The performance of carbon-coated carbon ink must first meet the requirements of the printing process, and it has the characteristics of good printability, difficulty in picking up, strong rewriting capability, and stable performance. There are mainly the following indicators for evaluating the printability and performance of carbon-coated replication inks.
1. Flow performance (transfer performance)
The flowability of carbon-coated carbon ink refers to the extended performance of the ink on the printing plate and the ability to form a uniform and smooth ink layer on the paper, which is an important indicator of ink rewriting. At a suitable temperature, during the process of transfer of the transfer ink from the printing plate to the paper, proper fluidity is maintained so that a smooth ink layer can be formed on the paper. However, the fluidity of the ink is related to the temperature. When the temperature is too low, due to the decrease of the fluidity, the ink is solidified on the paper when it is transferred from the printing plate to the paper. The formed ink layer is not smooth enough and has no gloss. The printing surface will show the state of surface undulations, and the front part of the print will contaminate the guide roller of the printing machine, and the scratches will be easily contaminated after processing. On the contrary, if the printing temperature is too high, the viscosity of the ink will decrease, most of the ink will penetrate into the paper, and the printing ink layer will be uneven and lose luster. The temperature control model of carbon-coating ink printing process is shown in Figure 1.
In the printing process, the viscosity of the ink is closely related to the transferability of the ink. Generally, the higher the speed of the ink, the faster the ink separation speed. The viscosity of the ink is required to be properly reduced, otherwise the transfer effect is not satisfactory. In addition, the surface strength of the paper is low, and the viscosity of the ink is also required to be low, otherwise paper picking may easily occur. Therefore, in the production process, in order to ensure the printing effect and obtain a smooth ink layer, the viscosity of the ink must be strictly controlled, and the printing temperature is appropriately adjusted.
2. Anti-backing performance
The ink transferred from the printing plate to the paper, if the fixing time is not sufficient, the ink layer is not completely dried, the ink on the previous printing product can be easily transferred to the back of the next printing product, causing the back of the ink to be dirty. The degree of back-scratching is mainly related to the adhesion of the ink to the paper and the cohesion of the ink. In addition, the thixotropy of the ink also has an effect on the back-scratch prevention performance.
During the printing process, the ink is constantly grinded by the ink roller while being subjected to a sustained shearing force, and the internal structure of the ink is destroyed and the viscosity decreases. After that, the internal pressure of the ink tends to ease, the internal structure gradually returns to its original state, the viscosity increases, and a firm ink film is quickly fixed on the paper. If the thixotropy of the ink is small, the viscosity of the ink film transferred to the printed matter rises slowly, and the fixation of the conjunctiva is slow, and it is easy for the back surface to be dirty. The fundamental way to prevent back-scratching is to improve the ink's performance, make it fast fix, or reduce its adhesion to the back of the paper, such as adding paraffin in the formula, help to form a single molecular wax layer on the ink surface, reduce the ink film and Adhesion to the back of the paper; some solid fillers, such as calcium carbonate, magnesium carbonate, barium sulfate, etc., can also be added as needed to improve the thixotropy of the ink.
3. Repeatability
Reproducibility refers to the fact that the ink is peeled off by pen pressure or printing and transferred to the underlying paper. This is an important use performance of carbon paper, which determines the number of rewritable bonds. The factors affecting the ink rewriting performance include the hardness of the ink, the content of the mineral oil in the formulation, the cohesive strength of the auxiliary binder, and the paper properties.
Paper has a great influence on the reproducibility of carbon ink, so the most easily printed paper should be selected according to the characteristics of the ink. In addition to meeting the general printing performance requirements, paper also requires slightly higher requirements in terms of whiteness, opacity, tightness, sizing, and frontal smoothness; and it requires paper to have good resistance to permeation, smooth and delicate surfaces, and high strength. , Has toughness; After high-temperature carbon coating is not yellowed, not brittle, anti-aging properties; good adhesion with the replication ink.
The copying ink's copying ability will change slowly with time. The initial change occurs in the 2 to 3 days after the printing is completed. This period can be regarded as the maturation period of the ink. After this period of time, the hand is not likely to cause contamination. Phenomenon. However, if the printed ticket is placed under high temperature conditions, its repeatability will continue to decrease because the ink will become softer as the temperature increases, the ability of the wax to maintain the grease will decrease, and the grease will penetrate the paper, thus changing the ink system. The balance ratio between the binder and the pigment reduces the softness of the ink film, causes the ink film to harden, and reduces the rewriting ability. In order to stabilize the reproducibility of the rewritable ink, a wax composition having a good ability to retain oil can be obtained by adjusting the composition and ratio of the wax, so as to avoid degradation of the rewrite capability at high temperatures.
4. Pollution performance
Back-scratching occurs during the printing process, and contamination occurs during use and storage. When the copying ability of the copy paper is strong, since the softness and the characteristic adhesiveness of the ink itself increase, the contamination performance also increases, and there is a possibility that the outside of the copying portion is contaminated. It is conventionally considered that the method of increasing the hardness of the ink can reduce the adhesion, but in fact, the reproducibility of such an ink, the clarity of the multi-pass reproduction may be reduced. To this end, Japan has developed a peelable type of carbon-coated carbon ink, which cleverly solves this contradiction, that is, the ink is designed to be printed in two parts of a peeling layer and an ink layer, as shown in FIG. 2 . Its rewriting principle is the same as that of ordinary copy inks, but because it is separated into a soft peeling layer and a harder surface layer, the contamination is minimal.
5. Penetration performance
Paper is made of fibers intertwined with many pores of different sizes and shades. During the printing process, the ink is pressed into the larger pores of the paper under the influence of the printing pressure, and it penetrates through the capillary action of the pores of the paper. Penetration is beneficial to the fixation of the ink on the paper, but excessive penetration will cause “printing out†and “powdering†and reduce the quality of the printed product. The most common fault in reprinting ink printing is “printing throughâ€. The main reason is that the texture of the paper is loose and the oil absorption is too strong; the printing temperature is too high and the ink viscosity is too low. "Through print" will destroy the balance ratio between the linking material and the pigment and reduce the softness of the ink film, thus affecting the rewriting effect.
To prevent the occurrence of these two kinds of failures, the most effective method is to make the distribution of paper capillary pores and the distribution of ink pigment particles suitable. In addition to the selection of special carbon-coated carbon copy base paper, it is also possible to adjust the composition and proportion of the linking material to obtain a formulation that has a good ability to maintain the grease, so that it has a suitable viscosity and drying speed, and even if printed at a high temperature, it will not happen. "Printing through" and "powdering" phenomenon.
6. Gloss
The glossiness of the carbon-coated ink ink layer affects the appearance quality of the printed product. The glossiness is good, the color is bright, the ink layer is smooth, and the print grade is high. The gloss of the carbon-coated ink layer is determined by the carbon-coated printing process and the ink formulation. In addition to proper control of the temperature of the carbon-coated printing, the ink formulation can also be improved, for example, by selecting a wax with high gloss. Improve the gloss of the product.
Source "Printing Technology"