Analysis of Physical and Chemical Changes in Color Printing

Color printing is a comprehensive technology that includes theories of mechanics, physics, and chemistry. With the continuous development of color printing technology, many physical and chemical changes are often involved, and it is more important to be able to grasp the laws of these changes. To reduce the unnecessary waste of ink resources, to seek solutions and improve product quality, it is necessary to explore and study these issues in order to further master and apply them theoretically and practically. Several common problems are discussed in general and solutions are discussed.

First, the relationship between ink color and spectral color

The spectrum consists of seven colors: red, orange, yellow, green, cyan, blue, and violet. There are various different subtle changes from one color to another, resulting in many different colors.

The color in printing is the result of the selective absorption and reflection of light by pigment particles. The composition of the ink color and the spectrum color is different, for example, when the yellow light and the blue light are mixed, white light is generated, and when the yellow ink and the blue ink are mixed, a green ink is generated because the ink color is formed by the pigment molecules. The composition, because the yellow ink molecule and the blue ink molecule are mixed, each has its reflected light wave reflected in human vision, and there are three types of nerve endings on the retina according to the visual principle. If one of the nerve endings is stimulated, it will produce a red feeling. The second stimulus produces a blue sensation, and the third stimulus produces a yellow sensation. If three nerve endings are stimulated to the same degree at the same time, a black sensation is produced. If three or two nerve endings are stimulated to varying degrees, a variety of different color sensations are produced.

In our production, for the discrimination of two or more colors, the judgment is made in terms of hue, brightness and saturation, that is, the terms thick, light, dark, and light in printing. In essence, the so-called dark and light colors are used to refer to the position of the pigment that absorbs light waves in the spectrum. The shorter the spectral wavelength absorbed by the pigment, the lighter the color. For example, yellow and orange are called light colors because It absorbs violet and blue light (shortest light wave). Conversely, the longer the wavelength at which the pigment absorbs light waves, the darker the color, and the reason why blue and green are called dark colors is because of the absorption of yellow light (longer light waves). Therefore, the order from light to dark is yellow, orange, red, purple, blue, and green.

As for the shade of color, it is actually the intensity of the color, and the degree to which the object absorbs light in a certain band is the size of the extinction coefficient. The greater the extinction coefficient, the darker the color. If a dark yellow color, it must be very good absorption of blue light, which is a large extinction coefficient of blue light. Conversely, the smaller the extinction coefficient.

Second, the color print color formation

With three or four different color inks, you can print a rich variety of screens. Why use three or four colors to print thousands of colors? The reason is:

1. Different dot sizes of the printing plate form the shade of ink.

2. Different colors, juxtaposed or overlapped reflect different colors of light, so that people's visual experience many colors.

3. Misc. Blending: When we stack a piece of yellow glass and a piece of blue glass and look into the sunlight, we see green. This is because when white light passes through the yellow glass, blue, blue and purple are blocked and only passed. Red, orange, and yellow were also blocked, leaving only the green light. So the green we see is not a mixture of yellow and blue light, but the light that is subtracted from the rest.

The yellow and blue inks used for printing are basically transparent, like a layer of color film. Due to the reflective effect of white paper, it is like looking at yellow and blue glass, which makes us see the green color. This is caused by the subtractive color method. This is the color The main cause of printing. In fact, in the printing, the yellow dot and the blue dot are not completely overlapped. Why can they appear green? This is because of the cause of the shading. Although the yellow and blue dots are very small, they reflect different light waves and enter our eyes, which can still form a green sensation on the retina. This is called "space mixing."

