Drying of the hottest ink and rubbing on the back

  • Detail

The drying of the ink and the back of the printed matter rubbing dirty (Part 1)

the ink in the ink conveying system of the printing machine will solidify rapidly 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 process is called drying time. Short drying time, fast ink drying speed; Long drying time and slow drying speed of ink

ink drying is generally carried out in two stages. The first stage is the stage when the binder in the ink penetrates into the paper layer (or the solvent in the ink volatilizes), the adhesion of the ink film increases and the fluidity loses, which is called the fixation of the ink. The second stage is the physical and chemical changes of the ink, which is firmly bonded to the surface of the substrate, which is called the curing of the ink. It is difficult to distinguish the fixation and solidification of ink in the whole process of ink drying

the drying speed of the imprint has a great influence on the printing quality of the printed matter. Drying is too slow, which may cause the back of the printed matter to rub dirty, or cause the printing sheet to stick to the page; Drying too fast, the ink crusts in the layout and blocks the pattern, which not only reduces the image definition, but also causes the ink to accumulate, resulting in "ink spots" on the printed products. Therefore, by analyzing and studying the law of imprint drying, controlling the speed of imprint drying in production and making the imprint dry in time, we can obtain high-quality prints with full ink color, bright color, rich layers and high gloss

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

I. factors affecting the drying speed of imprint

the drying speed of ink refers to the time required for the ink on the printed matter to completely solidify. The shorter the time, the faster the imprint drying promotes the promulgation of the Interim Regulations on express delivery

1. Ink drying form. On different substrate surfaces, different printing methods are used for printing, and the drying form of ink is also different. For common printing such as lithography, relief, intaglio and hole printing, ink on the printed matter generally depends on penetration, oxidative polymerization, volatilization and UV (ultraviolet) drying

① osmotic drying. After the ink is transferred to the porous substrate surface, under the action of capillary, part of the ink is connected into the paper, and the other part of the binder is fixed on the substrate surface together with the pigment. The drying of the ink mainly depends on the capillary absorption of the substrate. This kind of ink drying is called osmotic drying. The process of osmotic drying is the process that the substrate absorbs the binder in the ink. Because there are also tiny voids between the ink pigment particles, the capillary effect formed by these voids is to prevent the ink from penetrating. 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 substrate, and the ink on the printed matter dries

in the actual printing process, the penetration of ink into the substrate goes through two different stages. During the short time when the ink on the printing plate contacts with the paper, the ink penetrates into the surface of the substrate under the action of printing pressure, which is pressure penetration; When the substrate leaves the embossing area, the ink transferred to the substrate penetrates into the inner layer of the substrate by the force of capillary, which is the time of free penetration, pressurized penetration and free penetration, which can be expressed by the following formula:

pressurized penetration time:

tpressure =4 η H2 pressurization/p-2r()

free permeation time:

tfree =4 η H2free/r γ ()

see equations () and () for the derivation of the above two equations. Where η Is the viscosity of the ink, h is the penetration depth of the ink into the substrate under pressure, h is the free penetration depth, R is the porous "China attaches great importance to the development of the new material industry, and the average capillary radius of the substrate, γ Is the surface tension of the ink. The formulas () and () show that the printing ink with low viscosity can be dried quickly on the substrate with loose texture and strong porous absorption. High speed relief rotary ink, the binder is composed of mineral oil and asphalt resin. This ink is used to print on paper with good absorption. The drying of the imprint is completely completed by infiltration drying. However, if the printing pressure is too high and the viscosity of the binder is too small, the "through printing" fault often occurs

② oxidative polymerization drying. The ink with dry vegetable oil as binder will undergo oxidative polymerization with oxygen in the air, so that the dry vegetable oil molecules distributed in three-dimensional space will become giant molecules with three-dimensional structure, which will be dried and fixed on the surface of the substrate. This use of oxidative polymerization reaction, so that the ink from liquid to solid, in the substrate surface to form a solid imprint, called oxidative polymerization drying

the reaction of the dry vegetable oil in the ink from liquid to solid is very complex. Generally, it needs to be polymerized by peroxide bridge or the industrial grade lithium carbonate production line with an annual output of 10000 tons of lithium mica produced by tonggong Jiangxi Jinhui lithium industry was put into operation in early September. The conjugated double bond addition polymerization takes place on the double bond of unsaturated acid. With the progress of the reaction, the molecular chain increases gradually, and the viscosity of the ink also increases gradually until all the double bonds of unsaturated acid are reacted, and the ink is completely dry. Therefore, the more double bonds of dry vegetable ink, the faster the drying speed of oxidative polymerization

