Factors affecting ink fading and discoloration
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The printing ink is composed of a coloring agent (pigment), a binder, a filler and an auxiliary material according to the suitability of the printing process and the printing substrate, so that the coloring agent (pigment) and the like are uniformly dispersed in the binder composed of the polymer compound. And a stable suspended colloidal substance is formed.
First, the pigment of the ink
There are two kinds of pigments for inks: 1 inorganic pigments; 2 organic pigments. The pigment itself is not luminescent. It is the color of the pigment object's selective absorption and reflection from light (including artificial light such as sunlight, fluorescent lamps, incandescent lamps, xenon lamps, xenon lamps, etc.), and is the expression of the structural form of the object.
1. The role of pigments in inks:
1 color the ink, and determine the concentration of the ink according to the amount of the ink;
2 to give the ink a certain thickness and other physical properties;
3 to maximize the durability of the ink;
4 to a certain extent affect the drying of the ink, the most obvious oxidized conjunctival type drying.
2. The binder of the ink is a fluid medium having a certain viscosity which acts as a dispersing pigment in the ink. Its role is as follows:
1 to give the ink a certain viscosity, viscosity, flow properties, thixotropic properties;
2 determines the drying type and drying speed of the ink;
3 determines the gloss of the ink, wear resistance, firmness and the like.
3. The role of auxiliary materials in the ink:
The auxiliary materials in the ink make some adjustments to the color tone, viscosity, consistency, dryness, and fluidity of the ink.
Requirements for auxiliary materials:
1 The fineness of the particles must be similar to that of the ink, not too thick;
2 can not affect the coloring power, adhesion and other properties of the ink;
3 can be well fused with the ink without any chemical reaction;
4 can not play a role in corrosion and chemical reaction on the plate.
Second, the ink fades, the cause of discoloration
The fading and discoloration of printed products are mainly ink performance problems, but some are due to the art of the printer, and the ink encounters acid and alkali, sulfides, alcohols such as methanol, ethanol (alcohol), glycerol, surface activity. Chemical substances such as substances are also highly susceptible to fading and discoloration problems:
1. Discoloration of ink penetration and conjunctival process: In the printing process, the ink color is darker when the printed matter is printed. However, the color of the ink will become lighter after a period of time. This is not the reason why the ink is not resistant to light, mainly ink. The reason for penetration and conjunctiva during the drying process. The light is irradiated onto the pigment particles through the binder, and the color light which is reflected and reflected by the pigment to the daylight is reflected. The deeper the light passes through the ink film, the more saturated the reflected light will be. Therefore, the thicker color of the ink layer is thicker, and the ink layer is more vivid and brighter than the transparent color. Just printed, the ink layer is thicker. However, after drying, a large part of the binder in the ink will penetrate into the inside of the paper fibers, so that the ink layer after drying is shallower, and thus the color becomes lighter. When printing, the color should be controlled to a degree slightly thicker than the original color, and then the ink will be dried to the original color requirements.
2. The fading of the ink due to the inability to resist light: When we adjust the ink, we should try to use the ink with good light resistance. Any ink will have different degrees of fading under the illumination of sunlight or other light sources. The ink should be estimated in the light color. Light resistance after the ink is diluted. For example, it is best to use phthalocyanine blue in the pale lake blue, not peacock blue, because phthalocyanine blue is more resistant to light and not easy to change color. Gray ink can add black ink and phthalocyanine ink in white ink; green color can be used in monochrome green phthalocyanine green, not yellow can add bright resin yellow ink, bright color can also add some white ink, if you use peacock blue and chrome yellow ink It lasts for a long time and is easy to yellow.
If the products we print are advertisements, posters, and new pictures that are attached to the outdoors, these products are attached to the outdoors and exposed to direct sunlight. Under the action of ultraviolet rays in the sunlight, the wind blows and rains for a long time. The bright color is easy to fade, and only the inks of black and blue are still present. Light color, yellow, and red inks are all faded, so in order to reduce the fading after the sun's ultraviolet rays are irradiated, it is best to print the y, m colors first in the four-color overprinting, and the C and BK colors are placed first. Printing in the back, in this way, the two colors of the last C and BK are light-resistant and heat-resistant, thereby reducing the fading of the printed product.
3. Paper loose and rough discoloration: If the printed paper is poorly smooth, the paper surface is rough, loose offset paper, letterpress paper, newsprint, kraft paper, etc., the paper is highly absorbent, the ink pigment particles are fine, and the drying process is slow. The ink gradually absorbs with the paper together with the binder, so the ink color fades. The absorption of ink by the paper and the penetration of the ink into the paper increase the separation of the binder and the colorant, causing the ink film to become thin, discolored, and the gloss is poor, and the absorption and penetration time of the paper is reduced, so that the corresponding increase must be made. Drying speed, increase the amount of red and white dry oil in the ink.
