Discussion on the Process of Non-photohardening of Carbon Paper
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The so-called matt hardening (ie, dark reaction) is a sensitized carbon paper that has a thicker layer thickness after development without exposure. The concentration of potassium dichromate solution is large, the free acid is too much, and the gelatin of the animal is different, and the photohardening is increased. In the actual production process, care must be taken to promote the photohardening process of the carbon paper layer. This requires the operator to pay attention not only to the indoor temperature and humidity, but also to the storage time of the sensitized carbon paper during the process of preserving the sensitized carbon paper, because it is related to whether it can reach the corrosion process. Stabilize and get high quality prints.
The reaction process inside the rubber layer is as follows. Potassium dichromate exhibits an ionic state in an aqueous solution: K2Cr2O7→2K++Cr2O7--, wherein: Cr2O7--+H2O-2CrO4--+2H+. Potassium dichromate has different ionic forms in acid and alkaline solutions: when the solution is alkaline, the dichromate ions react in the direction of generating chromic acid ions. The oxidation of the colloid by the dichromate ion is stronger than the oxidation of the colloid by the chromate ion. Therefore, in order to reduce the oxidation of dichromate ions and slow down the dark reaction, an appropriate amount of ammonia water may be added to the potassium dichromate solution to enhance the alkalinity of the solution. There are three functions:
1. The reaction is carried out in the direction in which the chromate ions are formed. At this time, the orange potassium dichromate exhibits a pale yellow color, and the dark reaction inside the carbon paper layer is weakened.
2. Improve the adhesion of the colloidal substance so that the film can adhere tightly to the plate.
3. Inhibit the activity of microorganisms to prevent deterioration of the photosensitive solution.
After the photosensitive solution is added with ammonia water, the pH is usually between 7.5 and 8.8, which is slightly alkaline. In the drying process, the ammonia water is gradually reduced and becomes slightly acidic, so the photographic performance of the film is gradually increased, which does not affect the use effect. Higher room temperatures also result in the separation of pure chromic acid from chromic acid, which requires a freezer. It must be noted that the humidity inside the refrigerator should be consistent with the relative humidity of the studio.
The exposed carbon paper will not be developed after being stored for 20 hours in a humid environment. Practice has proved that if the carbon paper is stored in the wet season for one day and then goes to the sun, the carbon paper is not easy to stick to the plate cylinder, even if it has been attached to the drum, it is not obvious. Shadow. Therefore, it is required that the carbon paper that has been sun-dried and the positive image should be immediately printed, so that the intermittent time between the printing and the printing is not prolonged. Excessive time will cause difficulties in development and corrosion work. Another way to prevent dark hardening of the adhesive layer is to add 0.3% potassium citrate as a cleaning agent to the 4% sensitizing solution to prevent the latent shadow of the adhesive layer. The sensitizing property of the adhesive layer can be increased by appropriately increasing the acidity of the potassium dichromate (K2Cr2O7) solution, but the dark hardening of the adhesive layer is accelerated accordingly.
There is currently a new pre-sensitized carbon paper; the back is a film of Rope film (DuPont Rotofilm), which is a patented film for the plate. On the other hand, Rotofilm has been used in many printing factories for several years, and it can be replaced by a continuous yang image and a magenta or gray contact screen instead of two images (Durgen Dultgen mesh method) Version law). It is a positive color film that must be flushed under dark red security light. The magenta screen used with the Rope film is exposed through a K2 or #3385 frosted glass yellow filter, while the continuous tempering image is exposed through a frosted glass #5840 UV filter. After rinsing, the diaphragm has the following advantages: it can be stored for standby; it can be shipped without refrigeration; there is no dark reaction and continuous reaction like carbon paper; the density value can be read on a densitometer to check the exposure and Whether the development is accurate or not; only one continuous tempering picture is needed, instead of the screened embossing in the two positive images of the transverse net, it has a large contrast characteristic and therefore cannot be like carbon paper. Frequently copied highlights. In order to make up for this shortcoming, it is necessary to have a relatively high gloss portion on the continuous tempering map, otherwise the correct satisfactory corrosion is not obtained at a density of 0.30 or 0.35. It is also more sensitive than carbon paper in the process of dyeing and UV. After exposure and development, the Rope film is essentially the same as carbon paper in terms of printing, hot water development, and etching.