การใช้สีอินทรีย์ในหมึกพิมพ์

One: Preface
With the emergence and development of ink. The pigment industry – especially the organic pigment industry – has grown considerably. At present, the widely used ink varieties are: offset printing ink, gravure ink, ultraviolet light curing ink, flexo ink, screen ink and special ink (such as printing ink).

Two: pigment selection of the ink system
Due to the system and application of the ink, the following main requirements for organic pigments are as follows:
(1) Color: Pigment is the chromophore of the ink, which is first required to be bright. Bright and well saturated;
(2) The coloring power pigment coloring power directly affects the amount of pigment in the ink, which in turn affects cost and ink;
(3) Transparency and hiding power are different for the transparency and hiding of the pigment due to the difference in printing method and substrate;
(4) Gloss: Due to the improvement of the gloss requirement of the printed matter, the requirements for the gloss of the pigment are also improved;
(5) Oil absorption: The oil absorption is generally related to the pigment particle dispersion, wettability, and moisture on the surface of the water. When the oil absorption of the pigment is large, the concentration of the ink is not easily improved, and the ink adjustment is difficult;
(6) Dispersibility: Dispersibility is directly related to the stability of ink performance is an important indicator. Generally related to the wettability of the pigment, particle size, crystal size, etc.;
(7) Physicochemical properties The application of printed matter is more and more extensive, so there are more and more requirements for the physicochemical properties of pigments, including: light resistance, heat resistance, solvent resistance, acid and alkali resistance and migration resistance.

The organic pigment used in the ink is mainly composed of an azo pigment (monoazo, disazo, condensed azo, benzimidazolone), a phthalocyanine pigment, a lake pigment (acid lake, alkaline lake). The following is a brief introduction to the pigment selection of several major inks.

(1) Offset printing ink
Offset inks currently have the largest dosage, and the amount used in the world market accounts for about 40% of the total ink, and domestically reaches about 70%. The choice of pigments used mainly considers the following:
1. The solvent of the system is mainly mineral oil and vegetable oil, so its system contains a certain carboxyl group (-COOH). Therefore, it is not possible to use a large alkaline pigment;
2. In the printing process, the ink should be in contact with the water supply roller, so the water resistance is good;
3. The ink layer is thinner during printing, so the concentration is high;
4. Offset printing uses more overprinting, so it requires good transparency. Especially yellow pigments.

(2) Solvent-based gravure ink
The solvents in such inks are mainly various organic solvents such as benzenes, alcohols, esters, ketones, etc. Different system solvents have different requirements for pigment selection, but in summary, the following should be considered as a whole. point:
1. The viscosity of the gravure ink itself is low, which requires the dispersibility of the pigment to be good. Good fluidity in the binder and no flocculation and precipitation during storage;
2. Due to the printing material, the solvent-based gravure ink is mainly volatile and dry, so it is required to have good solvent release when the system is dry;
3. Solvent resistance is better, no discoloration or fading occurs in the solvent system;
4. In the printing process, it should be in contact with the metal roller. The free acid in the pigment should not corrode the metal cylinder.
Alcohol-soluble and ester-soluble inks in solvent-based gravure inks are less toxic to humans. It is the future direction of development.
(3) UV curing ink (y ink)
UV inks have been widely used throughout the world in recent years. The annual growth rate of more than 10% is much higher than the total growth rate of ink. It mainly has three forms of offset printing, flexo printing and silk screen printing. Its drying method determines the pigment selection mainly considering the following factors:
1. The pigment will not change color under ultraviolet light. 2. In order to avoid affecting the curing speed of the ink, a pigment having a small absorption rate in the ultraviolet spectrum should be selected.
(4) Water-based ink
The water-based ink mainly adopts two types of flexographic printing and gravure printing. Since the aqueous ink is generally alkaline, it is not suitable to use a pigment containing ions which are easily reacted in an alkaline environment: in addition, the aqueous ink contains an alcohol-like solvent, so the pigment is required. Alcohol resistant. In the long run, water-based inks and UV inks are extremely environmentally friendly due to their extremely low VOC, and are the future development direction of inks. The development of organic pigments should also move closer in this direction.

Third: the structure of the pigment and the surface treatment of the same chemical structure and different crystals of the pigment, its color and performance are very different, such as copper phthalocyanine a-type is red light blue solvent unstable B type is green blue solvent stable. The important properties of the pigment's tinting power, transparency, oil absorption and weather resistance are directly related to the particle size of the pigment. The general rules are as follows:

1. The relationship between pigment particle size, shape and performance: the smaller the particle size, the better the light resistance and weather resistance. The solvent dispersibility is also relatively poor. The relationship between particle size and color light is relatively complicated.

Table 3:Relationship between particle size and shade
PigmentLarge particle sizeSmall particle size
YellowReddishGreenish
RedBluishYellowish
BlueReddishGreenish

The relationship between particle size and hiding power depends primarily on the critical value of the particle size. Above the critical value, the opacity increases with the decrease of the particle size, and reaches the maximum value at the critical value. Thereafter, as the particle size decreases, the opacity decreases and the transparency increases. In the ink system, the coloring power is the strongest when the particle diameter is from 0.05 μm to 0.15 μm. Further, when the particle diameter of the pigment is small, the inter-particle gap is large and the oil absorption amount is large.

2. Relationship between structure and properties of pigments The various properties of pigments have a great relationship with their molecular structure. We can improve its performance by introducing various groups into the pigment molecule:
(1) Introducing an amide group, a sulfonamide group or a cyclized amide group, which can increase the polarity of the molecule, thereby improving the light resistance, heat resistance, solvent resistance and migration resistance of the pigment:
(2) Introducing chlorine or other halogens to improve light and solvent resistance:
(3) Introduction of sulfonic acid groups or carboxyl groups can improve solvent resistance and heat resistance
(4) The introduction of nitro group can improve light and solvent resistance.

3. Dispersion and Surface Treatment of Pigments Currently, inks, especially gravure inks, tend to have low viscosity and high pigment content, and thus the dispersibility of pigments is increasingly demanding.
There is now a way to produce inks using pigmented wet cakes to improve the gloss and flow of the ink. From the general point of view, pigments for inks have an organic tendency, while the trend of organic pigments is environmentally friendly. Each pigment manufacturer should produce environmentally friendly pigments.