The ink in the ink system of the printing press is quickly solidified after being transferred to the substrate, forming an ink film resistant to mechanical friction. This change from fluid to solid is called ink drying. The time required to complete this change is called the drying time. The drying time is short, the ink drying speed is fast, the drying time is long, and the ink drying speed is slow.

The drying of the ink is generally performed in two stages. In the first stage, the binder in the ink penetrates into the paper layer (or the solvent in the ink is volatilized). The adhesiveness of the ink film layer increases, and the stage of losing fluidity is called anchoring of the ink. The second stage is the stage where the ink undergoes physical and chemical changes and firmly adheres to the surface of the substrate, which is called ink curing. The fixation and curing of the ink are difficult to distinguish strictly throughout the ink drying process.

The drying speed of the print has a great influence on the print quality of the print. Drying is too slow, light to make the back of the printed product dirty, heavy sheets are caused by sticking; drying too fast, the ink in the layout of the crust, block patterns, not only to reduce the image clarity, but also make the ink pile up, prints appear " Ink blots." Therefore, analyzing the law of imprinting drying, controlling the drying speed of imprinting in production, and making the imprinting dry timely, it is possible to obtain high-quality prints with full ink, bright colors, rich layers, and high gloss.

The drying process of ink is an extremely complicated physical and chemical process. The drying speed of the ink is related to the form of the ink drying, the performance of the printing material, the printing process conditions, the temperature and humidity of the environment and other factors.

First, the factors that affect the drying rate of the print

The drying speed of the ink refers to the time required for the ink on the printed matter to completely cure. The shorter the time, the faster the drying of the print.

1. Ink drying form. Different substrates are printed on different substrates, and the ink is dried in different ways. Plain printing, letterpress printing, gravure printing, and stencil printing, the printing inks generally rely on infiltration, oxidation polymerization, volatilization, and UV (ultraviolet) drying.

1 infiltration drying. After the ink is transferred to the surface of the porous substrate, under the action of the capillary, part of the ink penetrates into the interior of the paper, and the other part of the ink is fixed on the surface of the substrate together with the pigment. The drying of the ink mainly depends on the capillary absorption of the substrate. Such ink drying is called osmotic drying. The process of osmotic drying is the process by which the substrate absorbs the binder in the ink. Because there are also tiny gaps between the ink pigment particles, the capillary action of these voids is to prevent ink penetration. When the capillary action of the substrate is balanced with the capillary action between the ink pigment particles, the binder in the ink stops penetrating into the interior of the substrate and the ink on the print is dried.

In the actual printing process, the penetration of the ink into the substrate undergoes two different stages. In the short period of time during which the ink on the plate is in contact with the paper, the ink penetrates into the surface of the substrate under the effect of the printing pressure. Pressurized infiltration; When the substrate leaves the embossing zone, the ink transferred to the substrate penetrates the inner layer of the substrate by the force of the capillary, for free penetration, pressure penetration and free penetration time, can be used below The formula shows:

Pressure penetration time:

tPressure=4ηh2 Pressurize/P-2r (7-9)

Free penetration time:

Tfree = 4ηh2free/rγ (7-10)

For the derivation of the above two formulas, see (7-3) and (7-4). Where η is the viscosity of the ink, h is the pressure at which the ink penetrates the interior of the substrate under pressure, h is the free penetration depth, r is the capillary mean radius of the porous substrate, and γ is the surface tension of the ink. The (7-9) and (7-10) formulas show that the use of low-viscosity inks allows printing on substrates with strong texture and porous absorption, and the inks can be dried quickly. High-speed letterpress rotary printing ink, which consists of mineral oil and asphalt resin, is printed on newsprint with good absorption. Drying of the print is completely based on osmotic drying. However, if the printing pressure is too high and the viscosity of the binder is too small, “through-printing” failures often occur.

2 oxidative polymerization type drying. The use of dry vegetable oil as the linking material produces an oxidative polymerization reaction with oxygen in the air, which causes the three-dimensionally distributed dry vegetable oil molecules to become giant molecules of three-dimensional network structure, and is dried on the surface of the substrate. This use of oxidative polymerization, the ink from the liquid to solid state, forming a solid imprint on the surface of the substrate, called oxidative polymerization drying.

The reaction of dry vegetable oils in inks from liquid to solid state is complex and generally requires the polymerization of peroxy bridges or the addition of co-conjugated double bonds. The reactions occur on the double bonds of unsaturated acids. As the reaction progresses, the molecular chain gradually increases, and the viscosity of the ink gradually increases until the unsaturated double bond of the acid is completely reacted and the ink is completely dried. Therefore, the more double bonds the dry vegetable ink has, the faster the oxidative polymerization will dry.

