Several important factors affecting the gloss of printed matter

During the printing process, the ink film is transferred to the surface of the paper and fills the unevenness of the paper surface, making the surface of the printed matter quite smooth. The specular reflection of light on the surface of the printed ink film determines the gloss of the printed matter. The surface of the ink film is related to the printing conditions, paper, ink properties and the combination of the two.

1. Printing conditions.

①Ink film thickness. Ink film thickness is the main factor affecting the gloss of printed matter. After the paper absorbs the ink binder as much as possible, the remaining binder remains in the ink film, which can effectively improve the gloss of the printed matter. The thicker the ink film, the more the remaining binder, which is more conducive to improving the gloss of the printed matter. The study found that the tendency of gloss to increase with ink film thickness varies with different papers and different inks. No matter what kind of paper, there is a certain limit to the amount that the gloss increases with the thickness of the ink film.

The gloss of prints formed by printing inks with different pigment contents is very different. Apparently this difference is due to the different diameters of the ink pigments. In order to accurately obtain the trend of the gloss of printed matter changing with the thickness of the ink film, it is necessary to compare the magnitude of the gloss formed under different ink film thicknesses.

②Other printing conditions. There is a lack of systematic research on the influence of other printing conditions on the gloss of printed matter. Printing pressure has little effect on the gloss of printed matter formed by printing on coated paper with small absorption capacity; for non-coated paper, increasing printing pressure will reduce the gloss of printed matter; The impact is not very significant. Relevant information points out that the operating conditions of the printing workshop are also relevant factors. Increased relative humidity in the workshop will reduce the gloss of printed matter. The American Institute of Paper Chemistry (IPC) researched and explained that this is the result of increased paper porosity due to increased relative humidity. In addition, the increase in workshop temperature will lead to a decrease in ink viscosity, which will also reduce the gloss of printed matter.

2. Ink properties.

From the previous discussion, we have noticed that the gloss of printed matter depends not only on the printing conditions, but also on the properties of ink and paper. In order to further understand the mechanism of the gloss formation of printed matter, it is necessary to specifically discuss the influence of ink properties on the gloss of printed matter and clarify the relationship between the two. The gloss of printed matter depends on the smoothness of the ink film, and the retention of the binder in the ink film is conducive to the improvement of smoothness. Therefore, the ink should contain uniformly dispersed fine pigments, and have sufficient viscosity and fast drying speed to avoid excessive penetration of the binder into the pores of the paper. In addition, the ink should also have good fluidity so that the printed ink can be leveled and a smooth ink film can be formed. The components of the ink determine the properties of the ink, and the various properties of the ink affect each other. It is difficult to figure out its influence on the gloss of the printed matter when studying a certain property in isolation, and a comprehensive study is required.

① The viscosity of the binder and the content of the pigment. According to the principle of interfacial chemistry, the capillary permeation velocity decreases with the increase of liquid viscosity. Therefore, it is generally believed that the gloss of printed matter will increase with the viscosity of ink and binder. However, the actual research has come to the opposite conclusion. A group of inks with different viscosities and different pigment contents are printed on the same kind of coated paper. The pigment is also a kind of carbon black pigment. It can be seen that although the content of the binder in ink C is not high and the viscosity of the binder is also low, the gloss of the printed matter is very high. However, the gloss of the printed matter formed by the printing of ink B with high content of binder and high viscosity is low. This is due to the high pigment content of Ink C, which makes more small capillaries formed in the ink film. Compared with the ink with low pigment content, Ink C can make the ink film retain more binder. On the contrary, for inks with low pigment content (such as ink B), although the viscosity of the linking material is high and the content is high, the penetration of the imprint is small, but the capillary penetration is slow after leaving the embossing area. The capillary is large, so that the linking material that penetrates into the pores of the paper increases. It can be seen from here that the capillary network structure formed between ink pigment particles is an important aspect that determines the gloss of printed matter. At the moment of embossing, the ink is pressed into the larger pores of the paper as a whole; after embossing, the binder 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 vehicle separation. The capillary retains the binder much more than the printing pressure presses the binder into the pores of the paper.