Third, the relationship between pigments and binders in ink

In the ink, the pigment is suspended in a highly dispersed state in a viscous binder, and the surface of the pigment particle strongly adsorbs a binder having a certain thickness to become a very firm protective film. The protective film prevents the mutual attraction of the pigment particles to a large extent to a certain extent, ensuring the transferability of the pigment particles during the printing process. The more peripheral material of the protective film is the dispersion medium. Because the binder material is quite viscous, and because the dispersion medium and the protective film of the outer layer of the pigment particles attract each other, they become a stable suspension colloid. The structure of the ink can be said to be "a thick and stable colloidal system having a protective film layer pigment particles highly dispersed and suspended in a sticky binder material".

Because of the different structural shapes of pigment particles, some are regular crystals, some are irregular polygons, some are smooth and some are porous, so the protective film that is composed of it is also thick and thin, and it is solid or not. Minute. The pigment is irregularly polygonal or loosely porous. The protective film is thick and firm. The more stable the suspension colloid is (such as the carbon ink is irregular and porous. The black ink made of charcoal ink is very stable, and the printing is transferred. Better;) pigments are regular crystalline or smooth surface, the protective film layer is relatively thin and relatively weak (such as relatively uniform group particles, the surface is smooth, the county blue ink made of ultramarine, poor colloidal stability, printing transfer Poor sex.)

Due to the difference in stickiness and thinness of the binder, the resulting pigment protective layer is also thick, thin, strong, and somewhat weak. At the same time, the binder is the dispersion medium in the colloidal system, and the dispersion medium is sticky. The colloidal structure is also relatively stable, otherwise it is unstable. For example, when adjusting the ink, adjusting the ink with too much thin varnish often affects the structure of the ink, causing the structure of the ink to be destroyed in the printing and causing a paste plate or a plate phenomenon.

Fourth, the ink's cohesion and adhesion

Cohesion is the interaction between molecules inside the material, and adhesion is the force of attraction between two different substances.

The ink is a suspended colloid whose cohesive force is related to the mutual attractive force between the pigment particles having the protective film, and also to the intermolecular attraction of the link material itself. In general, if there are few binders adsorbed by the pigment and the distance between the pigment particles with the protective layer is large, the cohesion is mainly related to the attraction between the molecules of the binder itself. The binder is more viscous and cohesive. If there are many binders adsorbed by the pigment and the distance between the pigment particles with the protective film is small, then the cohesion force has a considerable relationship with the attractive forces between these pigment particles in addition to the intermolecular molecular attraction of the binder.

Fifth, ink viscosity and fluidity

The resistance caused by the relative movement of a layer of an object and another layer of fluid, called friction, is proportional to the relative speed of the movement and the size of the contact surface. The viscosity of the fluid can be seen as a function of the internal friction of the fluid. Internal friction and cohesion are positively related under certain conditions, so fluid viscosity and cohesion are also positively related.

Physically, fluidity is called fluidity. It is the reciprocal of viscosity, so the viscosity has a large fluidity and the viscosity has a small fluidity. Since the ink is a non-ideal plastic fluid, its viscosity is different from that of an ideal fluid, and the force and the deformation rate are not linear but arc curves. The yield value of the ink can be considered to be due to the plasticity of the ink. The thicker the ink is, the larger the plasticity is and the greater the yield value is.

Fluidity is the reciprocal of viscosity in ideal fluids. However, the non-ideal plastic fluid such as ink is not so simple, and the fluidity is different from that used in daily production or usage. The fluidity of the ink is generally regarded as the degree of natural flow of a certain amount of ink on a flat surface over a certain period of time without external force. In addition to being inversely proportional to viscosity, plasticity and thixotropy are extremely important, and plasticity and thixotropy are less fluid.

In printing, the ink is not under the ink fountain. It is often regarded as poor fluidity. In fact, it can only be said to be poor in fluidity, or too plastic, or too thixotropy. Most fluid inks are generally more fluid, but they are not absolute. The fluidity of the ink is too large, the printing web is easy to expand, and the printing effect is not good. Excessive viscosity of the ink can cause problems such as flying ink, pulling the paper, etc. Viscosity is too small and can cause paste and other ills. Therefore, the fluidity and viscosity of the ink should be controlled within a certain range. This range varies with the color of the ink. The difference is also different.

