Water-based ink pigments (a)

One of the characteristics of solvent-based inks is that they contain 40-60% of volatile organic solvents. The ink is only dry after the solvent has evaporated. The large amount of organic solvents that have been volatilized is seriously polluting the environment. Moreover, most of the organic solvents are flammable and explosive substances. They are not safe during the production, transportation, storage, and use of inks.

Water-based inks are liquid inks that use water instead of organic solvents. In recent years, with the continuous development of the synthetic resin industry and the efforts of printing ink manufacturers, the performance objectives of water-based inks are becoming more and more perfect, and they are comparable to solvent-based inks. .

Water-based inks mainly include aniline inks, gravure inks for publication and gravure inks for packaging, etc. Although water-based aniline inks have a long history of production, they still have production, and there are still some problems in terms of variety and quality. In particular, water-based gravure inks for plastic films and other materials have yet to be improved. It can be considered that the prospect of water-based inks as pollution-free inks is broad.

Various types of aqueous inks generally contain the following components: water-based resins, alkali components, pigments, solvents, and additives. Waterborne resins are generally styrene and acrylic resins, and the basic components are organic amines such as dimethylethanolamine and monoethanolamine. The solvents are water and alcohol solvents. The additives include defoamers, dispersants and waxes. The performance of water-based inks depends mainly on the resins and pigments.

This article describes the pigments used in water-based inks, including methods for the preparation of high solids pigments, the surface treatment of pigments, and the use of pigments in aqueous inks. Since pigments are similar in the adaptation of aqueous aniline inks and water-based gravure inks, and the manufacturing principles of these two types of inks are similar, they are commonly referred to as aqueous inks without special distinction in this article.

First, the physical form of the pigment

1, powder pigment

The pigment used as a colorant is generally used in the form of a dry powder, which is obtained by subjecting a water-containing pigment filter cake to a dry-pulverization process. Such pigments generally do not contain any resin binders to avoid compatibility problems. This type of pigments are prone to dust pollution in the environment when used.

2, granular pigment

This is another dry pigment developed to solve the problem that powdery pigments are prone to dust flying. It is a hollow pigment aggregate that has been granulated by a special post-treatment process, due to the pigment particles that make up the aggregates. The mechanical force is very weak and easily dispersed in the binder.

3, Paste pigment

It is a pigment dispersion containing dispersants and ethylene glycol. Ethylene glycol prevents the pigment from drying and freezing. This form of pigment is only suitable for printing aqueous inks for absorbent substrates because the component ethylene glycol prevents the ink from drying on substrates such as polyethylene and aluminum foil. Pasty pigments containing resin binders and very small amounts of ethylene glycol have been developed.

4, pigment filter cake

Dispersing the pigment cake directly into the aqueous ink is a very attractive method. Since the pigment is in the form of an aqueous phase, the drying, crushing and redispersion of the pigment into the aqueous binder are avoided, the process is simplified, the drying is reduced, etc. The energy consumption of the process. Due to the unheated drying, the particles of the pigment are superior to those originally formed. However, the solid content of the pigment filter cake is generally about 18 to 25%, and it is incapable of acting as a water-based ink requiring a high pigment content.

The preferred aqueous pigment is a high solids pigment filter cake.

Second, high solids pigment cake

The high solids content pigment cake is made by the ordinary dehydration and thickening process of ordinary pigment filter cake, and the pigment content is generally controlled at about 50%. This type of pigment contains only pigments, water and a small amount of surfactant and does not contain resin binders or fillers.

1, the advantages of high solids pigment filter cake

(1) Widespread use. Because it does not contain resin binders and fillers, it can be used for various types of aqueous inks such as flexographic, gravure and other inks. In addition, it is also suitable for water-based printing pastes and water-based latex paints.

(2) Eliminating dust pollution to the environment. Because it is a semi-dry form, there is no dust flying in the application, which improves the operating environment and ensures the health of the ingredients.

(3) Improve product quality. Compared to the corresponding dry powder pigments, the aqueous inks made from high solids pigment filter cakes have higher tinctorial strength (generally 5 to 15% higher) and gloss, bright and bright, and have a higher dispersion stability. This is because when the filter cake is used, the particle size and shape of the pigment are mostly the same as when the pigment is formed, and when the pigment is heated and dried, a large amount of pigment aggregates and rearrangements of the crystal form are generated, and these factors all generate coloring power, hue, The reasons for the change in gloss and dispersion stability.

(4) The viscosity of the ink is low. Aqueous inks with low viscosity or high pigment content can be prepared using a high solids pigment filter cake. The viscosity of the ink containing a 31.5% by weight dry basis of a high solids filter cake is tested, as well as an ink containing 25% by weight dry pigmented pigments. The same value of viscosity.

(5) Simplify the manufacturing process of water-based inks. These pigments are easy to disperse, they only need to be dispersed at a high speed to achieve the fineness of the ink, simplify the ink manufacturing process, and greatly increase labor productivity.

2, the manufacture of high solids pigment filter cake

Methods for producing high solids filter cakes include mechanical dehydration, vacuum dehydration and post-treatment surfactant dehydration.

The KUTOBUKI rotary vane filter press in the United States is more effective than ordinary press basket filter presses in increasing the solids content of the pigment cake. See Table 1.

A U.S. company uses a vacuum-rolled two-roller machine to produce a high-solid-content pigment filter cake, as shown in Figure 1.

12 is a stainless steel roller, which is slotted 10, hole in the slot (11), and wrapped with a net cloth; 3 is a hopper with a vibrator; 4 hydraulic device, adjust the roller gap; 5 governor; 6 vacuum pump, The degree of vacuum is adjusted by 7; 9 is a doctor blade.

Example: green phase copper phthalocyanine blue pigment. The initial solid content of the filter cake is 30.7%. After dehydration and concentration by the device, the solid content of the filter cake is 46%.

Japanese companies used a surfactant to treat the pigment cake and then a second filtration process to produce a high solids filter cake.

Surfactants used include anionic surfactants such as naphthalene sulfonic acid formaldehyde condensate, sodium lauryl sulfate and the like; cationic surfactants such as lauryl trimethylammonium chloride; non-particulate surfactants such as polyethylene oxide Alkyl ethers and the like.

Example: A suspension of benzidine yellow pigment with a content of 5% was filtered at 3 kg/cm2 to obtain a filter cake with a solids content of 32.5%, and then 0.2% polyoxyethylene alkyl ether was stirred and mixed therewith. A second filtration under the same conditions gave a 52.5% high solids filter cake.

Another method is to mix the pigment dispersant with the pigment paste and concentrate with a semipermeable semi-permeation of the polymer. The permeable membrane is a polyacrylonitrile film.

The U.S. company has applied for a patent for a rosin derivative dispersant for preparing a high solids filter cake. The dispersant is formed by reacting a poly(lower alkylene) imine having a molecular weight of 1,000 to 15,000 with a polyester. The ester is the esterification product of hydroxystearic acid and tall oil rosin.