Analysis of printing consumables: factors affecting the fading of ink
Printing ink is composed of colorant (pigment), linking material, filler and auxiliary materials according to the suitability requirements of printing process and printing substrate material, so that colorant (pigment) and so on are evenly dispersed in the linking material composed of polymer compound. And formed a stable suspended colloidal substance.
1. Ink pigment
Ink pigments generally have two kinds: â‘ inorganic pigments; â‘¡ organic pigments. The pigment itself does not emit light. 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, dysprosium lamps, etc.), which is the expression of the structure of the object.
1. The role of pigments in ink:
â‘ Give ink color, and determine the ink concentration according to the amount of use;
â‘¡ Give the ink a certain thickness and other physical properties;
â‘¢ Maximize the durability of the ink;
â‘£To a certain extent, it affects the drying of ink, and the most obvious is the oxidative conjunctiva drying.
2. The ink binder is a fluid medium with a certain viscosity that acts as a pigment dispersion in the ink. Its role is as follows:
â‘ Give the ink a certain viscosity, viscosity, flow properties, thixotropic properties;
â‘¡ Determine the drying type and drying speed of the ink;
â‘¢ Determine ink gloss, abrasion resistance, firmness, etc.
3. The role of auxiliary materials in ink:
The auxiliary materials in the ink make some adjustments to the color, viscosity, consistency, dryness and fluidity of the ink.
Requirements for auxiliary materials:
â‘ The fineness of the particles must be similar to the ink, not too thick;
â‘¡Can not affect the performance of ink coloring force, adhesion, etc .;
â‘¢ Can be well fused with the ink without any chemical reaction;
â‘£ Can not play the role of corrosion and chemical reaction to the printing plate.
2. Causes of ink fading and discoloration
The discoloration and discoloration of printed products are mainly due to the performance of the ink, but some of them are caused by the art of printing, and the ink encounters acids, alkalis, sulfides, alcohols such as methanol, ethanol (alcohol), glycerin, surface activity Substances and other chemical substances are also very easy to cause discoloration and discoloration:
1. Ink penetration and discoloration in the conjunctiva process: In the printing process, the ink color is darker immediately after the print is printed, however, the ink color will become lighter after a period of time. This is not the reason why the ink is not lightfast, mainly the ink Causes of penetration and conjunctiva during drying. The light irradiates the pigment particles through the connecting material, and the colored light reflected by the selective absorption of sunlight by the pigment then exhibits color. The deeper the light passes through the ink film, the more saturated the colored light will be. Therefore, the thicker the ink layer is, the thicker the color is, and the ink layer is more transparent, the color is more vivid and dazzling. The ink just printed is thicker. However, after drying, a large part of the binder in the ink will penetrate into the interior of the paper fibers, so the ink layer after drying should be lighter, so that the color will become lighter. When printing, the color should be controlled to be slightly thicker than the original color sample, then the ink will meet the original color requirements after drying.
2. Ink discoloration due to light resistance: When adjusting ink, we should try to use inks with good light resistance. Any ink will have different degrees of fading under the sunlight or other light sources. Lightfastness after the ink is diluted. For example, it is best to use phthalocyanine blue for light lake blue, and do not use peacock blue, because phthalocyanine blue is relatively light-resistant and difficult to change color. Gray ink can add black ink and phthalocyanine ink slightly in white ink; emerald green color can use monochromatic phthalocyanine green; if it is not yellow enough, it can add bright resin yellow ink; if it is brighter, you can add white ink. , Long time, easy to yellow.
If the products we print are outdoor advertising posters, posters, new year pictures, etc., because these products are posted outdoors, they are directly exposed to sunlight, under the action of ultraviolet rays in the sunlight, the wind blows and rains for a long time. The bright color fades easily, and only black and blue inks are found. Light colors, yellow, and red inks all fade, so to reduce fading after exposure to ultraviolet rays from the sun, it is best to print y and m first in the color sequence for four-color overprinting, and put C and BK in the color sequence. After printing, in this way, the last two colors of C and BK overprinting have the effect of light and heat resistance, thereby reducing the fading of the printed matter.
3. Loose and rough discoloration of the paper: If the printed paper has poor smoothness, the surface of the paper is rough and loose, offset paper, letterpress paper, newsprint, kraft paper, etc., the paper has a large absorbability, and the ink pigment particles are fine, in the slow drying process As the binder is absorbed by the paper, the ink color becomes lighter. The absorption of ink by paper and the penetration of ink into paper will increase, which will lead to the separation of the connecting material and the color material, resulting in thinning, discoloration, and poor gloss of the ink film. To reduce the absorption and penetration time of the paper, it must be increased accordingly. Drying speed, increase the amount of red and white dry oil in the ink.
