Red shift of photography

Study on physical development process and silver accumulation morphology of silver salt CTP plate

In photosensitive science, the development process of silver halide can be divided into two categories: chemical development and physical development. The difference between them is that in the process of chemical development, silver ions come from the lattice of silver halide, which catalyzes the reduction of silver ions to the latent image center produced by exposure of silver halide crystal, and then develops to obtain filamentous silver; In the process of physical development, silver ions exist in the solution in the form of complex ions, and the complex silver ions and the developer undergo redox reaction under the catalysis of the physical development core (heavy metal sulfide or heavy metal colloid particles), and the generated metallic silver is deposited on the surface of the physical development core, which is generally granular. The silver salt diffusion and transfer system based on physical development principle has been widely used in one-step photography, direct plate-making printing and so on. Especially with the digitalization of printing technology, computer-to-plate (CTP) has become the development direction of printing technology, and silver salt diffusion CTP plate has been paid attention to because of its unique advantages. In the silver salt CTP system, the aggregation form and density of silver have great influence on the oleophilic and hydrophilic properties and printing resistance of the plate. Chen Ping [3] and others found that in the diffusion and transfer system, the physical development nuclei and media have different effects on the accumulation form of silver images. In the experiment of Hunsel① (1), the inhibitor makes the absorption peak position of physically developed silver red-shifted, indicating that the accumulation form of silver has changed. In this paper, the effects of exposure and complexing agent on the morphology of silver accumulation during the physical development of silver salt CTP system were observed by high resolution scanning electron microscope (SEM), and the physical properties of the plate were monitored in real time by one-dimensional linear CCD(ChargeCoupledDevice).

1 experimental part

The experimental principle of 1. 1

The experimental silver salt CTP plate is composed of aluminum substrate, physical development core layer and silver halide photosensitive layer, as shown in figure 1. After the printing plate is exposed, the silver halide in the exposed part is chemically developed by using a treatment solution with a combination of transparency and orientation, and the silver halide in the unexposed part is complexed by a silver complexing agent in the treatment solution to form freely movable complexed silver ions, which diffuse and transfer to the physical development core layer. The developer is reduced to silver. After washing off the emulsion layer with water lotion, the physically developed silver image attached to the aluminum substrate is exposed. When printing, the image area is lipophilic and the non-image area is hydrophilic, so that the transfer of graphics and text from printing plate to paper can be realized.

Figure 1 Schematic Diagram of Processing Flow of Silver Salt CTP Plate

1.2 experimental instruments and drugs

1.2. 1 The plate used in the experiment is a self-made aluminum-based silver salt CTP plate, and its structure is shown in figure 1.

1.2.2 The main components of the developer are potassium hydroxide, hydroquinone, sodium sulfite and complexing agent.

1.2.3 the instrument used for scanning electron microscope is JSM 630 1F scanning electron microscope.

1.2.4 The instrument for monitoring the physical development process is a self-assembled 2048-unit linear CCD device, and its principle was introduced in detail in the previous article.

2 Results and discussion

2. 1 physical development silver accumulation mode under different exposure.

Because the unexposed silver halide forms a positive image on the physical development core layer during diffusion and transfer, its photosensitive characteristic curve is opposite to the negative characteristic curve obtained by chemical development, that is, the image density of the unexposed area physically developed silver is high, and the density of the exposed area physically developed silver is low. After CTP plate was exposed by optical wedge, the structure of physically developed silver at different exposure levels was observed by scanning electron microscope, and the results are shown in Figure 2. The black part is the base, and the reflection density of the silver image is marked in the corresponding photo. As can be seen from the figure, the physically developed silver is granular for different exposure levels. Under the weak exposure level, the physical development of silver particles is numerous and densely arranged, which is also the main reason for the oleophilicity of the image area.

With the increase of exposure, the number of silver particles decreases gradually, and the reflection density of the image also decreases gradually. This is because with the increase of exposure, the chemical development process in the emulsion layer is enhanced, the silver halide is consumed, and the concentration of complex silver ions is reduced, so the number of silver particles deposited in the physical development core layer is reduced. In the strong exposure area, only a few physically developed silver particles are scattered on the plate surface, and the silver image density is only 0. 18.

Fig. 2 Silver electron microscope photos of body development under different exposure levels.

2.2 physical development accumulation of silver by complexing agent

In the silver salt diffusion transfer system, the main function of complexing agent is to complex and transport unexposed silver halide to the physical development core layer. Therefore, the complexing agent will have an important influence on the morphology of physically developed silver. When CTP plates are treated in developing solutions containing different complexing agents, the electron microscope photos of the plates are shown in Figure 3. When the complexing agent is sodium thiosulfate, the size of physically developed silver is about 80 ~ 150 nm, and its shape is regular. When the complexing agent is organic amine, the physically developed silver is dendritic and dispersed on the surface of the electrode plate, and some areas gather into loose clusters (Figure 3b), which may be related to the low complexing ability of organic amine; When the complexing agent is pyrimidine compound, after long-term development, the density of silver image is still very low, and no electron microscope photos can be taken.