In actual printing, varnish is often used to increase the gloss of printed matter, which is completely different from increasing the pigment content of ink. When these two methods of increasing the gloss of printed matter are applied, they should be selected according to the components of the ink and the thickness of the printing ink film. Due to the need for color reproduction in color printing, methods for increasing pigment content are limited. In addition to the lower ink film thickness, the gloss is significantly improved, which is due to the large amount of binder required to cover large pigment particles.

②The size of the pigment particles and their degree of dispersion. As mentioned earlier, ink capillary action is an important factor in the formation of print gloss. Small ink particles can form more small capillaries, which is conducive to improving the gloss of printed matter. However, the performance size of the pigment particles in the dispersed state is more important, which 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 gloss of printed matter. In addition, the pigment particles are small, well-dispersed, and conducive to the formation of a smooth ink film, which is also very beneficial for improving gloss. Studies by Hammel et al. have confirmed that these effects exist, pointing out that the pH value of the pigment particles and the content of volatile substances in the ink The content directly affects the degree of dispersion of pigment particles. The pH value of the pigment is low, and the content of volatile substances in the ink is high, which is conducive to the dispersion of pigment particles. This effect is very noticeable on coated papers and has little effect on uncoated papers. This shows that paper is also very important to the formation of gloss in printed matter. On non-coated paper with strong absorption capacity, the retention of the linking material in the ink film increases with the increase of pigment content, and a different trend may appear only when the size of the pigment particles itself varies greatly; on coated paper with weak absorption capacity In general, the gloss of the printed matter is very sensitive to the condition of the pigment. At this time, the gloss of the printed matter can be affected from two aspects: one is to increase the pigment content to retain the link material in the ink film, and the other is to increase the dispersion of the particles and use small particles to retain more ink. More binders to form a smooth ink film.

③ drying time. The effect on the gloss of the printed matter when the ink dries is very obvious. The rapid drying of the ink film on the paper surface can reduce the amount of the binder penetrating into the pores of the paper, thereby improving the gloss of the printed matter. In the process of forming the gloss of the printed matter, the curing of the binder is also very important.

To sum up, it is quite complicated to adjust a certain property of ink without affecting other properties. It is necessary to conduct a comprehensive analysis of the ink components and their possible changes after printing in order to adjust them well.

3. Paper properties.

① Due to the interweaving of fibers to form a capillary network structure, the paper has a large number of pores, which becomes the basis for its ink absorption. The balance between the absorption time of the ink binder by the paper capillary network and the fixation time of the binder determines the degree of penetration of the binder when the gloss of the printed matter is formed. Generally, the gloss of printed matter decreases with the increase of paper absorption capacity. Tollenaar has conducted research in this area for many years. He pointed out that it is not the average size of paper pores that has a particularly large impact on the gloss of printed matter, but the size distribution and number of pores.

However, current paper quality control methods show poor correlation between void count or absorbency measurements and print gloss.

Smoothness and shine. Paper smoothness and its own gloss also affect the gloss of printed matter. High smoothness is conducive to the formation of uniform and smooth ink film, thereby improving the gloss of printed matter. This has been confirmed in the studies of many researchers. The good gloss of the paper itself can improve the reflectivity of the ink film, especially for transparent inks. There is an excellent correlation between the gloss of the print and the gloss of the paper itself, better than the correlation between the gloss of the print and the absorbency of the paper.

From the above discussion, it can be found that there is also a correlation between paper smoothness, gloss and absorbency. This is easily explained from the paper making process. For example, calendering improves the smoothness and gloss of the paper, while also reducing the paper's porosity, which reduces its absorbency. In addition, the pH value of the paper surface is also a factor that cannot be ignored in the formation of gloss in printed matter. A high pH value is conducive to ink drying, which is conducive to improving the gloss of printed matter.

4. The relationship between paper and ink.

More and more studies have found that the relationship between paper and ink has the greatest impact on the gloss of printed matter. The results of the same group of studies by Feskp et al. are listed below to illustrate this problem. The higher the gloss of the printed matter; while another kind of ink printing is hardly affected by the type of cardboard. This shows that using only one kind of paper cannot predict the effect on the gloss of printed matter formed by ink printing.