4. The ink is not resistant to fading and discoloration of acid and alkali: Peacock blue ink will turn green when it is acid, the wetting fluid of offset printing is often acidic, PH value is controlled between 4.5 ~ 6, acidic wetting fluid will affect the discoloration of ink The ideal pH value of the wetting fluid is 7, which is neutral. The acid paper and wetting fluid can also resist the drying of ink.
Under normal circumstances, the inks are not resistant to alkali. The gold ink printed with gold and the galvanized aluminum foil will become pale yellow when it is alkaline, and it is dull and dull. The medium blue ink will fade when it meets alkali, and the chrome yellow ink The alkali color turns red, so it can't be used to print the packaging of alkaline substances. Pink, emerald green, green lotus, magenta, and light blue inks are not alkaline resistant, and peacock blue has good alkali resistance. Most papers are weakly alkaline. If printing soap, toilet soap, white alkali, etc. For the packaging of sexual substances, the alkali resistance and saponification resistance of printed products should be considered.
In the post-printing process, it is necessary to avoid the influence of the alkaline-containing adhesive on the color change of the printed product ink. For example, the bubble base adhesive is alkaline and cannot be used. Otherwise, it is easy to change color. PH value = 7, which is suitable for neutral and environmentally friendly adhesives.
5. The ink is not resistant to alcohol and changes color: there are ethanol and isopropyl alcohol in the offset printing dampening solution. Alcohol wetting has: â‘ good wetting performance, and good spreading performance on the layout; â‘¡ due to the certain volatility of alcohol, after the moisture of the layout is transferred to the blanket, the moisture has a certain degree of evaporation, so the paper will absorb water Reduce; â‘¢ When the alcohol is volatilized, it can take away a lot of heat, so that the temperature of the printing plate is reduced, and the flow performance of the ink is guaranteed.
In addition to ethanol, offset printing dampening fluids also use alcohols such as glycerin and polyethylene glycol. The printed matter printed by the alcohol dampening fluid changes color after a long time, indicating that the ink has poor alcohol resistance.
6. The ink is not heat-resistant and discolors and fades: under normal circumstances, the heat resistance of the ink is about 120 ℃, the heat resistance of the golden red ink is worse, only about 70 ℃, the ink is on the ink roller and the printing plate 5,000 sheets during the printing process / Hour high-speed operation, friction will generate heat at 40 ~ 50 ℃, the ink will release heat during the drying process, so the ink piled printed products should not be stacked too thick, should be ventilated and ventilated, reduce the temperature, prevent heat discoloration. If the printed product is too thick during stacking and drying, it will be discolored and faded when it is too close to 60 ~ 70 ℃. The pale red color mixed with golden red ink + white ink + yellow ink, and other light pastel inks often appear. The printing time is relatively bright, and after a period of time, the light flesh color will become yellowish, which is due to the heat resistance of the golden red ink. When the pressure cooker, fire stove, rice cooker and other kitchen utensils are used in packaging, the ink color is often not heat-resistant and changes color and fades.
7. Discoloration caused by drying oil added to the ink: dark color ink should be added with red dry oil, light color ink should be added with white dry oil, the amount of dry oil should not exceed 5% of the amount of ink, if the amount exceeds 20% The above changes the color of the ink. If the red ink is added to the red oil, it is easy to turn brown. The red oil has a deeper purplish red color. It will be affected by the light ink, but the effect of the small amount of dry oil is not too big. The white dry oil does not seem to have any color. Its essence is light brown after the conjunctiva. Therefore, when a lot of white dry oil is used to adjust the light color ink, the drying of the oxidized conjunctiva of the ink film is accelerated, and the color looks bright. However, after drying Yellowish brown.
8. Discoloration caused by chemical substances in paper: due to rinsing and processing during papermaking, the pulp often contains sulfides or other compounds. Due to different manufacturing methods, some papers are acidic or alkaline. The ideal paper PH value = 7, it is neutral, and the acidity and alkalinity of the paper will affect the ink color, especially the light color ink.
The sulfide or acid substances in the paper react with the inorganic pigments in the ink. When the ink binder and colorant penetrate into the paper fiber and the oxidized conjunctiva of the paper surface is dried, the color changes and the color becomes darker.
The paper contains ferrite to make the light-colored ink brown, and the gold-printed gold ink and the gold foil galvanized aluminum foil are easily eroded by sulfide and become black and have poor gloss. If the white paint and compound on the surface of the coated paper have strong alkalinity, they will react with the acid pigment iron blue, etc., and they are prone to discoloration.
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