The rate of oxidative polymerization of dry vegetable oils is relatively slow. This is because dry vegetable oils contain trace phosphonate organic antioxidants, together with other retarding drying effects, they absorb less oxygen and are the induction period of the drying process. Dry vegetable oils do not change substantially during this phase. Therefore, the drying time of oxidative polymerization of dry vegetable oils generally takes several hours or even several days. To speed up the reaction rate of oxidative polymerization, it is necessary to destroy the antioxidants so that the dry vegetable oil absorbs oxygen in the air to form peroxides and initiates the polymerization reaction of the double bonds with peroxide as a medium.

In dry inks with dry vegetable oils, adding driers (ie, drying oils) can accelerate the drying of the ink. The addition of driers can, on the one hand, oxidize the antioxidants in the ink or combine with antioxidants to form precipitates, remove the interference of the antioxidants on the oxidative polymerization reaction, and shorten the induction period; on the other hand, the driers can also Promotes polymerization.

Lithographic printing is mostly based on coated paper with relatively close texture and low absorption. The ink on prints mainly depends on oxidative polymerization and drying.

3 volatile drying. The ink transferred to the surface of the substrate, the organic solvent molecules with higher energy in the binder material, overcomes the mutual attraction between molecules in the ink and escapes to the air. The remaining resin, pigment, and other components quickly thicken and consolidate. This form of drying the ink by solvent evaporation is called volatile drying.

Volatile drying inks, the link material contains volatile solvents, such as: water, benzene, xylene, esters, low carbon chain alcohols. The rate of volatilization and drying depends primarily on the volatilization rate of the solvent and the latent heat of evaporation of the solvent.

Table 7-16 Boiling Point, Latent Heat of Evaporation, Volatilization Rate of Common Ink Solvents

Solvent name Boiling range (°C) Latent heat of vaporization (kcal25/kg) Volatilization rate (s)

Ethyl acetate 72～80 401 85

Toluene 109～112 400 180

Ethanol 75～80 810 240

Water 100 2291 1120

The volatilization rate of the solvent can be expressed as the number of seconds (s) used to completely volatilize 1 ml of solvent on the filter paper. Latent heat of vaporization refers to a certain amount of solvent, and all of it becomes heat energy needed for gas. The same type of solvent, the lower the boiling point, the higher the volatilization rate; but different types of solvents, the boiling point is low, but the volatilization rate is not necessarily fast. Tables 7-16 list the boiling points, latent heat of evaporation, and volatilization rates for the solvents commonly used in several inks. The ethanol and ethyl acetate in the table have similar boiling points, but the volatilization rate of ethyl acetate is more than twice that of ethanol, because the latent heat of evaporation of ethanol is higher than the latent heat of evaporation of ethyl acetate. The smaller the latent heat of evaporation of the solvent, the higher the evaporation rate of evaporation.

Volatile drying ink, the main components of solvents, resins and pigments, due to the presence of resin molecules and pigment molecules in the ink surface layer, solvent molecules and resin molecules, pigment molecules have a strong traction between the solvent molecules difficult to escape to the air, Therefore, the volatilization rate of the ink is lower than the volatilization rate of the solvent.

Different resins have different degrees of slowness in solvent evaporation. The greater the solubility of the resin, the more difficult it is to remove the solvent. When solvents with different boiling points are used to adjust the volatilization drying rate of the ink, the resin in the ink must have a certain release property to the selected solvent. Otherwise, during the printing process, the solvent is difficult to remove, and the ink on the printed matter is poorly dried, and it is easy to have the problem of “back-scratching”.

In addition, the volatilization rate of the solvent is also related to the outside temperature and vapor pressure. The greater the outside vapor pressure, the higher the temperature and the faster the solvent volatilization speed.

Printing on the surface of non-absorbent substrates, the ink on prints mainly depends on the volatilization of solvents to complete the drying.

With the development of high-speed, multi-color and multi-type printing, most of the printing uses mixed drying ink printing. For example, letterpress resin inks mainly rely on osmotic drying, but they also contain oxidative polymerization drying; fast fixing of resin ink, the use of osmotic drying to accelerate the fixation of the ink, the use of oxidative polymerization drying to complete the ink curing; thermosetting ink is the use of volatile drying To accelerate the initial drying of the ink. Hybrid ink drying forms make the drying of prints more complicated.