the oxidative polymerization reaction speed of dry vegetable oil is relatively slow, because dry vegetable oil contains a small amount of phosphorus ester organic antioxidants. In addition, due to other effects of blocking drying, it absorbs less oxygen, which is the induction period of drying process. During this stage, dry vegetable oil basically does not change. Therefore, the drying time of oxidative polymerization of dry vegetable oil generally takes several hours, or even as long as several days. In order to accelerate the reaction speed of oxidative polymerization, the antioxidant must be destroyed to make the dry vegetable oil absorb the oxygen in the air and form peroxide, which is used as the medium to initiate the polymerization of double bonds

adding a drying agent (i.e. drying oil) to the ink with dry vegetable oil as the binder can accelerate the drying of the ink. The addition of driers, on the one hand, can oxidize the antioxidant in the ink or combine with the antioxidant to form precipitation, so as to remove the interference of the antioxidant on the oxidative polymerization reaction and shorten the induction period; On the other hand, driers can also promote polymerization

for lithography, coating paper with relatively tight texture and low absorption is mostly used for printing, and the ink on the printed matter is mainly dried by oxidation polymerization

③ dry volatiles. For the ink transferred to the substrate surface, the organic solvent molecules with large energy in the binder overcome the mutual attraction between the molecules in the ink and escape to the air, and the remaining resin, pigment and other components are thickened and consolidated rapidly. This depends on solvent evaporation to complete ink drying, which is called evaporation drying

volatile dry inks and binders contain highly volatile solvents, such as water, benzene, xylene, esters, alcohols with low carbon chains, etc. The speed of evaporation drying mainly depends on the speed of solvent evaporation and the latent heat of solvent evaporation

table boiling point, latent heat of evaporation and volatilization rate of common solvents for inks

solvent name boiling point range (℃) latent heat of evaporation (kcal25/kg) volatilization rate (s)

ethyl acetate 72 ~ 8040185

toluene 109 ~ 112400 180

ethanol 75 ~ 80810240 in recent years

water 100 22911120

volatilization rate of solvent can be expressed by the number of seconds (s) it takes 1ml of solvent to completely volatilize on filter paper. Latent heat of evaporation refers to the heat energy required for a certain amount of solvent to become a gas. For the same solvent, the lower the boiling point, the higher the volatilization rate; However, different types of solvents have low boiling points, but the volatilization rate is not necessarily fast. The table lists the boiling point, latent heat of evaporation and evaporation rate of solvents commonly used in several inks. The boiling points of ethanol and ethyl acetate in the table are similar, but the evaporation rate of ethyl acetate is more than twice that of ethanol, because the latent heat of evaporation of ethanol is higher than that of ethyl acetate. The smaller the evaporation latent heat of the solvent, the higher the evaporation drying rate

volatile dry ink is mainly composed of solvent, resin and pigment. Since resin molecules and pigment molecules exist on the surface of the ink, there is strong traction between solvent molecules, resin molecules and pigment molecules, which makes it difficult for the solvent molecules to escape into the air. Therefore, the volatilization speed of the ink is lower than that of the solvent

different resins slow down the solvent volatilization rate differently. The greater the solubility of the resin, the more difficult it is for the solvent to be removed from it. When using solvents with different boiling points to adjust the evaporation and drying speed of the ink, the resin in the ink must have a certain release to the selected solvent. Otherwise, during the printing process, it is difficult to remove the solvent, and the ink on the printed matter is not dry well, which is easy to cause the fault of "back rubbing dirty"

in addition, the volatilization rate of the solvent is also related to the external temperature and vapor pressure. The higher the external vapor pressure is, the higher the temperature is, the faster the volatilization rate of the solvent is

when printing on the surface of non absorbent substrate, the ink on the printed matter mainly depends on the volatilization of solvent to complete the drying

with the development of high-speed, multi-color and multi variety printing, mixed dry ink is mostly used in printing. For example, letterpress resin printing ink mainly relies on penetration drying, but also includes oxidative polymerization drying; Fast fixing resin ink, using penetration drying to accelerate the fixation of the ink, using oxidative polymerization drying to complete the curing of the ink; Thermosetting ink is the use of volatile drying to accelerate the initial drying of ink. The mixed ink drying form makes the drying of imprinting more complicated

Copyright © 2011 JIN SHI