There are many factors that affect the viscosity of the ink and can be summarized as follows:

1. The viscosity of the ink is related to the binder: For inks using the same pigment, the viscosity of the binder is large, and the viscosity of the ink produced is also large. The linker viscosity referred to here means the total viscosity of the liquid medium including the solvent in the ink component.

2. The viscosity of the ink and the amount of pigment and filler used: For the same binder, if the amount of pigment and filler is not so large as to look like an ink, the amount of pigment and filler is increased. The greater the viscosity of the ink.
3. The viscosity of the ink and the pigment are related to the particle size of the filler: For the same binder, if the particles of the pigment and filler are larger, the viscosity of the ink becomes smaller. "China Printing Industry Annual Report", the more particles Small, the greater the viscosity. The particle size here refers to its original particle size rather than the size of the particles that the pigments and fillers reassemble into a mass during the production process.

4. The viscosity of the ink and the dispersion of the pigment and the filler: The better the pigment and filler are dispersed in the ink binder, the lower the viscosity of the ink, and vice versa. This is for the same pigment and binder at a certain ratio.

Six, ink plasticity and thixotropy

When solids are subjected to an external force and exceed their elastic limit, they will undergo unrecoverable deformation. This property is called plasticity. Without external force, the fluid deforms automatically according to the shape of the container under the action of its own gravity, which is called fluidity. The ink is a thick suspension colloid between fluid and semi-solid, so it has both fluidity and plasticity. The higher the pigment and other solids content, the thicker the ink, the greater the plasticity; the lower the pigment and other solids content, the thinner the ink, and the less malleable. Offset printing inks, in general, should have a certain degree of plasticity, so that the printing dot can not be enlarged during the printing.

Thixotropy refers to the gradual thickening of the ink after standing for a certain period of time, the viscosity becomes large, and after the agitation, the viscosity of the ink becomes slightly reversible. Because the pigment particles are irregular, even though a layer of binder material is adsorbed, it is not a regular sphere. Therefore, after a certain time of standing still, the pigment particles will be in close proximity or contact with each other in certain parts and attract each other. Impedes the free movement of the particles and therefore becomes sticky. However, if this temporary stable structure is stirred by external forces, it will be quickly destroyed and the mutual attraction between the particles will be lifted. The free movement of the particles is relieved and becomes thin and easy to flow. In general, the more irregular and porous the pigment particles, the greater the thixotropy of the ink (such as black ink). In contrast, the influence of the different binders on the thixotropy is also great. For example, the thixotropy of the polymer vegetable oil is small. , Rosin resin and other polymer compounds dissolved in mineral oil engine oil and other rare media made of the varnish, the thixotropy of the ink produced.

In general, inks are expected to be small, and the thixotropy of the printed newspapers and inks will not attract ink when they are turned on. The thixotropy of offset ink is generally not large, and if the thixotropy is too large, it will cause poor inking. After starting the machine for a long time, the ink will not smoothly turn with the ink roller, or even leave the ink roller, causing the phenomenon of overprint and fade.

Seven, quick-drying light ink characteristics

Fast-drying glossy resin ink is a new type of ink, which has the characteristics of light resistance, acid resistance and solvent resistance, in addition to bright colors, extremely high concentration, and exceptionally strong gloss. It is made of a diphenol-based phenol resin based on tung oil, linseed oil or a part of oleic acid resin and a solvent as an edge binder.

The so-called quick-drying, in fact, means that once the ink is printed on the paper, it can be quickly fixed in a short time (fixing speed of about 35 minutes), this time, although the ink has not yet fully dried, but if you do not have strong friction, Will not stain, thus ensuring the smooth progress of multi-color high-speed printing. The so-called fixation, in fact, is that when the ink is printed on the paper, the high-boiling oil portion of the resin binder in the ink penetrates quickly into the gaps of the paper fibers, so that the concentration of the resin binder immediately becomes larger and thickens, and is fixed with the pigment. on the paper.

