The causes and solutions of yellowing of film

In the process of plate-making, the yellowing or browning of copying, photography, and computer output films is mainly caused by the presence of a water-insoluble sulfide in the photosensitive emulsion film-sulfur. In actual work There are several factors that cause the yellowing of the photosensitive film.

1. The developer is not prepared strictly according to the ratio of the formula. The developer's alkalinity is too strong, the developer is easily oxidized and discolored, the developer has been used for too long, the development effect is significantly reduced, the developer is oxidized into reddish brown, and the development efficiency is also very easy to weaken or even disappear The film shown in these developing solutions will be dyed yellow or brown due to the discoloration of the surface drug film.

In general, rigid line films are used for photography, copying, and electrical separation. High-contrast, high-stability developing solutions are used. One bag of medicine is used to continuously develop 8m2 of film. The developer should be used after a long time Replace with new medicine.

2. The fixing solution uses sodium thiosulfate + water + sodium sulfite. The formula of the fixing solution: sodium thiosulfate: 350g, sodium sulfite: 8g, water: 1000mL.

The purpose of fixing is to permanently fix the image with proper density and contrast obtained after development. The role of the fixing solution is to neutralize the alkaline substance of the developing solution with its acidic substance to stop the development, fix the image and dissolve the unsensitized silver halide. The sodium thiosulfate in the fixing solution is exposed to too strong acidic substances or the temperature is too high, and free sulfur will be precipitated. These free sulfur can react with the silver particles on the negative film to produce silver sulfide. The color of silver sulfide is brown, so the color of the film will be dyed brown, so the acidity should not be too strong when preparing the fixing solution, and the temperature of the fixing solution should not be too high.

3. Inappropriate concentration of the fixing solution can also cause the film to turn yellow. Because the fixing speed of the film is related to the amount of sodium thiosulfate in the fixing solution, the concentration of the fixing solution is high, and the fixing speed is fast. If the concentration of the fixing solution exceeds 45%, after the fixing solution is too thick, the film The unsensitized silver halide has the lowest solubility, but reduces the solubility, which makes it adversely affected. Therefore, if the fixing concentration is not properly controlled, the effectiveness of the fixing solution can be weakened, and the fixing is not deep and yellowing occurs.

4. The fixing solution is outdated and insufficiently fixed, and the film can turn yellow even if it is not thorough. After using the fixing solution for a long time, the performance of fixing decreases, and the effectiveness of fixing is lacking, so that the unsensitized silver halide is completely dissolved. These undissolved silver salts remain in the emulsion film, causing the film to appear yellow after processing.

5. After fixing the photosensitive film into the fixing basin, please pay attention to the shaking operation method. It is required to shake the developing tank to shake the fixing solution, or to flip the film up and down. The purpose is to fully fix the photosensitive film completely. The unexposed silver halide is completely dissolved in the fixing solution. If the photosensitive film is placed in the fixing basin, it will not be flipped or shaken. Fixing in a static state will result in insufficient fixing, and residual sulfide will adhere to the film, causing yellowing and browning.

6. Yellowing caused by insufficient water washing after fixing. After the film has been sufficiently fixed, it should be placed in the processing tank.

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