Factors that affect ink discoloration and discoloration

Printing inks are made of colorants (pigments), binders, fillers, and auxiliary materials in accordance with the suitability requirements of the printing process and printing substrates, so that colorants (pigments) are uniformly dispersed in the binder composed of macromolecular compounds. The formation of a stable suspension of colloidal material.

First, the ink pigment

Ink pigments generally have two kinds: 1 Inorganic pigments; 2 Organic pigments. The pigment itself does not emit light. It is the color that the pigment object presents to the selective absorption and reflection of light (including sunlight, fluorescent light, incandescent light, xenon light, and xenon light, etc.), and is a manifestation of the structure of the object.

1. The role of pigments in inks:

1 to give ink color, and according to the amount of use to determine the ink concentration;

2 give the ink a certain thickness and other physical properties;

3 to maximize the ink has a strong durability;

4 To a certain extent, the dryness of the ink is affected, and the oxidative conjunctiva type drying is the most obvious.

2. The ink binder is a fluid medium having a certain viscosity which acts as a dispersing pigment in the ink. Its role is as follows:

1 to the ink with a certain viscosity, viscosity, flow properties, thixotropic properties;

2 determine the ink drying type and drying speed;

3 determine the ink gloss, wear resistance, firmness and so on.

3. The role of auxiliary materials in ink:

The auxiliary material in the ink makes some adjustments to the hue, 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, and cannot be too coarse;

2 can not affect the ink color strength, adhesion and other properties;

3 can be well mixed with the ink, without any chemical reaction;

4 can not play corrosion and chemical reaction on the printing plate.

Second, ink fade, color change causes

The discoloration and discoloration of printing products are mainly ink performance problems. However, some are caused by the art of the printing industry, and the inks encounter acids, alkalis, sulfides, and alcohols such as methanol, ethanol (alcohol), glycerin, and surface activity. Substances and other chemical substances can also easily cause discoloration and discoloration problems:

1. Ink penetration and discoloration of the conjunctiva process: In the printing process, the color of the ink is darker when the print is printed. However, the ink color becomes lighter after a certain period of time. This is not the reason why the ink is not resistant to light. The main reason is ink. The cause of penetration and conjunctiva in the drying process. The light is irradiated onto the pigment particles through the connecting material, and the colored light reflected by the pigment and then absorbed by the light is reflected in color. The deeper the light passes through the ink film, the more saturated the reflected light is. Therefore, the ink layer is thicker and thicker, and the ink layer is more vivid than the transparent color. The newly printed product has a thick ink layer. However, after the drying, a large part of the binder in the ink will penetrate into the inside of the paper fiber, so the ink layer after drying will be lighter and thinner, so that the color will also become lighter. When printing, the color should be controlled to be slightly thicker than the original color sample. After the ink is dried, it will meet the original color requirements.

2. Ink fade due to light resistance: we should try to use ink with good lightfastness. Any ink under sunlight or other light source will have different degrees of fading. In the preparation of light color ink should be estimated The lightfastness of the ink is diluted. Such as the lake blue is best to use phthalocyanine blue, do not use peacock blue, because the phthalocyanine blue is more light, not easy to change color. Gray ink can be slightly black ink and white ink in the white ink; green color can use a single color phthalocyanine green, not enough yellow can add bright resin yellow ink, bright can also add a little white ink, if peacock blue plus chrome yellow ink , long days, easy to yellow.

If the products we print are outdoor posters, posters, pictures, etc., because these products are directly exposed to the sun's rays outdoors, under the effect of ultraviolet light in the sun, wind, rain, long time The bright color tends to fade, and only black and blue inks are still found. Faint colors, yellow and red inks are faded, so in order to reduce the fading after the irradiation of the sun's ultraviolet rays, it is better to print the y and m colors on the row when the four-color overprinting sequence, and C and BK are placed in two colors. Printed at the back so that overprinting in the final C and BK colors has a light and heat resistant effect, thereby reducing the fading of the prints.

3. Paper loose, rough discoloration: If the smoothness of printed paper is poor, rough paper surface, loose offset paper, letterpress paper, newsprint paper, kraft paper, etc., the absorption of paper is large, the pigment of ink pigment is fine, and the drying process is slow. As the binder is gradually absorbed by the paper, the ink becomes lighter. The absorption of ink by paper and the penetration of ink into the paper increase the separation of the binder and the colorant, causing the ink film to become thin, discoloration, poor gloss, and reduce the absorption and penetration time of the paper, which must be increased accordingly. Drying speed, increase the amount of red, white dry oil in the ink.