Fast-drying glossy resin inks are suitable for papers with a certain absorption and smooth surface (such as coated paper). The printed product has good dryness and gloss. However, for papers with large absorbing surface porosity, it is not easy to form a flat ink film and solidification is slow, so the gloss is not normal and the dryness is not good. In addition, for paper with a low sizing bottom and poor strength, paper pull-out tends to occur.

The use of such inks, the best combination of use, transfer ink, dilute the agent must be the same type, because different types of varnish or dilute the dry film and other different performance, will affect the printing effect.
The dry form of the bright ink is mainly solidified, supplemented by oxidation. Under normal circumstances, under normal circumstances, the use of light in the use of dry ink does not need to add, but it can also be based on the specific circumstances of the local, appropriate to add dry oil, the amount should be added to reduce the amount of dry resin than general resin ink. Dry oil and no dry oil have the following differences:

1.Single-colored prints without drying oil can be placed in three days or so and then printed in the second color. The third color will not cause crystallization phenomenon and cannot be printed on. The placement time of monochrome printing after adding dry oil must be strictly controlled. Otherwise, crystallisation will occur and it will not be printed.

2. The ink without drying oil has slow conjunctiva time in the rubber roller and the ink fountain, and the time for adding dry oil and conjunctiva is fast.

Eight, fast fixing the use of ink

Fast fixing ink is also a kind of resin offset printing ink. The main characteristic of resin ink in general is: not simply rely on adding desiccant to increase the drying speed of ink, it mainly depends on the penetration of solvent. This type of ink contains much higher solvent (high-boiling point kerosene) than conventional inks. When the ink is transferred to the paper, the solvent in the ink quickly penetrates into the gaps in the paper fibers, and the resin, oil, and pigment instantaneously thicken and stick to the surface of the paper, forming a pseudo-dry phenomenon. Under certain printing process conditions, fast fixing, non-backing paper, high-speed multi-color printing or monochrome printing can be used to shorten the printing cycle.

As mentioned above, the ink is fastened by the penetration of the solvent to form a false dry, but it is not a true oxidized conjunctiva. When the surface of the blot is strongly rubbed, it will still be contaminated, and the oxidized conjunctiva will have to dry for a while. Therefore, it is required that a piece of printing should be printed before several colors of the first color completely dry. Otherwise, over-drying the ink film, the solvent penetration will be greatly slowed, and the fixing speed will be slowed down accordingly. In this way, the pre-imposed fast fix requirements are not achieved, and quality accidents may also occur. It is a good criterion for how much time a print should be printed in full color. This depends on the drying time of the first color oxidized conjunctiva. For example, the drying time is 24 hours and all printing must be completed within 24 hours. The same ink, due to different temperatures, the oxidative conjunctiva time difference. With high temperatures, short drying time for the oxidized conjunctiva, the overprint cycle can be shortened, which is good for fast fixation. Under the relative temperature and humidity, the oxidized conjunctiva time of yellow, red, blue, and black inks increases in order.

The use of such fast-fixing inks, such as stickiness, can be adjusted with a small amount of high-boiling kerosene. If the consistency is large and the viscosity is not large, a small amount of high-boiling kerosene and a small amount of #6 resin varnish can be used to adjust it, but when adding auxiliary materials such as #6 varnish and other mineral oil, they cannot be added at will, so as not to affect the fixing speed. Should pay attention to the ink can not be adjusted too thin, otherwise the color concentration is greatly reduced, in the printing needs to increase the amount of ink to meet the original requirements. In this way, it is easy to appear dirty.

In short, as much as possible requires a larger concentration, a thin layer of ink in order to facilitate the fast fixing properties of the ink. Especially in the light red, light blue ink blending, should pay attention to a little darker, give a little less ink.

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