In the production process of gravure ink, the affinity between carbon black particles is very strong. After dispersion in the medium, it is still easy to return to thick and return to thick, thereby affecting the film's color, luster and leveling of the wet film, and the occurrence of pull strips in printed products. Phenomenon, carbon black particles re-aggregated by van der Waals forces and hydrogen bonds, swelling and polymerization of carbon black will make carbon black particles flocculate and phase change. Take aggregates, carbon black particles will occur more serious flocculation phenomenon.
The close forces of the pigment particles include Van der Waals forces, electrostatic forces, and steric resistance. The van der Waals force between similar particles increases with the particle size, and decreases with the separation distance between the particles. Therefore, carbon black as a pigment plays an important role in the size and shape of carbon black particles in the grinding process.
1. Carbon black microstructure
Carbon black is well dispersed to obtain good ink properties. The dispersion of carbon black is the most difficult to disperse in all pigments. The reason is that the carbon black particles are fine (below 20 micrometers), the specific surface area is large, and the particles have a strong aggregation force to form a Cluster cluster.
There is a strong aggregation force between the carbon black particles, and it is more obvious when the carbon black is ground and dispersed using a conventional process. The dispersed carbon black particles are in an original dry hard state. After the ink is stored for a period of time, the interior is gradually swollen by the solvent and then wetted by the base resin. The swollen carbon black particles may cause the base material to be insufficiently wetted due to the increase in volume, so that part of the carbon black is exposed. Similar particles are recombined by van der Waals forces and hydrogen bonds.
1.1. The physical form of carbon black
There are two main physical forms of carbon black (see Table 1): fluffy and granular.
The carbon black produced by the oil furnace method is in the form of fluffy particles with high air content and low density. For ease of transportation, the air removed from carbon black is processed into spherical carbon black, and can also be processed into granular carbon black. Carbon blacks obtained by pyrolysis of hydrocarbons are generally platelet-shaped.
Table I Comparison of two physical properties of carbon black Low density of fluffy tablets with high granularity, difficulty in transport, difficulty in transport, and slightly higher blackness
The fluffy carbon black has a slightly higher brightness than the flake-like carbon black because the slightly dispersed state of the fluffy carbon black is slightly better than the flake-like carbon black.
The fluffy carbon black is densified into platelet-like carbon black, which improves the tightness of the aggregates, accelerates the wetting speed, and increases the attraction of van der Waals. All this must be overcome during the dispersion process. The granular carbon black is difficult to achieve. dispersion.
1.2. Primary Particles and Ultimate Particles
The basic structure of carbon black should remain as an aggregate of primary particles rather than individual particles. Primary particles are stable particles that do not split or aggregate.
The particle size of carbon black is determined by the diameter of the primary particle, because the primary particles tend to agglomerate to form an aggregate, that is, a Cluster cluster. Aggregates are related to carbon black structure. Carbon black formed by many primary particles forms aggregates with a large number of branches. It is called high-structure carbon black, and relatively few primary particles aggregate to form aggregates. Carbon black is called low-structure carbon black. .
The degree of dispersion of carbon black has a critical value. Carbon black is moderately dispersed into a single aggregate structure. The dispersion process should not destroy the aggregate structure of carbon black and make it an aggregate structure similar to primary particles, maintaining a dynamic and stable equilibrium. The aggregate structure becomes a kind of ultimate particle.
The primary particle carbon black structure is very dense, the size and shape of the primary particles determine the performance of the carbon black, and the carbon black structure affects the amount (viscosity) gloss and dispersibility of the carbon black (see Table 2).
Table II Structure and Performance of Carbon Black Low Structure and High Structure Air Content Wetting Properties Difficult to Use Difficult Glossiness Dispersion Easiness
Particle size affects the color and dispersibility of carbon black. The smaller the carbon black particle size, the weaker the reflected light, the darker the color, and the higher the color intensity, but the dispersion is also more difficult.
The low structural carbon black requires a lower amount of resin, a higher amount of pigment, and a higher gloss imparted to the ink. Low structural carbon black is less entrapped in air and more wettable, with lower fiscal costs. Aggregates of structural carbon black are more dense and difficult to disperse.
2. Flocculation mechanism
The electrostatic force between the pigment particles depends on the thickness of the double layer formed around the particles. The reduction in the thickness of the double layer leads to dispersion stability of the particles in the dispersed system. When some water is present on the surface of the pigment, the ionization of the ionizable species connected to the surface of the pigment is strongly promoted, resulting in a decrease in the thickness of the double layer. In order to obtain better dispersion stability of the carbon black, it is necessary to maintain the thickness of the double electric layer between the pigment particles at a certain value, and to remove or reduce the moisture between the pigment particles is advantageous for the dispersion.
The steric resistance comes from the steric resistance of a layer of adsorbent on the surface of the pigment particles to the neighboring particles. The system formed by space resistance and electrostatic force under certain conditions tends to be stable. The system is stable and space resistance must be formed. To form space resistance, each particle is required to be the ultimate particle, ie no more dispersed or aggregated.
A good dispersion stability effect requires that each newly formed particle surface is quickly wetted by the medium during the dispersion process, ie, it is isolated by the dispersed medium to prevent the regeneration of larger particles, and secondly, sufficient energy is required. Can prevent particles from colliding with each other and re-aggregating.
3, pigment dispersion and dispersion stability method
In practical applications, the fineness of carbon black is required to be 15 microns or less, and the high requirement is less than 5 microns. Actually, the degree of dispersion of carbon black has a critical value, and the pigment particle size can satisfy a certain particle size. The moderate dispersion of carbon black means that carbon black is dispersed into a single aggregate particle, and the surface of each aggregate is entirely composed of a resin. Covering, dispersion process does not destroy the carbon black aggregate structure into the ultimate particles.
The dispersion of the pigment particles in the medium includes the following three steps: first, the mixing stage, also called the wetting stage, the pigment aggregates are wetted by the medium, the entrained air is removed, and the carbon black is dispersed in the resin system; In the dispersion phase, also called the agglomeration phase, the carbon black aggregates are opened into independent primary particles or smaller aggregates by mechanical forces and eventually become particles; third, in the anti-agglomeration phase, the agglomerates break down into primary phases. Aggregates need enough energy to overcome the attraction between the aggregates, adsorb enough paint (which can be resin) to completely cover its surface, stabilize the primary particles or smaller aggregates, and prevent reaggregation.
The wetting of the pigment allows the air adsorbed on the surface of the raw material particles to be gradually replaced by the dispersion medium. There are many influencing factors, such as the particle shape, surface chemical polarity, the amount of air adsorbed on the surface, and the polarity of the dispersion medium. Winkler et al., gave the probability of dispersion of aggregates. From both energy and volume considerations, the direction of increasing the dispersion efficiency was given. Surface treatment of pigments weakened the agglomeration forces of the particles, which undoubtedly significantly reduced the dispersion. Energy consumption.
To reduce the aggregation of carbon black, carbon black needs to be pretreated. Since water is a flocculant, the moisture in carbon black should be reduced. On the one hand, the alcohols form hydrogen bonds with the water adsorbed on the surface of carbon black, and the water is replaced. On the one hand, the alcohol infiltrate the carbon black and the solvent in the binder resin occupies the position of the water, plays the role of dehydration, and then the binder resin Wet carbon black surface.
During dispersion grinding, the carbon black particles have been swollen by the solvent. Under the effect of grinding shear force, they are split into smaller final particles and completely wetted by the base resin to prevent secondary swelling and achieve anti-flocculation. purpose.
Source: cigarette packets ink information network
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