Fig. 3 silver electron microscope photos of physical development of different complexing agents

A: sodium thiosulfate, b: organic amine, c: sodium thiosulfate+pyrimidine compound, d: organic amine+pyrimidine compound.

Table 1 lists the complexing constants of three complexing agents, among which the complexing ability of organic amine is weaker than that of sodium thiosulfate. When sodium thiosulfate with strong complexing ability is used, unexposed silver halide particles in the emulsion layer are rapidly complexed and dissolved.

High density silver images were obtained by catalytic reduction. Organic amines with weak complexing ability can only transport a small amount of silver halide to obtain low-density silver images. Although the complex constant of pyrimidine compounds is relatively high, there is no silver image when pyrimidine compounds are used alone in this experiment. This is because the molecular volume of the complex formed by pyrimidine compound and silver ion is large and the diffusion coefficient in gelatin layer is low. The speed of transferring silver ions to the image receiving layer is also very low. Therefore, even after long-term development, high-density images cannot be obtained. This shows that complexing agent has two main effects on the physical development process of diffusion transfer system: the complexing speed of complexing agent with silver halide and the diffusion transfer speed of complexing silver ion. Therefore, when choosing a complexing agent, it is required that the complexing agent should have both sufficient complexing ability and high diffusion ability.

When pyrimidine compound is added to the developer as an auxiliary complexing agent, the morphology of silver particles has changed greatly. Compared with sodium thiosulfate alone, after adding pyrimidine compound, the silver particle size is reduced to 50 ~ 120 nm, and the packing is more compact (Figure 3c). Compared with organic amine alone, the amount of dendritic silver increased obviously after adding pyrimidine compounds, and the image density also increased from 0.27 to 0.69. And some large silver particles with a diameter of 300nm are produced, as shown in fig. 3d. This phenomenon may be because pyrimidine compounds promote the complexing process of silver halide in emulsion layer, and have a great influence on organic amines with poor complexing ability. The influence of complexing agent on the physical development process is also manifested in the development speed. We observed the development process with self-assembled CCD equipment, and obtained the results as shown in Figure 4. Among them, the faster the curve drops, the faster the formation speed of physically developed silver is. When the complexing agent is pyrimidine compound, the curve does not change during the monitoring time, indicating that there is basically no physically developed silver on the surface of the plate.

Fig. 4 Effect of complexing agent on development process

When the complexing agent is organic amine or sodium thiosulfate, the development speed is faster at the initial stage of development. With the development process, compared with sodium thiosulfate, the development process of organic amine as complexing agent quickly slows down and tends to end. We think this is due to the different complexation constants of the two complexing agents. With the development, the concentration of halide ions in the emulsion layer increases, which is affected by the complex balance. Organic amine with low complexation constant can't complex silver halide into freely movable complexed silver ions, which makes the physical development process come to an end. Because of the high complexation constant of sodium thiosulfate, its complexation balance is not affected by the concentration of halogen ions in the emulsion layer, so more silver halide can be complexed and transferred to the image receiving layer for physical development to generate more silver. When sodium thiosulfate and pyrimidine are used at the same time, the initial development speed is improved, which is obviously due to the synergistic effect of the two complexing agents.

2.3 the influence of temperature

When sodium thiosulfate is used as complexing agent, the color is developed at 20℃ and 40℃ respectively, and the electron microscope photo is shown in Figure 5. As can be seen from the figure, the silver obtained by development is regular spherical particles, which are closely arranged, indicating that the temperature has no obvious influence on its accumulation state within this temperature range. The particle size obtained at 20℃ is 100 ~ 200 nm, while the particle size obtained at 40℃ is 100 ~ 200nm.

A: 20 degrees Celsius B: 40 degrees Celsius.

Electron microscope photos of physically developed silver at different temperatures.

Fig. 6 shows the dynamic process of body development monitored at 20℃, 30℃ and 40℃ respectively. As can be seen from the figure, with the increase of temperature, the curve decreases rapidly, indicating that the speed of body development is accelerated, and the image density is also increased, which is 0.88, 0.98, 65,438+0.05 in turn. Obviously, the increase of temperature accelerates the complexation, diffusion and reduction of silver ions.

3 Conclusion

3. 1 Observe the accumulation state of physically developed silver in different exposure areas with high resolution scanning electron microscope. In the weak exposure area, there are many silver particles and dense accumulation, which is the main reason why the image is lipophilic.

3.2 The complexing agent has a significant influence on the morphology of silver particles: when the complexing constant is large, the development speed is fast, forming high-density and closely packed silver particles; When the complex constant is small, the development speed is slow, forming low-density dendritic silver. The diffusion ability of silver complex also has a great influence on the transfer process.

The higher the temperature, the faster the development speed, the higher the density of silver image and the smaller the silver particles.