4. Ink is not acid and alkali fading, discoloration: peacock blue ink will turn green when the acid, offset wetting fluid is often acidic, PH value is controlled between 4.5 to 6, acidic wetting liquid will affect the ink discoloration The ideal wetting fluid PH value = 7, is neutral, acidic paper, wetting liquid at the same time resist the role of ink drying.

Under normal circumstances, the ink is not alkali-resistant, gold gold ink, bronzing foil encounter alkaline will become pale yellow, and dull matt degree, the blue ink met alkali will fade, chrome yellow ink case Alkaline color is red, so it can not be used to print alkaline packaging materials. Pink, green, green lotus, blue, light blue and other ink are not alkali-resistant, peacocks blue alkali resistance is better, most of the paper is weakly alkaline, if printed soap, soap, alkali and other alkali Sex packaging materials, we must consider the alkali resistance and saponification resistance of printed products.

In the post-press processing to avoid the use of alkaline adhesives on the ink color discoloration of printed products, such as basal adhesive binder is alkaline can not be used, otherwise easy to change, should choose those natural, no pollution, PH value = 7, suitable for neutral environment-friendly adhesives.

5. Ink is not resistant to alcohol and discoloration: Epoxy and isopropyl alcohol are present in the offset wetting fluid. Alcohol wetting has: Wetting performance is good, in the layout has a good spread performance; 2 because alcohol has a certain degree of volatility, the layout of moisture transferred to the blanket, the water has a certain degree of volatilization, so the paper will absorb the amount of water Reduce; 3 When the alcohol is volatile, it can take away a lot of heat, so that the plate temperature decreases, ensuring the ink's flow properties.

In addition to ethanol, offset wetting fluids also use alcohols such as glycerin and polyethylene glycol, and the printed products printed in the alcohol-wetting solution are discolored after a long period of 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°C, and the heat resistance of the gold-red ink is even worse, only about 70°C. In the printing process, the ink is 5,000 sheets on the ink roller and printing plate. / Hours of high-speed operation, the friction will be heated in general 40 ~ 50 °C, the ink will release heat during the drying process, so that the printing ink piled up not too thick, air ventilation, lower temperatures, prevent heat discoloration. If the printed product is too thick in the process of stacking and drying close to 60~70°C, the golden red ink will change color and fade, and the red and white ink + yellow ink will be mixed with the pale flesh color, and other light-colored inks will appear. When printing is relatively bright, after being placed for a period of time, the pale flesh color will turn yellowish, which is why the golden red ink is not heat-resistant. Pressure cookers, fire stoves, rice cookers and other kitchen utensils are often used when the color of the ink is not heat-resistant and discolored and faded.

7. The discoloration caused by the desiccant drying oil added in the ink: dark-colored ink should be added with red dry oil, light-colored ink is added with white dry oil, and the amount of dry oil added is not more than 5% of the amount of ink, if the amount exceeds 20% The above changes the color of the ink. If the golden red ink is easily added to the red dry oil, it will turn brown, and the red dry oil itself will have a dark purple color. The deployment of the pale color ink will have some influence, but the small amount of dry oil will not be too great. White dry oil does not look any color, its essence is light brown after conjunctiva, so that when the pale color ink is dry with a lot of white oil, it accelerates the drying of the oxidized conjunctiva of the ink film, the color looks bright, but after drying it will Yellowish brown.

8. Discoloration caused by chemical substances in paper: During the rinsing and processing of paper, pulp often contains sulfides or other compounds. Due to the difference in manufacturing methods, some papers are acidic or alkaline, and the ideal paper pH value = 7, is neutral, the acidity of the paper will affect the ink, especially the impact of light-colored ink.

Sulfide or acids in the paper react with the inorganic pigments in the ink, causing discoloration and darkening of the color when the ink binder and colorants dry on the oxidized conjunctiva that penetrates into the paper fibers and the surface layer of the paper.

The inclusion of iron in the paper can turn the pale ink into brown, and the gold foil of the gold and the galvanized aluminum foil are eroded by sulfides and become black and have poor gloss. The white paint on the surface of the coated paper and the rubber compound are strongly alkaline, and react with the acidic pigment iron blue, etc., and are prone to discoloration.