4. The ink is not resistant to the fading and discoloration of acid and alkali: the peacock blue ink will turn green when it meets acid, and the dampening solution of the offset printing is often acidic. The pH value is controlled between 4.5 and 6. The acidic dampening solution will affect the discoloration of the ink. The ideal dampening solution PH value = 7 is neutral, and the acid paper and dampening solution also resist the drying of the ink.
Under normal circumstances, the ink is not alkali-resistant, gold-plated ink, gold-plated electro-aluminized foil will become pale yellow when exposed to alkali, and dull and dull, medium-blue ink will fade when exposed to alkali, chrome ink The color of the base turns red, so it cannot be used to print printed products of alkaline substances. Ink, emerald green, green lotus, royal blue, radiance blue and other inks are not alkaline, peacock blue has good alkali resistance, most of the paper is weakly alkaline, if printing soap, soap, alkali and other alkali For the packaging of sexual substances, it is necessary to consider the alkali resistance and saponification resistance of the printed products.
In post-press processing, it is necessary to avoid the influence of the alkaline binder on the discoloration of the printed product. For example, the saponin binder is alkaline and cannot be used, otherwise it is easy to change color, and should be selected as natural, non-polluting, PH value = 7, a neutral environmentally friendly adhesive is preferred.
5. The ink is not resistant to alcohol and discoloration: there is ethanol and isopropyl alcohol in the offset dampening solution. Alcohol wetting has: 1 good wetting performance, good spreading performance in the layout; 2 due to the certain volatility of alcohol, after the moisture of the plate is transferred to the blanket, the water has a certain degree of volatilization, so the water absorption of the paper will Reduce; 3 alcohol can take away a lot of heat when volatilized, so that the plate temperature is lowered, ensuring the flow properties of the ink.
In addition to ethanol, the offset dampening solution uses alcohols such as glycerol and polyethylene glycol. The printed products printed with alcohol dampening liquid change color after a long period of time, indicating that the ink has poor alcohol resistance.
6. The ink is not resistant to heat and discoloration and fading: under normal circumstances, the heat resistance of the ink is about 120 ° C, the heat resistance of the gold red ink is even worse, only about 70 ° C, the ink is in the ink roller and the printing plate 5000 sheets during the printing process. / hour high speed operation, friction will generally heat at 40 ~ 50 ° C, the ink will release heat during the drying process, so that the printed ink does not accumulate too thick, to breathe through the air, reduce the temperature, to prevent heat discoloration. If the printed product is too thick and too close to 60~70 °C during the drying process, the gold-red ink will change color and fade, the light red color of the gold-red ink + white ink + yellow ink, and other light-colored inks often appear. The printing time is more vivid. After a period of time, the light meat color will turn yellow. This is because the gold red ink is not heat resistant. When the pressure cooker, the fire stove, the rice cooker and the like are packaged, the ink color is not heat-resistant and discolored and faded.
7. The discoloration caused by the desiccant dry oil added to the ink: the dark color ink should be added with red dry oil, the light color ink should be added to the white dry oil, the dry oil should not be added more than 5% of the ink amount, if the dosage exceeds 20% The above makes the ink discoloration. If the red red ink is added to the red dry oil, it will become brown. The red dry oil itself has a darker purple color. The blending of the light color ink will have some influence, but the small amount of dry oil is not too big. White dry oil does not look like any color, its essence is light brown after the conjunctiva, so when the light color ink is used with a lot of white dry oil, it accelerates the drying of the oxidized film of the ink film, and the color looks bright, but after drying Yellowish brown.
8. Discoloration caused by chemical substances in paper: During the process of rinsing and processing, the pulp often contains sulfides or other compounds. Due to different manufacturing methods, some papers are acidic or alkaline, and the ideal paper PH value. =7, neutral, the acidity and alkalinity of the paper will affect the color of the ink, especially the light color ink.
The sulfide or acid substance in the paper reacts with the inorganic pigment in the ink, and when the ink binder and the color material are dried while the oxidized conjunct film penetrating into the paper fiber and the surface of the paper is dried, the color is darkened.
The inclusion of iron in the paper can turn the pale ink into brown, and the gold-plated and bronched aluminum foil of the gold-plated gold is easily blackened by the sulfide and has poor gloss. If the white paint and the rubber on the surface of the coated paper are strong in alkali, they react with the acidic pigment iron blue, which is prone to discoloration.