Analysis of Surface Glossiness of Printed Products (3)

2 other printing conditions. There is a lack of systematic research on the effect of other printing conditions on the gloss of prints. Printing pressure has less effect on the gloss of prints formed by printing paper with less absorption capacity; for non-coated paper, increasing the printing pressure will reduce the gloss of printed matter; when the change in printing speed does not result in a change in the thickness of the printed ink film, it will be glossy on the printed matter. The impact is not very significant. According to relevant information, the operating conditions of the printing shop are also relevant factors. Increased relative humidity in the shop will reduce the gloss of the printed matter. The American College of Paper Chemistry (IPC) studied and explained that this is due to the increase in relative humidity that leads to increased paper porosity. In addition, the increase in the temperature of the shop will cause the viscosity of the ink to drop and also reduce the gloss of the printed matter.

2. Ink properties. From the previous discussion we noticed that the gloss of the print depends not only on the printing conditions but also on the properties of the ink and the paper. In order to further understand the mechanism of the formation of glossy prints, it is necessary to specifically discuss the impact of ink properties on the gloss of prints and clarify the relationship between the two. The gloss of prints depends on the smoothness of the ink film, and the retention of the binder in the ink film facilitates the improvement of smoothness. The ink should therefore contain finely dispersed fine pigments, and have sufficient viscosity and faster drying speed to avoid excessive penetration of the binder into the paper pores. In addition, the ink should also have good fluidity in order to level the printed ink and form a smooth ink film. The composition of the ink determines the nature of the ink, and the various properties of the ink affect each other. Studying a property in isolation makes it difficult to ascertain its effect on the gloss of the printed matter and requires a comprehensive study.

1 The viscosity of the binder and the content of the pigment. According to the interfacial chemistry principle, the capillary permeation rate decreases as the viscosity of the liquid increases. Thus it is generally believed that the gloss of the print will increase as the viscosity of the ink and the binder increase. However, actual research has drawn the opposite conclusion. Figure 4-35 shows the experimental results of a set of binders with different viscosities and inks with different pigment contents printed on the same coated paper. The pigment is also a carbon black pigment. It can be seen from the figure that although the content of the ink C is not high, the viscosity of the binder is low, and the gloss of the printed matter is high. The printed matter produced by the printing of ink B with a high content of binder and a high viscosity has a lower gloss. This is due to the high pigment content of Ink C, which results in the formation of more small capillaries in the ink film. In contrast to inks with a low pigment content, Ink C allows the ink film to retain more of the binder material. In contrast, inks with low pigment content (such as ink B), although the viscosity of the binder is high, the content is high, the imprinting instantaneous penetration is small, but the capillary penetration after leaving the embossed zone is slow, and ultimately the pigment content is small, and the ink film is formed. Large capillaries increase the amount of binder that penetrates the pores of the paper. It can be seen from here that the capillary network structure formed between the ink pigment particles is an important aspect that determines the gloss of the printed matter. At the moment of imprinting, the ink is pressed into the larger pores of the paper as a whole; after imprinting, the binder material begins to separate from the ink and penetrates into the smaller pores of the paper. The size of the ink film capillary determines the amount of separation of the binder. Capillary retention of binders is much more important than printing pressure to press the binder into the pores of the paper.

In actual printing, the gloss of the printed matter is often increased by using a brightening oil method. This method is completely different from increasing the pigment content of the ink. These two methods for increasing the gloss of prints are selected according to the composition of the ink and the thickness of the ink film to be printed. Due to the need for color printing in color printing, the method of increasing the pigment content is limited. From Fig. 4-34, it can be seen that the ink formulated with small particle pigments has a reduced gloss when the pigment content is reduced, and a high gloss can be produced only when the ink film is relatively thick. Therefore, for this case, the method of increasing the content of the pigment can be used to increase the gloss of printed matters. However, the amount of pigment can only be increased to a certain limit. Otherwise, the pigment particles cannot be completely covered by the binder, and the phenomenon of light scattering on the surface of the ink film is intensified, resulting in a decrease in gloss of the printed matter. The situation of inks formulated with larger pigment particles is different. When the pigment content is reduced from 20% to 15%, although the color intensity is reduced, the gloss of the printed matter is not affected much; when the pigment content is reduced to 10%, 5 %, in addition to the lower thickness of the ink film, the gloss is significantly improved, due to the large amount of pigment particles required to cover a large amount of binder.

2 The size of the pigment particles and their degree of dispersion. As mentioned earlier, ink capillary action is an important factor in the gloss of prints. Smaller ink particles can form more small capillaries, which helps improve the gloss of prints. However, the size of the pigment particles in the dispersed state is more important, and it directly determines the state of the capillary of the ink film. Therefore, the pigment particles are well dispersed in the ink, which reduces the flocculation phenomenon and helps to improve the luster of the printed matter. In addition, pigment particles are small, well dispersed, and conducive to the formation of a smooth ink film, is also very beneficial for improving the gloss, Hammel et al.'s research confirmed that these effects exist, pointing out the pH of the pigment particles and the volatile substances in the ink The content directly affects the degree of dispersion of pigment particles. The low pH of the pigment, the high content of volatile substances in the ink, is conducive to the dispersion of pigment particles. This effect is very noticeable on coated paper and has little effect on non-coated paper. This shows that paper is also very important for the formation of print gloss. On non-coating papers with strong absorptive capacity, the amount of binder retained in the ink film increases with the increase of pigment content. Only when the size of the pigment particles varies greatly, different trends may occur; in the weak absorption capacity of the coated paper On the other hand, the gloss of the printed matter is quite sensitive to the condition of the pigment. At this time, the gloss of the printed matter can be affected in two ways: firstly, increasing the pigment content to keep the binder in the ink film; secondly, increasing the dispersion degree of the particles and adopting small particles to retain more. A lot of binders form a smooth ink film.

3 drying time. The effect of ink on the gloss of prints is very obvious. The rapid drying of the ink film on the surface of the paper can reduce the amount of material that penetrates the pores of the paper, thereby increasing the gloss of the print. As can be seen from Table 4-7, no matter what kind of paper, dry oil is added to the binder in the ink film to reduce the penetration of the binder in the pores of the paper. It can be seen that the curing of the binder is also very important during the formation of the print gloss.

Table 4-7 Change of 7O° Angle Gloss (%) of Prints after Adding Dry Oil

Paper type without dry oil Add 2% dry oil paint cardboard 80 90
Sizing paperboard 50 80
Non-coated cardboard 30 50
In summary, it is a very complicated matter to adjust a certain property of ink without affecting other properties. It is necessary to perform a thorough analysis of the ink components and the changes that may occur after printing in order to adjust well. (one two three four)