In the packaging and printing industry, sensitized carbon paper is used to ensure that the melting and photosensitivity of the adhesive layer are not affected by light. The so-called matte hardening (i.e. dark reaction) is that the sensitized carbon paper has a thick adhesive layer after development without exposure. Solution to the problem of matte hardening of carbon paper
The concentration of potassium dichromate solution is high, there are many free acids, and different animal colloids will be added without light hardening. Therefore, attention must be paid to promoting the matte hardening process of carbon paper layer in the actual production process. This requires the actual operator to pay attention not only to the indoor temperature and humidity, but also to the storage time of the sensitized carbon paper in the process of storing the sensitized carbon paper, because it is related to whether it can achieve stability in the corrosion process and obtain high-quality printing.Solution to the problem of matte hardening of carbon paper.
The reaction process inside the adhesive layer is shown as follows. In aqueous solution, potassium dichromate is in ionic state: K2Cr2O7 → 2K + + Cr2O7 –, in which: Cr2O7 — + h2o-2cro4 — + 2H +. In other words, the ionic forms of potassium dichromate in acid and alkaline solutions are different: when the solution is alkaline, dichromate ions react in the direction of generating chromate ions. The oxidation effect of dichromate ion on colloid is stronger than that of chromite ion. Therefore, in order to weaken the oxidation of dichromate ions and slow down the dark reaction, an appropriate amount of ammonia can be added to the potassium dichromate solution to enhance the alkalinity of the solution. Solution to the problem of matte hardening of carbon paper.
It has three functions:
1. The reaction is carried out in the direction of generating chromate ions. At this time, the orange potassium dichromate shows a light yellow color, and the dark reaction inside the carbon paper adhesive layer is weakened.
2. Improve the adhesion of colloidal substances so that the adhesive film can adhere closely to the layout.
3. Inhibit the activity of microorganisms to prevent the deterioration of photosensitive solution.
After ammonia is added to the photosensitive solution, the pH value is usually between 7.5 and 8.8, which is slightly alkaline. In the drying process, ammonia gradually decreases and becomes slightly acidic. Therefore, the photosensitivity of the film gradually increases, which does not affect the use effect. Because higher room temperature will also lead to the separation of pure chromic acid from chromic acid, it is necessary to set a refrigerator. It must be noted that the humidity inside the refrigerator should be consistent with the relative humidity of the studio.Solution to the problem of matte hardening of carbon paper.
1、 Carbon paper continues to harden after exposure
If the exposed carbon paper is stored for 20h in a certain humid environment and then printed, it will not be developed. Practice has proved that if carbon paper is stored for one day after drying the mesh in the wet season, and then go to the sun map, this kind of carbon paper is often not easy to adhere to the plate cylinder, even if it has been attached to the cylinder.
Therefore, it is required that the carbon paper that has been exposed to the mesh and positive map should be published immediately, so as not to delay the interval between printing and printing for a long time. If the delay time is too long, it will bring some difficulties to the development and corrosion work. Plate makers must cooperate with each other, strengthen work planning, timely clean the drum and do a good job of degreasing; Prepare for plate development by pouring the developing hot water in advance.
Another way to prevent dark hardening of adhesive layer is to add 0.3% potassium citrate to 4% sensitization solution
(K3C6H5O7. H2O) as a cleaning agent can prevent the latent image of the adhesive layer. The photosensitivity of the adhesive layer can be improved by increasing the acidity of potassium dichromate (K2Cr2O7) solution, but it will also accelerate the dark hardening of the adhesive layer.Solution to the problem of matte hardening of carbon paper.
2、 A new substitute for carbon paper — presentiment light gravure anti-corrosion diaphragm
Several pre photosensitive gravure resist films have been tried in the past. At present, there is a new kind of carbon paper with preview light; There is also a carbon paper substitute, with a film rotofilm on the back, which is a patented film for printing. German premonitory light carbon paper has been tried in China, but there is not enough data on its effective performance.
On the other hand, rotofilm has been used in many printing houses for several years. It can replace the screen positive picture with continuous positive adjustment picture and magenta or gray contact screen (two positive picture printing method in dulchen dultgen mesh method).Solution to the problem of matte hardening of carbon paper.It is a positive color film, so it must be washed under the dark red safety light. The magenta screen used with roto film is exposed through K2 or #3385 ground glass yellow filter, while the continuous positive adjustment picture is exposed through ground glass #5840 UV filter.
After flushing, this diaphragm has the following advantages:
1. It can be stored for standby.
2. It can be shipped without refrigeration.
3. There is no dark reaction and continuous reaction like carbon paper.
4. The density value can be read out on the densitometer to check the accuracy of exposure and development.
5. Only one continuous positive image is needed, instead of the screen positive image used in the horizontal screen two positive image method.
It has large contrast characteristics, so it can not copy the highlight details as often as carbon paper. In order to make up for this disadvantage, it is necessary to have enough highlight parts on the continuous positive adjustment diagram, otherwise the correct and satisfactory corrosion will not be obtained at the density of 0.30 or 0.35. It is also more sensitive to trimming dyes and UV factors than carbon paper. After exposure and development, Roto film is basically the same as carbon paper in over plate, hot water development and corrosion treatment.Solution to the problem of matte hardening of carbon paper.
Kronaville is another corrosion-resistant film recently invented by DuPont to replace the existing photosensitive corrosion-resistant film. After exposure on the positive diagram, it can be etched on copper or other surface layers without development. After the plate is passed to the drum, all operations are the same as the carbon paper plate passing technology used now.
The back of the diaphragm can be stripped by fingers. Only the corrosion-resistant film is left on the copper surface, which can be corroded by ferric chloride without hot water or chemical developer. After the corrosion is completed, the remaining corrosion-resistant film can be washed away with acetic acid and aqueous solution, saving the development time with hot water, It can also eliminate all kinds of shortcomings due to hot water development, and ensure uniform corrosion effect.
This special kronaville corrosion resistant film is currently only suitable for high contrast and line images. The emulsion used for continuous positive chart can not be used by all printing units, because this emulsion will produce dust particles, which may cause halo or spot when corroding the copper surface.Solution to the problem of matte hardening of carbon paper.
According to the manufacturer’s explanation, because the emulsion film is too thin, this defect has been corrected to increase the thickness, so as to better adhere to the copper surface. It can be expected that the improved kronaville corrosion resistant film can replace other corrosion resistant films such as carbon paper.
The kronaville corrosion resistant film with high contrast can add positive screen at long-distance transverse dot at the same time, which can achieve the effect obtained by direct corrosion. As for the reproduction curve, although it can not be compared with the results obtained by the two positive chart method and carbon paper corrosion-resistant film, it can also get a fairly good replica. If the main tone level and details are not emphasized, the reproducibility in corrosion can be determined without problem.Solution to the problem of matte hardening of carbon paper.