Water based ink pigments (2)

Third, the surface treatment of pigments

Ink manufacturers have to deal with a variety of pigments. For pigments that are not surface treated, they can be difficult to use, such as higher viscosity of the ink system, poor dispersion, and poor deflocculation stability. Pigments can only be suitable for aqueous ink systems after special surface treatments.

Waterborne resin binders are generally polar compounds. Of course, solvent water and isopropyl alcohol are also polar materials. As shown in FIG. 2, these resins are ionic carboxylic acid ammonia salts with many polar groups.

The pigment molecules generally include non-polar and polar substituents such as chloro, keto, amide, azo, and even ionic groups such as carboxyl, sulfonic acid, etc. 3 shows. Individual pigment molecules are stacked to form pigment crystals. Therefore, the pigment crystals include polar regions and non-polar regions.

The polar regions of the pigment are easily wetted by the polar aqueous binder, the non-polar regions are hardly wetted by the aqueous binder, and high surface free energy is maintained in the binder. Such a system is not stable. Therefore, pigment dispersions that have not been surface-treated produce flocculation to reduce their free energy.

1, the surface treatment mechanism

The surface treatment of pigments acts as an interface or bridge between the non-polar areas of the pigment surface and the aqueous binder, see Figure 4.

The left side shows that the non-polar region of the simple pigment crystals is in contact with the polar binder. The surface treatment is the “head” of the special treatment agent covering the non-polar region of the pigment, making it compatible with the polar binder. In addition, these "heads" also help the pigment particles to be isolated in space, allowing the pigment particles to easily disperse into the aqueous vehicle.

2, the surface treatment method

(1) Treatment with surfactants

Surfactants include anionic, cationic, amphoteric and non-particulate types. In general, surfactants suitable for pigments are anionic and nonionic, and the latter are particularly suitable for organic pigments.

One of the criteria for the selection of non-ionic surfactants is to make the HLB value (hydrophilic-lipophilic balance), each pigment has its specific HLB value, and if the selected surfactant has the same HLB value, it has the best The processing effect. British companies have published HLB values ​​for several typical pigments, as shown in Table 2.

U.S. companies applied for a patent in this regard. They used alkyl phenol polyoxyethylene derivatives and fatty alcohol polyoxyethylene derivatives as nonionic surfactants to treat pigments and make them easily dispersible in aqueous vehicles.

The alkyl tail of the surfactant is adsorbed in the non-polar region of the pigment and the "head" is oriented outward into the aqueous vehicle, see FIG.

(2) Treated with acrylic copolymer

There is a patent applied for the treatment of pigments with acrylic copolymers. The skeleton of the acrylic copolymer is adsorbed on the non-polar region of the pigment, and the carboxylic acid ions stick out and enter the aqueous binder, effectively shielding the non-polar regions of the pigment, as shown in FIG. 6 .

(3) Treated with Rosin Malay Resin

Since the resin molecule contains several carboxyl groups, the acid value is relatively high and a water-soluble ammonium salt is prepared to coat the pigment crystals. The polycyclic part of the resin is adsorbed, and the carboxylic acid ions protrude outwards, so that the non-polar regions of the pigment are isolated from the polar ones, as shown in FIG. 7 .

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