is 0.2–0.7μm, and that of carbon black is even smaller, at 0.02–0.08μm. However, the commonly used mica titanium pearlescent pigment (grade F) has a particle size of 25μm and a thickness of 0.2–0.5μm. The relevant parameters in printing must be adjusted, otherwise the transfer of pearlescent pigments will be greatly affected.
(3) Pigment arrangement
This is related to the thin flake pigment structure. When the pigment is evenly distributed in the ink coating, and most of the pigment particles are arranged parallel to the surface of the substrate, the luster obtained is the best. Otherwise, it will be greatly reduced. Therefore, attention should be paid to the leveling of the ink after transfer. Good leveling can ensure the quality of the thin flake pigment arrangement.
(4) Transparency of the pigment
The pearlescent effect mainly comes from the refraction and interference of incident light. If the ink coating has low transparency, the light that would otherwise be sufficient will be absorbed and lost. When selecting an ink vehicle or varnish, it is important to choose a material with as much transparency as possible. Therefore, when preparing UV pearlescent inks, transparent UV varnishes are commonly used as vehicles.
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