Seven factors affecting ink fading and discoloration
Time:2025-09-02
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There are generally two types of pigments in ink: inorganic pigments and organic pigments. Pigments generally have the following functions in ink.
1. Give ink various colors, and the amount used in the formula determines the color concentration of the ink;
2. Endowing ink with physical properties such as a certain thickness;
3. It is important to protect and give ink strong durability;
4. Some pigments affect the dryness of ink to a certain extent, with the oxidation film drying method being more pronounced.
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The fading and discoloration of ink is a common phenomenon, which is mainly caused by changes in the performance of the ink. There are also many reasons for the fading and discoloration of pigments in the ink.
1. Ink penetration and discoloration during the conjunctival process. During the printing process, the ink color of the printed material is darker when it is first printed, but after a period of time, the ink color becomes lighter. This is not the reason why the ink is not light resistant, but rather the reason why the ink penetrates and forms a film during the drying process. Light is directed onto the pigment particles through the connecting material, and the color light that is selectively absorbed by the pigment and then reflected appears as color. The deeper the light passes through the ink film, the more saturated the reflected color light becomes. So when the ink layer is thick, the color is darker, and when the ink layer is more transparent, the color is more vibrant and eye-catching. The ink layer of the newly printed product is thicker, but after drying, some of the connecting material in the ink will penetrate into the interior of the paper fibers. Therefore, the ink layer after drying should be shallower, and the color will become lighter. When printing, the color should be controlled to be slightly darker than the original color sample, so that the ink will meet the requirements of the original color after drying.
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2. Ink fading caused by lack of light resistance. For inks that require high resistance to fading and color loss, especially for outdoor applications, ink with good light resistance should be selected when adjusting the ink. Any ink will experience varying degrees of fading when exposed to sunlight or other light sources, so when mixing light and pale colored ink, the light resistance of the ink after dilution should be estimated. It is best to use phthalocyanine blue instead of peacock blue for light lake blue, as phthalocyanine blue is more light resistant and less prone to discoloration; Gray ink can be mixed with black ink and phthalocyanine ink in white ink; Green color can be achieved with monochromatic phthalocyanine green. If it is not yellow enough, bright resin yellow ink can be added. If it is more vivid, white ink can also be added. If peacock blue and chrome yellow ink are used, it is prone to yellowing over time.
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3. Ink may fade or discolor due to its lack of acid and alkali resistance. Malachite blue ink turns green when exposed to acid. The wetting solution used in offset printing is often acidic, with a pH value controlled between 4.5 and 6. Acidic wetting solution can affect the color change of the ink. In general, ink is not resistant to alkalinity. The gold ink used for gold printing and the electroplated aluminum foil used for hot stamping will turn into an ancient yellow color when exposed to alkaline substances, and will be dull and lack luster. The medium blue ink will fade when exposed to alkaline substances, and the chrome yellow ink will turn red when exposed to alkaline substances. Therefore, they cannot be used to print packaging prints containing alkaline substances. Taohong, emerald green, Qinglian, Pinlan, Sheguang blue and other inks are not alkali resistant, while peacock blue has better alkali resistance. Most of the paper is weakly alkaline. If printing alkaline substance packaging products such as soap, soap, and white alkali, the alkali resistance and saponification resistance of the printed products should be considered. During post press processing, it is important to avoid the use of alkaline adhesives that can cause discoloration of the printed product ink. If the foam alkali adhesive is alkaline and cannot be used, otherwise it is prone to discoloration. It is advisable to choose a pure natural, pollution-free, neutral and environmentally friendly adhesive with a pH value of around 7.
4. Loose and rough discoloration of paper. If the surface of the printed paper is rough and loose, such as offset paper, letterpress paper, news paper, kraft paper, etc., these types of paper have high absorption capacity, and the particles of ink pigments are small. During the slow drying process, they are gradually absorbed by the paper along with the connecting material, so the ink color will become lighter after drying. The absorption of ink by paper and the increase in the penetration of ink into paper can lead to the separation of binders and pigments, resulting in thinning, discoloration, and poor glossiness of the ink film.
5. Ink is not resistant to alcohol and changes color. There are ethanol, isopropanol, etc. in offset printing wetting solution. The printed products printed with alcohol wetting solution will change color after a long time, indicating poor alcohol resistance of the ink.
6. Color change caused by the addition of dry oil to ink. Dark colored ink should be mixed with red dry oil, and light colored ink should be mixed with white dry oil. The amount of dry oil added should not exceed 5% of the ink volume. If the amount exceeds 20%, the ink can change color. If Jin Hong ink is added with red dry oil, it is easy to turn brown. Red dry oil itself has a deep purple red color, and mixing with light ink may have some impact, but the effect is not significant when the amount of dry oil is small; White dry oil may not appear to have much color, but it appears light brown behind the conjunctiva. Therefore, when using white dry oil to adjust light colored ink, it accelerates the oxidation of the ink film and dries the conjunctiva, resulting in a bright color. However, after drying, it tends to be yellowish brown.
7. Ink is not heat-resistant and may discolor or fade. In general, the heat resistance of ink is around 120 ℃, while the heat resistance of golden red ink is even worse, only around 70 ℃. During the printing process, the ink runs at a high speed of 5000 sheets per hour on the ink roller and printing plate, generating heat through friction and resulting in a printing temperature difference of generally 40-50 ℃. Additionally, the ink also releases heat during the drying process. So, do not stack the printed matter with ink too thick. It should be ventilated, cool down, and prevent the ink from changing color due to heat.
8. Discoloration caused by chemicals in paper. During the papermaking process, pulp often contains sulfides or other compounds. The manufacturing methods are different, some papers are acidic or alkaline, and the ideal paper pH is 7, which is neutral. The acidity and alkalinity of paper can affect ink color, especially for light colored ink. Sulfides or acidic substances in paper react with inorganic pigments in ink. When ink binders and pigments penetrate into the paper fibers and the oxidized film on the surface of the paper and dry, they change color and darken the color; Iron compounds in paper can turn light colored ink brown, while gold ink for gold printing and electroplated aluminum foil for hot stamping are prone to blackening and have poor luster due to sulfide corrosion; If the white coating and adhesive on the surface of copperplate paper have strong alkalinity and react with acidic pigments such as iron blue, it is prone to discoloration.
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